Coronal Heating

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More than half a century ago, astronomers knew that the corona is a very hot, rarefied gas. So they knew what the corona is, but not why it exists, tte problem, which has not yet been completely solved, is to explain why the corona is so hot.

tte visible solar disk, the photosphere, is closer to the Sun's center than the million-degree corona, but the photosphere is several hundred times cooler, and this comes as a big surprise, tte essential paradox is that energy should not flow from the cooler photosphere to the hotter corona anymore than water should flow uphill. When you sit far away from a fire, for example, it warms you less.

tte temperature of the corona is just not supposed to be so much higher than that of the atmosphere immediately below it. It violates common sense, as well as the second law of thermodynamics, which holds that heat cannot be continuously transferred from a cooler body to a warmer one without doing work. Ms unexpected aspect of the corona has baffled scientists for decades, and they are still trying to explain where all the heat is coming from.

We know that visible sunlight cannot resolve the heating paradox. Light from the photosphere does not go into the corona; it goes through the corona, ttere is so little material in the corona that it is transparent to almost all of the photosphere's radiation. Sunlight therefore passes right through the corona without depositing substantial quantities of energy in it, traveling out to warm the Earth and to also keep the photosphere cool.

So, radiation cannot resolve the heating paradox, and we must look for alternate sources of energy. Possible mechanisms involve either the kinetic energy of moving material or the magnetic energy stored in magnetic fields. Indeed, the photosphere seethes with continual motion, and magnetism threads its way through the entire solar atmosphere. Unlike radiation, both of these forms of energy can flow from cold to hot regions.

For several decades, sound waves provided a widely accepted explanation for heating the low-density, million-degree corona. In 1948-49, for example, the German astrono mer Ludwig Biermann (1907-1986), the French astronomer Evry Schatzman (1920- ) and the German-born American astronomer Martin Schwarzschild (1912-1997) independently proposed that the high coronal temperature could be maintained by acoustical noise produced by solar convection, tte up and down motion of the piston-like convection cells, called granules, will generate a thundering sound in the overlying atmosphere, in much the same way that a throbbing high-fidelity speaker drives sound waves in the air. tte sound, or acoustic, waves should accelerate and strengthen as they travel outward through the increasingly rarefied solar atmosphere, until supersonic shocks occur that resemble sonic booms ofjet aircraft. It was thought that these shocks would dissipate their energy rapidly, and perhaps generate enough heat to account for the high-temperature corona.

Although the majority of the sound waves are reflected back and remain trapped in the Sun, a small percentage of them manage to slip through the photosphere, dissipating their energy rapidly within the chromosphere and generating large amounts of heat. And the low chromosphere does indeed seem to be heated by sound waves that are generated in the convective zone and dissipated by shocks in the chromosphere.

tte Sun's chromosphere is composed of hundreds of thousands of tiny spicules, energized and thrust outward by the sound-driven shocks, ttis method of chromosphere heating is generally consistent with the fact that other stars with outer convection zones have chromospheres; while for stars without outer convection zones no chromosphere is detected.

However, since the Skylab observations of ultraviolet spectral lines in the 1970s, it has become apparent that there is very little acoustic energy left over by the time the shock waves reach the upper chromosphere, and sound waves apparently cannot reach the corona, tte steep temperature and density gradient in the transition region would reflect sound waves, keeping them from propagating into the corona.

Magnetic fields probably play a pivotal role in heating the solar corona, tte hottest and densest material in the low corona is located where the magnetic field is strongest, and it is intense magnetism that molds the corona, producing its highly structured, inhomogeneous shape, tte other key ingredient for coronal heating is change, tte dynamic corona is magnetically linked to, and driven by, the underlying photosphere and convective zone whose turbulent motions can push the magnetic fields around and shuffle them about. Observations indicate that the corona is always changing on all observed spatial and temporal scales, seething and writhing in tune with the Suns magnetism.

So, recurring themes in explaining the million-degree temperatures of the solar corona are highly structured magnetism and turbulent change, tte ultimate source of the energy that heats the corona is convective motions in the solar interior, and that energy is somehow channeled by magnetic fields from the cool photosphere to the hot corona. And the solution to the corona's heating crisis is related to just how energy is transferred to, stored in, and released by the magnetic fields that dominate the corona.

