arXiv subjects

Ivan Y. Vasko

Publications and source records attributed to Ivan Y. Vasko.

13 recordsLinked to original sources

arXiv2026

Compressive Structures in the Foreshock of Collisionless Shocks

Collisionless shocks are fundamental accelerators of energetic particles; yet, the observations of nonlinear foreshock structures, which are essential in acceleration processes, differ significantly between Interplanetary (IP) shocks and planetary bow shocks. We present a direct comparison of two high-Mach-number, quasi-parallel shocks: an IP shock observed by Solar Orbiter and the Earth's bow shock measured by the Magnetospheric Multiscale (MMS) mission during the 2024-2025 ``string-of-pearls'' campaign. We show that Foreshock Compressive Structures (FCSs) initiate upstream of both shocks at similar normalized distances ($\lesssim$50 ion inertial lengths, $d_i$) when the suprathermal ($>10$ keV) ion density exceeds $\sim$1\% of the background. However, the IP shock lacks the fully evolved, high-amplitude Short Large Amplitude Magnetic Structures (SLAMS) characteristic of the terrestrial foreshock. We demonstrate that the ``growth zone'' capable of sustaining these structures is spatially limited ($\sim$135 $d_i$), which, due to the high speed of the propagating IP shock, corresponds to a brief observational window of $<10$ s. Beyond this observational constraint, we suggest an additional physical mechanism that can inhibit foreshock maturity at IP shocks. The lack of global curvature prevents the lateral supply (``cross-talk'') of energetic ions from different shock regions. These findings suggest that while the fundamental physics of FCS initiation is unified across collisionless shocks, the achievement of full nonlinearity can be regulated by the unique shock geometry and upstream properties, while ultimately remaining observationally challenging to identify.

physics.space-ph↗

arXiv2024

Extended Cyclotron Resonant Heating of the Turbulent Solar Wind

Circularly polarized, nearly parallel propagating waves are prevalent in the solar wind at ion-kinetic scales. At these scales, the spectrum of turbulent fluctuations in the solar wind steepens, often called the transition-range, before flattening at sub-ion scales. Circularly polarized waves have been proposed as a mechanism to couple electromagnetic fluctuations to ion gyromotion, enabling ion-scale dissipation that results in observed ion-scale steepening. Here, we study Parker Solar Probe observations of an extended stream of fast solar wind ranging from 15-55 solar radii. We demonstrate that, throughout the stream, transition-range steepening at ion-scales is associated with the presence of significant left handed ion-kinetic scale waves, which are thought to be ion-cyclotron waves. We implement quasilinear theory to compute the rate at which ions are heated via cyclotron resonance with the observed circularly polarized waves given the empirically measured proton velocity distribution functions. We apply the Von Karman decay law to estimate the turbulent decay of the large-scale fluctuations, which is equal to the turbulent energy cascade rate. We find that the ion-cyclotron heating rates are correlated with, and amount to a significant fraction of, the turbulent energy cascade rate, implying that cyclotron heating is an important dissipation mechanism in the solar wind.

astro-ph.SR↗

arXiv2023

Solar wind current sheets: MVA inaccuracy and recommended single-spacecraft methodology

We present the analysis of 2,033 current sheets (CS) observed aboard four Cluster spacecraft in a pristine solar wind. Four-spacecraft estimates of the CS normal and propagation velocity are compared with different single-spacecraft estimates. The Minimum Variance Analysis (MVA) of the magnetic field is shown to be highly inaccurate in estimating the normal. The MVA normal often differs by more than 60 degrees from the normal obtained by multi-spacecraft timing method, likely due to ambient turbulent fluctuations. In contrast, the cross-product of magnetic fields at the CS boundaries delivers the normal with the uncertainty of less than 15 degrees at the confidence level of 90%. The CSs are essentially frozen into plasma flow, since their propagation velocity is consistent with local ion flow velocity within 20% at the confidence level of 90%. The single-spacecraft methodology based on the cross-product method and frozen-in assumption delivers the CS thickness and current density amplitude within 20% of their actual values at the confidence level of 90%. The CSs are kinetic-scale structures with half-thickness $λ$ from a few tenths to tens of local proton inertial length $λ_{p}$ and scale-dependent shear angle and current density amplitude, $Δθ\propto (λ/λ_p)^{0.5}$ and $J_0\propto (λ/λ_{p})^{-0.5}$. The classification of the CSs in terms of tangential and rotational discontinuities remains a challenge, because even the four-spacecraft normal has too large uncertainties to reveal the actual normal magnetic field component. The presented results will be valuable for the analysis of solar wind CSs, when only single-spacecraft measurements are available.

