Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsh21a0740l&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SH21A-0740
Other
7509 Corona, 7534 Radio Emissions, 7843 Numerical Simulation Studies, 7867 Wave/Particle Interactions
Scientific paper
Electron beams play an important role in generating solar radio emissions. For Type III bursts, an electron beam ejected by a solar flare propagates through the corona, generating Langmuir waves at the local plasma frequency via a streaming instability. These Langmuir waves are then converted into radio emissions. However, multiple open questions are posed by fine structures in the dynamic spectra. For instance, in Type III decimetric and metric storms, multiple events at high frequency are often combined into a single event at low frequency, but the dynamics of this process is unclear. Motivated by this observation, numerical simulations based upon quasilinear theory are performed to investigate multiple electron beam propagation in a plasma and the emission of plasma waves. We find that the main features of the plasma waves generated by multiple beams can be understood on the basis of two beams. For two beams with the same parameters but different injection points, each injection is clearly and separately visible in the plasma wave profile in the region close to the injections where no interaction between the two beams has occurred. At later times and greater distances, the trailing beam merges with and is `cannibalized' by the leading one, enhancing the wave levels at the front but suppressing them at the back. The enhanced wave levels last for a long period of time, are strongest in the leading beam region near the mean beam speed, and weaken gradually with time and in other regions. These characteristics can be explained in terms of quasilinear relaxation of multiple hot beams. As the beams propagate, fast particles from the trailing beam overrun slower particles from the leading beam. The enhanced wave levels associated with the leading beam rapidly quasilinearly relax the trailing beam, enhancing the wave levels and moving the trailing beam particles towards the speed range of the leading beam. Specifically, particles from the trailing beam enlarge the width and slope of the bump and plateau created by the leading beam and so the levels and wavenumber range of the Langmuir waves. This enlargement is greatest in the maximal wave level region for the leading beam alone, due to the large number of beam particles in this speed region. In regions sufficiently far from the injected beams, the two distinct injections appear as a single event, and the wave levels are same as from two separate beams. Accordingly, the trailing beam appears to be cannibalized by the leading beam, so that multiple beam injections lead to enhancement of the leading beam and suppression of individual signatures for the other injections. This scenario appears directly relevant to the above observations of Type III storms at decimetric and metric wavelengths.
Cairns Iver H.
Li Baowen
Robinson Adam P.
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