Displays

Foreshock transients are frequently observed upstream from the bow shock (such as Hot Flow Anomalies, foreshock cavities, and foreshock bubbles). They play a significant role in the mass, energy and momentum transport from the solar wind into the magnetosphere and impact the whole magnetosphere-ionosphere system. This presentation will discuss the great progress made recently toward answering some specific outstanding science questions. Some outstanding questions are listed below. What are the physical differences and relationships between different transient phenomena at the bow shock? What are the formation conditions for the transient phenomena at the bow shock? How do the magnetosphere and ionosphere respond to transient phenomena generated at bow shock? How do transient phenomena at the bow shock evolve with time?

How to cite: Zhang, H.: Foreshock Transients and Their Geoeffects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4367, https://doi.org/10.5194/egusphere-egu2020-4367, 2020.

  Magnetic reconnection occurring during the development of a Hot flow anomaly (HFA) has been generated in hybrid simulation, but has never been observed by spacecraft. Using MMS we report an ion scale flux rope like structure, which is Earthward moving, embedded within the trailing edge of a hot flow anomaly (HFA) upstream from the quasi-parallel bow shock. The driver discontinuity of the HFA, a tangential discontinuity, is observed in the solar wind, but no flux rope signatures are observed around it. This suggests that the earthward moving flux rope was generated inside the HFA. This flux rope is close to a one-dimensional structure and expands due to a strong magnetic pressure gradient force. Solar wind ions are decelerated inside the flux rope by the static electric field likely caused by the charge separation of solar wind particles. Our observations imply that magnetic reconnection may have occurred inside the HFA. Reconnection and flux ropes may play a role in particle acceleration/heating inside foreshock transients.

How to cite: Bai, S.-C., Shi, Q., Liu, T., Zhang, H., Yue, C., Sun, W.-J., Tian, A., Degeling, A., Bortnik, J., Rae, J., and Wang, M.: Ion Scale Flux Rope Observed at the Trailing Edge of the Hot Flow Anomaly, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6535, https://doi.org/10.5194/egusphere-egu2020-6535, 2020.

The foreshock is a region of intense wave activity, situated upstream of the quasi-parallel sector of the terrestrial bow shock. The most common type of waves in the Earth's ion foreshock are quasi-monochromatic fast magnetosonic waves with a period of about 30 s. In this study, we investigate how the foreshock wave field is modified when magnetic clouds, a subset of coronal mass ejections driving the most intense geomagnetic storms, interact with near-Earth space. Using observations from the Cluster constellation, we find that the average period of the fast magnetosonic waves is significantly shorter than the typical 30 s during magnetic clouds, due to the high magnetic field strength inside those structures, consistent with previous works. We also show that the quasi-monochromatic waves are replaced by a superposition of waves at different frequencies. Numerical simulations performed with the hybrid-Vlasov model Vlasiator consistently show that an enhanced upstream magnetic field results in less monochromatic wave activity in the foreshock. The global view of the foreshock wave field provided by the simulation further reveals that the waves are significantly smaller during magnetic clouds, both in the direction parallel and perpendicular to the wave vector. We estimate the transverse extent of the waves using a multi-spacecraft analysis technique and find a good agreement between the numerical simulations and the spacecraft measurements. This suggests that the foreshock wave field is structured over smaller scales during magnetic clouds. These modifications of the foreshock wave properties are likely to affect the regions downstream - the bow shock, the magnetosheath and possibly the magnetosphere - as foreshock waves are advected earthward by the solar wind.

How to cite: Turc, L., Roberts, O., Archer, M., Palmroth, M., Battarbee, M., Brito, T., Ganse, U., Grandin, M., Pfau-Kempf, Y., Escoubet, P., and Dandouras, I.: Observations and simulations of foreshock waves during magnetic clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9300, https://doi.org/10.5194/egusphere-egu2020-9300, 2020.

