Further results referring to the neutral density depletions attributed to plasma bubbles

The interaction between the ionized and neutral components of the upper atm osphere (ion drag, air drag) is not limited to the large-scale motions, but it works also among the small-scale motions. This is demonstrated by neutr al density depletions (NDD) revealed by us in the neutral density measureme nts of great time resolution of the San Marco V satellite. The morphologica l and statistical investigations indicate that NDDs and plasma bubbles have similar characteristics, On the other hand, according to modelling, deplet ions of the total neutral density discovered by us below a height of about 350 km (collision-dominated case) in the vicinity of the equator might be r eally due to equatorial plasma bubbles. The relation of the NDDs to plasma bubbles (ion density depletions) is studied partly by direct comparison and modelling, partly indirectly by the distribution of the occurrence of NDDs according to local solar time, to season, to height and to longitude. All of them are arguing in favour of the plasma bubble origin of the NDDs. (C) 2000 Elsevier Science Ltd. All rights reserved.