Density depletions at the 10‐m scale induced by the Arecibo heater

Abstract

In June 1992 a NASA sponsored sounding rocket was flown through the Arecibo heater beam to study the structure of the heated volume. The rocket carried an instrument payload and traversed the 5.1-MHz reflection height at 268.5 kin. Data from the plasma density probe are presented in this paper. The rocket passed through several regions of disturbed plasma both above and below the reflection level. In these regions, over 180 deep filamentary density depletions were detected. Measured perpendicular to the magnetic field, these depletions or filaments have a mean width at half maximum of 7 m which is roughly equal to twice the ion gyroradius (0 +) and a mean depletion depth of 6%. The ratio of parallel to perpendicular scale for these structures exceeds 20,000, and the spacing between the filaments is around 15 m. A power spectrum of the rocket data clearly shows the spectral content of the filaments and also reveals peaks at longer wavelengths which we interpret as the spacing between the bunches and between sets of filaments within a given bunch. We believe that previous scintillation and satellite measurements emphasized these longer wavelengths. The power spectrum mea. sured by the rocket instrumentation falls off as k -4 for wavenumber k larger than 0.4/m and remains above the system noise for structure down to 1 m. It is clear that VHF backscatter from these structures can be explained by our data, as can many features of heater-related, field-aligned irregularities found in the literature. 17,367 17,374 KELLEY ET AL.' DENSITY DEPLETIONS AT THE 10-M SCALE Perkins, F. W., and M. V. Goldman, Self-focusing of radio waves in an underdense ionosphere, J. Geophys. Res., 86, 600, 1981. Perkins, F. W., and E. J. Valeo, Thermal self-focusing of electromagnetic waves in plasma, Phys. Rev. Lett., 3oe, 1234, 1974. Robinson, T. R., The heating of the high latitude ionosphere by high power radio waves, Phys. Rep., 179(2), 79, 1989. Rose, G., B. Grandal, E. Neske, W. Ott, K. Spenner, J. Holtet, K. M•eide, and J. Troim, Experimental results from the HERO project: In situ measurements of ionospheric modifications using sounding rockets, J. Geophys. Res., 90, 2851, 1985. Szuszczewicz, E. P., and J. C. Holmes, Observations of electron temperature gradients in mid-latitude E, layers, J. ) M. Sulzer, Arecibo Observatory, Arecibo, PR 00613. (email: sulzer@naic.edu) (

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

“…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). The spectrum is essentially flat with increasing wave number up to a first "break-point" at scales of a few metres to tens of metres when an inverse power-law dependence takes over.…”

“…Such theories do not take into account the possible effects on plasma transport due to the excitation of plasma turbulence in the region where the pump wave interacts with the plasma. In addition, the small-scale irregularities involve steep gradients in plasma density which may be unstable to, for example, the drift-wave instability, as has been suggested by Kelley et al (1995). Such instabilities could prevent striations reaching the stationary state, as predicted by current theories.…”

Radar backscattering from artificial field‐aligned irregularities

“…Although it is not readily apparent from this linear plot, the filaments in all three patches have similar relative depletions between 3% and 10%. Their mean width at half maximum is 15 m, and the spacing between filaments across the magnetic field is roughly 45 m. We found no obvious correlation between the width of a filament and its depletion depth [Kelley et al, 1995]. Figure 2 is the one-dimensional wavenumber spectrum computed from the Langmuir probe time series.…”

“…This agrees with a vast set of experimental and theoretical studies [see Robinson, 1989] that concluded that the filaments are highly field aligned. The elongation ratio of the filaments is likely to be •20,000 since meter-scale gradients were detected in all three patches [Kelley et al, 1995].…”

Temporal evolution of pump beam self‐focusing at the High‐Frequency Active Auroral Research Program

“…Plasma irregularities set in within $10 s of pumping [Utlaut et al, 1970] and self-focusing of the pump wave in the magnetic zenith can occur for small plasma depletions of order <1% [Georges, 1970]. Rocket and satellite measurements at Arecibo of pump-induced striations revealed smallscale plasma depletions up to 12% (mean 6%) [Kelley et al, 1995] and 3% [Farley et al, 1983], respectively, for low latitudes. Rocket measurements at EISCAT show smallscale pump-induced plasma depletions up to 2% (typically <1%) [Rose et al, 1985] for high latitudes.…”