Abstract
Abstract. On 07 September 2001 the Cluster spacecraft observed a "bursty bulk flow" event in the near-Earth central plasma sheet. This paper presents a detailed study of the coincident ground-based observations and attempts to place them within a simple physical framework. The event in question occurs at ~22:30 UT, some 10min after a southward turning of the IMF. IMAGE and SAMNET magnetometer measurements of the ground magnetic field reveal perturbations of a few tens of nT and small amplitude Pi2 pulsations. CUTLASS radar observations of ionospheric plasma convection show enhanced flows out of the polar cap near midnight, accompanied by an elevated transpolar voltage. Optical data from the IMAGE satellite also show that there is a transient, localised ~1 kR brightening in the UV aurora. These observations are consistent with the earthward transport of plasma in the tail, but also indicate the absence of a typical "large-scale" substorm current wedge. An analysis of the field-aligned current system implied by the radar measurements does suggest the existence of a small-scale current "wedgelet", but one which lacks the global scale and high conductivities observed during substorm expansions. Key words. Ionosphere (auroral ionosphere; ionospheremagnetosphere interactions; plasma convection)
Highlights
A major goal of solar-terrestrial physics is to improve the understanding of the nature of the coupled magnetosphereionosphere system
We investigate the ionospheric counterpart of a bursty bulk flows (BBFs) which occurred during a substorm growth phase, largely circumventing any such effects
Upstream interplanetary magnetic field data from the Advanced Composition Explorer (ACE) spacecraft showed this to be an interval of southward field, consistent with one of active convection in the magnetosphere
Summary
1993, 1995; Moen et al, 1995; Provan et al, 1998; Neudegg et al, 1999, 2000; Milan et al, 2000). Grocott et al (2003) have found intervals of northward IMF during which fast (∼1000 m s−1) flow bursts lasting a few tens of minutes are observed in the nightside ionosphere These bursts were not accompanied by any classic substorm phenomena, such as auroral enhancements, particle injections or magnetic bays, yet were shown to be associated with field reconfiguration after reconnection in the tail. Grocott et al.: Ionospheric observations of a bursty bulk flow of equatorward-directed flow in nightside Sondrestrom radar data, which take place during a “quiet-time” interval in which the transverse components of the IMF were small and directed mainly northward These flow bursts were initiated near the nightside open-closed field line boundary, had amplitudes up to several 100 m s−1, and recurred at ∼1 h intervals, lasting on each occasion for a few tens of minutes. Significant ionospheric flow and auroral signatures were observed and these are considered below, along with a discussion of potential driving mechanisms and their implications for our understanding of substorm physics
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