The Sun : Part 1 - Dynamic wonderland of the Sun
Dr.Benu Chatterjee
The Sun is at the heart of the solar system. Humans were aware of the Sun since the dawn of civilization. Some thousands of years ago, Sun was worshiped by the ancient Egyptians as the most important God, Ra who was considered to be the creator of everything. Sun is personified by mythological names such as Helios by the Greeks and Sol by the Romans.
As a very active star (defined as a source of heat and light) with a nuclear furnace at its core, Sun is always doing something interesting. Life on Earth is greatly affected by Sun. In the course of human history, it has been well recognised that without the energy from sunlight, no vegetation can grow, and without vegetation there is no source of nourishment for any earthly creatures to survive. However, what we know today that our ancestors did not is just how far reaching is the scope of Sun’s influence.
Despite the fact that the warm, life-giving nuclear factory we call the Sun is essential to all life on Planet Earth, yet it is million times more violent and destructive than any other force our planet faces. This is the same Sun that eradicated the atmosphere on Mars some four billion years ago when that planet lost its magnetic field. Earth sits much closer to the sun than Mars does and is thus more intensely subjected to Sun’s formidable power which has a core temperature of over 15 million degree Celsius ( 0C) and a surface temperature of more than 5,000 oC.
One wonders if the Sun with all its energetic functions is a friend, a threat or a bit of both to Planet Earth. Sun’s behaviour broadly reflects two extreme features : 1) normal mundane daily pursuit of radiating the Earth with sunlight, and 2) occasional showering of our planet with solar outbursts generated from dynamic activity inside Sun’s body. In a two part series, the present article discusses all the unique activities of the glorious Sun. In part 2, prospect of replicating artificial Sun on Earth is reviewed.
1. Introduction
The structure of Sun consists of layers, much like the Earth, but without any sharply defined solid surface because it is too hot to be anything but gas. At present, it is about 70% hydrogen and 28% helium by mass with everything else including metals amounts to less than 2%. The composition changes slowly over time as more hydrogen is converted to helium at Sun’s core via nuclear fusion.
Besides acting as a source of heat and light, Sun also emits a low density stream of charged particles (mostly electrons and ions) known as solar plasma which propagates as solar wind through the solar system at about 450 km/sec. This solar wind along with much higher energy particles ejected in the form of solar flares dramatically affect Earth’s power distribution grids, GPS (Global Positioning System) satellites etc, and at the same time could result in creating beautiful Northern Lights (Aurora Borealis).
The various aspects of Sun’s energetic are described in the present article along with a comparative assessment of physical properties of the Sun and Earth. This is followed by a review of the science of solar radiation in terms of sunlight. Finally, the unique dynamism of Sun is discussed with references to activities such as solar wind, flares, sunspots etc.
2. Layers of Sun’s atmosphere
The physical characteristics including a temperature profile of the Sun are highlighted next in terms of six regions with no sharp boundaries between them :
1) fusion core where all the energy is generated from thermonuclear fusion of hydrogen producing helium with the temperature reaching ~ 15*10^6 o C
2) radiation zone where energy from the core travels outward via radiation as photons through a dense plasma of mostly hydrogen, making it very hot ~ 6*10^6 oC
3) convection zone where energy from the radiation shell flows out by a convection process as thermal energy in the form of a circulating plasma toward the surface, producing ~ 1*10^6 oC 4) photosphere (sphere of light) which is Sun’s surface emitting lights as photons where the expected temperature is ~ 5500oC 5) chromosphere above photosphere, emitting reddish glow at temperature ~ 30,000 oC , and
6) corona which is a superhot region registering ~ 10^6 oC and emitting x-rays (possibly because of interactions with the sun's magnetic field). Corona shows up in the upper portion of sun’s atmosphere.
2.2 Brief account of solar layers
Photosphere - This is the deepest outer layer of the Sun that we observe directly as sunlight. It is marked by bright, bubbling granules of plasma and darker, cooler sunspots which emerge when sun’s magnetic field breaks through the surface. Photosphere is also the source of solar flares that produce bursts of x-rays, UV radiation, electromagnetic radiation and radio waves.
Chromosphere - It is present between about 250 miles (400 km) and 1300 miles (2100 km) above the solar surface i.e. the photosphere. It emits reddish glow as super-heated hydrogen burns off. However, the glow is too weak to be seen against the brighter photosphere, except during a total solar eclipse when it is visible as a red rim.
Transition Region - The transition region is a very narrow (60 miles / 100 km) layer between the chromosphere and the corona where the temperature rises abruptly from about 7700 to 500,000 0 C (14,000 to 900,000 degrees F).
Corona - This is the outermost layer of the Sun’s atmosphere, starting at about 1300 miles (2100 km) above the solar surface i.e. photosphere and is very hot ~ 10 ^6 oC. The corona cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph. It appears as white streamers or plumes of ionized gas that flows outward into space.
2.2 Unusual feature about temperature distribution
When Sun’s core can get as hot as 15*10^6 oC, its outer layers are cooler because heat normally passes outwardly from hot to cold. The surface layer (photosphere) is thus the coolest layer as expected. However, Sun’s outermost atmospheric layers such as chromospheres and corona are, as indicated above, much hotter than photosphere. In fact, corona at about 10^6 oC is almost 200 times hotter than photosphere despite being further away from Sun’s heat core. Flow of heat from cooler region (photosphere) to a hotter zone (corona) is a total violation of second law of thermodynamics. So there must be some form of non-thermal mechanism operating that carries energy from cooler solar surface to hotter corona. This enigma has puzzled solar physicists for decades. The latest proposed mechanisms are based on wave heating and magnetic reconnection, both of which require knowledge of physics in some depth which is beyond the scope of the present article
3. Origin, magnitude and dissipation of solar energy
3.1 Energy at Sun’s core
The major source of energy that powers Earth’s oceans, atmosphere, land and biological cycle comes from the Sun whose main engine is at its inner core where a colossal amount of energy around 3.8*10^26 joules per second or watts is generated by thermonuclear fusion of about 700 million tons of hydrogen atoms into 695 million tons of helium each second.
Nuclear fusion is a way of producing energy by fusing two light nuclei such as heavy hydrogen /isotope of hydrogen namely deuterium, to make one heavier nucleus namely helium. (Deuterium can be denoted as 21H where 2 represents atomic weight i.e. mass number containing one proton and one neutron, while 1 denotes charge number or atomic number i.e. number of protons; helium He is represented as 42 He). Energy is created because the mass of the two deuterium nuclei is slightly heavier than the mass of one nucleus of helium (this issue is discussed in detail in Part 2). Einstein’s famous equation relating energy (E) to change in mass (∆m) times the square of speed of light (c^2) i.e. E = ∆m*c^2 indicates that as speed of light squared is a vast number, even a miniscule loss of mass ∆m during fusion would produce a massive amount of energy. Thus our Sun, like all stars, involves fusion of elements to create denser element, and thereby enough energy to maintain its high temperature.
Fusion is possible in the Sun (and any star for that matter) only because of its immense mass, leading to an astronomically large gravitational field. The size of the star causes extremely high pressure and temperature within its core, where large amounts of hydrogen are present. Temperature and pressure at Sun’s core is ~ 15 million i.e. 15*10^6 0C and 250 billion i.e. 250* 10^9 atmospheres respectively. Under these conditions, hydrogen atoms can overcome the electromagnetic repulsion of similar charges between themselves, and undergo nuclear fusion. The magnitude of the repelling force increases with the electrical charges on the two nuclei. To keep this force small, the interacting nuclei should, therefore, have the lowest possible charge (or atomic number).
