Dipolarization front (DF) is an important region involved with energy conversion and flux transport in the planet's magnetotail. Using observation data from the recent Magnetospheric Multiscale (MMS) ...mission, we have identified and analyzed 215 DF events. Significant magnetic energy conversion and transport are observed at the DFs, which show strong dawn‐dusk asymmetry. We find that the transport process dominates the change of local magnetic energy. Nearly 70% of the events show net Poynting flux flowing into the DF region. Such effect is more significant than local magnetic energy dissipation, averagely. When DF approaches the near‐Earth region, the net flow‐in Poynting flux decreases and magnetic energy dissipation increases. Our results suggest that the downstream magnetic energies of transient magnetic reconnections in the midtail may be transported to the near‐Earth region by one DF event after another. These intensive strikes to the magnetosphere in the near‐planet region may drive stronger storms, compared with the previous knowledge about DF.
Plain Language Summary
Dipolarization front (DF), a sharp boundary with a scale of ion inertial length, is frequently observed at the nightside of the terrestrial magnetosphere. It is widely believed that DF plays a key role in magnetic flux transport and energy conversion in the magnetotail. Reliable data from Magnetospheric Multiscale (MMS) give us an opportunity to investigate the magnetic energy conversion and transport associated with DF. We have statistically studied 215 DF events and found that the magnetic dissipation increases when DF moves toward the near‐Earth region. It is interesting that the change of local magnetic energy mainly results from the net inflow Poynting flux, which is much larger than the magnetic energy dissipation in the midtail. These results suggest that DF has a long lifetime during the transport in the magnetotail and might produce a more intense impact on the inner magnetosphere and ionosphere.
Key Points
Conversion and transport of magnetic energy at the DF in the magnetotail plasma sheet have been statistically investigated
Poynting flux divergence term dominates the change of local magnetic energy in DF region
Energy conversion is more significant in the near‐Earth region, and ion is the major contributor to the dissipation
The current sheet is crucial in releasing magnetic free energy in cosmic plasmas via fast magnetic reconnection or wave excitation. This research investigates the characteristics of the Martian tail ...current sheet by analyzing data acquired from the Mars Atmosphere and Volatile EvolutioN spacecraft over 7 years. For the first time, we find that approximately 15% of the current sheet events display reconnection signatures, including Hall magnetic fields and fast proton flows. These reconnecting current sheets are detected more frequently on Mars than on Earth. The Martian tail current sheet is first demonstrated to be on the proton scale, which can explain the high reconnection occurrence rate. The research also reveals that the current sheets are thinner in regions closer to Mars and in the –E hemisphere. On average, reconnecting current sheets carry high fluxes of protons rather than oxygen ions.
Plain Language Summary
As a ubiquitous plasma configuration in various cosmic plasma environments, the current sheet is critical in facilitating the release of magnetic energy through explosive processes, such as fast magnetic reconnection. On Earth, magnetic reconnection in the tail current sheet can be responsible for triggering substorms and generating auroras. Recent observations have indicated that reconnection in the Martian tail enhances ion escape from the planet’s atmosphere. However, the frequency of reconnection events in the Martian tail and their involvement in ion loss is still not fully understood based on previous few research. In this study, we use data from the Mars Atmosphere and Volatile EvolutioN spacecraft to study the Martian tail current sheet. For the first time, we find that almost 15% of the current sheet events on Mars have reconnection signatures, happening more often than on Earth. The high occurrence rate of magnetic reconnection at the Martian tail current sheet can be attributed to its extremely thin structure, which is on the scale of protons. Magnetic reconnection may drive high fluxes of hydrogen ions and thus potentially impact the evolution of the Martian atmosphere.
Key Points
MAVEN recorded 880 current sheet events in the Martian tail in the past 7 years and about 15% of them show reconnection features
The average thickness of the current sheet is on the proton scale and is thinner in the −E hemisphere
The reconnecting tail current sheet carries a higher net tailward flux of hydrogen ions
It has been long theorized, but not directly observed, that low-frequency magnetosonic plasma waves can steepen and form shocks. We show an example of small-amplitude, sinusoidal magnetosonic waves ...at the proton gyrofrequency upstream of the Martian bow shock. We hypothesize that these waves are produced by an ion beam instability associated with the ionization of hydrogen atoms by charge exchange with solar wind protons, solar photoionization, and/or electron impact ionization. As the waves are convected toward the planet by solar wind flow, the wave amplitude grows due to additional free energy put into the system by further ion beam particles. Finally, the steepened waves form shocks. Because of their development, the shocks are periodic with the separation at the proton gyroperiod. These observations lead to the conclusion that newborn ions may play a crucial role in the formation process of some collisionless plasma shocks in astrophysical and space plasmas.
Abstract
Magnetic field signatures over impact craters provide constraints for the history of the Martian dynamo. Due to limitations of the spatial resolution of magnetic field models, previous ...studies primarily focused on large impact craters (mostly ≥ 500 km in diameter). To fill the impact crater age gaps of previous studies, we investigate the magnetic field signature of 23 intermediate-sized craters (150–500 km in diameter) on Mars using both MAVEN data and a magnetic field model. Ten impact craters located in the South Province, the unmagnetized primordial crust, exhibit no or weak magnetic field signatures. The other 13 impact craters produce stronger magnetic anomalies, with the ratio of the averaged magnetic field inside and outside the craters (
B
in
/
B
out
) ranging from 0.4 to 1.2. The
B
in
/
B
out
values exhibit correlation coefficients of −0.54, −0.57, and −0.69 with the diameters of craters, calculated from the MAVEN data, the crustal field model at the surface, and 150 km altitude, respectively. A
B
in
/
B
out
larger than 1.0 usually appears in craters with smaller diameters, which is also demonstrated by the forward modeling in this study. Furthermore, the results of the forward modeling indicate that the craters of stronger magnetizations show a larger
B
in
/
B
out
. According to this, the Martian dynamo can be associated with the magnetization of craters of different ages, and the characteristic time of the dynamo can be limited. Our study supports the hypothesis that the Martian dynamo weakened or ceased at ∼4.0 Ga and a late dynamo was perhaps active at ∼3.7 Ga.
