Storm-time Substorms Research Articles

Simultaneous DMSP and CRRES observation of broadband electrons during a storm-time substorm on March 25, 1991

Broadband electrons observed by the DMSP satellites during storm-time substorms are characterized by an unusually intense, flux of precipitating electrons over the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (Shiokawa et al., 1996; 1997). During the broadband electron event observed by the DMSP-F9 satellite at 21 MLT on March 25, 1991, the CRRES spacecraft was at a geocentric distance of 6.3 RE and in the same local time as that of DMSP. The CRRES satellite observed 1) a large enhancement of field-aligned electron flux, 2) highly turbulent electric fields up to 10 mV/m, 3) intense west-ward excursion (∼100 nT) of magnetic field (suggesting intense field-aligned current generation) in a highly tail-like field configuration, 4) large O+ energy density, and 5) intense low-frequency (below 300 Hz) electrostatic waves. We discuss possible production mechanisms of broadband electrons on the basis of these observations.

Current understanding of magnetic storms: Storm‐substorm relationships

This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high‐intensity, long‐duration, continuous auroral electrojet activity) and the so‐called Russell‐McPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood. (2) A serious controversy exists as to whether the successive occurrence of intense substorms plays a direct role in the energization of ring current particles or whether the enhanced electric field associated with southward IMF enhances the effect of substorm expansions. While most of the Dst variance during magnetic storms can be solely reproduced by changes in the large‐scale electric field in the solar wind and the residuals are uncorrelated with substorms, recent satellite observations of the ring current constituents during the main phase of magnetic storms show the importance of ionospheric ions. This implies that ionospheric ions, which are associated with the frequent occurrence of intense substorms, are accelerated upward along magnetic field lines, contributing to the energy density of the storm‐time ring current. An apparently new controversy regarding the relative importance of the two processes is thus created. It is important to identify the role of substorm occurrence in the large‐scale enhancement of magnetospheric convection driven by solar wind electric fields. (3) Numerical schemes for predicting geomagnetic activity indices on the basis of solar/solar wind/interplanetary magnetic field parameters continue to be upgraded, ensuring reliable techniques for forecasting magnetic storms under real‐time conditions. There is a need to evaluate the prediction capability of geomagnetic indices on the basis of physical processes that occur during storm time substorms. (4) It is crucial to differentiate between storms and nonstorm time substorms in terms of energy transfer/conversion processes, i.e., mechanical energy from the solar wind, electromagnetic energy in the magnetotail, and again, mechanical energy of particles in the plasma sheet, ring current, and aurora. To help answer the question of the role of substorms in energizing ring current particles, it is crucial to find efficient magnetospheric processes that heat ions up to some minimal energies so that they can have an effect on the strength of the storm time ring current. (5) The question of whether the Dst index is an accurate and effective measure of the storm time ring current is also controversial. In particular, it is demonstrated that the dipolarization effect associated with substorm expansion acts to reduce the Dst magnitude, even though the ring current may still be growing.

Substorms, Storms, and the Near-Earth Tail.

More than 50, 000 measurements made with the AMPTE IRM satellite in the magnetotail at radial distances between 10 and 19 RE during 40 substorms have been used in a super-posed epoch analysis, separating storm-time substorms from those occurring during intervals of low Dstt levels. It is found that most of the tail signatures reported previously as typical for substorms are strongly influenced by magnetic storm activity. The magnetic field dipolarization and the Earthward convection in the near-Earth tail are much more pronounced for substorms which occur during the main phase of magnetic storms than for non-storm substorms. Moreover, only for storm-time substorms the lobe magnetic pressure decreases during the expansion phase. These findings indicate that not all substorms are alike and that the near-Earth neutral line scenario may apply only to the storm-time substorms.