Foreshock Transients Research Articles

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

Magnetospheric Field‐Aligned Current Generation by Foreshock Transients: Contribution by Flow Vortices and Pressure Gradients

AbstractForeshock transients can result in significant dynamic pressure perturbations downstream, causing the magnetopause to move locally outward and inward. These near‐magnetopause phenomena in turn generate magnetospheric field‐aligned currents (FACs). FACs driven by solar wind impulses are commonly found to be due to flow vortices, but it remains unclear whether the FACs driven by those localized foreshock transients are contributed by flow vortices or pressure gradients. We report on a fortuitous conjunction between the Magnetospheric Multiscale (MMS) mission, which was observing a foreshock transient at the flank of the bow shock, and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, immediately downstream of MMS, which was observing magnetopause disturbances arising from that transient. Using observations from the three THEMIS spacecraft to calculate local current density perturbations within the outward motion region of the magnetosphere, we find that flow vortices play a dominant role in generating the current there; the contribution from pressure gradients is one order of magnitude smaller. Using a global hybrid simulation that reproduces the observed foreshock transient perturbations, we traced the simulated FACs generated by the transient's interaction with the magnetopause. We find that in the outward magnetopause motion region the simulated FACs are driven by flow vortices, in agreement with THEMIS observations. Deeper inside the magnetosphere, the faster convection of bipolar flow vortices than the local magnetospheric flow leads to reversal of the simulated FACs. Our results improve our understanding of how foreshock transients disturb and energize the magnetosphere‐ionosphere system.

MMS Observations of Lower Hybrid and Whistler Waves Inside a Foreshock Transient

AbstractWe report Magnetospheric Multiscale observations of lower hybrid and whistler waves at the compression region of a foreshock transient. Lower hybrid waves (LHWs) are observed at the boundaries of the compression region and excited by the instability associated with the density gradient. Electron fluxes parallel to the magnetic field exhibit oscillations at the frequency band close to the LHWs. Associated with LHWs, electron plateau‐like distributions in the parallel direction are observed during burst intervals. Evidence suggests that the LHWs likely contribute to parallel electron heating through Landau damping. Within the compression region, whistler waves are found driven by the electron perpendicular temperature anisotropy associated with the magnetic field pileup. These two types of plasma waves are intrinsically linked to the expansion of the transient. These results provide insights on the kinetic‐scale wave‐particle interactions associated with foreshock transients.

Magnetosheath Filamentary Structures Driven by Foreshock Energetic Ions

AbstractPrevious simulation studies have revealed the formation of magnetosheath filamentary structures by foreshock energetic ions. We report Cluster and Double Star observations of three such events triggered by a foreshock bubble, a hot flow anomaly and a traveling foreshock, respectively. In each case, in association with the onset of foreshock energetic ions, the magnetosheath energetic ion fluxes have quasi‐periodic modifications. Their azimuthal and pitch angular distributions indicate that these populations flow into the magnetosheath along the magnetic field lines. As a result, filamentary plasma structures form and display as a series of ion density and pressure oscillations anticorrelated to temperature oscillations. Moreover, our statistical analyses of their properties show that filamentary structures primarily form in the magnetosheath with the magnetic field lines connecting to the quasi‐parallel bow shock and under favorable formation conditions similar to those of foreshock transients. The distributions of the angles between the filamentary structure normals and the local magnetic field directions peak at 80°–90°. The transverse sizes of those filaments are in the range of 0.2–1.6 RE. Magnetosheath filamentary structures are often associated with large‐amplitude ion density and temperature perturbations comparable to the unperturbed background levels.

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

The MATLAB code for the paper "Agyrotropic electron distributions in the Terrestrial Foreshock Transients"

Hybrid Simulations of a Tangential Discontinuity‐Driven Foreshock Bubble Formation in Comparison With a Hot Flow Anomaly Formation

