During the powerful Halloween solar storm of 2003, eerie auroras like these lit up the skies as far south as Texas. Credit: Christer Olsen, CC BY-SA 2.0
Source: Journal of Geophysical Research: Space Physics

During Halloween of 2003, it wasn’t the Moon that had space scientists raving—it was the Sun. Erupting with flares from its surface and particles bursting from its outer corona, it unleashed some of the most powerful solar storms ever recorded between October 19 and November 7. At their peak, the Halloween storms of 2003 knocked out power grids in Sweden and caused satellites to fail. Sky gazers reported auroras as far south as Texas and the Mediterranean.

The storm even had long-lasting effects on Earth’s magnetic field: The Sun’s outbursts injected it with massive amounts of high-energy electrons that remained trapped for years. In the storm’s aftermath, scientists studied this new population of trapped particles using satellites, including NASA’s Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite.

But now Selesnick reports that at least seven SAMPEX studies were based on a faulty interpretation of the data. Most of the high-energy Halloween electrons that scientists thought SAMPEX had detected were probably instrumental “ghosts”—artifacts caused by coincidence, when two slower particles struck at once, giving the appearance of a high-energy electron.

The misconception stems from data collected by an instrument called the Proton-Electron Telescope. It has multiple detectors, each of which are calibrated to different sensitivities. A low-energy electron will be able to trigger only the most sensitive detector; an electron with an energy greater than 1 MeV will trip two of them in sequence. Using these data, many studies reported that in the aftermath of the 2003 Halloween storm, the space around Earth was flooded with electrons with energies as high as 6 MeV.

But if two electrons happen to come in at nearly the same time—roughly a millionth of a second apart or less—the instrument may register a high-energy electron when none existed. When the author calculated how often these ghost particles should appear in the months following the 2003 Halloween storm, he found that they were nearly as frequent as the reported rates of actual particles. In other words, SAMPEX hadn’t detected actual high-energy electrons at all.

The author notes that this doesn’t necessarily mean that there were no Halloween electrons at all—just that SAMPEX couldn’t see them. And the SAMPEX data are far from useless—the author shows that during calmer space weather conditions and in the inner regions of Earth’s magnetic field where background radiation levels are low, SAMPEX can measure electrons up to about 1.6 MeV. But scientists will need to design hardier instruments in the future to measure storms in Earth’s intense radiation belts. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA021387, 2015)

—Mark Zastrow, Freelance Writer

Citation: Zastrow, M. (2015), The curious case of the Halloween ghost electrons, Eos, 96, doi:10.1029/2015EO038435. Published on 30 October 2015.

Text © 2015. The authors. CC BY-NC 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.