We present a comprehensive statistical study of magnetic holes, defined as localized decreases of the magnetic field strength of at least 50%, in the solar wind near Mercury, using MESSENGER orbital ...data. We investigate the distributions of several properties of the magnetic holes, such as scale size, depth, and associated magnetic field rotation. We show that the distributions are very similar for linear magnetic holes (with a magnetic field rotation across the magnetic holes of less than 25°) and rotational holes (rotations >25°), except for magnetic holes with very large rotations (≳140°). Solar wind magnetic hole scale sizes follow a log‐normal distribution, which we discuss in terms of multiplicative growth. We also investigate the background magnetic field strength of the solar wind surrounding the magnetic holes, and conclude that it is lower than the average solar wind magnetic field strength. This is consistent with finding solar wind magnetic holes in high‐β regions, as expected if magnetic holes have a connection to magnetic mirror mode structures. We also present, for the first time, comprehensive statistics of isolated magnetic holes in a planetary magnetosheath. The properties of the magnetosheath magnetic holes are very similar to the solar wind counterparts, and we argue that the most likely interpretation is that the magnetosheath magnetic holes have a solar wind origin, rather than being generated locally in the magnetosheath.
Key Points
We present a comprehensive statistical study of magnetic holes in the near‐Mercury solar wind and magnetosheath
We find that magnetic hole scale sizes are log‐normally distributed
We suggest that a majority of the magnetosheath magnetic holes have a solar wind origin
A study of dipolarization fronts of the Earth's magnetotail has been performed using seven years (2001–2007) of Cluster data. Events both with and without high-speed earthward flows are included. A ...superposed epoch analysis of the data shows that the dipolarization is preceeded by a decrease of Bz before the increase. The duration of the dipolarization tends to be decreasing with increasing velocity of the plasma flows. The thickness of the dipolarization front is on average 1.8 plasma inertial lengths, independent of the plasma velocity. We find that the events fall into two categories: Earthward and tailward moving dipolarizations. The dipolarization fronts can be assumed to be tangential discontinuities and the currents on the front have mainly a perpendicular component.
Context.
On December 27, 2020, Solar Orbiter completed its first gravity assist manoeuvre of Venus (VGAM1). While this flyby was performed to provide the spacecraft with sufficient velocity to get ...closer to the Sun and observe its poles from progressively higher inclinations, the Radio and Plasma Wave (RPW) consortium, along with other operational in situ instruments, had the opportunity to perform high cadence measurements and study the plasma properties in the induced magnetosphere of Venus.
Aims.
In this paper, we review the main observations of the RPW instrument during VGAM1. They include the identification of a number of magnetospheric plasma wave modes, measurements of the electron number densities computed using the quasi-thermal noise spectroscopy technique and inferred from the probe-to-spacecraft potential, the observation of dust impact signatures, kinetic solitary structures, and localized structures at the bow shock, in addition to the validation of the wave normal analysis on-board from the Low Frequency Receiver.
Methods.
We used the data products provided by the different subsystems of RPW to study Venus’ induced magnetosphere.
Results.
The results include the observations of various electromagnetic and electrostatic wave modes in the induced magnetosphere of Venus: strong emissions of ∼100 Hz whistler waves are observed in addition to electrostatic ion acoustic waves, solitary structures and Langmuir waves in the magnetosheath of Venus. Moreover, based on the different levels of the wave amplitudes and the large-scale variations of the electron number densities, we could identify different regions and boundary layers at Venus.
Conclusions.
The RPW instrument provided unprecedented AC magnetic and electric field measurements in Venus’ induced magnetosphere for continuous frequency ranges and with high time resolution. These data allow for the conclusive identification of various plasma waves at higher frequencies than previously observed and a detailed investigation regarding the structure of the induced magnetosphere of Venus. Furthermore, noting that prior studies were mainly focused on the magnetosheath region and could only reach 10–12 Venus radii (
R
V
) down the tail, the particular orbit geometry of Solar Orbiter’s VGAM1, allowed the first investigation of the nature of the plasma waves continuously from the bow shock to the magnetosheath, extending to ∼70
R
V
in the far distant tail region.
