Subj: (Skyopen) SOLAR EVENTS DISRUPT RADIO SPECTRUM
Date: 98-05-11 23:33:14 EDT
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from the ARRL letter 5/11/98
SOLAR EVENTS DISRUPT RADIO SPECTRUM
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 (firstname.lastname@example.org), 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