At the very edge of the exosphere, the outermost layer of Earth’s atmosphere, sunlight scatters off hydrogen atoms. This effect creates the geocorona, a luminous glow when seen in far-ultraviolet light. Its brightness and hydrogen density vary over time, especially after geomagnetic storms, but previous sounding rockets and satellites have never had the temporal resolution to study those shifts in detail.
Japan’s Hisaki satellite, launched in 2013, was intended to study the atmospheres and magnetospheres of the other planets in our solar system from a low orbit around Earth. In a lucky coincidence, the images captured by its Extreme Ultraviolet Spectroscope for Exospheric Dynamics (EXCEED) happened to contain information about Earth’s corona as well, masquerading as EXCEED’s foreground contamination. In a new study, Kuwabara et al. examine this useful information about geocoronal emission to assess observed brightness and density changes.
The researchers analyzed EXCEED observations during a 5-day time period in February 2014, during which three separate geomagnetic storms occurred. They found that between approximately 2 and 6 hours after changes in magnetic activity, the geocorona’s hydrogen density abruptly increased, indicated by an increase in brightness when seen in far-ultraviolet light.
After geomagnetic storms, huge amounts of energy that are injected into high-latitude regions in the thermosphere expand, transferring energy to neutral particles in the exosphere. This heating process affects the composition of the exosphere, but because the process isn’t complete until more than 10 hours after the storm, this slow energy transfer couldn’t be the cause of the geocorona’s observed sudden brightening. So what exactly was the cause?
The authors found that the brightening may have something to do with the plasmapause, the boundary marking where the atmosphere’s plasma density drops precipitously. Following the geomagnetic storms covered in this study, the EXCEED data showed that the plasmapause tightened around Earth, squeezing from 5 Earth radii above the surface to only 2. This change, like the changes in the geocorona, occurred between 2 and 6 hours after the onset of a geomagnetic storm at the same time as the geocorona’s brightening.
EXCEED observed charge exchange between hydrogen ions in the plasmasphere and hydrogen atoms in the exosphere, which then pushed hot particles to the geocorona. The time frame of the charge exchange supports the authors’ idea that the contraction of the plasmapause could cause the abrupt increase in exospheric hydrogen after geomagnetic storms. Further research into the behavior of Earth’s atmosphere after geomagnetic storms will help us better predict and prepare for them. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023247, 2017)
—Leah Crane, Freelance Writer