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Are Solar Energetic Electron Injection Times Non-Simultaneous in the Corona?
Edmond C. Roelof
The onset times measured by ACE/EPAM of beam-like solar energetic electron
events (38- 315 keV) are not always consistent with simultaneous coronal
injection at all energies onto interplanetary field lines. Only electron
events in which the beam-like anisotropies endure into the event maximum
phase are analyzed in this study. In this class of events, the intensity
distributions have characteristic angular widths about the magnetic field
direction that are comparable to the aperture full-angles of the detector
collimators (48 deg). The actual pitch-angle distributions must have
a smaller angular width. Consequently, the electron velocity at 1 AU parallel
to the magnetic field is nearly equal to the full particle velocity (v).
Because the center energies of the four electron channels cover almost
a factor of two in velocity (0.39 < v/c < 0.73) or, equivalently,
path transit times (21.2 min/AU > T > 11.4 min/AU), these inconsistencies
in onset times at 1AU correspond to statistically significant differences
of several minutes in coronal injection times. When the peak intensity
histories are displaced in time back to the sun assuming a parallel velocity
of v, the lower energies are injected earlier. Although the length of the
path from the Sun affects the absolute injection time, it cannot change
the relative ordering of the injection times as a function of energy (assuming
that the path length is independent of energy). We consider it unlikely
that the path length is energy dependent, because the faster electrons
would have to have the longer path lengths. This effect should be manifested
at 1 AU in broader pitch-angle distributions at the higher energies, and
we have not yet been able to establish such a systematic trend. On the
other hand, if the path is energy-independent, then the dispersion in the
injection times is real. In any stochastic acceleration process, the intensity
must build up first at the lower energies before it can increase at the
higher energies. Consistent with this picture is the independent observation
that the intensity maximum (at the Sun) is reached simultaneously over
our full range of energies. This time would then mark the end of the primary
phase of the electron injection.
Authors: E. C. Roelof, S. E. Hawkins III, D.
K. Haggerty, and R. E. Gold
Organization: The Johns Hopkins University Applied Physics Laboratory
Telephone: (240) 228-7886
Address: 11100 Johns Hopkins Road
Laurel MD, 20723-6099
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