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ACE News #67 - Jan 2, 2003

ACE News Archives

Suprathermal Electrons in High-Speed Streams: Counter-Streaming on Open Field Lines at 1 AU

Where fast solar wind streams run into slower wind ahead, co-rotating interaction regions (CIRs) form. At heliocentric distances beyond ~2 AU, CIRs are usually bounded by forward and reverse shocks. At large heliocentric distances, field-aligned beams of suprathermal electrons with energies up to ~1 keV are commonly found upstream of both forward and reverse CIR shocks. The beams result from electrons that are energized at the shocks and leak out of the CIR into the upstream solar wind. That leakage produces field-aligned electron beams directed away from the CIR. The diagram above shows schematically the field line geometry of a CIR, and illustrates that CIR-leaked electrons move sunward for both the forward and reverse shock cases. Thus, the enhanced suprathermal electron fluxes produced by the CIR are observed to stream counter to the normally present anti-sunward electron strahl. The character of these counter-streaming electrons can be quite similar to that observed by a spacecraft encountering closed magnetic field lines within a coronal mass ejection (CME).

It has previously been argued that CIRs should not be a source of counter-streaming electrons on open field lines at 1 AU, because the CIRs are not typically bounded by shocks at 1 AU. We have examined 9 intervals in which counter-streaming electrons were observed immediately following passage of a CIR over the ACE spacecraft and continuing for 1-2 days within the high-speed solar wind. Pitch angle distributions for 272 eV electrons during a stream in 2002 are shown above. The anti-sunward suprathermal electron strahl is seen throughout at 0° pitch angle. A CIR-leaked beam is seen at 180° pitch angle beginning midday on March 31 (Day 90) immediately following a reverse wave, and lasting until midday on April 2 (Day 92).

Because the counter-streams are observed immediately after a CIR's passage, the CIR is concluded to be the source. Many hours later ACE is magnetically connected to the CIR at a distance where a well-formed reverse shock should be present, and the counter-streaming electrons must travel a field-aligned distance of several AU to reach ACE. Because magnetic moment conservation and particle scattering should act to restrict backstreaming electrons from reaching 1 AU as a field-aligned beam, it is extremely surprising to find these CIR-leaked electrons in a field-aligned beam when the connection to the CIR is very distant.

Contributed by John Steinberg, Jack Gosling, and Ruth Skoug of Los Alamos National Lab.

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Last modified 2 January 2003, by Andrew Davis