A solar flare is heading our way, so conditions will be terrible.

Have you heard s fellow amateur radio operator make statement, A solar flare is heading our way, so conditions will be terrible? Even the news media makes statements like this, as do popular blog: " A spectacular coronal mass ejection on June 7 sent a solar flare heading our way at a speed of 1400 km/s. The flare, said to be of medium size, is likely to result in spectacular aurora and other space weather effects with the potential to cause some communications, navigation, and power-grid problems." The problem with these statement is that they are inaccurate.

Solar flares do not head our way that these statement imply. Faithful readers of this columm know that solar flares often trigger a complex series of events that may lead to the release of solar plasma clouds do head our way, but also know that the flare itself is a nearly instant release of energy much like the burst of light of a flash bulb on a camera.

Solar flares are good examples of some of the most energetic natural explosive events known to man. Complex magnetic looping structures concentrated in an active sunspot region suddenly snap apart. Radiationis emitted across virtually the entire electromagnetic spectrum, from longwave radio frequencies, through the optical spectrum (the bright flash of a solar flare that is seen by the naked eye), to x-rays and gamma rays at tthe shortest wavelenght end. The amount of energy released is the equivalent  of millions of 100-megaton hydrogen bombs exploding at the same time!

This release of energy is nearly instantaneous and because this radiation of light and radio energy travels at the speed of light, it takes only about eight minutes for the energy to reach Earth. When an x-ray flare occurs on the Earth-facing side of the Sun, it takes only eight minutes for full impact of the flare's energy to reach our planet! That's why it is misleading for someone to say, " a solar flare is heading our way and the HF bands are going to be terrible tonight," as if sometimes in the coming hours, the results of the flare are going to make it here.

The x-rays from these events penetrate into the lower ionosphere and cause the D-region, which acts as a sponge that soaks up radio signals, to become more energized. The more ionized the D-region, the higher the frequencies that are absorbed and the stronger the absorption of the lower frequencies. Thus, radio signal from distant locations that travel through a flare-enhanced ionosphere are absorbed and become inaudible. These fideouts, which used to be called radio blackouts but are now known as Sudden Ionospheric Disturbances (SIDs), last only minutes for minor flares, to maybe an hour or so for the largest of flares. Once the flare is exhausted, the x-ray radiation fades and the ionosphere recovers to its normal level of ionization.

It might be that there is confusin about what actually may be heading toward the Earth when people talk about a " solar flare is heading our way." As all of this magnetic energy is being released during the explosive moment of a solar x-ray flare, and the Sun heats and accelerates the Sun's plasma caught up by the complex tangle of magnetic fields, and sometimes released these plasma particles as huge clouds out into interplanetary space. These billion-ton clouds are known as coronal mass ejections (CMEs), and they ride the solar wind out away from the Sun.

A CME travels very fast, but not as fast as the speed of light. Depending on how intensely the plasma is ejected during the explosive solar flare, the coronal mass ejection could take anywhere from one to four days arrive at Earth, if the CME was directed toward Earth. Not all CMEs are ejected toward Earth.

If a CME is directed toward Earth, it can cause a lot of havoc. When it hits our magnetosphere, we could see the geomagnetic activity turn stormy, which will cause longer-term degradation of HF propagation, as well as trigger auroral conditions. Geomagnetic activity has the effect of lowering the ionization of teh various ionospheric layers, which brings down the maximum usable frequency (MUF) over a given signal path. This lowering is much like what happens at night, when the ultravioler radiation of the Sun is blocked and ionosphere settles down. The stronger and longer the geomagnetic storm, the more depressed the ionospheric propagation becomes.

Some people think of the coronal mass ejection and the resulting geomagnetic storms collectively as a "solar storm." This may lead some to think of this as the "solar flare that is heading our way leading our way leading to bad radio conditions." However, now we know that a solar flare is a nearly instant event that effects radio propagation on the sunlit side of the Earth, with a possible longer-term event known as the coronal mass ejection. But wait! There is one more related event.

Readers of this column know that the Sun is always generating a solar wind. The solar wind is made up of protons and other particles that stream out away from the Sun with varying speeds. When CMEs plow through interplanetary space, they plow through this gaseous material, first in the Sun's atmosphere and then out in out the solar wind. Shock waves in front of the CME can accelerate these protons in our direction, causing what we know as "the proton storm."

When these accelerated protons are aimed toward Earth, they penetrate our magnetosphere and are funneled down toward the polar regions, forced along the Earth's magnetic field lines. These highly energized protons cause the D-region of the ionosphere to become highly ionized, effectively absorbing first the lowest wavelenghts of the high-frequency spectrum, then moving on the highest of the shortwave spectrum if the proton storm is extremely intense. This is known as a polar cap absorption (PCA) event, and this can occur as quickly as within hours of an x-ray flare, or perhaps day after the flare. When a PCA occurs, and radio signals over the polar region are absorbed, it shuts down radio paths between DX locations, depending on the polar paths, such as between central Europe and the United States.

Page 95-96 August 2011

Artikel ini mungkin memberi jawapan persoalan tentang fenomena ribut solar kesan kepada propagasi. Harap rakan ham dapat kepuasan jawapan oleh NW7US  seorang kolumnis di majalah CQ.

73, Jojet, 9W2DYA

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