New Dynamic Aspects of Intertropical F₂ Ionization

Abstract

Diurnal changes in F2 ionization distribution as measured during June and July 1966 over the Tamanrasset meridian reveal two types of asymmetric phenomena distorting the crest‐and‐trough system (formerly known as the equatorial F2 ionization anomaly). The first type corresponds to a general time variation of the latitude asymmetry. Our observations yield four independent parameters of this asymmetry; for 80% of the time, the variation can be attributed to ion drag due to a time‐varying meridian air wind. Magnetic activity is correlated with the amplitude of this wind, and the low‐altitude thermospheric circulation is discussed. Day‐to‐day fluctuations of the diurnal evolution pattern suggest a possible long‐term control of the equatorial electric field by the general dynamo current system. A second type of asymmetry shows more intense, local, and short‐lived phases superimposed on the fountain and neutral wind fluctuations. It has the character of a transitory forced‐diffusion process. We believe that the factor responsible is electron temperature gradients produced by conjugate photoelectron heating.

Interhemispheric asymmetry of the equatorial ionization anomaly in solstices observed by COSMIC during 2007–2012

“…A prominent longitudinal feature of the EIA with zonal component of wave structures was discovered and widely studied [e.g., Sagawa et al, 2005;Immel et al, 2006;Lin et al, 2007b;Liu and Watanabe, 2008;Kil et al, 2008]. Additionally, a summer-winter difference of the EIA has been reported in previous studies [e.g., Walker, 1981;Walker et al, 1994;Tsai et al, 2001;Lei et al, 2007;Lin et al, 2007a;Tulasi Ram et al, 2009;Zhao et al, 2009;Huang et al, 2013], which showed that EIA crests develop earlier in the winter hemisphere. This is generally attributed to the modulation of the summer to winter transequatorial circulation on the equatorial fountain effect.…”

“…A large number of studies have shown that the EIA varies with local time, season, and solar activity [e.g., Vila , , ; Walker , ; Walker et al ., ; Tsai et al ., ; Liu et al ., ; Lin et al ., ; Zhao et al ., ; Wu et al ., ; Tulasi Ram et al ., ; Huang et al ., ]. A prominent longitudinal feature of the EIA with zonal component of wave structures was discovered and widely studied [e.g., Sagawa et al ., ; Immel et al ., ; Lin et al ., ; Liu and Watanabe , ; Kil et al ., ].…”

“…Equatorward neutral winds tend to move plasma in the F 2 region to a higher altitude, where the recombination rate is lower, and then enhance the EIA crests [Kelley, 1989;Walker et al, 1994;Balan et al, 2013]. On the other hand, relatively more equatorward winds in the summer hemisphere, which are associated with the transequatorial neutral winds, can often cause net transport of the plasma from summer to winter hemisphere; thus, the EIA crest can be more intensified in the winter hemisphere than in the summer hemisphere [e.g., Tsai et al, 2001;Lin et al, 2007a;Tulasi Ram et al, 2009]. In solstices, the transequatorial winds have an opposite effect on modulating the EIA crests between the summer and winter hemispheres.…”

FORMOSAT‐3/COSMIC observations of seasonal and longitudinal variations of equatorial ionization anomaly and its interhemispheric asymmetry during the solar minimum period

“… Vila [1971a], from 20 days of campaign mode investigations over African region during the low solar activity ( F 10.7 = 100) period June to July 1966, reported the early development of EIA crests in the winter hemisphere during the morning hours and stronger EIA crests in the summer hemisphere during the afternoon hours. They have attributed this asymmetry reversal to the wind reversing during the early afternoon [ Vila , 1971a, 1971b]. Walker et al [1991] from their 3 quiet days of observations using ionosondes during the SUNDIAL‐87 campaign, also reported the crest asymmetry reversal around 1500–1600 LT.…”

“…The asymmetry of the crests, both in magnitude and latitudinal location was investigated in the past mainly using ground‐based data as well as by modeling [ Vila , 1971a, 1971b; Anderson and Roble , 1974; Aydogdu , 1988; Walker et al , 1991; Su et al , 1997]. Vila [1971a], from 20 days of campaign mode investigations over African region during the low solar activity ( F 10.7 = 100) period June to July 1966, reported the early development of EIA crests in the winter hemisphere during the morning hours and stronger EIA crests in the summer hemisphere during the afternoon hours.…”