Date: 98-05-11 23:33:14 EDT

From: (Skywatch International Inc.)


Reply-to: (Skywatch International Inc.)

To: (sky open list)


from the ARRL letter 5/11/98






Huge solar flares in late April and early May wreaked havoc on the ham bands

and other radio spectrum here on Earth. But the aftereffects of the solar

storms on April 30, May 2, and May 6--the first major geomagnetic storm in

years--continued for several days to keep HF noise levels higher than normal

and to disrupt HF skip propagation. "It has been an amazing week for solar

flares and geomagnetic disturbances," said propagation reporter Tad Cook,

K7VVV. "Suddenly the earth is getting bombarded by protons, and the immense

solar wind just doesn't let up." (see Tad's propagation report below).


Paul Harden, NA5N, of the National Radio Astronomy Observatory in Socorro,

New Mexico (, says several huge flares--explosions of

solar mass on the sun's surface--took place, one on April 30, two on May 2,

and three on May 6. As they occur, the flares emit high-energy radiation

from X-rays down to HF, producing about 20 minutes of "strong, bursty

static" here on Earth. But that's not the end of it.


The explosion throws heavy particles into the sun's atmosphere. Harden

explains that many of these particles get trapped in the sun's magnetic

field, spiraling along the flux lines and generating RF energy from about

800 MHz downward to--in this case--about 20 MHz. "This is called a Type III

storm," Harden said. "The RF sweeps downward in frequency about 20 MHz per

second, so if you were in a QSO, this Type III sweep would sound like a big

burst of static at regular intervals, almost like ignition noise." A Type

III storm lasts about 10 to 20 minutes following a solar disturbance.


The mass of electrons and protons traveling through the sun's magnetic field

produces electrical currents which, in turn, generate RF over a wide band of

frequencies simultaneously. "This is called continuum radiation or a Type IV

storm," Harden said. This produces the wideband noise on Earth--an elevated

noise level over much of the HF spectrum. A Type IV storm persists for about

an hour, Harden explained. But our troubles are not over here on Earth.


The "shock wave" of electrons and protons continues into space. "If the

trajectory is right, it can smack right into Earth, triggering a geomagnetic

storm." Harden says not all flares result in geomagnetic storms, however,

and the ones on April 30 and May 2 were not a direct hit. This is how flares

continue to make themselves known--and heard--for several days. A couple of

days or so after a flare, the shock wave hits Earth's magnetic field "just

like a big gust of wind," Harden said. "This causes our magnetic field to

wiggle and tremble like it was a sphere of Jello." The resulting electric

currents generate gobs of wideband noise. Electrons and protons traveling

along the magnetic field fall inwards into the ionosphere at the poles and

bunch up on the D layer. This makes it dense and difficult for radio signals

to pass through to the E and F layers, shutting down skip propagation.


Harden says D-layer absorption can tend to come and go during a geomagnetic

storm. "With a large solar disturbance, these electrons and protons keep

getting pumped into the earth at the poles for many hours--sometimes for

days--keeping this condition active," he said. While HF signals can't get

through the D layer, VHF can, sometimes resulting in unusual propagation

feats in that part of the spectrum. Many hams reported auroral conditions on

VHF during the recent storms. Harden says that in a geomagnetic storm, the

lowest usable frequency or LUF--normally about 2 MHz--can rise to 30 MHz.

"That would be a blackout, which many experienced," he said.


Harden compared forecasting such solar events to predicting the stock

market. It's not yet known if the shock wave from the May 6 flares will hit

Earth, but the forecast was calling for major to severe storming by May 8 or

9 and potential HF blackout conditions. Harden says that with the polar caps

already charged up, the May 6 events could trigger some aurora in the middle

latitudes. Effects tend to linger a bit in higher latitudes.


Cook suggests the recent events are part of Nature's give and take. "We are

seeing a big increase in solar activity, but with the increased sunspots

comes a downside, with flares disrupting HF communications, often to the

point of total blackout." To check the latest solar forecast, see