DYNAMICS OF LOW ENERGETIC IONS AND ELECTRONS NEAR SUBSTORM ONSET

Substorm associated behavior of the low energetic particles (30 eV - 28.5 keV) near the earthward edge of the plasma sheet is examined using the data from the CRRES during the late growth and early expansion phases of a substorm on March 12, 1991. The significance of these particles for substorm onset mechanism is discussed. During this substorm, the CRRES was located on L ~ 6.3 and ~ 20 O to the west from substorm onset region and observed three small bursts (100-120 s) of the ions (0.633- 9.6 keV) with quasi-period of 3-4 min. The first, less energetic (0.633-0.831 keV) ion burst occurred 2 min before the substorm onset, at the moment of weak brightening of the most equatorial arc near the latitude of ~62 O . The ion bursts preceded by 1-2 min the electron (7.31- 21.7 keV) bursts. The alternation of ion and electron bursts may be a signature of a ballooning-type instability developing at the inner edge of the plasma sheet near substorm onset.