Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

Abstract

Abstract. The companion paper by Zou et al. shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external in¯uences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noon NmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations of NmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric ®elds play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions aect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual eect in NmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the eect of the composition changes is modi®ed by the large solar zenith angle at midwinter. The zenith angle eect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle eect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/ molecular ratio. This leads to a semiannual variation of NmF2. A dierent situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values of NmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations.

On the mechanism of seasonal and solar cycleNmF2variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

“…[24] Our analysis has shown that seasonal differences in daytime N m F 2 solar activity variations are due to the different roles which neutral composition (O, O 2 , N 2 ) and temperature play in summer and in winter, and the observed seasonal difference in N m F 2 is not simply reduced to seasonal O/N 2 variations [e.g., Millward et al, 1996;Rishbeth et al, 2000;Richards, 2001;Yu et al, 2004].…”

“…[2] Seasonal and December anomalies in N m F 2 variations have been studied since the beginning of ionospheric observations [Appleton and Naismith, 1935;Seaton and Berkner, 1939;Yonezawa and Arima, 1959]; however, the mechanisms of their formation are still discussed in publications [Richards et al, 1994c;Rishbeth et al, 2000;Richards, 2001;Kawamura et al, 2002;Yu et al, 2004;Pavlov and Pavlova, 2005;Rishbeth and Müller-Wodarg, 2006;Zhao et al, 2007]. Among different explanations proposed during the decades of the problem analysis [see Rishbeth, 1998], the suggestion by Rishbeth and Setty [1961] that F 2 layer seasonal anomaly is due to neutral composition variation may be considered as generally accepted [Rishbeth et al, 2000].…”

“…Among different explanations proposed during the decades of the problem analysis [see Rishbeth, 1998], the suggestion by Rishbeth and Setty [1961] that F 2 layer seasonal anomaly is due to neutral composition variation may be considered as generally accepted [Rishbeth et al, 2000]. At present, one can see only a detailed elaboration of this concept [e.g., Pavlov and Pavlova, 2005].…”

A Statistical Comparison of Zonal Mean and Tidal Signatures in FORMOSAT-3/COSMIC and Ground-Based GPS TECs

“…The zonal mean TECs maximize around the equinoxes (days 90 and 300), and minimize around the solstices (days 30 and 210). This variation in zonal mean TEC amplitude corresponds to the known semiannual variation in the ionosphere that has been connected to seasonal variations in the O/N 2 ratio (Rishbeth et al 2000). The twin bands of the EIAs on either side of the magnetic equator are not distinct in the case of the zonal mean TECs, likely due to the diurnal nature of the equatorial fountain mechanism (Appleton 1946).…”

Ionospheric F2-Layer Semi-Annual Variation in Middle Latitude by Solar Activity

“…According to Rishbeth et al (2000) the electron density variation (or anomaly) of the F2 layer can be divided into 3 types: seasonal variation (or winter variation), semiannual variation and annual or non-seasonal variation. Seasonal variation refers to the phenomenon that the NmF2, which is the maximum electron density in the F2 layer, is larger in winter than in summer usually in the day time in the middle latitudes.…”