Effects of atmospheric oscillations on the field-aligned ion motions in the polar F-region

Abstract

Abstract. The ®eld-aligned neutral oscillations in the F-region (altitudes between 165 and 275 km) were compared using data obtained simultaneously with two independent instruments: the European Incoherent Scatter (EISCAT) UHF radar and a scanning FabryPerot interferometer (FPI). During the night of February 8, 1997, simultaneous observations with these instruments were conducted at Troms , Norway. Theoretically, the ®eld-aligned neutral wind velocity can be obtained from the ®eld-aligned ion velocity and by di usion and ambipolar di usion velocities. We thus derived ®eld-aligned neutral wind velocities from the plasma velocities in EISCAT radar data. They were compared with those observed with the FPI (k 630:0 nm), which are assumed to be weighted height averages of the actual neutral wind. The weighting function is the normalized height dependent emission rate. We used two model weighting functions to derive the neutral wind from EISCAT data. One was that the neutral wind velocity observed with the FPI is velocity integrated over the entire emission layer and multiplied by the theoretical normalized emission rate. The other was that the neutral wind velocity observed with the FPI corresponds to the velocity only around an altitude where the emission rate has a peak. Di erences between the two methods were identi®ed, but not completely clari®ed. However, the neutral wind velocities from both instruments had peak-to-peak correspondences at oscillation periods of about 10±40 min, shorter than that for the momentum transfer from ions to neutrals, but longer than from neutrals to ions. The synchronizing motions in the neutral wind velocities suggest that the momentum transfer from neutrals to ions was thought to be dominant for the observed ®eld-aligned oscillations rather than the transfer from ions to neutrals. It is concluded that during the observation, the plasma oscillations observed with the EISCAT radar at di erent altitudes in the F-region are thought to be due to the motion of neutrals.

Observations of storm time midlatitude ion‐neutral coupling using SuperDARN radars and NATION Fabry‐Perot interferometers

“…In the recovery phase, modeling studies using the National Center for Atmospheric Research Thermosphere Ionosphere General Circulation Model (NCAR‐TIGCM) have found that the neutrals are capable of driving the westward ionospheric drifts for 5–6 h after the magnetospheric convection declines [ Deng et al , ; Lyons et al , ]. On the observational side, Oyama et al [] used ISR and Fabry‐Perot interferometer (FPI) observations at high latitudes to determine that neutrals drive field‐aligned ion oscillations on a time scale of ∼40 min. Most of these previous works relied heavily on thermospheric models and/or colocated coverage of ISR and FPI instruments at high latitudes.…”

Field‐aligned ion motions in the polar E‐F transition region: Mean characteristics

“…The time constant for momentum transfer from neutral particles to ions is considerably shorter than the EISCAT radar time resolution that is normally longer than a few tens of seconds. This means that neutral wind drag causes observed FA ion oscillations in phase with thermospheric wind variations for oscillate at periods longer than the time constant for neutral‐ion collision effects [ Oyama et al , 2000]. The magnitude of FA ion diffusion velocity caused by plasma pressure gradients and gravity becomes comparable or larger than effects from FA neutral wind drag with increasing height above the F2 peak [ Lilensten and Lathuillere , 1995].…”