Session TO06: Heliospheric Physics

Characterizing velocity space structures of ion cyclotron turbulence in the Earth's magnetosheath plasma

Understanding and characterizing the dissipation of turbulent energy in fusion, space, and astrophysical plasmas is of fundamental importance for the respective environments in which turbulence plays a role in transferring energy from large to small scales. We apply the novel field-particle correlation technique to electromagnetic field and plasma particle data from the Magnetospheric Multiscale mission while the spacecraft were located in the Earth's magnetosheath. Using the field-particle correlation technique, through distinct velocity-space signatures, we identify which dissipation mechanisms are presently removing energy from the turbulent field fluctuations and consequently energizing the plasma particles. Characteristic signatures of ion cyclotron damping are identified and presented as one channel of the dissipation of turbulent energy mediated by the perpendicular electric field fluctuations. We further quantify this energization rate and compare it to the turbulent energy cascade rate to find the percentage of energy dissipated via this channel.   

Radial evolution of switchbacks in the inner heliosphere: observations from PSP to Ulysses

Measurements from Parker Solar Probe have shown the ubiquitous presence of the so-called switchbacks. These are magnetic field lines which are strongly perturbed to the point that they lead to local inversions of the radial magnetic field. The corresponding signature in the velocity field is that of a local radial speed jet displaying the well-known velocity/magnetic field correlation that characterizes Alfvén waves propagating away from the Sun. While there is not yet a general consensus on the origins of switchbacks and their connection to coronal activity, a first necessary step is to understand how they evolve and how long they can propagate undisturbed in the solar wind. Characterizing the dynamical evolution of switchbacks in the solar wind can help us determine whether they are generated in-situ or not, and whether they contribute to the turbulent cascade by evolving nonlinearly. In this work, we have analyzed magnetic field data from the first six encounters of Parker Solar Probe, three fast streams observed by Helios 1 and 2, and two Ulysses south polar passes, covering the range of heliocentric distances 0.1 < R < 3 au. We have compared the radial evolution of the magnetic energy density of switchbacks with that of the overall turbulent fluctuations, and we have characterized the radial evolution of the occurrence rate of switchbacks as a function of their duration. Our results show that switchbacks both decay and reform in-situ in the inner heliosphere, in-situ generation being more efficient at the larger scales. Our results confirm that switchbacks can be generated in the inner heliosphere by the expansion, although other types of switchbacks, generated closer to the sun, cannot be ruled out.   

Ion-Driven Instabilities as Observed by Helios

Instabilities described by linear theory have been studied for several decades as an essential part of the kinetic description of the solar wind. We diagnose unstable behavior of solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it to fits of 1.5M proton and alpha particle Velocity Distribution Functions (VDFs) observed by Helios I and II. The variation of the fraction of unstable intervals with radial distance from the Sun is linear, signaling a gradual decline in the activity of unstable modes. When calculated as functions of the solar wind velocity and Coulomb number, we obtain more extreme, exponential trends in the regions where collisions appear to have a notable influence on the VDF. Instability growth rates demonstrate similar behavior, and significantly decrease with Coulomb number. We identify three separate regimes with respect to collisions and their role in regulating instabilities in the solar wind: 1) the collisionless regime, where rapidly growing instabilities dictate the limits of plasma parameters, 2) the transition regime where collisions drive the plasma towards Local Thermodynamic Equilibrium (LTE), significantly decreasing unstable behavior and 3) the collisionally processed regime, where plasma is close to LTE, but occasionally driven towards marginal stability states via local plasma processes. We demonstrate that the observed limits on solar wind plasma parameters are imposed by the interplay between multiple kinds of instabilities and collisions, with the relative importance of these mechanisms varying as the plasma travels further from the Sun.   

Preliminary Results for Experiment at WiPPL

The Sun, being an active star, undergoes eruptions of magnetic fields and charged

particles that reach the Earth and cause the aurora near the poles. Some eruptions may

be more powerful than others, resulting in an Interplanetary Coronal Mass Ejection

(ICME) that can cause major damage to our modern electrical systems without warning.

We want to form a better understanding of how the ICMEs interact to create a more

predictive model for them to more effectively defend our systems from a potentially

devastating ICME. To explore solar phenomena, we use the Big Red Ball (BRB) facility

at the Wisconsin Plasma Physics Laboratory (WiPPL). The BRB is a 3-meter diameter

plasma confinement system equipped with around 200 ports for diagnostic acess. Using

the BRB, we can explore the interactions between an ICME and the Earth's field by

using a spherical permanent magnet as the analog to the Earth, a light background

plasma as the analog to the interplanetary medium (IM), and a compact torus (CT) of

plasma as the ICME. My presentation will detail the desired parameters as well as

describe preliminary results of the upcoming experiment.   