physics.space-ph↗

arXiv2023

Particle-In-Cell Simulations of Sunward and Anti-sunward Whistler Waves in the Solar Wind

Spacecraft observations showed that electron heat conduction in the solar wind is probably regulated by whistler waves, whose origin and efficiency in electron heat flux suppression is actively investigated. In this paper, we present Particle-In-Cell simulations of a combined whistler heat flux and temperature anisotropy instability that can operate in the solar wind. The simulations are performed in a uniform plasma and initialized with core and halo electron populations typical of the solar wind. We demonstrate that the instability produces whistler waves propagating both along (anti-sunward) and opposite (sunward) to the electron heat flux. The saturated amplitudes of both sunward and anti-sunward whistler waves are strongly correlated with their {\it initial} linear growth rates, $B_{w}/B_0\sim (γ/ω_{ce})^ν$, where for typical electron betas we have $0.6\lesssim ν\lesssim 0.9$. The correlations of whistler wave amplitudes and spectral widths with plasma parameters (electron beta and temperature anisotropy) revealed in the simulations are consistent with those observed in the solar wind. The efficiency of electron heat flux suppression is positively correlated with the saturated amplitude of sunward whistler waves. The electron heat flux can be suppressed by 10--60% provided that the saturated amplitude of sunward whistler waves exceeds about 1% of background magnetic field. Other experimental applications of the presented results are discussed.

physics.space-ph↗

arXiv2022

Double layers in the Earth's bow shock

We present Magnetospheric Multiscale observations of electrostatic double layers in quasi-perpendicular Earth's bow shock. These double layers have predominantly parallel electric field with amplitudes up to 100 mV/m, spatial widths of 50-700 m, and plasma frame speeds within 100 km/s. The potential drop across a single double layer is 2-7% of the cross-shock potential in the de Hoffmann-Teller frame and occurs over the spatial scale of ten Debye lengths or one tenth of electron inertial length. Some double layers can have spatial width of 70 Debye lengths and potential drop up to 30% of the cross-shock potential. The electron temperature variation observed across double layers is roughly consistent with their potential drop. While electron heating in the Earth's bow shock occurs predominantly due to the quasi-static electric field in the de Hoffmann-Teller frame, these observations show that electron temperature can also increase across Debye-scale electrostatic structures.

physics.space-ph↗

arXiv2022

Electron-driven instabilities in the solar wind

The electrons are an essential particle species in the solar wind. They often exhibit non-equilibrium features in their velocity distribution function. These include temperature anisotropies, tails (kurtosis), and reflectional asymmetries (skewness), which contribute a significant heat flux to the solar wind. If these non-equilibrium features are sufficiently strong, they drive kinetic micro-instabilities. We develop a semi-graphical framework based on the equations of quasi-linear theory to describe electron-driven instabilities in the solar wind. We apply our framework to resonant instabilities driven by temperature anisotropies. These include the electron whistler anisotropy instability and the propagating electron firehose instability. We then describe resonant instabilities driven by reflectional asymmetries in the electron distribution function. These include the electron/ion-acoustic, kinetic Alfvén heat-flux, Langmuir, electron-beam, electron/ion-cyclotron, electron/electron-acoustic, whistler heat-flux, oblique fast-magnetosonic/whistler, lower-hybrid fan, and electron-deficit whistler instability. We briefly comment on non-resonant instabilities driven by electron temperature anisotropies such as the mirror-mode and the non-propagating firehose instability. We conclude our review with a list of open research topics in the field of electron-driven instabilities in the solar wind.

