This blog post is part of our regular “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
More evidence of increasing solar activity
The lengthening days of summer and the increase in activity as Solar Cycle 24 gets under way have produced some welcome results for observers of Sudden Ionospheric Disturbances (SIDs). SIDs are caused by dramatic increases in solar radiation hitting the upper regions of the atmosphere, which change the way radio waves are reflected, and hence the strength of the received signal at ground level monitored by SID observers.
The past 18 months have tended to be characterised by the plots shown in Figure 1, with sunrise and sunset effects dominating a midday ‘desert’. However two SID events are instantly recognisable in the plot for May 5 (Figure 2). A C8.8 flare at 11:40 UTC resulted in strong increases in received signal levels from two Very Low Frequency (VLF) radio transmitters: St Assise in France (at a frequency of 20.9kHz), and Skelton in Cumbria (22.1kHz). There was an even more pronounced reduction in the signal received from Anthorn (19.6kHz), also in Cumbria. An M1.3 event at 17:15 produced even larger disturbances.
Figure 1. Received signal levels on 2010 May 1. Paul Hyde, Basingstoke, UK
Figure 2. Received signal levels on 2010 May 5. Paul Hyde, Basingstoke, UK
Not quite so obvious is the disturbance caused by a C2.3 event at 07:10, though closer inspection shows the telltale signs of a fast initial change in signal level as ionisation levels in the ‘D’ Layer of the Earth’s atmosphere quickly rise, followed by a much slower return to normal signal levels. There are now more than a dozen SID observers providing data to John Cook who produces a monthly consolidated report for members. His report for the last 12 months can be found in the June edition of the Journal. Further information on SIDs and the equipment for observing them can be found on the RadioAstronomy Group website, www.britastro.org/radio.
Paul Hyde, Coordinator, Radio Section
This blog post is part of our regular Thursday “From the BAA Journal” series. This series features a selection of articles, news, reviews and letters from the Journal of the British Astronomical Association, published six times a year.
News from the Radio Astronomy Group, by John Cook
Over the last 12 months, solar cycle 24 has at last started to show some real flare activity as active areas on the Sun begin to develop in complexity and size. Fig 1 below shows our recorded Sudden Ionospheric Disturbances (SIDs) over the last 5 years.
As the SIDs that we record are from solar flares occurring only while the Sun is above the horizon, monthly figures are skewed by the varying day length over the year. 2008 ended with a single C-class flare in December, then followed 6 months with no recorded flares. In 2009 July we recorded 2 C-class flares, while August and September were again blank. October included 3 B-class and 3 C-class flares, November was again blank, while December produced 6 C-class flares. The chart includes 2010 January which, despite the short day length, produced the highest level of activity in 31 months with 3 M-class and 10 C-class flares.
Fig 2 below shows the visual relative sunspot number (R) over the same period, as recorded by the Solar Section. Visual activity is clearly rising through the first half of 2009 despite our lack of recorded flares. The active areas seen over this period were mostly small spots which were inactive, showed little complexity, and did not develop much while visible. The active areas in October and December showed more complexity (hence the higher ‘R’ recorded), while those in 2010 January grew quite rapidly. Note that the smoothed curve for ‘R’ is derived over 13 months, and so the last 6 months shown are provisional.
Fig 1: VLF flare activity recorded 2005–2010
Relative sunspot no. 2005–2010
Observers in 2009 were Roberto Battaiola, Colin Clements, John Cook, Mark Edwards, Paul Hyde, Mike King, Peter King, Bob Middlefell and John Wardle.
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