Laboratory nano-flares generated from multiple braided current loops

Braided loop structures are observed in a new Caltech lab experiment replicating solar coronal loops. A single strand of a braided magnetic structure is observed to undergo a magnetic Rayleigh-Taylor Instability (MRTI) driven by the effective gravity of hoop force acceleration.  A burst of keV X-rays and a ~ 4 kV inductive voltage spike are detected simultaneously with the MRTI. These observations reveal a clear MHD to non-MHD path for generating solar energetic particles and X-ray bursts: a solar flux rope is a braid of current-carrying fractal-scale flux ropes with the finest braid strand being larger than ion skin depth d_i or β^-1/2 d_i.  Current path curvature creates a hoop force accelerating strands creating a large effective gravity that can cause a fast-growing MRTI to choke the strand to less than d_i or β^-1/2 d_i. This choking is presumed to cause kinetic instabilities that increase the local effective resistivity. This effective opening switch abruptly reduces the current in the strand and large-scale exterior circuit inductive energy dumps into the increased resistivity region. The inductive voltage spike LdI/dt accelerates charged particles to extreme energies, and electron bremsstrahlung produces X-ray bursts.   

Rapid plasma bursts and lingering debris clouds driven by hypervelocity dust impacts on Parker Solar Probe: an unintentional active experiment in the inner heliosphere

As the Parker Solar Probe spacecraft passes near the Sun, it encounters dense populations of interplanetary dust at high velocities.  When dust grains impact the spacecraft at these speeds, the impactor is vaporized and ionized, along with a fraction of the spacecraft surface material.  This creates a plume of rapidly expanding plasma.  Some of the impactors encountered by Parker Solar Probe are relatively large, resulting in plasma plumes dense enough to (i) refract natural plasma waves away from the spacecraft, (ii) produce transient magnetic signatures, (iii) and drive plasma waves during plume expansion.  Further, some impacts liberate clouds of macroscopic spacecraft material which can result in electrostatic disturbances near the spacecraft that can linger for up to a minute, which is ~10,000 times longer than the transient plasma plume.  In this presentation, we discuss the characteristics and statistics of debris-producing events, evidence supporting their origin from dust impacts, plasma plume expansion, and the electromagnetic processes by which both the plasma and macroscopic impact products disturb the spacecraft environment.   

Laboratory study of carbon ablation in Jupiter-like heating environment

Here we report on experiments where ablation of carbon-based materials (typically used for spacecraft heat shields) was studied at high heating environments available in the DIII-D tokamak. Due to inherent properties of the tokamak plasma (e.g., high temperature, rotation, and fast flows), the heat flux deposited to the material samples is comparable to that experienced by the Galileo probe during its entry into Jupiter’s atmosphere. In this presentation, we discuss scaling between laboratory and space conditions, specifics of the experimental design, and calculations of material ablation as a function of incident heat flux for three types of carbon: porous, glassy, and ATJ graphite. The validity of several analytical ablation models available in the space community is tested against mass loss rates recorded in the experiments. The experimental results are further compared to a numerical model of meteor ablation adapted for entry into Jupiter’s atmosphere, which allows for scaling between laboratory and space conditions. We find that the predictions from the analytical ablation models are reasonably consistent with experimental measurements of material mass loss rates.

    

Not Participating

Results obtained by adopting an electron velocity distribution function that faithfully reproduces those recently observed during the PSP Encounter 1, and following its evolution with heliocentric distance, will be presented. They were obtained with the purpose of shedding light on the kinetic processes that influence the electron dynamics during solar wind expansion from the inner heliosphere onward. In order to do so, we carried out a two spatial dimensions, three velocity components (2D3V) fully kinetic Expanding Box Model simulation through which we could monitor the nonlinear development of kinetic instabilities and study their effects on the solar wind electrons. We observe that the expansion drives the solar wind into successive regimes where whistler heat flux instabilities are triggered. These instabilities produce sunward whistler waves initially characterized by predominantly oblique propagation with respect to the interplanetary magnetic field. The excited waves interact with the electrons via resonant scattering processes. As a consequence, strahl electrons are scattered in the direction perpendicular to the magnetic field, and an electron halo is formed. Wave-particle interaction processes are accompanied by a substantial reduction of the solar wind heat flux.    

Stability of superthermal strahl electrons in the solar wind

We present a kinetic stability analysis of the solar wind electron distribution function consisting of the Maxwellian core and the magnetic-field aligned strahl, a superthermal electron beam propagating away from the sun (Schroeder et. al. 2021, submitted to MNRAS). We use an electron strahl distribution obtained as a solution of a weakly collisional drift-kinetic equation in Boldyrev & Horaites 2019, representative of a strahl affected by Coulomb collisions but not by possible broadening from turbulence. This distribution function is essentially non-Maxwellian and varies with the heliospheric distance. The stability analysis is performed with the linear Vlasov-Maxwell solver LEOPARD (Astfalk & Jenko 2017). We find that depending on the heliospheric distance, the core-strahl electron distribution becomes unstable with respect to sunward-propagating kinetic-Alfvén, magnetosonic, and whistler modes, in a broad range of propagation angles. The wavenumbers of the unstable modes are close to the ion inertial scales, and the radial distances at which the instabilities first appear are on the order of 1 AU. However, we have not detected any instabilities driven by resonant wave interactions with the superthermal strahl electrons. Instead, the observed instabilities are triggered by a relative drift between the electron and ion cores necessary to maintain zero electric current in the solar wind frame, indicating that the electron strahl obtained as a solution of the kinetic equation is stable.