Space Physics Seminar - fall-2021
Energetic Electron Precipitation in Earth’s Radiation Belts: New Insights from ELFIN
Oct. 8, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Xiaojia Zhang - Department of Earth, Planetary, and Space Sciences, UCLA
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Dynamic flux variations in Earth’s electron radiation belts are controlled by concurrently operating acceleration and loss processes. Wave-particle resonant interaction contributes significantly to both processes, providing electron acceleration or precipitation to the atmosphere. Modern radiation belt models, which are largely based on near-equatorial spacecraft measurements, have been successful in explaining the long-term dynamics in energetic electron fluxes by electron diffusive scattering from low-intensity whistler-mode and electromagnetic ion cyclotron (EMIC) waves. Equatorial observations, however, cannot directly resolve the precipitating electrons inside the loss cone; such measurements of electron precipitation can be only performed by low-altitude spacecraft capturing electron fluxes to the atmosphere. In this presentation, we overview first results from UCLA’s ELFIN CubeSat Mission, which provides the first pitch-angle resolved energetic electron measurements at low altitudes. These measurements demonstrate both electron precipitation as predicted by the diffusion model and new patterns of precipitation related to non-diffusive (nonlinear) electron resonant interaction with whistler-mode waves.
Solar Wind Triggering of Substorms
Oct. 15, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Robert McPherron - Department of Earth, Planetary, and Space Sciences, UCLA
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We investigate whether substorms can be triggered by the solar wind. We conclude many are. A new program to identifies negative bay onsets in the AL index and finds omh106,527 onsets from 1981 to 2019. This list controls a superposed epoch analysis of the solar wind AL coupling function obtaining its average behavior relative to onset. Only 68,700 events have enough data to calculate coupling. Its average is a peak of about 4 hour’s duration with maximum at substorm onset. This is the origin of the concept of triggering by northward turnings. Close examination shows that the peak is preceded by a plateau followed by a sharp rise starting 13 minutes before onset. The average of the coupling derivative has two features. The first is a sinusoidal variation corresponding to the gradual increase and decrease in coupling. This the expected from regression to the mean. Superimposed on this is a positive pulse starting at -13 minutes, peaking at 7 minutes, and passing through zero at onset. The pulse is present in all subsets of the list except weak substorms. It is present at all levels of correlation of the average with every event. Solar wind averages reveal that this trigger is caused by a transient event where the speed increases by 10% over an interval of 30 minutes. The transverse field increases by a similar amount and the clock angle factor increases by 25%. We speculate that the onset is triggered by a rapid increase in convection due the increase in coupling. We use statistical procedures to estimate the number of substorms that are triggered in this manner.
The generation of two-band chorus waves in the Earth’s outer radiation belt
Oct. 22, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Jinxing Li - Department of Earth, Planetary, and Space Sciences, UCLA
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Whistler-mode chorus waves typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a long-standing question. Fundamental questions include: 1) Are chorus spectral gaps formed near the equatorial source region or during propagation away from the equator? 2) How is the two-band structure formed? 3) Why are chorus spectral gaps typically located just below 0.5 fce (fce: electron-gyrofrequency)? By analyzing Van Allen Probes data, we show that chorus spectral gaps are observed in the source region where chorus waves propagate both in the parallel and anti-parallel directions to the magnetic field. Observations show that banded chorus waves are commonly accompanied by two separate anisotropic electron components. We explain that initially generated chorus waves quickly isotropize the electron distribution through Landau resonant acceleration. The altered electron distribution, after returning to the magnetic equator, exhibits two anisotropic populations which lead to the generation of two-band chorus waves. The Landau resonant velocities are close to 0.5 vAe (vAe: electron Alfven velocity), which increase as latitude. Consequently, Landau resonances at high latitudes lead to electron isotropization above 0.5 vAe and wave spectral gaps formation below 0.5 fce at equator. Using 1D and 2D PIC simulations, we demonstrate that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppresses the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components, and lead to the formation of two-band chorus waves.
