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Ann. Geophys., 25, 1199-1214, 2007
www.ann-geophys.net/25/1199/2007/
© European Geosciences Union 2007
Energetic electron precipitation during sawtooth injections
A. J. Kavanagh1,*, G. Lu1, E. F. Donovan2, G. D. Reeves3, F. Honary4, J. Manninen5, and T. J. Immel6
1High Altitude Observatory (HAO), National Center for Atmospheric Research (NCAR), P.O. Box 3000, Boulder, CO, 80307-3000, USA
2University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada
3Los Alamos National Laboratory, Los Alamos, New Mexico, USA
4Department of Communication Systems, Lancaster University, Lancaster, UK
5Sodankylä Geophysical Observatory, Tähteläntie 62, 99600 Sodankylä, Finland
6University of California at Berkeley, Space Science Lab, Berkeley, CA 94720, USA
*now at: Department of Communication Systems, Lancaster University, Lancaster, UK
Abstract. We present simultaneous riometer observations of cosmic noise absorption in
the nightside and dawn-noon sectors during sawtooth particle injections
during 18 April 2002. Energetic electron precipitation (>30 keV) is a
feature of magnetospheric substorms and cosmic radio noise absorption acts
as a proxy for qualitatively measuring this precipitation. This event
provides an opportunity to compare the absorption that accompanies periodic
electron injections with the accepted paradigm of substorm-related
absorption. We consider whether the absorption is consistent with the
premise that these injections are quasi-periodic substorms and study the
effects of sustained activity on the level of precipitation. Four
consecutive electron injection events have been identified from the LANL
(Los Alamos National Laboratory) geosynchronous data; the first two showing
that additional activity can occur within the 2–4 h sawtooth periodicity.
The first three events have accompanying absorption on the nightside that
demonstrate good agreement with the expected pattern of substorm-absorption:
discrete spike events with poleward motion at the onset followed by
equatorward moving structures and more diffuse absorption, correlated with
optical observations. Dayside absorption is linked to gradient-curvature
drifting electrons observed at geostationary orbit and it is shown that low
fluxes can lead to a lack of absorption as precipitation is suppressed;
precipitation begins when the drifting electron flux surpasses some critical
level following continuous injections of electrons from the magnetotail. In
addition it is shown that the apparent motion of absorption determined from
an azimuthal chain of riometers exhibits an acceleration that may be
indicative of an energisation of the drifting electron population.
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