Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStochastic ion and electron heating on drift instabilities at the bow shock
169
0
0.0
(
0
)
Added by
Krzysztof Stasiewicz
Publication date
2020
fields
Physics
and research's language is
English
Authors
Krzysztof Stasiewicz
Ask ChatGPT about the research
No Arabic abstract
The analysis of the wave content inside a perpendicular bow shock indicates that heating of ions is related to the Lower-Hybrid-Drift (LHD) instability, and heating of electrons to the Electron-Cyclotron-Drift (ECD) instability. Both processes represent stochastic acceleration caused by the electric field gradients on the electron gyroradius scales, produced by the two instabilities. Stochastic heating is a single particle mechanism where large gradients break adiabatic invariants and expose particles to direct acceleration by the DC- and wave-fields. The acceleration is controlled by function $chi = m_iq_i^{-1} B^{-2}$div($mathbf{E}$), which represents a general diagnostic tool for processes of energy transfer between electromagnetic fields and particles, and the measure of the local charge non-neutrality. The identification was made with multipoint measurements obtained from the Magnetospheric Multiscale spacecraft (MMS). The source for the LHD instability is the diamagnetic drift of ions, and for the ECD instability the source is ExB drift of electrons. The conclusions are supported by laboratory diagnostics of the ECD instability in Hall ion thrusters.
rate research
Read More
The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale (MMS) observations at Earths bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection.
Solar wind plasma at the Earths orbit carries transient magnetic field structures including discontinuities. Their interaction with the Earths 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 Earths 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 Earths magnetosphere and may have implications for the processes in astrophysics.
We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquists instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and He$^{2+}$ temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He$^{2+}$ components on stability. Of 309 randomly selected spectra from the Wind spacecraft, $53.7%$ are unstable when the ion components are modeled as drifting bi-Maxwellians; only $4.5%$ of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates $gamma$ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively-large He$^{2+}$ drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.
Based on in-situ measurements by Wind spacecraft from 2005 to 2015, this letter reports for the first time a clearly scale-dependent connection between proton temperatures and the turbulence in the solar wind. A statistical analysis of proton-scale turbulence shows that increasing helicity magnitudes correspond to steeper magnetic energy spectra. In particular, there exists a positive power-law correlation (with a slope $sim 0.4$) between the proton perpendicular temperature and the turbulent magnetic energy at scales $0.3 lesssim krho_p lesssim 1$, with $k$ being the wavenumber and $rho_p$ being the proton gyroradius. These findings present evidence of solar wind heating by the proton-scale turbulence. They also provide insight and observational constraint on the physics of turbulent dissipation in the solar wind.
The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating are not yet fully understood. We present for the first time a statistical test for one of the proposed mechanisms, stochastic ion heating. We use the amplitude of magnetic field fluctuations near the proton gyroscale as a proxy for the ratio of gyroscale velocity fluctuations to perpendicular (with respect to the magnetic field) proton thermal speed, defined as $epsilon_p$. Enhanced proton temperatures are observed when $epsilon_p$ is larger than a critical value ($sim 0.019 - 0.025$). This enhancement strongly depends on the proton plasma beta ($beta_{||p}$); when $beta_{||p} ll 1$ only the perpendicular proton temperature $T_{perp}$ increases, while for $beta_{||p} sim 1$ increased parallel and perpendicular proton temperatures are both observed. For $epsilon_p$ smaller than the critical value and $beta_{||p} ll 1$ no enhancement of $T_p$ is observed while for $beta_{||p} sim 1$ minor increases in $T_{parallel}$ are measured. The observed change of proton temperatures across a critical threshold for velocity fluctuations is in agreement with the stochastic ion heating model of Chandran et al. (2010). We find that $epsilon_p > epsilon_{rm crit}$ in 76% of the studied periods implying that stochastic heating may operate most of the time in the solar wind at 1 AU.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
