Short Term Variations

Variations in layer critical frequencies will arise on an hour-to-hour and day-to-day basis, especially for the F2 region. Day-to-Day F region variability is exhibited in Figure 3-12 for a period of maximum solar activity and for midlatitudes. It appears that much of this variability owes its existence to the impact of geomagnetic storms, traveling ionospheric disturbances (TIDs), and miscellaneous F region dynamic effects. TIDs are one of the more fascinating features of the ionosphere. They are the ionospheric tracers of neutral atmospheric gravity waves, which derive from a number of sources in the upper atmosphere. These sources include localized heating effects, atmospheric explosions, enhanced auroral activity, and other atmospheric phenomena that are associated with relatively rapid and nonuniform changes in atmospheric pressure. Figure 3-13 shows the variation of foF2 as a function of time showing the impact of TIDs. Figure 3-14 shows the effect of a large geomagnetic storm.

Figure 3-11: Mean diurnal variation of foE, foFl, and foF2 for summer and winter under Northern Hemispheric solar maximum conditions. The seasonal anomaly is clearly evident with the wintertime values of the F2-laver peak electron density being larger than the summertime values. At the same time we note that the E-layer peak density is larger in summer than in wintertime, as suggested by Chapman theory. A late afternoon (pre-midnight) bulge in foF2 occurs for summertime solar maximum conditions. From Jursa |1985|.

Figure 3-11: Mean diurnal variation of foE, foFl, and foF2 for summer and winter under Northern Hemispheric solar maximum conditions. The seasonal anomaly is clearly evident with the wintertime values of the F2-laver peak electron density being larger than the summertime values. At the same time we note that the E-layer peak density is larger in summer than in wintertime, as suggested by Chapman theory. A late afternoon (pre-midnight) bulge in foF2 occurs for summertime solar maximum conditions. From Jursa |1985|.

Figure 3-12: Variations in the hourly values of foF2 as a function of the time of day, for January solar maximum conditions, and Northern Hemispheric sounder site. The range of day-to-dav variability in foF2 is ~ ± 10%, suggesting a variation in NmaxF2 of ~ ± 5%. From Davies [1965],

Figure 3-12: Variations in the hourly values of foF2 as a function of the time of day, for January solar maximum conditions, and Northern Hemispheric sounder site. The range of day-to-dav variability in foF2 is ~ ± 10%, suggesting a variation in NmaxF2 of ~ ± 5%. From Davies [1965],

Figure 3-13: Variations in the ionosphere thought to be associated with traveling ionospheric disturbances. The fof'2 variations shown here are of the order of ± 2% and have periods of- 20 minutes. The NF2max variations are ~ ± 1%. From Paul [1989],
Peak Density Nr
Total Electron Content Space Weather

Figure 3-14: Eflect ot a large geomagnetic storm on Ntnax and the total electron content Ay. The curves represent average perturbations (%) at hourly intervals, as reckoned from the mean of the 7 days prior to the storm onset. There were some selectivity requirements. The main one was that the storm must begin (i.e., day-1) from a few hours before sunrise to a tew hours before sunset. Nocturnal storm onsets generate complications that make a general summary problematic. Illustration derived from Mendillo and Klobuchar [1974b]. (See Section 3.10.)

Figure 3-14: Eflect ot a large geomagnetic storm on Ntnax and the total electron content Ay. The curves represent average perturbations (%) at hourly intervals, as reckoned from the mean of the 7 days prior to the storm onset. There were some selectivity requirements. The main one was that the storm must begin (i.e., day-1) from a few hours before sunrise to a tew hours before sunset. Nocturnal storm onsets generate complications that make a general summary problematic. Illustration derived from Mendillo and Klobuchar [1974b]. (See Section 3.10.)

Was this article helpful?

0 0

Post a comment