The mirror modes are fundamental process in space, and play important roles in solar physics, planetary, interplanetary, astrophysical and laboratory plasmas over the past half century. Although theoretical studies and numerical simulations further revealed their kinetic effects, they are generally regarded as magnetohydrodynamics (MHD) scale process. However, if the electron distribution is anisotropic, the electrons could become unstable and excite kinetic scale mirror modes to remove the free energy. This instability was presented for more than thirty years, but so far few unambiguous observational evidence has been found. In this study, we provide high-resolution Magnetospheric Multiscale (MMS) observations of electron mirror mode structures. These structures: (1) are train-like features similar to the MHD-scale mirror-mode; (2) are anti-correlation between electron and magnetic pressure; (3) satisfy electron trapping conditions and theoretical excitation of the mirror modes; (4) are “frozen” in the plasma flow frame. They were observed during the Corotating Interaction Region events (CIRs) near the Earth’s foreshock and its downstream turbulence, and could involve with the interaction between Earth’s magnetosphere and solar wind.

How to cite: Yao, S., Shi, Q., Yao, Z., Guo, R., Zong, Q., Wang, X., Alex, D., Rae, J., Russell, C., Tian, A., Zhang, H., Hu, H., Liu, J., Liu, H., Li, B., and Giles, B.: The observational evidence of electron mirror mode, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4394, https://doi.org/10.5194/egusphere-egu2020-4394, 2020.

Particle reflection at the bow shock provides a source of free energy to drive local instabilities and turbulence within the foreshock. A variety of low-frequency fluctuations (up to 16 mHz) results from the interactions of back-streaming ions with the incoming solar wind flow. We report observations of low-frequency magnetosonic waves observed during intervals of a radial interplanetary magnetic field in the foreshock. A case study of simultaneous dual THEMIS spacecraft observations of asymmetrical fluctuations in V_y is complemented by a statistical study of similar solar wind deflections in the foreshock.  Our moment calculations do not include the reflected particles as well as heavier ions, revealed the modulation of a solar wind core and deflection of the solar wind in the foreshock. This effect decreases with the distance from the bow shock. We conclude that large asymmetrical Vy velocity component fluctuations are typical for the foreshock formed by the radial IMF. The asymmetry of fluctuations changes the mean direction of the incoming solar wind flow within the foreshock leading to preconditioning prior to its encounter with the bow shock.

How to cite: Gutynska, O., Urbář, J., Šafránková, J., and Němeček, Z.: ULF waves in the foreshock formed by the radial IMF: their effect on solar wind sheath-like deflection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3687, https://doi.org/10.5194/egusphere-egu2020-3687, 2020.

Foreshock bubbles (FBs) are kinetic transient phenomena formed due to the interaction between IMF discontinuities and backstreaming energetic ions in Earth’s foreshock region. FBs can be driven by both rotational discontinuities and tangential discontinuities and are typically observed under higher solar wind speed conditions. They play important roles in the solar wind-magnetosphere coupling because of very large dynamic pressure variations associated with them. The trailing edge of an FB is usually a fast shock which forms due to the expansion of the thermal plasma in the core. Using data from Magnetospheric Multiscale (MMS) mission, we investigate an FB structure with a particle foreshock region upstream its trailing edge. Distinct wave activity is observed in the particle foreshock region and wave analysis shows that the waves with periods of a few seconds may be generated by shock-reflected ion instabilities. The ions reflected at FB shock are observed and the acceleration mechanism needs to be analyzed.

How to cite: Wang, M. and Shi, Q.: MMS observations of wave activity in the particle foreshock bubble foreshock region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10305, https://doi.org/10.5194/egusphere-egu2020-10305, 2020.

The Earth’s magnetosphere occasionally experiences sudden movements from localized sources. For example, the impact of the interplanetary shock on the magnetosphere starts from a localized region on the dayside magnetopause, where the perturbations rapidly propagate inside the magnetosphere as the pressure front moves farther away from the Sun. The impulses generated from these sources propagate through the inhomogeneous plasma and can be detected in many corners of the magnetosphere. These impulses often mark the beginning of large-scale reconfigurations in the magnetosphere and the ionosphere, such as magnetic/ionospheric storms and substorms. The propagation of these impulses, such as that through MHD waves, is fast but not instantaneous. The propagation paths in the highly inhomogeneous magnetosphere may not be straightforward. Nonetheless, past studies have demonstrated that the impulse propagation in the dayside magnetosphere can be characterized by the Tamao model.

In this study, we examine the signatures of sudden impulses in the data from a network of spacecraft in the magnetosphere, including THEMIS, Van Allen Probes, MMS, Geotail, and GOES. The ACE and Wind data are also used for solar wind conditions. Observations from Polar, FAST, GOES, Cluster, Swarm, IMP-8, and ground-based magnetometers are also examined whenever they are available. The observations of impulse propagation time will be compared against the modeled Tamao travel time to understand how much the two agree with each other and how the comparison varies with the properties of the solar wind discontinuity.

