EFFECT OF LOWER-HYBRID CAVITIES ON CORE PLASMA OBSERVED BY FREJA

Sounding rocket experiments in the topside ionosphere have established previously a one-to-one relation between transversely heated ions and packets of lower hybrid waves trapped in localized density depletions (lower hybrid cavities (LHCs)). LHCs were observed routinely by the F reja wave instrument between 1000 and 1750 km; however, the effect of these LHCs on the core ion population is not as obvious as observed by sounding rockets. We have searched for signatures of LHC-induced ion heating in data from the Freja cold plasma analyzer, which measures tw o-dimensional integrated ion flux (0-20 eV) every 10 m along the satel lite's trajectory. We identify a possible signature of localized ion h eating in the form of isolated bursts and depletions of integral ion f lux with amplitudes of the order of 10(8) cm(-2)s(-1) and lasting a fe w to tens of milliseconds or 30 to hundreds of meters along the trajec tory when interpreted as spatial structures, with smaller features occ urring more frequently. LHCs exist over a small subset of these scales , with a mean width of 54 m and a variance of only 15 m; in this case the spatial interpretation is supported by measurements of time delays between spatially separated density probes. Ion flux bursts are found in LHC-containing regions; however, they also occur elsewhere. That i s, the relation between flux bursts and LHCs is not one to one. Thus L HCs may cause some, but not all, ion flux bursts. If flux bursts are d ue at least in part to LHCs, there are several possible reasons why we observe no one-to-one relation between the two phenomena, including i nstrumental and sampling constraints and the fact that LHCs are not as intense as those observed at lower altitudes (several to tens versus hundreds of millivolts per meter peak to peak). Regardless of their ca use, ion flux bursts provide an upper Limit to the amount of heating c aused by LHCs observed by Freja. We show that if flux bursts are cause d by transverse heating of the core ion population, this heating canno t exceed similar to 1 eV, although stronger heating of only a fraction of the ion population is also possible.