On the mechanism of the post-midnight winter <i>N<sub>m</sub>F<sub>2</sub></i> enhancements: dependence on solar activity

Abstract

Abstract. The mechanism of the N m F 2 peak formation at dierent levels of solar activity is analyzed using Millstone Hill IS radar observations. The h m F 2 nighttime increase due to thermospheric winds and the downward plasmaspheric¯uxes are the key processes responsible for the N m F 2 peak formation. The electron temperature follows with the opposite sign the electron density variations in this process. This mechanism provides a consistency with the Millstone Hill observations on the set of main parameters. The observed decrease of the nighttime N m F 2 peak amplitude with solar activity is due to faster increasing of the recombination eciency compared to the plasmaspheric¯ux increase. The E Â B plasma drifts are shown to be inecient for the N m F 2 nighttime peak formation at high solar activity.

Midlatitude nighttime enhancement in F region electron density from global COSMIC measurements under solar minimum winter condition

“…The corresponding h m F 2 of the two observations had higher peak heights at lower latitudes. At latitudes between 30°and 40°during the postmidnight hours, the COSMIC observed a decrease of the h m F 2 accompanying the maximum enhanced N m F 2 , which was consistent with previous observations [e.g., Mikhailov et al, 2000b]. Note that at 30°and 35°t he two enhancement peaks of N m F 2 from COSMIC are short-lived and may be caused by the uneven distribution of the observed samples.…”

“…Neutral meridional winds can also contribute to the enhancements of electron density by transporting the plasma to higher altitudes where the recombination rate of ions is small. It is suggested that the relative balance between the downward plasma flux and the recombination determines the characteristics of the enhancements, and in turn affects the seasonal and solar cycle variations of the nighttime enhancements Mikhailov et al, 2000b;Farelo et al, 2002]. The postmidnight enhancements of electron density have been found to be contributed by larger downward plasma flux during the enhancement periods at midlatitudes in winter at solar minimum condition.…”

“…[2] Nighttime enhancements of the ionospheric F 2 region electron density have been observed by ionosondes, incoherent scatter radar (ISR), GPS total electron content (TEC) and also tomographic techniques [e.g., Evans, 1965;Balan and Rao, 1987;Bailey et al, 1991;Balan et al, 1991;Su et al, 1994Su et al, , 1995Mikhailov et al, 2000aMikhailov et al, , 2000bFarelo et al, 2002;Dabas and Kersley, 2003;Pavlov and Pavlova, 2007]. The peak electron density (N m F 2 ) of the ionospheric F 2 layer and the TEC have been found to increase after sunset in all seasons.…”

“…[3] Several mechanisms have been proposed to explain the observed nighttime enhancements of the electron density. At midlatitudes, the enhancements are mostly considered to be the result of the net increase of electron density from the downward plasma fluxes relative to the loss from the recombination [e.g., Mikhailov et al, 2000b;Farelo et al, 2002;Dabas and Kersley, 2003]. At equatorial anomaly latitudes, an upward E Â B drift in the evening raises the equatorial F region to higher altitudes and the subsequent downward movement of plasma at later hours gives rise to the postsunset/premidnight enhancements of TEC Su et al, 1994Su et al, , 1995Balan and Bailey, 1995;Balan et al, 1995].…”

“…The postmidnight enhancements of electron density have been found to be contributed by larger downward plasma flux during the enhancement periods at midlatitudes in winter at solar minimum condition. Several simulation studies have taken efforts to explain how and why this larger downward plasma flux source occurs at midlatitude over stations in the North American sector Mikhailov et al, 2000b;Richards et al, 2000a].…”