How to cite: Flores, E. and Chi, P.: Multi-point Observations of Sudden Impulses and Implications for Signal Travel Time in the Magnetosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12703, https://doi.org/10.5194/egusphere-egu2020-12703, 2020.

Both simulations and observations had shown that step function-like increase/decrease of solar wind dynamic pressure pulse would excite flow vortex pairs in the dawn and dusk high latitude ionosphere simultaneously. However, some plasma structures, hot flow anomaly, sheath jets etc. existing in the solar wind or magnetosheath are often accompanied with spike-like changes of the dynamic pressure. Whether they can drive the ionospheric vortices or not is still unclear. In this work we report a traveling convection vortex like (TCV-like) event that was induced by a positive-negative pulse pair of dynamic pressure(△p/p~1) accompanying a large scale (~9min) magnetic hole in the solar wind. It is found that following the magnetic hole, two traveling convection vortices first in anticlockwise then in clockwise rotation were detected by geomagnetic stations located along the 10:30MLT meridian. Meanwhile, another pair of ionospheric vortices azimuthally seen up to 3 MLT first in clockwise then in anticlockwise rotation were appeared in the afternoon sector (~14MLT) centered at ~75MLAT with a trend of poleward moving. The duskside vortices were also confirmed by SuperDARN radar data. The processes following magnetosphere struck by a positive-negative pulse pair were simulated and it found that two pairs of flow vortices in the dawn and dusk magnetosphere may provide the field-aligned currents(FACs) required for the flow/current vortices observed in ionosphere. This work provides a way to understand how the momentum and energy injects to the ionosphere under spike-like dynamic pressures imposing on the magnetosphere.

How to cite: Tian, A., Degeling, A., Shi, Q., and Xing, Z.: TCV-like event induced by positive-negative pulse pair of solar wind dynamic pressure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2816, https://doi.org/10.5194/egusphere-egu2020-2816, 2020.

Recent observations from the Van Allen Probes mission have established that Pc3-5 ultra-low-frequency (ULF) waves can energize ions and electrons via drift-resonance and drift-bounce resonance. The extent to which these waves contribute to the space weather of the belts is relatively poorly understood and requires sophisticated modelling and characterization of the dominant wave modes that arise in the development and recovery phase of geomagnetic storms. Despite more than four decades of observations and theoretical analysis of ULF waves, there is no framework for accurately assessing the global distribution of ULF waves and their influence on the ring current. 
In this presentation, we describe a new global model of ULF waves that incorporates non-dipolar geomagnetic fields. The model is constrained using the GCPM of cold plasma density model and a specification of the ionosphere using the IRI and MSIS models. An algorithm is applied to adjust the initial plasma state to a quasi-static equilibrium that is then driven by a global convection electric field and ULF wave source. For specific observations by the Van Allen Probes and ARASE mission, the effect of these ULF waves on radiation belt ions and electrons is evaluated utilizing test-particle methodology and Liouville's theorem, which enables the phase space density to be followed and compared one-for-one with the satellite observations.  

How to cite: Rankin, R. and Degeling, A.: ULF waves and their influence on radiation belt dynamics in Earth's magnetosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13703, https://doi.org/10.5194/egusphere-egu2020-13703, 2020.

How to cite: Ma, X., Zong, Q., Hao, Y., Claudepierre, S., and Liu, Y.: Electron drift echoes induced by negative solar wind dynamic pressure pulses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5698, https://doi.org/10.5194/egusphere-egu2020-5698, 2020.

Plasmaspheric hiss waves is important in the radiation belt. Previous papers have shown that considering the variability of wave parameters will improve the effectiveness of modeling wave-particle interactions in the Radiation Belt, but less is known about how rapidly (and by how much) wave characteristics vary. We use measurements from the Van Allen Probe mission to study the correlation and ratio of wave amplitudes over a range of frequencies covering the plasmaspheric hiss band as a function of separation and time delay between two satellites. A total of 1851 events with small separation (<1R_E) were found. The statistical results show that, as separation between spacecraft increase, the characteristics of hiss change in both correlation of the wavepacket and amplitude. Moreover, we find that the coherence between spacecraft is strong at low-L, and decreases strongly with increasing L. We investigate the coherence of plasmaspheric hiss on geomagnetic indices and solar wind driving. We discuss the ramifications of our results with relevance to understanding the global characteristics of plasmaspheric hiss waves.

