[10] The newly defined mechanism for electrical coupling between the troposphere and the electrically active mesosphere, which reveals the effect of electron cooling in the ionospheric D region, consists of the following.
[11] 1. Large mesospheric electric fields, which have a probability of occurrence of 0.7-0.75, ensure that the ionospheric D region departs from local thermodynamic equilibrium with enhanced electron temperatures. However, this state may be modified by large disturbances in the tropospheric conductivity when coupling between the troposphere and the lower ionosphere develop.
[12] 2. A significant increase in the tropospheric conductivity, by 1 or 2 orders of magnitude, as over seismically active regions or at nuclear power plants with the discharge of radioactive materials, results in grounding the mesospheric current source and in a significant decrease in the intensity of large mesospheric electric fields. This process may be accompanied by a large-scale redistribution of the inherent mesospheric electric potentials.
[13] 3. The reduction in large mesospheric electric fields leads to a decrease in the electron temperature and effective collision frequency with a characteristic time constant of less than 1 ms. The corresponding variations in the high- and low-frequency (down to DC) conductivity of the lower ionosphere are remotely detected by sensing instruments employing radio wave techniques.
[14] Our observations show that the large mesospheric electric fields are absent during 25-30% of the entire measurement interval, and therefore the ionospheric D region is in the state of local thermodynamic equilibrium. Consequently, the troposphere-mesosphere electrical coupling, which is discussed in this paper, does not occur during these time intervals.
[15] Fuks and Shubova [1994], Martynenko et al. [1994, 1996], and Fuks et al. [1997] observed the localized disturbances of this kind in the VLF perturbations. The perturbations were caused by rapid VLF conductivity enhancements resulting from the electron cooling in the lower ionospheric D region over seismically active regions and nuclear power plant accidents with radioactive fallout.
[16] The model of electrical coupling between the troposphere and the mesosphere proposed in this study is in agreement with earlier measurements. Prior to and during the 17 January 1995 Kobe earthquake with 7.2 magnitude, Maeda and Tokimasa [1996] observed two sequences of 22 MHz radio bursts at a distance of 77 km from the epicenter. They may be explained by two sequences of reductions in the ionospheric D -region HF conductivity, and hence in the HF absorption due to the decreases in electron collision frequencies.
[17] Warwick et al. [1982] were the first to report radio wave disturbances associated with seismic activity. The large-scale ionospheric disturbances caused by strong seismic activity persisted for a few days prior to and during the 22 May 1960 Chile earthquake with magnitude 9.6. The measurements were taken using a net of 18 MHz riometers in North America and spaced by thousands kilometers from each other. The increases in the signal amplitude by a factor of up to 2 over a background noise were observed to correlate with the seismic disturbances. Warwick et al. [1982] and Maeda and Tokimasa [1996] interpreted the HF bursts to be generated in the seismically active region, which is difficult to reconcile with the current descriptions of seismic activity. We propose that the enhancements in HF signal amplitudes may be due to the effect of electron cooling that could cause a large-scale reduction in the HF total absorption in the ionospheric D region.
[18] We believe that the proposed mechanism for troposphere-mesosphere coupling through large mesospheric electric fields could provide the basis for developing coupled troposphere-mesosphere-ionosphere electrodynamic models under disturbed conditions. The results would be of use for developing new techniques for monitoring the near-Earth environment and remotely sensing disturbances from various physical origins.
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