Sub‐ion magnetic holes are rich in the terrestrial plasma sheet and magnetosheath. Here, we statistically investigate 60 sub‐ion magnetic holes in the solar wind at 1 AU using the high‐resolution ...data measured by the Magnetospheric Multiscale mission. We find that they are observed with a duration of 0.1 –0.5 s, and the lengths of their cross‐section are ~0.1 –0.6 ion gyroradius. These structures prefer to occur in the slow solar wind with a weak ambient magnetic field strength. They also prefer to occur in the marginally mirror stable or unstable environments. Electron vortices as well as an enhancement of the electron perpendicular temperature and electron fluxes at ~90° pitch angle tend to be observed inside some magnetic holes with a large ambient magnetic field strength. By contrast, there are no clearly observational electron vortices as well as the electron fluxes at ~90° pitch angle inside some magnetic holes with a weak ambient magnetic strength. The current density with a value of ~10 –50 nA/m2 reveals that the corresponding maximum electron velocity is <10 km/s inside some magnetic holes, lower than the level of the observational electron velocity noise, which prevents the detection of the weak electron vortex. We suggest that electron vortices exist inside all the sub‐ion magnetic holes in the solar wind. The generation of these sub‐ion magnetic holes can be explained by the electron magnetohydrodynamics soliton and the electron vortex magnetic hole.
Key Points
Electron vortices are a common feature inside the sub‐ion magnetic hole in the solar wind at 1 AU
The magnetic holes prefer to occur in the marginally mirror stable and weak magnetic field strength environments during the slow solar wind
The electron magnetohydrodynamics soliton and electron vortex magnetic hole are two potential generation mechanisms of the magnetic hole
Abstract
Small-scale magnetic holes (SSMHs) are frequently observed in the solar wind at 1 au, as well as the terrestrial current sheet and magnetosheath. These kinetic-size structures play an ...important role in energy dissipation and particle transportation. Here, we report the existence of SSMHs in the upstream regime of the Martian bow shock and statistically investigate these SSMHs based on 5 yr observations of the Mars Atmosphere and Volatile EvolutioN spacecraft. A total of 549 SSMHs are found, and their durations and sizes obey the lognormal distribution. The median duration is ∼0.46 s, and the median size is ∼2 ion inertial lengths. We regard an isolated SSMH or a train of SSMHs as a SSMH event. The average occurrence rate of the SSMH events is ∼0.6 event per day. The occurrence rate is much larger in the region belonging to the ion foreshock on average, suggesting that the ion foreshock is an important source of SSMHs. The occurrence of the SSMH events tends to be larger when the ion number density and the interplanetary magnetic field (IMF) strength are larger, indicating that the generation of SSMHs might be associated with ions and the IMF strength. Although their generation mechanism is still unclear, the finding of the link between the occurrence rate of the SSMH events and ion number density, as well as the IMF strength, might provide a clue to further reveal the generation mechanism of SSMHs in the solar wind or planetary foreshock.
Abstract
The coronal heating region is able to generate mirror mode structures by ion mirror instabilities. Linear magnetic holes are believed to be the remnants of mirror mode structures, thus they ...are believed to be messengers from the coronal heating region. They can be convected to ∼9 au with the solar wind flow, indicating that a stabilizing mechanism is necessary to make the magnetic holes survive for such a long time. Here, we investigate a magnetic hole with a size of ∼6.7
ρ
i
in the solar wind based on observations by the Magnetospheric Multiscale mission. The unprecedented high-resolution data enable us to reveal the existence of the ion vortex inside the structure for the first time. Such an ion vortex forms a ring-like current, which is consistent with the magnetic field depression. The self-consistent structure of the magnetic hole contributed by the ion vortex can help to further shed light on the mechanism of the long-term survival of magnetic holes in the astrophysical plasma.
Electron vortices are a key element in the sub‐ion magnetic hole. Here, we investigate the electron velocity inside two sub‐ion magnetic holes in the solar wind based on the Magnetospheric Multiscale ...(MMS) mission. We find that the observational electron velocities inside the magnetic holes are contributed by the combination of the electron diamagnetic (Ve,dia), E × B (Ve,E), magnetic gradient (Ve,▽B) and curvature (Ve,R) drifts. Ve,dia, Ve,▽B, and Ve,R are comparable, while Ve,E is very small inside the hole. The weak Ve,E could result from the electric field approximately perpendicular to the magnetic field inside the structure. The value of Ve,▽B + Ve,R is near 0; thus, Ve,dia is approximately equal to the observational electron velocity. The current density contributed by the electron diamagnetic, magnetic gradient and curvature drift motions is self‐consistent with the magnetic depression inside the hole, suggesting that these three electron drift motions play a crucial role in stabilizing the magnetic hole in the mirror‐stable astrophysical plasma environment.
Key Points
Electron vortices exist in the sub‐ion‐scale magnetic holes in the solar wind
The electron vortex is mainly contributed by the combination of the electron diamagnetic, magnetic gradient, and curvature drifts
The magnetic gradient and curvature drifts almost cancel each other out; thus, the electron vortex can be explained by the diamagnetic drift
We here present a new type of kinetic-scale (∼1 ion gyroradius) flux rope (KFR) in the Earth's dayside magnetosheath boundary layer with Magnetospheric Multiscale high temporal cadence data. This ...structure exhibits a slight twist of magnetic field that is possibly generated by a field-aligned current, which differs from typical dayside flux ropes usually observed within the current sheet where magnetic reconnection can occur. The perpendicular electron fluxes within 19-52 eV are increased ∼10% inside the KFR. Detailed analysis shows that these perpendicular electrons may encounter their mirror point (at the position of the KFR, strong field region) when traveling from the magnetosheath toward the ionosphere and will be reflected to the magnetosheath. A possible scenario is that this KFR is different from previous flux ropes that transfer electron flux to the magnetosphere but could intercept magnetosheath large pitch angle electron flux to the magnetosphere.
