| 1. |
- Dimmock, A. P., et al.
(author)
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Analysis of multiscale structures at the quasi-perpendicular Venus bow shock Results from Solar Orbiter's first Venus flyby
- 2022
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 660
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Journal article (peer-reviewed)abstract
- Context. Solar Orbiter is a European Space Agency mission with a suite of in situ and remote sensing instruments to investigate the physical processes across the inner heliosphere. During the mission, the spacecraft is expected to perform multiple Venus gravity assist maneuvers while providing measurements of the Venusian plasma environment. The first of these occurred on 27 December 2020, in which the spacecraft measured the regions such as the distant and near Venus magnetotail, magnetosheath, and bow shock. Aims. This study aims to investigate the outbound Venus bow shock crossing measured by Solar Orbiter during the first flyby. We study the complex features of the bow shock traversal in which multiple large amplitude magnetic field and density structures were observed as well as higher frequency waves. Our aim is to understand the physical mechanisms responsible for these high amplitude structures, characterize the higher frequency waves, determine the source of the waves, and put these results into context with terrestrial bow shock observations. Methods. High cadence magnetic field, electric field, and electron density measurements were employed to characterize the properties of the large amplitude structures and identify the relevant physical process. Minimum variance analysis, theoretical shock descriptions, coherency analysis, and singular value decomposition were used to study the properties of the higher frequency waves to compare and identify the wave mode. Results. The non-planar features of the bow shock are consistent with shock rippling and/or large amplitude whistler waves. Higher frequency waves are identified as whistler-mode waves, but their properties across the shock imply they may be generated by electron beams and temperature anisotropies. Conclusions. The Venus bow shock at a moderately high Mach number (similar to 5) in the quasi-perpendicular regime exhibits complex features similar to the Earth's bow shock at comparable Mach numbers. The study highlights the need to be able to distinguish between large amplitude waves and spatial structures such as shock rippling. The simultaneous high frequency observations also demonstrate the complex nature of energy dissipation at the shock and the important question of understanding cross-scale coupling in these complex regions. These observations will be important to interpreting future planetary missions and additional gravity assist maneuvers.
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| 2. |
- Dimmock, Andrew P., et al.
(author)
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Backstreaming ions at a high Mach number interplanetary shock : Solar Orbiter measurements during the nominal mission phase
- 2023
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 679
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Journal article (peer-reviewed)abstract
- Context: Solar Orbiter, a mission developed by the European Space Agency, explores in situ plasma across the inner heliosphere while providing remote-sensing observations of the Sun. The mission aims to study the solar wind, but also transient structures such as interplanetary coronal mass ejections and stream interaction regions. These structures often contain a leading shock wave that can differ from other plasma shock waves, such as those around planets. Importantly, the Mach number of these interplanetary shocks is typically low (1-3) compared to planetary bow shocks and most astrophysical shocks. However, our shock survey revealed that on 30 October 2021, Solar Orbiter measured a shock with an Alfven Mach number above 6, which can be considered high in this context.Aims: Our study examines particle observations for the 30 October 2021 shock. The particles provide clear evidence of ion reflection up to several minutes upstream of the shock. Additionally, the magnetic and electric field observations contain complex electromagnetic structures near the shock, and we aim to investigate how they are connected to ion dynamics. The main goal of this study is to advance our understanding of the complex coupling between particles and the shock structure in high Mach number regimes of interplanetary shocks.Methods: We used observations of magnetic and electric fields, probe-spacecraft potential, and thermal and energetic particles to characterize the structure of the shock front and particle dynamics. Furthermore, ion velocity distribution functions were used to study reflected ions and their coupling to the shock. To determine shock parameters and study waves, we used several methods, including cold plasma theory, singular-value decomposition, minimum variance analysis, and shock Rankine-Hugoniot relations. To support the analysis and interpretation of the experimental data, test-particle analysis, and hybrid particle in-cell simulations were used.Results: The ion velocity distribution functions show clear evidence of particle reflection in the form of backstreaming ions several minutes upstream. The shock structure has complex features at the ramp and whistler precursors. The backstreaming ions may be modulated by the complex shock structure, and the whistler waves are likely driven by gyrating ions in the foot. Supra-thermal ions up to 20 keV were observed, but shock-accelerated particles with energies above this were not.
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| 3. |
- Ganushkina, N. Y., et al.
