4. Determination of the Variations in W From the Variations in hmF2

[7]  The calculations were performed using the main principles ionospheric model which has been developed by Rishbeth [1967] and Rishbeth et al. [1978] and which is usually called a servo model. According to the servo model, the variations in the velocity W of the plasma vertical drift in quasi-stationary conditions are related to the variations in hmF2 by the following formula [Buonsanto et al., 1989]:

eq001.gif

eq002.gif

where Dam is the ambipolar diffusion coefficient Da at the hmF2 height, H is the scale height for atomic oxygen which controls the diffusion, I is the Earth's magnetic field inclination, and k represents the scale height for the effective recombination coefficient and is equal to 1.875. The balance height, hm0, is determined as a height where the following relation between the recombination and diffusion processes is fulfilled:

eq003.gif

The constant s is taken equal to 0.160 and 1.077 for the nighttime and daytime conditions, respectively. The values of the atmospheric parameters for calculation were taken from the MSIS model [Hedin, 1991], and the values on Te were taken from the Cosmos 900 data [Karpachev et al., 1997].

[8]  The variations in the balance height hm0 are mainly determined by the changes in the composition and temperature of the thermosphere and are described in the following way [Karpachev and Gasilov, 1998]:

eq004.gif(1)

where h0 is some reference height (in the calculations it was taken h0 = 300 km), b0 is the recombination coefficient at the height h0, and c1 and c2 are constants. Since all the parameters in (1) depend on longitude, hm0 depends on longitude also. Taking into account that one can rewrite (1) in a form more simple and convenient for the analysis:

eq005.gif

[9]  Determination of the balance height hm0 raises no difficulties for purely daytime or nighttime conditions, when according to the servo model the coefficient s takes the values 1.077 or 0.160, correspondingly. However, as it has been noted above, the auroral ionosphere in summer near-midnight conditions is partly sunlit ( Z le 95o ). So in calculations of hm0 for invariant latitudes 60o and 65o, the values of the coefficient s were determined depending on the illumination level using a linear interpolation between the daytime and nighttime values. Such procedure provides smooth variations in hm0 with longitude, because transferring from the nighttime to the daytime conditions the average value of hm0 decreases but the character of its longitudinal variations does not change strongly.

AGU

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