Solar wind directional discontinuities can generate transient mesoscale structures upstream of Earth's bow shock, which can have a global impact on the near-Earth environment. Understanding the formation conditions of these transient structures is crucial to evaluate their contribution to solar wind-magnetosphere coupling. Hot flow anomalies (HFAs) are thought to be created only by tangential discontinuities, and develop where the discontinuities intersect the shock. Foreshock bubbles, on the other hand, are associated with both tangential and rotational discontinuities, and are generated before the discontinuities reach the bow shock, as they form due to foreshock suprathermal ions accumulation on the upstream side of the discontinuities. Here we present the results of a global 2D hybrid-Vlasov simulation of the interaction of a rotational discontinuity with near-Earth space performed with the Vlasiator model. As the discontinuity enters the simulation domain, a foreshock bubble forms duskward of the Sun-Earth line, where the foreshock is initially located. Shortly after the discontinuity makes first contact with the bow shock at the subsolar point, we find that a structure with enhanced temperature and strongly deflected flows develops at the intersection of the discontinuity with the bow shock. This structure displays typical features of an HFA. This suggests that both a foreshock bubble and an HFA can be generated concurrently by a single directional discontinuity, and that a rotational discontinuity can lead to HFA formation in some conditions. We compare the ion distribution functions inside the foreshock bubble and the HFA, and find significant solar wind core heating within the HFA, as expected from spacecraft observations. We discuss how the properties of the structures vary spatially and temporally, providing global context to localised spacecraft measurements.

How to cite: Turc, L., Archer, M., Zhou, H., Kajdic, P., Blanco-Cano, X., Pfau-Kempf, Y., Liu, T., Lin, Y., Omidi, N., LaMoury, A., Lee, S., Raptis, S., Sibeck, D., Zhang, H., Hao, Y., Silveira, M., Wang, B., and Palmroth, M.: A rotational discontinuity can generate both a foreshock bubble and a hot flow anomaly, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20429, https://doi.org/10.5194/egusphere-egu24-20429, 2024.

The foreshock is a turbulent region located upstream of the quasi parallel bow shock. It is dominated by waves and reflected back-streaming particles that interact with each other and result in the creation of different foreshock phenomena. Xirogiannopoulou et al. (2023) found that the deceleration of the solar wind in the foreshock region plays a major role in the creation of subsolar structures with enhanced density or/and magnetic field magnitude, like plasmoids, SLAMS and mixed structures. Moreover, they have found that their formation is increasing with increased velocity of the pristine solar wind. Previous studies, established that foreshock structures are connected with MSH jets (Raptis et al., 2022).  Simultaneously, Koller et al. (2023) researched the connection between the MSH jets and solar wind structures and concluded that the high-speed streams (HSS) create a more favorable environment for the jet creation. Following these results, we use measurements of the Magnetospheric Multiscale Spacecraft (MMS) and OMNI solar wind database between the years 2015 and 2018 and present a statistical analysis considering the presence of solar wind phenomena and their effect in the appearance rate of the compressive foreshock structures. We attempt to explore the origin of these structures and analyze their connection with the notable decrease (10–15 %) of the solar wind speed inside the foreshock region.

How to cite: Xirogiannopoulou, N., Goncarov, O., Safrankova, J., and Nemecek, Z.: Solar wind deceleration in the foreshock as the source of the foreshock compressive structures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8084, https://doi.org/10.5194/egusphere-egu24-8084, 2024.

The day-to day variability of quiet-time ionosphere is surprisingly high even during periods of negligible solar forcing. Relatively well understood is the high-latitude variability where the solar wind is directly driving the high latitude currents, convection electric field or polar aurorae. But the current
understanding does not allow to accurately model the ionospheric state during the quiet-time conditions at mid- and low-latitudes. Surprising effects remain even at mid-latitudes, including for instance double daily maxima of ionospheric critical frequency.
European Space Agency's SWARM satellite constellation measurements allow the characterization of the upper atmospheric conditions and dynamics for already more than 10 years. The analysis of SWARM electron density and electric field data already showed that the ionosphere at mid-latitudes shows a non-negligible variability without a-priori solar driver, during negligible variations both in X-ray/EUV fluxes and missing disturbances in the solar wind. This often significant ionospheric variability currently remains unexplained, and further studies need to evaluate contributions by couplings from below comparing with those from above, i.e. checking the frequencies matching the foreshock waves with the local field-line resonances.

