Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh41c..06n&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SH41C-06
Physics
Plasma Physics
[7514] Solar Physics, Astrophysics, And Astronomy / Energetic Particles, [7845] Space Plasma Physics / Particle Acceleration, [7851] Space Plasma Physics / Shock Waves, [7867] Space Plasma Physics / Wave/Particle Interactions
Scientific paper
Solar energetic particle intensities at 1 AU often show an early temporal plateau where the intensity is limited. This early intensity limit may provide a valuable time window for astronauts to seek shelter before large shock-associated intensity increase (if any). The Ng and Reames (1994) time-dependent model of SEP transport through self-amplified Alfvén waves predicts a maximum proton intensity of ~ 250 particles /(cm^2 s str MeV) at ~ 1 MeV, in agreement within a factor of 2 with the observational survey by Reames and Ng (1998). In fact, streaming-limited intensity is implicit in the steady-state shock-acceleration solution of Bell (1978) and Lee (1983). Further studies on the effect of self-amplified waves on SEP intensity spectra have been made by Ng, Reames and Tylka (2003), Vanio (2003), and Lee (2005). Intensities exceeding the Ng and Reames (1994) limit have been reported (e.g., Lario et al. 2009). We present new observations of multi-species SEP spectra at the temporal intensity plateau. We also present new theoretical results on how the streaming limit depends on ion species and energy, ambient wave intensity spectrum, Alfvén speed, solar-wind speed, shock speed, and the presence of interplanetary shocks and interaction regions. Among the new interesting observations is the strong suppression of ion intensities near 1 MeV/amu in events that have high 10-100 MeV proton intensity. New modeling results confirm that this is due to these low-energy ions being strongly scattered at small pitch angles by waves amplified by 10-100 MeV protons at large pitch angles. As the high-energy protons travel upstream and scatter from small to large pitch-angles, they simultaneously amplify waves en route over a range of wavenumbers, including those that are resonant with low-energy protons. Thus, wave amplification by streaming protons and the pitch-angle dependence of the wave-particle resonance condition are essential factors in understanding the limiting behavior. We thank Glenn Mason for his advice on ACE/ULEIS observations. Work by CKN, DVR, and AJT are supported in part by NASA grants NNX09AU98G, NNX08AQ02G, and NNH09AK79I, respectively.
Ng Chee K.
Reames Donald. V.
Tylka Allan J.
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