How to cite: Zhang, S., Rae, J., Watt, C., Degeling, A., Tian, A., and Shi, Q.: Determining the global coherence of plasmaspheric hiss waves in the magnetosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18504, https://doi.org/10.5194/egusphere-egu2020-18504, 2020.

Magnetic cavities, also termed magnetic holes, dips or depression structures, have an observable magnetic field decrease in a short time span and have been widely observed in the solar wind plasmas, comet magnetospheres, terrestrial/planetary magnetosheaths, magnetospheric cusps and magnetotail plasmas since 1970s. In early observations, the structures were found in MHD scale, from tens to thousands of ρi (proton gyroradius) with corresponding temporal scales from seconds to tens of minutes. Later, kinetic scale magnetic cavities were detected in the earth’s magnetotail and magnetosheath, with size less than ρi and sometimes close to several ρe (electron gyroradius) and often associated with a significant electron vortex around the structure. Surprisingly, it has been found that such a small structure contains an abundance of phenomena, including different kinds of ion and electron distributions, electron or ion vortices, various types of waves, and even particle acceleration and declarations. In this presentation, we will show our recent observations of magnetic cavities from MHD scale to kinetic scale in the solar wind, magnetosheath, cusp and magnetotail. In the magnetosheath, downstream of the bow shock, the mirror mode instability can generate magnetic dip and peak trains. Using data from the new NASA satellite constellation MMS, we have found that electrons exhibit a new ‘donut’ shaped distribution function related to particle deceleration processes. Using boundary normal and velocity determination techniques, we found that MHD scale magnetic cavity structures can expand or shrink, and they can enter the cusp regions along with the entry plasmas. In the turbulent magnetosheath and quiet magnetotail, we have observed kinetic scale magnetic cavity structures with scales comparable or less than one ρi. An EMHD model and other theories will also be introduced and compared. We found that in the sheath the electron scale magnetic cavity has a circular cross section and it is a magnetic bottle in 3-D. We have also found that these structures shrink due to increases in the surrounding magnetic field, and this shrinkage of the small scale magnetic cavity can induce an electric field that accelerates the electrons to a significantly higher energy. Qualitatively distinct from other acceleration mechanisms, this process indicates a new type of non-adiabetic acceleration, and has been confirmed by the observed electron distribution function and test particle simulations. This discovery in space physics also has implications for understanding energy conversion in astrophysical plasmas, the origin of cosmic high-energy particles and plasma turbulence.

How to cite: Shi, Q. and al, E.: Recent observations of magnetic cavities: from MHD to kinetic scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6406, https://doi.org/10.5194/egusphere-egu2020-6406, 2020.

The subsolar magnetosheath is oftentimes permeated by jets. These are localized entities of enhanced dynamic pressure with respect to the ambient plasma. Magnetosheath jets are thought to arise from bow shock ripples and/or foreshock structures. They can easily propagate through the entire magnetosheath and impact on the magnetopause, where they can cause large amplitude indentations, launch magnetopause surface waves, or modulate magnetopause reconnection. The scale size distributions of magnetosheath jets observed by single spacecraft are relatively well modeled by exponential functions with characteristic scales of 0.71 Earth radii (RE) and 1.34 RE in the directions parallel and perpendicular to the jet propagation direction, respectively. However, these functions do not represent the actual, true jet scale size distributions, because of two reasons: (1) Spacecraft are much more likely to observe large scale jets rather than small scale jets. Hence, the observed scale size distributions are biased towards larger scales. (2) The distributions modeled by exponential functions highly overestimate observation probabilities of jets of smallest scales (on the order of 0.1 RE). We overcome both shortcomings by replacing the exponential functions by log-normal functions, which can be corrected for the bias. By re-analyzing THEMIS multi-spacecraft data, we obtain, for the first time, unbiased, i.e., actual jet scale size distributions. Our results reveal a large population of jets of smallest scales that have not been accounted for, so far. Consequently, we find median scale sizes of jets to be about an order of magnitude smaller than previously thought: 0.15 and 0.12 RE in the parallel and perpendicular directions, respectively.

