Observations from Millstone Hill during the geomagnetic disturbances of March and April 1990

Abstract

The incoherent scatter radars at Millstone Hill operated continuously during the periods March 16–23 and April 6–12, 1990, providing observations of large‐scale ionospheric structure and dynamics over a large portion of eastern North America. Major geomagnetic storms occurred during each of these periods, with deep nighttime ionospheric troughs and large magnetospheric convection electric fields observed equatorward of Millstone. The Millstone observations provide a comprehensive data set detailing storm‐induced ionospheric effects over a 35° span of latitude during both of these intervals. At the latitude of Millstone the ionospheric peak height hmF2 rose above 600 km in the trough on March 22 and 23 and reached ≈500 km at night on April 11 and 12. Increased recombination, apparently due to the strong electric fields, the temperature dependent recombination rate coefficient, and neutral composition changes, greatly depleted the F2 region over a wide latitude range during the day on April 10, 1990. This resulted in an ionosphere dominated by molecular ions, with ionospheric peak heights below 200 km on this day. A number of frictional heating events during the disturbed periods are seen from comparison of ion temperature and velocity measurements. The most intense event took place near 1200 UT (≈0715 LMT) on April 10, 1990, when Kp reached 8. At 0110 UT on March 21, line of sight ion velocities in excess of 500 m s−1 were observed at the extreme southern limit of the Millstone steerable radar's field of view (40° apex magnetic latitude at an altitude of 700 km). These could be due to penetration of magnetospheric electric fields or electric fields associated with ring current shielding in the storm‐time outer plasmasphere. About an hour later, ion outflow was observed just equatorward of Millstone. This is most likely due to heating from a latitudinally confined region of intense westward convection. Neutral meridional winds above Millstone were obtained by three different techniques employing radar and Fabry‐Perot measurements. The latitude variation of the winds was also estimated from radar measurements of hmF2 and electric fields using the servo model method. Strong equatorward nighttime neutral wind surges were found during both the March and April disturbances, which reached the equatorward limit of the observations at F peak heights.

A quantitative study of ionospheric density gradients at midlatitudes

“…The location of Millstone Hill is such that significant ionospheric structure falls well within the radar's field of view. The ionospheric total density trough is observed nearly overhead at Millstone Hill almost every night, and the equatorward offset of the north geomagnetic pole nearly along the Millstone Hill meridian brings high-latitude and auroral phenomena into the observatory's near field of view during geomagnetic storms [e.g., Buonsanto et al, 1992]. During disturbed conditions, intense convection electric fields occur at previously subauroral latitudes, resulting in the formation of a deep, narrow F region ionization trough.…”

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].…”

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

“…6. The method of x derivation at Millstone Hill is described by Buonsanto (1990) and Buonsanto et al (1992a). The calculated x repeats Millstone Hill day-to-day x , the relative variation being more poleward on March 16, 17 and 19, coincides on March 18 and 20, but di ers in sign on March 22.…”

“…Such an accuracy is, in principle, quite su cient for the ®rst step of solution ®nding. On the other hand, daytime electric ®elds were small for the period in question (Buonsanto et al, 1992a), so the frictional term was ignored in Eq (2 ] values, which are ®nally speci®ed at the second step using a standard multi-regressional method with physical constraints on the parameters.…”

“…Incoherent scatter observations along with F2-layer theoretical modelling provide an excellent opportunity for an F2-layer storm e ect analysis and this is being conducted by scientists in the framework of the CEDAR program. Periods of low and high (Buonsanto et al, 1992a;Richards et al, 1994b) solar activity were analyzed using Millstone Hill radar observations.…”