After efforts to properly select such "solar-quiet" periods, we compare the SWARM-detected variability trying to relate them to observations of magnetospheric ULF waves and configurations of the Earth foreshock mainly infered from the NASA's THEMIS satellite fleet and Wind solar wind observations.

How to cite: Urbar, J.: Evaluating the effects of the Earth's foreshock configurations on the "quiet-time" ionosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18654, https://doi.org/10.5194/egusphere-egu24-18654, 2024.

With the help of a two-dimensional (2-D) particle-in-cell (PIC) simulation model, we investigate the long-time evolution of a quasi-parallel shock. Part of upstream ions are reflected by the shock front, and their interactions with the incident ions excite low-frequency magnetosonic waves in the upstream. Detailed analyses have shown that the dominant wave mode is caused by the resonant ion-ion beam instability, and the wavelength can reach tens of the ion inertial lengths. Although these plasma waves are directed toward the upstream in the upstream plasma frame, they are brought by the incident plasma flow toward the shock front, and their amplitude is enhanced during the approaching. The interaction of the upstream plasma waves with the shock leads to the cyclic reformation of the shock front. When crossing the shock front, these large-amplitude plasma waves are compressed and evolve into current sheets in the transition region of the shock. At last, magnetic reconnection occurs in these current sheets, accompanying with the generation of magnetic islands. Simultaneously, there still exist another kind of plasma waves with the wavelength of several ion inertial lengths in the ramp of the shock. The current sheets in the transition region are distorted and broken into several segments when this kind of plasma waves are transmitted into the downstream, where magnetic reconnection and the generated islands have a much smaller size.

How to cite: Lu, Q., Guo, A., Yang, Z., Lu, S., and Wang, R.: Upstream plasma waves and downstream magnetic reconnection at a reforming quasi-parallel shock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-85, https://doi.org/10.5194/egusphere-egu24-85, 2024.

We use the Magnetospheric Multiscale mission to study electron kinetic entropy across Earth's quasi-perpendicular bow shock. We perform a statistical study of how the change in electron entropy depends on the different plasma parameters associated with a collisionless shock crossing. We find that the change in electron entropy exhibits strong correlations with upstream electron plasma beta, Alfvén Mach number, and electron thermal Mach number. The source of entropy generation is investigated by correlating the change in electron entropy across the shock to the measured electric and magnetic field wave power strengths for different frequency intervals within different regions in the shock transition layer. The electron entropy change is observed to be greater for higher electric field wave power within the shock ramp and shock foot for frequencies between the lower hybrid frequency and electron cyclotron frequency. This implies electrostatic waves are important for electron kinetic entropy generation at Earth's quasi-perpendicular bow shock but also for the non-adiabatic electron heating at quasi-perpendicular shocks.

How to cite: Lindberg, M., Wallner, A., Berglund, S., and Vaivads, A.: Statistical Study of Electron Kinetic Entropy Generation at Earth's Quasi-perpendicular Bow Shock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1836, https://doi.org/10.5194/egusphere-egu24-1836, 2024.

3D velocity distribution functions (VDFs) of He⁺ pickup ions (PUIs) were used to trace local physical processes around interplanetary shocks. He⁺ PUIs are measured by PLasma And SupraThermal Ion Compostion (PLASTIC) instrument onboard the Solar Terrestrial Relations Observatory (STEREO) in an unprecedented cadence and angular/velocity resolutions with mass and charge state unambiguously determined. We focused on the VDFs in terms of the interplanetary magnetic field orientation in the solar wind frame and indipendently evaluated the acceleration, heating, and pitch-angle scattering for both perpendicular and parallel shocks with notable differences. For the perpendicular shock: (a) Reflected and energized particles were found in the upstream, and the PUI properties were isolated between upstream and downstream, (b) Strong heating is observed in the sheath region, (c) Suprathermal particles are found in the sheath region and attributed to the compression within the solar wind, and (d) Universal pitch-angle scattering were found through out the ICME. For the parallel shock: (a) Locaized heating and acceleration were found around the shock location, (b) Harder suprathermal particle distributions were found near the shock, suggesting diffusive shock processes, (c) Weak or no sheath heating was observed, and (d) Pitch angle scattering were correlated with magnetic field power spectral density at the Helium cyclotron.

