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ACE News #46 - June 30, 2000

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Energetic Electrons in 3He-Enhanced Solar Energetic Particle Events

In the solar wind, the 3He/4He ratios are found to be ~4x10-4. However in certain impulsive solar energetic particle (SEP) events the 3He/4He ratio can be orders of magnitude greater than the coronal value. Most 3He enrichment theories involve some form of wave-particle resonant interaction. The most recent theories exploit the fact that electrons are also present in impulsive SEPs, as indicated by accompanying type III radio bursts. These electrons are responsible for the generation of Electromagnetic Ion Cyclotron (EMIC) waves that resonant directly with 3He. In these models, 3He and other heavy ions are accelerated directly by the fundamental and higher harmonics of the EMIC waves. Because of the possible vital importance of electrons in the 3He-enhancement mechanism, we investigate the relationships between electrons and He ions within 3He-rich SEP events.

Using the ULEIS and EPAM instruments on ACE, we investigated 27 3He-rich SEP events from November 1997 to December 1999. The majority (17) of these events have a clear association with impulsive injections of energetic electrons exhibiting velocity-dispersed intensity maxima and beam-like anisotropies, confirming the results of earlier studies. The left figure shows a 3He-rich SEP event observed by ULEIS in September 1999. The onset time difference agrees with the expected difference (6.2 hr) in scatter-free propagation times for 230-450 keV/nuc He and 38-53 keV electrons over 1.2 AU. We find no significant correlation between the peak electron intensities with those of 3He or 4He in these events (although all vary by a factor ~100), indicating differences between the electron and He acceleration and/or coronal escape. Moreover, as shown in the right figure, there is no significant correlation between the peak electron intensities and the 3He/4He ratios. Therefore, if the peak electron intensities at ACE are proportional to the number of electrons present in the 3He-acceleration site, these observations do not support the EMIC resonant-acceleration model. However, if the EMIC mechanism is responsible for the 3He/4He enhancement, then the observations indicate that the electrons which generate the EMIC waves are not the same population as the electrons that escape the corona and are measured at ACE.

Contributed by George C. Ho and Edmond C. Roelof of the Johns Hopkins University Applied Physics Laboratory, and Glenn M. Mason of the University of Maryland.

For more information on the and EPAM and ULEIS instruments, see the EPAM and ULEIS Homepages.

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Last modified June 30 2000, by Andrew Davis