Equatorial Spread-F Irregularities As Observed By Three Different Rocket-Borne Plasma Density Probes

Some new results obtained from in-situ measurements of the height variation of the ionospheric electron density made<br>with rocket-borne electron density probes during a campaign conducted from Alcantara (2.31oS; 35.2oW) in Brazil are<br>presented here. The campaign designated Guará was conducted in collaboration with NASA. In addition to several<br>plasma diagnostic instruments provided by other participating institutes the Aeronomy Division in the Instiuto Nacional de<br>Pesquisas Espaciais in Brazil provided a High Frequency Capacitance probe that measured the height profile of the<br>electron density. During the Guara campaign, a Black Brant X sounding rocket was launched on 14-th October 1994 at<br>1955hrs (LT) to investigate into the phenomenon of high-altitude equatorial spread-F events. The rocket, as expected<br>passed through an active topside spread-F event, monitored simultaneously by several ground-based instruments. The<br>electron density height profile and the amplitude of the electron density fluctuations were measured simultaneously by<br>three different plasma density probes; a High Frequency Capacitance (HFC) probe, a conventional Langmuir Probe (LP)<br>and a Plasma Frequency Probe (PFP). While the PFP provided the absolute electron density, the LP gave the relative<br>variation in the electron density. The electron density profile obtained from the HFC probe measurements is practically<br>absolute except for a plasma sheath factor. But this technique does not provide the small scale electron density<br>fluctuation amplitude. The three experiments provided data, which could be used not only to obtain reliable electron<br>density data, but also to arrive at some of the inherent difficulties associated with each of these techniques. For example<br>the elctron density profiles estimated from the HFC and PFP experiments are almost identical except for a small factor<br>varying with altitude. The amplitude of large scale fluctuations provided by the LP measurements is considerably less<br>than that provided by HFC and PFP. The k-spectra of the plasma irregularities were obtained by the spectral analysis of<br>the electron density fluctuation data. The electron density irregularites associated with the plasma bubbles were seen to<br>have rather sharp lines in their k-spectra extending over a wide range of altitude. What one would expect from the<br>existing theories on the generation of small scale irregularities by the cascading process is a flat k-spectrum. Present<br>results may be indicative of the presence of preferred wave modes in developing plasma bubbles.