We analyze two ion scale dipolarization fronts associated with field‐aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on 10 August 2016. The first event ...corresponds to a fast dawnward flow with an antiparallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing and with a smaller lower hybrid drift wave activity. Electromagnetic electron phase‐space holes are detected near these low‐frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet we cannot rule out the possibility that the drift waves are produced by the antiparallel current associated with the fast flows, leaving the source for the electron holes unexplained.
Key Points
Dipolarization fronts associated with field‐aligned currents are observed at the plasma sheet edge with a few ion inertial length scale
Intense lower hybrid drift waves are detected at the front and can accelerate electrons parallel to B
Electromagnetic electron phase‐space holes are detected near the lower hybrid drift waves and could be a latter by‐product of these
Solar Orbiter’s first Venus flyby Volwerk, M; Horbury, T S; Woodham, L D ...
Astronomy and astrophysics (Berlin),
12/2021, Letnik:
656
Journal Article
Recenzirano
Odprti dostop
Context. The induced magnetosphere of Venus is caused by the interaction of the solar wind and embedded interplanetary magnetic field with the exosphere and ionosphere of Venus. Solar Orbiter entered ...Venus’s magnetotail far downstream, > 70 Venus radii, of the planet and exited the magnetosphere over the north pole. This offered a unique view of the system over distances that had only been flown through before by three other missions, Mariner 10, Galileo, and BepiColombo. Aims. In this study, we study the large-scale structure and activity of the induced magnetosphere as well as the high-frequency plasma waves both in the magnetosphere and in a limited region upstream of the planet where interaction with Venus’s exosphere is expected. Methods. The large-scale structure of the magnetosphere was studied with low-pass filtered data and identified events are investigated with a minimum variance analysis as well as combined with plasma data. The high-frequency plasma waves were studied with spectral analysis. Results. We find that Venus’s magnetotail is very active during the Solar Orbiter flyby. Structures such as flux ropes and reconnection sites were encountered, in addition to a strong overdraping of the magnetic field downstream of the bow shock and planet. High-frequency plasma waves (up to six times the local proton cyclotron frequency) are observed in the magnetotail, which are identified as Doppler-shifted proton cyclotron waves, whereas in the upstream solar wind, these waves appear just below the proton cyclotron frequency (as expected) but are very patchy. The bow shock is quasi-perpendicular, however, expected mirror mode activity is not found directly behind it; instead, there is strong cyclotron wave power. This is most likely caused by the relatively low plasma-β behind the bow shock. Much further downstream, magnetic hole or mirror mode structures are identified in the magnetosheath.
In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by ...bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 di, where di is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma..
We present detailed observations of dynamic, fine‐scale inner magnetosphere‐ionosphere coupling at ∼3.9 RE in the Region 2 Birkeland field‐aligned current (FAC). We find that observed electrodynamic ...spatial/temporal scales are primarily characteristic of magnetically mapped ionospheric structure. On 15 September 2015, conjugate Magnetospheric Multiscale (MMS) spacecraft and Millstone Hill radar observations show plasmasphere boundary region subauroral polarization stream (SAPS) electric fields at L = 4.0–4.2 near 21 MLT. MMS observations reveal high‐altitude ∼1 mV/m fine‐scale radial and azimuthal electric field perturbations over ≤0.15 L with high spatial coherence over ≥2–3 min and show outward motion within a broader FAC of ∼0.12 μA/m2. Our analysis shows that MMS electric field fluctuations are most likely reflective of SAPS ionospheric structure at scales of ∼22 km and with ionospheric closure of small‐scale filamentary FAC perturbations. The results highlight the ionosphere's importance in regulating fine‐scale magnetosphere‐ionosphere structure.
Key Points
SAPS electric fields have small‐scale structure in the plasmasphere boundary region
SAPS perturbations can have spatiotemporal structure governed by the ionosphere
SAPS Region 2 filamentary current closure may occur on ∼22 km ionospheric spatial scales
Observations by the four Magnetospheric Multiscale spacecraft are used to investigate the Hall physics of a magnetopause magnetic reconnection separatrix layer. Inside this layer of currents and ...strong normal electric fields, cold (eV) ions of ionospheric origin can remain frozen-in together with the electrons. The cold ions reduce the Hall current. Using a generalized Ohms law, the electric field is balanced by the sum of the terms corresponding to the Hall current, the v x B drifting cold ions, and the divergence of the electron pressure tensor. A mixture of hot and cold ions is common at the subsolar magnetopause. A mixture of length scales caused by a mixture of ion temperatures has significant effects on the Hall physics of magnetic reconnection.
