Because the currents of electrons that cause auroral emissions are very energetic, with energies in the kiloVolt range. This means that it is possible to examine the details of the collisions betwen the very energetic electrons with atoms of nitrogen and oxygen by detecting the feeble x-ray light that these collisions produce. Because the atmosphere does not produce x-rays even in daylight, it is possible to observe auroral physics even in the daytime! The fact that sunlight is present, normally washing out the optical emission of auroras, is irrelevant to detecting the same aurora as they move from nightime into daylight. This allows almost constant monitoring of auroral physics by looking at their X-ray light.