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 a ect 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 e ect 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 e ect of the composition changes is modi®ed by the large solar zenith angle at midwinter. The zenith angle e ect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle e ect 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 di erent 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.

Anomalous variations of <I>Nm</I>F2 over the Argentine Islands: a statistical study

“…Millward et al, 1996;Zou et al, 2000;Rishbeth et al, 2000;Tao Yu et al, 2000). On the other hand, Zou et al (2000) compared the coupled thermosphere-ionosphere-plasmasphere model (CTIP) results with ionosonde data from mid-latitudes for geomagnetically quiet conditions.…”

“…Croom et al, 1960;King, 1961;Torr and Torr, 1973;Millward et al, 1996;Zou et al, 2000;Rishbeth et al, 2000;Tao Yu et al, 2000;Pavlova, 2005, 2008a, b). The winter/summer variations in the foF2 monthly noon medians were studied by Torr and Torr (1973) using the data of 140 ionosondes in 1958, 1964, and 1969.…”

Summer‐winter hemispheric asymmetry of the sudden increase in ionospheric total electron content and of the O/N2 ratio: Solar activity dependence

“…As shown by Figures 3b and 3d and Table 1, [20] The dayside ionosphere at midlatitudes is formed by the balance between the production rate due to the photoionization of atom oxygen O and molecular N 2 from the E region through the F region and the loss rate due to the chemical reaction of molecular N 2 and oxygen ion O + in the F region under the strong influence of ambipolar diffusion processes. The F region electron density and TEC at midlatitudes are well known to be correlated with O/N 2 ratio in the ionospheric F region [e.g., Rishbeth et al, 2000;Mendillo et al, 2005]. Sudden increases in the EUV radiation (assumed to be coincident with the X-ray) during solar flares cause immediate enhancements of ion/electron density in the lower F region and the E region rather than around the F2 region.…”

“…This seasonal (or hemispheric) asymmetry of O/N 2 ratio is well known to be responsible for the F region electron density winter anomaly at midlatitudes, that is, the winter daytime density is higher than that in summer. Rishbeth et al [2000] investigated the annual variations in the ionospheric F2 region using a coupled thermosphere-ionosphere computational model (CTIP), and suggested that the hemispheric asymmetry of total solar radiation (daily insolation) associated with the solar declination is the principal driver of the thermospheric circulation. Since the magnitude of this hemispheric asymmetry depends on the absolute solar radiation, the solar activity affects the thermospheric circulation, and accordingly the SWHA of O/N 2 ratio.…”

Seasonal and solar cycle variations of the ionospheric peak electron density: Comparison of measurement and models

“…The model was able to reproduce many of the features of NmF2 but could not explain the increased prevalence of winter maxima of noon NmF2 at higher solar activity and the semiannual variation of hmF 2. Rishbeth et al [2000] have given a thorough physical explanation of the changes in neutral composition that are responsible for the observed seasonal changes in NmF2. At solstice the solardriven circulation transports oxygen rich gas from the summer to the winter hemisphere.…”