physics.space-ph↗

arXiv2021

Kinetic-scale current sheets in the solar wind at 1 AU: Scale-dependent properties and critical current density

We present analysis of 17,043 proton kinetic-scale current sheets collected over 124 days of Wind spacecraft measurements in the solar wind at 11 Samples/s magnetic field resolution. The current sheets have thickness $λ$ from a few tens to one thousand kilometers with typical value around 100 km or from about 0.1 to 10$λ_{p}$ in terms of local proton inertial length $λ_{p}$. We found that the current density is larger for smaller scale current sheets, $J_0\approx 6\; {\rm nA/m^2} \cdot (λ/100\;{\rm km})^{-0.56}$ , but does not statistically exceed critical value $J_A$ corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units, $J_0/J_{A}\approx 0.17\cdot (λ/λ_{p})^{-0.51}$. The current sheets are statistically force-free with magnetic shear angle correlated with current sheet spatial scale, $Δθ\approx 19^{\circ}\cdot (λ/λ_{p})^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale current sheets in the solar wind, while mechanisms limiting the current density remain to be understood.

physics.space-ph↗

arXiv2021

Solar wind discontinuity transformation at the bow shock

Solar wind plasma at the Earth's orbit carries transient magnetic field structures including discontinuities. Their interaction with the Earth's bow shock can significantly alter discontinuity configuration and stability. We investigate such an interaction for the most widespread type of solar wind discontinuities - rotational discontinuities (RDs). We use a set of in situ multispacecraft observations and perform kinetic hybrid simulations. We focus on the RD current density amplification that may lead to magnetic reconnection. We show that the amplification can be as high as two orders of magnitude and is mainly governed by three processes: the transverse magnetic field compression, global thinning of RD, and interaction of RD with low-frequency electromagnetic waves in the magnetosheath, downstream of the bow shock. The first factor is found to substantially exceed simple hydrodynamic predictions in most observed cases, the second effect has a rather moderate impact, while the third causes strong oscillations of the current density. We show that the presence of accelerated particles in the bow shock precursor highly boosts the current density amplification, making the postshock magnetic reconnection more probable. The pool of accelerated particles strongly affects the interaction of RDs with the Earth's bow shock, as it is demonstrated by observational data analysis and hybrid code simulations. Thus, shocks should be distinguished not by the inclination angle, but rather by the presence of foreshocks populated with shock reflected particles. Plasma processes in the RD-shock interaction affect magnetic structures and turbulence in the Earth's magnetosphere and may have implications for the processes in astrophysics.

physics.space-ph↗

arXiv2020

The dynamics of electron holes in current sheets

We present 1.5D Vlasov code simulations of the dynamics of electron holes in non-uniform magnetic and electric fields typical of current sheets and, particularly, of the Earth's magnetotail current sheet. The simulations show that spatial width and amplitude of electron holes do not substantially vary in the course of propagation, but there arises a double layer localized around the electron hole and manifested as a drop of the electrostatic potential along the electron hole. We demonstrate that electron holes produced around the neutral plane of a current sheet slow down in the course of propagation toward the current sheet boundaries. The leading contribution to electron hole braking is provided by the non-uniform magnetic field, though electrostatic fields typical of the current sheets do provide a noticeable contribution. The simulations also show that electron holes with larger amplitudes are slowed faster. The simulation results suggest that some of slow electron holes recently reported in the Earth's plasma sheet boundary layer may appear due to braking of initially fast electron holes in the course of propagation in the current sheet.

