Session BI01: Invited: Parker Solar Probe and Distribution Functions

Live

A primary goal of the Parker Solar Probe Mission is to determine the heating mechanism that accounts for the very high temperature of the plasma in the solar corona. Various heating mechanisms have been suggested but one that is gaining increasing credence is associated with the dissipation of low frequency magnetohyrodynamic (MHD) turbulence. However, the MHD turbulence models come in several flavors. Two basic MHD turbulence transport models have been developed, one in which outwardly propagating Alfven waves experience reflection from the large-scale flow and density gradients associated with the solar corona, and the resulting counterpropagating Alfven waves couple nonlinearly to produce quasi-2D turbulence that dissipates and heats the corona. The second approach eschews a dominant outward flux of Alfven waves but argues instead that quasi-2D turbulence dominates the lower coronal plasma, is generated in the constantly upwelling magnetic carpet, experiencing dissipation as it is advected through the corona, leading to temperatures in the corona that exceed a million degrees. We will review the two theoretical turbulence models, describe the basic modeling that has been done, and describe tests of both models against Parker Solar Probe observations.   

Live

Perhaps the most striking observation made by Parker Solar Probe during its first few orbits is that of the prevalence of extremely large amplitude oscillations in the radial magnetic field, leading to reversals in sign not connected to crossings of the heliospheric current sheet but rather to kinks of the field line themselves, as demonstrated by the permanence of the electron pitch angle. Such rapid folds in the field, also called switchbacks, are seen with periods going from seconds to more than an hour, while an analysis of the corresponding velocity field shows that the fluctuations in radial velocity, $\delta V_r$ are well correlated to those of the radial magnetic field, with a correlation with $\delta B_r$ corresponding to Alfv\'en waves propagating away from the Sun. Switchbacks however belong to a well - developed power spectrum, so the appropriate description is that of Alfv\'enic turbulence. In addition, the magnitude of the total magnetic field often remains almost constant, i.e., the compressibility of the fluctuations is very small. The present talk will discuss these intriguing Probe observations, including the prevalence of high velocity magnetic field correlation even in extremely low speed wind, to suggest scenarios for the origin and evolution of such fluctuations in the solar wind, and their potential role in coronal heating and solar wind acceleration.   

Live

Type III radio bursts have long been used as remote probes of electron acceleration in the solar corona. Most studies of periodic and quasiperiodic behavior of radiation from the sun have focused on large events -- large flares or coronal mass ejections. The occurrence of periodic behavior in interplanetary Type III bursts observed by Parker Solar Probe when there are no observable flares provides a unique opportunity to diagnose small-scale acceleration of electrons in the corona. Because these events are not associated with observable flares, the acceleration process must be associated either with small nanoflares, or with some other mechanism such as kinetic Alfven waves. The correlated periodicities in coronal EUV lines suggest that these small scale processes may be important for coronal heating. We focus on coordinated observations of Type III radio bursts from the Fields instrument on Parker Solar Probe, of EUV emissions in the 211A and 171A bands by the SDO/AIA, and of solar flare x-rays by Nuclear Spectroscopic Telescope Array (NuSTAR) on April 12, 2019. Periodicities of \textasciitilde 5 minutes in the EUV in several areas of an active region are well correlated with the repetition rate of the Type III radio bursts. NuSTAR x-rays provide evidence for a flare during the interval of Type III bursts, but there is not a 1-1 correspondence between the x-rays and the Type-III bursts. Collaborators: L. Glesener, B. Leiran, PSP Fields Team   

Live

In collisionless and weakly collisional plasmas, the particle distribution function is a rich tapestry of the underlying physics. However, actually leveraging the particle distribution function to understand the dynamics of a weakly collisional plasma is challenging. The equation system of relevance, the Vlasov-Maxwell system of equations, is difficult to numerically integrate, and traditional methods such as the particle-in-cell method introduce counting noise into the distribution function. Motivated by the physics contained in the distribution function, we have implemented a novel continuum Vlasov-Maxwell method in the Gkeyll simulation framework. I will present results of simulations of collisionless shocks which utilize both the high fidelity representation of the particle distribution function provided to us by a continuum method, and modern phase space diagnostics such as the field-particle correlation, to ascertain the details of the energy exchange via these collisionless shocks. These results, in addition to providing insight into the phase space structure observed in the ever higher quality distribution function data from spacecraft missions such as the Magnetospheric Multiscale mission, highlight the strength of this complementary, noise-free, approach to kinetic simulations.   

Live

Electron velocity distribution functions (EDF's) are the foundation of most plasma physics. Here we present the first measurements of the complete EDF without any assumptions on its shape or the underlying physics that generated it. This first-principle measurement showed that during significant inverse bremsstrahlung heating by laser beams, the bulk of the distributions were measured to be super-Gaussian in shape, while the electrons above 3\texttimes the thermal velocity were distributed according to Maxwell--Boltzmann (Maxwellian) statistics. When the inverse bremsstrahlung rates were negligible, Maxwell EDF's were measured. To enable single-shot temporally and spatially resolved measurements, an optical diagnostic was invented that uses the collective nature of plasmas and the angular dependence of the scattering to enable the EDF to be determined over several orders of magnitude without any assumptions on its form. This novel Thomson-scattering technique encoded the electron motion to the frequency of scattered light and used collective scattering to increase the signal at velocities where the number of electrons are limited. The ability to measure the EDF beyond 4\texttimes the thermal velocity with high precision allowed the results to address long-standing physics questions regarding the relaxation of high-velocity electrons toward a Maxwellian. These results are in excellent agreement with pioneering computational work by J. P. Matte \textit{et al.}\footnote{ J. P. Matte \textit{et al.}, Plasma Phys. Control. Fusion \textbf{30}, 1665 (1988).} that describe the evolution of the bulk electrons to a non-Maxwellian distribution due to inverse bremsstrahlung heating. The response of high-velocity (\textgreater 3$v_{\mathrm{th}})$ electrons was compared with Fourkal \textit{et al.},\footnote{ E. Fourkal \textit{et al.}, Phys. Plasmas \textbf{8}, 550 (2001)} which suggested electron--electron collisions dictate the shape of the tail while the isotropy of the electric field dictates the amplitude of the tail. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

A longstanding conundrum in magnetic reconnection is its accompaniment by anomalous ion heating much faster than conventional collisional heating. It is well known that in-plane Hall electric fields that inherently develop during collisionless magnetic reconnection are associated with ion energization, but the exact mechanism behind the observed ion heating has not been clear. It is first shown via a zero-beta, two-fluid analysis that the Hall electric fields intrinsically satisfy the criterion for ``stochastic heating'' of ions --- a process in which sufficiently strong electric fields chaotize particle orbits and effectively heat the particles. Second, a kinetic calculation is presented which illustrates that stochastic ion heating is also intrinsic to the Harris equilibrium. The spatial extent of the heating is found to decrease as the equilibrium thickness and/or the guide field increases, and above a threshold guide field, stochastic heating is not predicted. Third, the mechanism is verified by particle-in-cell simulations of collisionless reconnection. Extreme ion heating is observed at the regions where stochastic heating is predicted. In line with the kinetic calculation, both the predicted and the observed heating extent decreases as the guide field increases, and at a certain guide field strength, extreme ion heating disappears. Test-particle simulations show that ions that satisfy the stochastic heating criterion indeed undergo chaotic motion, as evidenced by finite Lyapunov exponents and violation of orbit adiabatic invariance. The present work shows that Hall electric fields intrinsic to collisionless reconnection intrinsically cause stochastic heating, thus establishing it as the generic ion heating mechanism.