(author)
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SHARP Shock Database
- 2024
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 129:7
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Journal article (peer-reviewed)abstract
- Despite more than half a century of Collisionless shock (CS) research, our understanding of the processes of the shock energy dissipation into the charge particle heating and acceleration remains incomplete. To help to address the problem of the rate of the data analysis on CSs being well below of the rate of the data acquisition, an open-source high-level database of shocks and a centralized source of advanced tools for the purpose of analyzing shock structure and dynamics have been developed. The database is called SHARP shock database by the name of the project SHARP (Shocks: structure, AcceleRation, dissiPation) funded by the European Union's Horizon 2020 program. The SHARP shock database contains shock crossings and corresponding parameters obtained from Cluster and MMS (Magnetospheric Multiscale) missions for terrestrial bow shocks, THEMIS (Time History of Events and Macroscale Interactions during Substorms)/ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) missions for interplanetary shocks, and MAVEN (Mars Atmosphere and Volatile EvolutioN) and VEX (Venus Express) missions for shocks at non-magnetized planets. The SHARP shock database can be accessed via https://sharp.fmi.fi/shock-database/.
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| 4. |
- Graham, Daniel B., et al.
(author)
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Ion Dynamics Across a Low Mach Number Bow Shock
- 2024
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 129:4
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Journal article (peer-reviewed)abstract
- A thorough understanding of collisionless shocks requires knowledge of how different ion species are accelerated across the shock. We investigate a bow shock crossing using the Magnetospheric Multiscale spacecraft after a coronal mass ejection crossed Earth, which led to solar wind consisting of protons, alpha particles, and singly charged helium ions. The three species are resolved upstream of the shock. The low Mach number of the bow shock enabled the ions to be partly distinguished downstream of the shock due to the relatively low ion heating. Some of the protons are specularly reflected and produce quasi-periodic fine structures in the velocity distribution functions downstream of the shock. Heavier ions are shown to transit the shock without reflection. However, the gyromotion of the heavier ions partially obscures the fine structure of proton distributions. Additionally, the calculated proton moments are unreliable when the different ion species are not distinguished by the particle detector. The need for high time-resolution mass-resolving ion detectors when investigating collisionless shocks is discussed. One of the ongoing challenges when investigating collisionless shocks is determining the energy partition between electromagnetic fields and different particle species. Resolving this question requires detailed observations of the electromagnetic fields and particle distributions, and is challenging when multiple ion species are present. We investigate a crossing of Earth's bow shock for unusual solar wind conditions; three ion species are observed in the solar wind and behind the bow shock, namely protons, alpha particles, and singly charged helium ions. We investigate the ion dynamics and show that a small fraction of protons are reflected by the electric field associated with the shock, which results in complex ion distributions. However, since the highest time-resolution ion detectors cannot distinguish between different ion species, the heavier ions partly obscure the fine structure of the protons. The heavier ions lead to errors when calculating the bulk properties (e.g., moments) of protons. These observations illustrate the need for high time-resolution ion detectors, which can distinguish different ion species when studying shocks. Protons, singly charged helium ions, and alpha particles are observed upstream and downstream of a bow shock crossing The alpha particles and helium ions partly obscure the fine structure of the downstream proton distributions High time-resolution mass-resolving ion detectors are needed to study the ion dynamics across collisionless shocks
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| 5. |
- Johlander, Andreas, 1990-, et al.
(author)
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Electron Heating Scales in Collisionless Shocks Measured by MMS
- 2023
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In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 50:5
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Journal article (peer-reviewed)abstract
- Electron heating at collisionless shocks in space is a combination of adiabatic heating due to large-scale electric and magnetic fields and non-adiabatic scattering by high-frequency fluctuations. The scales at which heating happens hints to what physical processes are taking place. In this letter, we study electron heating scales with data from the Magnetospheric Multiscale (MMS) spacecraft at Earth's quasi-perpendicular bow shock. We utilize the tight tetrahedron formation and high-resolution plasma measurements of MMS to directly measure the electron temperature gradient. From this, we reconstruct the electron temperature profile inside the shock ramp and find that the electron temperature increase takes place on ion or sub-ion scales. Further, we use Liouville mapping to investigate the electron distributions through the ramp to estimate the deHoffmann-Teller potential and electric field. We find that electron heating is highly non-adiabatic at the high-Mach number shocks studied here.Plain Language SummaryShock waves appear whenever a supersonic medium, such as a plasma, encounters an obstacle. The plasma, which consists of charged ions and free electrons, is heated by the shock wave through interactions with the electromagnetic fields. In this work, we investigate how electrons are heated at plasma shocks. A key parameter to electron heating is the thickness of the layer where the heating takes place. Here, we use observations from the four Magnetospheric Multiscale spacecraft that regularly cross the standing bow shock that forms when the supersonic plasma, known as the solar wind, encounters Earth's magnetic field. We find that the thickness of the shock is larger than previously reported and is on the scales where ion physics dominate. We also find that the electron heating deviates significantly from simple adiabatic heating.
