F region and magnetosphere, backscatter results
Authors: Gordon Wm. E. 
Published: February 2008

The F region and the magnetosphere are under regular observation by the incoherent‐backscatter radar technique at three stations near 70°W longitude and ranging in magnetic latitude from the equator (Jicamarca, Peru), through 30°N (Arecibo, Puerto Rico), to 55°N (Millstone Hill, Massachusetts), and three other stations: Prince Albert in Canada, Nancay in France, and Malvern in England.

The total power backscattered from the electrons in a pulse volume at a particular height is related to the number of electrons in the volume when corrected for two factors, each of which usually does not exceed 2. The corrections provide for the lack of equilibrium in the electron and ion temperatures and for the ratio of the Debye shielding distance to the operating wavelength. The corrections can be deduced from a spectral analysis of the returned signal. Alternately, and particularly in the F region, the electron density is determined by observing the differential Faraday rotation of the polarization of the signal, or, under certain conditions by measuring the weak spectral line at the electronic plasma frequency.

The total scattered power can be considered to arise from two components: one component having a frequency spectrum spread around the transmitted frequency in a band corresponding to, and produced by, the electronic thermal motions, and the second component having a frequency spectrum spread around the transmitted frequency in a band corresponding to, and produced by, the ionic thermal motion. For all of the operating observatories, the conditions are such that the ionic component dominates in the F region and lower magnetosphere, and, since ionic thermal motion depends not only on the ion temperature but also on the ion mass, it is possible to identify the dominant ion and the ionic composition where mixtures are present. The ionic thermal motion fixes the characteristic width of the spectrum, but the shape of the central region of the spectrum is largely due to the ratio of the electron to ion temperatures. Hence from spectra at various heights it is possible to deduce electron temperature, ion temperature, and ionic composition all as functions of height.

The three stations near 70°W longitude have regular observational programs that have extended over a year or more. Although the results of these programs are not published for all stations, the principal results will be summarized.

Electron density profiles show values of 0.5 × 104 at 10,000 cm−3 at 10,000 km (daytime, Jicamarca) and of 5 × 104 and 1 × 104 at 1000 km (daytime and night, Jicamarca). At all stations diurnal patterns are apparent in the height of the maximum of the F2 layer, including rapid changes near sunrise. Ion production and loss rates are inferred for certain times and heights.

The ion temperature during the day increases from about 1000 degrees near the F peak to a few thousand degrees at about 800 kilometers and remains isothermal to the limits of the observation (usually less than 1500 km). At night the values are about one‐half these values. The electron temperature near the F peak is higher than the ion temperature by a factor that may reach 3 in the daytime but is only slightly above 1 at night. At greater heights the electron and ion temperatures agree.

The dominant ion in the F region is O+, changing to H+ at about 1000 km during the day and at about 700 km at night. He+ is observed in the transition region but in amounts of 10% or 20% of the ion content. These observations are made during quiet solar conditions.

Traveling ionospheric disturbances having a wave structure have been observed at heights ranging from 150 km to the limit of observation (about 800 km) and are associated with predictable magnetic storms. They are interpreted in terms of gravity waves on the interface of the cool lower F region and warmer magnetosphere.

Photoelectrons produced in the sunlit hemisphere contribute a measurable amount of heat at the conjugate point before its local dawn.