Session GO8: Space Plasmas

\textbf{The Nature and Source of Solar Magnetic Phenomena}

The heliosphere appears to be powered by coaxial helicity injection from a negative helicity injector in the northern hemisphere and a positive one in the southern. The injector flux for both is the measured solar polar flux and the injector voltage is generated by a simple dynamo effect due to the differential rotation of the solar surface with the polar flux. The dynamo current is estimated from the solar motion that it causes. This current also appears to sustain a thin, shallow global magnetic structure over most of the solar surface that has the form of a 1D minimum energy state. The current channel appears to be destroyed and reformed every 11 years. The currents and magnetic fields reverse in this solar cycle. A brief discussion of surface phenomena observed during this cycle is given. Plasma self-organization is briefly discussed and used in this analysis of solar data. The magnetic phenomena discussed include torsional oscillations; the heat source for the chromosphere and the corona; filaments; meridional flow; the solar cycle; sunspots; CMEs; and flares.   

Stellar Rotation Effects on the Stellar Wind

We discuss the role of the azimuthal stellar wind flow in the stellar-rotation breaking mechanism. We make use of Parker-Weber-Davis ([1], [2]) MHD stellar wind model. The stellar rotation is shown to cause the slow magnetosonic critical point to occur lower in the corona and hence lead to enhanced stellar wind acceleration. For strong rotators, this process is shown to occur in a narrow shell adjacent to the star. \\ \\ $[1]$ E. N. Parker: Astrophys. J. 128, 664, (1958). \\ \\ $[2]$ E. J. Weber and L. Davis: Astrophys. J. 148, 217, (1967).   

The Ubiquity of Ion-Driven Microinstabilities in the Inner Heliosphere

The relative lack of collisions in low-density, high-temperature plasmas, such as the solar wind, allow systems to depart from local thermodynamic equilibrium (LTE); these departures act as sources of free energy driving unstable growth. Understanding the behavior of these instabilities is necessary for fully characterizing the transport and dissipation of energy in these systems. In the solar wind, the presence of unstable wavemodes are frequently inferred. To understand the role instabilities play in the evolution of this system, we must investigate how their frequency of occurrence changes with distance from the Sun. Given the large number of free energy sources that can drive these instabilities, e.g. temperature anisotropies, temperature disequilibrium between species, and relative drifts, we use an automated implementation of the Nyquist criterion instead of more traditional parametric models to determine stability. We apply this method to tens of thousands of ion spectra measured by the Helios spacecraft at distances ranging from 0.3 to 0.9 au. These results will be expanded upon using forthcoming measurements from Parker Solar Probe to refine our understanding of the role instabilities play in shaping the evolution of the expanding solar wind.   

Spectral/Discontinuous Galerkin approach to fully kinetic simulations of magnetized plasmas

We propose a new method for the solution of the Vlasov-Maxwell equations for magnetized plasmas. It is based on a spectral expansion of the velocity space with Asymmetrically-Weighted Hermite Polynomials (AWHP), together with the discontinuous-Galerkin (DG) approximation for the spatial coordinate. The spectral expansion allows significant reduction in the number of degrees of freedom required to represent velocity space, while still retaining kinetic effects. Moreover, the spectral expansion is isomorphic to the classical fluid-moment expansion, hence providing a fine control over the fluid to kinetic regime transition through the number of polynomials/moments used. The DG approximation guarantees high accuracy and the ability to handle complex geometries. At the same time, its high locality ensures parallel efficiency leading to optimal scalability, which is desirable for large scale kinetic simulations on modern HPC architectures. Here, we demonstrate the properties and capabilities of the Spectral Plasma Solver framework based on the Hermite-DG approximation (SPS-DG) on the classical example of decaying magnetized plasma turbulence, the Orszag-Tang vortex test. We further compare SPS-DG with fully kinetic standard tools, such as Particle-In-Cell codes.   

Beam physics in support of active experiments in space

Recent advances in accelerator technologies as well as space diagnostic instruments open possibilities for new electron beam experiments in space. Two examples of such experiments include the CONNection EXplorer (CONNEX) experiment and the Beam Plasma Interaction Experiment (BeamPIE). The CONNEX experiment aims to study magnetic field-line connectivity between magnetosphere and ionosphere. BeamPIE will fly an advanced electron beam on an ionospheric rocket in order to generate whistler and X-mode plasma waves. Our theoretical and modeling work supports the design of both experiments and other applications such as using electron beams to remediate artificial and natural radiation belts. In the presentation we will focus on the beam dynamics for these applications. We perform particle simulations of the transverse beam dynamics coupled with a simple model of longitudinal beam dynamics. We discuss how the beam dynamics affects the beam-generated wave-source region in the BeamPIE experiment and how it can be maximized by optimizing the beam parameters. We also show that the transverse beam dynamics plays an important role in for beam stability in the CONNEX experiment.   

