In Situ Observation Of Electrostatic Waves Associated With Ionospheric Plasma Irregularities

In-situ measurements of the height variation<br>of the ionospheric electric field and electron density<br>variations were made with a rocket-borne electric<br>field double probe and two different types of electron<br>density probes. A Brazilian made SONDA III rocket<br>carrying these experiments in addition to other<br>airglow experiments was launched on 18-th<br>December, 1995 at 2117 hrs (LT) from the equatorial<br>rocket launching station, Alcantara (2,31oS;44,4oW)<br>in Brazil. The rocket reached an apogee altitude of<br>557km and covered a horizontal range of 589km.<br>Several ground equipments were operated during the<br>launch campaign with the specific objective of<br>knowing the ionospheric conditions at the time of<br>launch and thereby to launch the rocket into an Fregion<br>prone to the presence of plasma bubbles. The<br>rocket in fact passed through several medium scale<br>plasma bubbles and the electric field double probe<br>and the electron density probes detected the presence<br>of a wide spectrum of electric field and electron<br>density irregularities. In the base of the F-region the<br>electric field double probe measurements clearly<br>indicated the presence of large amplitude fluctuations,<br>closely associated with large amplitude electron<br>density irregularities But in the height region close to<br>the rocket apogee though the electron density profile<br>showed the presence of large scale spatial structures,<br>the electric field measurements did not show<br>fluctuations of similar amplitude. In the nighttime Fregion<br>one would expect the electron density<br>irregularities, if generated by the well-known crossfield<br>instability mechanism, in height regions where<br>the electron density gradient is downward, i.e in the<br>same direction as the ambient Hall electric field. An<br>FFT algorithm was then used to estimate the spectral<br>distribution of the electric field and electron density<br>fluctuations. Some new results on the association of<br>these fluctuations with the plasma bubbles are<br>presented here.