Equatorial plasma bubble rise velocities in the Indian sector determined from multistation scintillation observations


  • R. S. Dabas,

  • B. M. Reddy


Systematic time differences observed in the onset of postsunset VHF scintillations, recorded simultaneously at a meridian chain of stations in the Indian sector, have been analyzed in order to determine the plasma bubble rise velocities over the magnetic equator. The method is based on the assumption that the postsunset scintillations up to several degrees on either side of the magnetic equator are generated by the equatorial plasma bubble and associated irregularities which are strongly field aligned. As the plasma bubble rises in the equatorial ionosphere, after its generation in the bottomside of the F layer, the low-latitude extremities of the bubble propagate away from the equator in such a way that the upper height limit of the irregularities defines the latitudinal limit of the scintillation occurrence associated with the given event. From the relative time delays observed in the onset of postsunset scintillations at a chain of different latitude stations situated nearly along a common meridian plane and the altitudinal differences over the magnetic equator between their respective field lines corresponding to the F region heights, we have determined the average bubble rise velocities between three altitudinal slabs. The velocities so determined are found to vary from about 128 to 416 m/s between the altitudes of 450 and 550 km, 38 to 327 m/s between 550 and 1140 km, and 15 to 200 m/s between 1140 and 1270 km, which shows that the bubble rise velocities decrease with altitude. The results are found to be comparable to the values measured by VHF radar and satellite techniques. The calculated values of bubble rise velocity during the early postsunset hours are found to have good correlation with the equatorial F layer vertical drift velocities. Also, as compared to the results derived by using spaced ionosondes, present results should yield the plasma bubble rise velocities with better precision because of better spatial and temporal resolution. Thus the present technique provides an effective and rather simple way of investigating the equatorial ionospheric dynamics.