Nature and Role of Turbulence in the Inner Heliosphere: Parker Solar Probe Results
Oct. 29, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Chris Chen - Queen Mary University of London
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Parker Solar Probe, launched in 2018, has become the first spacecraft to measure the solar wind in situ closer than 0.28 AU to the Sun, and has currently reached distances down to 15 solar radii from the solar surface. These measurements have revealed much about the young solar wind, the physical processes occurring within it, and how they evolve into the well-known features seen at 1 AU. One particular aspect of interest is the solar wind turbulence, which PSP has shown to continually intensify with the decreasing solar distances measured so far. In this talk, I will present what we have learned so far about turbulence in the young solar wind. Some of these findings have confirmed our expectations from current models, but some have been unexpected, forcing us to think of new explanations for the fundamental behaviour of turbulence in this regime. Furthermore, turbulence is thought to play a key role in some of the questions at the heart of the PSP mission - the heating of the solar corona, the acceleration of the solar wind, and the large-scale structure of the heliosphere. PSP's early orbits allowed us to test turbulence-driven models for the origin of the solar wind at far closer distances than previously possible, generally finding a good match for the required energy fluxes and other properties. With the more recent orbits encountering different classes of solar wind, we have now also began to be able to test the generality of such findings, and there are indications that other driving mechanisms in addition to the turbulence may be required in some wind types. Overall, there have been many interesting new findings, but also new questions raised, which further work and data from closer to the Sun in the coming years of the mission should help us resolve.
Laboratory Realization of an Ion-ion Hybrid Alfvén Wave Resonator
Nov. 5, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Stephen Vincena - Department of Physics and Astronomy, UCLA
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In a magnetized plasma with two ion species, shear Alfvén waves (or guided electromagnetic ion cyclotron [EMIC] waves) have zero parallel group velocity and experience a cut-off near the ion-ion hybrid frequency $\omega_{ii}$ [1]. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is possible, in principle, for a magnetic well configuration to behave as an Alfvén wave resonator in a two-ion plasma. The important role played by the wave cut-off at $\omega=\omega_{ii}$ in determining the structure of low frequency wave spectra has long been recognized in space plasma studies. For instance, Temerin and Lysak [2] identified that the narrow-banded ELF waves seen in the S3-3 satellite were generated by the auroral electron beam in a limited spatial region determined by the local value of $\omega_{ii}$ for a mix of H+-He+ ions. In addition to playing a key role in magnetospheric resonators, EMIC waves and the existence of multiple ion species are also important in the scattering of high-energy electrons in the earth's inner magnetosphere [3].The present study demonstrates [4] such a resonator in a controlled laboratory experiment (in the Large Plasma Device at UCLA) using a H+-He+ mixture. The resonator response is investigated by launching monochromatic waves and sharp tone-bursts from a magnetic loop antenna. The topic is also investigated theoretically, and the observed frequency spectra are found to agree with predictions of a theoretical model of trapped eigenmodes. Results of the experiment and theory will also be discussed in their relation to the ion-ion resonator feature proposed for planetary magnetospheres [5-6] and to magnetic confinement devices containing multiple ion species [7].
Charged Particle Isotropization in Collisionless Environments: The Role of Turbulence as well as Very Short Radius-of-Curvature Toroidal Field Geometries
Nov. 12, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- William Newman - Departments of Earth, Planetary, and Space Physics; Physics and Astronomy; and Mathematics, UCLA
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The kinematics of individual particles responding to magnetic fields with very short radius-of-curvature R lines of force is highly complex, and the statistical nature of observed kinetic energy partitioning is often noted as being isotropic. Suppose a charged particle with a velocity v is injected into an environment with an associated gyrofrequency Om where with no electric field present. Northrop (1963) demonstrated, when the dimensionless constant Psi = v/Omega R << 1, that the behavior would be adiabatic and largely field-aligned. A natural question, then, is how the particle motion would evolve if its kinetic energy were increased making Psi very large. We approach this question in two steps. (a) Consider the situation where the magnetic field undergoes abrupt changes in orientation, as a model for turbulence, intermittently rendering its radius-of-curvature R=0. This produces to a field configuration that could be described literally as a ``stick-model.'' We show analytically via orbit integration methods in conjunction with Brownian motion methodologies that the particle's velocity distribution becomes isotropic. (b) Consider a toroidally symmetric electric field with radius-of-curvature R as might be encountered in a tokamak or as an approximation to field geometries encountered in space-based observations as well as their simulations. Schmidt (1979) noted that the associated Hamiltonian is "integrable" and every particle satisfies three conservation laws. He showed that these presented bounds on how the kinetic energy would evolve in time. Others have followed suit and, in some instances, presented numerical solutions to illustrate the complex behavior that emerged. However, an explicit analytic solution for the particle trajectories has eluded investigations owing to technical complexities. Exploiting the scaling properties of the field, we are able obtain analytically the evolution of the field and calculate explicitly how the distribution of its kinetic energy components varies as a function of Psi and become isotropic as Psi becomes infinite, including situations where the relativistic Lorentz factor must be included. While this latter result employs a specific model for the field geometry, the KAM theory for perturbations to that integrable model provides substantial assurance that its conclusions remain robust in other toroidal geometries.