When a magnetic field is disturbed, a tension acts to pull it back, generating magnetic waves that can propagate upward and dissipate energy in the corona, ttese waves are called Alfven waves after the Swedish theoretician Hannes Alfven (1908-1995) who first described them mathematically. He pioneered the study of the interaction of hot gas, or plasma, and magnetic fields, in a discipline called magneto-hydrodynamics, and was awarded the Nobel Prize in Physics in 1970 for this work.

Instruments on a number of spacecraft have detected Alfven waves from the low corona to more distant regions of interplanetary space. However, once you get energy into an Alfven wave, it is hard to dampen the wave and extract energy from it. Like radiation, the Alfven waves seem to propagate right through the low corona without being noticeably absorbed or dissipated there, and without depositing enough energy into the coronal gas to heat it up to the observed temperatures.

Nevertheless, magnetic waves are still viable candidates for some coronal heating, and other energy-carrying waves such as sound (acoustic), magneto-acoustic (magnetic-sound) and shock waves, may play a role. Hot, oscillating magnetic loops have, for example, been observed swaying in the corona under the influence of periodic waves, sometimes for periods of five or ten minutes characteristic of internal sound waves, ttese back and forth motions have been attributed to magneto-acoustic waves.

Magnetic loops can heat the corona by coming together and releasing stored magnetic energy when they make contact in the corona. Motions down inside the con-vective zone twist and stretch the overlying magnetic fields, slowly building up their energy. When these magnetic fields are pressed together in the corona, they can merge and join at the place where they touch, releasing their pent-up energy to heat the gas. tte magnetic fields reform or reconnect in new magnetic orientations, so this method of coronal heating is termed magnetic reconnection.

Instruments aboard the SOlar and Heliospheric Observatory, abbreviated SOHO, have shown that tens of thousands of small magnetic loops are constantly being generated, rising up and out of the photosphere, interacting, fragmenting and disappearing within hours or days (Fig. 6.8). tte numerous magnetic loops in this "magnetic carpet" are always being replaced, in a sort of self-cleaning action initiated from below. Motions in the photosphere and underlying turbulent convection push the carpet loops around, and when the magnetic fields of adjacent loops meet, they can break and reconnect with each other into simpler magnetic configurations, releasing enough energy to heat the corona. Since the magnetic carpet is continuously replenished every 40 hours, forming new magnetic connections all the time and all over the Sun, it can provide a continual source of coronal heating.

According to another scenario, the steady heating of the hot solar corona is produced by coronal magnetic interactions that produce frequent, numerous, small-scale explosions, called nanoflares or microflares, which occur at seemingly random locations, tte coronal magnetic fields can braid, twist and writhe in a perpetual dance, energized from below, rising, falling, intertwining and coupling together to cumulatively produce numerous small explosions that heat the high-temperature corona.

So there is no lack of possible mechanisms for explaining how the corona becomes so hot and stays heated. And that's a good thing, for the corona would collapse in minutes if energy weren't constantly dumped into it to maintain its high temperature.

But exactly where is the corona heated? It becomes hottest at locations where the magnetic fields are strongest. And most of this heating, at least in magnetic loops, is often confined to the lower parts of the corona, near the chromosphere and transition region.

FIG. 6.8 Magnetic carpet Magnetic loops of all sizes rise up into the solar corona from regions of opposite magnetic polarity (black and white) in the photosphere (green), forming a veritable carpet of magnetism in the low corona. Energy released when oppositely directed magnetic fields meet in the corona, to reconnect and form new magnetic configurations, is one likely cause for making the solar corona so hot. (Courtesy of the SOHO EIT and MDI consortia. SOHO is a project of international cooperation between ESA and NASA.)

FIG. 6.8 Magnetic carpet Magnetic loops of all sizes rise up into the solar corona from regions of opposite magnetic polarity (black and white) in the photosphere (green), forming a veritable carpet of magnetism in the low corona. Energy released when oppositely directed magnetic fields meet in the corona, to reconnect and form new magnetic configurations, is one likely cause for making the solar corona so hot. (Courtesy of the SOHO EIT and MDI consortia. SOHO is a project of international cooperation between ESA and NASA.)

And this brings us to the ubiquitous coronal loops that structure the solar atmosphere and the coronal holes that provide an open gateway to interplanetary space.

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