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Earth's magnetosphere is a cavity of space carved out of the supersonic solar wind, which forms a bow shock as it is slowed and deflected around the magnetospheric obstacle. Since Earth's bow shock is typically supercritical in nature, it accelerates and reflects a portion of the incident solar wind ions, resulting in the formation of the ion foreshock upstream of the bow shock. The foreshock consists of a combination of incident solar wind plasma plus the superthermal, counter-streaming ions reflected by the shock. In this talk, I'll quickly review our understanding of the ion foreshock and focus in on large-scale kinetic phenomena that can arise within it due to the interaction between the incident, discontinuous solar wind plasma and the superthermal foreshock plasma. In particular, I'll discuss shocklets, hot flow anomalies (HFAs), and foreshock bubbles (FBs) and new insights and perspectives we've been afforded on these transient ion foreshock phenomena by NASA's Magnetospheric Multiscale (MMS) mission and the state-of-the-art in global hybrid simulations.
Electron dynamics and energization are a key component of magnetic field dissipation in collisionless reconnection. In 2D reconnection, the main mechanism that limits the current density and provides an effective dissipation is most probably the electron pressure tensor term, that has been shown to break the frozen-in condition at the x-point. In addition the electron- meandering-orbit scale controls the width of the electron dissipation region, where the electron temperature is observed to increase both in recent MMS observations as well as in laboratory experiments (MRX). By means of two-dimensional, full-particle simulations in an open system (Pei et al. 2001; Ohtani and R. Horiuchi 2009), we investigate how the energy conversion and particle energization depends on the guide field intensity. We study the energy transfer from the electromagnetic field to the plasma, and the threshold guide field separating when parallel and perpendicular energy transfers dominate, confirming recent MRX results, in agreement with MMS observations. We calculate the energy partition between fields and kinetic and thermal energy of different species, from the electron scales to ion scales, showing there is no significant variation for different guide field configurations. We study electron distribution functions and self consistently evolved particles orbits for high guide field configuration, investigating possible mechanisms for electron perpendicular heating. Finally I will give an idea of our 2D simulations with plasmoids for which the setup can be easily extended to study 3D reconnection, thanks to GPU technology.
Electromagnetic ion cyclotron (EMIC) waves are observed in the magnetosphere and in the ionosphere with a frequency range of 0.2-5 Hz. EMIC wave-particle interactions have been considered as a significant loss process of ring current particles through pitch-angle scattering. Thus, understanding the generation of EMIC waves is important to study the radiation belt’s dynamics. In this talk, we present our recent reports of EMIC waves in the inner magnetosphere under the different geomagnetic environments (e.g., inside or outside the plasmasphere, and EMIC waves associated with or without energetic particle injections) using the Van Allen Probes and Geostationary Operational Environmental Satellites. He+ EMIC waves associated with injection are usually accompanied by an increase in energetic H+ fluxes (E~0.1-50 keV) with intense wave power at 14-16 MLT inside the plasmasphere. They were predominantly observed with left-hand polarization and higher wave normal angles. On the other hand, H+ EMIC waves were predominantly observed outside the plasmasphere on the dayside, and showed a mixture of left-hand and linear polarizations with lower wave normal angles regardless of injections. More than half of the events outside the plasmasphere showed no H+ flux enhancement at energies of 0.1-50 keV, but they were accompanied by a solar wind dynamic pressure enhancement (ΔPsw=0.5 nPa). Based on these observations, we discuss that possible generation processes of EMIC waves under different geomagnetic environments in the inner magnetosphere.
