Statistical study of high-latitude plasma flow during magnetospheric substorms

Abstract

Abstract.We have utilised the near-global imaging capabilities of the Northern Hemisphere SuperDARN radars, to perform a statistical superposed epoch analysis of high-latitude plasma flows during magnetospheric substorms. The study involved 67 substorms, identified using the IMAGE FUV space-borne auroral imager. A substorm co-ordinate system was developed, centred on the magnetic local time and magnetic latitude of substorm onset determined from the auroral images. The plasma flow vectors from all 67 intervals were combined, creating global statistical plasma flow patterns and backscatter occurrence statistics during the substorm growth and expansion phases. The commencement of the substorm growth phase was clearly observed in the radar data 18-20 min before substorm onset, with an increase in the anti-sunward component of the plasma velocity flowing across dawn sector of the polar cap and a peak in the dawnto-dusk transpolar voltage. Nightside backscatter moved to lower latitudes as the growth phase progressed. At substorm onset a flow suppression region was observed on the nightside, with fast flows surrounding the suppressed flow region. The dawn-to-dusk transpolar voltage increased from ∼40 kV just before substorm onset to ∼75 kV 12 min after onset. The low-latitude return flow started to increase at substorm onset and continued to increase until 8 min after onset. The velocity flowing across the polar-cap peaked 12-14 min after onset. This increase in the flux of the polar cap and the excitation of large-scale plasma flow occurred even though the IMF B z component was increasing (becoming less negative) during most of this time. This study is the first to statistically prove that nightside reconnection creates magnetic flux and excites high-latitude plasma flow in a similar way to dayside reconnection and that dayside and nightside reconnection, are two separate time-dependent processes.

Towards a synthesis of substorm electrodynamics: HF radar and auroral observations

“…Indeed, the fact that these flows clearly cross the open-closed field line boundary implies reconnection driven flow. The development of these flows over the course of the expansion phase, rather than an instantaneous appearance at the time of substorm onset, is corroborated by other recent studies such as those of Jayachandran et al (2003) and Provan et al (2004) which also show a large-scale reaction in the flows over the ∼10-20 mins following substorm onset. What is clear from this study, however, is the addition to these flows of other large-scale flow features, which have previously only been discussed individually.…”

“…Grocott et al (2000Grocott et al ( , 2002 presented analyses of Super Dual Auroral Radar Network (SuperDARN) flow data obtained during isolated substorms, and have found evidence for the excitation of twin-vortex flow cells centred in the nightside ionosphere, which enhance the transpolar voltage by ∼40 kV compared with pre-onset values. Following this work, Provan et al (2004) conducted a statistical study of substorm flows and found them to become enhanced across the polar cap and in the low-latitude return flow region. They also found a systematic increase in the transpolar voltage from ∼40 kV 2 min before onset to ∼75 kV 12 min after, and attributed this to the removal of open flux from the polar cap by nightside reconnection.…”

Superposed epoch analysis of the ionospheric convection evolution during substorms: onset latitude dependence

“…Further studies using SuperDARN have produced mixed results, however, by providing evidence for both the excitation (e.g. Grocott et al, 2006;Provan et al, 2004) and reduction (e.g. Lyons et al, 2001;Bristow and Jensen, 2007) of the flows following substorm onset.…”

The pulsed nature of the nightside contribution to polar cap convection: repetitive substorm activity under steady interplanetary driving

“…The two currently available techniques of direct CPCP measurements, i.e., via satellite ion drift data (Hairston et al, 1998) and ground-based radars Grocott et al, 2002;Provan et al, 2004), are both incomplete. This is due to the limited temporal resolution of the satellite measurements, on one hand, and the missing spatial coverage at high latitudes of the present radars, on the other.…”

“…The relative contributions to the cross-polar cap potential (CPCP) from the dayside (CPCP/day) and nightside (CPCP/night) sources during intervals of substorm activity have been studied in recent years, applying different observational techniques (Bristow et al, 2004;Lockwood et al, 2009;Kullen et al, 2010;Gordeev et al, 2011;Andalsvik et al, 2011Andalsvik et al, , 2012. Fox et al (1999), Grocott et al (2002) and Provan et al (2004) estimated the convection response to isolated substorms by continuous observations, applying a ground-based radar technique. Grocott et al (2010) studied the convection responses in the dayside and nightside sectors of the polar cap to substorms occurring during intervals of positive and negative interplanetary magnetic field (IMF) B y polarity, applying a superposed epoch analysis.…”