Observational evidence for an inside-out substorm onset scenario

M G Henderson

doi:10.5194/angeo-27-2129-2009

Abstract

Abstract. We present observations which provide strong support for a substorm expansion phase onset scenario in which a localized inner magnetospheric instability developed first and was later followed by the development of a Near Earth Neutral Line (NENL) farther down-tail. Specifically, we find that the onset began as a localized brightening of an intensified growth phase arc which developed as a periodic series of arc-aligned (i.e. azimuthally arrayed) bright spots. As the disturbance grew, it evolved into vortical structures that propagated poleward and eventually morphed into an east-west aligned arc system at the poleward edge of the auroral substorm bulge. The evolution of the auroral intensity is consistent with an exponential growth with an e-folding time of around 188 s (corresponding to a linear growth rate, γ of 5.33×10−3 s−1). During the initial breakup, no obvious distortions of auroral forms to the north were observed. However, during the expansion phase, intensifications of the poleward boundary of the expanding bulge were observed together with the equatorward ejection of auroral streamers into the bulge. A strong particle injection was observed at geosynchronous orbit, but was delayed by several minutes relative to onset. Ground magnetometer data also shows a two phase development of mid-latitude positive H-bays, with a quasi-linear increase in H between the onset and the injection. We conclude that this event provides strong evidence in favor of the so-called "inside-out" substorm onset scenario in which the near Earth region activates first followed at a later time by the formation of a near-to-mid tail substorm X-line. The ballooning instability is discussed as a likely mechanism for the initial onset.

Highlights

A satisfactory understanding of the physical processes that lead to substorm expansion phase onset and subsequent expansion phase activity has eluded researchers since substorms were first discovered by Akasofu (1964)
Since the auroral onset occurred; (1) deep within the closed field line region; (2) developed without any discernable distortions of the auroral emissions on the poleward side of it; and (3) developed in the form of periodic azimuthally arrayed bright spots which evolved into vortical structures that propagated poleward, we are led to the interpretation that the initial breakup during this substorm very likely occurred as a result of the growth of a localized inner magnetospheric instability. (See the yellow text in Figs. 2 and 3.)
We have presented observations that provide strong evidence that the auroral expansion phase onset was associated with an “inside-out” substorm onset scenario

Summary

Introduction

A satisfactory understanding of the physical processes that lead to substorm expansion phase onset and subsequent expansion phase activity has eluded researchers since substorms were first discovered by Akasofu (1964). For example; the “Original” Near Earth Neutral Line (ONENL) model (e.g. see Hones, 1977) violated the first constraint (onset was not as close to the Earth as required); early versions of the Current Disruption (CD) model (e.g. see Lui, 1978, and Lui et al, 1988) and the MIcoupling model of Kan et al (1988) largely disregarded the second constraint (onset was near-Earth, but X-lines did not play an integral role); the Thermal Catastrophe (TC) model (Smith et al, 1986; Goertz and Smith, 1989) and the Boundary Layer Dynamics (BLD) model (Rostoker and Eastman, 1987), essentially ignored both constraints (both placed onset in the Plasma Sheet Boundary Layer (PSBL) and neither invoked X-line formation) Since these models were proposed, the general consensus has emerged that a successful substorm model needs to, at least approximately, satisfy both of the observational constraints listed above. We present detailed observations of a single substorm which provides compelling evidence that the “inside-out” model of substorm development does operate for at least some substorms

Terminology

Observations

Discussion

Conclusions