Magnetic holes (MHs), with a scale much greater than ρi (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic‐size magnetic holes (KSMHs), previously called small‐size magnetic holes, with a scale of the order of magnitude of or less than ρi have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission, which provides three‐dimensional (3‐D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10–20 ρe (electron gyroradii) and lasted 0.1–0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90° pitch angle, the flux of electrons with energy 34–66 eV decreased, while for electrons of energy 109–1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self‐consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10–20 ρe.
We use Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms data over 15 years (1995–2009) to statistically investigate convective ion flows (V⊥xy<200 km/s) in the magnetotail plasma sheet under the influence of a clearly nonzero dawn‐dusk interplanetary magnetic field (IMF By). We find that IMF By causes an interhemispheric asymmetry in the flows, which depends on the direction of IMF By. On the average, one magnetic hemisphere is dominated by a dawn‐dusk flow component, which is oppositely directed compared to that in the other hemisphere. This asymmetry is observed for both earthward and tailward flows. A comparison to tail By reveals that the region where the asymmetry in the average flows appears agrees with the appearance of the tail By direction collinear to IMF By. The results imply that IMF By has a major influence on the direction of the magnetic flux transport in the magnetotail.
This study analyzes 25 ion‐scale flux ropes in the Magnetospheric Multiscale (MMS) observations to determine their structures. The high temporal and spatial resolution MMS measurements enable the application of multispacecraft techniques to ion‐scale flux ropes. Flux ropes are identified as quasi‐one‐dimensional (quasi‐1‐D) when they retain the features of reconnecting current sheets; that is, the magnetic field gradient is predominantly northward or southward, and quasi‐2‐D when they exhibit circular cross sections; that is, the magnetic field gradients in the plane transverse to the flux rope axis are comparable. The analysis shows that the quasi‐2‐D events have larger core fields and smaller pressure variations than the quasi‐1‐D events. These two types of flux ropes could be the result of different processes, including magnetic reconnection with different dawn‐dusk magnetic field components, temporal transformation of flattened structure to circular, or interactions with external environments.
Based on Cluster observations, the propagation velocities and normal directions of hot flow anomaly (HFA) boundaries upstream the Earth's bow shock are calculated. Twenty‐one young HFAs, which have clear leading and trailing boundaries, were selected, and multispacecraft timing method considering errors was employed for the investigation. According to the difference in the propagation velocity of the leading and trailing edges, we categorized these events into three groups, namely, contracting, expanding, and stable events. The contraction speed is a few tens of kilometers per second for the contracting HFAs, and the expansion speed is tens to more than hundred kilometers per second for expanding events. For the stable events, the leading and trailing edges propagate at almost the same speed within the error range. We have further investigated what causes them to contract, expand, or stay stable by carefully calculating the thermal pressure of the young HFAs which have two distinct ion populations (solar wind beam and reflected flow). It is found that the extreme value of the sum of the magnetic and thermal pressure inside the HFAs compared with that of the nearest point outside of the leading edges is higher for expanding events and lower for contracting events, and there is no significant difference for the stable events, and the total pressure (sum of thermal, magnetic, and dynamic pressure) variation has a significant effect on the evolution for most (70%) of the HFAs, which implies that the pressure plays an important role in the evolution of young HFAs.
Using the Cluster data during the period from January to April between 2001 and 2006, we find an observation of solar wind entry due to magnetic reconnection occurred in the terrestrial high‐latitude magnetospheric lobes, tailward of the cusps under northward interplanetary magnetic field (IMF). Occurrence rate of solar wind entry events in this study is of the same order as that for the Cluster orbital interval from August to October between the years of 2002 and 2004 as reported by Shi et al. (2013). In this paper, we further study the role of the IMF Bx and By components in the control of solar wind plasma entry based on the investigations of different magnetic dipole tilt variations between our database and Shi et al. (2013). This study shows that the asymmetry distribution of solar wind entry events in the northern and southern lobes could be caused by the variation of magnetic dipole tilt, which could influence the locations of the reconnection site on the high‐latitude lobe magnetopause. On the other hand, IMF Bx can also affect the solar wind plasma entry rate, which is also consistent with previous results. Therefore, we conclude that the “north‐south asymmetry” of solar wind entry events in the lobes could be the combined result of magnetic dipole tilt and IMF Bx. In addition, the IMF By component can influence the entry events in conjunction with the variation of IMF Bx component, which is in line with the Parker Spiral of the IMF.
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