September 2010

Propagation Summary
August has been an interesting month with a massive M1 Solar Flare on August 7th which narrowly missed earth and resulted in the Northern Lights being visible as far south as Iowa, USA. A Persuid meteor shower also passed the Earth over a few days around 12th August which was actually visible to the naked eye in some parts of the world, which should have produced some interesting results for Radio Amateurs. (The next Persuid Meteor shower is forecast for 5-17 September, peaking on the 9th. http://www.imo.net/calendar/2010#julsep )

The NOAA report that On Saturday, August 14, 2010 a small solar flare erupted on the Sun at about 6am EDT. Associated with this flare was a coronal mass ejection (CME) that was partially directed towards the Earth. Also associated with this event was a S1 or minor solar radiation storm on the NOAA Space Weather Scales. The only impacts expected for a solar radiation storm of this magnitude are minor impacts to HF radio communications in the polar regions. However, this is the first solar radiation storm of Solar Cycle 24 and the first solar radiation storm since December of 2006. http://www.swpc.noaa.gov/ .

However, the sunspot number progression chart at Solarcycle24.com still shows sunspot activity to be actually lower than predicted. ( http://solarcycle24.com/sunspots.htm ) Apart from a noticeable disturbance around the 23rd, August should end with relatively steady conditions with the Boulder A index at 5 and the K index at 2, and is forecast to remain the same until mid September. The Solar Flux is forecast to return to a steady 85 during this period. WM7D.net

Solar Cycle 25
Peaking around 2022, Solar Cycle 25 could be one of the weakest in centuries.
The Sun's Great Conveyor Belt has slowed to a record-low crawl, according to research by NASA solar physicist David Hathaway. "It's off the bottom of the charts," he says. "This has important repercussions for future solar activity." http://www.physorg.com/news66581392.html

Radio Fadeouts and Solar Flares
Solar flares produce copious amounts of electromagnetic radiation, the X-ray component of which increases the ionisation of the ionospheric D layer. HF communication generally depends on the reflection of signals from the higher F layer and such signals must travel through the D layer at least twice. Increased ionisation results in greater absorption of the signal in the D layer.
This effect is known as a short-wave fadeout (SWF) and is observed as an increased attenuation of HF signals particularly at the lower frequencies. The fadeout follows closely the pattern of the solar flare, being observed at the same time as the flare. Fadeouts mostly have a rapid onset of a few minutes and a slower decline lasting perhaps an hour (this is highly variable).
A property of SWFs is that they affect the lower HF frequencies more than the higher ones which may not be affected at all. The high frequencies are the last to be affected and the first to recover.
An important feature of SWFs is that the HF circuit is affected only if there is an ionospheric reflection point for the signal in the sunlit hemisphere. No effect is observed if all the reflection points are located in the night hemisphere which is shadowed from the X-rays from the flare.
The intensity of flares at X-ray wavelengths allows us to estimate the extent (both geographical and in frequency) of a fadeout. This intensity is now measured by satellite and so IPS has established a page on the Internet to show the extent of fadeouts in near real-time at:
http://www.ips.gov.au/HF_Systems/6/2/1

Links to these articles and more can be found at www.jameswelsh.org.uk