About

Vassilis Angelopoulos is a Professor and Vice Chair for Teaching at the UCLA Department of Earth, Planetary, and Space Sciences. His role involves academic leadership and teaching within the department. He is associated with the Experimental Space Physics Group, indicating a focus on space physics research. His contact information includes a phone number (310-794-7090) and email (vassilis@g.ucla.edu). The department is located at UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567. The webpage highlights his involvement in research areas related to space physics and his contribution to the academic community at UCLA.

Research topics

• Physics
• Astronomy
• Atmospheric sciences
• Mathematics
• Geometry
• Statistical physics
• Nuclear physics
• Mathematical analysis
• Classical mechanics
• Optics
• Geophysics
• Quantum mechanics

Selected publications

• Parker Solar Probe Observations of Compound Reconnection Exhaust Boundaries and Mirror-mode Structures in the near-Sun Heliospheric Current Sheet

  The Astrophysical Journal Letters · 2026-05-20

  articleOpen access

  Abstract Magnetic reconnection is a fundamental physical process that can drive rapid conversion of magnetic energy into plasma bulk flows, thermal heating, and particle acceleration in space and astrophysical plasmas. Classical reconnection theory predicts that the Alfvénic reconnection exhausts are bounded by pairs of slow-mode shocks. However, identifying and characterizing these shocks through in situ spacecraft observations remains a challenge. Here, we report Parker Solar Probe observations of a reconnection exhaust embedded in the heliospheric current sheet at a heliocentric distance of 12.2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⨀</mml:mo> </mml:mrow> </mml:msub> </mml:math> . The reconnection exhaust is bounded on both boundaries by compound magnetic structures rather than a pair of pure slow shocks. Each boundary consists of a rapidly evolving, steep, inner slow shock, whose Mach numbers and shock-normal angles change significantly within several minutes, and an outer, gradual compound structure that comprises a slow shock and a rotational discontinuity. These slow shocks are quasi-perpendicular and are accompanied by enhanced proton perpendicular heating. Deep within the reconnection exhaust, high perpendicular temperature together with large plasma <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>β</mml:mi> </mml:math> trigger mirror instability and generate mirror-mode structures. These observations provide new insights into the structure of reconnection exhaust boundaries and their role in energy conversion in the near-Sun plasma.

  PublisherDOI

• High‐Time‐Resolution Observations of Plasma Convection in the Nightside High‐Latitude Ionosphere

  Journal of Geophysical Research Space Physics · 2026-04-01

  articleSenior author

  Abstract Using high‐time‐resolution Super Dual Auroral Radar Network (SuperDARN) radar data, we investigated the dynamic evolution of ionospheric convection under various conditions, including an interplanetary magnetic field (IMF) southward turning, a substorm onset, quasi‐steady southward IMF, and Pc5 ultra‐low frequency (ULF) waves. The SuperDARN high‐resolution convection revealed that during an IMF southward turning, convection enhancements form narrow, transient flow channels in the polar cap rather than broad, uniform flows, with velocity peaks moving equatorward and crossing the nightside open‐closed boundary. These flow peaks are linked to poleward boundary intensifications. The convection response was nearly simultaneous across latitudes. During a substorm onset, enhanced equatorward flow across the polar cap boundary preceded auroral onset, suggesting that flow observations may be a more sensitive indicator of pre‐onset conditions than auroral data. Under quasi‐steady southward IMF, both aurora and plasma velocity show significant unsteady behavior, with flow channels moving equatorward beyond the extent of their corresponding auroral streamers. For Pc5 ULF waves, the data reveal fine‐scale velocity structures within larger poleward‐moving enhancements, which correspond to breaks in the ULF magnetic field. This study emphasizes that high‐time‐resolution convection observations are essential for accurately capturing the rapid evolution of ionospheric convection, which is often underestimated using conventional lower‐resolution methods.

