Intermediate-<I>m</I> ULF waves generated by substorm injection: a case study
A case study of SuperDARN observations of Pc5 Alfvén ULF wave activity generated in the immediate aftermath of a modest-intensity substorm expansion phase onset is presented. Observations from the Hankasalmi radar reveal that the wave had a period of 580 s and was characterized by an intermediate...
Main Authors: | , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2010-08-01
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Series: | Annales Geophysicae |
Online Access: | https://www.ann-geophys.net/28/1499/2010/angeo-28-1499-2010.pdf |
Summary: | A case study of SuperDARN observations of Pc5 Alfvén ULF wave activity
generated in the immediate aftermath of a modest-intensity substorm expansion
phase onset is presented. Observations from the Hankasalmi radar reveal that
the wave had a period of 580 s and was characterized by an intermediate
azimuthal wave number (<I>m</I>=13), with an eastwards phase propagation. It had a
significant poloidal component and a rapid equatorward phase propagation
(~62° per degree of latitude). The total equatorward phase
variation over the wave signatures visible in the radar field-of-view
exceeded the 180° associated with field line resonances. The wave
activity is interpreted as being stimulated by recently-injected energetic
particles. Specifically the wave is thought to arise from an eastward
drifting cloud of energetic electrons in a similar fashion to recent
theoretical suggestions (Mager and Klimushkin, 2008; Zolotukhina
et al., 2008; Mager et al., 2009). The
azimuthal wave number <I>m</I> is determined by the wave eigenfrequency and the
drift velocity of the source particle population. To create such an
intermediate-<I>m</I> wave, the injected particles must have rather high energies
for a given L-shell, in comparison to previous observations of wave
events with equatorward polarization. The wave period is somewhat longer than
previous observations of equatorward-propagating events. This may well be a
consequence of the wave occurring very shortly after the substorm expansion,
on stretched near-midnight field lines characterised by longer
eigenfrequencies than those involved in previous observations. |
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ISSN: | 0992-7689 1432-0576 |