Nature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude

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.Comment: Accepted for publication in The Astrophysical Journal Letter

Magnetic Reconnection May Control the Ion-Scale Spectral Break of Solar Wind Turbulence

The power spectral density of magnetic fluctuations in the solar wind exhibits several power-law-like frequency ranges with a well defined break between approximately 0.1 and 1 Hz in the spacecraft frame. The exact dependence of this break scale on solar wind parameters has been extensively studied but is not yet fully understood. Recent studies have suggested that reconnection may induce a break in the spectrum at a "disruption scale" $\lambda_D$, which may be larger than the fundamental ion kinetic scales, producing an unusually steep spectrum just below the break. We present a statistical investigation of the dependence of the break scale on the proton gyroradius $\rho_i$, ion inertial length $d_i$, ion sound radius $\rho_s$, proton-cyclotron resonance scale $\rho_c$ and disruption scale $\lambda_D$ as a function of $\beta_{\perp i}$. We find that the steepest spectral indices of the dissipation range occur when $\beta_e$ is in the range of 0.1-1 and the break scale is only slightly larger than the ion sound scale (a situation occurring 41% of the time at 1 AU), in qualitative agreement with the reconnection model. In this range the break scale shows remarkably good correlation with $\lambda_D$. Our findings suggest that, at least at low $\beta_e$, reconnection may play an important role in the development of the dissipation range turbulent cascade and causes unusually steep (steeper than -3) spectral indices.Comment: Accepted in ApJ

Transition of Solar Wind Turbulence from MHD to Kinetic Scales

Turbulence is a ubiquitous process in space plasmas that could potentially explain the large temperatures in many astrophysical systems such as the solar corona and solar wind. Turbulent fluctuations of the magnetic field occur over a wide range of spatial scales, which are usually classified as the outer scale, magnetohydrodynamic (MHD) scale and kinetic scale (including ion and electron scales). The outer scale feeds energy into the turbulent cascade that is transferred through MHD scales without dissipation. At kinetic scales the fluctuations undergo a major transition: conservation of energy across scales breaks down, heating mechanisms start operating and the dispersion relation of fundamental wave modes change. In this dissertation we analyze emph{in situ} solar wind observations from Wind and Parker Solar Probe to characterize the physical mechanisms that operate in the turbulent cascade at the connection of MHD and kinetic scales. 1) We present the first statistical study on stochastic proton heating in the solar wind and identify the critical gyroscale turbulence amplitude when the first adiabatic invariant is violated and perpendicular heating takes places. Our results suggest that stochastic heating operates 76% of the time at 1 AU meaning that it has significant contribution to the non-adiabatic temperature profile of the solar wind. 2) The precise scale where MHD turbulence transitions into the kinetic range is a matter of considerable debate. Recent turbulence models suggested that current sheetlike structures form in the inertial range and get disrupted when the timescale of the tearing mode instability is shorter than the eddy turnover time. Our results suggest that these models can explain the ion-scale spectral break of the magnetic energy spectrum in 41% of the time. We also find that the disruption process may generate large amplitude ion-scale coherent structures. 3) Very little is known about the transition of proton velocity fluctuations from MHD to kinetic scales due to the scarcity of available measurements. We use a special operation mode of the Faraday Cup onboard Parker Solar Probe and develop a novel approach to study high frequency ($>1$ Hz) velocity fluctuations and their correlation with magnetic fields. Our results imply that the highly Alfv'{e}nic nature of the turbulence breaks down near the ion-scale spectral break potentially due to the demagnetization of protons and the onset of kinetic effects.PHDClimate and Space Sciences and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155255/1/dvech_1.pd

Large-scale Control of Kinetic Dissipation in the Solar Wind

In this Letter we study the connection between the large-scale dynamics of the turbulence cascade and particle heating on kinetic scales. We find that the inertial range turbulence amplitude ($\delta B_i$; measured in the range of 0.01-0.1 Hz) is a simple and effective proxy to identify the onset of significant ion heating and when it is combined with $\beta_{||p}$, it characterizes the energy partitioning between protons and electrons ($T_p/T_e$), proton temperature anisotropy ($T_{\perp}/T_{||}$) and scalar proton temperature ($T_p$) in a way that is consistent with previous predictions. For a fixed $\delta B_i$, the ratio of linear to nonlinear timescales is strongly correlated with the scalar proton temperature in agreement with Matthaeus et al., though for solar wind intervals with $\beta_{||p}>1$ some discrepancies are found. For a fixed $\beta_{||p}$, an increase of the turbulence amplitude leads to higher $T_p/T_e$ ratios, which is consistent with the models of Chandran et al. and Wu et al. We discuss the implications of these findings for our understanding of plasma turbulence.Comment: Accepted in ApJ

Prediction of plasmaspheric hiss spectral classes

We present a random forests machine learning model for prediction of plasmaspheric hiss spectral classes from the Van Allen Probes dataset. The random forests model provides accurate prediction of plasmaspheric hiss spectral classes obtained by the self organizing map (SOM) unsupervised machine learning classification technique. The high predictive skill of the random forests model is largely determined by the distinct and different locations of a given spectral class (&ldquo;no hiss&rdquo;, &ldquo;regular hiss&rdquo;, and &ldquo;low-frequency hiss&rdquo;) in (MLAT, MLT, L) coordinate space, which are the main predictors of the simplest and most accurate base model. Adding to such a base model any other single predictor among different magnetospheric, geomagnetic, and solar wind conditions provides only minor and similarly incremental improvements in predictive skill, which is comparable to the one obtained when including all possible predictors, and thus confirming major role of spatial location for accurate prediction. &nbsp;</p

