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

Abstract

[1] Geomagnetic field-aligned (FA) ion motions in the transition region between E and F regions (from 150 to 250 km; hereafter termed the E-F transition region) in the polar ionosphere are analyzed statistically using data from European Incoherent Scatter (EISCAT) radar from 1987 to 1999. We use all available EISCAT data sets that satisfy the definition of data selection criteria (i.e., 24-hour observation periods beginning at 1200 UT); therefore it has not been feasible to investigate seasonal, solar, and geomagnetical activity dependences. FA ion motions above and below the E-F transition region (250-300 km and 100-150 km, respectively) are also analyzed to compare with lower-altitude ion motions in the E-F transition region. The FA ion velocity that is observed with the EISCAT radar can be written as the sum of the FA ion diffusion velocity and FA component of the thermospheric wind using the ion momentum equation. The main purpose of this paper is to understand quantitatively the relative contributions of ion diffusion and neutral wind on FA ion motions in the E-F transition region. We derive the amplitude and phase of the first three daily harmonics, i.e., the 24-, 12-, and 8-hour periodic oscillations in observed FA ion velocities and estimated FA ion diffusion velocities. The spectral characteristics are determined with a Fast Fourier Transfer (FFT) method after averaging velocity data in 1-hour bins. The amplitude of the 24-hour periodic oscillation is the largest of the three harmonic components for the FA ion velocity as well as FA ion diffusion velocity. While the amplitude of the 24-hour periodic oscillation in FA ion diffusion velocity below 230 km is considerably smaller than the values of the observed FA ion velocities, the amplitude from 230 to 300 km is comparable to that in observed FA ion velocities. From 230 to 300 km the phase of the 24-hour periodic oscillation in FA ion diffusion velocity has about 180°difference from that in the observed FA ion velocity. This spectral analysis results suggest that, in the E-F transition region, FA ion diffusion velocity is not the major contributor to the three harmonic oscillations in the observed FA ion velocities; thus FA component of thermospheric winds or tides prove to be the primary contributor.

Simultaneous FPI and TMA Measurements of the Lower Thermospheric Wind in the Vicinity of the Poleward Expanding Aurora After Substorm Onset

“…A large vertical shear occurs in the winds between 98 and 106 km and is particularly evident in the meridional component. We attribute the shift of the wind rotational pattern between 106 and 118 km to the mode change in the tidal wind gradually from semidiurnal to diurnal one (Oyama et al, ). The large shear in the horizontal wind is a typical signature, which can be seen at all latitudes (Larsen, ).…”

“…At E region heights, in particular below 110 km, the diurnal tide is weakened due to less in situ solar EUV absorption by the atmosphere, making the semidiurnal component induced by solar UV absorption by ozone in the stratosphere dominant. Thus, a dominant tidal component shifts from semidiurnal to diurnal with increasing height in the lower thermosphere at high latitudes (Oyama et al, , and references therein). During geomagnetically quiet periods, tidal winds are dominant at all thermospheric heights, and the vertical wind is considerably smaller in magnitude than the horizontal wind because the wind tends to flow on the isobar, which is almost parallel to the terrestrial surface.…”

Application of a new beam configuration to estimate lower thermospheric vertical velocities at high latitudes with monostatic incoherent scatter radars

“…The neutral wind velocity is derived from the steady state ion momentum equation neglecting ambipolar diffusion [ Rino et al , 1977; Nozawa and Brekke , 1995, 1999]. This assumption is acceptable at E region heights because statistics using EISCAT radar data suggest that the ion diffusion velocity along the magnetic field line is considerably smaller than the observed field‐aligned ion velocity below 230 km [ Oyama et al , 2003]. where U , V , B , and E are vectors of the neutral wind velocity, the ion velocity, the magnetic field, and the electric field, respectively, B is magnitude of the magnetic field, Ω i is the ion gyrofrequency, and ν in is the ion‐neutral collision frequency, which is calculated using data from the Mass Spectrometer Incoherent Scatter (MSIS) model [ Hedin , 1991].…”

Fabry‐Perot Interferometer Observations of Thermospheric Horizontal Winds During Magnetospheric Substorms

“…One of the outstanding question is how the energy dissipation disturbs thermospheric winds during substorms. The thermospheric wind is a key parameter that significantly affects mass transportation (e.g., Lilensten & Lathuillere, ; Oyama et al, ), ionospheric currents (Deng et al, ; Peymirat et al, ), and electromagnetic energy exchange (Aikio et al, ; Cai et al, ; Thayer & Semeter, ).…”