The fluxgate magnetometer MGF on board the Mio spacecraft of the BepiColombo mission is introduced with its science targets, instrument design, calibration report, and scientific expectations. The ...MGF instrument consists of two tri-axial fluxgate magnetometers. Both sensors are mounted on a 4.8-m long mast to measure the magnetic field around Mercury at distances from near surface (initial peri-center altitude is 590 km) to 6 planetary radii (11640 km). The two sensors of MGF are operated in a fully redundant way, each with its own electronics, data processing and power supply units. The MGF instrument samples the magnetic field at a rate of up to 128 Hz to reveal rapidly-evolving magnetospheric dynamics, among them magnetic reconnection causing substorm-like disturbances, field-aligned currents, and ultra-low-frequency waves. The high time resolution of MGF is also helpful to study solar wind processes (through measurements of the interplanetary magnetic field) in the inner heliosphere. The MGF instrument firmly corroborates measurements of its companion, the MPO magnetometer, by performing multi-point observations to determine the planetary internal field at higher multi-pole orders and to separate temporal fluctuations from spatial variations.
The electron dynamics within thin current sheets plays a key role both for the process of magnetic reconnection and other energy transfer mechanisms but, from an observational point of view, is not ...well understood. In this paper we report observations of a reconnecting current sheet with intermediate guide field BG=0.5Bin, where Bin is the magnetic field amplitude in the inflow regions. The current sheet width is comparable to electron spatial scales. It shows a bifurcated structure and is embedded within the magnetopause current layer with thickness of several ion scales. The electron scale current sheet has strong out‐of‐plane and in‐plane currents, Hall electric and magnetic fields, a finite magnetic field component normal to the current sheet, and nongyrotropic electron distributions formed due to finite gyroradius effects at the boundary of the current sheet. Comparison between test particle simulations and electron data shows that electrons approaching from the edge of the largest magnetic curvature are scattered to perpendicular pitch angles in the center of the current sheet while electrons entering from the opposite side remain close to field aligned. The comparison also shows that an observed depletion in phase space at antiparallel pitch angles can be explained if an out‐of‐plane electric field, which due to the guide field is close to antiparallel to the magnetic field, is present in the center of the current sheet. This electric field would be consistent with the reconnection electric field, and we therefore interpret the depletion of electron phase space density as a manifestation of ongoing reconnection.
Key Points
A reconnecting electron scale current sheet is observed inside a filamented magnetopause current layer
Due to a guide magnetic field and related magnetic curvature, electrons entering from opposite sides have different behavior
The presence of an out‐of‐plane electric field can explain observed electron distributions
Energetic (greater than tens of keV) magnetospheric particle escape into the magnetosheath occurs commonly, irrespective of conditions that engender reconnection and boundary-normal magnetic fields. ...A signature observed by the Magnetospheric Multiscale (MMS) mission, simultaneous monohemispheric streaming of multiple species (electrons, H+, Hen+), is reported here as unexpectedly common in the dayside, dusk quadrant of the magnetosheath even though that region is thought to be drift-shadowed from energetic electrons. This signature is sometimes part of a pitch angle distribution evolving from symmetric in the magnetosphere, to asymmetric approaching the magnetopause, to monohemispheric streaming in the magnetosheath. While monohemispheric streaming in the magnetosheath may be possible without a boundary-normal magnetic field, the additional pitch angle depletion, particularly of electrons, on the magnetospheric side requires one. Observations of this signature in the dayside dusk sector imply that the static picture of magnetospheric drift-shadowing is inappropriate for energetic particle dynamics in the outer magnetosphere.
We present multimission observations of field-aligned currents, auroral oval, and magnetopause crossings during the 17 March 2015 magnetic storm. Dayside reconnection is expected to transport ...magnetic flux, strengthen field-aligned currents, lead to polar cap expansion and magnetopause erosion. Our multimission observations assemble evidence for all these manifestations. After a prolonged period of strongly southward interplanetary magnetic field, Swarm and AMPERE observe significant intensification of field-aligned currents .The dayside auroral oval, as seen by DMSP, appears as a thin arc associated with ongoing dayside reconnection. Both the field-aligned currents and the auroral arc move equatorward reaching as low as approx. 60 deg. magnetic latitude. Strong magnetopause erosion is evident in the in situ measurements of the magnetopause crossings by GOES 13/15 and MMS. The coordinated Swarm, AMPERE, DMSP, MMS and GOES observations, with both global and in situ coverage of the key regions, provide a clear demonstration of the effects of dayside reconnection on the entire magnetosphere.
PDF
