Is Ionospheric Propagation In Decline?
2001, a small group of LF experimenters met at the HF Convention held at Old Windsor,
Berkshire. This group of old timers comprised: Bill G0AKY; John G3BDQ; Colin G3KMP; Dave
G3YMC; and myself. At some point, the conversation digressed from 73 and 136 kHz topics to
our memories of HF band conditions when we were first licensed. There was general
agreement that HF radio propagation appears to have been in decline over recent decades,
and a number of possible causes were discussed.
Since that meeting, I have taken the opportunity to discuss this matter with other old
timers. I was interested to discover that all those asked supported the view that the
ionospheric mirror is not as effective as it used to be. This view is shared by Ron G6RO,
and Vic G8IK - both having personal experience of low power HF operation dating back to
the 1930s. Vic has given the matter some thought, and wonders whether a decline in HF
radio propagation might be related to long-term changes in the Earth's magnetic field.
I am interested in hearing from others who have a definite view as to whether HF
ionospheric propagation has, or has not, been subject to long term decline over several
decades, with brief supporting remarks. I am especially interested in hearing from those
having access to relevant signal measurement data.
I propose to summarise the responses received, and post the results on this web page.
My email address is:
Last September, British scientists announced that the ionosphere, the
upper layer of Earth's atmosphere, has actually dropped! Located about 90
kilometers (56 miles) above the planet's surface and reaching up to 500 km
(310 mi), the ionosphere has fallen by 8 km (4.8 mi) in the last 38 years.
The culprit, according to scientists from the British Antarctic Survey
(BAS): global warming.
Fossil fuels like coal and oil belch carbon dioxide into the atmosphere
as they burn and trap heat in Earth's lower atmosphere. Result: the cooling
of the ionosphere, so that atmospheric pressure (pressure caused by the
weight of air) decreases. The ionosphere then contracts and drops in
altitude. "It's another indication that humans may be changing the very
environment . which supports us, even out to the edge of space," claims
Martin Jarvis, the BAS lead researcher.
To measure the ionosphere's height, researchers in the Falkland Islands
and Antarctica beamed radio waves up 250 km (150 mi) into the atmosphere.
Ultraviolet rays from the sun remove or add electrons (negatively charged
particles) to atoms in the ionosphere --a process called ionization. Ionized
atoms cause radio waves to bounce back to Earth's surface. By measuring the
time it takes radio signals to rebound to Earth, researchers were able to
track changes in the ionosphere's height.
The sinking ionosphere shouldn't harm life on Earth, says Jarvis, and the
ionosphere could bounce back up if humans stop pumping greenhouse gases into
the atmosphere. At the moment, that's a big if. Maybe Chicken Little has the
COPYRIGHT 1999 Scholastic, Inc.
Graph of annual sunspot numbers 1700 - 1995
HAARP facility: Index of Information About the Ionosphere + loads of links
Solar data from NGDC
Steve Rawlings, GW4ALG
9th February 2005