How to cite: Plaschke, F. and Hietala, H.: On the scale sizes of magnetosheath jets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5026, https://doi.org/10.5194/egusphere-egu2020-5026, 2020.

The supersonic solar wind is decelerated and thermalized when it encounters the Earth's magnetosphere and cross the bow shock. Sometimes, however, due to discontinuities in the solar wind, bow shock ripples or ionised dust clouds carried by the solar wind, high speed jets (HSJs) are observed in the magnetosheath. These HSJs have typically a Vx component larger than 200 km s-1 and their dynamic pressure can be a few times the solar wind dynamic pressure. They are typically observed downstream from the quasi-parallel bow shock and have a typical size around one Earth radius (RE) in XGSE. We use a conjunction of Cluster and MMS, crossing simultaneously the magnetopause, to study the characteristics of these HSJs and their impact on the magnetopause. Over one hour-fifteen minutes interval in the magnetosheath, Cluster observed 21 HSJs. During the same period, MMS observed 12 HSJs and entered the magnetosphere several times. A jet was observed simultaneously by both MMS and Cluster and it is very likely that they were two distinct HSJs. This shows that HSJs are not localised into small regions but could span a region larger than 10 RE, especially when the quasi-parallel shock is covering the entire dayside magnetosphere under radial IMF. During this period, two and six magnetopause crossings were observed respectively on Cluster and MMS with a significant angle between the observation and the expected normal deduced from models. The angles observed range between from 11° up to 114°. One inbound magnetopause crossing observed by Cluster (magnetopause moving out at 142 km s-1) was observed simultaneous to an outbound magnetopause crossing observed by MMS (magnetopause moving in at -83 km s-1), showing that the magnetopause can have multiple local indentation places, most likely independent from each other. Under the continuous impacts of HSJs, the magnetopause is deformed significantly and can even move in opposite directions at different places. It can therefore not be considered as a smooth surface anymore but more as surface full of local indents. Four dust impacts were observed on MMS, although not at the time when HSJs are observed, showing that dust clouds would have been present during the observations. No dust cloud in the form of Interplanetary Field Enhancements was however observed in the solar wind which may exclude large clouds of dust as a cause of HSJs. Radial IMF and Alfvén Mach number above 10 would fulfill the criteria for the creation of bow shock ripples and the subsequent crossing of HSJs in the magnetosheath.

How to cite: Escoubet, C.-P. and the Cluster-MMS team: Magnetosheath high speed jets observed simultaneously by Cluster and MMS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5101, https://doi.org/10.5194/egusphere-egu2020-5101, 2020.

Upstream of Earth’s bow shock, the foreshock is filled with particles that have been reflected at the bow shock and are streaming away from it. Interaction of these particles with solar wind particles and discontinuities within this region can cause foreshock transients to form. Downstream of Earth’s bow shock, localized magnetosheath jets with high dynamic pressure are frequently observed. When such a fast magnetosheath jet compresses the ambient magnetosheath plasma, an earthward compressional bow wave/shock can form. Here we present that foreshock transients and magnetosheath jets can accelerate particles through shock drift acceleration, Fermi acceleration, and the betatron acceleration. Foreshock transients and magnetosheath jets therefore can increase the particle acceleration efficiency of the parent shock by providing additional acceleration. The shock environment relevant for particle acceleration is not just the shock itself, but also the nonlinear transient structures both upstream and downstream of it.

How to cite: Liu, T. Z., Angelopoulos, V., Hietala, H., Lu, S., and Turner, D.: Particle acceleration by transient structures around Earth’s bow shock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2508, https://doi.org/10.5194/egusphere-egu2020-2508, 2020.

Downstream of the Earth's quasi-parallel shock, transients with higher earthward velocities than the surrounding magnetosheath plasma are often observed. These transients have been named magnetosheath jets. Due to their high dynamic pressure, jets can cause multiple types of effects when colliding into the magnetopause. Recently, jets have been linked to triggering magnetopause reconnection in case studies by Hietala et al. (2018) and Nykyri et al. (2019). Jets have been proposed to affect magnetopause reconnection in multiple ways. Jets can compress the magnetopause and make it thin enough for reconnection to occur. Jets could also affect the magnetic shear either by indenting the magnetopause or via the magnetic field of the jets themselves. Here we want to study whether the magnetic field of jets can statistically affect magnetopause reconnection. In particular, we are interested in whether jets could enhance reconnection during more quiet northward IMF conditions.