AbstractHot flow anomalies (HFAs) and foreshock bubbles (FBs) are significant foreshock transients that can accelerate particles and disturb the magnetosphere‐ionosphere system. Yet, their early formation mechanisms are still not fully understood. To investigate the formation of tangential discontinuity (TD)‐driven FBs and HFAs, we use 2‐D local hybrid simulations where a reflected or an injected warm foreshock ion beam can interact with a TD whose half‐thickness is comparable to the ion inertial scale. We show that the foreshock ions perform a partial gyration within, or across, the TD. Bulk motion differences between partially‐gyrating foreshock ions and fluid‐electrons lead to the generation of currents. As the trigger, these foreshock‐driven currents change the magnetic field topology around the TD and force the frozen‐in solar wind plasma to redistribute along with the field lines, shaping the foreshock transient. This confirms a recently proposed kinetic formation model. The extent of the magnetic field direction change across the TD within the foreshock ion gyromotion determines the current profile and thus the type of foreshock transient that forms. For a thin TD, the foreshock ions generate a current that is much stronger on the upstream side than the downstream side, forming an FB with one upstream compressional boundary. For the same foreshock ion gyroradius and magnetic shear, a thick TD yields comparable foreshock‐driven currents on the upstream and downstream sides, forming an HFA with two compressional boundaries. Our study suggests that the TD thickness is one of the factors that determine the formation of FBs and HFAs.

Statistical Study of Foreshock Density Holes

AbstractDensity holes (DHs), characterized by the correlating depletion of plasma density and magnetic field strength, are one of the transient structures frequently observed upstream of Earth's bow shock. The low dynamic pressure of DHs may disturb the bow shock and magnetopause, and have potential geoeffects. In this paper, we perform a statistical study of 411 density hole events, identified by the Magnetospheric Multiscale Mission. The average occurrence rate of DHs is ∼5.4 events/day. All DHs are associated with foreshock ions. DHs tend to occur on the dawn side than on the dusk side. The occurrence rate is higher for faster solar wind, larger magnetic shear angle across DHs, and lower magnetic field strength. The spatial scale of DHs is several RE. Within DHs, electron heating and density depletion are correlated. Most DHs do not have significant flow deflections. Most DHs with discontinuities have the convection electric field pointing toward the discontinuity on at least one side. About two thirds of the 411 events show different characteristics compared to other types of foreshock transients that also have correlated density and field strength depletion.

Electron Resonant Interaction With Whistler Waves Around Foreshock Transients and the Bow Shock Behind the Terminator

AbstractInvestigation of electron energization at (and around) the Earth's bow shock is critical to our understanding of space weather and astrophysical phenomena. The traditional adiabatic mechanisms for such energization compete with transient wave‐particle interactions there. One of the most intense wave modes resonating with electrons is the high‐frequency whistler mode, which is widely observed at (and around) the Earth's bow shock. Here, we examine these interactions in the context of the strong magnetic field gradients often found near the bow shock and at foreshock transients. Using THEMIS and ARTEMIS wave measurements, we quantify the nonlinear effects of resonant interactions between ≥100 eV electrons and intense coherent whistler waves. Such nonlinear interactions include the electron phase trapping by waves. As a result, the trapped electrons gain an energy up to several hundreds of eV. We estimate the main characteristics of the proposed acceleration mechanism and discuss its applicability to realistic plasma and magnetic field distributions.

Statistical Study of Foreshock Bubbles, Hot Flow Anomalies, and Spontaneous Hot Flow Anomalies and Their Substructures Observed by MMS

AbstractForeshock transients such as foreshock bubbles (FBs), hot flow anomalies (HFAs), and spontaneous hot flow anomalies (SHFAs) display heated, tenuous cores and large flow deflections bounded by compressional boundaries. THEMIS and Cluster observations show that some cores contain local density enhancements which can be studied to better understand the evolution processes of foreshock transients. However, closer examinations of these substructures were not feasible until the availability of the higher resolution data from the Magnetospheric Multiscale mission (MMS). We identify 164 FB‐like, HFA‐like, and SHFA events from two MMS dayside phases for a statistical study to investigate their solar wind conditions, properties, and substructure properties. Occurrence rates of the three event types are higher for lower magnetic field strengths, higher solar wind speeds and Mach numbers, and quasi‐parallel bow shocks. Events usually span up to 3 RE along the bow shock surface and extend up to 6 RE upstream from the bow shock. Though events with and without substructures exhibit similar solar wind conditions, events with substructures are more likely to have longer core durations and larger sizes. Substructure densities display a positive correlation with bulk flows and a negative correlation with temperatures. Substructure sizes vary between 4 and 24 ion inertial lengths, indicating multiple formation mechanisms. Substructures could be the boundary between two foreshock transient events that have merged into a single event, fast‐mode variations, generated by slow or mirror mode instabilities, or produced from instabilities due to parameter gradients at the compressional boundaries or shocks.