  PublisherDOI

• Evidence for a Localized Burst of Relativistic Electrons Produced in Earth's Plasma Sheet During a Substorm

  Journal of Geophysical Research Space Physics · 2026-04-01

  articleOpen accessSenior author

  Abstract Earth's magnetotail and its plasma sheet are highly dynamic, influenced by both the solar wind and the inner magnetosphere. Periodically, energy is explosively released in the magnetotail during substorms. However, the extent to which electrons are accelerated in the magnetotail remains an open question, with recent observations revealing acceleration to energies exceeding all previous theoretical and simulation estimates. Here, we investigate the possible origin and spatial scale of relativistic electron bursts by combining in situ plasma sheet measurements taken by the Magnetospheric Multiscale (MMS) mission, with low Earth orbit electron precipitation measurements taken by the Electron Losses and Fields Investigation (ELFIN). On 17 July 2021 at 19:41 UT, ELFIN detected a transient and intense burst of 3 MeV electrons precipitating into the atmospheric loss cone. These relativistic electrons had energies higher than the surrounding plasma sheet, and had fluxes higher than the nearby Van Allen radiation belt. By comparing the electron spectra between MMS and ELFIN, we suggest the burst originated from the plasma sheet, which is supported by the SST19 magnetic field model. Lastly, MMS did not directly observe the burst, despite observing the central plasma sheet at . Altogether, these results suggest that the plasma sheet may be capable of effectively accelerating electrons to relativistic energies over a localized region.

  PublisherDOI

• Statistical Characteristics of the Proton Isotropy Boundary

  Journal of Geophysical Research Space Physics · 2025-12-01

  article

  Abstract We present a statistical study of 272 proton isotropy boundary (IB) events, spanning 50 keV up to 2 MeV observed by ELFIN on the nightside magnetosphere, extending the energy range beyond the 120 keV population considered in prior works. Viewed from Low Earth Orbit, the IB is the magnetic latitude poleward of which persistently isotropic pitch‐angle distributions () are first detected. We characterize the IB distribution in local time, energy, geomagnetic activity, and 50 keV precipitation from isotropic protons. We find these IBs primarily exhibit negative energy‐latitude dispersion patterns consistent with equatorial magnetic field‐line curvature (FLC) scattering, with a 20%–30% chance of any particular energy channel exhibiting positive dispersion. The lowest latitude and most energetic IBs were detected in the pre‐midnight sector, consistent with the typical location of maximal cross‐tail current‐sheet thinning. We identify that proton IBs form the lower‐latitude boundary of a region with significant FLC‐caused proton precipitation between the outer ring current and inner edge of the plasma sheet (“PS2RC”), resulting in perpetual loss of protons exceeding typical plasma sheet energies. We show this 50 keV precipitation is often sufficiently intense and distributed to produce ionization enhancements over a range of altitudes at auroral/sub‐auroral latitudes. We also compare proton IB properties to those of electron IBs observed by ELFIN, finding similar trends across local time and activity. These results demonstrate that the IB and precipitation of isotropic particles in their poleward vicinity can be geophysically significant in connecting the magnetosphere and ionosphere.

  PublisherDOI

• Microbursts Near the Electron Isotropy Boundary: Colocation of Curvature and Whistler‐Mode Scattering