We statistically study the magnetic field within jets in the subsolar magnetosheath using measurements from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and OMNI solar wind data from 2008–2011. We investigate jets next to the magnetopause and find that the magnetic field within jets is statistically different compared to the non-jet magnetosheath. Our results suggest that during southward IMF, the non-jet magnetosheath magnetic field itself has more variation than the jets. This suggests that jets should have no statistical, neither enhancing nor suppressing, effect on reconnection during southward IMF. However, during northward IMF, the magnetic field within jets is statistically favorable for enhancing magnetic reconnection at the subsolar magnetopause as around 70 % of these jets exhibit southward fields close to the magnetopause.

How to cite: Vuorinen, L., Hietala, H., and Plaschke, F.: Magnetic field within magnetosheath jets during northward and southward interplanetary magnetic field conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11030, https://doi.org/10.5194/egusphere-egu2020-11030, 2020.

Plasmoids, defined as plasma entities with a higher anti-sunward velocity component than the surrounding plasma, have been observed in the magnetosheath in recent years. Among other denominations, plasmoids are also called “magnetosheath jets” and can be classified by transient localized enhancements in dynamic pressure. Propagating through the magnetosheath, jets do not only affect the magnetopause and magnetosphere. Jets pushed slower ambient magnetosheath plasma out of their way. As a result, plasma moves around the jets, and it is slowed down or could even be pushed in the sunward direction. Consequently, jets may create anomalous flows and be a source of additional turbulence. Using the magnetosheath measurements by the Magnetospheric Multiscale (MMS) and THEMIS spacecraft, and comparing several criteria, we have identified several thousand events in the wide range of bow shock distances. Previous statistical studies have shown that jet occurrence is almost exclusively controlled by the angle between the IMF and the Earth–Sun line (cone angle), and jets are predominantly observed when this cone angle is small. However, high-speed jets downstream of the quasi-perpendicular bow shock are very common. Our statistical analysis shows differences of jets evolution in the quasi-parallel and quasi-perpendicular magnetosheath regions. We discuss their properties, nature and relation to anomalies regions in the magnetosheath.

How to cite: Goncharov, O., Gunell, H., Hamrin, M., Norenius, L., and Gutynska, O.: Modification of the magnetosheath due to the high-speed jets propagation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3335, https://doi.org/10.5194/egusphere-egu2020-3335, 2020.

Numerous simulation results have shown that the spatial profiles of a thermal pressure from the bow shock to the magnetopause are determined by the upstream interplanetary magnetic field (IMF) orientation. The southward IMF conditions lead to an increasing trend of thermal pressure with a maximum near the magnetopause. In contrast, the thermal pressure increases on the plasma entry to the  magnetosheath, but this trend turns to a decreasing one after passing a middle point between the bow shock and magnetopause during northward IMF intervals. In the present study, we show the observation results, both particular events and statistics, to check the variations of pressure profiles in the magnetosheath and their relation to upstream IMF conditions. THEMIS-C observartions during 2007–2009 are used. The profiles near the Sun–Earth line and the global pressure distribution in the dayside equatorial magnetosheath are shown.

How to cite: Pi, G., Němeček, Z., and Šafránková, J.: Variations of Pressure Profiles from the Bow Shock to Magnetopause, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2443, https://doi.org/10.5194/egusphere-egu2020-2443, 2020.

The magnetic-to-particle energy conversion is one of the most fundamental physics processes to laboratory, space and astrophysical contexts. Adiabatic acceleration processes in moderate varying environment merely play significant roles to generate devastating cosmic rays and spectacular aurorae, etc. More commonly, when the violent variation or strongly inhomogeneity in electromagnetic field distorts the trajectory of the particles, non-adiabatic acceleration processes function more transiently and drastically on particle energization trigger explosive phenomena like sudden solar flares. However, without high-resolution simultaneous measurements on plasma and field at previous space missions, the small/fast scale of the non-adiabatic processes make it difficult to be analyzed to reach a comprehensive understanding to most of the underlying non-adiabatic acceleration mechanisms in space and astrophysical contexts. Here, using MMS data with unprecedented high temporal resolutions, we report such finding of acceleration for electrons trapped in a kinetic-size magnetic holes which at the same time is the acceleration region, and demonstrate the validity of the acceleration process by numerical simulation, achieving the reproduction for the observation.