physics.plasm-ph↗

arXiv2019

Nonlinear evolution of the whistler heat flux instability

We use the one-dimensional TRISTAN-MP particle-in-cell code to model the nonlinear evolution of the whistler heat flux instability that was proposed by Gary et al. (1999, 2000) to regulate the electron heat flux in the solar wind and astrophysical plasmas. The simulations are initialized with electron velocity distribution functions typical for the solar wind. We perform a set of simulations at various initial values of the electron heat flux and $β_{e}$. The simulations show that parallel whistler waves produced by the whistler heat flux instability saturate at amplitudes consistent with the spacecraft measurements. The simulations also reproduce the correlations of the saturated whistler wave amplitude with the electron heat flux and $β_{e}$ revealed in the spacecraft measurements. The major result is that parallel whistler waves produced by the whistler heat flux instability do not significantly suppress the electron heat flux. The presented simulations indicate that coherent parallel whistler waves observed in the solar wind are unlikely to regulate the heat flux of solar wind electrons.

physics.space-ph↗

arXiv2019

Statistical Study of Whistler Waves in the Solar Wind at 1 AU

Whistler waves are intermittently present in the solar wind, while their origin and effects are not entirely understood. We present a statistical analysis of magnetic field fluctuations in the whistler frequency range (above 16 Hz) based on about 801,500 magnetic field spectra measured over three years aboard ARTEMIS spacecraft in the pristine solar wind. About 13,700 spectra (30 hours in total) with intense magnetic field fluctuations satisfy the interpretation in terms of quasi-parallel whistler waves. We provide estimates of the whistler wave occurrence probability, amplitudes, frequencies and bandwidths. The occurrence probability of whistler waves is shown to strongly depend on the electron temperature anisotropy. The whistler waves amplitudes are in the range from about 0.01 to 0.1 nT and typically below 0.02 of the background magnetic field. The frequencies of the whistler waves are shown to be below an upper bound that is dependent on $β_{e}$. The correlations established between the whistler wave properties and local macroscopic plasma parameters suggest that the observed whistler waves can be generated in local plasmas by the whistler heat flux instability. The whistler wave amplitudes are typically small, which questions the hypothesis that quasi-parallel whistler waves are capable to regulate the electron heat flux in the solar wind. We show that the observed whistler waves have sufficiently wide bandwidths and small amplitudes, so that effects of the whistler waves on electrons can be addressed in the frame of the quasi-linear theory.

physics.space-ph↗

arXiv2019

Whistler wave generation by halo electrons in the solar wind

We present an analysis of simultaneous particle and field measurements from the ARTEMIS spacecraft which demonstrate that quasi-parallel whistler waves in the solar wind can be generated locally by a bulk flow of halo electrons (whistler heat flux instability). ARTEMIS observes quasi-parallel whistler waves in the frequency range $\sim 0.05 - 0.2 f_{ce}$ simultaneously with electron velocity distribution functions that are a combination of counter-streaming core and halo populations. A linear stability analysis shows that the plasma is stable when there are no whistler waves, and unstable in the presence of whistler waves. In the latter case, the stability analysis shows that the whistler wave growth time is from a few to ten seconds at frequencies and wavenumbers that match the observations. The observations clearly demonstrate that the temperature anisotropy of halo electrons crucially affects the heat flux instability onset: a slight anisotropy $T_{\parallel}/T_{\perp}>1$ may quench the instability, while a slight anisotropy $T_{\parallel} / T_{\perp}<1$ may significantly increase the growth rate. These results demonstrate that heat flux inhibition is strongly dependent on the microscopic plasma properties.

physics.space-ph↗

arXiv2018

Electrostatic steepening of whistler waves

We present surprising observations by the NASA Van Allen Probes spacecraft of whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. The wave power at harmonics is due to nonlinearly steepened whistler electrostatic field that becomes possible in the two-temperature electron plasma due to whistler wave coupling to the electron-acoustic mode. The simulation and analytical estimates show that the steepening takes a few tens of milliseconds. The hydrodynamic energy cascade to higher frequencies facilitates efficient energy transfer from cyclotron resonant electrons, driving the whistler waves, to lower energy electrons.

physics.space-ph↗