Abstract
AbstractWe examine the magnetotail using data from the Hot Ion Analyzer on Cluster 1 during 2001–2009. We develop and utilize an algorithm in order to identify times during which Cluster 1 is in the magnetotail lobe but observes plasma, which is hotter than our expectations of the lobe. We analyze the prevailing Interplanetary Magnetic Field (IMF) conditions for our algorithm and a reference algorithm (with no particle energy criteria) and find that the periods we select are, on average, nT more toward northward IMF. Examining the temperature in the magnetotail for our periods shows that the morphology of the average temperature is consistent with the Milan et al. (2005, https://doi.org/10.1029/2004JA010835) model of a magnetotail structure during northward IMF, in which closed field lines are prevented from convecting to the dayside, causing them and the plasma trapped on them to protrude into the magnetotail lobes. We also find evidence that of our identified periods may be driven by direct entry into the magnetosphere from the solar wind.
Highlights
The configuration of the magnetosphere during southward Interplanetary Magnetic Field (IMF) is well-understood, and was first described by Dungey (1961)
Zhu et al (1997) presented a review of polar cap arcs, outlining one potential magnetotail configuration in which closed field lines get trapped in the magnetotail and these field lines intrude upon the polar cap as a result, causing transpolar arcs; this is consistent with Frank et al (1982, 1986) observations of open field lines on either side of a transpolar arc on closed field lines
Is anomalously hot plasma observed in the magnetotail lobes generally a result of trapped closed field lines unable to convect around Earth due to asymmetries in return flow during northward IMF? Or is it generally a result of direct entry into the magnetotail during periods of high-latitude reconnection during northward IMF? The object of this study is to examine this question statistically, and to do so using an algorithm that can capture the full range of anomalously hot plasma in the lobes
Summary
The configuration of the magnetosphere during southward Interplanetary Magnetic Field (IMF) is well-understood, and was first described by Dungey (1961). Fear et al (2014) presented a case study of anomalously hot electrons and ions in the magnetotail lobes using Cluster data in order to differentiate between the Milan et al (2005) and Shi et al (2013) mechanisms They demonstrated that this hot plasma was on closed field lines which were located at higher latitudes than the typical plasma sheet, and noted that this was consistent with the idea of a closed field line trapped in the magnetotail lobe. They used Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) data to show that there was a transpolar arc simultaneous with the Cluster observations, and demonstrated that the Cluster footprint was coincident with the arc using the T96 model (Tsyganenko, 1996). Algorithm within that study; namely, we concatenate periods that are separated by 10 min and discard periods 5 min long, which achieves the best correspondence with the provided event list
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