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Tracing the Boundary of the South Atlantic Anomaly Region with SPHEREx Transient Flagging Rates

This paper utilizes six months of transient flagging data from the NASA SPHEREx satellite to map the boundary of the South Atlantic Anomaly and identify additional high-transient regions near the geomagnetic poles, providing a critical reference for the design and operation of future space instruments.

Original authors: Chi H. Nguyen, James Bock, Sean Bryan, Walter R. Cook, Brendan Crill, C. Darren Dowell, Olivier Doré, Beth Fabinsky, Candice Fazar, Howard Hui, Phillip Korngut, Steve Padin, Michael Zemcov

Published 2026-07-07

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CC BY 4.0

Original authors: Chi H. Nguyen, James Bock, Sean Bryan, Walter R. Cook, Brendan Crill, C. Darren Dowell, Olivier Doré, Beth Fabinsky, Candice Fazar, Howard Hui, Phillip Korngut, Steve Padin, Michael Zemcov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth is surrounded by an invisible, protective force field called the magnetic field. This field acts like a giant shield, deflecting dangerous particles from space (like cosmic rays) that could fry our satellites. However, there is a giant "weak spot" in this shield over the South Atlantic Ocean, known as the South Atlantic Anomaly (SAA). Think of it like a thin patch in a raincoat; when you walk through a storm, water (radiation) leaks through that specific spot much more easily than anywhere else.

This paper tells the story of how the SPHEREx satellite, a space telescope launched in March 2025 to take pictures of the entire universe, accidentally became a master cartographer of this weak spot.

The Satellite's "Sunburn"

SPHEREx is designed to stare at the sky and take long-exposure photos of distant galaxies and stars. To do this, it uses very sensitive digital cameras (detectors). But when the satellite flies over the SAA, it gets "sunburned" by high-energy particles.

In the world of digital cameras, a "transient" is like a sudden, bright flash of light that isn't a star—it's a particle hitting the sensor.

Normal flight: The camera sees a few stray flashes here and there (less than 10% of the pixels get hit).
Flying over the SAA: The camera gets pummeled. Up to 80% of the pixels get hit by these particles in a single photo. The image becomes too "noisy" to use for science, so the satellite's computer automatically marks these photos as "trash" and plans to take them again later.

The Accidental Discovery

The scientists running SPHEREx had a simple rule: "If the satellite is over the South Atlantic Anomaly, throw away the photos." They used a map of where the SAA should be to decide this.

However, after a year of collecting data, they noticed something interesting. Even when the satellite was not over the pre-defined South Atlantic zone, some photos were still getting "noisy" (about 1.5% to 2% of the time).

When they plotted exactly where these "noisy" photos were taken, they didn't just see a blob over the South Atlantic. They saw a detailed map that traced:

The SAA itself: The big weak spot.
The Magnetic Poles: The North and South poles of Earth's magnetic field. Just like the SAA, the magnetic field lines dip down near the poles, letting particles sneak in.
Other weak spots: Some smaller, scattered areas in the Southern Hemisphere where the magnetic shield is also a bit thin.

The Analogy: The "Noise" Map

Imagine you are walking through a city with a very loud, annoying radio playing in the background.

The SAA is like a specific neighborhood where the radio is blasting at maximum volume. You know to avoid it.
The SPHEREx data is like a map drawn by people who only marked the spots where the radio was loud enough to ruin their conversation.

The paper shows that by looking at where the radio was loud enough to ruin the photos (the "transient flags"), the scientists could draw a much more accurate map of the "loud zones" than the old map they were using. They found that the "loudness" isn't just in one big circle; it stretches out to the poles and has other smaller pockets of noise.

What They Found

The Map: Using over 200,000 observations, they created a 2D map showing exactly where the radiation hits hardest. This map confirms the location of the SAA and shows the "oval" shapes around the magnetic poles.
Solar Storms: They checked if big solar storms (like solar flares) changed the shape of this weak spot. They found that while the amount of noise went up during storms, the shape and location of the weak spot stayed surprisingly steady.
Camera Differences: They noticed that the cameras designed to see longer infrared wavelengths (like a "night vision" mode) got hit by particles more often than the others, likely because their sensors are more sensitive to the specific type of energy coming from space.

Why This Matters

While SPHEREx is an astronomy mission, this paper shows that its "trash" data (the photos it threw away) is actually a treasure trove for understanding Earth's magnetic field.

The authors conclude that by simply counting how many "glitches" happen in a satellite's camera, we can trace the invisible boundaries of Earth's magnetic shield. This provides a new, real-time way to map the South Atlantic Anomaly and the magnetic poles, which is crucial for keeping future satellites safe and understanding how Earth's magnetic field is changing over time.

In short: SPHEREx didn't just look at the stars; it used its own "bruises" from space radiation to draw a new, detailed map of Earth's invisible magnetic shield.

