Effects of the May 2024 Solar Storm on the Earth's Radiation Belts Observed by CALET on the International Space Station

This paper presents observations from the CALET instrument on the International Space Station detailing how the May 2024 solar storm generated a new, long-lasting component of multi-MeV relativistic electrons in Earth's radiation belts that extended down to L=2.2 and persisted for several months.

The Gist

The Big Picture: A Solar "Superstorm" Hits Earth

Imagine the Sun as a giant, restless neighbor who occasionally throws tantrums. In early May 2024, this neighbor threw a massive tantrum, sending a huge wave of energy (a solar storm) hurtling toward Earth. This was the most intense storm we've seen in over 20 years.

When this wave hit Earth's magnetic shield (the magnetosphere), it didn't just bounce off; it squeezed the shield hard and pushed a massive amount of high-speed particles deep into the space around our planet.

The "Forbidden Zone" Gets Filled

Normally, the space around Earth has two main "parking lots" for dangerous, high-speed electrons (tiny particles with a negative charge):

1. The Inner Belt: Close to Earth.
2. The Outer Belt: Farther away.

Between these two lots is a "no-parking zone" called the Slot Region. Think of this slot as a quiet, empty highway lane between two busy traffic jams. Usually, it's empty because the particles there get lost quickly.

What happened in May 2024?
The solar storm was so powerful that it forced a massive crowd of high-speed electrons from the Outer Belt to crash into this empty "Slot Region." It was like a sudden traffic jam forcing cars into a quiet, empty lane where they aren't supposed to be.

The "Storage Ring" That Wouldn't Leave

Usually, when these particles get pushed into the Slot Region, they disappear (fall into the atmosphere) within days or weeks. But this time, something unusual happened.

Using a special telescope called CALET (which is currently riding on the International Space Station), scientists watched this new crowd of electrons. They found that these particles didn't just vanish; they formed a long-lasting "storage ring."

• The Analogy: Imagine you spill a bucket of marbles on a smooth floor. Usually, they roll away and stop quickly. But in this case, the floor was tilted in a way that kept the marbles rolling in a circle for months.
• The Result: This new ring of electrons stayed in the "forbidden zone" for over five months. Some of the fastest, most energetic electrons (multi-MeV) stayed for more than a month, while the slightly slower ones hung around for the full five months.

The "Traffic Lights" of Energy and Location

The scientists didn't just watch the crowd; they studied how long different types of particles stayed in the ring. They looked at particles with different energy levels (speeds) and at different distances from Earth.

They discovered a surprising pattern, like a traffic light changing colors depending on where you are:

• Close to Earth (Lower L-shells): The faster, more energetic particles actually stayed longer than the slower ones. It's like a race car having better brakes than a bicycle in this specific zone.
• Farther from Earth (Higher L-shells): The pattern flipped. Here, the slower particles stayed longer, and the fast ones disappeared quickly.

This "switch" in behavior helps scientists understand the invisible forces (like magnetic waves) that act as a vacuum cleaner, sucking these particles out of space.

The "South Atlantic Anomaly" Got a Makeover

The paper also noticed changes in a specific area called the South Atlantic Anomaly (SAA). Think of the SAA as a "pothole" in Earth's magnetic shield where radiation naturally leaks through.

After the storm, this pothole didn't just get deeper; it changed shape. The storm pushed solar particles (protons) deeper into this hole than usual, making the radiation there sharper and more intense on one side. It's as if a storm surge pushed water further up a beach than it ever had before, leaving a new, wetter mark on the sand.

Why This Matters

This study is important because:

1. It's Rare: We haven't seen a "storage ring" of electrons this deep in the Slot Region since the famous Halloween storm of 2003.
2. It Lasted: The fact that these particles stayed for months tells us that the "vacuum cleaners" (loss mechanisms) in this part of space are less efficient than we thought for certain types of particles.
3. Safety: Understanding how long these dangerous particles stick around helps us protect satellites and astronauts who might fly through these zones.

In summary: A massive solar storm in May 2024 forced a crowd of high-speed electrons into a quiet zone between Earth's radiation belts. Instead of disappearing quickly, they formed a long-lasting ring that persisted for months, revealing new secrets about how Earth's magnetic environment handles extreme space weather.

Technical Summary

Technical Summary: Effects of the May 2024 Solar Storm on the Earth's Radiation Belts Observed by CALET

Problem Statement
The Earth's radiation belts typically consist of two distinct populations: a stable inner belt centered near $L \approx 1.5$ and a highly variable outer belt at $L \gtrsim 4$, separated by a "slot region" ($L \approx 2-3$) that is generally devoid of relativistic electrons during geomagnetically quiet times. While major geomagnetic storms can occasionally inject outer-belt electrons into the slot region, these populations usually decay within tens to hundreds of days. The May 10–11, 2024, event represented the most intense geomagnetic storm in two decades (Dst = -412 nT, Kp = 9), offering a critical opportunity to observe the formation, persistence, and decay mechanisms of a new relativistic electron population injected deep into the slot region. Previous observations, such as those following the 2003 Halloween storm, documented similar events, but the longevity and specific energy-dependent dynamics of the 2024 event required detailed characterization using continuous low-Earth orbit (LEO) monitoring.

