Relative Ionospheric Opacity Measurements

The riometer technique for examining electron density enhancements in the ionosphere is based on the absorption of cosmic radio noise, the broadband RF energy radiated by stellar sources in the galaxy. Riometer measurements are usually made at frequencies in the range of 20 to 50 MHz; the absorption of radio energy at these frequencies is sensitive to changes in electron density in the ionospheric D- and E-regions. Auroral absorption is caused by the precipitation of energetic (> 10 keV) electrons from the magnetosphere, which increases the ionospheric electron density between about 70 and 120 km altitude. Riometers are usually operated with broad-beam antennas, with beamwidth on the order of 60 degrees. These integrate absorption activity over a large portion of the ionosphere, so small scale details of the actual physical distribution of electron density enhancements are lost. Narrow beam antennas, with beamwidths of 10 to 20 degrees, are sensitive to smaller scale features of ionization, but are generally limited in the ionospheric area which can be examined at one time. The data from several riometers operated at different frequencies, but examining the same sky with broadbeam antennas, show effects which have been interpreted as being due to small scale spatial structure in the electron precipitation region which does not completely fill the antenna beam. Results analyzed from narrow beam and multiple frequency riometer data, in conjunction with optical data from all-sky cameras and photometers, indicate that the spatial scale of ionospheric electron density perturbations can be very small, on the order of a few kilometers.

A recent trend in riometry is toward the use of antennas providing several narrow beams, which examine different parts of the sky. Some of these systems have been constructed to provide several fixed beams, and others scan the ionosphere in a linear path with one or more steerable beams. The IRIS (Imaging Riometer for Ionospheric Studies) system was designed to operate as a fast-scan multiple-beam instrument to examine the entire ionospheric sky, out to about 45 degrees from the zenith. The IRIS antenna is a sophisticated phased-array which produces 49 narrow beams, on the order of 12 degrees beamwidth, all of which are sampled once a second. This system is capable of examining ionospheric electron density perturbations in fine time scale, as well as small spatial scale. A picture of a typical IRIS riometer antenna is depicted above.

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