wpo - Aurora

atmospheric phenomena

2003 Oct 29/30: Aurora images by colleague Mike Fantham during trip to Scotland following a powerful solar outburst 48 hours earlier.

2001 Oct 21: Following a phone tip-off from Ron Johnson I went outside about 10pm [2100UT] to see a rare [for the London area] aurora in the NW through to NNE.  Above a yellow sodium illuminated cloud over London, red rays were weakly seen reaching nearly to Polaris - changing from moment to moment.

Ron Johnson sequence via SLR camera with f/2.8; 28mm fl lens+ 200 ISO print film; exposures 20s - 60s from Ewell Court - Surrey.

Maurice Gavin's sample image below via Minolta DiMAGE 7 digital still camera [auto mode - f/2.8; 7.2mm fl - 28mm equiv; 4s exp.] reveal the rays clearer than seen visual.

Neil Bone emails following above event - Well, further to our conversation on Saturday, guess what? Aurora from Chichester (and many other points in an unexpectedly clear, if hazy, south of England!) last night. Not seen from Scotland due to overcast, where it would undoubtedly have been spectacular.  From the field round the back of our house, I saw a low, fairly bright glow, mostly under the Plough to about 25 degrees altitude. Interludes of diffuse, reddish rays stretching long to Polaris came and went. My impression is that the display was at its brightest around 2040 UT, when the northern sky was decidedly pink with diffuse light.

Best site is <www.sec.noaa.gov> and most useful part accesses SpaceWeather Now. This features a couple of very useful real-time displays, based on live data from the ACE satellite at the L1 point, 1.5 million km upstream of Earth in the solar wind (in terms of particle arrival times, about a 1-hour warning - useful for satellite operators).

On the left hand side of this screen is a 'meter' for the solar wind magnetic field polarity and strength. If the needle's pointed southwards, and particularly if it's close to the stops at 50 nT, then things are looking encouraging for solar wind/terrestrial field reconnection.

Another useful guide is the panel with the solar wind velocity.  Under normal conditions, this is around 400 km/s; sometimes it drops to about 200 km/s if things are unusually quiet. If the solar wind velocity creeps up to 600 km/s or faster, then there could be some fireworks provided this is accompanied by a southerly-directed interplanetary magnetic field. Instances of high solar wind velocity alone will compress the magnetosphere, but not necessarily lead to the particle overload which stresses the system and leads to low-latitude aurora. Also shown is a guesstimate of where the auroral oval should be, based on these data but shouldn't be taken too literally.

From the SWN page, connect to 'Today's Space Weather'. which gives a general overview of what's happening. Solar X-ray activity is particularly worth following, as one can readily see when big flares are going off. They've recently appended the flare category scale to the (logarithmic) Y-axis, so it's easy to distinguish the M- and X-class events which are of particular relevance. These will only produce aurora, of course, if accompanied by Earth-directed halo CMEs (occurrence of these is usually given in the text forecast, and a look at the SOHO site is always a useful, informative backup).

Also on here are GOES satellite data for their local magnetic environment. Usually this shows a gradual diurnal fluctuation. When a geomagnetic storm sets in, however, the values start to fluctuate very rapidly and erratically (cf last night - this was my first warning that something was afoot!). Based on these, the site has a running histogram near the bottom of the page, showing estimated Kp: once this goes into the red, chances of aurora improve, especially if the values hit 7 or more.

As an aside, one of the more fascinating bits is the proton flux, measured at three energy levels. Quite often, the big flares are followed within minutes by the red proton trace jumping to high levels - this a result of near-lightspeed protons ejected during the flare.  These are what should send astronauts running for their lead underwear!

Taking all these bits together, then, it is possible to make a reasoned guess as to the state of activity and the likelihood of seeing low-latitude aurora. However, as I say endlessly when I lecture on the subject, there are no guarantees in all this: even the professional forecasters are only making educated guesses !  Neil Bone


images/text copyright - Ron Johnson/ Neil Bone/ Maurice Gavin - October 2001/2003