How to cite: Ogasawara, K., Kucharek, H., Klecker, B., Dayeh, M., and Ebert, R.: Helium pickup ion 3D velocity distributions at interplanetary shocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2220, https://doi.org/10.5194/egusphere-egu24-2220, 2024.

Short Large-Amplitude Magnetic Structures (SLAMS) are common magnetic field signatures observed in the foreshock region of collisionless quasi-parallel shocks. SLAMS are non-linear isolated structures, defined to have an amplitude of more than 2 times the background magnetic field. They have been suggested to grow from ultra-low frequency (ULF) waves that are common in the foreshock region, and are believed to play important roles in the formation of quasi-parallel shocks, influencing the properties and dynamics of the shock and also associated particle energization.   

In this work, we use data from the Cluster and Magnetospheric Multiscale (MMS) missions to statistically study the properties of SLAMS in the foreshock region of Earth. We use an automated method to detect SLAMS in the data, producing a comprehensive database of SLAMS detections. The size, morphology and propagation velocity of SLAMS are then studied using statistical approaches, investigating the dependence on several parameters, such as amplitude, upstream conditions and distance from the bow shock. 

How to cite: Bergman, S., Karlsson, T., and Wong Chan, T. K.: Statistical properties of Short Large-Amplitude Magnetic Structures (SLAMS) in the foreshock region of Earth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3125, https://doi.org/10.5194/egusphere-egu24-3125, 2024.

Ion reflection is known to be a key component of particle energisation and heating in super-critical collisionless shockwaves. As a source of free energy, reflected ions drive stream instabilities in the shock foot, create magnetic perturbations, and contribute to magnetic field amplification in the upstream and downstream regions of quasi-perpendicular shocks. Where ions reflect from the shock ramp multiple times, a longer dwell time in the shock foot allows for more opportunities for interaction with non-stationary and turbulent shock processes, which can result in injection into processes such as diffusive shock acceleration and shock drift acceleration. To characterise the pathway to multiple ion reflection, we perform a series of 2D and 3D hybrid particle-in-cell simulations (fluid electron, particle ions) over a parameter range typical of Earth’s bow shock, varying Mach number, plasmas betas, and the angle between the shock normal and upstream magnetic field (θ_Bn). This enables a parametric investigation of the density fraction of multiply reflected ions both upstream and downstream of the shock, for protons and for heavier ion components of the solar wind such as He²⁺. We discuss methods for identification of multiply-reflected ions in kinetic plasma simulations, corresponding analogues for observations (where available), and investigate their impact on shock energetics. By examining the partial moments of reflected ions in two- and three-dimensional simulations, we also explore which shock processes drive multiple reflection.

How to cite: Gingell, I. and Madanian, H.: Hybrid simulations of multiple reflection of protons and heavy ions at Earth’s bow shock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3966, https://doi.org/10.5194/egusphere-egu24-3966, 2024.

Transient upstream mesoscale structures (TUMS) are an important topic in the field of research of the near-Earth environment. These events form upstream of the Earth's bow shock and can perturb regions downstream it, i.e. magnetosheath and magnetopause. They can even affect the magnetosphere and ionosphere causing a range of space weather phenomena during periods without noticeable solar activity. There is still much to learn about the TUMS and the way they interact with the near-Earth environment. We are only beginning to understand how the different types of the TUMS relate to each other. In the past it has been shown that traveling foreshocks may contain foreshock cavitons, spontaneous hot flow anomalies and foreshock compressional boundaries (FCB). Here we present the first evidence, that traveling foreshocks may be bounded on at least one of their edges by hot flow anomalies (HFA) and by events that look like hybrids between HFAs and FCBs. We show two case studies observed by the Cluster and MMS constellations. Such studies enable us to better understand all the ways in which the solar-terrestrial interactions occur.

