Abstract
Abstract. We present Cluster spacecraft observations from 12 October 2006 of convective plasma flows in the Earth's magnetotail. Earthward flow bursts with a dawnward v⊥y component, observed by Cluster 1 (C1), are inconsistent with the duskward flow that might be expected at the pre-midnight location of the spacecraft. Previous observations have suggested that the dusk–dawn sense of the flow can be governed by the interplanetary magnetic field (IMF) By conditions, with the related “untwisting hypothesis” of magnetotail dynamics commonly invoked to explain this dependence, in terms of a large-scale magnetospheric asymmetry. In the current study, observations of the upstream solar wind conditions from OMNI, magnetic field observations by Cluster and ionospheric convection data using SuperDARN indicate a large-scale magnetospheric morphology consistent with positive IMF By penetration into the magnetotail. At the pre-midnight location of Cluster, however, the dawnward flow observed below the neutral sheet by C1 could only be explained by the untwisting hypothesis in a negative IMF By scenario. The Cluster magnetic field data also reveal a flapping of the magnetotail current sheet, a phenomenon known to influence dusk–dawn flow. Results from the curlometer analysis technique suggest that the dusk–dawn sense of the J×B force was consistent with localised kinks in the magnetic field and the flapping associated with the transient perturbations to the dusk–dawn flow observed by C1. We therefore suggest that the flapping overcame the dusk–dawn sense of the large-scale convection which we would expect to have been net duskward in this case. We conclude that invocation of the untwisting hypothesis may be inappropriate when interpreting intervals of dynamic magnetotail behaviour such as during current sheet flapping, particularly at locations where magnetotail flaring becomes dominant.
Highlights
Convective magnetotail plasma flows at Earth, driven by the closing of magnetic flux via reconnection as part of the Dungey Cycle (Dungey, 1961), have been studied extensively for many years (e.g. Angelopoulos et al, 1992, 1994; Sergeev et al, 1996; Petrukovich et al, 2001; Cao et al, 2006; McPherron et al, 2011; Frühauff and Glassmeier, 2016)
We suggest that the dynamic behaviour of (J × B)y is consistent with the localised kinks and flapping in the magnetic field that are associated with the transient perturbations to the dusk–dawn flow observed by Cluster 1 (C1)
We have presented a case study from 12 October 2006 revealing a dynamic interval of plasma flows and current sheet flapping, observed by the Cluster 1 spacecraft
Summary
Convective magnetotail plasma flows at Earth, driven by the closing of magnetic flux via reconnection as part of the Dungey Cycle (Dungey, 1961), have been studied extensively for many years (e.g. Angelopoulos et al, 1992, 1994; Sergeev et al, 1996; Petrukovich et al, 2001; Cao et al, 2006; McPherron et al, 2011; Frühauff and Glassmeier, 2016). Drivers of current sheet flapping have been widely investigated, with possible causes ranging from external solar wind/IMF changes (Runov et al, 2009), induction of hemispheric plasma asymmetries (Malova et al, 2007; Wei et al, 2015), fast earthward flow (Nakamura et al, 2009) and periodical, unsteady magnetotail reconnection (Wei et al, 2019) Studies such as Volwerk et al (2008) and Kubyshkina et al (2014) have illustrated that flapping of the current sheet can be associated with variable dusk–dawn flow, potentially overriding or preventing any IMF By control of the flow. We instead suggest that the current sheet flapping was exciting the variable dusk–dawn flow, overriding the expected large-scale duskward convection at the location of Cluster 1
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