Magnetic flux ropes are bundles of twisted magnetic fields and their associated currents. They are common on the surface of the sun (and presumably all other stars) and are observed to have a large range of sizes and lifetimes. One or more flux ropes are routinely generated in the Large Plasma Device at UCLA. The ropes are kink unstable and when they collide fully 3D magnetic reconnection occurs. The time dependent magnetic field, plasma flow, electron temperature, plasma density, and the space charge and inductive electric fields were measured at over 42,000 spatial positions throughout the plasma volume over several million rope collisions. Magnetic field lines are followed and used to derive quasi-seperatrix layers, locations where reconnection occurs. The complete data set was used to evaluate all the terms in Ohm’s law, which resulted in unphysical plasma resistivity. It was then determined that in this situation Ohm’s law is non-local. The resistively was properly evaluated using the fluctuation dissipation theorem (Kubo resistivity). Time domain structures (TDS) , sharp pulses in potential and magnetic field are generated during reconnection events and subsequent move from the reconnection region into the rope currents. The probability distribution function of the spike amplitudes is log-normal, the same as the fold of crumpled paper. A amplitude counting method is used to create a vector map of the magnetic field of the spikes. TDS observed by satellites are ubiquitous in the plasma surrounding the earth.
The dynamics of multi-MeV electrons in the Earth’s outer radiation belt is extremely variable. Electrons can remain trapped for long periods of time but can also vary several orders of magnitude on timescales of a few hours to a few days. This extreme variability is ultimately driven by the solar wind and since prediction of relativistic electron enhancements is a problem of great interest to the scientific community, understanding the connections between solar wind and magnetospheric parameters, and flux levels of MeV electrons can lead to improvements in our prediction potential. Using in-situ measurements from the solar wind (OMNI database) and flux levels from the outer radiation belt (GOES, Van Allen Probes) we explore the relationship between different geomagnetic perturbations and the increases in fluxes of relativistic (MeV) electrons. We aim to (1) establish what solar wind parameters are statistically relevant for the enhancement and depletion of MeV electrons at geostationary orbit independent of storm-time condition and (2) study the extent of this solar wind influence outer belt.
For space physicists, a bow shock generates a beam of reflected ions, which excite ultra-low-frequency waves in the upstream plasma that flows into the shock. At the Large Plasma Device, our goal is to achieve the opposite: We use a dense beam of laser-ablated ions to excite ion-cyclotron waves and study the early stages of the formation of a parallel collisionless shock. These waves actually lie in the HF band (λ ~ decameter scale) for our magnetic field strength, which allows us to probe their spatial structure. After a short theory primer on how left- and right-hand polarized waves can form a shock front, I will analyze the saturation of the instabilities behind these waves in hybrid simulations and present LAPD measurements of the right-hand resonant instability and the current-density structure that it induces in the background plasma.
Ion dynamics are controlled by the energy-dependent source, transport, energization and loss processes. Systematic changes in the ion dynamics are essential to understand the ring current variations in the inner magnetosphere. The Van Allen Probes mission, which orbits near the equatorial plane inside the geosynchronous orbit, has a wide energy coverage with high energy resolution and state-of-the-art ion composition instrumentation. It provides a great opportunity to investigate the plasma dynamics and the associated wave properties. In this talk, I will present some of our recent studies on the ion dynamics and the associated EMIC wave properties during geomagnetic quiet and active times. We have found that H+ with energies from 50 keV to several hundred keV is the dominant component of the symmetric ring current during both quiet and active times, while O+ ions with energies from 10 keV to 50 keV always make a non-negligible contribution to the ring current when sym-H < -60 nT. Besides, the EMIC waves excited by the proton anisotropy instability constrain the ion distributions to a marginally stable state. The properties of EMIC wave are organized into patterns in the parameter space of proton anisotropy and parallel proton beta, which reveals several important features regarding the wave excitation and propagation.
UCLA AOS and The Aerospace Corporation
The high-energy tail of the plasma in near-Earth space is trapped by the geomagnetic field, forming the Van Allen radiation belts that encircle the Earth. Various physical processes can rapidly accelerate these charged particles to prodigious energies, in excess of one megaelectron volt, on timescales of one day or less. Owing to the hazard that this radiation poses to manmade technologies in space, there is considerable interest in understanding the processes that govern the particle dynamics. To that end, NASA launched the twin Van Allen Probes in 2012 to improve radiation belt predictability, and radiation belt science has evolved significantly over that time. With the mission soon coming to an end, it is time to survey the landscape to take stock of where we have been, and where we are going. We will thus present an overview of recent advances in radiation belt science, highlighting current research trends and several unresolved issues. In particular, we will examine the various theories for particle acceleration and loss, with an emphasis on what has been revealed in the Van Allen Probes observations.