On the relative importance of convection and temperature to the behavior of the ionosphere in North America during January 6-12, 1997

This paper is primarily concerned with the causes of the large density and temperature enhancements that are often observed during magnetically quiet periods on winter nights at midlatitudes in the North American sector. Meas urements from a network of Digisondes and an incoherent scatter radar are c ompared with the field line interhemispheric plasma (FLIP) model for Januar y 6-12, 1997, in order to examine the temporal evolution and geographical e xtent of the enhancements in eastern North Amer ica. Postsunset measurement s at Millstone Hill show high electron temperatures accompanied by rapid de nsity decay until midnight followed by a rapid temperature decay accompanie d by a pronounced density enhancement in the early morning hours. The FLIP model reproduces the nighttime density enhancement well, provided the model is constrained to follow the topside electron temperature and also that th e overlying plasmaspheric flux tube is full. The dramatic reduction in plas maspheric heat flux near midnight results in a sharp decrease in ionospheri c temperature, inducing a large downward flow of plasmaspheric ions which c reates the nighttime enhancement in ionospheric density. We find that the n ighttime plasmaspheric heat flux variation drives the nighttime ionospheric density variation, which is opposite to conclusions in previously publishe d work. Although the plasmaspheric heat flux variation can explain the iono spheric density variation, the reasons for this heat flux variation are not understood. Convection of plasma from higher magnetic latitudes is now inc luded in the FLIP model but is not needed to produce the observed nighttime density maximum. We have found that the fraction of light ions in the tops ide ionosphere at 500 km altitude in the model is very close to that obtain ed from chemical equilibrium and agrees well with the measured fraction. Th e model generally reproduces the daytime electron density very well at all stations except Bermuda, where the difference is as much as 50%.