We present Magnetospheric Multiscale observations of an electron‐scale reconnecting current sheet in the mixing region along the trailing edge of a Kelvin‐Helmholtz vortex during southward ...interplanetary magnetic field conditions. Within this region, we observe intense electrostatic wave activity, consistent with lower‐hybrid waves. These waves lead to the transport of high‐density magnetosheath plasma across the boundary layer into the magnetosphere and generate a mixing region with highly compressed magnetic field lines, leading to the formation of a thin current sheet associated with electron‐scale reconnection signatures. Consistencies between these reconnection signatures and a realistic, local, fully‐kinetic simulation modeling this current sheet indicate a temporal evolution of the observed electron‐scale reconnection current sheet. The multi‐scale and inter‐process character of this event can help us understand plasma mixing connected to the Kelvin‐Helmholtz instability and the temporal evolution of electron‐scale reconnection.
Plain Language Summary
Like wind blowing over water, the stream of ionized gas released from the Sun, called the solar wind, can lead to waves and rolled‐up vortex structures at the boundary of Earth's magnetosphere, called the magnetopause. These so‐called Kelvin‐Helmholtz waves have been shown to be connected to various different plasma processes on different scales. This multi‐scale and multi‐process character makes them an ideal candidate to study the relation between these processes from both spacecraft observations and simulations. By using spacecraft data from the Magnetospheric Multiscale mission, which was designed for the study of small‐scale plasma processes in Earth's magnetosphere, we show observations of electron‐scale magnetic reconnection, an explosive energy conversion process in plasmas, in a region along the trailing edge of these waves. These observations shed new light on the multi‐scale and multi‐process character of the Kelvin‐Helmholtz instability and the energy conversion processes along its boundary.
Key Points
A reconnecting electron‐scale current sheet is observed by Magnetospheric Multiscale (MMS) in mixing plasma along the trailing edge of a Kelvin‐Helmholtz vortex
Realistic 2.5D fully‐kinetic simulation shows reasonable agreement with MMS data
Consistencies between the simulation and MMS indicate a temporal evolution of the reconnecting current sheet
The second Venus flyby of the BepiColombo mission offer a unique opportunity to make a complete tour of one of the few gas-dynamics dominated interaction regions between the supersonic solar wind and ...a Solar System object. The spacecraft pass through the full Venusian magnetosheath following the plasma streamlines, and cross the subsolar stagnation region during very stable solar wind conditions as observed upstream by the neighboring Solar Orbiter mission. These rare multipoint synergistic observations and stable conditions experimentally confirm what was previously predicted for the barely-explored stagnation region close to solar minimum. Here, we show that this region has a large extend, up to an altitude of 1900 km, and the estimated low energy transfer near the subsolar point confirm that the atmosphere of Venus, despite being non-magnetized and less conductive due to lower ultraviolet flux at solar minimum, is capable of withstanding the solar wind under low dynamic pressure.
We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low‐energy plasma in the near‐nucleus coma of comet ...67P/Churyumov‐Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1–2·10−6, at a cometocentric distance of 10 km and at 3.1 AU from the Sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low‐energy plasma around the comet.
Key Points
The spatial distribution of plasma around comet 67P is highly structured
Local ionization of neutral gas dominates the plasma environment
Plasma falls off with cometocentric distance as 1/r
Draping features of the interplanetary magnetic field around nonmagnetic bodies, especially Venus, have been studied in detail in numerical simulations and also from observations. Existing analytical ...and numerical work for nonperpendicular interplanetary magnetic field and solar wind velocity direction show a kink in the draped fieldlines in the near magnetosheath on the quasi‐parallel side of the bow shock. Here long‐term magnetic field data from the Venus Express mission (2006–2014) are analyzed in the near‐nightside region of the magnetosheath, searching for differences in the draping pattern between the quasi‐parallel and quasi‐perpendicular side of the shock. From these magnetometer (MAG) data, the kink in the fieldlines occurring only on the quasi‐parallel side is clearly identified from the change of sign in the field component parallel to the solar wind velocity. Furthermore, an asymmetry in the deflection of the out‐of‐plane field component due to the slipping of the fieldlines over the planetary obstacle is also found, which confirms predictions from numeral studies and from earlier work.
