[51] Our data agree with the results of measurements of the
absolute amplitude
dI
of TEC variations obtained
at transionospheric UHF sounding by signals of geostationary
satellite ETS 2 [Afraimovich et al., 1999] and
also with the data of the analysis of the variation of TEC spectra
according to the GPS data [Afraimovich and Karachentsev,
3; Afraimovich et al., 2001].
[52] Unexpectedly, the inverse dependence of the relative
amplitude of the TEC variations in quiet period was found: the
maximum values of
dI/I are observed at night but
not in the daytime. The latter means that the mechanisms of
generation and propagation of AGW differ significantly in the
daytime and at night. This is in agreement with conclusions of
other researchers, for instance, based on the data of TEC
measurements in Los Alamos (35.9oN, 106.3oW). Jacobson et al. [1995] claimed that the seasonal
variations in MS TID occurrence rate and propagation directions
are different in the daytime and at night. They showed that the
daytime MS TID are formed mainly during the winter solstice and
propagate southward, whereas the nighttime MS TID occur mainly
during the summer solstice till the fall equinox and propagate
northwestward.
[53] Kelley and Miller [1997] noted that the difference between the direction of MS TID propagation in the daytime and at night is responsible for the difference in MS TID generation. The amplitude of gravity waves grows at a depletion of the neutral density [Hines, 1960]. Larger amplitude of gravity waves is able to generate larger disturbances in plasma density, i.e., in the MS TID activity. On the other hand, the rate of the linear growth of a Perkins instability is also inversely proportional to the neutral density [Perkins, 1973]. Most probably the Perkins instability is a source of nighttime MS TID though the rate of the linear grow is very small.
[54] Kotake et al. [2006] have found that MS TIDs activity during daytime is different from that during nighttime with respect to seasonal, solar activity, longitudinal, and latitudinal dependencies. Daytime MS TIDs activity is high in winter. On the other hand, seasonal variation of nighttime MS TIDs activity is coupled with its longitudinal variation. In the Japanese and Australian longitudinal sector, nighttime MS TIDs are most active near the June solstice, whereas in the European longitudinal sector it is most active near the December solstice. Nighttime MS TIDs activity at the Japanese and Australian longitudinal sector shows negative correlation with solar activity, whereas solar activity dependence is not seen in daytime MS TIDs activity. These results suggest that mechanisms causing MS TIDs could be different between daytime and nighttime.
[55] Hernandez-Pajares et al. [2006] have shown that the MS TIDs, which occur at daytime in local winter and nighttime in local summer, are related to the solar terminator and are modulated by the solar cycle.
[56] Then our conclusion regarding the difference mechanisms of generation and propagation of AGW in the daytime and at night agrees with data obtained by the Kelley and Miller [1997], Kotake et al. [2006], and Hernandez-Pajares et al. [2006].
[57] The delay by 2 hours of the sharp peak of
dI/I relative quick variations in the magnetic field
during the 30 October 2003 storm can be explained in the following
way. Under the averaging, the main weight brings the midlatitude
zone of GPS stations. This zone is located at a distance of about
2000 km from the southern boundary of the auroral source of TID
that occurred during geomagnetic disturbances. The TID generated
at the appearance of this source moves equatorward with velocity
of about 300-400 m s-1 [Afraimovich et al.,
2001; Hunsucker, 1]. However,
there is a large evidence of other mechanisms besides TID which
could have the same observational features. For instance, Foster and Rideout [2005] have shown that for the
same storm discussed in our paper (30 October 2003), TEC increase
is observed at postnoon local times and related to subauroral
electric fields.
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