How to cite: Kajdic, P., Blanco-Cano, X., Rojas Castillo, D., Archer, M., Turc, L., Pfaum-Kempf, Y., LaMoury, A., Liu, T., Raptis, S., Vinicius, M., Lee, S., Shutao, Y., Hao, Y., Sibeck, D., Zhang, H., Omidi, N., Wang, B., Lin, Y., and Escoubet, P.: Hot Flow Anomalies Delimiting Traveling Foreshocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4145, https://doi.org/10.5194/egusphere-egu24-4145, 2024.

In recent years we have learnt that foreshock transients play an influential role in solar wind coupling with Earth’s magnetosphere. These transients include Spontaneous Hot Flow Anomalies (SHFAs) which are characterized by dips in magnetic field magnitude and ion density, enhanced temperature, and are surrounded by ultra low frequency waves. SHFAs share some characteristics with hot flow anomalies but their formation mechanism does not require a discontinuity in the solar wind.  SHFAs evolve from caviton interaction with backstreaming ions. We use Cluster and MMS magnetic field and plasma data to study SHFAs internal structure and their evolution. We find that SHFA can occur not only deep in the foreshock as initially thought, but they can also been observed near the foreshock boundary where higher frequency waves (f ∼ 1 Hz) exist. We also investigate the properties of velocity distribution functions inside these transients, and their influence on bow shock structure.

How to cite: Blanco-Cano, X., Rojas-Castillo, D., and Kajdic, P.: Spontaneous Hot Flow Anomalies at Earth’s foreshock., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6483, https://doi.org/10.5194/egusphere-egu24-6483, 2024.

In this paper, two-dimensional hybrid simulations are used to study wave excitation and evolution throughout a low-Mach number quasi-parallel shock. Simulation results show that quasi-parallel fast magnetosonic waves, ion Bernstein waves with harmonics and possible Alfven/ion cyclotron waves can be excited in the upstream region, and their small phase velocities compared to injected flow velocity results in the convection to the shock front where they are mode converted into several groups of downstream waves, including Alfven waves along the directions parallel to downstream average magnetic fields and perpendicular to the shock normal, the quasi-perpendicular kinetic slow waves and possible kinetic Alfven waves. We suggest that downstream Alfven waves originate from the mode conversion of upstream quasi-parallel fast magnetosonic waves with left-hand polarization in the downstream rest frame under helicity conservation, while the downstream left-hand polarized kinetic slow waves and right-hand polarized kinetic Alfven waves can be from the upstream quasi-perpendicular ion Bernstein waves.

How to cite: Hao, Y., Lu, Q., Wu, D., and Xiang, L.: Wave Activities Throughout a Low-Mach Number Quasi-Parallel Shock: 2-D Hybrid Simulations , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9451, https://doi.org/10.5194/egusphere-egu24-9451, 2024.

Reflection of a fraction of incoming ions is a vital dissipation mechanism for super-critical shocks. Such reflected ions provide a significant contribution to the downstream ion temperature increase. Understanding ion dynamics is crucial for the characterization of the shocks. It is needed to provide an equation of state that connects the upstream and downstream parameters for given shock parameters. The reflection depends on the detailed structure of the electromagnetic fields in the shock transition region. The ion-scale structure is strongly affected by the shock non-stationarity. To characterize the spatiotemporal evolution of the shock structure and ion reflection, we combine observations of several in-situ shock events by MMS with hybrid simulations performed for the specific parameters of the observed shocks. We find that the ion reflection is strongly affected by the structure imposed by the ripples. The reflection is very patchy, with regions of strong and weak ion reflection. 

How to cite: Khotyaintsev, Y., Graham, D., Trotta, D., Lalti, A., and Dimmock, A.: Ion dynamics in quasi-perpendicular nonstationary shocks: comparison between MMS observations and hybrid modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16745, https://doi.org/10.5194/egusphere-egu24-16745, 2024.