Advances in Simulating Magnetosheath Plasma Entry

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Onsager et al. [1993] demonstrated that the entry of the magnetosheath ions into the dayside magnetosphere can be modeled fairly accurately in three steps:

Trace the particle backward from the ionosphere to the magnetopause entry point using Stern magentic field model [1985] and a simple dawn-dusk electric field.
Compute the particle acceleration due to the magnetopause crossing, assuming the particle pitch angle and energy are conserved in the de Hoffman-Teller reference frame [e.g., Hill and Reiff, 1977; Cowley, 1989].
Compute the phase space density of the particle using Spreiter and Stahara magnetosheath model [1985].

The results of the original Onsager model is shown here.

The ion spectogram above looks realistic (an example of the spectrogram of real data from the DMSP satellite will be shown shortly), but the electron spectrogram does not. The electron spectra should but do not show a sharp dropoff poleward of the cusp and flux remains much too high in the mantle and polar rain regions.

Our first improvement was to impose charge quasi neutrality (e.g. Reiff et al., 1977). This is done by having a potential drop between the magnetopause and the earth and adjusting the potential until the electron and ion densities are the same. The result is shown here.

This makes quite an improvement, since the sharp dropoff at the cusp/mantle interface appears in the electrons. The potential succesfully limits the number of electons that can enter the polar rain region, but now there are too few of them. The potential cannot be adjusted in such a way as to realistically simulate polar rain using core solar wind electrons.

The second improvement that we made was to add the suprathermal electrons. The solar wind or magnetosheath electrons consist of the thermal and suprathermal components. The suprathermal component, which makes up only a small part of the solar wind, has higher temperature and lower density than those of the main (thermal or core) component. Hotter electrons can overcome the parallel electric potential which prevents the core component from entering.

The result of incorporating both improvements to the original Onsager model is shown below. For comparison with the real data, we first show the DMSP spectrogram and then the model spectrogram.

Now not just the ions but the electrons also look realistic. The model can succesfully produce the open field line LLBL, the cusp, the mantle, and the polar rain. The region equatorward of the cusp, where the electron flux is less intense is the open field line LLBL.

The results also indicate that each of the dayside open field line regions, namely open field line LLBL, cusp, mantle, and polar rain, can be characterized uniquely in terms of their potential and particle composition. The following table summarizes the characteristics of the regions.

Due to the space constraints, some of the details from the above table have been omitted.

The potential is high in the open field line LLBL region because in this region the field lines are so close to the merging site that no or few ions have yet reached the ionosphere. Therefore, a high potential is needed to prevent some electrons from entering. Further poleward, in the cusp, it turns out that no or little potential is needed to maintain charge quasi neutrality. In this region the bulk of the ions would have reached the ionosphere and the same with the electrons. However, the ions and electrons already maintain charge quasi neutrality in the magnetosheath. Further poleward still, in the mantle region, the ions start to drop because of the increasing tailward flow in the magnetosheath. The potential rises to a threshold level to prevent some electrons from entering. Finally, in the polar rain region, where few ions can enter, the potential remains at a threshold level to maintain charge quasi neutrality. The potential does not change very much because the suprathermal electrons maintain more or less the same density and temperature through out the tail.

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