Fusion primarily takes place during the main sequence of a star which is the time for a star to convert its available hydrogen into helium, which in turn produces light and heat energy. The lower the mass of a star, the longer it takes to burn up all its hydrogen, chiefly because of the relationship between mass and gravity. For example, Sun is about halfway through its 10 billion year main sequence. During this stage, the Sun is in hydrostatic equilibrium. This would imply that its gravity and the opposing thermal pressure created by
fusion are pushing against each other equally to provide stability within the star. Gravity pulls all the mass of Sun inward at the core, creating a density ~ 150g/cm^3 raising a colossal pressure and intense temperature in the core as mention above.
If the super high temperature is not maintained, the sun will collapse in on itself. On the other hand, one would expect the extreme temperature inside Sun’s nuclear plant to blow out Sun’s core apart. But surprisingly, this does not happen because of the enormous pressure of solar matter that surrounds the core. Some of the mass is lost from the Sun and reappears as energy following Einstein’s equation of E=∆m*c^2. Also, at this extreme temperature, gas is ionized, and turns into plasma where atoms break apart into charged particles allowing both species of ions and electrons to coexist. Thus the Sun instead of being solid, liquid or gas, is instead made up of plasma, the “fourth state of matter” which are formed when atoms are stripped down to bare protons and electrons. All these charged particles make solar plasma a splendid conductor of electricity. (Although the same name, plasma in our blood is totally different.)
3.2 Dissipation of energy from Sun’s core
The huge energy from the core is initially radiated in the form of gamma rays which are continuously absorbed and re-emitted at lower and lower temperatures. The energy change involved in reaching Sun’s outer surface i.e. photosphere mostly via convection rather than the initial radiation route, is displayed as a round, golden solar disk which we observe from Earth. Photosphere is the visible surface of the Sun that we are most familiar with. It is, like the Sun (a ball of gas), is not a solid surface but contains gaseous layer of ~100km thickness which is extremely thin compared to Sun’s radius of ~ 7*10^5 km The total quantity of energy emitted from photosphere is about 6.33*10^7 W/m^2 with the surface temperature around 5500oC.
3.3 Solar energy hitting Earth’ extraterrestrial atmosphere
The intensity of solar radiation emitted from photosphere becomes weaker as it spreads out into space. The average amount of electromagnetic solar energy (light and heat) eventually hitting Earth’s top i.e. extraterrestrial atmosphere is termed as solar constant. It can be calculated by Newton’s Inverse Square Law according to which (I1 / I2) = (D2/D1) ^2 where I1 and I2 are energy intensities at corresponding increasing distances D1 and D2 respectively. The above equation can be re-written as I2 = I1* D1^2 / D2^2. In the present context, I2 represents energy received at Earth’s outer atmosphere i.e. the solar constant, I1 denotes energy from Sun’s photosphere (6.33*10^7W/m^2), D1 is the radius of Sun’s sphere (6.96*10^8 m) and D2 is the radius from Sun to Earth i.e. Sun – Earth distance (1.5 *10^11 m). Using these data, solar constant I2 is calculated as 1368.8 W/m^2 which is of course based on following assumptions : 1) as Sun’s rays spread out into space, the radiations by the time they reach the top of Earth’s atmosphere are considered to be parallel and perpendicular to the atmosphere 2) surface receiving the radiation is at a mean Sun- Earth distance of 1.5*10^11m 3) radiation (6.33*10^7 W/m^2)from Sun’s photosphere remains constant, and 4) calculation is made for the top of atmosphere and not for the surface of Earth. The calculated value is similar to the generally accepted value of 1368 W/m^2 which is averaged yearly via NASA satellite measurements. It It is important to remember that the solar constant of 1368 W/m^2 is not the intensity of energy per square metre of Earth’s surface, but per square metre of extraterrestrial atmosphere which can be considered as a sphere of radius of 1.5*10^11 m with the Sun at its centre.
3.4 Solar energy per unit area of Earth’s surface
If the Earth were a flat disc facing the Sun, then 1368 watts of solar energy would be received by every square metre of Earth’s surface. In such a case, with r denoting radius of the Earth, the total solar radiation incident on Earth’s disc would be the solar constant multiplied by area of a flat circle namely π* r^2. In its real shape as a sphere, Earth will still receive the same amount of solar radiation as a flat disc except the radiation i.e. the solar energy would be expected to spread out over a larger area (4π* r^2) for a sphere. Thus the average energy from the Sun that fall upon every square metre of the spherical surface of Earth would be (1368 * π* r^2) / 4π r^2 = 1368 / 4 = 342 W/m^2
3.5 Actual amount of solar energy striking Earth’s surface
It is assumed in sec.3.4 that the Earth did not have any atmosphere. However, in reality, while travelling towards Earth’s surface, some of the solar energy is lost in its path when Earth’s atmosphere itself absorbs ~ 67 W/m^2 and reflects back ~ 77W/m^2 to space. This leaves 342 – (67+77) = 198 W/m^2 which is 198*100/342 ~ 58% of the solar energy intercepted by the Earth, reaches Earth’s surface. Of this quantity, 168 W/m^2 is absorbed and 30 W/m^2 is reflected back by the Earth to space.
Assuming Earth as a disk, the cross section area (π* r^ 2) that would intercepts the solar radiation, would be 168 * π r^2 watts where r is the radius of Earth (6376 km). This would result in generating a colossal amount of solar power of ~21 quadrillions (2.1* 10^16) watts that would strike the entire area of the Earth. Thus the Sun even though so far from the Earth, still acts as an immensely powerful source of energy. It is interesting to note that the trip of photons from Sun’s photoshere to Earth’s extraterrestrial atmosphere which covers a distance of 1.5*10^11 m, takes about 8.5 minutes.
Of course, with all this massive energy input, Earth should just get hotter all the time. But from our common experience we know that when an object heats up, it starts to give off heat energy by radiation. So the more Earth heats up, the more rapidly it loses energy into space. In this way, a proper energy balance is restored between the incoming solar energy and outgoing thermal (heat) energy.
4. Basic facts about Sun and Earth
The Sun is by far the largest object in the solar system with a diameter of 696*10^6 m which is about 109 times bigger than Earth (see Table below). Interestingly, ratio of the corresponding volumes indicates that about one million Earths could theoretically fit inside an imaginary hollow Sun.
The total mass of the Sun is 2*10^30 kg which is about 330,000 times the mass of Earth, and accounts for 99.8% of the mass of the solar system. About three quarters of Sun’s mass consist of hydrogen while the rest is mostly helium, and > 2% of elements such as oxygen, carbon, neon, iron
Radius (m)
696*10^6 : Sun
637.6*10^4 : Earth
109 : Sun/Earth
Mass (kg)
2*10^30 : Sun
6*10^24 : Earth
333,000 : Sun/Earth
Volume (m³)
1.4*10^27 : Sun
1.1*10^21 : Earth
1.3 million : Sun/Earth
Average density (kg/m³)
1400 : Sun
5506 : Earth
1/4 : Sun/Earth
Gravitational acceleration at surface (m/s²)
274 : Sun
9.81 : Earth
28 : Sun/Earth
Rotation period (days)
26 (at equator) : Sun
1 : Earth
1/26 : Sun/Earth
Temperature at surface (C)
Photosphere ~5500 : Top of Sun's surface
~20 : Earth
275 : Sun's surface/Earth
Escape velocity at surface (km/s)against gravity
~618 : Sun
11.2 : Earth
~55 : Sun/Earth
and others. The Sun is neither a solid nor a gas but is actually made of plasma (see sec.3.1 below) which is tenuous and gaseous near the surface but gets denser down towards the Sun’s fusion core.
