|ACE News Archives||
ACE News #56 - November 7, 2001
|ACE News Archives|
|ACE Science Nuggets||
|ACE Science Nuggets|
Up to energies of ~2 keV two suprathermal electron populations can usually be distinguished in the solar wind: (1) an intense beam, known as the strahl, that is directed outward from the Sun along the heliospheric magnetic field, and (2) a more tenuous and roughly isotropic component known as the halo. The figure illustrates a newly recognized phenomenon in the ACE SWEPAM suprathermal electron data - depletions of halo electrons centered on and roughly symmetric about 90° pitch angle (PA) relative to the heliospheric magnetic field. Such symmetric depletions are present on open field lines at least 10% of the time in the ACE data and provide important clues to the origin of the electron halo.
The 90° PA depletions on open field lines are most logically interpreted in terms of adiabatic focusing and mirroring associated with field line connections to regions of stronger magnetic fields than at the observers location. The sketch in the figure illustrates how the field strength might vary along a field line connecting into a compressive stream interaction region and the resulting adiabatic changes in halo electron pitch angle distributions along the field line. In constructing the pitch angle distributions it is assumed that: (1) the one-sided strahl consists of solar electrons that are focused into a field-aligned beam as they propagate outward from the Sun, (2) beyond the peak in the field enhancement scattering from the strahl and/or heating (for example, by a shock) produces a more tenuous, sunward-directed population of electrons (the backscattered halo - dark blue) that is isotropic over the sunward facing hemisphere along the field line, and (3) adiabatic motion governs the subsequent evolution of the halo population.
Changes in field strength along the field line first focus the backstreaming halo electrons emerging from the compression region and then mirror them, producing the mirrored halo population (dark red) and, in steady state, a depletion in halo electrons centered on and symmetric about the direction perpendicular to the magnetic field. These observations provide strong evidence that the antisunward-directed portion of the electron halo at times results primarily from mirroring inside 1 AU of backscattered strahl and/or heated electrons from far out in the heliosphere rather than from scattering out of the strahl inside 1 AU.
The 90° PA depletions produce the appearance of counterstreaming electron beams (along the magnetic field) even though no enhancement in particle flux occurs opposite to the strahl. On open field lines this counterstreaming can be confused with counterstreaming associated with the closed field lines that commonly thread coronal mass ejections in the solar wind. The difference is that in the case of depletions there is no enhancement in the electron flux counter to the strahl, whereas there are such flux enhancements on field lines connected to the Sun at both ends.
Contributed by Jack Gosling, Ruth Skoug, and Bill Feldman of Los Alamos National Laboratory.
Last modified 7 November 2001, by