A global magnetosphere‐ionosphere coupling model of substorms

Abstract

A global model of substorms is proposed on the basis of observational synthesis and theoretical modeling. Since the theoretical basis of the present model is the magnetosphere-ionosphere coupling (MIC) process, it will be called the MIC model of substorms. Substorms can occur in the MIC model without a new X line formed in the near-Earth plasma sheet, in contrast to the highly popular near-Earth neutral line (NENL) model of substorms. Following enhanced dayside reconnection, the ionosphere overloads both the solar wind on open field lines and the plasma sheet on closed field lines. The solar wind responds to the overload by providing more driven energy from the dynamo action on open field lines. The plasma sheet responds to the overload by collapsing its eft, i.e., dipolarizing its field configuration to form the substorm current wedge. The explosive intensification during the expansion phase is powered by releasing the magnetic energy stored on closed field lines in the plasma sheet. The stored energy is released by the unloading instability driven by a positive feedback in the substorm current wedge. INTRODUCTION Substorm phenomenon is a global electrodynamic response of the coupled magnetosphere-ionosphere (M-I) system to the dynamic forcing by the solar wind. Substorm activities in the ionosphere are linked to substorm activities in the magnetosphere by field-aligned currents [lijima and Potemra, 1978]. Auroral substorms, polar magnetic substorms, field reconfiguration and plasma heating and jetting in the plasma sheet are all integral parts of the magnetospheric substorm. Auroral substorm morphology, describing the recurring pattern of the auroral activity, was discovered almost three decades ago by Akasofu [1964] based on observations from the ground. This discovery was confirmed on the global scale by aurora imaging from space: ISIS 2 [Anger et al., 1973], DMSP [Rogers et al., 1974], KYOKKO [Kaneda, 1979], DE [Frank et al., 1981], HILAT [Meng and Huffman, 1984], VIKING [Anger et al., 1987], and AKEBONO [Kaneda and ¾amamoto, 1991]. The concept of the magnetospheric substorm has been a fundamental framework for studying the solar-terrestrial relationship. Substorm morphology in the plasma sheet, describing configurational and dynamical changes in the plasma sheet during substorms, was pioneered in 1967 by Hones and collaborators, as summarized in the near-Earth neutral line (NENL) model of substorms [e.g., Hones, 1979, 1984]. A long-standing controversial issue of the plasma sheet substorm morphology is, "Where and when is the near-Earth X line formed during substorms?" There are several versions of the NENL models [McPherron et al., 1973; Hones, 1979, 1984, Baker et al., 1987]. In some versions of the NENL model the new X line is no longer in the near-Earth plasma sheet. The canonical version was proposed by Hones [1984] in which a new X line is formed in the near-Earth plasma sheet (< 20 R E radial distance) at the substorm expansion onset. The most recent version of the NENL model [Baker et al., 1992] sug...

Observational evidence for an inside-out substorm onset scenario

“…Numerous models or mechanisms have been introduced over the past 40 years in attempts to explain the observed phenomenology (e.g. Swift, 1967;Hones et al, 1973;Russell and McPherron, 1973;McPherron et al, 1973;Hones, 1977;Lui, 1978;Lui et al, 1988;Roux, 1985;Smith et al, 1986;Rostoker and Eastman, 1987;Kan et al, 1988;Goertz and Smith, 1989;Lui, 1991;Roux et al, 1991;Kan, 1993;Hones and McPherron, 1994;Lyons, 1995;Miura, 2001;Zhu et al, 2007), and while some of these models have been rejected or abandoned over the years, others have been continuously modified and updated. Nevertheless, there is still no clear consensus within the substorm community about which model is most appropriate.…”

Indirect mapping of the source of the oppositely directed fast plasma flows in the plasma sheet onto the auroral display

“…Roux et al, 1991;Golovchanskaya et al, 2004), or instabilities in the magnetosphereionosphere system (e.g. Kan, 1993).…”

Substorm dynamics revealed by ground observations of two-dimensional auroral structures on 9 October 2000

“…Suggestions for the particular type of NGOPS instability include the cross-field current instability (Lui et al, 1991), the drift-Alfvén-ballooning (DAB) instability (Roux et al, 1991), the shear flow ballooning instability (Samson et al, 1996), and the magnetosphere-ionosphere coupling feedback instability (Kan, 1993). The ballooning instability initially proposed by Roux et al (1991) was criticized by Ohtani and Tamao (1993) etc.…”