Efficient non-thermal particle acceleration mediated by the kink instability in jets
Nov. 19, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Paulo Alves - Department of Physics and Astronomy, UCLA
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Astrophysical jets shine across the entire electromagnetic spectrum and are among the most powerful particle accelerators in the universe. Yet, the mechanisms underlying their particle acceleration are not well understood. MHD simulations suggest that the development of the current-driven kink instability (KI) can play an important role in the dissipation of the jet’s internal magnetic field, but it remains clear if such process could lead to efficient non-thermal particle acceleration, required to explain observations. In this talk, I will present 3D particle-in-cell simulations that capture the self-consistent particle acceleration associated with the development of the current-driven kink instability (KI) in magnetic field geometries relevant to recollimation regions of relativistic jets. These simulations reveal that the development of the KI mediates the efficient dissipation of the magnetic field into high-energy particles. Non-thermal particles are accelerated by a coherent inductive electric field that develops at the core of the current flow during the nonlinear stage of the KI. Acceleration by the large-scale inductive electric field is made efficient by the highly tangled magnetic field structure that characterizes the nonlinear phase of the KI, which allows particles to experience rapid curvature-drift motions across the magnetic field lines and parallel to the electric field. This results in a spectral power-law tail that is robust for a large range of initial conditions and system sizes. I will present scaling laws for this process with system size and magnetization, and discuss the implications of our results for relativistic astrophysical jets. Finally, I will discuss the possibility of exploring this physics in high-energy-density (HED) laboratory experiments.
Understanding the Relativistic Electron Precipitation: Combined effect of Electromagnetic Ion Cyclotron and Whistler-mode Waves
Dec. 3, 2021
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Muhammad Fraz Bashir - Department of Earth, Planetary, and Space Sciences, UCLA
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Energetic electron losses in the Earth's inner magnetosphere are determined by outward radial diffusion and scattering into the atmosphere by various electromagnetic waves. The two most important wave modes responsible for electron scattering are electromagnetic ion cyclotron (EMIC) waves and whistler-mode waves (whistler waves) that, acting together, can provide rapid electron losses over a wide energy range from few keV to few MeV. Wave-particle resonant interaction resulting in electron scattering is well described by quasi-linear diffusion theory using the cold plasma dispersion, whereas the effects of nonlinear resonances and hot plasma dispersion are less well understood. In this presentation, I consider the combined effect of EMIC and whistler-mode waves, including nonlinear resonances and hot plasma effects. The first part of the presentation is focused on an event during which both wave-modes are quasi-periodically modulated by ULF waves, observed by THEMIS. Based on observed wave properties and plasma parameters, the test particle simulations and hot plasma EMIC waves dispersion analysis reveal that nonlinear phase trapping of 300-500 keV electrons through resonances with whistler waves may accelerate and make them resonant with EMIC waves that, in turn, quickly scatter those electrons into the loss-cone. The second part of the presentation is focused on the hot plasma model for EMIC waves applicable to a wide range of plasma and hot ion characteristics. I show preliminary results of a parametric study based on few events observed by THEMIS and discuss how this model may provide a generalized hot plasma effect for realistic quasi-linear diffusion rate calculations. The third part of the presentation considers events with conjugated equatorial measurements of ULF-modulated EMIC and whistler-mode waves and low-altitude measurements of electron precipitations by UCLA ELFIN CubeSats.