Kinetic‐Scale Magnetic Holes Inside Foreshock Transients

AbstractIn Earth’s foreshock, there are many foreshock transients that have core regions with low field strength, low density, high temperature, and bulk velocity variation. Through dynamic pressure perturbations, they can disturb the magnetosphere–ionosphere system. They can also accelerate particles contributing to particle acceleration at the bow shock. Recent Magnetospheric Multiscale (MMS) mission observations showed that inside the low field strength core region, there are usually kinetic‐scale magnetic holes with even lower field strength (<1 nT). However, their nature and effects are unknown. In this study, we used MMS observations to conduct case studies on these magnetic holes. We found that they could be subion‐scale current sheets without a magnetic normal component and guide field, driven by the motion of demagnetized electrons. These magnetic holes can also be subion‐scale flux ropes or magnetic helical structures with weak axial field. The low field strength inside them can be either driven by external expansion or electron mirror mode. Electrons inside them show flux depletion at 90° pitch angle resulting in an “electron hole” distribution. These magnetic holes can play a role in electron dynamics, wave excitation, and shaping the foreshock transient structures. Our detailed study of such features sheds light on the turbulent nature of foreshock transient cores.

Impact of Foreshock Transients on the Flank Magnetopause and Magnetosphere and the Ionosphere

Mesoscale (on the scales of a few minutes and a few RE) magnetosheath and magnetopause perturbations driven by foreshock transients have been observed in the flank magnetotail. In this paper, we present the 3D global hybrid simulation results to show qualitatively the 3D structure of the flank magnetopause distortion caused by foreshock transients and its impacts on the tail magnetosphere and the ionosphere. Foreshock transient perturbations consist of a low-density core and high-density edge(s), thus, after they propagate into the magnetosheath, they result in magnetosheath pressure perturbations that distort magnetopause. The magnetopause is distorted locally outward (inward) in response to the dip (peak) of the magnetosheath pressure perturbations. As the magnetosheath perturbations propagate tailward, they continue to distort the flank magnetopause. This qualitative explains the transient appearance of the magnetosphere observed in the flank magnetosheath associated with foreshock transients. The 3D structure of the magnetosheath perturbations and the shape of the distorted magnetopause keep evolving as they propagate tailward. The transient distortion of the magnetopause generates compressional magnetic field perturbations within the magnetosphere. The magnetopause distortion also alters currents around the magnetopause, generating field-aligned currents (FACs) flowing in and out of the ionosphere. As the magnetopause distortion propagates tailward, it results in localized enhancements of FACs in the ionosphere that propagate anti-sunward. This qualitatively explains the observed anti-sunward propagation of the ground magnetic field perturbations associated with foreshock transients.

Energy Modulations of Magnetospheric Ions Induced by Foreshock Transient‐Driven Ultralow‐Frequency Waves

AbstractAlthough foreshock transients can generate strong magnetospheric Pc5 ultralow‐frequency (ULF) waves, whether they can modulate the energy of magnetospheric ions is still poorly understood. In this study, we analyze the strong magnetospheric ion energy modulations in a foreshock transient event on October 30, 2008, based on the magnetospheric observations by the time history of events and macroscale interactions during substorms A and D in the prenoon sector. ULF wave‐induced drift accelerated the cold ions up to ∼10 keV and the enhanced ion fluxes have wave‐like patterns. There is another portion of enhanced ion fluxes from ∼0.8 to ∼10 keV but with strong energy dispersions in this event. By comparing the observations and the theoretical prediction, we for the first time found that the drift‐bounce resonances played a major role in modulating the energy of those ions with energy dispersions, during the interactions between the ions and the foreshock transient‐driven Pc5 ULF wave with growing and damping effects.

