A fundamental question for the atmospheric evolution of Venus is how much water‐related material escapes from Venus to space. In this study, we calculate the nonthermal escape of H+ and O+ ions ...through the Venusian magnetotail and its dependence on the solar cycle. We separate 8 years of data obtained from the ion mass analyzer on Venus Express into solar minimum and maximum. The average escape of H+ decreased from 7.6 · 1024 (solar minimum) to 2.1 · 1024 s−1 (solar maximum), while a smaller decrease was found for O+: 2.9 · 1024 to 2.0 · 1024 s−1. As a result, the H+/O+ flux ratio decreases from 2.6 to 1.1. This implies that the escape of hydrogen and oxygen could have been below the stoichiometric ratio of water for Venus in its early history under the more active Sun.
Plain Language Summary
An open issue for the atmospheric evolution of Venus is how the presumably large water content was lost. In this study, we use 8 years of data collected by the ion mass analyzer instrument onboard Venus Express. We investigate the escape of hydrogen H+ and oxygen O+ ions, the components of water, from the Venusian atmosphere to space. If water is escaping from Venus entirely as ions, the hydrogen‐to‐oxygen ratio should be close to 2. We find that the ratio of H+/O+ ion escape is 2.6 for the solar minimum period, while the escape is close to 1‐to‐1 for solar maximum, when the Sun is more active. Thus, Venus is currently on average losing water as ions from its atmosphere. However, in the early history of the solar system, when the Sun was in an even more active state, the escape ratio was probably below the water ratio. This implies that the oxygen did not enrich the atmosphere and surface, and instead hydrogen remained in the Venusian system over its history.
Key Points
The escape rate of O+ is almost stable over the solar cycle, while H+ escape rate decreases by a factor of 3.6 from solar minimum to maximum
The H+/O+ escape rate ratio decreases from 2.6 at solar minimum to 1.1 at solar maximum
The historic escape rate has presumably been below the stoichiometric ratio of water
Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from XGSM = −30 RE to XGSM = −7 RE. How these DFs are formed is still poorly understood. Three possible mechanisms have ...been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near‐Earth region (XGSM > −10 RE) are primarily attributed to jet braking, while the DFs detected in the mid‐ or far‐tail region (XGSM < −15 RE) are primarily attributed to transient reconnection or spontaneous formation. In the jet‐braking mechanism, the high‐speed flow “pushes” the preexisting plasmas to produce the DF so that there is causality between high‐speed flow and DF. In the transient‐reconnection mechanism, there is no causality between high‐speed flow and DF, because the frozen‐in condition is violated.
Key Points
DFs are observed inside reconnection diffusion region
Three formation mechanisms of DF are compared
Causality between flow and DF is discussed
We investigate the lagged correlation between a selection of geomagnetic indices and solar wind parameters for a complete solar cycle, from 2000 to 2011. We first discuss the mathematical assumptions ...required for such a correlation analysis. The solar wind parameters and geomagnetic indices have inherent timescales that smooth the variations of the correlation coefficients with time lag. Furthermore, the solar wind structure associated with corotating interaction regions and coronal mass ejections, and the compression regions ahead of them, strongly impacts the lagged correlation analysis results. This work shows that such bias must be taken into account in a correct interpretation of correlations. We then evidence that the magnetospheric response time to solar wind parameters involves multiple timescales. The simultaneous and quick response of the PC and AE indices to solar wind dynamic pressure with a delay of ~5 min suggests that magnetospheric compression by solar wind can trigger substorm activity. We find that the PC and AE indices respond to interplanetary magnetic field (IMF) BZ with a response time of respectively ~20 and ~35 min. The response of the SYM‐H index takes longer (~80 min) and is less sharp, SYM‐H being statistically significantly correlated to the IMF BZ observed up to more than ~10 h before. Our results suggest that the solar wind velocity's dominant impact on geomagnetic activity is caused by the compression regions at the interface of fast/slow solar wind regimes, which are very geo‐effective as they are associated with high solar wind pressure and strong interplanetary magnetic field.
