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 Freja 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 two-dimensional integrated ion flux (0-20 eV) every 10 m along the satellite's trajectory. We identify a possible signature of localized ion heating in the form of isolated bursts and depletions of integral ion flux with amplitudes of the order of 10(8) cm(-2)s(-1) and lasting a few to tens of milliseconds or 30 to hundreds of meters along the trajectory when interpreted as spatial structures, with smaller features occurring 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 is, the relation between flux bursts and LHCs is not one to one. Thus LHCs may cause some, but not all, ion flux bursts. If flux bursts are due at least in part to LHCs, there are several possible reasons why we observe no one-to-one relation between the two phenomena, including instrumental 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 cause, ion flux bursts provide an upper Limit to the amount of heating caused by LHCs observed by Freja. We show that if flux bursts are caused by transverse heating of the core ion population, this heating cannot exceed similar to 1 eV, although stronger heating of only a fraction of the ion population is also possible.