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The Ionosphere
May 13, 2013
As a
teenager of
my generation, my choices for
entertainment were limited. There was no
Internet, there were just three
television stations, two of which were very fuzzy, and the local
radio stations played music that could have been
background music at an
ice skating rink or in an
elevator. As a consequence, I did a lot of reading, which, in retrospect, wasn't a bad thing. Now, I can
split infinitives with the best
bloggers.
Another refuge besides reading was
AM radio after
sunset. The
US AM radio broadcast band presently has
frequency assignments between 540 and 1700
kilohertz (kHz); but, at that time, only between 540 kHz to 1600 kHz. Why after sunset? At
night, the only active layer of
Earth's ionosphere is its highest layer, the
F-layer, and this allows long distance AM radio reception.
The F-layer consists of
ionization at
altitudes from about 120
miles (200
kilometers) to about 310 miles (500 km). This ionization will reflect
radio waves, so
transmissions from about double these distances can be received after a single "
bounce." This allowed my reception of the more powerful
Detroit (450 miles) and
Chicago (650 miles) radio stations from the west, and
Boston (250 miles) radio stations from the east.
The ionosphere doesn't reflect all radio frequencies, just those above a certain
wavelength. The
electrons in the ionosphere are ineffective at reflecting radio waves above a certain
critical frequency, which is a simple function of the
square-root of the
electron density. Under some conditions, it's possible for all radio waves below about 50
megahertz (MHz) to be reflected; but, since the
very high frequencies (VHF) are defined to start at 30 MHz, we usually consider that point to be a useful cut-off.
So, we're blessed in two ways. First, the easily generated frequencies (those used by
Marconi and his colleagues),
propagated much farther than expected. Then, the transparency of the ionosphere above a certain frequency means that the
Earth is not isolated from the radio
universe. At high enough frequencies, we still have
radio astronomy and the ability to signal our
satellites and
spacecraft.
Radio astronomy pioneer,
Grote Reber, who was the world's second radio astronomer (
Karl Jansky was the first), decided that he wasn't going to let the ionosphere come between him and the low frequency radio waves of
outer space. I wrote about Reber in a
previous article (LOFAR, Not LOTR, March 2, 2011).
Grote Reber's first radio telescope antenna, built at his home in Wheaton, Illinois, a suburb of Chicago, in 1937.
Reber was an an amateur radio operator (W9GFZ). He built this 31.4 foot diameter parabolic antenna from iron, which was a cheap, conducting metal.
(NRAO photograph by Grote Reber, via Wikimedia Commons.)
Reber's first step in his quest for extraterrestrial radio signals around the AM broadcast frequencies was to move to
Tasmania. In that region of the Earth, most of the ionosphere will neutralize during long and cold
winter nights. Tasmania was also removed from much of mankind's
interfering radio signals. Reber made his low frequency observations, generally in the range 0.3-3 MHz, with support from the
University of Tasmania.
A recently commissioned radio telescope,
LOFAR, has been designed for observations in the 10-240 MHz frequency range, which includes frequencies below the ionospheric cutoff. LOFAR is composed of thousands of
antennas distributed across
The Netherlands,
Germany,
Great Britain,
France and
Sweden.
Digital signal processing by a
Blue Gene/P supercomputer combines these signals into one huge
interferometer.
Because of its low frequency monitoring capability, LOFAR can observe changes in ionospheric transparency. One application of this is detection of distant
gamma ray bursts.
Gamma radiation incident on
Earth's atmosphere will cause additional ionization, so extraterrestrial signals will be blocked, and the strength of long-distance terrestrial signals will be enhanced.
This effect has been noted for several sources; namely,
SGR 1806-20, a
magnetar and gamma ray repeater detected on December 27, 2004;[vlf.stanford]
GRB 030329, a gamma-ray burst detected on March 29, 2003; GRB 830801, detected on August 1, 1983; XRF 020427, an Xray flare detected on April 27, 2002; and a flare from
SGR 1900+14, a
soft gamma ray repeater, detected on Aug. 27, 1998.[4]
These gamma ray events can pack quite a punch. SGR 1806-20 is about 50,000
light years from Earth, but it ionized the atmosphere down to about 50,000 feet (20 kilometers), nearly down to the level of
commercial aircraft.[5] The signal had a peak lasting a few seconds, followed by smaller intensity signals for an hour. This gamma ray event increased the density of ions at 60 kilometer altitude by a huge factor, from 0.1 to 10,000 free electrons per
cubic foot.[5]
The ionosphere also has small-scale, local variations in its properties. One example is the ionization created by
meteors,[6] but the transition time from the daytime to the nighttime ionosphere can be turbulent, with violent ionospheric storms evident in the
equatorial F-layer a few hours after sunset. These storms can affect even high frequency communications.[7]
Local disturbances in the ionosphere cause problems for even higher frequency signals, such as those of the Global Positioning System.
(Air Force Research Laboratory image.)
NASA launched a
sounding rocket mission, called
EVEX (Equatorial Vortex Experiment), on May 1-9 from the
Kwajalein Atoll in the
Marshall Islands to study these ionospheric disturbances. One portion of EVEX consisted of two sounding rockets, launched a few minutes apart. One of these sounding rockets traveled to a high altitude, and the other to about half that altitude, to record data about
electric field strength and the density of the
charged particles.[7-8]
These rockets also released a
tracer chemical,
trimethylaluminum, which decomposes into fine particles of
aluminum oxide. The tracer stream was optically
triangulated to show
wind direction.[7] This wind, called a neutral wind, is thought to be an important element in the formation of ionospheric storms.[7]
Stanford University has a program in which
high school students can participate in ionosphere research. High schools can participate in the
Space Weather Monitor Program by installing an inexpensive
receiver to monitor a very low frequency (VLF)
transmitter. Data are collected on a
personal computer, and they are uploaded to the program site.[9]
References:
- Grote Reber, "Cosmic Static," Astrophysical Journal, vol. 91 (1940), pp. 621ff.
- LOFAR Web Site.
- Gamma-ray Burst Effects on the Ionosphere, Stanford University VLF Web Site.
- Doug Welch, "GRB030329 observed as a sudden ionospheric disturbance (SID)," GCN GRB Observation Report No. 2176, MIT Space Web Site.
- Dawn Levy, "Big gamma-ray flare from star disturbs Earth's ionosphere," Stanford Report, March 1, 2006.
- FM Radio Detection of Meteors.
- Karen C. Fox, "NASA Mission to Study What Disrupts Radio Waves," NASA Goddard Space Flight Center Press Release, April 25, 2013.
- Marshall Islands Campaign Completed, NASA Goddard Space Flight Center Press Release, May 9, 2013.
- Web Site of the Space Weather Monitor Program.
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