How to cite: Liu, J., Yao, S., Shi, Q., Wang, X., Zong, Q., Feng, Y., Liu, H., Guo, R., Yao, Z., Rae, J. I., Degeling, A. W., Tian, A., and Woodham, L.: Electron acceleration in small-size magnetic holes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2719, https://doi.org/10.5194/egusphere-egu2020-2719, 2020.

The magnetopause is usually at the point where the pressure of the magnetospheric magnetic field is balanced by a sum of the thermal plasma and magnetic pressures on the magnetosheath side. However, statistics from THEMIS magnetopause crossings have showed that about 2 % of them exhibit a larger magnetic field in the magnetosheath than in the magnetosphere in the subsolar region (Y_GSM < 5 R_E) and thus, the pressure from the magnetosheath side seems to be uncompensated. In our study, we compare parameters of those unusual crossings with the rest of our statistic in that region with motivation to highlight the possible sources and mechanisms of this apparent pressure imbalance, which can be caused either by specific upstream solar wind conditions or by the state of the magnetosphere. We also compare our THEMIS results with the sets of magnetopause crossings observed by other spacecraft (e.g., Cluster, MMS).

How to cite: Grygorov, K., Němeček, Z., Šafránková, J., and Šimůnek, J.: Subsolar magnetopause under an inverse gradient of the magnetic field: Statistical study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3683, https://doi.org/10.5194/egusphere-egu2020-3683, 2020.

Under steady-state conditions the magnetopause location is described as a pressure balance between internal magnetic pressures and the external dynamic pressure of the solar wind. The question is, does this approximation hold during more dynamic solar wind features?

Under more extreme solar wind driving, such as high solar wind pressures or strong southward-directed interplanetary magnetic fields, this boundary is significantly more compressed than in steady-state, playing a significant role in the depletion of magnetospheric plasma from the Van Allen Radiation Belts, via magnetopause shadowing. Large step-changes in solar wind conditions enable the real magnetopause to have a significant time-dependence which empirical models cannot capture.

We use a database of ~20,000 magnetopause crossings, to determine how the measured magnetopause differs from a statistical model, and under which conditions. We find that observed magnetopause is on average 6% closer to the radiation belts,  with a maximum of 42%, during periods of sudden dynamic pressure enhancement, such as during storm sudden commencement. Our results demonstrate that empirical magnetopause models such as the Shue et al. [1998] model should be used cautiously to interpret energetic electron losses by magnetopause shadowing. 

How to cite: Staples, F., Rae, J., Forsyth, C., Smith, A., Murphy, K., Raymer, K., Plaschke, F., Case, N., Rodger, C., Wild, J., Milan, S., and Imber, S.: Do Statistical models capture magnetopause dynamics during sudden magnetospheric compressions? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21977, https://doi.org/10.5194/egusphere-egu2020-21977, 2020.

The Earth’s magnetopause is highly variable in location and shape, and is modulated by solar wind conditions. On 8 March 2012, the ARTEMIS probes were located near the tail current sheet when an interplanetary shock arrived under northward interplanetary magnetic field (IMF) conditions, and recorded an abrupt tail compression at ~(-60, 0, -5) R_E in Geocentric Solar Ecliptic (GSE) coordinate in the deep magnetotail. Approximately 10 minutes later, the probes crossed the magnetopause many times within an hour after the oblique interplanetary shock passed by. The solar wind velocity vector downstream from the shock was not directed along the Sun-Earth line, but had a significant Y component. We propose that the compressed tail was pushed aside by the appreciable solar wind flow in the Y direction. Using a virtual spacecraft in a global magnetohydrodynamic (MHD) simulation, we reproduce the sequence of magnetopause crossings in the X-Y plane observed by ARTEMIS probes under oblique shock conditions, demonstrating that the compressed magnetopause is sharply deflected at lunar distances in response to the shock and solar wind V_Y effects. The results of the two different global MHD simulations show that the shocked magnetotail at lunar distances is mainly controlled by the solar wind direction with a timescale of about a quarter hour, which appears to be consistent with the windsock effect. The results also provide some references for investigating interactions between the solar wind/magnetosheath and lunar nearside surface during full moon time intervals, which should not happen in general.

