Search for geomagnetic storm effects on lower thermospheric winds at midlatitudes

Abstract

Neutral winds in the lower thermosphere during several geomagnetic storms of different intensity are derived from ion velocity measurements made by the Millstone Hill incoherent scatter radar $\text{(42.6°}\text{N}\text{,}\text{71.5°}\text{W}\text{)}$. The reliable detection of storm effects on midlatitude winds in the altitude region 100–130 km is difficult since the response at these altitudes is expected to be small. A search for such effects has been conducted using observations made during the past eight years during moderate and intense geomagnetic conditions. When geomagnetic activity is moderate, as that occurred in May 1995 and March 1999, there is no strong evidence for a response in the ion drifts or lower thermosphere winds. When the geomagnetic disturbances are intense, exceeding a geomagnetic index Kp of 6, such as that occurred in June 1991 and September 1998, there is clear evidence of a response. The primary finding is that the zonal wind component is enhanced, and the magnitude of the enhanced winds and their duration depends on the intensity and duration of the storm. In September 1998, both the ion and neutral velocities in the lower thermosphere are found to respond to the disturbance, and the largest enhancement of daytime zonal winds is observed roughly 13 h after the onset of the storm impulse.

Introduction

Our knowledge about the response of the lower thermosphere at altitudes from 100 to 150 km to geomagnetic storms on a global basis derives primarily from studies using general circulation models and from a small set of observations. Simulations of the magnetic storm of March 22, 1979, by Roble et al. (1987) revealed a two-cell circulation pattern in the winds at 120 km, with maximum winds of 275 m/s and a pronounced asymmetry between the cells on the dawn and dusk sides of the polar cap. The storm effects in the lower thermosphere were largely confined to high latitudes. Using a coupled ionosphere–thermosphere model, Fuller-Rowell (1995) and Fuller-Rowell 1994, Fuller-Rowell 1997 examined the response of the lower thermosphere to a storm forcing imposed for a period of 12 h, and found that the neutral winds at 135 km altitude accelerated to 400–500 m/s at high latitudes in a predominantly clockwise vortex. Zonal winds in excess of 250 m/s were found to penetrate to midlatitudes in the dusk sector due to advective transport by the meridional winds. Studies of geomagnetic activity effects on thermospheric tides using a coupled ionosphere–thermosphere model were also conducted by Fesen 1991, Fesen 1993 and Fesen (1997) for several latitudes. At mid- and low latitudes, winds and temperatures in the lower thermosphere were found to be relatively less sensitive to the level of auroral forcing compared to effects at higher altitudes in the upper thermosphere and at higher latitudes. There is some evidence for the penetration of the perturbation effects due to geomagnetic storms to lower altitudes as the level of activity increases, although the effects below 130 km are not significant. At high latitudes, increasing levels of geomagnetic activity cause tidal amplitudes to increase strongly in both the lower and upper thermosphere.

Observations of the effects of geomagnetic storms on the earth's lower thermosphere and ionospheric E-region are relatively sparse compared to a very rich data base of such effects on the upper thermosphere and ionospheric F-region as illustrated by the recent review by Buonsanto (1999). Incoherent scatter radars have contributed some of the important observations of the dynamics of the ionospheric E-region and the lower thermosphere at altitudes between 100 and 150 km. At high latitudes, measurements at Chatanika, Alaska (65°N), Sondrestrom, Greenland (67°N), and EISCAT, Scandinavia (69.5°N) by Johnson et al. (1987) and Johnson and Virdi (1991) suggest that the strongest response to geomagnetic activity is found in the zonal wind component with enhanced eastward mean flow, and that temporal oscillations are a balance between tidal components and ion drag forces driven by the polar plasma convection patterns. At EISCAT, Kunitake and Schlegel (1991) also report significant correlations of the zonal diurnal amplitudes at heights of 117–120 km with Kp but not at lower altitudes. Oscillation amplitudes reaching 200 m/s for the diurnal components and 150 m/s for the semidiurnal meridional components are reported during the most active conditions studied, represented by average Kp reaching values of 5. Kirkwood (1996) reports a distinct dependence of temperatures in the 110–120 km altitude range on magnetic activity in all seasons, and even at lower altitudes in winter. Correlations of the meridional winds in the lower thermosphere with geomagnetic indices were not statistically significant.

At midlatitudes, Salah et al. (1996) observed enhanced winds in the altitude region 100–130 km above Millstone Hill (42.6°N) during a sustained intense magnetic storm in June 1991 (Kp=6-8). Zonal neutral winds reached levels $\text{>150}\text{m}\text{/}\text{s}$ compared to peak winds of 70–80 m/s generally observed in summer, and retained the tidal wave characteristics such as the phase of the semidiurnal oscillation that have dominated lower thermosphere dynamics at midlatitudes. A previous study at Millstone Hill by Wand (1983) examined the average tidal behavior of winds in the lower thermosphere during geomagnetic storms with Kp ranging from 3 to 5, and reported a reduction of 20–50% in the semidiurnal amplitudes of the neutral winds between 105 and 115km, and a westward shift of the daytime mean zonal winds above 115km by about 25m/s. At Saint-Santin, France (45°N), Mazaudier et al. (1987) report case studies of the meridional wind in the lower thermosphere during two geomagnetic storms where Kp reached levels of 5 and 6, resulting in southward-directed mean winds above 115km and return northward flow below that altitude. At Arecibo (18°N), Morton and Mathews (1993) studied the effects of a major storm on the E-region ionization layers. The layer near 100 km usually seen in the evening disappeared completely and the layering structure at higher altitudes was drastically changed, suggesting that the change in layering activity is due to a large electric field.

In an effort to further search for the response of the lower thermosphere at midlatitudes to geomagnetic storms of different intensities and characteristics and to contribute to the data base on this topic, we examine in this paper the ion and neutral dynamics during four geomagnetically active periods based on data collected with the Millstone Hill incoherent scatter radar $\text{(42.6°}\text{N}\text{,}\text{71.5°}\text{W}\text{)}$. We compare the ion drift velocities and neutral winds in the altitude region between 100 and 150km during the storms with data during quiet periods. Emphasis is on the altitude profiles of the plasma and neutral velocities rather than on tidal structures that have been discussed in previous reports (Salah et al., 1996; Goncharenko and Salah, 1998).