arXiv subjects

I. Y. Vasko

Publications and source records attributed to I. Y. Vasko.

10 recordsLinked to original sources

arXiv2024

Resonance of low-frequency electromagnetic and ion-sound modes in the solar wind

Parker Solar Probe measurements have recently shown that coherent fast magnetosonic and Alfvén ion-cyclotron waves are abundant in the solar wind and can be accompanied by higher-frequency electrostatic fluctuations. In this letter we reveal the nonlinear process capable of channelling the energy of low-frequency electromagnetic to higher-frequency electrostatic fluctuations observed aboard Parker Solar Probe. We present Hall-MHD simulations demonstrating that low-frequency electromagnetic fluctuations can resonate with the ion-sound mode, which results in steepening of plasma density fluctuations, electrostatic spikes and harmonics in the electric field spectrum. The resonance can occur around the wavenumber determined by the ratio between local sound and Alfvén speeds, but only in the case of {\it oblique} propagation to the background magnetic field. The resonance wavenumber, its width and steepening time scale are estimated, and all indicate that the revealed two-wave resonance can frequently occur in the solar wind. This process can be a potential channel of energy transfer from cyclotron resonant ions producing the electromagnetic fluctuations to Landau resonant ions and electrons absorbing the energy of the higher-frequency electrostatic fluctuations.

astro-ph.SR↗

arXiv2024

Slow electron holes in the Earth's magnetosheath

We present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale spacecraft in the Earth's magnetosheath. Applying single-spacecraft interferometry to several hundred solitary waves collected in about two minute intervals, we show that almost all of them have the electrostatic potential of positive polarity and propagate quasi-parallel to the local magnetic field with plasma frame velocities of the order of 100 km/s. The solitary waves have typical parallel half-widths from 10 to 100 m that is between 1 and 10 Debye lengths and typical amplitudes of the electrostatic potential from 10 to 200 mV that is between 0.01 and 1\% of local electron temperature. The solitary waves are associated with quasi-Maxwellian ion velocity distribution functions, and their plasma frame velocities are comparable with ion thermal speed and well below electron thermal speed. We argue that the solitary waves of positive polarity are slow electron holes and estimate the time scale of their acceleration, which occurs due to interaction with ions, to be of the order of one second. The observation of slow electron holes indicates that their lifetime was shorter than the acceleration time scale. We argue that multi-spacecraft interferometry applied previously to these solitary waves is not applicable because of their too-short spatial scales. The source of the slow electron holes and the role in electron-ion energy exchange remain to be established.

physics.space-ph↗

arXiv2023

Density Enhancement Streams in The Solar Wind

This letter describes a new phenomenon on the Parker Solar Probe of recurring plasma density enhancements that have $Δ$n/n ~10% and that occur at a repetition rate of ~5 Hz. They were observed sporadically for about five hours between 14 and 15 solar radii on Parker Solar Probe orbit 12 and they were also seen in the same radial range on both the inbound and outbound orbits 11. Their apparently steady-state existence suggests that their pressure gradient was balanced by the electric field. The EX electric field component produced from this requirement is in good agreement with that measured. This provides strong evidence for the measurement accuracy of the density fluctuations and the X- and Y-components of the electric field (the Z-component was not measured). The electrostatic density waves were accompanied by an electromagnetic low frequency wave which occurred with the electrostatic harmonics. The amplitudes of these electrostatic and electromagnetic waves at $\ge$ 1 Hz were greater than the amplitude of the Alfvenic turbulence in their vicinity so they can be important for the heating, scattering, and acceleration of the plasma. The existence of this pair of waves is consistent with the observed plasma distributions and is explained by a magneto-acoustic wave theory that produces a low frequency electromagnetic wave and electrostatic harmonics.

astro-ph.SR↗

arXiv2022

Kinetic-scale current sheets in near-Sun solar wind: properties, scale-dependent features and reconnection onset

