Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusmsh44a..02r&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SH44A-02
Physics
7514 Energetic Particles (2114), 7519 Flares, 2114 Energetic Particles (7514), 2134 Interplanetary Magnetic Fields
Scientific paper
The critically important information contained in the rise-to-maximum phase of solar energetic particle (SEP) events (electrons >50 keV and ions > 5 MeV/nucleon) that are magnetically well-connected from the injection site to a spacecraft near 1 AU may be understood in terms of two concepts. The first is the scatter- free approximation that is justified by the observed beam-like collimation during the early rise phase of the intensity. This collimation results from the over-powering of any pitch-angle scattering mechanism by the focusing produced by ~1/r2 dependence of the interplanetary magnetic field. The second is magnetic back-scatter from beyond 1AU which occurs because the field is tending towards a ~1/r dependence, so the magnetic focusing becomes quite weak. The important parameter is the logarithmic distance scale 1/L=-dlnB/dz, where z is measured along the field line. It is a curious accident of the Parker field that inside 1AU, L~r/2, whereas beyond 1AU L~r2, so where the focusing length was 0.25AU at r=0.5AU, it becomes about 4AU at r=2AU. Thus, beyond 1 AU the energetic particle begins to see a more constant mean field along its path, so that compressions and irregularities with δB/B~1 will strongly affect its pitch angle, i.e., back-scattering. In more extreme cases, this is how particle "reservoirs" are produced late in large SEP events. The familiar diminution of the strong initial anisotropy at the intensity maximum occurs because (1) the solar injection has passed its maximum and (2) the back-scattered particles (from the early part of the injection) have begun to return to 1AU and then reverse direction by magnetic mirroring in the stronger mean field inside 1AU. The intensity-anisotropy history within a given SEP event therefore depends on the relation between the characteristic rise time of the injection and the return-time of the back-scatter. Nolte and Roelof [1975] presented a functional equation that describes this propagation situation, and Dennis Haggerty and I have recently applied it to extract the injection histories of energetic electrons and ions in beam-like SEP events.
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