Observations of neutral circulation at mid‐latitudes during the Equinox Transition Study

Abstract

Measurements of ion drift velocity made by the Millstone Hill incoherent scatter radar have been used to calculate the meridional neutral wind velocity during the Sept. 17-24,1984 period. Strong daytime southward neutral surges were observed during the magnetically disturbed days of September 19 and 23, in contrast to the small daytime winds obtained as expected during the magnetically quiet days. The surge on September 19 was also seen at Arecl'bo. In addition, two approaches have been used to calculate the meridional wind component from the radarderived height of the F-layer electron density peak. Results confirm the wind surge, particularly when the strong electric fields measured during the disturbed days are included in the calculations. The two approaches for the F-layer peak wind calculations are applied to the radarderived electron density peak height as a function of latitude to study the variation of the southward daytime surges with latitude. INTRODUCI'IONGood progress has been made in the last decade in advancing our knowledge [Oliver and Salah, 19881. Satellite data has recently been used by Hedin et al. [1988] to create a global empirical wind model. A semi-empirical approach, which combines observations of the F2 peak height with MSIS neutral densities and temperatures and ionospheric models has also proven fruitful [Buonsanto, 1986;Miller et a]., 1986; Forbes et al., Studies of mid-latitude winds during quiet times reveal a generaI pattern of solar EW-driven day-to-night circulation with zonally averaged winds directed from the summer to the winter hemisphere, and a seasonal transition which the NCAR TGCM predicted to occur abruptly near the equinoxes [Roble et al, 1977). On the theoretical side, Thermospheric General Circulation ModelsIt was to investigate this transition period that the Equinox Transition Study (ETS), a global, coordinated, multi-instrument campaign, was organized and carried out during the period Sept. [17][18][19][20][21][22][23][24]1984. This turned out to be an ideal period for detailed study on account of the variety of geomagnetic conditions which occurred.After a period of geomagnetic quiet, a small storm (Ap=36) occurred on September 19. Following a period of recovery, a major storm (Ap==112) occurred on September 23.While the diurnally or zonally averaged global patterns of thermospheric neutral winds are becoming better understood, the diurnal and seasonal variations, particularly under varying geomagnetic conditions, are still poorly known. At high latitudes during storms, ion drag momentum forcing by the magnetospheric convection ionization drifts [eg., Killeen et al., 1984) is important in driving the neutral winds. The associated Joule heating and heating from particle precipitation results in post-midnight equatorward surges in the neutral wind, which have been modeled by the NCAR TGCM [Killeen and Roble, 1986, Roble et al., 1987al Theoretical models do not generally predict large scale changes in the day-time meridional winds during storms at mid-l...

On the mechanism of seasonal and solar cycleNmF2variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

“…Equation (1) uses n O+,O collision frequency, which, as the other ones, is taken from Banks and Kockarts [1973]. However, in some publications devoted to ISR data analyses [Burnside et al, 1987[Burnside et al, , 1988[Burnside et al, , 1991Buonsanto et al, 1989Buonsanto et al, , 1992aBuonsanto et al, , 1992b it was suggested that n O+,O collision frequency should be increased by a factor of 1.7. But later a much lower factor of 1.2-1.4 was recommended by Pesnell et al [1993], Reddy et al [1994], and Davis et al [1995].…”

Dayside enhancements of thermospheric O/N2 following magnetic storm onset

“…At middle and low latitudes the electric field is usually small, and the height of the ionosphere can be used to infer meridional wind speeds if the electric field is ignored [Miller et al, 1986[Miller et al, , 1987Buonsanto et al, 1989]. Ionosondes typically make hourly soundings and report the peak electron density in the E and F regions, together with the base height of the F region (h'F).…”

Day-to-day thermosphere parameter variation as deduced from Millstone Hill incoherent scatter radar observations during March 16-22, 1990 magnetic storm period

“…It should be recalled that all collision frequencies used in our calculations are taken from Banks and Kockarts (1973) and my À y, in particular. But in recent works devoted to the incoherent scatter data analysis (Burnside et al, 1987(Burnside et al, , 1988Buonsanto et al, 1989;Burnside et al, 1991b, a;Buonsanto et al, 1992a, b) it is suggested that this value should be increased by a factor of 1.7. The possible eect of such correction is discussed by Belley et al (1992).…”