Other interesting data : Earth-Sun Distance: 93 million miles (150 million km) Total luminosity (energy radiated per second) of Sun : ~ 4 x 10³³ erg/s. [As bright as 4 trillion trillion 100-watt light bulbs ]bulbs] Diameter of Photosphere : ~ 1.39 * 10^6 km ; Solar cycle of high and low activity : Repeats about every 11 years Age of Sun: 4.5 billion years and would, according to astrophysicist, become a red giant in the next about 5 billion years
5. Radiation from the Sun
5.1 Sunlight
It is worth mentioning straightway that the "sunlight" produced at the core doesn't just fly off into space. As has been seen, the proton-proton chain doesn't even produce light in the visible part of the spectrum. Instead, the high-energy protons and resultant kinetic energy produced induces other atoms in the Sun to vibrate and, in turn, release photons of many different frequencies, including those in the visible part of the spectrum. These photons are re-absorbed and re-emitted by adjacent atoms, each, on average, slightly closer to the surface of the Sun. Finally, after around 100 to 200 thousand years an atom at the surface of the Sun absorbs and then re-emits a photon, which flies off into space. Then, if heading in our direction, it takes around 8.5 minutes for the photon to reach the Earth. All of the daylight that we see started its journey a very long time ago.
The most outstanding characteristic of the Sun is the fact that it emits huge quantities of electromagnetic radiation of all wavelengths including visible range and beyond the rainbow spectrum of VIBGYOR, namely IR (below red) and UV (above violet) ranges. The energy emitted by the Sun is divided approximately into 40% visible light, 50 % infrared (IR), 9% ultraviolet (UV) and 1% x-ray, radio etc. The glow of a hot object like the Sun indicates how hot it is. For planets and stars, physicist’s theory of “black body radiation” is used as a first approximation for the energy they emit. Based on this concept, the glow of a body is taken as the way a red-hot object produces light. The hotter the object, the brighter it shines. In the case of Sun, the colour of Sun’s surface i.e. photosphere suggests a temperature around 5500 C.
Almost all of Sun’s energy warms up the Earth and, as mentioned above, is the driving force behind all our weather, climate and ecosystem. The sunlight reaching the top of Earth’s atmosphere provides about 2,500 times as much energy as the total of all other sources combined. This staggering statistics remains quite steady. It varies only by about 0.1% over an entire course of a stable (11-year long) solar cycle.
Most of the solar UV radiation is absorbed by the concentrated layer of ozone gas found in the stratosphere. Earth’s atmosphere is estimated to block 98.7% of Sun’s UV radiation from penetrating through its atmosphere. IR radiation is partially absorbed by carbon dioxide, ozone and water vapour in Earth’s atmosphere.
A totally different source of IR radiation is produced from the earth. As the ground on Earth is heated by sunlight, it begins to radiate. But being too cool to radiate even a dull red, Earth, unlike Sun’s predominant UV rays, radiates in the IR range which travel back from the Earth to the outer space. In this way a balance is restored between the absorbed sunlight and emitted infrared radiation, otherwise an imbalance would result that could cause global warming (“Green house” effect) or global cooling.
5.2 Effect of UV exposure
With 98.7% of Sun’s UV radiation being blocked by Earth’s atmosphere, the rest namely 2.3% that gets through has both harmful and beneficial effects. The molecules of a substance when hit by UV rays start to vibrate back and forth. However the UV energy is quite often high enough to knock electrons away from the atoms or cause molecules to split instead of just being shaken up. As a result, a change in the chemical structure of the molecule occurs which can be detrimental to living organisms causing cell damages, making UV radiation from the Sun an environmental human carcinogen. It cam damage immune system, cause sunburn, speed up aging of skin etc. However, there are some credit sides of Sun’s UV if used in moderation. UV is needed by our bodies to produce vitamin D which helps to strengthen bones, muscles etc. It is used to treat some skin condition such as psoriasis. Many insects use UV emission from celestial objects as reference for navigation in flight. UV has positive applications in the fields of disinfection and sterilisation.
6. Dynamism of Sun
Sun would appear to change little from our viewing position on Earth. As a provider of life and heat to Planet Earth, Sun appears quite placid – it is, however, anything but. Though laying a very long way from Earth, Sun’s unceasing activity has had a dramatic impact on our planet. In normal circumstance, we are all worried about the weather, but there is another kind of weather to worry about which comes not from the horizon but from the Sun some 150 million km (93 million miles) away. The greatest threat to Earth sits right at the heart of our solar system namely the Sun. A violent activity on Sun’s surface producing solar storms could disrupt our technological civilization.
The Sun with a huge nuclear bomb at its heart is the worst place in the solar system when it comes to stormy weather. Much of Sun’s tempestuous nature comes from its core which produces dense, electrically charged gas in the form of a special form of matter called plasma. This rolling, boiling plasma generates Sun’s powerful magnetic field that produces various solar activities defined below namely solar flares, solar prominence, solar wind, coronal mass ejection (CME) etc.
6.1 Activity of magnetic field
Origin of magnetic Field
Everyday physics tells us that magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a magnet originates from the motion of negatively charged electrons in the magnet. Although the origin of Earth's magnetic field is not completely understood, it is, however, thought to be associated with the so called dynamo effect in the outer core of Earth’s interior. The source of such field appears to be electrical currents and magnetic fields mixed in turbulent motions of hot conducting metal fluids of iron and nickel in Earth’s outer core influenced by the rotation of Earth.
Earth's magnetosphere
Earth’s magnetic field acts like a giant invisible bubble called magnetosphere that shields the planet from much of the dangerous cosmic radiation spewing from the Sun in the form of solar wind. The solar wind as mentioned above is a stream of ionized gases i.e. plasma that blows outward from the Sun at about 450 km/second – its intensity varies with the degree of surface activity on the Sun. When the solar wind encounters magnetosphere, it is deflected like water around the bow of a ship. The imaginary surface at which the solar wind is first deflected is called the bow shock. The corresponding region of space sitting behind the bow shock and surrounding the Earth represents a region of space dominated by the Earth's magnetic field called magnetosphere which largely prevents the solar wind from entering.
Sun’s magnetic field
The amplitude and spatial configuration of Sun’s magnetic field varies with times scales ranging from a fraction of a second to billions of years. Sun, like Earth, has its north and south poles. Evolution of Sun’s activity is directly linked to changes in its magnetic field that undergoes a complete makeover about every 11 years when the polarity flips with North Pole becoming South Pole and vice versa. Changes to the field's polarity ripple all the way out to the Voyager probes, on the doorstep of interstellar space. The exact internal mechanism of such reversal of polarity is not clearly understood. The domain of the sun's magnetic influence (also known as the "heliosphere") extends billions of kilometers beyond Pluto.