How to cite: Shang, W., Tang, B., Shi, Q., Tian, A., Zhou, X., Yao, Z., Degeling, A. W., Rae, I. J., Fu, S., Lu, J., Pu, Z., Fazakerley, A. N., Dunlop, M. M., Facsko, G., Liu, J., and Wang, M.: Unusual location of the geotail magnetopause at lunar distance: ARTEMIS observation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1890, https://doi.org/10.5194/egusphere-egu2020-1890, 2020.

Although the responses of the transpolar arcs (TPAs) to the north-south or dawn-dusk interplanetary magnetic field (IMF) orientations are relatively well known, the effects of the Sun-Earth IMF component on the TPA formation are still poorly understood. On 29 October 2005, the IMF pointed nearly earthward over seven hours from 08:20 to 15:40 UT. In this time interval, the Defense Meteorological Satellite Program (DMSP) satellite and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite observed two clear TPA structures (one near the magnetic pole and the other near the dawnside auroral oval) in the northern hemisphere and one clear TPA structure in the dawnside southern hemisphere. Precipitating particle data reveal that the TPA in the southern hemisphere and that near the dawnside auroral oval in the northern hemisphere are associated with precipitating electrons and ions, but the TPA near the magnetic pole in the northern hemisphere is associated with electron-only precipitation. These observational results imply that the formation of TPAs is not limited to northward IMF conditions and that the TPAs could be located not only on open field lines connected to the northward draped IMFs over one hemisphere magnetopause, but also on closed field lines rooted on both hemispheres even under the radial IMF conditions.

How to cite: Park, J.-S., Shi, Q. Q., Nowada, M., Shue, J.-H., Kim, K.-H., Lee, D.-H., Zong, Q.-G., Degeling, A. W., Tian, A. M., Pitkänen, T., Zhang, Y., Rae, I. J., Bai, S., and Yao, S.: Transpolar arcs under a long-duration radial IMF interval: A case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4478, https://doi.org/10.5194/egusphere-egu2020-4478, 2020.

High-latitude dayside aurora (HiLDA) are large-scale discrete arcs or spot-like aurora poleward of the cusp, observed previously in the northern hemisphere by the Viking UV imager [Murphree et al., 1990] and by the IMAGE FUV [Frey et al., 2003]. The particular interest on HiLDA is to understand its formation related to the dayside reconnection and the resulted field-aligned currents (FACs) configuration in the polar cap (open field line region). In addition, the occurrence of HiLDA in the southern hemisphere is not well known.

In this study, we investigate the properties of HiLDA using DMSP/SSUSI images from the satellites F16, F17, F18, and F19. The combined data with auroral images from DMSP/SSUSI, ion drift velocity from SSIES, magnetic field perturbations from SSM, and energetic particle spectrum from SSJ make it possible to study the electrodynamics in the vicinity of the HiLDA and its connection the dayside cusp. HiLDA is formed due to monoenergetic electron precipitation (inverted-V structures) with the absence of ion precipitation. The field-aligned potential drop can be up to tens of keV. Applying the current-voltage relation, we suggest accelerated polar rain as the source of HiLDA, indirectly controlled by the solar wind/magnetosheath plasma population. The upward field-aligned current associated with the potential drop is a part of the cusp current system, produced by the dayside reconnection. Both lobe reconnection and reconnection on the duskside flanks play a role in the formation of HiLDA.

The occurrence of HiLDA is highly associated with the sunlit hemisphere and IMF By dominated conditions. Our results agree with previous observations, which show that HiLDA occurs during positive By dominated conditions in the northern summer hemisphere. We also confirmed that HiLDA occurs during negative By dominated conditions in the southern hemisphere. In addition, the fine structures of HiLDA are studied.

References

Murphree, J. S., Elphinstone, R. D., Hearn, D., and Cogger, L. L. ( 1990), Large‐scale high‐latitude dayside auroral emissions, J. Geophys. Res., 95( A3), 2345– 2354, doi:.

Frey, H. U., Immel, T. J., Lu, G., Bonnell, J., Fuselier, S. A., Mende, S. B., Hubert, B., Østgaard, N., and Le, G. ( 2003), Properties of localized, high latitude, dayside aurora, J. Geophys. Res., 108, 8008, doi:, A4.

How to cite: Cai, L., Kullen, A., Zhang, Y., Karlsson, T., and Vaivads, A.: DMSP/SSUSI observations of the high-latitude dayside aurora (HiLDA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12369, https://doi.org/10.5194/egusphere-egu2020-12369, 2020.