Abstract. A large increase in electron temperature measured in filamentary aurora with the European incoherent scatter radar has been modeled with a one-dimensional electron transport and ion chemistry code. To account for the observed changes in electron temperature, while also reproducing the measured E region electron density profiles, a source of electron heating is required in addition to local heating from energy degradation of the precipitating electrons. We show that ohmic heating in a strong field-aligned current can account for the required heat source.

Abstract. Ionosonde data from sixteen stations are used to study the semiannual and annual variations in the height of the ionospheric F2-peak, hmF2. The semiannual variation, which peaks shortly after equinox, has an amplitude of about 8 km at an average level of solar activity (10.7 cm¯ux = 140 units), both at noon and midnight. The annual variation has an amplitude of about 11 km at northern midlatitudes, peaking in early summer; and is larger at southern stations, where it peaks in late summer. Both annual and semiannual amplitudes increase with increasing solar activity by day, but not at night. The semiannual variation in hmF2 is unrelated to the semiannual variation of the peak electron density NmF2, and is not reproduced by the CTIP and TIME-GCM computational models of the quiet-day thermosphere and ionosphere. The semiannual variation in hmF2 is approximately``isobaric'', in that its amplitude corresponds quite well to the semiannual variation in the height of ®xed pressure-levels in the thermosphere, as represented by the MSIS empirical model. The annual variation is not``isobaric''. The annual mean of hmF2 increases with solar 10.7 cm¯ux, both by night and by day, on average by about 0.45 km/¯ux unit, rather smaller than the corresponding increase of height of constant pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle variations of thermospheric winds. Although geomagnetic activity, which aects thermospheric density and temperature and therefore hmF2 also, is greatest at the equinoxes, this seems to account for less than half the semiannual variation of hmF2. The rest may be due to a semiannual variation of tidal and wave energy transmitted to the thermosphere from lower levels in the atmosphere.

Abstract. Naturally enhanced incoherent scatter spectra from the vicinity of the dayside cusp/cleft, interpreted as being due to plasma turbulence driven by short bursts of intense field-aligned current, are compared with high-resolution narrow-angle auroral images and meridian scanning photometer data. Enhanced spectra have been observed on many occasions in association with nightside aurora, but there has been only one report of such spectra seen in the cusp/cleft region. Narrow-angle images show considerable change in the aurora on timescales shorter than the 10-s radar integration period, which could explain spectra observed with both ion lines simultaneously enhanced. Enhanced radar spectra are generally seen inside or beside regions of 630-nm auroral emission, indicative of sharp F region conductivity gradients, but there appears also to be a correlation with dynamic, small-scale auroral forms of order 100 rn and less in width.

Abstract. Ionosonde data from sixteen stations are used to study the semiannual and annual variations in the height of the ionospheric F2-peak, hmF2. The semiannual variation, which peaks shortly after equinox, has an amplitude of about 8 km at an average level of solar activity (10.7 cm¯ux = 140 units), both at noon and midnight. The annual variation has an amplitude of about 11 km at northern midlatitudes, peaking in early summer; and is larger at southern stations, where it peaks in late summer. Both annual and semiannual amplitudes increase with increasing solar activity by day, but not at night. The semiannual variation in hmF2 is unrelated to the semiannual variation of the peak electron density NmF2, and is not reproduced by the CTIP and TIME-GCM computational models of the quiet-day thermosphere and ionosphere. The semiannual variation in hmF2 is approximately``isobaric'', in that its amplitude corresponds quite well to the semiannual variation in the height of ®xed pressure-levels in the thermosphere, as represented by the MSIS empirical model. The annual variation is not``isobaric''. The annual mean of hmF2 increases with solar 10.7 cm¯ux, both by night and by day, on average by about 0.45 km/¯ux unit, rather smaller than the corresponding increase of height of constant pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle variations of thermospheric winds. Although geomagnetic activity, which aects thermospheric density and temperature and therefore hmF2 also, is greatest at the equinoxes, this seems to account for less than half the semiannual variation of hmF2. The rest may be due to a semiannual variation of tidal and wave energy transmitted to the thermosphere from lower levels in the atmosphere.