Space Physics Seminar - fall-2023
Energy cascade from whistler waves to smaller scale electrostatic waves
Oct. 13, 2023
3:30 p.m. - 4:30 p.m.
Slichter Hall # 3853
Presented By:
- Donglai Ma - AOS, UCLA
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Fast plasma flows in the Earth’s plasma sheet transport energetic particles into the inner magnetosphere and form injection fronts at mesoscales. The abruptly enhanced fluxes of high-energy ions and electrons can provide free energy to a zoo of plasma waves including whistler waves. Although the excitation of whistler waves has been extensively studied, the process of energy transfer from whistler waves to smaller scale electrostatic waves, and eventually into electron thermal energy, remains poorly understood. Using particle-in-cell simulations guided by spacecraft observations, we demonstrate the excitation mechanism of two kinds of electrostatic waves driven by whistler waves: electron acoustic waves and electron cyclotron harmonic waves. Such secondary electrostatic instabilities driven by whistler waves provide a path to deposit the energy associated mesoscale flows into smaller scales
Global modeling of transport in the magnetosphere: The large effect of small scales
Oct. 20, 2023
3:30 p.m. - 4:30 p.m.
Zoom only: https://ucla.zoom.us/j/99437182144?pwd=NDFEZmZ2YUl4cHdJMFlZZU9WS2MzQT09
Presented By:
- KAREEM SORATHIA - APL, Johns Hopkins University
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During geomagnetically active periods, plasma is transported from the magnetotail into the ring current in the inner magnetosphere. The transport of plasma into the ring current occurs at different spatial and temporal scales, from global quasi-steady convection to bursty bulk flows (BBFs), with typical cross-tail extents of 1-3 Earth radii. During its enhancement, the ring current plays a critical role in regulating the coupling between the magnetosphere and ionosphere. Ring current ions build up plasma pressure in the inner magnetosphere and will drive field-aligned currents which must close in the ionosphere, while electrons will lead to diffuse precipitation and enhanced ionospheric conductance which shape the ionospheric path of current closure. The ionospheric current closure will in turn couple to the thermospheric neutral population, via Joule heating, and alter the dynamics of the plasmasphere, via the penetration electric field in the inner magnetosphere. Understanding the relative role of convection at different spatial scales in both the buildup of the ring current and its broader effects on geospace coupling is an area of active interest and one of the core science questions of the Center for Geospace Storms. In this talk I will describe how addressing this question has informed the development of the Multiscale Atmosphere Geospace Environment (MAGE) model and highlight several recent modeling studies which illustrate the central role of mesoscale processes in magnetospheric transport.
Foreshock ULF waves as observed in global hybrid-Vlasov simulations
Oct. 27, 2023
3:30 p.m. - 4:30 p.m.
Slichter Hall # 3853
Presented By:
- Kun Zhang - EPSS UCLA
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Solar wind ions can be reflected and accelerated when they hit the Earth’s bow shock, forming ion beams streaming backward toward the Sun in the ion foreshock region. Ultralow frequency (ULF) waves are commonly observed in the foreshock region due to counter-streaming solar wind and foreshock ions, and these waves can have large amplitudes and spatial scales. The ULF waves are carried Earthward by the solar wind and thus potentially have significant impact on the Earth’s magnetosphere. In addition, the foreshock and foreshock waves exist in many other plasma environments in the universe, such as at other planets and comets. Therefore, it is important to obtain a comprehensive understanding of the properties of the foreshock ULF waves. Properly measuring the properties of the ULF waves requires at least two spacecraft that are spatially aligned with the interplanetary magnetic field (IMF) direction, especially for properties such as phase speed and growth rate. Due to the limited number of available spacecraft, the properties and the spatial variations of the foreshock ULF waves cannot be well resolved solely based on satellite observations. In this talk, we present a global survey of the ULF wave properties using simulation results from Vlasiator, a hybrid-Vlasov code. Wave properties such as obliquity, polarization, phase speed and early-phase growth rate will be discussed. Our recent findings on the oblique waves near the edge of the foreshock show how spatial variations can affect the measurement of wave properties, and this effect is also demonstrated using ARTEMIS observations as an example. In addition, we conduct detailed investigation on the early-phase growth of the ULF waves and compare the measured properties with wave dispersion relation calculated based on ion distributions using the state-of-art dispersion solver, LEOPARD, to demonstrate the role that linear physics plays in the early growing phase of foreshock waves.
