Ring current intensification and convection-driven negative bays: Multisatellite studies

[1] The original view on the cause of ring current intensifications was a frequent occurrence of intense substorm expansion phases. Results from many studies have supported this view. However, whether this is the only mechanism of ring current buildup has been a controversy. Kamide [ 1992] asserted that ring current intensification is due to "sustained, southward IMF, not because of frequent occurrence of intense substorms." Lui et al. [ 2001] have shown that the ring current can be intensified during enhanced convection without substorm occurrence. Tsurutani et al. [ 2003] have found that there was a lack of substorm expansion phases for long periods of time ( up to 7 hours) in 5 out of 11 storm main phases ( in 1997) that were induced by the smoothly varying B-z component of the interplanetary magnetic field ( IMF) within interplanetary magnetic clouds. In this paper, a relatively weak magnetic storm event ( with minimum SYM-H at - 47 nT) that occurred on 15 July 1997 is studied using ground-based magnetograms, polar cap potentials from Super Dual Auroral Radar Network, and Los Alamos National Laboratory (LANL) geosynchronous energetic particle data as well as the Polar UV imaging ( for aurorae) and Wind ( for the solar wind) data. It is shown that during the storm main phase, there was a lack of substorm expansion phase activity ( from imaging and the ground-based data) and a lack of energetic particle injections at the geostationary orbit. The most prominent auroral forms were north-south aligned auroral patches and torches. Dawn and dusk aurorae were more intense than the aurorae near midnight, where auroral gaps occurred. In addition, this paper shows that there was a significant directly driven activity during the storm main phase when the IMF was continually southward. We argue that during this event the ring current intensification was more strongly associated with enhanced magnetospheric convection than with impulsive energy unloading. Three scenarios are suggested to explain the relatively low intensity of the magnetic storm induced by a magnetic cloud. They are ( 1) weak nightside auroral zone ionospheric ion outflows ( due to lack of substorms), ( 2) choked penetration of the tail plasma flow ( due to lack of substorms), and ( 3) retarded magnetospheric convection ( due to reduced solar wind-magnetosphere reconnection). The observed saturation of the polar cap potential drop is in support of this latter mechanism.