Tag: Arase processed data

In May 2024, Earth experienced its most intense geomagnetic superstorm since the 2003 Halloween storms (often called the Gannon or Mother’s Day storm). This caused a sudden, dramatic loss (“dropout” or draining) of highly energetic electrons from the outer Van Allen radiation belt.

The Van Allen belts are doughnut- shaped regions of energetic electrons and protons (mostly from the solar wind) trapped by Earth’s magnetosphere. Geomagnetic storms can reconfigure the magnetosphere and rapidly change electron dynamics in the outer belt, creating hazards for satellites.

Satellite observations (particularly from JAXA’s Arase satellite) showed this extreme depletion of MeV electrons during the main phase of the storm. Researchers (Lyu et al.) provided the first comprehensive analysis of the rapid electron transport, published in AGU Advances, and summarized by Sarah Stanley for Eos (American Geophysical Union) on 14 September 2026.

They used the Versatile Electron Radiation Belt (VERB) model to simulate the event and identify the drivers. Two main processes caused the electron loss:

Magnetopause shadowing: This transported electrons outward, releasing them into interplanetary space. It was tied to intense compression of Earth’s magnetosphere (the magnetopause was pushed inward significantly, with estimates around L ~ 3.7 in related work).

Local wave scattering (e.g., by plasmaspheric hiss and other waves): This scattered electrons so their pitch angles changed, allowing them to precipitate into Earth’s upper atmosphere.

The relative importance of each process varied by region of the magnetic field. Critically, accurate reproduction of the extreme dropout required that the sudden outward transport from magnetopause shadowing occur nearly simultaneously with the strong magnetospheric compression, followed by the wave scattering.

Standard empirical models of electron transport (often parameterized by the Kp index and used in space weather forecasting) do not capture this tightly coupled timing of superstorm dynamics.

The findings highlight the need for improved models that better account for the interplay of magnetopause compression, radial diffusion/transport, and local scattering during extreme events. This could improve forecasting and risk assessment for satellites and other infrastructure during future superstorms.

Note that the same storm also injected particles and temporarily created new radiation belt structures in other regions, which persisted for months in some cases before later storms depleted them; the “draining” specifically refers to the rapid outer- belt electron dropout analyzed here.

Magnetopause shadowing is a key non- adiabatic loss process for energetic particles (especially relativistic electrons) in Earth’s outer Van Allen radiation belt. It occurs when the magnetopause, the boundary separating Earth’s magnetosphere from the solar wind, moves inward during periods of elevated solar wind dynamic pressure (e.g., during geomagnetic storms or interplanetary shocks), allowing previously trapped particles to escape into interplanetary space.

Charged particles in the radiation belts spiral along magnetic field lines while bouncing between the northern and southern magnetic mirror points and slowly drifting azimuthally around Earth (due to gradient and curvature drifts). Under quiet conditions, their drift paths are closed within the magnetosphere, so the particles remain trapped for long periods.

The magnetopause typically sits around 10 Earth radii ($R_E$) on the dayside. Strong solar wind pressure compresses the dayside magnetosphere, pushing the magnetopause inward (sometimes to $R_E$) on the dayside.

Strong solar wind pressure compresses the dayside magnetosphere, pushing the magnetopause inward (sometimes to $L^* \approx 4$ -6 or closer during extreme events).

There are two related forms:

Direct magnetopause shadowing
The compressed magnetopause intersects the particles’ previously closed drift shells. Particles whose drift orbits now cross the magnetopause are lost directly into the solar wind as they drift into the boundary. This can happen rapidly (on timescales of a particle’s drift period, often hours or less) when the magnetopause moves inward suddenly.

Indirect magnetopause shadowing (often coupled with outward radial diffusion)
Even if the magnetopause does not immediately intersect a given drift shell, the sudden loss of particles at higher $L^*$creates a steep negative radial gradient in phase space density (PSD). Ultra- low- frequency (ULF) waves then drive outward radial diffusion, transporting particles from lower $L^*$toward the last closed drift shell (LCDS, the outermost closed drift path). Once they reach the LCDS or the magnetopause itself, they escape.

The last closed drift shell (LCDS) is a useful concept for modeling: any particle whose $L^*$ exceeds the LCDS is considered lost.

Key Features and Consequences

Location preference: Most effective at higher $L^*$(outer parts of the outer belt, typically $L^* \gtrsim 4$ -5). Losses often start at high (L) and propagate inward.

Energy and pitch- angle dependence: Affects a broad range of energies (keV to multi- MeV). It can produce characteristic “butterfly” pitch- angle distributions (flux minima near 90° equatorial pitch angle) because of drift- shell splitting and preferential loss of certain pitch angles.

Speed: Can cause rapid, deep “dropouts” of electron flux (orders- of- magnitude decreases in hours) during the main phase of storms.

Additional factors: The detailed structure of the magnetopause (magnetic field gradients along the boundary, presence of a normal magnetic-field component, or induced electric fields during rapid compression) influences how easily particles cross the boundary. A normal field component facilitates escape along field lines; induced electric fields can sometimes inhibit or modify escape for lower- energy particles.

