Dynamics of variable dusk–dawn flow associated with magnetotail current sheet flapping

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.

Dynamics of Variable Dusk-Dawn Flow Associated with Magnetotail Current Sheet Flapping

Previous observations have provided a clear indication that the dusk-dawn (v⊥y) sense of both slow (< 200 km s−1) and fast (> 200 km s−1) convective magnetotail flows is strongly governed by the Interplanetary Magnetic Field (IMF) By conditions. The related “untwisting hypothesis” of magnetotail dynamics is commonly invoked to explain this dependence, in terms of a large-scale magnetospheric asymmetry. In the current study, we present Cluster spacecraft observations from 12 October 2006 of earthward convective magnetotail plasma flows whose dusk-dawn sense disagrees with the untwisting hypothesis of IMF By control of the magnetotail flows. During this interval, observations of the upstream solar wind conditions from OMNI, and ionospheric convection data using SuperDARN, indicate a large-scale magnetospheric morphology consistent with positive IMF By penetration into the magnetotail. Inspection of the in-situ Cluster magnetic field data reveals 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 flow perturbations may have been driven by the J x B force associated with a dawnward-propagating flapping of the magnetotail current sheet, locally overriding the expected IMF By control of the flows. We conclude that invocation of the untwisting hypothesis may be inappropriate when interpreting intervals of dynamic magnetotail behaviour such as during current sheet flapping.

Data Mining Reconstruction of Magnetotail Reconnection and Implications for Its First-Principle Modeling

Magnetic reconnection is a fundamental process providing topological changes of the magnetic field, reconfiguration of space plasmas and release of energy in key space weather phenomena, solar flares, coronal mass ejections and magnetospheric substorms. Its multiscale nature is difficult to study in observations because of their sparsity. Here we show how the lazy learning method, known as K nearest neighbors, helps mine data in historical space magnetometer records to provide empirical reconstructions of reconnection in the Earth’s magnetotail where the energy of solar wind-magnetosphere interaction is stored and released during substorms. Data mining reveals two reconnection regions (X-lines) with different properties. In the mid tail (∼30RE from Earth, where RE is the Earth’s radius) reconnection is steady, whereas closer to Earth (∼20RE) it is transient. It is found that a similar combination of the steady and transient reconnection processes can be reproduced in kinetic particle-in-cell simulations of the magnetotail current sheet.

Detecting near-tail current sheet formation using isotropic boundaries: lessons from global MHD.

A number of recent studies suggests an existence of magnetotail current sheet configurations with tailward Bz gradient during the growth phase of the substorm. Such configurations are especially interesting since they are potentially unstable for different types of instabilities and can lead to explosive reconfiguration of the magnetosphere. However, the observations are rare and ability to observe tailward gradients is very limited. Here we use the global MHD configuration with near-tail Bz minimum to investigate the regions with adiabatic and non-adiabatic behavior of energetic particles. Thus we estimate the locations of the isotropic boundaries for the modelled POES-type spacecraft flybys. We expect that the lessons learned from global MHD simulation may become helpful in exploration of non-monotonic tail current sheet configuration using observations on low-orbiting spacecraft.

Current sheet structure close to a reconnection point observed by MMS

&lt;p&gt;The typical picture of magnetic reconnection in the magnetosphere includes a classic Harris-type current sheet, where the current density is maximum at the magnetic equator (Bx=0). However, observations have shown that the magnetotail current sheet structure is much more complicated than this simple view. Therefore, revealing the structure of the current sheet is of importance to understand the reconnection process. Based on the four-point MMS high-resolution data, we present observations of a multiple reconnection event for which we study the structure of the current sheet as well as some of its characteristic scales. We show that the CS structure is highly dynamic during the reconnection process, changing from a bifurcated shape away from the reconnection site, to a more symmetric (Harris-type) structure near the X-line.&lt;/p&gt;

Electron anisotropy driven by kinetic Alfven waves in the Earth magnetotail

&lt;p&gt;Thermal and subthermal electron populations in the Earth&amp;#8217;s magnetotail is usually characterized by pronounced field-aligned anisotropy that contributes to generation of strong electric currents within the magnetotail current sheet. Formation of this anisotropy requires electron field-aligned acceleration, and thus likely involves field-aligned electric fields. Such fields can be carried by various electromagnetic waves generated by fast plasma flows interacting with ambient magnetotail plasma. In this presentation we consider one of the most intense observed wave emissions, kinetic Alfven waves, that accompany all fast plasma flows in the magnetotail.&lt;/p&gt;&lt;p&gt;Using two tail seasons (2018, 2019) of MMS observations we have collected statistics of 80 fast plasma flows (or BBF) events with distinctive enhancement of intensity of broadband electromagnetic waves sharing properties of kinetic Alfven waves. We show that a direct correlation the intensity of electric fields of kinetic Alfven waves and electron anisotropy distribution: the parallel electron anisotropy significantly increases with magnitude of the wave parallel electric field. The energy range of this electron anisotropic population is well within the range of resonant energies for observed kinetic Alfven waves. Our results show that kinetic Alfven waves can significantly contribute to shaping the magnetotail electron population.&lt;/p&gt;