|MadSci Network: Astronomy|
First let's review some facts we know about the solar corona. 1) It is made up of material ejected from the sun. 2) Some particles in the solar corona travel at speeds close to the speed of light. 3) The density of the solar corona is around 100 million particles per cubic centimeter near the Sun, but decreases quickly to less than 1 million per cubic centimeter at large distances according to measurements using Voyager 2 by Richard Woo (The Astrophysical Journal, Vol. 458, L87 ). This may sound a lot, but it is almost nothing if you compare it with the trillion particles per cubic centimeter that we have in air on Earth's surface. Now let's check what we do not know. The solar corona is a few million degrees, and we do not know what heats the gas. There are many ways to heat gas in space. One is by movement as you suggest. If a mass of gas collides in space with something else, the gas will heat up very much in the same way that gas heats up around an explosion. Those collisions are called shocks, and produce shock waves. As the shock wave travels, it compresses and heats the gas. That is one theory to explain coronal heating. Now we need to know how shock waves are produced by the Sun.
The environment around the Sun cannot be compared to that around a black hole. The gravitational field of the Sun is much weaker than that of a black hole. Light bending near the Sun is really too weak to be noticed. The photon sphere does not exist in the Sun. Light really goes almost straight near the Sun. We do not see anything falling onto the Sun as it happens when a black hole is near a star and attracts gas from the star. When gas falls onto a black hole, it will form a disk that rotates at high speed. As you say, the gas will heat up when it hits the disk. Friction of the gas in the disk also produces heating, but you really need a lot of gas to see this. And we do not see anything like this happening around the Sun.
Theories about coronal heating have come and gone as more information is gathered from the Sun over time. The energy emitted by the Sun every day is enough to provide the heat. The real problem is how part of that energy is transfered to the corona and converted into heat because nobody has observed something coming out of the Sun, and hitting coronal gas. One of the latest theories talks about what we do not see: magnetic fields. The magnetic field on the surface of the Sun is much stronger than fields we are familiar with on Earth, and we do not know a lot about magnetic fields in the Sun (or on Earth for that matter), so there is a lot of room for speculation. One strange theory about magnetic fields is magnetic line reconnection. Magnetic lines can be visualized when you scatter iron filings over a cardboard and put an iron bar under the cardboard. The iron filings tend to form lines from one end of the bar to the other. The number of magnetic lines increases with the magnetic field intensity. That is why you see the lines closer together near the ends of the bar. The electromagnetic theory says that two magnetic lines cannot cross each other, and the question is what happens is the magnetic field is so strong that we really force two lines to cross each other. Would it be possible to break one of the lines and reconnect it to the other? The speculation goes on by saying that if that happens a lot of energy will be released. In the end it is the magnetic field what does the trick.
Satellites observing the Sun have discovered that on the surface of the Sun there are small patches of magnetic fields that appear and disappear within periods of 40 hours, so there you have your magnetic line reconnections, say the scientists!. The energy released by magnetic field reconnection may give rise to jets of gas that hit the corona and heat the gas according to one team of scientists working with observations of the TRACE satellite ( Brekke, Kjeldseth-Moe, Tarbell, and Gurman, "Jets and Eruptions in the Transition Region Observed with CDS, EIT and TRACE," ASP Conference Series: "High Resolution Solar Physics: Theory, Observations, and Techniques," Vol. 183, p. 357, ). Now nobody knows how long will this theory survive before some observation prove it wrong.
Vladimir Escalante Ramírez
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