Studies of the structure and dynamics of the ...

4. Analysis

Figure 6

[16] The relation between the intensity of the X-ray radiation of the flare and anomalous ionization in the ionospheric D region needs more detailed consideration. To do this, it is interesting to compare the changes in the electron concentration Ne with the behavior of the intensity of the flare X-ray radiation I and a square root of the intensity I^0.5. For calculation of the dependencies at fixed altitudes of the D region, the 5-min data of the X-ray radiation flux and electron concentration were used. As an example, Figure 6 shows such dependencies for the X-ray radiation flux in the range 1-8 Å from the flare on 5 April 2004 at altitudes of 72.4 and 77.5 km. At these altitudes the significance coefficients were the maximal and equal to 0.98 and 0.94, respectively. The calculations show that at altitudes below 74 km the significance coefficient for the N_e(I) dependence is higher then the coefficient for the N_e(I^0.5 ) relation, that is there presents a linear relation in the considered region between the intensity of the X-ray radiation (proportional to the ionization rate) and electron concentration. At altitudes from 75 km to 81 km the significance coefficient is higher for the N_e(I^0.5 ) dependence; that is, a quadratic recombination law is observed. Thus linear recombination law and quadratic recombination law are observed in the lower part of the D region and at higher altitudes, respectively. It should be noted that the conclusion on the linear relation between the ionization rate and electron concentration variations in the daytime has been earlier obtained by Belikovich et al. [2003b].

[17] According to Belikovich et al. [2005] the linear recombination law in the D region may be explained on the basis of the hypothesis of intense recombination of positive and negative charges on dust-like particles. The number of particles in a volume unit is limited and is not related to the value of the electron concentration. The term prevailing in the balance equation and containing the concentration of dust-like particles leads to the linear recombination law. Above 80 km atomic oxygen presents in the atmosphere and prevents cluster formation. Because of that the quadratic recombination law begins to work.

[18] One can explain the increase in the electron concentration during flares by variations in the ion composition of the ionospheric D region. It is known [Danilov, 1989; Mitra, 1974] that the portion of the rapidly recombining complex clustered ions in the total amount of positive ions decreases. Theoretical models of the ion composition based on the ion chemistry [Smirnova et al., 1988], in particular conditions of the polar ionosphere, lead to the same conclusion. The calculations show that considerable destruction of clustered ions under the action of an X-ray solar flare of the M importance occurs in the lower ionosphere below 70 km. The depletion of the clustered ions content leads to a significant depletion of the effective recombination coefficient at altitudes of 65-95 km. The decrease in the effective recombination coefficient is accompanied by an increase in the electron concentration.

[19] The trough in the electron concentration vertical profile at a height of 80 km (see Figure 3) during the maximum of the 5 April flare can be related to the instrumental features of the partial reflections method and conditions in the environment. Apparently, the existence of high electron concentration and strong absorption during a solar flare limits the height up to which the data could be obtained by this method at a single frequency. However, the latter assumption requires a thorough checking.