Global properties of the magnetosphere during a substorm growth phase: A case study

Abstract

At approximately 0100 UT on December 29, 1976, a large injection of energetic (>30 keV) particles was observed by Los Alamos instrumentation onboard spacecraft 1976‐059 (35°W longitude) at geostationary orbit. This injection was closely associated with the onset of a major substorm (also at 0100 UT) identified by sharp negative bays in the H components of magnetic records at Leirvogur (22°W) and Narssarssuaq (45°W) and by the occurrence of a positive H component bay at 0100 UT in the mid‐latitude magnetogram record at M'Bour (17°W). This substorm expansion onset (and concomitant particle injection) was preceded (between 2330 and 0100 UT) by a pronounced ‘stretching’ of the magnetic field at synchronous orbit into a taillike configuration and by a development of highly cigarlike (field‐aligned) electron distributions at geostationary orbit that we have in the past identified with the substorm growth phase. Of principal importance in this case are two other auxiliary data sets. The first is a well‐timed set of DMSP auroral images taken during the course of the growth and expansion phases of the substorm. The images before and during the growth (cigar) phase, including one auroral zone crossing at ∼0050 UT, show quiet aurora with no observable substorm activity in the visible polar region. The second relevant data set is a broad set of riometer data from 13 separate stations in three general meridians (west coast Greenland, east coast Greenland, and northern Scandinavia) from magnetic latitudes of ∼65° to ∼90°. The riometer data also show clearly that there was no measurable substorm activity anywhere, either in longitude or latitude, as the magnetosphere developed its very stressed, growth‐phase configuration prior to substorm expansion onset. These results support the concept of a storage of energy (growth phase) prior to its rapid release at substorm onset. IMP 8 observations in the high southern magnetotail lobe at ∼35 RE geocentric radial distance provide strong corroborative evidence of this storage and subsequent release of magnetic energy.