Solar Energetic Particle Acceleration/Injection Time Profiles: Much Better Observed in the Inner Heliosphere

Details Solar Energetic Particle Acceleration/Injection Time Profiles: Much Better Observed in the Inner Heliosphere Solar Energetic Particle Acceleration/Injection Time Profiles: Much Better Observed in the Inner Heliosphere

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh53c..03r&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH53C-03

Physics

2114 Energetic Particles (7514), 7513 Coronal Mass Ejections (2101), 7514 Energetic Particles (2114), 7845 Particle Acceleration

Scientific paper

We have argued that during the beam-like anisotropic phase of solar energetic particle (SEP) events, the history of the field-aligned unidirectional intensity is essentially that of the injection history at the Sun (shifted backwards by the scatter-free transit time). We have concentrated so far mainly on the injection timing relative to that of solar electromagnetic signatures, because the onset of the first-arriving particles can be delayed by only a fraction of their scatter-free transit time by interplanetary scattering (i.e., 10 min to 1AU for relativistic particles). However, there is much more information than just the onset time contained in the field- aligned intensity time profile throughout the beam-like phase; it must echo that of the actual solar injection history. We have recently classified the intensity time profiles during the duration of the beam-like anisotropies of ACE/EPAM near-relativistic electrons into three broad categories: 1) Spikes (rapid and equal rise and decay); 2) Pulses (rapid rise, slower decay); and 3) Ramps (rapid rise followed by a plateau). These classes indicate a wide range (and possibly different mechanisms) for SEP acceleration/injection. The beam- like anisotropic phase ends when a sufficient number of particles have been backscattered from beyond 1AU where propagation is no longer essentially scatter-free (as it usually is inside 1AU). Preliminary estimates of the time difference between the event beam onset and the first arrival of back-scattered particles yield about 10 min or more. This gives us a 10 min window for an unambiguous view of the injection history (although we can actually extract considerably more of the injection history even in the presence of back-scatter). The >10 min elapsed time for relativistic particles places this back-scattering region somewhere roughly beyond 1.3 AU (a field-aligned round-trip distance of >1.0 beyond 1 AU). Now imagine a spacecraft like one of the Sentinels, Solar Orbiter, or Solar Probe observing such a beam-like event at 0.3 AU. The elapsed time between onset of the beam and the first-arriving backscatter will be about 7 min scatterfree outbound (0.3r>0.3) scatterfree inbound for a total of 24 min vs. only 10 min for a spacecraft at 1AU. This more than doubles the time window for unambiguous sampling of the SEP acceleration/injection process for any energetic particle species (because the transit times scale inversely with the particle velocity). We present examples demonstrating this greatly increased advantage from published beam-like SEP electron and proton events observed on the Helios 1/2 spacecraft [Roelof, 1979; Kallenrode, 1993,1997].

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