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

Abstract

Abstract. 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. Measurements from a network of Digisondes and an incoherent scatter radar are compared with the field line interhemispheric plasma (FLIP) model for January 6-12, 1997, in order to examine the temporal evolution and geographical extent of the enhancements in eastern North America. Postsunset measurements at Millstone Hill show high electron temperatures accompanied by rapid density decay until midnight followed by a rapid temperature decay accompanied 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 the overlying plasmaspheric flux tube is full. The dramatic reduction in plasmaspheric heat flux near midnight results in a sharp decrease in ionospheric temperature, inducing a large downward flow of plasmaspheric ions which creates the nighttime enhancement in ionospheric density. We find that the nighttime plasmaspheric heat flux variation drives the nighttime ionospheric density variation, which is opposite to conclusions in previously published work. Although the plasmaspheric heat flux variation can explain the ionospheric density variation, the reasons for this heat flux variation are not understood. Convection of plasma from higher magnetic latitudes is now included 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 topside ionosphere at 500 km altitude in the model is very close to that obtained from chemical equilibrium and agrees well with the measured fraction. The model generally reproduces the daytime electron density very well at all stations except Bermuda, where the difference is as much as 50%.

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

“…The mechanism explains also the observed maximum in the downward plasma velocity V z (and in the O ¯u x, correspondingly) coinciding in time with the N m F 2 peak. In this mechanism the electron temperature variation is dependent on the electron density variation contrary to the approach by Richards et al (2000), where the plasmaspheric heat¯ux variation drives the nighttime ionospheric density variation. Electron density and temperature are known to be tightly related likè`h orse-and-cart'', but in each case it should be speci®ed exactly where the``horse'' is and where the``cart'' is.…”

“…Night-tonight variations was related in our approach with the plasma compression/decompression mechanism under the action of the observed E Â B drift moving plasma from higher L shells to lower ones and squeezing it into the F 2 -region. In contrast, Richards et al (2000) when analyzing the same January 06±12, 1997, period came to the conclusion that the nighttime plasmaspheric heat ux variation drives the nighttime ionospheric density variation. However, they could not explain the reason for night-to-night plasmaspheric heat¯ux variation and their calculated nighttime¯uxes of O ions at 400 km are around 3 Â 10 8 cm À2 s À1 being by a factor of two larger than the observed ones.…”

Peculiarities of the nighttime winter foF2 increase over Irkutsk

“…Nighttime increases of the midlatitude F2 region electron density, Ne, (critical frequency, foF2, maximum electron density, NmF2, and total electron content, TEC) are wellknown phenomena (Evans, 1965;Da Rosa and Smith, 1967;Rao et al, 1982;Krinberg and Tashchilin, 1984;Bailey et al, 1991;Jakowski and Fö rster, 1995;Mikhailov and Fö rster, 1999;Mikhailov et al, 2000aMikhailov et al, , 2000bRichards et al, 2000;Belehaki and Tsagouri, 2002;Farelo et al, 2002;Dabas and Kersley, 2003;Pavlov and Pavlova, 2005;Luan et al, 2008;Pavlov et al, 2008;Yakovets et al, 2009). These phenomena are called also anomalous enhancements/increases of electron density, since the solar EUV radiation as the major source of ion production is absent during the nighttime.…”

New method in computer simulations of electron and ion densities and temperatures in the plasmasphere and low-latitude ionosphere

“…This computational grid dipole magnetic field line intersects the Earth's surface at middle geomagnetic latitudes (two geomagnetic latitudes of ± 40 • are used in this study). It is well known that, unlike the auroral and equatorial ionosphere, electric fields have little effect on the mid-latitude ionosphere, and even relatively strong electric fields measured by the Millstone Hill radar (43 • N, 288 • E) during a January 1997 magnetic storm had little effect on electron and ion densities (Richards et al, 2000). Therefore, we can suggest that the effects of the equatorial electric field on N i , T i and T e are negligible at middle geomagnetic latitudes.…”