Statistical Study of Foreshock Transients in the Midtail Foreshock

AbstractIn the dayside foreshock, many foreshock transients have been observed and simulated. Because of their strong dynamic pressure perturbations, foreshock transients can disturb the local bow shock, magnetosheath, magnetopause, and thus the magnetosphere‐ionosphere system. They can also accelerate particles contributing to shock acceleration. Recent observations and simulations showed that foreshock transients also exist in the midtail foreshock, which can continuously disturb the nightside bow shock, magnetosheath, and magnetopause while propagating tailward for tens of minutes. To further understand the characteristics of midtail foreshock transients, we studied them statistically using Acceleration Reconnection Turbulence & Electrodynamics of Moon’s Interaction with the Sun observations. We selected 111 events that have dynamic pressure decrease along the local bow shock normal by more than 50%. We show that the dynamic pressure decrease is contributed by both density decrease and speed decrease. Around 90% of the events have electron temperature increase by more than 10% with a temperature change ratio proportional to the solar wind speed. Midtail foreshock transients more likely occur at the dawnside than the duskside. They are more significant closer to the bow shock and rather stable along the tailward direction. They have similar formation conditions compared to the dayside foreshock transients, except the ones related to the bow shock geometry. Our study indicates that the characteristics of foreshock transients based on dayside observations need to be generalized. Our study also implies that foreshock transients can exist for tens of minutes (even longer for larger planar shocks), continuously disturbing the local shock and accelerating/heating particles.

Global Propagation of Magnetospheric Pc5 ULF Waves Driven by Foreshock Transients

AbstractPc5 (2–7 mHz) ultralow frequency (ULF) waves play a significant role in resonating with particles and transferring energy in the coupled magnetospheric and ionospheric system. Recent studies found that Pc5 ULF waves can be triggered by foreshock transients which can perturb the magnetopause through dynamic pressure variation. However, whether foreshock transient‐driven Pc5 ULF waves are geoeffective and can propagate globally is still poorly understood. In this study, we take advantage of the conjunction between in situ (by the THEMIS probes, Geotail satellite, GOES satellites, and Van Allen probes) and ground‐based (by the all‐sky imager at South Pole and ground‐based magnetometers) observations to simultaneously analyze the waves from the foreshock region to the dayside and nightside magnetosphere. Both of our two events show that the Pc5 ULF waves are generated by foreshock transients in the dayside magnetosphere. The in situ observations by THEMIS A and D and the 2‐D auroral signatures show that the compressional mode waves are likely broadband and coupled to the FLRs with different frequencies and different azimuthal phase speeds. This is the first report that foreshock transients can drive both low‐ and high‐m FLRs, with the azimuthal wave numbers varying from ~5 to ~23. Moreover, the Pc5 ULF waves propagated antisunward to midnight, this can potentially modulate magnetospheric and ionospheric dynamics globally.

ARTEMIS Observations of Foreshock Transients in the Midtail Foreshock

AbstractForeshock transients such as hot flow anomalies (HFAs) are frequently observed in the dayside foreshock. They can disturb the local bow shock, magnetopause, and consequently the magnetosphere‐ionosphere system through dynamic pressure perturbations. Recent multipoint observations found that such perturbations can even propagate from the dayside to the midtail. However, whether the drivers of such perturbations, foreshock transients, persist in the midtail foreshock has not been observed. Thus, it is unclear whether the observed nightside magnetosheath/magnetopause perturbations are traveling waves or continuously driven by a propagating foreshock transient. Using two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft, we report direct observational evidence of foreshock transients in the midtail foreshock. We present a case study showing an elongated mature HFA propagating with its driver discontinuity from TH‐C (X ~ −43 RE) to TH‐B (X ~ −48 RE). Our results confirm that foreshock transients disturb not only the dayside bow shock but also the nightside bow shock while propagating tailward.

Formation of Foreshock Transients and Associated Secondary Shocks

Abstract Upstream of shocks, the foreshock is filled with hot ions. When these ions are concentrated and thermalized around a discontinuity, a diamagnetic cavity bounded by compressional boundaries, referred to as a foreshock transient, forms. Sometimes, the upstream compressional boundary can further steepen into a secondary shock, which has been observed to accelerate particles and contribute to the primary shock acceleration. However, secondary shock formation conditions and processes are not fully understood. Using particle-in-cell simulations, we reveal how secondary shocks are formed. From 1D simulations, we show that electric fields play a critical role in shaping the shock’s magnetic field structure, as well as in coupling the energy of hot ions to that of the shock. We demonstrate that larger thermal speed and concentration ratio of hot ions favor the formation of a secondary shock. From a more realistic 2D simulation, we examine how a discontinuity interacts with foreshock ions leading to the formation of a foreshock transient and a secondary shock. Our results imply that secondary shocks are more likely to occur at primary shocks with higher Mach number. With the secondary shock’s previously proven ability to accelerate particles in cooperation with a planetary bow shock, it is even more appealing to consider them in particle acceleration of high Mach number astrophysical shocks.