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| 6. |
- Lalti, Ahmad, et al.
(author)
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A Database of MMS Bow Shock Crossings Compiled Using Machine Learning
- 2022
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:8
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Journal article (peer-reviewed)abstract
- Identifying collisionless shock crossings in data sent from spacecraft has so far been done manually or using basic algorithms. It is a tedious job that shock physicists have to go through if they want to conduct case studies or perform statistical studies. We use a machine learning approach to automatically identify shock crossings from the Magnetospheric Multiscale (MMS) spacecraft. We compiled a database of 2,797 shock crossings, spanning a period from October 2015 to December 2020, including various spacecraft-related and shock-related parameters for each event. Furthermore, we show that the shock crossings in the database are spread out in space, from the subsolar point to the far flanks. On top of that, we show that they cover a wide range of parameter space. We also present a possible scientific application of the database by looking for correlations between ion acceleration efficiency at shocks with different shock parameters, such as the angle between the upstream magnetic field and the shock normal theta(Bn) and the Alfvenic Mach number M-A. We find no clear correlation between the acceleration efficiency and M-A; however, we find that quasi-parallel shocks are more efficient at accelerating ions than quasi-perpendicular shocks.
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| 7. |
- Lalti, Ahmad, et al.
(author)
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Electron heating at quasi-perpendicular collisionless shocks
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Other publication (other academic/artistic)abstract
- Adiabatic and non-adiabatic electron dynamics have been proposed to explain electron heating across collisionless shocks. We analyze the evolution of the suprathermal electrons across 310 quasi-perpendicular shocks with $1.7
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| 8. |
- Lalti, Ahmad
(author)
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Electrostatic turbulence and electron heating in collisionless shocks
- 2022
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Licentiate thesis (other academic/artistic)abstract
- Collisionless shocks are one of the most peculiar phenomena in space where non-linear collective phenomena in the plasma dominate the dynamics. They are believed to be one of the most efficient particle accelerators in the universe, and have internal dynamics that are yet to be fully explored. In this project we aim to understand the interplay between the electrostatic turbulence in the shock ramp and the electron dynamics leading to thermalization across the shock. To do so we first use a machine learning technique to compile a database of shocks crossings observed by magnetospheric multiscale (MMS), which will facilitate both case studies and statistical studies of shocks using MMS. The database contains 2803 shock crossings spanning a period from October 2015 to December 2020. For each crossing we provide key parameters necessary for understanding shock dynamics such as Alfv\'{e}nic Mach number and the angle between the upstream magnetic field and the vector normal to the shock $\theta_{Bn}$. We then study whistler waves upstream of 11 quasiperpendicular supercritical shocks. We first apply four spacecraft timing method to magnetic field data from MMS to properly characterize the observed whistler waves. We determine their frequency in the plasma rest frame to range from 0.3 to 1.2 the lower hybrid frequency,their wavelength to range from 0.7 to 1.7 ion inertial length and $\theta_{kB}$ to range between $20^\circ$ and $42^\circ$. We then use particle data provided by MMS to show that a reflected beam component in the ion velocity distribution function is in resonance with the observed waves indicating that a kinetic cross field streaming instability (KCFSI) is behind the generation of such waves. Finally a kinetic solver is used to model to observed distribution and reinforce the previous conclusion that the KCFSI is behind the generation of the observed whistlers. We end this thesis by discussing the ongoing projects pertaining to the interaction of electrostatic wave mode determination in the shock ramp and the correlation between whistler waves and electrostatic waves around quasi-perpendicular shocks.
