Modeling variations observed in soft X-ray signals detected by satellites

Models capture how solar wind charge exchange events are observed.

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The German orbital X-ray telescope ROSAT started seeing significant signal changes in the soft X-ray region of the spectrum in the 1990s that lasted for a few days. These resembled the copious flux of soft X-rays from the comet Hyakutake that was found at about the same time.

It was suggested that these were caused by the solar wind, fluxes of charged particles coming from the sun, and how they interacted with neutral ions in the geocorona, the uppermost region of our atmosphere. The distinctive spectra of these occurrences, known as solar wind charge exchange events (SWCX), were verified by more thorough studies in the 2000s, and the process was widely recognized.

However, modeling how solar wind gives rise to the measurements taken by orbital telescopes proved much more difficult. It requires successfully capturing the arrival of solar wind events, how the charged particles interact with neutral atoms, and how that affects the magnetosphere.

Researchers from Tokyo Metropolitan University used numerical techniques to predict the fluctuations in soft X-ray signals picked up by X-ray satellites. They examined data from the Suzaku telescope and compared it with simulations of how solar winds would interact with the uppermost regions of our atmosphere.

By doing so, they were able to capture how the signal varied with the orbital motion of the satellite, with implications for how predictions can be made for future satellite experiments.

A highlight of the team’s work is the wide range of astrophysical events they can bring together and how they may be mapped onto real data.

A remarkable achievement in the field, the model demonstrated outstanding concordance with experimental data, recreating the signal seen up to a factor of two. Additionally, they could reproduce the unusually potent signal fluctuations when the satellite’s line of sight coincided with the polar cusps. There were some notable outliers, such as when a severe geomagnetic storm was seen; still, the variations’ ability to be successfully reproduced offers great promise for forecasting the results of the upcoming generation of X-ray observations in space.

Journal Reference:

  1. Daiki Ishi et al. Modeling of geocoronal solar wind charge exchange events detected with Suzaku. Publications of the Astronomical Society of Japan, psac095, DOI: 10.1093/pasj/psac095
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