Role in Storms (Including the 2024 Superstorm)

During intense geomagnetic storms, magnetopause shadowing is frequently a dominant contributor to the sudden draining of the outer radiation belt electrons, often acting together with local wave- particle scattering (which precipitates particles into the atmosphere). Accurate modeling requires event- specific magnetopause location (or LCDS) and realistic radial diffusion rates; standard empirical models often underperform if they do not capture the tight timing between magnetospheric compression and particle transport.

In short, magnetopause shadowing is the process by which a compressed magnetopause “cuts off” or “shadows” the outer drift paths of trapped radiation- belt particles, allowing them to stream away into space, directly or after being transported outward by waves.

Extreme Radiation Belt Dropout During the May 2024 Superstorm

This is the scientific study summarized in the Eos article by Sarah Stanley. Direct full- text retrieval from the Wiley platform was unsuccessful (likely due to access restrictions), but the content is consistent with the open EGU abstract, the associated Zenodo dataset (initial submission materials), and the Eos research spotlight.

Key Points from the Paper

Observations

• Data from the JAXA Arase (ERG) satellite showed that MeV electron fluxes in the outer radiation belt dropped by several orders of magnitude during the main phase of the May 2024 superstorm (the strongest geomagnetic storm in ~20 years).
• This extreme dropout coincided with intense magnetopause compression (minimum standoff distance ~ $L \approx 3.7$, estimated via the Space Weather Modeling Framework) and enhanced ULF Pc5 wave activity (from SuperMAG ground magnetometers).

Modeling Approach

• The team used the Versatile Electron Radiation Belt (VERB) model to simulate the event.
• They tested different drivers for radial diffusion and loss processes.

Main Findings

• Standard empirical radial diffusion models (parameterized by the Kp index) failed to reproduce the observed electron flux profiles.
• Accurate reproduction of the extreme dropout required enhanced radial diffusion that is properly timed relative to the magnetopause compression and storm- time wave activity.
• Magnetopause shadowing (outward transport and loss to the compressed magnetopause) was a primary driver, especially at higher $L^*$.
• Stronger plasmaspheric hiss scattering was also needed to account for losses at lower $L^*$.
• Overall, the extreme electron loss was controlled by the coupled timing of three processes:
  1. Magnetopause compression (enabling shadowing),
  2. Radial diffusion (outward transport of particles), and
  3. Local wave-particle scattering (primarily hiss- driven precipitation into the atmosphere).

Implications

The results emphasize that physically timed (event- specific) representations of transport and loss are essential for modeling extreme storms. Commonly used empirical parameterizations are insufficient for such intense events. Improved models incorporating these timed couplings could enhance space- weather forecasting and satellite risk assessment.

Supporting materials (Arase processed data, ULF wave measurements, VERB simulation outputs under different driving scenarios, magnetopause positions, etc.) are available on Zenodo as part of the initial submission.

This paper provides the detailed quantitative analysis behind the “draining” of the outer radiation belt electrons during the 2024 superstorm via the mechanisms we previously discussed (especially magnetopause shadowing combined with radial diffusion and local scattering).

Published:  AGU Advances

DOI: 10.1029/2026av002476

Provided: American Geophysical Union

Authors: Xingzhi Lyu, Dedong Wang, Bernhard Haas, Miroslav Hanzelka, Yixin Sun, Zikang Xie, Alwin Roy, Yoshizumi Miyoshi, Yihua Zheng, Christos Katsavrias, Sigiava Aminalragia-Giamini, Nana Higashio, Takefumi Mitani, Takeshi Takashima, Iku Shinohara, Yoshiya Kasahara, Shoya Matsuda, Atsuki Shinbori, Fuminori Tsuchiya, Atsushi Kumamoto, Ayako Matsuoka, Mariko Teramoto, Kazuhiro Yamamoto, Yuri Y. Shprits

Abstract

The geomagnetic superstorm in May 2024 represents the most extreme space weather event over the past two decades, offering a unique opportunity to investigate radiation belt electron dynamics under exceptionally strong solar wind driving conditions. Observations from the Arase satellite show that relativistic electron fluxes dropped by several orders of magnitude during the storm main phase. Although magnetopause shadowing and wave-driven losses are established mechanisms, the response of radiation belt electrons to superstorm-level solar wind driving remains poorly constrained. Using the Versatile Electron Radiation Belt model, we present the first physics-based simulation of the May 2024 electron dropout. Utilizing multi-satellite and ground-based observations, we show that the decades-long, widely adopted Kp-driven radial diffusion parameterization fails to represent radial transport under superstorm conditions, substantially misrepresenting both the timing and magnitude of enhanced diffusion relative to observed ultra-low-frequency wave activity. Accurate modeling of the observed extreme dropout requires precise specification of the onset of enhanced radial diffusion, which should coincide with the nearly simultaneous storm-time magnetopause compression. These coupled processes drive the dramatic depletion, followed by scattering from whistler-mode plasma waves.