Investigation of the Anisotropy of Solar Wind Turbulence with Multiple Point Spacecraft Observations

Validerat; 20160627 (global_studentproject_submitter

Space weather effects on the bow shock, the magnetic barrier, and the ion composition boundary at Venus

We present a statistical study on the interaction between interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus when the peak magnetic field of the magnetic barrier was anomalously large (>65nT). Based on the entire available Venus Express data set from April 2006 to October 2014, we selected 42 events and analyzed the solar wind parameters, the position of the bow shock, the size and plasma properties of the magnetic barrier, and the position of the ion composition boundary (ICB). It was found that the investigated ICMEs can be characterized with interplanetary shocks and unusually large tangential magnetic fields with respect to the Venus-Sun line. In most of the cases the position of the bow shock was not affected by the ICME. In a few cases the interaction between magnetic clouds and the induced magnetosphere of Venus was observed. During these events the small magnetosonic Mach numbers inside magnetic clouds caused the bow shock to appear at anomalously large distances fromthe planet. The positions of the upper and lower boundaries of the magnetic barrier were not affected by the ICMEs. The position of the ICB on the nightside was found closer to the planet during ICME passages which is attributed to the increased solar wind dynamic pressure. Key Points Statistical study of the ICME-Venus interaction Analysis of solar wind and magnetic barrier conditions during ICME passages Decreased altitude of the nightside ionosphere during ICME passages ©2015. American Geophysical Union. All Rights Reserved.Peer reviewe

Electrostatic Waves with Rapid Frequency Shifts in the Solar Wind Sunward of 1/3 AU

International audienceDuring its first five orbits, the FIELDS plasma wave investigation on board Parker Solar Probe (PSP) has observed a multitude of plasma waves, including electrostatic whistler and electron Bernstein waves (Malaspina et al. 2020), sunward propagating whistlers (Agapitov et al. 2020), ion-scale electromagnetic waves (Verniero et al. 2020, Bowen et al. 2020) and Alfven, slow and fast mode waves (Chaston et al. 2020).The importance of these waves lies in their potential to redistribute the energy of the solar wind among different particles species (wave-particle interactions) or different types of waves (wave-wave interactions). The abundance of waves and instabilities observed with PSP points to their central role in the regulation of this energy exchange.Here we present first observations of an intermittent, electrostatic and broadband plasma wave that is ubiquitous in the range of distances that PSP has probed so far. A unique feature of these waves (FDWs) is a frequency shift that occurs on millisecond timescales. In the frame of the spacecraft, FDWs usually appear between the electron cyclotron and electron plasma frequencies.We develop a detection algorithm that identifies the FDWs in low cadence spectra. We analyze them using various statistical techniques. We establish their phenomenology and compare the magnetic fluctuations of the background magnetic field at times of FDWs and at times without FDWs. We establish their polarization with respect to the background magnetic field and search for correlations with various plasma parameters and features in the electron, proton and alpha particle distribution moments. We also investigate possible plasma wave modes that could be responsible for the growth of FDWs and the instability mechanisms that could be generating them. Lily Kromyda*(1), David M. Malaspina (1,2), Robert E. Ergun(1,2) , Jasper Halekas(3), Michael L. Stevens(4) , Jennifer Verniero(5), Alexandros Chasapis(2) , Daniel Vech(2) , Stuart D. Bale(5,6) , John W. Bonnell(5) , Thierry Dudok de Wit(7) , Keith Goetz(8) , Katherine Goodrich(5) , Peter R. Harvey(5) , Robert J. MacDowall(9) , Marc Pulupa(5) , Anthony W. Case(4) , Justin C. Kasper(10) , Kelly E. Korreck(4) , Davin Larson(5) , Roberto Livi(5) , Phyllis Whittlesey(5)(1) Astrophysical and Planetary Sciences Department, University of Colorado, Boulder, CO, USA(2) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA(3) University of Iowa, Iowa City, IA, USA(4) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA(5) Space Sciences Laboratory, University of California, Berkeley, CA, USA(6) Physics Department, University of California, Berkeley, CA, USA(7) LPC2E, CNRS, and University of Orleans, Orleans, France(8) School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA(9) NASA Goddard Space Flight Center, Greenbelt, MD, USA(10) University of Michigan, Ann Arbor, MI, US

Measures of Scale-dependent Alfvénicity in the First PSP Solar Encounter

International audienceThe solar wind shows periods of highly Alfvénic activity, where velocity fluctuations and magnetic fluctuations are aligned or antialigned with each other. It is generally agreed that solar wind plasma velocity and magnetic field fluctuations observed by the Parker Solar Probe (PSP) during the first encounter are mostly highly Alfvénic. However, quantitative measures of Alfvénicity are needed to understand how the characterization of these fluctuations compares with standard measures from prior missions in the inner and outer heliosphere, in fast wind and slow wind, and at high and low latitudes. To investigate this issue, we employ several measures to quantify the extent of Alfvénicity—the Alfvén ratio rA, the normalized cross helicity σc , the normalized residual energy σr , and the cosine of angle between velocity and magnetic fluctuations $\cos {\theta }_{{vb}}$. We show that despite the overall impression that the Alfvénicity is large in the solar wind sampled by PSP during the first encounter, during some intervals the cross helicity starts decreasing at very large scales. These length scales (often >1000di ) are well inside inertial range, and therefore, the suppression of cross helicity at these scales cannot be attributed to kinetic physics. This drop at large scales could potentially be explained by large scale shears present in the inner heliosphere sampled by PSP. In some cases, despite the cross helicity being constant down to the noise floor, the residual energy decreases with scale in the inertial range. These results suggest that it is important to consider all these measures to quantify Alfvénicity