On the mechanism of the post-midnight winter NmF2 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.

Local empirical model of ionospheric plasma density derived from Digisonde measurements at Irkutsk

“…in the postmidnight hours. This means that the enhancement of postmidnight N m F 2 gets weaker with increasing solar activity, which agrees with the studies of Mikhailov et al (2000). The authors explained this phenomenon by suggesting that the nighttime recombination rate increases faster than the nighttime influx from the plasmasphere with increasing solar activity.…”

Global morphology of night-time <i>Nm</i>F2 enhancements

“…No clear dependence of any parameter on solar activity is apparent, except for a small decay of amplitude during solar maximum, which was explained in detail by Mikhailov et al (2000b). A small variation of these parameters with geographic location occurs and is described later.…”

“…A physical mechanism to explain this behaviour was proposed by Mikhailov et al (2000b). This pronounced seasonal and solar variability occurs at high and middle geomagnetic latitudes only.…”

Pre-storm <I>Nm</I>F2 enhancements at middle latitudes: delusion or reality?

“…Long-duration positive disturbances (type I) take place during both daytime and nighttime. They are related mostly to atomic oxygen abundance increase during daytime hours (Mikhailov et al, 2007) and to a plasmaspheric influx during nighttime (Mikhailov et al, 2000b). Some comments may be pertinent in relation with the discussed mechanisms.…”