Detection of EUV/Soft X-ray bremsstrahlung emission at terrestrial altitudes above 750 km

Abstract:

LYRA is a fast radiometer on-board the PROBA-2 mission designed to observe the solar activity from UV to Soft X-rays and consists of three redundant units of four different optical bandpasses each.

Since the start of operation in 2010, LYRA regularly observes disturbances with a characteristic signature that have no direct solar origin. Instead the frequency of occurrence correlates with the $A_p$ index of geomagnetic activity on Earth's surface and the location of these detections coincides with the McIlwain L $\approx$ 3 zon.

By comparing the wavelength sensitivity of the main PROBA-2 instruments, the wavelength range of the detected photons can be narrowed down to the range of 0.07-1 KeV (1-17 nm) and the altitudes of their source to those above PROBA-2's orbit (~750 km). A discussion on the magnetospheric origins of this emission is included.