Distant modulation of electron intensity during the expansion phase of an auroral substorm

Pulses in electron intensity, occurring during the expansion phase of an auroral substorm, were studied using a Skylark sounding rocket launched from Kiruna, Sweden at 2226:50 UT on 2 March 1972. The pulses were typically of 5 s duration and occurred sporadically with a typical interval of 15 s. The rocket carried main and ejected payloads, and the pulses, occurring simultaneously at both, are found to be temporal rather than spatial in origin. Pitch-angle distributions changed little during a pulse, remaining slightly peaked towards larger pitch-angles. Precipitation is thought to be caused by pitch-angle diffusion under conditions where scattering angles are large compared with the opening angle of the loss cone. Enhanced scattering and variations in the temperature of the source plasma are both considered as possible causes of the pulse-like modulation. A dispersion in the times of occurrence of the pulses at different electron energies (3·8, 5·7, 9·0, > 18 and > 48 keV) indicates that the pulses originated at distances which varied between 41,000 ± 4000 km and 90,000 ± 7000 km during the flight. The larger distances are seen as evidence for temporary distortion of the local (L = 5·4) geomagnetic field lines whereby they are stretched to twice their normal (near-dipole) length. It appears that over a period of 100 s the field lines expanded and contracted at a speed of approximately 1000 km s^?1. The further observation that the electron energy spectra were approximately Maxwellian, with a temperature equivalent to ～3 keV, helps to confirm the indications that the precipitation and its modulation were controlled directly by processes taking place in the plasma sheet. Other possible causes of the variable dispersion are considered, but found to be inconsistent with the present results.