Some new features of electron density irregularities over SHAR during strong spread F

Abstract

Abstract. An RH-560 rocket¯ight was conducted from Sriharikota rocket range (SHAR) (14°N, 80°E, dip latitude 5.5°N) to study electron density and electric ®eld irregularities during spread F. The rocket was launched at 2130 local time (LT) and it attained an apogee of 348 km. Results of electron density¯uctuations are presented here. Two extremely sharp layers of very high electron density were observed at 105 and 130 km. The electron density increase in these layers was by a factor of 50 in a vertical extent of 10 km. Large depletions in electron density were observed around 175 and 238 km. Both sharp layers as well as depletions were observed also during the descent. The presence of sharp layers and depletions during the ascent and the descent of the rocket as well as an order of magnitude less electron density, in 150±300 km region during the descent, indicate the presence of strong large-scale horizontal gradients in the electron density. Some of the valley region irregularities (165±178 km), in the intermediate scale size range, observed during this¯ight, show spectral peaks at 2 km and can be interpreted in terms of the image striation theory suggested by Vickrey et al. The irregularities at 176 km do not exhibit any peak at kilometer scales and appear to be of new type. The growth rate of intermediate scale size irregularities, produced through generalized Rayleigh Taylor instability, was calculated for the 200± 330 km altitude, using observed values of electron density gradients and an assumed vertically downward wind of 20 ms )1 . These growth rate calculations suggest that the observed irregularities could be produced by the gradient drift instability.

New Features of the ``Double-Humped Effect'' in the Magnetized Plasma

“…Vertical structures, in the scale size range of 5-10 km, are also quite prominent. The most intense irregularities occurred in three patches at 165-178 km, 210-257 km and 290-330 km [15,16]. Irregularities of a range of scale sizes starting from a few hundred meters to a few ten of kilometers are observed in this patch.…”

“…Irregularities of a range of scale sizes starting from a few hundred meters to a few ten of kilometers are observed in this patch. Studying the equatorial spread F irregularities using RH-560 rocket instrumented with Langmuir probes launched from SHAR it was established [15,16] that the relationship between the spectral index, p and the mean integrated spectral power (in 20 m to 200 m scale size range) could be represented by a Gaussian function.…”

Power Spectra of Ionospheric Scintillations

“…The incident electromagnetic wave has the frequency of 3 MHz ( [15] that the intermediate range irregularities (100 m -2 km) were observed in abundance in altitude regions 220-250 km and 290-320 km. Irregularities of a range of scale sizes starting from a few hundred meters to a few ten of kilometers are observed in these patches.…”

Rocket-borne Langmuir probe for plasma density irregularities

“…Prakash and Pal (1985) found (a) vertical scales up to 25 km wavelength in 100-140 km region and explained these in terms of wind shear mechanism, (b) vertical scales up to 100 km wavelength whose mechanism of generation was not clear (c) large variation in the height of the base F region, which was explained in terms of variation of downward drift velocity of plasma and (d) the presence of plasma depletions above 275 km where the electron density was depleted by factors as large as 15. Sinha et al (1999) and Raizada and Sinha (2000) used a cylindrical LP sensor along with LP double probes for electric field measurement along and across the spin axis of a RH 300 MK II rocket during a strong spread F event. Electron density irregularities in 30-300 m scale size range were observed in the F region valley over SHAR, a region which was earlier believed to be free from irregularities.…”

“…LP was also used to study irregularities during a number of spread F events by Prakash and Pal (1985), and Raizada and Sinha (2000) from Sriharikota. Prakash and Pal (1985) found (a) vertical scales up to 25 km wavelength in 100-140 km region and explained these in terms of wind shear mechanism, (b) vertical scales up to 100 km wavelength whose mechanism of generation was not clear (c) large variation in the height of the base F region, which was explained in terms of variation of downward drift velocity of plasma and (d) the presence of plasma depletions above 275 km where the electron density was depleted by factors as large as 15.…”