  Journal of Geophysical Research Space Physics · 2025-12-01

  article

  Abstract Field‐line curvature scattering (FLCS) within the plasma sheet–to–outer radiation belt transition region (hereafter PS2ORB ) serves as a key driver of energy‐latitude dispersion in energetic electron precipitation observed at low latitudes. This precipitation forms the isotropy boundary of electrons ( IBe pattern ), located between the isotropic keV electron fluxes of the plasma sheet and the anisotropic relativistic fluxes of the outer radiation belt. During geomagnetically active periods, the PS2ORB region becomes populated with plasma sheet injections that introduce various transient electron precipitation mechanisms, significantly complicating the structure of the IBe pattern . In this study, we show that the timescales of these precipitations can reach subsecond levels, allowing them to be interpreted as microbursts. Observations of such microbursts substantially enhance the spatial and temporal variability of the IBe pattern . By combining low‐altitude ELFIN satellite measurements with high‐temporal‐resolution (40 ms) near‐UV imaging photometer data from the Pulsating Aurora Imaging Photometers System project, we separate between FLCS‐driven precipitation patterns that form the IBe pattern and electron scattering by whistler‐mode waves, which generates microbursts. We identify, for the first time, the near‐colocation of these two precipitation mechanisms within the PS2ORB region–an important feature not previously reported.

  PublisherDOI

• Dynamics of Thermal Electron Anisotropy in the Magnetotail Current Sheet During Substorm Growth Phase

  Journal of Geophysical Research Space Physics · 2025-09-01

  articleSenior author

  Abstract The growth phase of the magnetospheric substorm is accompanied by the formation of a thin magnetotail current sheet, which is subsequently destroyed due to magnetic reconnection. The configuration and kinetic properties of this current sheet determine its stability and are important in the context of reconnection onset. This study focuses on the electron component of such thin current sheets. Observations in the near‐Earth magnetotail show that electrons are predominantly field‐aligned anisotropic. However, this anisotropy decreases as the current sheet becomes thinner. The observed isotropization cannot be explained by electron scattering, as anisotropy is provided by the subthermal electron population. We propose an adiabatic model for such isotropization based on observations from the Time History of Events and Macroscale Interactions during Substorms mission, and demonstrate that the inclusion of a self‐consistent electrostatic field (current sheet polarization) can explain the decrease in electron anisotropy. Our findings highlight the importance of the role of electron temperature anisotropy and current sheet polarization in regulating the magnetotail dynamics during the substorm growth phase.

  PublisherDOI

• Prolonged Intervals of Relativistic Electron Storm‐Time Flux Enhancements in the Magnetotail at Lunar Distance

  Geophysical Research Letters · 2025-08-15

  articleOpen access

  Abstract We report on prolonged enhancements of electron fluxes at energies at or above 500 keV, observed in the magnetotail by the lunar‐orbiting Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) during the recovery phase of a magnetic storm with minimum −200 nT during periodic auroral electrojet () activations. The enhanced energetic electron fluxes were omnidirectional and observed near the magnetic equator. No solar energetic particle background was detected. Although ARTEMIS detected earthward magnetic flux transport impulses exceeding 2 mV/m, along with associated broadband electrostatic fluctuations, no correlation was evident between these phenomena and the relativistic electron flux enhancements. Spectra obtained during the relativistic electron flux enhancements are fit by the Kappa function, = 3.75, similar to that of the quiet‐time plasma sheet electron population at lunar distance. Multiple reconnection events at large distances are, most likely, responsible for the electron heating.

  PublisherOA PDFDOI

• On Energetic Electron Precipitation in Auroral Arcs

  Geophysical Research Letters · 2025-09-18 · 1 citations

  articleOpen accessSenior author

  Abstract A key element of magnetosphere‐ionosphere coupling is the precipitation of electrons, which transfers energy from the collisionless, rarefied magnetospheric plasma into the dense, collisional ionosphere. Two distinct types of such precipitation are: auroral electrons, which carry field‐aligned currents and are responsible for auroral arc formation, and energetic electrons, which contribute to ionization in the lower ionosphere. Although these two electron populations are well separated in energy, this study reveals a close connection between them, likely due to the collocation of their equatorial drivers. Using low‐Earth orbit satellite measurements from Electron Losses and Fields Investigation of energetic (50–1,000 keV) electron precipitation and ground‐based all‐sky imager observations of auroral arcs, we demonstrate how the auroral arc structures and locations strongly correlate with the boundaries or gradients of keV precipitation. We identify three typical correlation patterns and discuss their implications for the physics of magnetosphere‐ionosphere coupling.

