ASTROPHOTOGRAPHY by Joseph Cali


Aurora (?) no  it's airglow, on 30/31 October 2021
The Sun has been very active lately.  I've been keeping my eye on the two orbiting LASCO coronagraphs. Following the a large coronal mass ejection on 28 October, I sent out a heads up to the Yass Valley Astronomical Society members to be on the lookout for an Aurora Australis display on the night of Saturday 30 October around and after midnight. Based on the solar wind predictions, it was predicted to arrive at the Earth's atmosphere at 11pm  at a Kp=5 intensity and strengthen to 6 during the night. the Kp scale is a scale used to describe the strength of geomagnetic storms.  At Kp=4,  and at my latitude 34 south, the aurora is detectable photographically in short exposures looking south. At Kp 5 it is easier and at Kp 6, it can cover the entire sky as was supposed to have occurred that night. I might have been looking for an aurora, what I got was a major airglow event and no aurora. Airglow is chemiluminescence. Cosmic rays and solar radiation break the molecular bonds of  oxygen, O2  and nitrogen, N2 into their consituent atoms.  The energy required to break these strong bonds is released when these atoms recombine into molecules again. This is seen as a glow that occurs at around 90km above the Earth's surface.

The Sun had been quite active during October and so I was keeping my eye on the SOHO LASCO C3 wide field coronagraph camera.  Just after 16:00UT on October 28, a large class X1 flare erupted  propagating in the general direction of Earth(left (16:30UT) photo below). By 23:00UT the cosmic ray wave had hit Earth causing all the noise visible in the right hand(23:18UT) photo. In that photo, the visible extent of the flare on the image is about 25 million km. However, the flare was propagating towards us so in reality it was much greater. 


An X1 solar flare is capable of producing aurora visible all the way to mid-latitudes. My property is at 34 south latutude. The solar wind that produces auroral displays takes about 36-48 hrs to reach us.  It was predicted to reach Earth in the early morning hours of Sunday 31 October. I relaxed in the early part of the night then went out on aurora watch at around 11pm.

Unfortunately, that flare didn't arrive until after sunrise at my location so no aurora but I was treated to the best airglow display I have ever seen. To the naked eye, there was a faint colourless glow, the light not bright enough to trigger my photopic or colour vision. However my scotopic or night vision could faintly perceive it.  It was however, easily recorded on a still camera in all its glory. 

Airglow is an atmospheric phenomena that occurs at around 90km altitude and is caused by a variety of chemical and photochemical reactions between atmospheric gases. Solar radiation during the day can photo-ionise gases, cosmic rays spallate through the atmosphere, and chemical luminescence can occur from reactions between oxygen, nitrogen and free radicals in the upper atmosphere. 

This web page at the University of Albany lists seven different processes thought to cause airglow events. 

Storm growing in intensity at 13:05UT 12:05 am local time.
Pentax K1   ISO 12800   20s  Samyang 14mm f2.8
The little streak near the pointers might be some starlink satellites.
Several appeared one after the other on the same orbital track.

Above:  Pentax K1   ISO 12800  30s  Samyang 14mm f2.8  This photo was taken at 13:14 UT about
10 mins after the first photo I posted (top) and when the event had strengthened.  In this image
you can see Orion  on the far left, Sirius and Canis Major left of centre with Canopus and the Magell
anic Clouds on the right.  This photo spans the horizon north east through east to south.


  Above:  Pentax K1   ISO 12800  30s  Samyang 14mm f2.8  This photo was taken at 13:46 UT about
an hour after the first photo I posted (top) and when the event had strengthened.  In this image
you can see Orion and Taurus above the north and north east horizon.

A multipanel panorama showing about 1/3 of the sky.  13:40UT   Pentax K1   ISO 12800  30s  Samyang 14mm f2.8

Above: This photo was taken at 13:57 UT about an hour after the
  first photo I posted (top) and when the event had strengthened.
In this image, you can see the Magellanic Clouds (top) Pointers(lower left)
Jupiter (lower right) above the southern horizon.
Pentax K1   ISO  25600   30s  Samyang 14mm f2.8