Ionospheric Scintillations 

Prior to the use of the phrase “space weather” to summarize all possible effects of solar-terrestrial phys­ics upon technological systems, the U.S. Department of Defense (DOD) created and maintained active programs in the application aspects of space physics. The person arguably most associated with those efforts was Jules Aarons, who died on 21 November 2008 at age 87 at his home in Newton, Mass. Jules was a re­search professor of astronomy and space physics at Boston University from 1981 to 2005, but it was as a civilian scientist at the Air Force Cambridge Research Laboratory (AFCRL) from 1946 to 1981 that Jules emerged as a true leader in studies of how the ionosphere can affect radio communications. He specialized in scintillations, those serious fluctuations of radio signal amplitudes and phases that cause dropouts in otherwise reliable com­munications links.

Prior to the space age, the Army, Navy, and Air Force required robust transmitter- to- receiver pathways for the high-frequency (3- to 30-megahertz) sig­nals governed by ionospheric reflections. When the radar technologies developed during World War II evolved into the new field of radio astronomy, Jules promoted the use of celestial radio sources (such as the radio star Cassiopeia A) as transionospheric probes of the regions on Earth where radio scintillations occur and the use of spaced receivers to study how the scintillations drift. When receivers were used at higher frequencies, the scin­tillations changed but did not go away, making it possible to study how different radio wavelengths probe the spatial scales of irregularities in the ionosphere.

Also prior to the launch of satellites, Jules and colleagues advanced the con­cept of using radio reflections from the Moon as a means of long-distance com­munications for DOD needs, as well as a way to study basic ionospheric structure. In addition to conducting experiments in the United States, Jules fostered interna­tional cooperation by arranging for U.S. support for the radio astronomy team of Bernard Lovell, Gerald Hawkins, and John Evans to conduct lunar reflection experiments at the Jodrell Bank radio observatory outside of Manchester in the United Kingdom. Their subsequent prom­inence in the fields of radio astrophysics, meteor science, and ionospheric physics, respectively, followed from equipment provided under Jules’s auspices.

Note: Text by Michael Mendillo, Professor of Astronomy at Boston University

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