First terrestrial soft X-ray auroral observation by the Chandra X-ray Observatory

Northern auroral regions of Earth were imaged with energetic photons in the 0.1–10 keV range using the High-Resolution Camera (HRC-I) aboard the Chandra X-ray Observatory at 10 epochs (each ∼20min duration) between mid-December 2003 and mid-April 2004. These observations aimed at searching for Earth's soft (<2keV) X-ray aurora in a comparative study with Jupiter's X-ray aurora, where a pulsating X-ray “hot-spot” has been previously observed by Chandra. The first Chandra soft X-ray observations of Earth's aurora show that it is highly variable (intense arcs, multiple arcs, diffuse patches, at times absent). In at least one of the observations an isolated blob of emission is observed near the expected cusp location. A fortuitous overflight of DMSP satellite F13 provided SSJ/4 energetic particle measurements above a bright arc seen by Chandra on 24 January 2004, 20:01–20:22 UT. A model of the emissions expected strongly suggests that the observed soft X-ray signal is bremsstrahlung and characteristic K-shell line emissions of nitrogen and oxygen in the atmosphere produced by electrons.