Towards Multi-point Investigations of the Lunar-distant Magnetotail: What Have We Learned from the ARTEMIS mission?
Nov. 17, 2023
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Andrei Runov - EPSS, UCLA
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Since July 2011 two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes have been orbiting the Moon in stable 26 hours period equatorial, high-eccentricity orbits, of ~100 km x 19,000 km altitude with inter-probe separations vary from 500 km to 10 Earth’s Radii (RE). With the Moon, the ARTEMIS probes are traversing the magnetotail at geocentric distances ~60 RE during three to four days every month. Having aboard flux gate and search-coil magnetometers, an electric field instrument, and a plasma instrument suite which includes an electrostatic analyser and a solid state telescope, ARTEMIS is capable to study magnetic structures, electromagnetic waves, and plasma environments of the magnetotail at lunar distances. The upcoming Lunar Gateway mission with the Heliophysics Environmental and Radiation Measurement Experiment Suite (HERMES) aboard will enable three-point measurements by ARTEMIS and HERMES to study plasma characteristics, particle energy spectra in the solar wind and in the magnetotail. In this presentation I will summarize ARTEMIS observations in the lunar-distant magnetotail and discuss perspectives of multi-point investigations of the lunar-distant magnetotail with ARTEMIS and Gateway/HERMES.
Moon-Magnetosphere Interactions at Outer Planet Moons
Dec. 1, 2023
3:30 p.m. - 4:30 p.m.
Slichter Hall # 3853
Presented By:
- Tom André Nordheim - JPL, Caltech
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The giant planets Jupiter, Saturn, Uranus, and Neptune are host to a large array of moons, some of which are believed to contain vast liquid water oceans beneath their icy surfaces. Much of what we know about these bodies comes from examining their surfaces and atmospheres using remote sensing observations. However, many outer planet moons orbit inside the magnetosphere of their parent planet, and are continuously bombarded by charged particle radiation that can chemically and physically alter their surfaces, modifying and potentially obscuring the identity and origin of surface constituents. For those moons with substantial atmospheres, such as Titan and Triton, magnetospheric particle precipitation also represents an important input to their ionospheres, with key implications for ionospheric and atmospheric chemistry. To understand how outer planet moons (and candidate ocean worlds) evolve over time, we must therefore first understand how they interact with their local magnetospheric environment. Here I will present an overview of some recent work on moon-magnetosphere interactions, as well as discuss implications for the interpretation of scientific observations by past and future missions to the outer planets.
Nonlinear Wave-Particle Interactions: Recently Developed Theoretical Results
Dec. 8, 2023
3:30 p.m. - 4:30 p.m.
Zoom Only
Presented By:
- Jay Albert - Air Force Research Laboratory
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Radiation belt electrons are strongly affected by resonant interactions with cyclotron-resonant waves. In the case of a particle passing through resonance with a single, coherent wave, the Hamiltonian can be approximated by that for a plane pendulum, leading to estimates of the change at resonance of the first adiabatic invariant I, energy, and pitch angle. In the case of small wave amplitude (relative to the spatial variation of the background magnetic field) the change in I is a diffusive random walk, but for large relative amplitude the behavior is more nearly deterministic, driven by the nonlinear dynamics associated with slow separatrix crossing. A general analytical treatment of slow crossings has long been available and can be used to give the changes associated with “phase bunching,” including the detailed dependence on phase. I will review this treatment and relate it to much simpler estimates. It will be shown that phase bunching can cause increase as well as decrease of I (and E and PA), even in the pendulum Hamiltonian approximation, though the fraction of particles with increase may be exponentially small. Recent numerical simulations have uncovered additional, complex behavior, not captured by the pendulum model, for particles with low pitch angle. Avoiding a commonly made approximation leads to a more general but still tractable “second fundamental model” Hamiltonian, which involves not one but two regions of phase trapping. Its trajectories encompass traditional phase bunching and phase trapping as well as the newly uncovered phenomena referred to as positive phase bunching and anomalous phase trapping, which will also be discussed.