Theoretical and computational predictions for the upcoming SMART experiment

The SMART (Space Measurement of A Rocket-released Turbulence) experiment will demonstrate the production of electromagnetic waves in the space environment via a cascade of plasma physics processes. A sounding rocket will release heavy atoms into the upper ionosphere at high speed (\textasciitilde 10 km/s) across the magnetic field. These atoms photoionize and the magnetic field traps them, leading to a ring distribution in velocity space. An ion ring distribution is unstable to electrostatic lower hybrid waves with perpendicular wavelengths much shorter than the electron skin depth. Nonlinear induced scattering of lower hybrid waves produces electromagnetic magnetosonic and whistler waves with wavelengths large compared to the skin depth. Due to their large group velocities, these electromagnetic waves rapidly escape the experimental (or source) region and propagate into the radiation belts where the amplitude is expected to be similar to large lightning generated whistlers. In this talk, we will describe these plasma processes, the experimental design of SMART, the theoretical and computational work underway to quantitatively predict the results of the experiment, and the ways in which these processes are expected to play important roles in the near-Earth space environment.   

Nonadiabatic motion of energetic ions in the inner magnetosphere

The problem of trapped charged particle motion in a dipole magnetic field is intrinsically insoluble and full solution of this problem can only be achieved by numerical integration. The theory of adiabatic invariants is often employed to describe orbits of charged particles in the inner magnetosphere. The first adiabatic invariant, magnetic moment, is associated with fast gyromotion around the magnetic field. It leads to the concept of 'adiabatic loss cone', which determines when trapped particles can become lost by scattering into the atmosphere. We first show that when the ring-current ion energy increases or radius of curvature of equatorial magnetic field decreases, the equatorial atmospheric loss cone shifts away from the direction of the magnetic field. This modification can be described by a simple quasi-adiabatic model which works well even for significant deviations from adiabaticity. Next we discuss how nonadiabatic effects can lead to diffusion of charged particles which occur when cyclotron motion of particles around magnetic field line resonates with longitudinal oscillations of bounce motion between mirror points. We show that such diffusion becomes important for the ring current decay during geomagnetic storms.   

A New N$+$ Band of Electromagnetic Ion Cyclotron Waves: Observations and Theory

The first observational evidence and theoretical modeling of a new N$+$ band for EMIC waves in multi-ion (He$+$, O$+$, N$+)$ hot plasma are provided as a new perspective to distinguish the relevant magnetospheric processes. The past magnetospheric missions such as Dynamic Explorer-1, Geotail and WIND provided observations showing abundances of N$+$ similar to those of O$+$. However, the instruments on board of the current missions (e.g. Cluster, Van Allen Probes, MMS) lack the possibility of reliably separate N$+$ from O$+$ due to less mass resolution. The wave observations from Van Allen Probes confirm the existence of N$+$ EMIC waves and also suggest looking for possible ways to improve capabilities of current missions in quantifying the relative contribution of N$+$ and O$+$. Theoretical modeling confirms that the presence of N$+$ leads to a new N$+$ band and its instability with additional cut-off, crossover, and resonance frequencies, and significantly changes the dispersion properties of other bands of EMIC waves. This new band has the potential to reduce the discrepancy of considering the wave activity close to the oxygen cyclotron frequency ($\Omega $O$+)$ as He$+$ band.   

Three-dimensional kinetic simulations of magnetotail dynamics

The Earth's magnetosphere is a dynamic environment that features a complex interplay between fundamental plasma processes. The accessibility of the magnetosphere for detailed satellite measurements makes it an ideal natural laboratory for studying nonlinear magnetized plasma dynamics. Disturbances at the bow shock from the impinging solar wind can propagate downstream and drive instabilities in the extended magnetotail - potentially resulting in the onset of magnetospheric substorms. After decades of research, the physical mechanism underlying substorm onset remains one of the of the most important unresolved issues in magnetospheric physics. We present simulations of the magnetotail that capture both the near-earth dipole and the extended tail regions. We study the impact of solar wind driving on this configuration using 2D and 3D kinetic particle-in-cell simulations and compare with results from fluid simulations. The interplay between reconnection and cross-tail ballooning modes is investigated to determine their roles in the onset of magnetospheric substorms. The implications of these simulations for elucidating substorm onset is discussed by direct comparison with satellite measurements.   

Impacts of Fine Particulate Matters on Atmospheric Ionic Mobilities in Direct Current Corona Discharges

Ionic mobilities are significant parameters to reveal the physical linkage between ionization processes and movement characteristics of diverse ions in electric fields. An investigation into the impacts of fine particulate matters on atmospheric ionic mobilities is presented. A measurement apparatus was designed utilizing direct current (DC) corona discharges at room temperature. Ions were generated from parallel 50 $\mu $m stainless steel wires applied to a high-voltage DC power supply. Fine particles with diameter magnitude of micrometers and even less were generated from burning insense in a closed laboratory and were analyzed by a scanning electron microscope (SEM, FEI Quanta 200F). Positive and negative ionic mobilities were extracted from the electric field strength and ion current density measured by the rotating field meter and ionic current plate respectively. With the presence of fine particles, atmospheric ionic mobilities were found to decrease exponentially with the increasing mass concentration of the particles. The distributions of electric field and space charge density, the contributions to the charge density from ions and fine particles were also attained.