[Paper Review] Highly relativistic electron flux enhancement during the weak geomagnetic storm of April-May 2017

Katsavrias, Christos, Sandberg, Ingmar+6 more|arXiv (Cornell University)|Feb 3, 2019

Ionosphere and magnetosphere dynamics43 citations

TL;DR

This study reveals that a weak geomagnetic storm (Sym-Hmin ≈ -50 nT) in April–May 2017 triggered a two-order-of-magnitude enhancement of relativistic and ultra-relativistic electrons up to 10 MeV, not detected in geosynchronous orbit. The enhancement resulted from two-stage acceleration: chorus waves first energized seed electrons to relativistic energies (1–2 MeV), followed by inward radial diffusion driven by Pc5 ULF waves, which further accelerated electrons to ultra-relativistic energies.

ABSTRACT

We report observations of energetic electron flux and Phase Space Density (PSD) to show that a relatively weak magnetic storm with $Sym-H_{min} \approx -50 $nT, resulted in a relativistic and ultra--relativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with $Sym-H_{min} \approx -235 nT$. This enhancement appeared at energies up to $\approx 10$ MeV, lasted for at least 24 days and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of PSD radial profiles and Fokker--Planck simulation, we show that the enhancement of relativistic and ultra--relativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of April 21--25 accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 ULF waves further accelerates them to ultra--relativistic energies.

Motivation & Objective

• Investigate the mechanisms behind a long-lasting, intense enhancement of relativistic and ultra-relativistic electrons during a weak geomagnetic storm with minimal Sym-H depression.
• Address the limitation of geosynchronous orbit monitoring, which failed to detect the electron flux enhancement.
• Determine whether local acceleration and radial diffusion jointly contributed to the observed electron energization.
• Assess the role of chorus waves and Pc5 ULF waves in driving electron acceleration under low geomagnetic activity.
• Demonstrate that extreme electron enhancements can occur independently of strong Dst or Sym-H indices, challenging conventional storm classification.

Proposed method

• Utilized high-resolution phase space density (PSD) measurements from Van Allen Probes' MagEIS and REPT instruments, with invariant-based analysis using TS05 magnetic field model.
• Applied wavelet analysis to identify Pc5 ULF wave power in the 2–7 mHz range, correlating with radial diffusion drivers.
• Inferred lower-band chorus wave amplitudes using the method of Li et al. (2013) from in situ electric and magnetic field data.
• Constructed radial diffusion simulations using a Kp- and µ-dependent diffusion coefficient model combining DE_LL (Liu et al., 2016) and DB_LL (Ozeke et al., 2014) to avoid overestimation at L > 4.
• Solved the Fokker-Planck equation with radial diffusion only, using boundary conditions at L* = 3.2 and 5.2, and assumed infinite loss time (τ = ∞) to exclude plasmaspheric hiss effects.
• Compared simulated PSD with in-situ RBSP measurements across L* = 4.2–4.8, focusing on the period April 19–29 when plasmapause was inside L* ≈ 4.

Experimental results

Research questions

• RQ1 Why did a weak geomagnetic storm (Sym-Hmin ≈ -50 nT) produce a relativistic electron flux enhancement comparable to the extreme St. Patrick’s Day storm (Sym-Hmin ≈ -235 nT)?
• RQ2 What physical mechanisms drove the acceleration of electrons to ultra-relativistic energies (>3 MeV) when conventional storm indices suggest minimal impact?
• RQ3 To what extent did chorus waves contribute to local acceleration of seed electrons during substorm injections?
• RQ4 How did Pc5 ULF waves influence inward radial diffusion and further energize electrons beyond 3 MeV?
• RQ5 Why was the flux enhancement not detected in geosynchronous orbit, despite its magnitude and duration?

Key findings

• A two-order-of-magnitude enhancement in relativistic and ultra-relativistic electron flux (up to 10 MeV) occurred during April–May 2017, lasting at least 24 days, despite a weak geomagnetic storm with Sym-Hmin ≈ -50 nT.
• The seed electron population (tens to hundreds of keV) was injected into the outer radiation belt during intense substorms from April 19–26, as indicated by AL index minima ≈ -1800 nT.
• Chorus wave activity during April 21–25 caused local acceleration, producing characteristic rising peaks in PSD radial profiles, confirming in situ energization to relativistic energies (1–2 MeV).
• Inward radial diffusion driven by Pc5 ULF waves (2–3.5 mHz) further accelerated electrons to ultra-relativistic energies, with the highest PSD ratios (≈1) observed at L* = 4.2–4.8 for µ = 5100 MeV/G.
• The simulation results show strong agreement with observations at L* = 4.2–4.8 for µ = 5100 MeV/G, confirming that radial diffusion was the dominant mechanism for ultra-relativistic electron acceleration.
• The study demonstrates that ultra-relativistic electron enhancements can occur independently of strong Dst or Sym-H indices, challenging the assumption that only extreme storms drive such events.