N. N. Murzaeva
Institute of Space Physics and Aeronomy, Yakutsk, Russia
The dependence of the lower ionosphere parameters on solar activity has already been studied during several decades. However, the problem has not been solved yet. Smirnova and Danilov  analyzed the conclusions made by a number of authors concerning the dependence of the quiet-time D region electron concentration on solar activity. Danilov et al. , Friedrich and Tokar [1992, 1997], Knyazev et al. [1994a, 1994b], Mechtly et al. , and Sengupta  considered the results of direct measurements of electron concentration and empirical models of the D region relying on these measurements. Bremer and Singer  and Lauter et al.  considered the patrol of the D region state by the radio wave propagation method (measurements of the radio wave absorption by the A1 and A3 methods, measurements of the reflection altitude hp ). Smirnova and Danilov  emphasize that different authors present substantially different amplitudes of increase in the electron concentration of the upper D region with solar activity and conclusions made by different authors on the sign of the solar activity effect in the lower D region also contradict each other.
Detection of the intensity of natural low-frequency electromagnetic emission propagating in the near-Earth waveguide is an easy method for continuous monitoring the D region condition. This paper describes the studies of variations in the lower ionosphere parameters with solar activity involving the analysis of solar cycle variations in the response of the regular noise background (RNB) of natural low-frequency (0.5-10 kHz) emission to solar flares inducing sudden ionospheric disturbances (SID). The RNB is a fluctuating component of the electromagnetic emission generated by lightning discharges and propagating in the near-Earth waveguide [Druzhin and Shapaev, 1988; Druzhin et al., 1986; Kozlov and Mullayarov, 1996; Vershinin and Ponomarev, 1966]. The character of solar cycle changes of natural low-frequency emission variations during solar flares is the evidence of existence of solar cycle variations in both the quiet-time D region parameters and their response to solar flares.
Our experiments have shown that there is a transition from an enhancement to a decrease in the RNB intensity during solar flares. During the solar cycle, the transition moves from 1.5 kHz for the periods of solar activity decline and minimum to ~3 kHz for the periods of solar activity rise and maximum.
It is known that in the frequency range above 10 kHz, i.e., outside the operating range of our equipment, there exists an inverse transition [Eryushev, 1960; Field, 1970; Mitra, 1977; Sao et al., 1970]. It is worth analyzing its dependence on solar activity. Since we could not get the necessary experimental data, we extrapolated the experimental data obtained to the range above 10 kHz. The data obtained by Rizzo Piazza and Kauffman  were used for the extrapolation.
Thus, we have drawn Figure 1b, using the results obtained by Rizzo Piazza and Kauffman , that is, extrapolating the curves shown in Figure 1a over points to 15 kHz for quiet period and 13.6 kHz for active period. To justify the use of the data for 1967 in processing the data for 1981 (the curve in Figure 1a), the following arguments were used. Exact values of the transition frequencies may vary from one solar cycle to another. The important thing is that the reverse transition frequency decreases with solar activity rise. The latter fact has been revealed from the data for 1961, 1965 and 1967, 1968 and then confirmed by the data for 1974. During the latter year the transition frequency increased again, and the records of signal amplitude during the 13 April 1974 flare exhibited a negative bay at the 13.6-kHz point which has not been observed in 1967 [see Rizzo Piazza and Kauffman, Figure 2]. Therefore, one can see that the transition frequencies obtained by Rizzo Piazza and Kauffman in 1961, 1965 and 1974 are similar to each other, and the 15-kHz point taken from the data of 1961 and 1965 may be used in combination with the curve measured in 1973-1974. A reason for this operation is also the fact that the data on natural signal (RNB) variations during 1973-1974 comply with the observations in 1983-1986 [Murzaeva, 1997]. This fact provides an additional evidence of a constancy of the forward (from a rise to a decrease) and reverse (from a decrease to a rise) transition frequencies.
In order to reveal solar cycle variations in the D region parameters and in their response to SIDs, we used the calculations of the field strength of low-frequency (0.5-10 kHz) emission propagating in the near-Earth waveguide obtained by the model of a flat waveguide with a sharp upper boundary suggested by D. S. Fligel' [Alpert et al., 1967; Murzaeva, 1991; Murzaeva and Fligel', 1980]. In this model the Earth's conductivity is infinite and conductivity of the upper wall of the near-Earth waveguide is determined by the factor N/n, where N is the electron concentration in cm-3, and n is the effective collision frequency in s-1. The distance to the emission source is assumed to be 1000 km. The emission source power is 1 kW. The calculations were carried out using the mode theory. The calculations involved only the mode n=0, because other modes scarcely contribute to the field strength at the distance r 1000 km. The field strength was calculated for a wide range of variations in the ionospheric conductivity and altitude. In case of quiet ionosphere, the parameters were typically taken as follows: N/n = 10-5 and the waveguide altitude h=70 km [Murzaeva, 1991].
The character of the near-Earth waveguide parameter variations revealed in this work is similar to that obtained by Trista and Lastovichka [1970, 1972]. They found that a decrease in the near-Earth waveguide height leads to a SDA (sudden decrease of atmospherics) occurrence, whereas an increase in the conductivity causes SEA (sudden enhancement of atmospherics). Our calculations show that during quiet-Sun periods the number of SEA are much lower the number of SDA and even negligible, whereas during active-Sun periods the number of SEA increases sharply.
The comparative analysis of experimental data and calculations performed using the model of a flat waveguide with a sharp upper boundary showed that the quiet-ionosphere ratio N/n at the upper boundary of the near-Earth waveguide is higher at the quiet-Sun periods than at the active-Sun periods.
During the periods of sudden ionospheric disturbances, the increase in the conductivity of the upper boundary of the Earth-ionosphere waveguide is greater during the rising phase and maximum of the solar cycle. At the same time the variations in the waveguide upper boundary height are, apparently, more important during the falling phase and minimum of the solar cycle.
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