Wind spacecraft observations of solar impulsive electron events associated with solar type III radio bursts

R. E. Ergun, D. Larson, R. P. Lin, J. P. McFadden, C. W. Carlson, K. A. Anderson, L. Muschietti, M. McCarthy, G. K. Parks, H. Reme, J. M. Bosqued, C. D'Uston, T. R. Sanderson, K. P. Wenzel, M. Kaiser, R. P. Lepping, S. D. Bale, P. Kellogg, and J. -L. Bougeret

Space Sciences Laboratory, University of California, Berkeley, CA


We present Wind spacecraft observations of solar impulsive electron events associated with locally generated Langmuir waves during solar type III radio bursts. The solar impulsive electrons had energies from ~600 eV to greater than 300 keV. Local Langmuir emissions associated with these fluxes generally coincided with the arrival of 2-12 keV electrons. A survey of 27 events over 1 yr shows that there were few occurrences of electron distributions (~96 s averaged) that were unstable to Langmuir waves and none that had a substantial growth rate (>3 X 10-2 s-1) or endured for more than 96 s. Intense solar impulsive electron events that occurred on 1995 April 2 are studied in detail. Marginally stable (plateaued) distributions occasionally coincided with a period of local Langmuir emissions, but the electron distributions were otherwise stable. These observations suggest that kinetic processes were modifying the electron distribution but also suggest that processes other than one-dimensional quasilinear relaxation were involved. We find that solar impulsive electron distributions were often unstable to oblique waves, such as quasi-electrostatic whistler waves or electromagnetic ion cyclotron waves, suggesting that competition between Langmuir and oblique emissions may be important. There are several other features in the Wind spacecraft solar impulsive electron observations that are noteworthy. Nondispersive flux modulations were visible in many of the events (also visible in the published IEEE 3 data) in ~1-4 keV electrons, suggesting that a local hydromagnetic instability may have accompanied the lowest energy solar impulsive electron fluxes. The Wind data differ from the ISEE 3 data in the energy spectra of the electron events. ISEE 3 recorded few events with only high-energy (> 10 keV) electron fluxes, whereas a survey of the Wind events shows a substantially higher ration of high-energy events. The high-energy events were often associated with solar flares that could not have been magnetically well connected with the satellite.

Astrophys. J., 503, 435-445, 1998