Abstract
Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct ...evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile EvolutioN (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere.
Abstract
Kelvin–Helmholtz (K-H) instability is a fundamental boundary instability between two fluids with different speeds, exchanging the mass, momentum, and energy across the boundary. Although the ...K-H instability has been suggested to play a critical role in atmospheric ion loss on Mars, the knowledge about its formation and evolution is still poor, due to the limitation of spacecraft missions and a dearth of dedicated simulation codes. In this study, we combine observations from the Mars Atmosphere and Volatile EvolutioN mission and global 3D kinetic simulations to investigate the solar wind–Mars interaction. For the first time, it is found that K-H waves prominently appear in the −E hemisphere, which is attributed to the stronger proton velocity shear therein associated with the asymmetric diamagnetic drift motion of protons. The K-H instability is mainly excited in the −E hemisphere and propagates downstream along the boundary, with the waves also able to be generated near the subsolar point. The K-H waves produce plasma clouds with a net oxygen ion escape rate of about 1.5 × 10
24
s
−1
, contributing to almost half of the global loss on present-day Mars. This heavy ion escape pattern associated with K-H instability is cyclic and could occur on other nonmagnetized planets, potentially influencing planetary atmosphere evolution and habitability.
It has been confirmed that dipolarization fronts (DFs) are the result of the interchange instability in the Earth’s magnetotail. In this paper, we use a Hall MHD model to simulate the evolution of ...the interchange instability that produces DFs along the leading edge. A test particle simulation is performed to study the physical phenomenon of ion acceleration at the DF. The numerical simulation indicates that almost all particles move earthward and dawnward and then drift to the tail. The DF-reflected ion population at the duskside appears earlier as a consequence of the asymmetric Hall electric field. Ions that are distributed in a dawn-dusk asymmetric semicircle behind the DF tend to be accelerated to higher energies (>13.5 keV). These high-energy particles eventually concentrate in the dawnside. Ions experience effective acceleration by the dawnward electric field, while they drift through the dawn flank at the front, toward the tail.
Abstract
We show observational evidence for a new form of collisionless shock in interplanetary space near Mars, small-scale shocks with periodic spacings. Pickup of new ionized hydrogen atoms in a ...magnetic field aligned with the solar wind direction causes the generation of a magnetosonic wave train through an ion beam instability. The waves have a frequency close to the local proton gyrofrequency. This is a similar physical process as for the formation of cometary plasma waves/turbulence. However, for the case of proton pickup near Mars, each individual magnetosonic wave cycle develops into a small-scale shock. So there is a string of fast mode shocks formed with proton gyroperiod spacings. These small-scale shocks display dissipation in the ions and dispersive whistlers. A fraction of ions trapped/reflected at the small-scale shocks are accelerated by the motional electric field. Observational results demonstrate that periodic shocks can perform the same functions as a single supercritical shock in a high-speed flow.
Abstract
In the Martian induced magnetosphere, the motion of planetary ions is significantly controlled by the ambient electric fields, which can be decomposed into three components: the motional, ...Hall, and ambipolar electric fields. Each of them is dominant in different regions and provides the ion acceleration with a particular effectiveness. Therefore, it is necessary to characterize the global distribution of these electric field components. In this study, a global multifluid Hall-MHD model is applied, which considers the motional, Hall, and ambipolar electric fields in ion transport and magnetic induction equations to self-consistently investigate the morphology of the electric fields in the Martian space environment. Numerical results suggest that the motional electric field is dominant in the upstream of the bow shock and in the magnetosheath along the
Z
MSE
direction, leading to the formation of the ion plume escape channel. At the bow shock, the ambipolar electric field points outward, to decelerate and deflect the solar wind plasma flow. In the magnetosheath region, the ambipolar and motional electric fields with inward direction tend to reaccelerate the solar wind ions. However, along the magnetic pileup boundary, the Hall electric field pointing outward prevents the solar wind ions from penetrating the Martian induced magnetosphere, which also prevails in the Martian magnetotail region, to accelerate the ions’ tailward escape. This is the first systematic investigation of the global distribution of electric fields, which is helpful to understand the processes of ion acceleration/deceleration and escape within the Mars–solar wind interaction.
The statistical properties of ULF waves observed upstream of Venus foreshock are investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. ...Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi‐monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2–3 times of the local proton cyclotron period. The transverse power dominates the power spectrum, and most of the waves display nearly circular or slightly elliptical polarization in the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B0 for parallel component (δB||/B0 less than 0.3), while the range of relative amplitudes for perpendicular component δB⊥/B0 is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshock form an important energy source for the generation of the waves.
Key Points
Properties of quasi‐monochromatic ULF waves in the Venusian foreshock are determined
The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock
All the wave properties are consistent with generation by foreshock backstreaming ions
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