The strength of Sun’s magnetic field is typically only about twice as strong as Earth’s field. However, it can become highly concentrated in small areas, reaching up to 3,000 times stronger than usual. Kinks and twist in the magnetic field are developed because the Sun spins more rapidly at the equator than at the higher latitudes, and also because the inner parts of the Sun rotating more quickly than the surface. Build up of these distortions throughout an 11- year cycle create regions of intense magnetic activity that show up as sunspots on Sun’s surface i.e. the photosphere.
6.2 Outburst from the Sun
One of the most dramatic properties of Sun’s activity from its complex magnetic activity is the existence of a solar cycle which is best seen through development of the pattern of sunspots. The powerful magnetic fields around sunspots produce active regions on the Sun which often lead to solar flares and CMEs. Over the course of about a month, sunspots disintegrate with regular spectacular release of massive amounts of
energy known as solar flares. Associated with these flares are violent bursts of plasma (CMEs) which travel as solar winds till they reach the Earth where they induce geomagnetic storms that could have major disturbance of Earth’s magnetosphere They can have a power to disrupt our electrical power grids, GPS satellites and commercial flights.
In summarizing, solar energy produced at the core by nuclear fusion, is radiated via radiation and convection to finally reach the surface i.e. photosphere from where solar radiation is emitted into space. On the photosphere, localized sunspots appear which burst into solar flares as plasma eruptions in the form of fiery looping rain on the Sun. Solar wind from the flares containing plasma of protons and electrons travels through the solar system. When encountered by Earth, it is mostly deflected by magnetosphere.
6.3 Brief accounts of various solar activities
Solar cycle : One of the most dramatic activities of the Sun is the existence of solar cycle. This can be best seen in the pattern of sunspots. Based on a study of the average number of sunspots over the last 300 years, it becomes apparent that sunspots vary significantly displaying a peak number about every 11 years. The Sun, thus, has a cycle of behaviour which repeats on average every 11 years incorporating solar maximum and solar minimum which refer respectively to periods of maximum and minimum sunspot counts. Cycles span from one minimum to the next. Astronomers at the Wilcox Solar Observatory (WSO) monitor the sun's global magnetic field on a daily basis. Magnetograms at Wilcox have been tracking Sun’s polar magnetism since 1976, and they have recorded three grand reversals—with a fourth in the offing. A reversal of the sun's magnetic field is, literally, a big event. According to Phil Scherrer of Stanford University, Sun’s polar magnetic fields gradually weaken to zero, and then emerge again with the opposite polarity. This reversal of Sun’s magnetic field is a regular part of the solar cycle.
Sunspots : These are originated from Sun’s magnetic field welling up to the photosphere. High magnetic pressure inside sunspots forces hot plasma out in order to balance the pressure. As a result, the matter inside the spots is less dense and somewhat cooler ~ 3500 C, making them appear as dark blotches (when viewed in optical wavelengths) in comparison with the brighter and hotter surrounding ~ 5500C of photosphere. Sunspots are associated with higher activity of the sun which means the more number of sunspots appearing, the more active the sun is. Sunspots vary in size ranging from the size of United Kingdom to several times the size of Earth. Normally they come in groups and can last from a few hours to several days.
Flares and Prominences : A solar flare appearing like giant flame is caused by a sudden eruption of intense, high-energy hot plasma radiation from Sun’s surface that stretch out from the surface. They can sometimes be trapped by nearby magnetic fields via convection currents within the sun, and pulled back to the sun in the form of giant arcs called prominences. These arcs reflect the effect of the hot plasma gas travelling along the magnetic field lines which may themselves remain invisible. Occasionally, these invisible magnetic fields can break and cause a prominence or a CME ejection.
Coronal mass injections (CMEs) : These originate from the snapping of local magnetic fields that have captured plasma (because it’s a charged, super-heated gas that can follow magnetic fields). CMEs containing charged ions are sent into space by the most powerful solar storms. The hot gas of CME is released into space as a gigantic explosion, sending material toward the Earth at speeds reaching 3200 Km/s. If Earth happens to be in the path of a CME, the charged particles can slam into our atmosphere with enough energy to cause a temporary disturbance of the Earth’s magnetic field, disrupt satellites in orbit and even cause them to fail, and bathe high-flying airplanes with radiation. They have the potential to overwhelm our magnetic field for a short period of time, causing problems with GPS satellites and power grids and disrupting telecommunications and navigation systems.
Travelling at the speed of light, it takes about eight minutes for the light (photons) of solar flare to reach Earth from photosphere. In contrast, the immense cloud of hot plasma containing magnetized particles hurled into space such as CME travels around million miles per hour taking up to three days to reach Earth.
Solar storm/ wind : This is associated with a release of streams of plasma particles (protons, electrons, neutrinos etc), energy, and radiation from Sun's surface. It travels past us at a rate between 300 and 800 km/s and varies from day to day with changes in sun's activity. The solar storm is the cause of a comet's tail and also the cause of the auroras (northern or southern lights). Earth would be more affected by the solar storm were it not for the magnetic field that surrounds us. The magnetic field causes the charged particles from the sun to flow around the Earth barely showing any effects at all, usually. The storm on the sun causes a type of storm on the Earth, known as a geomagnetic storm.
6.4 Devastation on Earth from solar outburst
Flares and CMEs have different effects on Earth. A solar flare could hit the Earth like a lightning bolt affecting the radio waves which would result in degradation or at worst, temporary blackouts of signals in navigation and communications. In contrast, a CME striking Earth’s atmosphere can cause disturbance of Earth’s magnetic field.
The record about power failures from solar storms commonly refers back to March 13, 1989 – 23 years ago which was the last time a huge flare/CME affected the Earth due to the failure of magnetosphere to provide the Earth with 100% protection. The entire province of Quebec, Canada suffered a devastating 12-hour total electrical power blackout - the electrical supply was cut off to over 6 million people for 9 hours. Parts of the northeast of U.S also suffered blackout from power failure.
Solar storms were even more powerful on August 28, 1859with larger solar flare than the one that caused the 1989 Québec and northeastern U.S. blackout. It was observed and recorded by Richard C. Carrington, and sometimes called as the 1859 Solar Super storm. The accompanying CME traveled to Earth in only 17 hours, rather than the usual three to four days. The largest recorded geomagnetic storm occurred with observations of auroras or northern lights in many parts of the world. Telegraph systems throughout Europe and North America failed.
7. Final comments
The basic sequence of Sun’s activity is as follows : Sunspots on photosphere → Solar flares and/or Prominences, CME →Solar storm/wind containing plasma of protons and electrons from the Sun that travel through the solar system. As a layman in solar physics, It has been a fascinating experience to review Sun’s dynamism in some simple basic details.
For the future, it has been suggested that a space observatory might include a radiometric imager for a better understanding of how solar variability might affect the Earth. One would like to think that such a device could essentially map the surface of the Sun and reveal contributions of the surface to Sun’s luminosity. It is interesting to note that with eye on the Sun, the presence of a filament (stable less than a week) of a length of 50 Earths side-by-side has been very recently reported (NASA 360, 29 October,2015) from Solar Dynamics Observatory (SDO). These are apparently elongated clouds of solar material tethered above the Sun by magnetic forces. This image makes Sun’s mysticism more intriguing.