We present statistical analysis of 11,200 proton kinetic-scale current sheets (CS) observed by Parker Solar Probe during 10 days around the first perihelion. The CS thickness $λ$ is in the range from a few to 200 km with the typical value around 30 km, while current densities are in the range from 0.1 to 10\;$μ{\rm A/m^2}$ with the typical value around 0.7\;$μ{\rm A/m^2}$. These CSs are resolved thanks to magnetic field measurements at 73--290 Samples/s resolution. In terms of proton inertial length $λ_{p}$, the CS thickness $λ$ is in the range from about $0.1$ to $10λ_{p}$ with the typical value around 2$λ_{p}$. The magnetic field magnitude does not substantially vary across the CSs and, accordingly, the current density is dominated by the magnetic field-aligned component. The CSs are typically asymmetric with statistically different magnetic field magnitudes at the CS boundaries. The current density is larger for smaller-scale CSs, $J_0\approx 0.15 \cdot (λ/100\;{\rm km})^{-0.76}$ $μ{\rm A/m^2}$, but does not statistically exceed the Alfvén current density $J_A$ corresponding to the ion-electron drift of local Alfvén speed. The CSs exhibit remarkable scale-dependent current density and magnetic shear angles, $J_0/J_{A}\approx 0.17\cdot (λ/λ_{p})^{-0.67}$ and $Δθ\approx 21^{\circ}\cdot (λ/λ_{p})^{0.32}$. Based on these observations and comparison to recent studies at 1 AU, we conclude that proton kinetic-scale CSs in the near-Sun solar wind are produced by turbulence cascade and they are automatically in the parameter range, where reconnection is not suppressed by the diamagnetic mechanism, due to their geometry dictated by turbulence cascade.

astro-ph.SR↗

arXiv2022

Core Electron Heating By Triggered And Ordinary Ion Acoustic Waves In The Solar Wind

Orbits six through nine of the Parker Solar Probe have been studied to show that solar wind core electrons emerged from 15 solar radii with temperatures that were constant to within ~10% although the solar wind speed varied from 300 to 800 km/sec. After leaving 15 solar radii, the core electrons were isotropically heated as much as a factor of two below 30 solar radii by triggered and natural ion acoustic waves. To distinguish this wave heating from processes occurring inside 15 solar radii, the electron temperature is modeled as having two components, the base temperature observed at 15 solar radii and the temperature resulting from wave interactions between 15 solar radii and the observing point. As further justification of this temperature model, there were about a dozen intervals during the four orbits, summing to about 250 hours of total time, during which there were few waves, TE/TI was small, and the core electron temperature was close to or slightly greater than the base temperature. Both this base temperature and the temperature resulting from wave heating decreased with radius due to the adiabatic expansion of the solar wind. The waves that produced the electron core heating were triggered [Mozer et al, 2021] or normal {Mozer et al, 2020A] ion acoustic waves. They are the dominant wave modes at frequencies greater than 100 Hz at solar distances between 15 and 30 solar radii.

astro-ph.SR↗

arXiv2021

Electrostatic solitary waves in the Earth's bow shock: nature, properties, lifetimes and origin

We present a statistical analysis of more than two thousand bipolar electrostatic solitary waves (ESW) collected from ten quasi-perpendicular Earth's bow shock crossings by Magnetospheric Multiscale spacecraft. We developed and implemented a correction procedure for reconstruction of actual electric fields, velocities, and other properties of ESW from measurements, whose spatial scales are typically comparable with or smaller than spatial distance between voltage-sensitive probes. We determined the optimal ratio between frequency response factors of axial and spin plane antennas to be around 1.65/1.8. We found that more than 95\% of the ESW in the Earth's bow shock are of negative polarity and present an in depth analysis of properties of these ESW. They have spatial scales of about 10--100 m that is within a range of $λ_{D}$ to $10λ_{D}$, amplitudes typically below a few Volts that is below 0.1 of local electron temperature, and velocities below a few hundreds km/s in spacecraft and plasma rest frames that is on the order of local ion-acoustic speed. The spatial scales of ESW are distinctly correlated with local Debye length $λ_{D}$. ESW with amplitudes of 5--30 V or 0.1--0.3 Te have the occurrence rate of a few percent. The ESW have electric fields generally oblique to local magnetic field and propagate highly oblique to shock normal ${\bf N}$; more than 80\% of ESW propagate within 30$^{\circ}$ of the shock plane. In the shock plane, ESW typically propagate within a few tens of degrees of local magnetic field projection ${\bf B}_{\rm LM}$ onto the shock plane and preferentially opposite to ${\bf N}\times {\bf B}_{\rm LM}$. We argue that the ESW of negative polarity are ion phase space holes produced in a nonlinear stage of ion-ion ion-streaming instabilities. We estimated lifetimes of the ion holes to be 10--100 ms, or 1--10 km in terms of spatial distance.