  PublisherDOI

• Excitation of Whistler‐Mode Waves by an Electron Temperature Anisotropy in a Laboratory Plasma

  Geophysical Research Letters · 2025-11-15

  articleOpen access

  Abstract Naturally occurring whistler‐mode waves in near‐Earth space play a crucial role in accelerating electrons to relativistic energies and scattering them in pitch angle, driving their precipitation into Earth's atmosphere. Here, we report on the results of a controlled laboratory experiment focusing on the excitation of whistler waves via temperature anisotropy instabilities–the same mechanism responsible for their generation in space. In our experiments, anisotropic energetic electrons, produced by perpendicularly propagating microwaves at the equator of a magnetic mirror, provide the free energy for whistler excitation. The observed whistler waves exhibit a distinct periodic excitation pattern, analogous to naturally occurring whistler emissions in space. Particle‐in‐cell simulations reveal that this periodicity arises from a self‐regulating process: whistler‐induced pitch‐angle scattering rapidly relaxes the electron anisotropy, which subsequently rebuilds due to continuous energy injection and further excites wave. Our results have direct implications for understanding the process and characteristics of whistler emissions in near‐Earth space.

  PublisherOA PDFDOI

• Latitudinal Profiles of Nightside Isotropy Boundaries: Comparison of Observations and Predictions of Adaptive Magnetospheric Model

  Journal of Geophysical Research Space Physics · 2025-10-01 · 2 citations

  articleSenior author

  Abstract There is significant interest in monitoring the instantaneous magnetic configurations and dynamic states of the magnetotail and understanding what controls them. A unique and attractive opportunity is provided by remote sensing of the radial profile of the equatorial magnetic field curvature based on low‐latitude energetic particle measurements of isotropy boundaries (IBs), providing that you can determine the origin of isotropic precipitation. To validate the magnetic field line curvature scattering (FLCS) as the main mechanism of the isotropy boundary formation, we compare coarse energy versus latitude IB profiles (in 3 + 3 energy channels) measured during a few dozen passes of POES and ELFIN spacecraft with the theoretical predictions of the adapted (AM03) magnetospheric model. Two studied intervals in August 2022 include substorm events of various intensities for which good spacecraft coverage in the near magnetotail helps reconstruct the adaptive model in the areas where the IBs are formed. We find a general agreement between the predicted and observed coarse IB profiles' shape and latitude, validating the FLCS hypothesis. Deviations are also observed, and we discuss the factors that can influence identification of the true FLCS profiles in observations and predictions, including limitations of adaptive modeling, non‐monotonic radial structure of the tail magnetic field, and interference of FLCS with other precipitation mechanisms related to wave‐particle interactions. Most can be avoided by improving the sensitivity, energy coverage, and resolution in future instruments.

  PublisherDOI

Recent grants

• Collaborative Research: Causes and Consequences of Relativistic Electron Precipitation as Revealed by the CubeSat Mission ELFIN’s Pitch-Angle Resolved Loss Cone Measurements

  NSF · $388k · 2020–2024

• CubeSat: Electron Losses and Fields INvestigation (ELFIN)

  NSF · $651k · 2014–2019

Frequent coauthors

• A. V. Artemyev

  University of California, Los Angeles

  903 shared

• X.‐J. Zhang

  Inner Mongolia Agricultural University

  529 shared

• A. Runov

  Planetary Science Institute

  467 shared

• Y. Nishimura

  Boston University

  297 shared

• C. T. Russell

  284 shared

• D. Mourenas

  271 shared

• E. Donovan

  University of Calgary

  240 shared

• H. J. Singer

  NOAA Weather Prediction Center

  230 shared

Education

• Ph.D., Earth and Space Sciences

  University of California, Los Angeles

  1990

• M.S., Earth and Space Sciences

  University of California, Los Angeles

  1986

• B.S., Physics

  University of California, Los Angeles

  1983