Instruments

• Introduction
• Riometers
• Magnetometers
• Searchcoil Magnetometer
• Fluxgate Magnetometer
• Optical Instruments

Riometers

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 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.

At South Pole Station, the University of Maryland operates broadbeam riometers at 20.5, 30, 38.2, and 51.4 MHz. The signals from these instruments are digitized and recorded at 1-Hz resolution by the station data acquisition system. The broadbeam antennas are located in a group about 1-km from the Cusp Lab. For the 20.5, 30 and 51.4 MHz systems, the broadbeam antenna comprises two adjacent parallel horizontal dipoles; the antenna for the 38.2 MHz system is a single element identical to those used with the IRIS phased array, including a chicken-wire ground plane located a quarter-wavelength below the horizontal dipole element plane. The antenna beam projects a nearly circular -3 dB locus onto the ionosphere, where the absorption of cosmic radio noise occurs; the -3 dB beamwidth is approximately 60-degrees, so the diameter of the typical ionospheric absorption region at 90 km altitude is about 100 km.