Key Points
For oblique solar wind velocity and IMF direction, fieldline draping in the nightside magnetosheath is asymmetric with kink only on the quasi-parallel shock side
Asymmetry not related to direction of motional electric field
Magnetotail dipolarization fronts (DFs) are referred as the sharp increase of the northward magnetic field component, embedded in bursts of fast earthward moving plasma flows, so called bursty bulk ...flows. Earlier studies often considered DFs as tangential discontinuities (TDs), which can be understood as thin vertical current layers of earthward moving flux tubes, so called dipolarzing flux bundles (DFBs), which separate the ambient plasma sheet plasma from the low entropy plasma within the DFB. Here we present a statistical study of 23 DFs observed by the Magnetospheric Multiscale mission during 2017 and 2018 when the apogee was at 25 RE in the magnetotail. We perform a test of the Walén relation to distinguish whether the observed DFs have rather a TD or rotational discontinuity character and evaluate the plasma flow across the DFs in detail. The results show that on MHD large scales, all 23 DFs can be considered as TD like, but sometimes may have a significant normal plasma flow across it: for 16 events (∼70%), the plasma flows mainly tangential to the DFs, while for seven events (∼30%), the plasma flows mainly across the DFs. Based on the findings present in this study, we further hypothesize that the DF structure becomes more distorted and unstable in a (locally) more dipolarized background magnetic field region, which may additionally facilitate the plasma flow across the front.
Key Points
Walén test shows that dipolarization fronts (DFs) can generally be classified as tangential discontinuity (TD)‐like structures
Plasma flow across DFs are observed, indicating that they can not be characterized as strict TDs
Plasma flow across DFs is larger when background northward magnetic field is stronger
Mercury is host to a rich and highly dynamic plasma environment, resulting from the interaction of the solar wind with Mercury's intrinsic magnetic field and tenuous exosphere. Ion cyclotron waves ...generated by the pick‐up of freshly photoionized neutrals are a common physical phenomenon in the exosphere of weakly and/or unmagnetized bodies. Here we study the properties of cyclotron waves generated by proton pick‐up, the so‐called proton cyclotron waves (PCWs), at Mercury. We surveyed 4 years of MESSENGER magnetometer data and identified 5455 PCW events observed over a wide spatial range, where ∼73% of the observed PCWs are located in the solar wind and ∼27% in the magnetosheath. By associating the wave properties with their spatial distribution, we evaluate the characteristics and differences of the PCWs in the two regions.
Plain Language Summary
Mercury possesses an internally generated magnetic field which constitutes an obstacle to the supersonic solar wind. Upstream of the planet a bow shock emerges where the supersonic solar wind slowed and then flows subsonically in the so‐called magnetosheath. Mercury is also surrounded by a tenuous exosphere, which contains a variety of species that find their origin in the solar wind, micrometeorites that impact the planet and from Mercury's surface itself. These neutral particles can get photoionized through the solar UV radiation, and will then be picked‐up by the solar wind. Because the freshly pick‐up particles have a different velocity than the solar wind, the solar wind plasma becomes unstable for the generation of plasma waves, in particular for the so‐called ion cyclotron waves. Here we report for the first time on ion cyclotron waves generated by the pick‐up of photoionized hydrogen at Mercury. They are detected upstream and downstream of the bow shock, in the solar wind and magnetosheath. From the results found in this study, we speculate that the pick‐up ion cyclotron waves in these two regions might originate from different excitation mechanisms.
Key Points
First report of ion cyclotron waves generated by the pick‐up of protons in the space environment of Mercury
In the solar wind the right‐hand beam resonant instability is dominating
In the magnetosheath the left‐hand ring‐beam distribution instability becomes important
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