Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm43b1326l&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SM43B-1326
Other
2704 Auroral Phenomena (2407), 2744 Magnetotail, 2764 Plasma Sheet, 2790 Substorms
Scientific paper
A good indicator of substorm expansion phase onset is a well-defined increase and/or energization of the HYDRA electron flux measured onboard POLAR when the satellite is on the night side in the central region of the near earth plasmasheet. This signature is usually, but not always, accompanied by a dipolarization of the magnetic field. Another clear indicator of expansion phase onset is a well-defined increase in the z-component of the magnetic field which is indicative of dipolarization on the night side at geostationary orbit. Substorm events for this study were selected using these two indicators. 34 expansion phase onsets were found using the HYDRA instrument and 119 onsets were found using GOES 10 satellite data. For event selection the GSM coordinates of POLAR were constrained as follows: -9 < x < -7, -2 < y < 2, -1 < z < 1 in units of earth radii. The GOES 10 location was subject to the requirement that the satellite was located within 3 hours either side of local midnight. As expected these onset times were found to be closely correlated with the onset of ground-based auroral zone enhanced Pi2 activity and magnetic bays. Multiple onset substorms were distinguished from single onset events by observing the occurrence of one or more additional subsequent Pi2 intensifications and negative bays corresponding to enhancements of the westward electrojet. For several events, when data was available, auroral brightenings at the equatorward edge of discrete arcs as observed by the FUV experiment onboard the IMAGE spacecraft were also found to be closely correlated with not only the initial Pi2 intensification but also with subsequent Pi2 intensifications. The ratio of multiple onset to single onset substorms was found to be 2.3:1. Using Pi2 and IMAGE FUV data it was found that the initial onset of a multiple onset substorm usually corresponds to Pi2 intensifications and auroral brightening signatures at a lower auroral zone latitude than for a single onset event. In addition the time interval between Pi2 intensifications and/or auroral brightenings for the multiple onset events was found to have a range of 8 to 30 min with an average of 19 min. 63 percent of the events selected by increases in the z-component of the magnetic field at the GOES 10 satellite were found while the satellite was located in the pre-midnight sector. The triggering of substorms was also studied using OMNI Solar Wind data. It was found that 54 percent of the substorm events were consistent with external solar wind triggering.
Engebretson Mark J.
Erickson K. N.
Frey Harald U.
Larson Richard B.
Russell Christopher T.
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