Figure 1. An artist’s concept of the Van Allen belts
with a cutaway section of the giant donuts of radiation that surround Earth.
Image Credit: NASA Goddard Space Flight
Center/Scientific Visualization Studio
A new instrument is using advanced
detection techniques and leveraging an orbit with specific characteristics to
increase our understanding of the Van Allen belts—regions surrounding Earth
that contain energetic particles that can endanger both robotic and human space
missions. Recently, the instrument provided a unique view of changes to this
region that were brought on by an intense magnetic storm in May 2024.
The discovery of the Van Allen
radiation belts by the U.S. Explorer 1 mission in 1958 marked a prominent
milestone in space physics and demonstrated that Earth’s magnetosphere
efficiently accelerates and traps energetic particles. The inner belt contains
protons in the MeV (million electric volt) to GeV (109 electric volt) range, and even higher concentrations of energetic
electrons of 100s of keV (1000 electric volt) to MeV are found in both the
inner belt and the outer belt.
The energetic electrons in these
belts—also referred to as “killer electrons”—can have detrimental effects on
spacecraft subsystems and are harmful to astronauts performing extravehicular
activities. Understanding the source, loss, and varying concentrations of these
electrons has been a longstanding research objective. High-energy resolution
and clean measurements of these energetic electrons in space are required to
further our understanding of their properties and enable more reliable
prediction of their intensity.
Overcoming the challenges of measuring relativistic electrons in the inner
belt
Measuring energetic electrons
cleanly and accurately has been a challenge, especially in the inner belt,
where MeV to GeV energy protons also exist. NASA's Van Allen Probes, which
operated from 2012 to 2019 in low inclination, geo-transfer-like orbits, showed
that instruments traversing the heart of the inner radiation belt are subject
to penetration by the highly energetic protons located in that region. The
Relativistic Electron Proton Telescope (REPT) and the Magnetic Electron and Ion
Spectrometer (MagEIS) instruments onboard the Van Allen Probes were heavily
shielded but were still subject to inner-belt proton contamination.
To attempt to minimize these
negative effects, a University of Colorado Boulder team led by Dr. Xinlin Li,
designed the Relativistic Electron Proton Telescope integrated little experiment
(REPTile)—a simplified and miniaturized version of REPT—to fly onboard the
Colorado Student Space Weather Experiment (CSSWE). An effort supported by the
National Science Foundation, the 3-Unit CSSWE CubeSat operated in a highly
inclined low Earth orbit (LEO) from 2012 to 2014. In this highly inclined
orbit, the spacecraft and the instruments it carried were only exposed to the
inner-belt protons in the South Atlantic Anomaly (SAA) region where the Earth’s
magnetic field is weaker, which greatly reduced the time that protons impacted
the measurement of electrons.
REPTile’s success motivated a team, also led by Dr. Xinlin Li, to design REPTile-2—an advanced version of REPTile—to be hosted on the Colorado Inner Radiation Belt Experiment (CIRBE) mission. Like CSSWE, CIRBE operates in a highly inclined low-Earth orbit to ensure the exposure to damaging inner-belt protons is minimized. The team based the REPTile-2 design on REPTile but incorporated two additional technologies—guard rings and Pulse Height Analysis—to enable clean, high-energy-resolution measurements of energetic electrons, especially in the inner belt.
Figure 2: PI observing two engineers testing the
interface between the CIRBE bus and REPTile-2 on September 29, 2021.
Image Credit: Xinlin Li, University of Colorado
Boulder
As shown on the left in Figure 3, the field of view (FOV) of REPTile-2 is 51o. Electrons and protons enter the FOV and are measured when they reach a stack of silicon detectors where they deposit their energies. However, very energetic protons (energy greater than 60 MeV) could penetrate through the instrument’s tungsten and aluminum shielding and masquerade as valid particles, thus contaminating the intended measurements. To mitigate this contamination, the team designed guard rings that surround each detector. These guard rings are electronically separated from the inner active area of each detector and are connected by a separate electric channel. When the guard rings are triggered (i.e., hit by particles coming outside of the FOV), the coincident measurements are considered invalid and are discarded. This anti-coincidence technique enables cleaner measurements of particles coming through the FOV.