Technical Summary: Tracing the Boundary of the South Atlantic Anomaly Region with SPHEREx Transient Flagging Rates

Problem Statement
The South Atlantic Anomaly (SAA) is a region where Earth's magnetic field strength drops significantly, allowing high-energy particles from the Van Allen radiation belts to penetrate to lower altitudes. This phenomenon poses a critical challenge for Low Earth Orbit (LEO) satellites, particularly those equipped with sensitive detectors, as it causes transient events (e.g., cosmic rays) that can saturate pixels and corrupt data. While the SAA's general location is known, its boundaries shift over time, and its effects vary with geomagnetic conditions. Existing monitoring relies on ground-based logs or specific particle flux measurements. The NASA SPHEREx satellite, launched in March 2025 to conduct an all-sky near-infrared survey, encounters these high transient rates frequently. The challenge addressed in this paper is the utilization of SPHEREx's operational data—specifically the real-time flagging of transient events—to map the SAA boundary and identify other geomagnetic regions of high particle flux, providing a reference for future missions.

Methodology
The study utilizes data collected by SPHEREx between May 2025 and April 2026, comprising 206,379 observations. The methodology relies on the satellite's onboard real-time transient detection algorithm applied to its six HAWAII-2RG (H2RG) detectors.

Transient Detection Algorithm: During 115-second integrations, the instrument control electronics (ICE) compute the photocurrent slope for every pixel. A statistical algorithm compares the current pixel value against a prediction based on the prior four reads. If the deviation exceeds a threshold defined by configurable noise parameters ( $K_{EN}$ for white noise and $K_{SN}$ for shot noise), a transient flag is set.
Data Filtering and Flagging:
- SAA Flagging: Exposures taken when the spacecraft is predicted to be within a pre-defined SAA zone (based on ground-track latitude/longitude) are flagged and excluded from standard science processing.
- High-Transient Flagging: A secondary cut was implemented for "non-SAA" observations. If any detector in an exposure exceeds a transient hit rate of $\sim10\%$ (approximately 435,000 pixels), the exposure is flagged for re-observation.
Mapping: The authors aggregate the transient flagging rates (percentage of flagged pixels per exposure) across all six detectors. They construct a two-dimensional map on a $2^\circ \times 2^\circ$ grid (approx. $250 \times 250$ km at 670 km altitude) to visualize the spatial distribution of transient rates.
Geomagnetic Correlation: The study correlates these transient rates with the spacecraft's ground-tracked location and compares the resulting maps against known SAA boundaries and geomagnetic pole locations. Variations are analyzed across different geomagnetic disturbance levels, including periods of low solar activity and periods of strong solar storms (e.g., November 2025, January-February 2026).

Key Results

Transient Rate Distribution: SAA observations exhibit extremely high transient rates, reaching $>80\%$ of pixels, rendering them unsuitable for astrophysical analysis. In contrast, non-SAA observations generally show rates $<10\%$ , though a subset ( $\sim1.5\%$ of total non-SAA exposures) exceeds the $10\%$ threshold.
Detector Variance: Detectors 4–6 (5.0 $\mu$ m cutoff) consistently record higher transient rates than Detectors 1–3 (2.5 $\mu$ m cutoff). This is attributed to the lower effective energy band gap of the 5.0 $\mu$ m arrays, which results in a lower well-depth and a higher probability of "snowball" events (cosmic rays spreading across multiple pixels).
SAA Boundary Tracing: The map of transient rates clearly traces the SAA. The centroid of the transient hit rate aligns with previous models. The pre-defined operational SAA boundary successfully captures $\sim77\%$ of the high-transient observations.
Geomagnetic Poles and Anomalies: Beyond the SAA, the transient rates reveal two distinct oval regions of elevated flux centered on the North and South geomagnetic poles. Additionally, scattered clusters of high transient rates were observed in the Southern Hemisphere west of the SAA, distinct from the poles, potentially indicating smaller-scale regions of weakened magnetic field strength.
Temporal Stability: While the absolute number of transient hits increases during periods of strong geomagnetic disturbance (solar storms), the spatial boundaries of the SAA (defined by 1% and 2% transient rate contours) remain relatively stable. The re-observation rate for non-SAA exposures tracks solar activity, rising from a mean of 1.45% in the first six months to 2.21% in the subsequent six months.

Significance and Claims
The paper posits that while SPHEREx is an astrophysics mission, its survey data provides a continuous, global measurement of the upper atmosphere's response to particle flux, offering a novel method for monitoring the SAA. The primary contribution is the presentation of the SAA boundary as traced by SPHEREx transient flags, serving as a reference for future space missions operating in LEO.

The authors modestly claim that this method demonstrates the utility of "incidental" data (transient flags intended for data quality control) for heliophysics and space weather research. The paper does not propose new operational protocols for other missions but suggests that the transient rates recorded by SPHEREx can be used to probe temporal variations in geomagnetic conditions. The work highlights that the SAA boundary, as defined by particle-induced transients, is a robust feature that persists even during solar storms, though the overall flux intensity varies. The study concludes that SPHEREx data can effectively map the SAA and geomagnetic poles, complementing existing models derived from particle flux measurements and magnetic field modeling.

The Satellite's "Sunburn"

The Accidental Discovery

The Analogy: The "Noise" Map

What They Found

Why This Matters

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