The Solar Wind Striking the Earth's Atmosphere

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The solar wind is hot plasma blowing out from the sun (it escapes from "coronal holes"). It contains a magnetic field, called the interplanetary magnetic field (IMF). When this field points southward, it is in the opposite direction as the Earths magnetic field:

These oppositely directed fields can cancel, leaving a few of the Earth's magnetic field lines connected up to the solar wind at high latitudes. At first the solar wind enters in full force -- in fact accelerated, because the annihilation of magnetic fields involved in breaking open the Earth's field adds energy to solar wind particles. This early intense entry is called the "cusp". However, the solar wind after all is blowing away from the sun. It quickly drags one end of the newly merged field lines downstream of the Earth (meaning further away from the sun). The solar wind is made up of electrons and ions. The ions are much heavier, and they soon find it difficult to make their way back upstream to reach the Earth. But you can't have electrons entering without ions for more than a fraction of a second. This is because the inbalance creates an electric field, which then keeps out most of the electrons as well. Before long only the hottest part of the solar wind electrons strike the Earth's atmosphere over the polar regions (forming the "polar rain").

Here is some real data from the Air Force DMSP satellite, measuring solar wind electrons and ions just before they hit the Earth's atmosphere (this causes the hardest type of all aurora to see, because you need to be at a spot that has dark skies even at noon. Obviously it has to be in winter and at very high latitude). The data is "typical", which in the context of presenting scientific data means that it is a particularly good case (the polar rain usually isn't so intense and easy to see on a spectrogram). Next to it is a simulation of the process:

The particle entry regions can be categorized into 4 divisions, namely "open field line LLBL (Low LatitudeBoundary Layer", "cusp", "mantle",and "polar rain". Each of these regions is labeled in the above pictures. The first one, "open field line LLBL", is the area equatorward of the cusp (left of the cusp in the above pictures) where the electrons are less intense.

The simulation requires the assembly of pieces from many researchers work over many years. The Earth's magnetic field is represented by the work of David Stern of Goddard Space Flight Center (not the most advanced in existence, but readily available and easy to work with). The solar wind condition upon striking the Earth's magnetic field is calculated from work by Spreiter and Stahara. The acceleration or de-acceleration associated with particles crossing from the solar wind into the magnetosphere is based on equations developed by Tom Hill and Pat Reiff of Rice University, and Stan Cowley of Imperial College in London. The code for mapping the solar wind particles down to the Earth was first developed by Terry Onsager, at the University of New Hampshire.

What did we add? Well when the pieces above are assembled you get a picture like this:

The bottom color panel, the ions, works well. But the electrons, the top color panel, don't work very well. They don't have the sharp dropoff that occurs at the cusp poleward boundary in the real data. Instead they have an intense but unrealistic looking presence throughout the mantle and polar rain. Because electrons are so much lighter than ions, they move faster, so a first calculation like this suggests lots more of them will enter than do for the ions. But that is not really possible, because it would quickly lead to impossibly large electric fields.

The first thing we did was introduce charge quasi-neutrality. This means that a voltage was added, with a value calculated to repell enough electrons (it also slightly attracts ions) so that the same number of electrons and ions enter. In effect the problem of an imbalance in the electrons and ions naturally takes care of itself. This gave a simulation result like this:

This makes quite an improvement, since the sharp dropoff at the cusp/mantle interface appears in the electrons. However now the polar rain electrons are not present. The potential cannot be adjusted in such a way as to realistically simulate polar rain from the main part of solar wind electrons (called the "core" component). The core has a temperature ~200,000 degrees Fahrenheit, which just isn't hot enough to be polar rain.

But the solar wind also contains a tiny portion heated to 1 or 2 million degrees Fahrenheit, called the halo or superthermal part. If we add these to the simulation mix, we get the result shown above next to the data. Now the particles striking the entire open field line part of the polar regions can be simulated accurately!

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