Figure 3. Left (adapted from Figure 1 of Khoo et al.,
2022): Illustration of REPTile-2 front end with key features labeled; Right:
REPTile-2 front end integrated with electronic boards and structures, a
computer-aided design (CAD) model, and a photo of integrated REPTile-2.
Image Credit: Xinlin Li, University of Colorado
Boulder
To achieve high energy resolution, the team also applied full Pulse Height Analysis (PHA) on REPTile-2. In PHA, the magnitude of measured charge in the detector is directly proportional to the energy deposited from the incident electrons. Unlike REPTile, which employed a simpler energy threshold discrimination method yielding three channels for the electrons, REPTile-2 offers enhanced precision with 60 energy channels for electron energies ranging from 0.25 – 6 MeV. The REPT instrument onboard the Van Allen Probes also employed PHA but while REPT worked very well in the outer belt, yielding fine energy resolution, it did not function as well in the inner belt since the instrument was fully exposed to penetrating energetic protons because it did not have the guard rings implemented.
Figure 4: The CIRBE team after a successful
“plugs-out” test of the CIRBE spacecraft on July 21, 2022. During this test the
CIRBE spacecraft successfully received commands from ground stations and
completed various performance tests, including data transmission back to ground
stations at LASP.
Image Credit: Xinlin Li, University of Colorado
Boulder
CIRBE and REPTile-2 Results
CIRBE's launch, secured through the
NASA CubeSat Launch Initiative (CSLI), took place aboard SpaceX's Falcon 9
rocket as part of the Transporter-7 mission on April 15, 2023. REPTile-2,
activated on April 19, 2023, has been performing well, delivering valuable data
about Earth's radiation belt electrons. Many features of the energetic
electrons in the Van Allen belts have been revealed for the first time, thanks
to the high-resolution energy and time measurements REPTile-2 has provided.
Figure 5 shows a sample of CIRBE/REPTile-2 measurements from April 2024, and illustrates the intricate drift echoes or “zebra stripes” of energetic electrons, swirling around Earth in distinct bunches. These observations span a vast range across the inner and outer belts, encompassing a wide spectrum of energies and electron fluxes extending over six orders of magnitude. By leveraging advanced guard rings, Pulse Height Analysis (PHA), and a highly inclined LEO orbit, REPTile-2 is delivering unprecedented observations of radiation belt electrons.
Figure 5: Color-coded electron fluxes detrended
between REPTile-2 measurements for a pass over the South Atlantic Anomaly
region on April 24, 2023, and their average, i.e., the smoothed electron fluxes
using a moving average window of ±19% in energy; Black curves plotted on top of
the color-coded electron fluxes are contours of electron drift period in hr.
The second horizontal-axis, L, represents the magnetic field line, which CIRBE
crosses. The two radiation belts and a slot region in between are indicated by
the red lines and arrow, respectively.
Image Credit: Xinlin Li, University of Colorado
Boulder
In fact, the team recently
announced that measurements from CIRBE/REPTile-2 have revealed a new temporary
third radiation belt composed of electrons and sandwiched between the two
permanent belts. This belt formed during the magnetic storm in May 2024, which
was the largest in two decades. While such temporary belts have been seen after big
storms previously, the data from CIRBE/REPTile-2 are providing a new viewpoint
with higher energy resolution data than before. Scientists are currently
studying the data to better understand the belt and how long it might stick
around — which could be many months.
PROJECT LEAD
Dr. Xinlin Li, University of
Colorado Laboratory for Atmospheric and Space Physics and Department of
Aerospace Engineering Sciences.
SPONSORING ORGANIZATIONS
Heliophysics Flight Opportunities for Research & Technology (H-FORT) program, National Science Foundation
By: NASA Science Editorial Team
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