Aurorae offer a beautiful spectacle to inhabitants of northern countries by
painting the night sky in red and green colors. What is less well known is
that aurorae are also X-ray emitters. Weak aurorae have a soft X-ray spectrum,
which drops off at an energy around 10 keV. Occasionally, however, the emission
can be much harder and can extend to more than 100 keV (see, e.g.,
Stadler et al. 1996;
Bhardwa et al. 2007).
This was the case on
20 November 2012,
when INTEGRAL was performing one of its new set of Earth observations (see
INTEGRAL POM of March 2006,
November 2013 and
February 2014).
Whereas this fortuitous emission is a disturbance for measuring the cosmic X-ray
background via occultation by the Earth, it is an interesting new source for
INTEGRAL science.
The right-side animation is a sequence of hard X-ray images - in the 17-60 keV
band - obtained with the IBIS/ISGRI instrument in short time intervals of only
5 minutes. The Earth position - shown by its coordinate grid - is kept fixed,
while it is drifting towards the border of the field of view (the nearing edge
at the bottom left). Its apparent size is decreasing due to the spacecraft receding
from Earth on its elongated orbit. Intense auroral emission is first visible on
the near side of the Earth (roughly around Iceland in the early evening), before
reappearing on the opposite side of the pole for more steady emission on a wider
area (in Siberia before dawn).
The animated plot on the left shows the evolution with time of the ISGRI/IBIS
count rate from the aurorae. The shaded area shows the time interval of the
simultaneous IBIS/ISGRI image displayed on the right. The three light curves
are in different energy bands: 20-32 keV (red), 32-60 keV (green), and 60-100 keV
(blue). The short flare on the near side of the Earth is found to be brightest at
lower X-ray energies. The second part of this auroral sub-storm event differs not
only geographically, but also spectrally with strong hard X-ray emission detected
up to 100 keV followed by a softer decay. These changes in intensity and X-ray
hardness are illustrated by the little sun-like symbol varying in size and color,
respectively.
Credits: M. Türler (ISDC, Geneva) and E. Churazov (IKI, Moscow & MPA, Garching)