Key Points
Data from one solar wind cycle are used to study the lagged (Pearson) correlation between solar wind parameters and geomagnetic indices
The impact of solar wind structuring and magnetospheric inherent response time on the result is analyzed
The solar wind structuring at the interface between fast/slow solar wind has strong impact on the results of the lagged correlation analysis
The generation of kinetic‐scale flux ropes (KSFRs) is closely related to magnetic reconnection. Both flux ropes and reconnection sites are detected in the magnetosheath and can impact the dynamics ...upstream of the magnetopause. In this study, using the Magnetospheric Multiscale satellite, 12,623 KSFRs with a scale <20 RCi are statistically studied in the Earth's dayside magnetosheath. It is found that they are mostly generated near the bow shock (BS), and propagate downstream in the magnetosheath. Their quantity significantly increases as the scale decreases, consistent with a flux rope coalescence model. Moreover, the solar wind parameters can control the occurrence rate of KSFRs. They are more easily generated at high Mach number, large proton density, and weak magnetic field strength of the solar wind, similar to the conditions that favor BS reconnection. Our study shows a close connection between KSFR generation and BS reconnection.
Plain Language Summary
Kinetic‐scale flux ropes (KSFRs) exist widely in near‐earth space and play an important role in mass transport, energy conversion, and dissipation during magnetic field reconnection. The KSFR in the magnetosheath can be generated by reconnection in three regions: the magnetopause, the magnetosheath, and the BS. The spatial distribution of KSFRs can indirectly reflect the reconnection situation in the magnetosheath. We use various methods to select the KSFRs and study their spatial distribution and generation in the magnetosheath. Our results show that BS reconnection plays an important role in generating the KSFR in the magnetosheath.
Key Points
Kinetic‐scale flux ropes observed in the magnetosheath are primarily generated near the bow shock (BS) and travel to downstream magnetosheath
The quantity of flux ropes significantly increases as their scale decreases, which is in accordance with the FR coalescence model
The occurrence of flux ropes is influenced by solar wind parameters, and could strongly correlate with BS reconnection
Plasma structures with enhanced dynamic pressure, density, or speed are often observed in Earth's magnetosheath. We present a statistical study of these structures, known as jets and fast plasmoids, ...in the magnetosheath, downstream of both the quasi‐perpendicular and quasi‐parallel bow shocks. Using measurements from the four Magnetospheric Multiscale (MMS) spacecraft and OMNI solar wind data from 2015–2017, we present observations of jets during different upstream conditions and in the wide range of distances from the bow shock. Jets observed downstream of the quasi‐parallel bow shock are seen to propagate deeper and faster into the magnetosheath and on toward the magnetopause. We estimate the shape of the structures by treating the leading edge as a shock surface, and the result is that the jets are elongated in the direction of propagation but also that they expand more quickly in the perpendicular direction as they propagate through the magnetosheath.
Plain Language Summary
The solar wind is a stream of charged particles continuously emitted from the upper atmosphere of the Sun. When it approaches Earth, it is slowed down and creates the bow shock. The region with high temperature and lower speed, downstream of the bow shock is called the magnetosheath. From time to time, plasma jets with speeds close to the solar wind speed are observed in this magnetosheath. They are thought to be formed at the bow shock, which is the boundary between the magnetosheath and the solar wind. In this article, we use data obtained by the four MMS spacecraft, while they passed through the magnetosheath, in a statistical study of the properties of the jets. We have found that they slow down as they move through the magnetosheath and that, in the beginning, they are elongated in the direction of their motion, but also that they expand to become rounder as they move along.
Key Points
The jets grow larger and slower as they move away from the bow shock
The deceleration of jets and fast plasmoids in the quasi‐perpendicular magnetosheath is twice as fast as in the quasi‐parallel magnetosheath
Jets propagate deeper into the magnetosheath for smaller angles between the interplanetary magnetic field and the bow shock normal
Previous studies have shown that the average dawn‐dusk component of the perpendicular plasma flow in the plasma sheet (V⊥) can vary depending on the distance relative to the neutral sheet and the ...dawn‐dusk component of the interplanetary magnetic field (IMF By). In this study, we combined 33 years of data from the Geotail, Time History of Events and Macroscale Interactions during Substorms, Cluster, and magnetospheric multiscale missions to study the slow (<200 km/s) ion flows perpendicular to the magnetic field. We find that IMF By has a hemispheric dependent influence on both the tail By and tail V⊥. Particularly, the influence is more prominent in the midnight sector (compared to both the pre‐ and post‐midnight sectors) and at distances far from the neutral sheet (compared to the distances close to the neutral sheet). However, at distances close to the neutral sheet, there is an increased dominance of duskward flows which dominates over the systematic influence of IMF By on tail V⊥. Our results indicate that IMF By has a major influence on the magnetic flux transport in the magnetotail, mainly at distances far from the neutral sheet. The influence is weaker at distances close to the neutral sheet.
