Image of the sun in visible and ultraviolet.
It was obvious to primitive man that the sun has an overwhelming influence upon life on earth, but we are still learning new ways in which the sun affects the terrestrial environment. This is only one of the reasons why it is so important to improve our knowledge of the sun and to better understand the fundamental physics determining its behavior. Activities such as sun spot formation and decay, solar flares, plasma heating, particle acceleration and magnetic field generation are common to various astrophysical objects so by studying the sun we can learn much about stellar processes and the evolution of the universe.
Image of the sun in visible and ultraviolet.
With the new generation of satellites that can image the sun in ultraviolet and x-ray wavelengths (TRACE, SOHO, YOKOH) we now have a very different picture of the sun. It is no longer the smooth, glowing sphere, but an dynamic, patchy atmosphere. The layers of the sun's atmosphere – the chromosphere, the transition layer, and the corona – rise above the visible surface, the photosphere, and are heated to temperatures much higher than the photosphere. No acceptable explanation for this phenomenon exists. The primary difficulty is that the atmosphere is almost always in a dynamic state, and the complex magnetic structures, that are too small to be resolved by current observation techniques, are thought to play an essential role. Two prominent theories suggest that either magnetic reconnection or magnetohydrodynamic waves are the source of these heating. A controversial subject is whether electric fields are important in the sun's atmosphere.
UNM faculty and students are seeking to understand the heating and dynamical behavior of the atmosphere in the presence of complex magnetic fields and to determine how electric fields contribute to the observed dynamics and energetics. This reseach includes laboratory studies of Alfven waves in the HelCat device, and whether or not these can heat plasma to high temperatures.
Sun's energy comes from fusion processes deep in the solar core. Here the temperature reaches 10,000,000°, hot enough to fuse four hydrogen atoms into one helium atom. Unlike fusion energy research on Earth, where magnetic fields confine the plasma, Sun is able to confine the plasma through the tremendous gravitational forces. Outside of about 1/4 of Sun's radius the fusion processes stop, and we reach a region called the radiation zone. The energy generated in the core, in the form of electromagnetic radiation, slowly leaks out towards the surface. In fact, it takes about a million years for aphoton (a light particle) to travel from the core across the radiation zone. The outer 1/4 of Sun is the convection zone. Here the dense plasma fluid, like very hot sticky syrup, slowly circulates between the radiation zone and the surface. This stirring motion brings energy from the solar interior to the photosphere.
The photosphere is the name for Sun's surface; it's the point where the light finally escapes. Sort of like looking into a very dense fog, and the photosphere is the deepest layer you can see to. At the photosphere the temperature of Sun has cooled from 10 million degrees to 5,600 degrees. Then, something remarkable happens. One would expect that as we move away from Sun's surface the temperature would get cooler – like moving away from a campfire. But exactly the opposite happens: the temperature of the solar atmosphere increases! The solar atmosphere is broken up into three regions: the chromosphere, the transition region and the corona. The figure shows the different regions and how their temperature changes as we go up in the solar atmosphere. The temperature of the atmosphere rises from 5,600° to over 1 million degrees in the corona.
The reason for this heating is not well understood, and that's one of the research topics we investigate at UNM in the Plasma & Fusion Sciences Lab on HelCat. Ultimately, the energy source for the heating is the convective motion of the plasma below the photosphere. This stirs the plasma and compresses and strengthens Sun's magnetic field through a process called the magnetic dynamo. This magnetic field forms the sunspots we see dotting the surface. Above these active magnetic regions are regions of hot plasma trapped by the magnetic field called prominences and loops. Magnetic reconnection can eventually lead to the formation of solar flares. Many researchers think that magnetic reconnection is the source of energy for heating the corona.
To be continued ...
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