The Van Allen radiation belts were first discovered by the U.S. Explorer 1 mission in 1958, revealing that Earth's magnetosphere accelerates and traps energetic particles. The inner belt is composed of protons ranging from MeV to GeV energy levels, while both the inner and outer belts contain significant numbers of energetic electrons in the 100s of keV to MeV range.
These energetic electrons, sometimes called "killer electrons," pose risks to spacecraft and astronauts conducting extravehicular activities. Research has long sought to understand the source, loss, and fluctuating intensities of these electrons. High-resolution measurements are essential for predicting their intensity and understanding their behavior.
Challenges in Measuring Relativistic Electrons
Accurately measuring energetic electrons, especially in the inner belt, is difficult due to the presence of high-energy protons. NASA's Van Allen Probes, operational from 2012 to 2019, faced proton contamination even with heavy shielding on instruments like the Relativistic Electron Proton Telescope (REPT) and the Magnetic Electron and Ion Spectrometer (MagEIS).
To address these issues, a team from the University of Colorado Boulder, led by Dr. Xinlin Li, developed the Relativistic Electron Proton Telescope integrated little experiment (REPTile). This miniaturized version of REPT flew aboard the Colorado Student Space Weather Experiment (CSSWE), a 3-Unit CubeSat supported by the National Science Foundation. Operating in a highly inclined low Earth orbit (LEO) from 2012 to 2014, the CubeSat reduced its exposure to inner-belt protons, particularly in the South Atlantic Anomaly (SAA), where Earth's magnetic field is weaker.
The success of REPTile led to the development of an advanced version, REPTile-2, for the Colorado Inner Radiation Belt Experiment (CIRBE) mission. Like its predecessor, REPTile-2 operates in a highly inclined LEO to minimize exposure to harmful protons. The REPTile-2 instrument incorporates two new technologies - guard rings and Pulse Height Analysis (PHA) - to achieve cleaner, high-energy-resolution measurements, particularly in the inner belt.
Technological Improvements in REPTile-2
Electrons and protons enter the field of view (FOV) and are measured when they hit a stack of silicon detectors. However, protons with energies greater than 60 MeV can penetrate the shielding and interfere with electron measurements. To prevent contamination, REPTile-2 uses guard rings around each detector. These rings are electronically separated from the inner active area and connected to a separate channel, allowing the system to discard invalid measurements caused by particles outside the FOV.
In addition, REPTile-2 applies full Pulse Height Analysis (PHA) to achieve high energy resolution. PHA measures the charge deposited by incoming electrons, providing precise energy readings. Unlike REPTile, which used a simpler method with three energy channels, REPTile-2 offers 60 energy channels for electrons in the 0.25 to 6 MeV range. While the REPT instrument on the Van Allen Probes also used PHA, it was less effective in the inner belt due to the lack of guard rings.
CIRBE and REPTile-2 Findings
CIRBE was launched on April 15, 2023, aboard SpaceX's Falcon 9 rocket during the Transporter-7 mission, supported by NASA's CubeSat Launch Initiative (CSLI). REPTile-2 was activated on April 19, 2023, and has been collecting critical data on radiation belt electrons. The instrument has provided unprecedented measurements, revealing features of the electrons in the Van Allen belts in greater detail than ever before.
In April 2024, REPTile-2 captured high-resolution images of energetic electron drift echoes, also known as "zebra stripes," showing electrons swirling around Earth in distinct clusters. These observations span both the inner and outer belts, covering a wide range of electron energies and fluxes.
Most notably, CIRBE/REPTile-2 has identified a temporary third radiation belt composed of electrons, forming between the two permanent belts after the intense magnetic storm in May 2024. Previous large storms have generated similar temporary belts, but the high-energy-resolution data from REPTile-2 provides new insights into this phenomenon. Scientists are now analyzing the data to determine the belt's potential longevity, which could extend for several months.
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