Plain Language Summary
The dawn‐dusk component of the interplanetary magnetic field (IMF By) can “penetrate and induce” the magnetic field (of same direction) in the magnetotail's plasma sheet. Consequently, IMF By can cause a dawn‐dusk asymmetry in the plasma sheet flow. Recently, it has been found that the dawn‐dusk asymmetry of the plasma sheet flow can also depend on the distance with respect to the neutral sheet. These two factors present a competing effect on the dawn‐dusk asymmetry of the plasma sheet flows. In this study, we will analyze these two competing factors (i.e., IMF By and distance to the neutral sheet) on the plasma sheet flows. We find that IMF By only has a hemispheric dependent influence on tail V⊥ at distances far from the neutral sheet. At distances close to the neutral sheet, duskward flows become more dominant and IMF By does not have a clear influence on tail V⊥. Our results indicate that IMF By has a major influence on the direction of the magnetic flux transport in the magnetotail, only at distances far from the neutral sheet.
Key Points
IMF By has a systematic interhemispheric influence on the By and V⊥y in the magnetotail's plasma sheet
The influences are more prominent in the midnight sectors and at distances far from the neutral sheet
At distances close to the neutral sheet, duskward diamagnetic drift dominates over the influence of IMF By on tail V⊥y
Magnetosheath jets are localized dynamic pressure enhancements in the magnetosheath. We make use of the high time resolution burst mode data of the Magnetospheric Multiscale mission for an analysis ...of waves in plasmas associated with three magnetosheath jets. We find both electromagnetic and electrostatic waves over the frequency range from 0 to 4 kHz that can be probed by the instruments on board the MMS spacecraft. At high frequencies we find electrostatic solitary waves, electron acoustic waves, and whistler waves. Electron acoustic waves and whistler waves show the typical properties expected from theory assuming approximations of a homogeneous plasma and linearity. In addition, 0.2 Hz waves in the magnetic field, 1 Hz electromagnetic waves, and lower hybrid waves are observed. For these waves the approximation of a homogeneous plasma does not hold anymore and the observed waves show properties from several different basic wave modes. In addition, we investigate how the various types of waves are generated. We show evidence that, the 1 Hz waves are connected to gradients in the density and magnetic field. The whistler waves are generated by a butterfly‐shaped pitch‐angle distribution and the electron acoustic waves by a cold electron population. The lower hybrid waves are probably generated by currents at the boundary of the jets. As for the other waves we can only speculate about the generation mechanism due to limitations of the instruments. Studying waves in jets will help to address the microphysics in jets which can help to understand the evolution of jets better.
Plain Language Summary
There is a constant plasma flow from the sun, the solar wind. The Earth's magnetic field deflects the solar wind as it flows toward Earth. As the solar wind plasma approaches Earth it gets decelerated and heated at the bow shock. Earthward of the bow shock, the magnetosheath is located where the flow diverges around Earth. In the magnetosheath plasma flows that are denser and faster than normal can sometimes be observed, so called magnetosheath jets. We investigate waves in plasmas in these magnetosheath jets and how they are generated. Studying these waves will help to understand the interaction of magnetosheath jets with their environment.
Key Points
MMS burst mode data is used to investigate waves in, and in the vicinity of, magnetosheath jets
0.2 Hz waves, 1 Hz waves, whistler waves, electron acoustic waves, lower hybrid waves, and solitary waves are observed
Waves with low frequencies cannot be explained by “basic” wave modes that are derived in homogeneous plasmas
We utilized 33 years of data obtained by the Geotail, THEMIS, Cluster and MMS missions to investigate the slow (<200 km/s) ion flows perpendicular to the magnetic field in Earth's magnetotail plasma ...sheet. By using plasma β as a proxy of distance to the neutral sheet, we find that the ion flow patterns vary systematically within the plasma sheet. Particularly, in regions farther from the neutral sheet, earthward (tailward) flows exhibit a strong tendency to diverge (converge) quasi‐symmetrically, with respect to the midnight meridional plane. As the distance becomes closer toward the neutral sheet, this tendency to diverge and converge gradually weakens. Moreover, duskward flows become the dominant components in both the earthward and tailward flows. These variations in ion flow patterns with distance to neutral sheet are hemispherically independent. We suggest that the spatial profiles of the electric and diamagnetic drift vary with distance to the neutral sheet and are therefore responsible for the varying ion flow patterns.
