Here is a reproduced article by ARRL.
Sunspot 1158 Produces Largest Flare of Solar Cycle 24; CME Headed Toward Earth02/15/2011
Over the past few days, there has been a lot of activity on the Sun. On Sunday, February 13 at 1738 UTC, sunspot 1158 unleashed an M6.6-level blast and on Tuesday, February 15, the same sunspot unleashed an X-class flare, the strongest solar flare in more than four years. On Monday, the solar flux index reached 113, the highest yet in Solar Cycle 24, dropping down to 112 for Tuesday. It is expected to continue to be at least 100 for the next few days.
The source of this activity -- sunspot 1158 -- is growing rapidly (click here to watch a video of the sunspot’s progression over a 48 hour period). Sunspot 1158 is in the Sun’s southern hemisphere, which has been lagging behind the northern hemisphere in activity for Solar Cycle 24. This active region is now more than 100,000 km wide, with at least a dozen Earth-sized dark cores in the group. More Earth-directed eruptions are likely in the hours ahead.
According to Spaceweather.com, Sunday’s eruption produced a loud blast of radio waves that was heard in shortwave receivers. That website reported that a New Mexico amateur radio astronomer recorded these sounds at 19-21 MHz, calling it “some of the strongest radio bursting of the new solar cycle.”
On Tuesday at 1516 UTC, the same sunspot unleashed an X2.2-class flare; X-flares are the strongest type of x-ray flare, and this is the first such eruption of Solar Cycle 24; the last X-class flare was December 13, 2006 (click here to watch a movie of this X-class solar flare). NASA’s Solar Dynamics Observatory (SDO) recorded an intense flash of extreme ultraviolet radiation. The expanding cloud may be seen in this movie from NASA’s STEREO-B spacecraft.
In addition to flashing Earth with UV radiation, data from data from NASA’s Solar Terrestrial Relations Observatory (STEREO) and its Solar Heliospheric Observatory (SOHO) show that the explosion also hurled a coronal mass ejection (CME) toward Earth. Geomagnetic storms are possible when the CME hits the Earth’s magnetic field on or about February 16, and auroras are possible.
What does this mean to radio amateurs? When the CME hits the Earth’s atmosphere, the low bands will be depressed and signals will be weaker the lower the frequency. The absorption rate will be most severe on 160 meters, less on 80 and somewhat better on 40 meters. The maximum usable frequency (MUF) -- the highest frequency by which a radio wave can propagate between given terminals by ionospheric propagation alone, independent of power -- will be lower and auroral propagation on the VHF bands is quite possible.
“Sunspot 1158 is firing off all sorts of flares and causing disruptions to the geomagnetic field,” said H. Ward Silver, N0AX, who edits the ARRL Contest Update. “Depending on how active it remains over the next couple of days, there may be significant impact to HF propagation for the ARRL International DX CW Contest this weekend. The length of the disruption by the CME is unknown. A head-on collision with a lot of plasma will keep things unsettled all weekend, while the recovery from a glancing blow or smaller amounts of plasma may occur relatively quickly. Polar path propagation on Friday morning will be the best indicator of conditions before the contest begins. Those operating on 10 meters at all the multi-multi stations are holding their collective breaths!”
What Is a Solar Flare?
A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time.
Solar flares extend out to the layer of the Sun called the corona. The corona is the outermost atmosphere of the Sun, consisting of highly rarefied gas. This gas normally has a temperature of a few million Kelvins. Inside a flare, the temperature typically reaches 10 or 20 million Kelvins, and can be as high as 100 million degrees Kelvin. The corona is not uniformly bright, but is concentrated around the solar equator in loop-shaped features. These bright loops are located within and connect areas of strong magnetic fields called active regions. Sunspots are located within these active regions and solar flares occur in active regions.
The frequency of flares coincides with the Sun’s 11 year cycle. When the solar cycle is at a minimum, active regions are small and rare and few solar flares are detected. These increase in number as the Sun approaches the maximum part of its cycle. According to NASA, the Sun will reach its next maximum this year, give or take one year.
Scientists classify solar flares according to their x-ray brightness. There are five categories of solar flares: X-class flares are big; these flares are major events that can trigger planet-wide radio blackouts and long-lasting radiation storms. M-class flares are medium-sized and can cause brief radio blackouts that affect Earth’s polar regions; minor radiation storms sometimes follow an M-class flare. Compared to X- and M-class events, C-class flares are small, with few noticeable consequences here on Earth. A- and B- class solar flares are not even noticeable on Earth. Each category for x-ray flares has nine subdivisions: C1-C9, M1-M9 and X1-X9. A brighter solar flare has a higher number, so an M6 solar flare is brighter than an M2 solar flare. Thanks to NASA, Spaceweather.com and SolarCycle24.com for the information