physics.space-ph↗

arXiv2020

On quasi-parallel whistler waves in the solar wind

The recent simulations showed that the whistler heat flux instability, which presumably produces the most of quasi-parallel coherent whistler waves in the solar wind, is not efficient in regulating the electron heat conduction. In addition, recent spacecraft measurements indicated that some fraction of coherent whistler waves in the solar wind may propagate anti-parallel to the electron heat flux, being produced due to a perpendicular temperature anisotropy of suprathermal electrons. We present analysis of properties of parallel and anti-parallel whistler waves unstable at electron heat fluxes and temperature anisotropies of suprathermal electrons typical of the pristine solar wind. Assuming the electron population consisting of counter-streaming dense thermal core and tenuous suprathermal halo populations, we perform a linear stability analysis to demonstrate that anti-parallel whistler waves are expected to have smaller frequencies, wave numbers and growth rates compared to parallel whistler waves. The stability analysis is performed over a wide range of parameters of core and halo electron populations. Using the quasi-linear scaling relation we show that anti-parallel whistler waves saturate at amplitudes of one order of magnitude smaller than parallel whistler waves, which is at about $10^{-3}\;B_0$ in the pristine solar wind. The analysis shows that the presence of anti-parallel whistler waves in the pristine solar wind is more likely to be obscured by turbulent magnetic field fluctuations, because of lower frequencies and smaller amplitudes compared to parallel whistler waves. The presented results will be also valuable for numerical simulations of the electron heat flux regulation in the solar wind.

physics.plasm-ph↗

arXiv2020

Shock Drift Acceleration of Ions in an Interplanetary Shock Observed by MMS

An interplanetary (IP) shock wave was recorded crossing the Magnetospheric Multiscale (MMS) constellation on 2018 January 8. Plasma measurements upstream of the shock indicate efficient proton acceleration in the IP shock ramp: 2-7 keV protons are observed upstream for about three minutes (~8000 km) ahead of the IP shock ramp, outrunning the upstream waves. The differential energy flux (DEF) of 2-7 keV protons decays slowly with distance from the ramp towards the upstream region (dropping by about half within 8 Earth radii from the ramp) and is lessened by a factor of about four in the downstream compared to the ramp (within a distance comparable to the gyroradius of ~keV protons). Comparison with test-particle simulations has confirmed that the mechanism accelerating the solar wind protons and injecting them upstream is classical shock drift acceleration. This example of observed proton acceleration by a low-Mach, quasi-perpendicular shock may be applicable to astrophysical contexts, such as supernova remnants or the acceleration of cosmic rays.

physics.space-ph↗

arXiv2020

Kinetic models of tangential discontinuities in the solar wind

Kinetic-scale current sheets observed in the solar wind are frequently approximately force-free despite the fact that their plasma $β$ is of the order of one. In-situ measurements have recently shown that plasma density and temperature often vary across the current sheets, while the plasma pressure is approximately uniform. In many cases these density and temperature variations are asymmetric with respect to the center of the current sheet. To model these observations theoretically we develop in this paper equilibria of kinetic-scale force-free current sheets that have plasma density and temperature gradients. The models can also be useful for analysis of stability and dissipation of the current sheets in the solar wind.

physics.space-ph↗

arXiv2019

Electrostatic turbulence and Debye-scale structures in collisionless shocks

We present analysis of more than one hundred large-amplitude bipolar electrostatic structures in a quasi-perpendicular supercritical Earth's bow shock crossing, measured by the Magnetospheric Multiscale spacecraft. The occurrence of the bipolar structures is shown to be tightly correlated with magnetic field gradients in the shock transition region. The bipolar structures have negative electrostatic potentials and spatial scales of a few Debye lengths. The bipolar structures propagate highly oblique to the shock normal with velocities (in the plasma rest frame) of the order of the ion-acoustic velocity. We argue that the bipolar structures are ion phase space holes produced by the two-stream instability between incoming and reflected ions. This is the first identification of the ion two-stream instability in collisionless shocks. The implications for electron acceleration are discussed.

astro-ph.SR↗