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| 9. |
- Lalti, Ahmad
(author)
-
Electrostatic turbulence and electron heating in collisionless shocks
- 2024
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Doctoral thesis (other academic/artistic)abstract
- When the supersonic solar wind interacts with Earth’s magnetosphere it forms a shock wave. However, due to the low densities in space, inter-particle collisions play an insignificant role in its dynamics. Earth's bow shock is an example of a collisionless shock, ubiquitous throughout the universe. Their dynamics are complex and their physics remains an active field of research. In this thesis, we use high-resolution measurements from NASA's Magnetospheric Multiscale (MMS) spacecraft to study the plasma wave activity across Earth’s bow shock and its effects on electron heating. In Paper I we train a convolutional neural network (CNN) to identify the different plasma regions that MMS crosses. In Paper II we use the results of this CNN to compile a database of time intervals in which MMS crosses Earth’s bow shock, which we use to find suitable events to tackle the science questions of interest. In Paper III we use multispacecraft methods to properly characterize obliquely propagating whistler waves running upstream of the shock. By analyzing the ion and electron distribution functions we find that their likely source is the instability between the incoming electrons and reflected ions. Shifting our focus to Debye scale electrostatic waves, in Paper IV we develop a method to measure their 3D wave vector based on single-spacecraft interferometry. We are in the process of using this method to study the evolution of Debye scale electrostatic waves across quasi-perpendicular shocks (see Chapter 7). Finally, in Paper V we investigate the electron heating mechanism across quasi-perpendicular shocks. We find the heating mechanism to depend on the Alfvénic Mach number in the deHoffman-Teller frame . We also find that at high the heating mechanism is consistent with the stochastic shock drift acceleration mechanism.
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| 10. |
- Lalti, Ahmad, et al.
(author)
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Short-Wavelength Electrostatic Wave Measurement Using MMS Spacecraft
- 2023
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 128:4
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Journal article (peer-reviewed)abstract
- Determination of the wave mode of short-wavelength electrostatic waves along with their generation mechanism requires reliable measurement of the wave electric field. We show that for such waves the electric field measured by Magnetospheric MultiScale becomes unreliable when the wavelength is close to the probe-to-probe separation. We develop a method, based on spin-plane interferometry, to reliably determine the full three-dimensional wave vector of the observed waves. We test the method on synthetic data and then apply it to ion acoustic wave bursts measured in the solar wind. By studying the statistical properties of ion acoustic waves in the solar wind, we retrieve the known results that the wave propagation is predominantly field aligned. We also determine the wavelength of the waves. We find that the nominal value is around 100 m, which when normalized to the Debye length corresponds to scales between 10 and 20 Debye lengths.
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| 11. |
- Lalti, Ahmad, et al.
(author)
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Whistler Waves in the Foot of Quasi-Perpendicular Supercritical Shocks
- 2022
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:5
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Journal article (peer-reviewed)abstract
- Whistler waves are thought to play an essential role in the dynamics of collisionless shocks. We use the magnetospheric multiscale spacecraft to study whistler waves around the lower hybrid frequency, upstream of 11 quasi-perpendicular supercritical shocks. We apply the 4-spacecraft timing method to unambiguously determine the wave vector k of whistler waves. We find that the waves are oblique to the background magnetic field with a wave-normal angle between 20 degrees and 42 degrees, and a wavelength of around 100 km, which is close to the ion inertial length. We also find that k is predominantly in the same plane as the magnetic field and the normal to the shock. By combining this precise knowledge of k with high-resolution measurements of the 3D ion velocity distribution, we show that a reflected ion beam is in resonance with the waves, opening up the possibility for wave-particle interaction between the reflected ions and the observed whistlers. The linear stability analysis of a system mimicking the observed distribution suggests that such a system can produce the observed waves.
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| 12. |
- Olshevsky, Viacheslav, et al.
(author)
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Automated Classification of Plasma Regions Using 3D Particle Energy Distributions
- 2021
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:10
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Journal article (peer-reviewed)abstract
- We investigate the properties of the ion sky maps produced by the Dual Ion Spectrometers (DIS) from the Fast Plasma Investigation (FPI). We have trained a convolutional neural network classifier to predict four regions crossed by the Magnetospheric Multiscale Mission (MMS) on the dayside magnetosphere: solar wind, ion foreshock, magnetosheath, and magnetopause using solely DIS spectrograms. The accuracy of the classifier is >98%. We use the classifier to detect mixed plasma regions, in particular to find the bow shock regions. A similar approach can be used to identify the magnetopause crossings and reveal regions prone to magnetic reconnection. Data processing through the trained classifier is fast and efficient and thus can be used for classification for the whole MMS database.
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