Dr.Benu Chatterjee
The Sun is at the heart of the solar system. Humans were aware of the Sun since the dawn of civilization. Some thousands of years ago, Sun was worshiped by the ancient Egyptians as the most important God, Ra who was considered to be the creator of everything. Sun is personified by mythological names such as Helios by the Greeks and Sol by the Romans.
As a very active star (defined as a source of heat and light) with a nuclear furnace at its core, Sun is always doing something interesting. Life on Earth is greatly affected by Sun. In the course of human history, it has been well recognised that without the energy from sunlight, no vegetation can grow, and without vegetation there is no source of nourishment for any earthly creatures to survive. However, what we know today that our ancestors did not is just how far reaching is the scope of Sun’s influence.
Despite the fact that the warm, life-giving nuclear factory we call the Sun is essential to all life on Planet Earth, yet it is million times more violent and destructive than any other force our planet faces. This is the same Sun that eradicated the atmosphere on Mars some four billion years ago when that planet lost its magnetic field. Earth sits much closer to the sun than Mars does and is thus more intensely subjected to Sun’s formidable power which has a core temperature of over 15 million degree Celsius ( 0C) and a surface temperature of more than 5,000 oC.
One wonders if the Sun with all its energetic functions is a friend, a threat or a bit of both to Planet Earth. Sun’s behaviour broadly reflects two extreme features : 1) normal mundane daily pursuit of radiating the Earth with sunlight, and 2) occasional showering of our planet with solar outbursts generated from dynamic activity inside Sun’s body. In a two part series, the present article discusses all the unique activities of the glorious Sun. In part 2, prospect of replicating artificial Sun on Earth is reviewed.
1. Introduction
The structure of Sun consists of layers, much like the Earth, but without any sharply defined solid surface because it is too hot to be anything but gas. At present, it is about 70% hydrogen and 28% helium by mass with everything else including metals amounts to less than 2%. The composition changes slowly over time as more hydrogen is converted to helium at Sun’s core via nuclear fusion.
Besides acting as a source of heat and light, Sun also emits a low density stream of charged particles (mostly electrons and ions) known as solar plasma which propagates as solar wind through the solar system at about 450 km/sec. This solar wind along with much higher energy particles ejected in the form of solar flares dramatically affect Earth’s power distribution grids, GPS (Global Positioning System) satellites etc, and at the same time could result in creating beautiful Northern Lights (Aurora Borealis).
The various aspects of Sun’s energetic are described in the present article along with a comparative assessment of physical properties of the Sun and Earth. This is followed by a review of the science of solar radiation in terms of sunlight. Finally, the unique dynamism of Sun is discussed with references to activities such as solar wind, flares, sunspots etc.
2. Layers of Sun’s atmosphere
The physical characteristics including a temperature profile of the Sun are highlighted next in terms of six regions with no sharp boundaries between them :
1) fusion core where all the energy is generated from thermonuclear fusion of hydrogen producing helium with the temperature reaching ~ 15*10^6 o C
2) radiation zone where energy from the core travels outward via radiation as photons through a dense plasma of mostly hydrogen, making it very hot ~ 6*10^6 oC
3) convection zone where energy from the radiation shell flows out by a convection process as thermal energy in the form of a circulating plasma toward the surface, producing ~ 1*10^6 oC 4) photosphere (sphere of light) which is Sun’s surface emitting lights as photons where the expected temperature is ~ 5500oC 5) chromosphere above photosphere, emitting reddish glow at temperature ~ 30,000 oC , and
6) corona which is a superhot region registering ~ 10^6 oC and emitting x-rays (possibly because of interactions with the sun's magnetic field). Corona shows up in the upper portion of sun’s atmosphere.
2.2 Brief account of solar layers
Photosphere - This is the deepest outer layer of the Sun that we observe directly as sunlight. It is marked by bright, bubbling granules of plasma and darker, cooler sunspots which emerge when sun’s magnetic field breaks through the surface. Photosphere is also the source of solar flares that produce bursts of x-rays, UV radiation, electromagnetic radiation and radio waves.
Chromosphere - It is present between about 250 miles (400 km) and 1300 miles (2100 km) above the solar surface i.e. the photosphere. It emits reddish glow as super-heated hydrogen burns off. However, the glow is too weak to be seen against the brighter photosphere, except during a total solar eclipse when it is visible as a red rim.
Transition Region - The transition region is a very narrow (60 miles / 100 km) layer between the chromosphere and the corona where the temperature rises abruptly from about 7700 to 500,000 0 C (14,000 to 900,000 degrees F).
Corona - This is the outermost layer of the Sun’s atmosphere, starting at about 1300 miles (2100 km) above the solar surface i.e. photosphere and is very hot ~ 10 ^6 oC. The corona cannot be seen with the naked eye except during a total solar eclipse, or with the use of a coronagraph. It appears as white streamers or plumes of ionized gas that flows outward into space.
2.2 Unusual feature about temperature distribution
When Sun’s core can get as hot as 15*10^6 oC, its outer layers are cooler because heat normally passes outwardly from hot to cold. The surface layer (photosphere) is thus the coolest layer as expected. However, Sun’s outermost atmospheric layers such as chromospheres and corona are, as indicated above, much hotter than photosphere. In fact, corona at about 10^6 oC is almost 200 times hotter than photosphere despite being further away from Sun’s heat core. Flow of heat from cooler region (photosphere) to a hotter zone (corona) is a total violation of second law of thermodynamics. So there must be some form of non-thermal mechanism operating that carries energy from cooler solar surface to hotter corona. This enigma has puzzled solar physicists for decades. The latest proposed mechanisms are based on wave heating and magnetic reconnection, both of which require knowledge of physics in some depth which is beyond the scope of the present article
3. Origin, magnitude and dissipation of solar energy
3.1 Energy at Sun’s core
The major source of energy that powers Earth’s oceans, atmosphere, land and biological cycle comes from the Sun whose main engine is at its inner core where a colossal amount of energy around 3.8*10^26 joules per second or watts is generated by thermonuclear fusion of about 700 million tons of hydrogen atoms into 695 million tons of helium each second.
Nuclear fusion is a way of producing energy by fusing two light nuclei such as heavy hydrogen /isotope of hydrogen namely deuterium, to make one heavier nucleus namely helium. (Deuterium can be denoted as 21H where 2 represents atomic weight i.e. mass number containing one proton and one neutron, while 1 denotes charge number or atomic number i.e. number of protons; helium He is represented as 42 He). Energy is created because the mass of the two deuterium nuclei is slightly heavier than the mass of one nucleus of helium (this issue is discussed in detail in Part 2). Einstein’s famous equation relating energy (E) to change in mass (∆m) times the square of speed of light (c^2) i.e. E = ∆m*c^2 indicates that as speed of light squared is a vast number, even a miniscule loss of mass ∆m during fusion would produce a massive amount of energy. Thus our Sun, like all stars, involves fusion of elements to create denser element, and thereby enough energy to maintain its high temperature.
Fusion is possible in the Sun (and any star for that matter) only because of its immense mass, leading to an astronomically large gravitational field. The size of the star causes extremely high pressure and temperature within its core, where large amounts of hydrogen are present. Temperature and pressure at Sun’s core is ~ 15 million i.e. 15*10^6 0C and 250 billion i.e. 250* 10^9 atmospheres respectively. Under these conditions, hydrogen atoms can overcome the electromagnetic repulsion of similar charges between themselves, and undergo nuclear fusion. The magnitude of the repelling force increases with the electrical charges on the two nuclei. To keep this force small, the interacting nuclei should, therefore, have the lowest possible charge (or atomic number).