An understanding of the transport of solar wind plasma into and throughout the terrestrial magnetosphere is crucial to space science and space weather. For non-active periods, there is little ...agreement on where and how plasma entry into the magnetosphere might occur. Moreover, behaviour in the high-latitude region behind the magnetospheric cusps, for example, the lobes, is poorly understood, partly because of lack of coverage by previous space missions. Here, using Cluster multi-spacecraft data, we report an unexpected discovery of regions of solar wind entry into the Earth's high-latitude magnetosphere tailward of the cusps. From statistical observational facts and simulation analysis we suggest that these regions are most likely produced by magnetic reconnection at the high-latitude magnetopause, although other processes, such as impulsive penetration, may not be ruled out entirely. We find that the degree of entry can be significant for solar wind transport into the magnetosphere during such quiet times.
We investigate the characteristics of the magnetic flux rope in the Venusian ionosphere near the terminator observed by the Venus Express while previous works about flux rope by PVO are mainly in the ...subsolar region. The probability to observe the flux rope becomes larger as solar activity strengthens. A statistical work during solar maximum presents the following characteristics. (1) The flux ropes in the terminator region have a lower spatial occurrence compared with those in the subsolar region, and the spatial occurrence of the flux ropes is also getting smaller when altitude increases. (2) The scale size of the flux rope is larger in the terminator region than that in the subsolar region and becomes larger when altitude increases. (3) In the terminator region, the flux rope appears to have a quasi‐horizontal orientation but with some cases can be vertical at low altitude. (4) The flux ropes in high solar zenith angle regions are confirmed to have a lower helicity compared with those in low solar zenith angle regions.
Key Points
An overall statistic work about characteristics of the flux rope at the terminator
We find a less occurrence and a larger‐scale size of flux ropes in terminator region
The flux rope's orientation can be vertical at low altitude, and flux ropes in terminator region are found less twisted
Abstract
One Martian induced magnetosphere boundary (IMB) crossing at the subsolar region is analyzed here with multiple instruments on board MAVEN. Properties of the magnetic field and particles ...around the IMB are evaluated. We find different trends of variation in magnetic field components at the two sides of an interface coincident with the previously defined ion composition boundary. This case shows the IMB at the Martian dayside could be divided into three parts: two regions (denoted as R1 and R2), with different field and plasma properties, and an interface between them. Currents found in R1 and R2 are flowing in antiparallel, and the current density in R2 (at lower altitude) is significantly larger than that in R1 (at higher altitude). Results indicate the interaction between Mars and the solar wind could induce strong currents in the IMB, which are with antiparallel current directions and separated by an interface where the ion composition changes. This could be a typical feature that occurred during the interaction between the solar wind and the nonmagnetized planets.
We study the evolution of the temporal properties of MAXI J1820+070 during the 2018 outburst in its hard state from MJD 58,190 to 58,289 with Insight-HXMT in a broad energy band 1-150 keV. We find ...different behaviors of the hardness ratio, the fractional rms and time lag before and after MJD 58,257, suggesting a transition occurred around this point. The observed time lags between the soft photons in the 1-5 keV band and the hard photons in higher energy bands, up to 150 keV, are frequency-dependent: the time lags in the low-frequency range, 2-10 mHz, are both soft and hard lags with a timescale of dozens of seconds but without a clear trend along the outburst; the time lags in the high-frequency range, 1-10 Hz, are only hard lags with a timescale of tens of milliseconds; they first increase until around MJD 58,257 and decrease after this date. The high-frequency time lags are significantly correlated to the photon index derived from the fit to the quasi-simultaneous NICER spectrum in the 1-10 keV band. This result is qualitatively consistent with a model in which the high-frequency time lags are produced by Comptonization in a jet.
Abstract
Electron-scale magnetic peaks (ESMPs) with spatial sizes less than one local ion gyroradius have been recently revealed to exist in the terrestrial magnetosheath and solar wind at 1 au. ...Whether they widely exist in the astrophysical plasma is unclear. Here, we investigate the magnetic peaks with a period of 0.1–100 s upstream of Mercury’s bow shock by using the magnetic field data from the MESSENGER spacecraft. Based on the distribution of their durations, these magnetic peaks can be divided into two groups: one with durations less than 0.6 s and the other with durations larger than 0.6 s. The durations in each group obey a log-normal distribution. The magnetic peaks with durations less than 0.6 s are inferred to be electron scale, suggesting that ESMPs exist in the solar wind at Mercury’s orbit. The median duration of these ESMPs is ∼0.3 s. The ESMPs have a larger occurrence rate near the bow shock and prefer to occur when the ambient interplanetary magnetic field (IMF) can be connected to the bow shock, which suggests that the foreshock could be one source region of these ESMPs. Their occurrence rate also tends to be larger when the IMF strength is weaker. Our observations also suggest that some ESMPs originate from the upstream solar wind. The properties of the ESMPs found here could help to shed light on their generation mechanisms and their roles in the astrophysical plasma.
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