Hot Plasma in the Magnetotail Lobes Shows Characteristics Consistent With Closed Field Lines Trapped in the Lobes

Abstract

We 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) BZ conditions for our algorithm and a reference algorithm (with no particle energy criteria) and find that the periods we select are, on average, 00∼2 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 00∼0.5% of our identified periods may be driven by direct entry into the magnetosphere from the solar wind.

The Association of Cusp‐Aligned Arcs With Plasma in the Magnetotail Implies a Closed Magnetosphere

“…Coxon et al. (2021) recently showed that hot plasma consistent with trapping on closed field lines is frequently seen at | Z | > 5 R E during northward IMF; here we have shown that CAAs are the auroral signature of this closed flux, and that at such times the magnetosphere is likely (almost) entirely closed. We believe that this closure is achieved by DLR, as proposed by Milan, Carter, Bower, et al.…”

“…This trapped plasma will provide a source for the CAA auroral emission, and as the auroral emission (and associated precipitation) covered the polar regions, we further suggest that the magnetosphere was almost entirely closed. Coxon et al (2021) recently showed that hot plasma consistent with trapping on closed field lines is frequently seen at |Z| > 5 R E during northward IMF; here we have shown that CAAs are the auroral signature of this closed flux, and that at such times the magnetosphere is likely (almost) entirely closed. We believe that this closure is achieved by DLR, as proposed by Milan, Carter, Bower, et al (2020) and Milan et al (2022).…”

“…It has been suggested that polar cap arcs—a catch‐all term for high latitude auroral phenomena which tend to be elongated in a sun‐aligned or cusp‐aligned direction—can come in different varieties which may be associated with either open or closed magnetic flux (e.g., Bower, Milan, Paxton, & Imber, 2022; Carlson & Cowley, 2005; Reidy et al., 2020). Evidence suggests that the most prominent of polar cap arcs, those also known as transpolar arcs or theta auroras, are likely associated with closed magnetic flux (Coxon et al., 2021; Fear et al., 2014; Fryer et al., 2021), proposed to be produced by magnetic reconnection in the magnetotail (Fear & Milan, 2012a, 2012b; Milan et al., 2005). Closed magnetic flux traps plasma in the magnetotail which then acts as a source for auroral precipitation.…”

3D GUMICS Simulations of Northward IMF Magnetotail Structure

“…(2021), and Coxon et al. (2021). However, the configuration we observe in this simulation differs from that proposed by Milan et al.…”

“…(2021) and Coxon et al. (2021). Therefore, in this study, we compare the output of the GUMICS model under varying northward IMF conditions with the characteristics that have been previously observed based on the Milan et al.…”