Radio frequency scattering from a heated ionospheric volume: 3, Cross section calculations

Abstract

The proper normalization procedure for determining per‐unit‐volume properties of aspect‐sensitive scattering media from experimental measurements is discussed. This is complicated by the fact that, because of the aspect sensitivity, it is not clear a priori what the per‐pulse scattering volume actually is. In general, the correct normalization requires quantitative knowledge of the aspect sensitivity which may not be possible to determine because of fundamental limitations imposed by the spatial‐resolution capability of the measurement system on the extent to which this property of the scattering medium can actually be determined. The identical problem exists in radio‐auroral measurements. This problem is considered here, and an exact expression from which the desired per‐unit‐volume quantities of interest can be obtained from measurements is given. It is shown that proper choice of the experimental parameters leads to a simplification whereby the normalization can be carried out without knowledge of the aspect sensitivity being necessary. The results are applied to calculating from experimental data the per‐unit‐volume scattering properties of a heated ionospheric volume as a function of frequency, from which a transverse scale size for the scattering medium of 3 m is estimated. For these experiments, under maximum heating conditions, a representative value of 1% for the rms fractional electron density deviation is calculated; an upper bound is also established, showing that values as large as 4 or 5% were probably never achieved. It is shown that, for an electron density distribution axially symmetric with respect to the geomagnetic field, B, the cross section for backscatter within a plane containing B uniquely determines the cross section for bistatic scattering axially around B. Experimental results for small axial‐bistatic angles are presented showing good agreement between calculated and measured values.

Multi-frequency HF radar measurements of artificial F-region field-aligned irregularities

“…Fialer (1974) reported that the backscatter power fell by at least 20 dB when the deviation from orthogonality reached 3 • and Minkoff et al (1974) found that attempting to measure the vertical extent of the irregularities by varying the elevation angle of the radar beam resulted in nothing more than a measurement of the radar's radiation pattern in elevation! Further radar backscatter measurements established the field-transverse wave number spectrum of the irregularities (Minkoff, 1974). These observations have been confirmed by in-situ rocket measurements (Kelley et al, 1995;Franz et al, 1999).…”

“…Another break-point occurs at higher wave numbers where the power-law index changes. Franz et al (1999) showed that this spectral form for their in-situ measurements made at Arecibo was similar to the Platteville results of Minkoff (1974), except that the break-points occurred at a higher wave number at Platteville. Franz et al (1999) speculated that the difference may be due to the differing magnetic dip angle at the two locations and that higher dip angles may lead to the break-points moving to higher wave numbers.…”

Radar backscattering from artificial field‐aligned irregularities

“…In the case of the Arecibo heater this corresponds to data taken using a VHF radar on the island of Guadaloupe, as reported, for example, by Coster et al [1985] and Noble and Djuth [1990]. We then compare the results with the multiradar Platteville results obtained more than 2 decades ago [Minkoff, 1974]. Finally, we review existing theoretical results and then compare the waveform of observed structures with a recent theory by Gurevich et al [1995].…”

“…Our goal here is to compare in as much detail as is practical previously published backscatter radar data from heating experiments with the rocket re- Minkoff [1974] for the Platteville heating experiments. These display both similarities to and intriguing differences from the Arecibo data.…”

Effects of multiple scatter on the propagation and absorption of electromagnetic waves in a field-aligned-striated cold magneto-plasma: implications for ionospheric modification experiments

“…They have been regularly observed by coherent backscatter radars during ionospheric modification experiments (Fialer, 1974;Minkoff et al, 1974;Hedberg et al, 1983;Robinson et al, 1997). Moreover, Minkoff (1974) has shown that the spectral power of the irregularities produced in heating experiments is primarly confined to crossfield scale sizes in excess of 1 m. This result is also consistent with the estimate of a few tens of km for the scale length of the irregularities along the geomagnetic field line (Jones et al, 1984), since their aspect ratio is expected to be of the order of the ratio of the electron-neutral collision frequency, ν en , to the electron gyro-frequency, . This ratio is around a few times 10 −4 in the F-region.…”