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ACE News #76 - October 31, 2003

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Very Low-Energy Solar Electron Bursts and Solar Wind Structure

Impulsive solar electron bursts nominally associated with solar activity and type III (rapid drift) radio events were first observed at energies > ~40 keV. More than 330 solar electron bursts have been identified in the ACE/SWEPAM data at energies below 1.4 keV, including several events where the bursts were detected at energies as low as 100 eV. At energies < 1.4 keV the bursts are superimposed on the solar wind electron strahl (the narrow beam at 0 degree pitch angle at the start of the left figure above).

That figure illustrates some of the typical features of these bursts at low energies: 1) a strong association with intense type III radio events that extend down close to the local plasma frequency (the enhancement running nearly horizontally across the radio spectra in the bottom panel at ~18 kHz); 2) energy and angle dispersion at onset with the most energetic and field-aligned electrons arriving first; 3) relatively structureless profiles having rapid rises and more gradual falls; 4) persistent anti-sunward streaming with beam pitch-angle distributions broader than those of the underlying strahl; and 5) a more isotropic and generally weaker back- scattered component superimposed on the tenuous and nearly isotropic solar-wind suprathermal electron halo population.

Collectively these characteristics indicate that: 1) the bursts are the low-energy extensions of the electrons that excite the type III radio emission; 2) the sources of the energized electrons must at times endure in the vicinity of the Sun for many hours; and 3) the burst electrons experience greater scattering en route from the Sun than do the strahl electrons, presumably because of an enhanced wave field self-generated by the burst electrons.

There have been predictions that burst spectra should turn over at energies below 1 keV owing to losses to coulomb collisions as the electrons escape from the corona. The right figure above illustrates, however, that burst peak spectra in most of these events are power laws in energy down to the lowest detected energies. Such spectra can be interpreted as evidence that these electrons are accelerated at very great heights (>> 1 solar radius) in the solar atmosphere. An alternate possibility is that processes such as instabilities and energy losses to waves and the interplanetary potential also affect observed burst spectra at low energies and that the bursts actually originate relatively low in the solar atmosphere.

We find that these low-energy solar bursts occur within both low and high-speed solar wind flows and within both recurrent streams and transient disturbances. However, the bursts are preferentially found in low- density solar wind and are seldom found close to the heliospheric current sheet. This indicates that if the bursts are a result of magnetic reconnection in the solar atmosphere, the reconnection must usually occur at current sheets that do not extend out into the solar wind. That is, the reconnection must usually occur at current sheets separating open and closed field lines in the corona.

Submitted by Jack Gosling and Ruth Skoug of Los Alamos National Laboratory.

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