Fusion primarily takes place during the main sequence of a star which is the time for a star to convert its available hydrogen into helium, which in turn produces light and heat energy. The lower the mass of a star, the longer it takes to burn up all its hydrogen, chiefly because of the relationship between mass and gravity. For example, Sun is about halfway through its 10 billion year main sequence. During this stage, the Sun is in hydrostatic equilibrium. This would imply that its gravity and the opposing thermal pressure created by
fusion are pushing against each other equally to provide stability within the star. Gravity pulls all the mass of Sun inward at the core, creating a density ~ 150g/cm^3 raising a colossal pressure and intense temperature in the core as mention above.
If the super high temperature is not maintained, the sun will collapse in on itself. On the other hand, one would expect the extreme temperature inside Sun’s nuclear plant to blow out Sun’s core apart. But surprisingly, this does not happen because of the enormous pressure of solar matter that surrounds the core. Some of the mass is lost from the Sun and reappears as energy following Einstein’s equation of E=∆m*c^2. Also, at this extreme temperature, gas is ionized, and turns into plasma where atoms break apart into charged particles allowing both species of ions and electrons to coexist. Thus the Sun instead of being solid, liquid or gas, is instead made up of plasma, the “fourth state of matter” which are formed when atoms are stripped down to bare protons and electrons. All these charged particles make solar plasma a splendid conductor of electricity. (Although the same name, plasma in our blood is totally different.)
3.2 Dissipation of energy from Sun’s core
The huge energy from the core is initially radiated in the form of gamma rays which are continuously absorbed and re-emitted at lower and lower temperatures. The energy change involved in reaching Sun’s outer surface i.e. photosphere mostly via convection rather than the initial radiation route, is displayed as a round, golden solar disk which we observe from Earth. Photosphere is the visible surface of the Sun that we are most familiar with. It is, like the Sun (a ball of gas), is not a solid surface but contains gaseous layer of ~100km thickness which is extremely thin compared to Sun’s radius of ~ 7*10^5 km The total quantity of energy emitted from photosphere is about 6.33*10^7 W/m^2 with the surface temperature around 5500oC.
3.3 Solar energy hitting Earth’ extraterrestrial atmosphere
The intensity of solar radiation emitted from photosphere becomes weaker as it spreads out into space. The average amount of electromagnetic solar energy (light and heat) eventually hitting Earth’s top i.e. extraterrestrial atmosphere is termed as solar constant. It can be calculated by Newton’s Inverse Square Law according to which (I1 / I2) = (D2/D1) ^2 where I1 and I2 are energy intensities at corresponding increasing distances D1 and D2 respectively. The above equation can be re-written as I2 = I1* D1^2 / D2^2. In the present context, I2 represents energy received at Earth’s outer atmosphere i.e. the solar constant, I1 denotes energy from Sun’s photosphere (6.33*10^7W/m^2), D1 is the radius of Sun’s sphere (6.96*10^8 m) and D2 is the radius from Sun to Earth i.e. Sun – Earth distance (1.5 *10^11 m). Using these data, solar constant I2 is calculated as 1368.8 W/m^2 which is of course based on following assumptions : 1) as Sun’s rays spread out into space, the radiations by the time they reach the top of Earth’s atmosphere are considered to be parallel and perpendicular to the atmosphere 2) surface receiving the radiation is at a mean Sun- Earth distance of 1.5*10^11m 3) radiation (6.33*10^7 W/m^2)from Sun’s photosphere remains constant, and 4) calculation is made for the top of atmosphere and not for the surface of Earth. The calculated value is similar to the generally accepted value of 1368 W/m^2 which is averaged yearly via NASA satellite measurements. It It is important to remember that the solar constant of 1368 W/m^2 is not the intensity of energy per square metre of Earth’s surface, but per square metre of extraterrestrial atmosphere which can be considered as a sphere of radius of 1.5*10^11 m with the Sun at its centre.
3.4 Solar energy per unit area of Earth’s surface
If the Earth were a flat disc facing the Sun, then 1368 watts of solar energy would be received by every square metre of Earth’s surface. In such a case, with r denoting radius of the Earth, the total solar radiation incident on Earth’s disc would be the solar constant multiplied by area of a flat circle namely π* r^2. In its real shape as a sphere, Earth will still receive the same amount of solar radiation as a flat disc except the radiation i.e. the solar energy would be expected to spread out over a larger area (4π* r^2) for a sphere. Thus the average energy from the Sun that fall upon every square metre of the spherical surface of Earth would be (1368 * π* r^2) / 4π r^2 = 1368 / 4 = 342 W/m^2
3.5 Actual amount of solar energy striking Earth’s surface
It is assumed in sec.3.4 that the Earth did not have any atmosphere. However, in reality, while travelling towards Earth’s surface, some of the solar energy is lost in its path when Earth’s atmosphere itself absorbs ~ 67 W/m^2 and reflects back ~ 77W/m^2 to space. This leaves 342 – (67+77) = 198 W/m^2 which is 198*100/342 ~ 58% of the solar energy intercepted by the Earth, reaches Earth’s surface. Of this quantity, 168 W/m^2 is absorbed and 30 W/m^2 is reflected back by the Earth to space.
Assuming Earth as a disk, the cross section area (π* r^ 2) that would intercepts the solar radiation, would be 168 * π r^2 watts where r is the radius of Earth (6376 km). This would result in generating a colossal amount of solar power of ~21 quadrillions (2.1* 10^16) watts that would strike the entire area of the Earth. Thus the Sun even though so far from the Earth, still acts as an immensely powerful source of energy. It is interesting to note that the trip of photons from Sun’s photoshere to Earth’s extraterrestrial atmosphere which covers a distance of 1.5*10^11 m, takes about 8.5 minutes.
Of course, with all this massive energy input, Earth should just get hotter all the time. But from our common experience we know that when an object heats up, it starts to give off heat energy by radiation. So the more Earth heats up, the more rapidly it loses energy into space. In this way, a proper energy balance is restored between the incoming solar energy and outgoing thermal (heat) energy.
4. Basic facts about Sun and Earth
The Sun is by far the largest object in the solar system with a diameter of 696*10^6 m which is about 109 times bigger than Earth (see Table below). Interestingly, ratio of the corresponding volumes indicates that about one million Earths could theoretically fit inside an imaginary hollow Sun.
The total mass of the Sun is 2*10^30 kg which is about 330,000 times the mass of Earth, and accounts for 99.8% of the mass of the solar system. About three quarters of Sun’s mass consist of hydrogen while the rest is mostly helium, and > 2% of elements such as oxygen, carbon, neon, iron
Radius (m)
696*10^6 : Sun
637.6*10^4 : Earth
109 : Sun/Earth
Mass (kg)
2*10^30 : Sun
6*10^24 : Earth
333,000 : Sun/Earth
Volume (m³)
1.4*10^27 : Sun
1.1*10^21 : Earth
1.3 million : Sun/Earth
Average density (kg/m³)
1400 : Sun
5506 : Earth
1/4 : Sun/Earth
Gravitational acceleration at surface (m/s²)
274 : Sun
9.81 : Earth
28 : Sun/Earth
Rotation period (days)
26 (at equator) : Sun
1 : Earth
1/26 : Sun/Earth
Temperature at surface (C)
Photosphere ~5500 : Top of Sun's surface
~20 : Earth
275 : Sun's surface/Earth
Escape velocity at surface (km/s)against gravity
~618 : Sun
11.2 : Earth
~55 : Sun/Earth
and others. The Sun is neither a solid nor a gas but is actually made of plasma (see sec.3.1 below) which is tenuous and gaseous near the surface but gets denser down towards the Sun’s fusion core.