Methodology
The study utilizes data from the Calorimetric Electron Telescope (CALET) aboard the International Space Station (ISS). CALET, a high-energy astrophysics instrument, continuously monitors the LEO radiation environment. The analysis focuses on count rates from three specific detector layers with distinct energy thresholds for electrons:

• CHDX: >1.5 MeV
• CHDY: >3.4 MeV
• IMC4X: >8.2 MeV

The authors analyzed data from May 1 to October 1, 2024, correlating CALET count rates with contextual space weather data, including GOES-18 proton fluxes, solar wind parameters (IMF magnitude, $B_z$, speed), and geomagnetic indices (Dst, Kp). To distinguish between electron and proton contributions, the study leverages the ISS orbit inclination, the specific energy thresholds of the detectors, and contextual data from other missions (e.g., PROBA-V/EPT, REPTile-2). The temporal evolution of the electron population was characterized by smoothing count rates with a four-day rolling average and fitting exponential decay models ($N(t) = N_0 e^{-t/\tau}$) to determine electron lifetimes ($\tau$) as a function of energy and McIlwain's L-shell.

Key Contributions and Results

1. Formation of a New Electron Storage Ring:
   Following the main phase of the May 10–11 superstorm, CALET observed a significant enhancement in relativistic electron fluxes in the slot region, specifically between $L \approx 2.2$ and $3.2$. This new population extended well below the nominal impenetrable barrier of $L=2.8$. The count rates in this region increased by approximately one order of magnitude compared to pre-storm levels.

2. Extended Persistence:
   Unlike the 2003 event where the slot region filling lasted roughly a month, the 2024 electron population persisted for significantly longer. The >1.5 MeV population remained enhanced for over five months (through September), while the >8.2 MeV population persisted for well over a month. The population was subject to subsequent geomagnetic disturbances (e.g., storms in June, August, and September), which caused temporary dropouts, but the >1.5 MeV population recovered and remained elevated longer than higher-energy components.

3. Energy-Dependent Decay and Lifetime Trends:
   The study calculated electron lifetimes ($\tau$) for the period between the storm recovery (May 26) and the next major disturbance (June 28).

   • Global Lifetimes: For the $L=2.8-3.2$ region, lifetimes were estimated at $\tau \approx 41.3$ days (>1.5 MeV), $\tau \approx 25.4$ days (>3.4 MeV), and $\tau \approx 19.0$ days (>8.2 MeV).
   • L-Shell Dependence: A distinct transition in lifetime trends was observed across $L \approx 2.5$.
     • At $L=2.2$, lifetimes increased with energy (e.g., IMC4X >8.2 MeV reached $\sim 72$ days at $L=2.3$).
     • At $L=3.0$, lifetimes decreased with energy (e.g., IMC4X >8.2 MeV dropped to $\sim 19$ days).
     • The >1.5 MeV channel showed a minimum lifetime at $L \approx 2.3$ before increasing at lower L values, a trend not seen in the higher-energy channels.

4. South Atlantic Anomaly (SAA) Modifications:
   The study also noted changes in the SAA, dominated by trapped protons. The eastern edge of the SAA sharpened, and a count-rate deficit appeared in the slot region ($L \sim 2.0-2.2$) between the SAA and the outer belt. This is attributed to rapid proton losses (e.g., magnetic curvature scattering) during the storm's main phase, followed by a slow recovery.

Significance and Claims
The paper asserts that the CALET observations provide a unique, continuous dataset revealing a long-lived relativistic electron population in the slot region that has not been observed since the 2003 Halloween storm. The primary significance lies in the characterization of the energy-dependent lifetime trends within the slot region. The observed transition—where lifetimes increase with energy at lower L-shells but decrease with energy at higher L-shells—offers new constraints on loss mechanisms.

The authors suggest this trend may be explained by a shift in dominant loss processes:

• At lower L-shells ($L < 2.5$), collisional losses may dominate, favoring longer lifetimes for higher-energy electrons.
• At higher L-shells ($L > 2.5$), wave-particle scattering (specifically by EMIC waves for multi-MeV electrons and hiss waves for MeV electrons) becomes more effective, leading to shorter lifetimes for higher-energy electrons.

The study concludes that while the derived lifetimes are generally consistent with theoretical estimates and previous observations (e.g., SAMPEX, Fennel et al.), the specific multi-MeV dynamics observed by CALET highlight the complexity of radiation belt evolution during extreme space weather events. The continuous monitoring capability of CALET on the ISS is highlighted as essential for investigating both short-term and long-term variations in the inner magnetosphere.