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ACE News #91 - Aug 30, 2005
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In a 1983 paper by M. A. Lee, it was shown that an anisotropic distribution of high-energy protons escaping upstream from an interplanetary shock can amplify anti-sunward propagating Alfven waves in the solar wind. In Lee's quasi-linear theory of shock-acceleration (here-in-after QLT83), these waves resonantly scatter subsequent ions escaping upstream, trapping them near the shock and thereby increasing the efficiency of the shock-acceleration process. Although these waves have become an essential element in the theory of solar energetic particle (SEP) events that are produced by acceleration at CME-driven shocks, the waves that are the cornerstone of this process are seldom observed directly at 1-AU. However, two of the largest SEP events from the recent solar maximum have provided an opportunity to observe these waves in situ, and test the predictions of QLT83.
In an analysis of data from the ACE magnetometer and from the CELIAS/HSTOF sensor on board SOHO by Bamert et al. (ApJ 601, L99-L102, 2004) it has been found that the QLT83 applies fairly well for the energy range of protons from about 60 keV to a few MeV and to the corresponding Alfven waves resonating with these protons. The approximate gain in wave power through amplification by suprathermal ions can be seen when comparing panels A and B in the range of wave numbers above 10-7 m-1. Panel A shows the typical level of magnetic fluctuations in the solar wind, while panel B shows the magnetic field fluctuations amplified by the upstream anisotropic suprathermal protons preferentially streaming away from the interplanetary shock. The same data of panel B indicate that weakly super-Alfvenic ions generate ion whistler waves. This is a natural consequence of the dispersion relation of left-handed Alfven waves, which enters the ion whistler (L-mode) branch if the wave frequency approaches the proton gyro-frequency (right figure). The QLT83 has been extended to apply to weakly super-Alfvenic ions in order to explain the observations (Kallenbach et al., Proc. 4th IGPP Conf. on the Physics of Collisionless Shocks).
It has also been found that the amplification of Alfvenic wave power right at the shock layer agrees fairly well with the model of Vainio and Schlickeiser (A&A. 343, 303-311, 1999) on self-consistent Alfven-wave transmission and test-particle acceleration at parallel shocks. This has been verified by comparing the magnetic fluctuation power of panel C, representing the turbulence region downstream of the shock, with that of panel B, representing the upstream region. The latter result has been confirmed for the interplanetary shocks of the Halloween 2003 events.
The results of this investigation lend further observational support to the idea that proton-amplified Alfven waves play a critical role in the acceleration of solar energetic particle events by CME-driven shocks.
Submitted by Reinald Kallenbach (International Space Science Institute, Bern) and Karin Bamert (University of Kiel). Address comments or questions to firstname.lastname@example.org.
Last modified 30 Aug 2005, by