Other interesting data : Earth-Sun Distance: 93 million miles (150 million km) Total luminosity (energy radiated per second) of Sun : ~ 4 x 10³³ erg/s. [As bright as 4 trillion trillion 100-watt light bulbs ]bulbs] Diameter of Photosphere : ~ 1.39 * 10^6 km ; Solar cycle of high and low activity : Repeats about every 11 years Age of Sun: 4.5 billion years and would, according to astrophysicist, become a red giant in the next about 5 billion years
5. Radiation from the Sun
5.1 Sunlight
It is worth mentioning straightway that the "sunlight" produced at the core doesn't just fly off into space. As has been seen, the proton-proton chain doesn't even produce light in the visible part of the spectrum. Instead, the high-energy protons and resultant kinetic energy produced induces other atoms in the Sun to vibrate and, in turn, release photons of many different frequencies, including those in the visible part of the spectrum. These photons are re-absorbed and re-emitted by adjacent atoms, each, on average, slightly closer to the surface of the Sun. Finally, after around 100 to 200 thousand years an atom at the surface of the Sun absorbs and then re-emits a photon, which flies off into space. Then, if heading in our direction, it takes around 8.5 minutes for the photon to reach the Earth. All of the daylight that we see started its journey a very long time ago.
The most outstanding characteristic of the Sun is the fact that it emits huge quantities of electromagnetic radiation of all wavelengths including visible range and beyond the rainbow spectrum of VIBGYOR, namely IR (below red) and UV (above violet) ranges. The energy emitted by the Sun is divided approximately into 40% visible light, 50 % infrared (IR), 9% ultraviolet (UV) and 1% x-ray, radio etc. The glow of a hot object like the Sun indicates how hot it is. For planets and stars, physicist’s theory of “black body radiation” is used as a first approximation for the energy they emit. Based on this concept, the glow of a body is taken as the way a red-hot object produces light. The hotter the object, the brighter it shines. In the case of Sun, the colour of Sun’s surface i.e. photosphere suggests a temperature around 5500 C.
Almost all of Sun’s energy warms up the Earth and, as mentioned above, is the driving force behind all our weather, climate and ecosystem. The sunlight reaching the top of Earth’s atmosphere provides about 2,500 times as much energy as the total of all other sources combined. This staggering statistics remains quite steady. It varies only by about 0.1% over an entire course of a stable (11-year long) solar cycle.
Most of the solar UV radiation is absorbed by the concentrated layer of ozone gas found in the stratosphere. Earth’s atmosphere is estimated to block 98.7% of Sun’s UV radiation from penetrating through its atmosphere. IR radiation is partially absorbed by carbon dioxide, ozone and water vapour in Earth’s atmosphere.
A totally different source of IR radiation is produced from the earth. As the ground on Earth is heated by sunlight, it begins to radiate. But being too cool to radiate even a dull red, Earth, unlike Sun’s predominant UV rays, radiates in the IR range which travel back from the Earth to the outer space. In this way a balance is restored between the absorbed sunlight and emitted infrared radiation, otherwise an imbalance would result that could cause global warming (“Green house” effect) or global cooling.
5.2 Effect of UV exposure
With 98.7% of Sun’s UV radiation being blocked by Earth’s atmosphere, the rest namely 2.3% that gets through has both harmful and beneficial effects. The molecules of a substance when hit by UV rays start to vibrate back and forth. However the UV energy is quite often high enough to knock electrons away from the atoms or cause molecules to split instead of just being shaken up. As a result, a change in the chemical structure of the molecule occurs which can be detrimental to living organisms causing cell damages, making UV radiation from the Sun an environmental human carcinogen. It cam damage immune system, cause sunburn, speed up aging of skin etc. However, there are some credit sides of Sun’s UV if used in moderation. UV is needed by our bodies to produce vitamin D which helps to strengthen bones, muscles etc. It is used to treat some skin condition such as psoriasis. Many insects use UV emission from celestial objects as reference for navigation in flight. UV has positive applications in the fields of disinfection and sterilisation.
6. Dynamism of Sun
Sun would appear to change little from our viewing position on Earth. As a provider of life and heat to Planet Earth, Sun appears quite placid – it is, however, anything but. Though laying a very long way from Earth, Sun’s unceasing activity has had a dramatic impact on our planet. In normal circumstance, we are all worried about the weather, but there is another kind of weather to worry about which comes not from the horizon but from the Sun some 150 million km (93 million miles) away. The greatest threat to Earth sits right at the heart of our solar system namely the Sun. A violent activity on Sun’s surface producing solar storms could disrupt our technological civilization.
The Sun with a huge nuclear bomb at its heart is the worst place in the solar system when it comes to stormy weather. Much of Sun’s tempestuous nature comes from its core which produces dense, electrically charged gas in the form of a special form of matter called plasma. This rolling, boiling plasma generates Sun’s powerful magnetic field that produces various solar activities defined below namely solar flares, solar prominence, solar wind, coronal mass ejection (CME) etc.
6.1 Activity of magnetic field
Origin of magnetic Field
Everyday physics tells us that magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a magnet originates from the motion of negatively charged electrons in the magnet. Although the origin of Earth's magnetic field is not completely understood, it is, however, thought to be associated with the so called dynamo effect in the outer core of Earth’s interior. The source of such field appears to be electrical currents and magnetic fields mixed in turbulent motions of hot conducting metal fluids of iron and nickel in Earth’s outer core influenced by the rotation of Earth.
Earth's magnetosphere
Earth’s magnetic field acts like a giant invisible bubble called magnetosphere that shields the planet from much of the dangerous cosmic radiation spewing from the Sun in the form of solar wind. The solar wind as mentioned above is a stream of ionized gases i.e. plasma that blows outward from the Sun at about 450 km/second – its intensity varies with the degree of surface activity on the Sun. When the solar wind encounters magnetosphere, it is deflected like water around the bow of a ship. The imaginary surface at which the solar wind is first deflected is called the bow shock. The corresponding region of space sitting behind the bow shock and surrounding the Earth represents a region of space dominated by the Earth's magnetic field called magnetosphere which largely prevents the solar wind from entering.
Sun’s magnetic field
The amplitude and spatial configuration of Sun’s magnetic field varies with times scales ranging from a fraction of a second to billions of years. Sun, like Earth, has its north and south poles. Evolution of Sun’s activity is directly linked to changes in its magnetic field that undergoes a complete makeover about every 11 years when the polarity flips with North Pole becoming South Pole and vice versa. Changes to the field's polarity ripple all the way out to the Voyager probes, on the doorstep of interstellar space. The exact internal mechanism of such reversal of polarity is not clearly understood. The domain of the sun's magnetic influence (also known as the "heliosphere") extends billions of kilometers beyond Pluto.