Plain Language Summary
The plasma sheet can be visualized as “layers” stacking on top of each other where the middle layer represents the neutral sheet. In this study, we investigate the plasma convection patterns in these various plasma sheet “layers” in terms of their location relative to the neutral sheet. Our study finds that the plasma convection patterns in the plasma sheet are different depending on their distance from the neutral sheet. Farther from the neutral sheet, earthward and tailward flows converge and diverge quasi‐symmetrically, respectively, with respect to the midnight plane. As distance becomes closer to the neutral sheet, the degree of convergence and divergence weakens. At distance closest to the neutral sheet, duskward flows become the dominant components. We suggest that the spatial profiles of the electric and diamagnetic drift vary with distance to the neutral sheet and are therefore responsible for the varying ion flow patterns.
Key Points
Slow plasma sheet ion flows (<200 km/s) perpendicular to the magnetic field vary systematically with distance to the neutral sheet
Farther from the neutral sheet, earthward (tailward) flows exhibit stronger flow divergence (convergence) with the midnight meridional plane
Closer to the neutral sheet, the diverging/converging tendency weakens, and the flows are dominated by duskward flow components
Ion Dynamics at the Magnetopause of Ganymede Fatemi, S.; Poppe, A. R.; Vorburger, A. ...
Journal of geophysical research. Space physics,
January 2022, 2022-01-00, 20220101, 2022, Letnik:
127, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We study the dynamics of the thermal O+ and H+ ions at Ganymede's magnetopause when Ganymede is inside and outside of the Jovian plasma sheet using a three‐dimensional hybrid model of plasma (kinetic ...ions, fluid electrons). We present the global structure of the electric fields and power density (E ⋅ J) in the magnetosphere of Ganymede and show that the power density at the magnetopause is mainly positive and on average is +0.95 and +0.75 nW/m3 when Ganymede is inside and outside the Jovian plasma sheet, respectively, but locally it reaches over +20 nW/m3. Our kinetic simulations show that ion velocity distributions at the vicinity of the upstream magnetopause of Ganymede are highly non‐Maxwellian. We investigate the energization of the ions interacting with the magnetopause and find that the energy of those particles on average increases by a factor of 8 and 30 for the O+ and H+ ions, respectively. The energy of these ions is mostly within 1–100 keV for both species after interaction with the magnetopause, but a few percentages reach to 0.1–1 MeV. Our kinetic simulations show that a small fraction (< $< $25%) of the corotating Jovian plasma reach the magnetopause, but among those >50% cross the high‐power density regions at the magnetopause and gain energy. Finally, we compare our simulation results with Galileo observations of Ganymede's magnetopause crossings (i.e., G8 and G28 flybys). There is an excellent agreement between our simulations and observations, particularly our simulations fully capture the size and structure of the magnetosphere.
Key Points
The Hall electric field dominates at the magnetopause and in the magnetotail of Ganymede, accelerating Jovian magnetospheric ions
Magnetopause interaction increases the energy of O+ and H+ ions by a factor of 8 and 30, respectively
The incident Jovian plasma upstream of Ganymede's magnetopause is highly non‐Maxwellian and accelerated up to 1 MeV at the magnetopause
The occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these ...BBFs (or TWABs – Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ∼75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical‐like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the “freshly” rebounded BBFs from the near‐Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear‐induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs.
Plain Language Summary
In Earth's magnetotail, the plasma sheet is generally populated with slow earthward moving plasma. This plasma sheet is often perturbed by short bursts of earthward high speed flows, commonly known as the bursty bulk flows (BBFs). These BBFs are closely associated with the formation mechanisms of tailward flows in the magnetotail. The dynamic interaction between these tailward flows and BBFs can affect the energy, mass and momentum transport in the coupled magnetosphere‐ionosphere system. In this work, we conduct a statistical study on the tailward flows with a particular focus on those observed in the vicinity of BBFs (or TWABs, tailward flows around BBFs in short). We find that TWABs are common. They have a flow convection pattern consistent with, but a velocity magnitude twice larger than the general tailward flows in the plasma sheet. We also find that the plasma density and temperature of TWABs are comparable with BBFs. Based on their characteristics, we suggest that TWABs primarily result from the rebound of earthward flows in the near‐Earth region, where the magnetic field is stronger.
Key Points
Tailward flows around BBFs (TWABs) can frequently be observed. They have high |V| and plasma properties similar to bursty bulk flows (BBFs) compared to the general tailward flows
It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference between them
TWABs are likely the “freshly” rebounded BBFs. They may represent the early stage of the evolution of tailward flows in the plasma sheet
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