Saturday, January 28, 2012

How do Solar Flares Affect Propagation

This is from Paul NA5N explaining how all this works in the Elecraft reflector - and so I am re-posting here for posterity

And I realize we have many new hams who are learning how propagation works on the HF bands, now that the solar flux is above like 65 :-( and of course what all this solar flare, CME,
geomagnetic storm stuff is all about.

So here goes.

Friday, there was a fairly large X2 (X1.7 to be exact) solar flare on 27 January at 1837Z. An X-class flare is the highest category and can cause radiation storms on earth and effect HF propagation - some of it good. Go here to see the x-ray emissions from this flare: The first chart is the x-ray emissions as detected by two different sensors on the GOES-15 satellite. The X-class flare is easily seen towards the end of the UTC day on 27 Jan. X-rays are ionizing radiation, that is, they can knock electrons away from their host atoms and molecules. In our upper ionosphere, this ionizing radiation knocks electrons away from oxygen, nitrogen and hydrogen atoms. These electrons just roam around in our ionosphere, being knocked around by the ionizing radiation from the sun. For this reason, they are called "free electrons," not currently being associated with a host atom. The more free electrons in the ionosphere, the more reflective are the E and F layers, and the higher the maximum usable frequency (MUF). During a solar flare, ionizing radiation increases almost immediately, producing more free electrons in our ionosphere, making the E and F layers even
more reflective, and often raising the MUF. This condition quickly improves HF propagation.

Therefore, for QRPers, solar flares are often a good thing. From the time of the flare until local sundown, enhanced HF propagation will be present. With higher reflectivity, this means QRP signals get reflected more efficiently for an environment of working longer skip distances (and new DX) than normal. Once the sun goes down ... that is, when the ionosphere above our heads is no longer illuminated by the sun and receiving the solar x-rays, the free electrons recombine with their host atoms, reflectivity and the MUF drops, and we fall back into normal night time propagation. The lack of ionizing radiation and free electrons is why the MUF drops at night, and the higher day time bands shut down. During the day, this ionizing radiation penetrates deep into our ionosphere, causing a layer of free electrons we know as the D-layer. Seldom do our signal bounce off this layer, but penetrates it. Unfortunately, the electron density of the D-layer does eat up (attenuates) some of our signal. At night, solar radiation and ionizing radiation is gone. The E and F layers combine, and with no deep penetrating radiation, the D-layer disappears. Without the attenuation of the D-layer, this is why signals appear stronger and less noisy at night - because they are!

As stated above, a solar flare is often a good thing from the time ofthe flare until local sundown ... except in those cases of a very strong solar flare. Its radiation can be so strong that the D-layer becomes nearly saturated with free electrons, such that signals can not pass through at all. Higher above our heads, the E and F layers are also saturated with free electrons and the MUF drops quickly, sometimes to a few MHz or less, or below the lowest usable frequency or LUF. This is a radio blackout. The D-layer consumes virtually all of your signal power, and the MUF can fall to below 3 MHz. This extreme case of a total radio blackout is fairly rare.

Friday's X-class flare was associated with a CME - a coronal mass ejection. As the name implies, a CME is where the flare belched out copious amounts of solar mass - mostly electrons and protons. This is usually an explosive event, forming a shock wave as the CME travels outward from the sun. While the x-rays from the sun travel at the speed of light, reaching the Earth in about 8 minutes, a CME travels much slower than light speed, reaching the Earth in about 3 days ... if the flare and the CME is located near the center of the sun. If the flare is located near the edges, or limbs, of the sun, the CME will travel outward into space, but away from the earth.

Today's X-class flare was a doozie. The shock wave was measured at 1,532 km/sec., about 950 miles per second, and about 3.5 million miles per hour. Anything over about 1,000 km/sec. is considered a strong shock wave and almost guaranteed to trigger a major geomagnetic storm about three days later - if it hits the earth.

Friday's X-class flare occurred in region 1402, located on the limb of the sun. In fact, that region will rotate out of view by tomorrow. Therefore, this strong shock wave is traveling away from the sun and away from the earth. It will not hit us, so it will not trigger a geomagnetic storm. If it were to hit the Earth, it would have produced a severe geomagnetic storm, the type that can even shut down portions of our power grid and knock satellites out of orbit. We escaped this one.

Go to:

The map of the sun on the left shows where the current active regions are located on the sun. As you can see, region 1402 is on the extreme edge of the sun and rotating out of view. Region 1408 is smack in the middle of the sun. Should a flare occur from 1408 in the next couple of days, that shock wave will hit the earth. (1408 is a weak, unorganized region and not likely to produce a major flare at present). Region 1410 is just now rotating into view and will be towards the center of the sun in a few more days. That is the area to watch for it to grow into an active region capable of producing flares by early next week.

NOAA issues a daily summary of solar and geomagnetic activity, and a forecast for the next three days, located at:

In Section IV "Penticton 10.7 cm Flux" you will see today's solar flux was 142 and the forecast for the next three days (i.e., the weekend) is 120, 120 and 120. Why would the solar flux drop from 142 to 120 so quickly? Like overnight? The reason is because Friday's X-class flare was also characterized as a "Ten Flare." This means the flare was so strong, it affected the solar flux as measured at 2880 MHz, or 10.7 cm (the Ten Flare thing), where the solar flux is measured. Thus, today's daily solar flux was elevated (contaminated) due to the enhanced ionizing radiation from the flare. Tomorrow, back to normal solar radiation and back to the normal solar flux of about 120. Though, that is still fairly high for the higher bands to be open during daylight hours.

So if you want to work into areas where your QRP signals don't normally reach, start keeping an eye on the solar x-ray emissions (at for an M- or X-class solar flare. For the rest of the day, you will most likely experience good signal propagation with perhaps an opening of the next highest band. For those of us who still work, I've noticed the good flares always happen like2-3 in the afternoon with only moments left in the day by the time I get home :-(

At the VLA radio telescope, we were observing today at L-band (1-2 GHz) and S-band (2-3 GHz). The solar flare did affect observing for about 20 minutes in that the "sky temperature" suddenly increased due to the flare. Our switched noise source calibration, which constantly measures system temperature, detected the sudden rise in sky temperature and flagged the data so it was not used for science for the duration of the flare. The biggest problem at the time is nobody knew what caused a 200 degree K shift in system temperature. Basically, the electronics system is always blamed until they realize it was a flare!

Keep an eye on the sun. Solar activity can be advantageous to QRPers if you learn a little bit of the physics and learn to read the "tea leaves." Don't let the news reports of solar flares scare you off the bands.

If you have any questions, ask them on the group and I'll answer, or one of the other seasoned QRPers.

72, Paul NA5N

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