The strength of Sun’s magnetic field is typically only about twice as strong as Earth’s field. However, it can become highly concentrated in small areas, reaching up to 3,000 times stronger than usual. Kinks and twist in the magnetic field are developed because the Sun spins more rapidly at the equator than at the higher latitudes, and also because the inner parts of the Sun rotating more quickly than the surface. Build up of these distortions throughout an 11- year cycle create regions of intense magnetic activity that show up as sunspots on Sun’s surface i.e. the photosphere.
6.2 Outburst from the Sun
One of the most dramatic properties of Sun’s activity from its complex magnetic activity is the existence of a solar cycle which is best seen through development of the pattern of sunspots. The powerful magnetic fields around sunspots produce active regions on the Sun which often lead to solar flares and CMEs. Over the course of about a month, sunspots disintegrate with regular spectacular release of massive amounts of
energy known as solar flares. Associated with these flares are violent bursts of plasma (CMEs) which travel as solar winds till they reach the Earth where they induce geomagnetic storms that could have major disturbance of Earth’s magnetosphere They can have a power to disrupt our electrical power grids, GPS satellites and commercial flights.
In summarizing, solar energy produced at the core by nuclear fusion, is radiated via radiation and convection to finally reach the surface i.e. photosphere from where solar radiation is emitted into space. On the photosphere, localized sunspots appear which burst into solar flares as plasma eruptions in the form of fiery looping rain on the Sun. Solar wind from the flares containing plasma of protons and electrons travels through the solar system. When encountered by Earth, it is mostly deflected by magnetosphere.
6.3 Brief accounts of various solar activities
Solar cycle : One of the most dramatic activities of the Sun is the existence of solar cycle. This can be best seen in the pattern of sunspots. Based on a study of the average number of sunspots over the last 300 years, it becomes apparent that sunspots vary significantly displaying a peak number about every 11 years. The Sun, thus, has a cycle of behaviour which repeats on average every 11 years incorporating solar maximum and solar minimum which refer respectively to periods of maximum and minimum sunspot counts. Cycles span from one minimum to the next. Astronomers at the Wilcox Solar Observatory (WSO) monitor the sun's global magnetic field on a daily basis. Magnetograms at Wilcox have been tracking Sun’s polar magnetism since 1976, and they have recorded three grand reversals—with a fourth in the offing. A reversal of the sun's magnetic field is, literally, a big event. According to Phil Scherrer of Stanford University, Sun’s polar magnetic fields gradually weaken to zero, and then emerge again with the opposite polarity. This reversal of Sun’s magnetic field is a regular part of the solar cycle.
Sunspots : These are originated from Sun’s magnetic field welling up to the photosphere. High magnetic pressure inside sunspots forces hot plasma out in order to balance the pressure. As a result, the matter inside the spots is less dense and somewhat cooler ~ 3500 C, making them appear as dark blotches (when viewed in optical wavelengths) in comparison with the brighter and hotter surrounding ~ 5500C of photosphere. Sunspots are associated with higher activity of the sun which means the more number of sunspots appearing, the more active the sun is. Sunspots vary in size ranging from the size of United Kingdom to several times the size of Earth. Normally they come in groups and can last from a few hours to several days.
Flares and Prominences : A solar flare appearing like giant flame is caused by a sudden eruption of intense, high-energy hot plasma radiation from Sun’s surface that stretch out from the surface. They can sometimes be trapped by nearby magnetic fields via convection currents within the sun, and pulled back to the sun in the form of giant arcs called prominences. These arcs reflect the effect of the hot plasma gas travelling along the magnetic field lines which may themselves remain invisible. Occasionally, these invisible magnetic fields can break and cause a prominence or a CME ejection.
Coronal mass injections (CMEs) : These originate from the snapping of local magnetic fields that have captured plasma (because it’s a charged, super-heated gas that can follow magnetic fields). CMEs containing charged ions are sent into space by the most powerful solar storms. The hot gas of CME is released into space as a gigantic explosion, sending material toward the Earth at speeds reaching 3200 Km/s. If Earth happens to be in the path of a CME, the charged particles can slam into our atmosphere with enough energy to cause a temporary disturbance of the Earth’s magnetic field, disrupt satellites in orbit and even cause them to fail, and bathe high-flying airplanes with radiation. They have the potential to overwhelm our magnetic field for a short period of time, causing problems with GPS satellites and power grids and disrupting telecommunications and navigation systems.
Travelling at the speed of light, it takes about eight minutes for the light (photons) of solar flare to reach Earth from photosphere. In contrast, the immense cloud of hot plasma containing magnetized particles hurled into space such as CME travels around million miles per hour taking up to three days to reach Earth.
Solar storm/ wind : This is associated with a release of streams of plasma particles (protons, electrons, neutrinos etc), energy, and radiation from Sun's surface. It travels past us at a rate between 300 and 800 km/s and varies from day to day with changes in sun's activity. The solar storm is the cause of a comet's tail and also the cause of the auroras (northern or southern lights). Earth would be more affected by the solar storm were it not for the magnetic field that surrounds us. The magnetic field causes the charged particles from the sun to flow around the Earth barely showing any effects at all, usually. The storm on the sun causes a type of storm on the Earth, known as a geomagnetic storm.
6.4 Devastation on Earth from solar outburst
Flares and CMEs have different effects on Earth. A solar flare could hit the Earth like a lightning bolt affecting the radio waves which would result in degradation or at worst, temporary blackouts of signals in navigation and communications. In contrast, a CME striking Earth’s atmosphere can cause disturbance of Earth’s magnetic field.
The record about power failures from solar storms commonly refers back to March 13, 1989 – 23 years ago which was the last time a huge flare/CME affected the Earth due to the failure of magnetosphere to provide the Earth with 100% protection. The entire province of Quebec, Canada suffered a devastating 12-hour total electrical power blackout - the electrical supply was cut off to over 6 million people for 9 hours. Parts of the northeast of U.S also suffered blackout from power failure.
Solar storms were even more powerful on August 28, 1859with larger solar flare than the one that caused the 1989 Québec and northeastern U.S. blackout. It was observed and recorded by Richard C. Carrington, and sometimes called as the 1859 Solar Super storm. The accompanying CME traveled to Earth in only 17 hours, rather than the usual three to four days. The largest recorded geomagnetic storm occurred with observations of auroras or northern lights in many parts of the world. Telegraph systems throughout Europe and North America failed.
7. Final comments
The basic sequence of Sun’s activity is as follows : Sunspots on photosphere → Solar flares and/or Prominences, CME →Solar storm/wind containing plasma of protons and electrons from the Sun that travel through the solar system. As a layman in solar physics, It has been a fascinating experience to review Sun’s dynamism in some simple basic details.
For the future, it has been suggested that a space observatory might include a radiometric imager for a better understanding of how solar variability might affect the Earth. One would like to think that such a device could essentially map the surface of the Sun and reveal contributions of the surface to Sun’s luminosity. It is interesting to note that with eye on the Sun, the presence of a filament (stable less than a week) of a length of 50 Earths side-by-side has been very recently reported (NASA 360, 29 October,2015) from Solar Dynamics Observatory (SDO). These are apparently elongated clouds of solar material tethered above the Sun by magnetic forces. This image makes Sun’s mysticism more intriguing.