The dynamo of the diurnal tide and its effect on the thermospheric circulation

Abstract

In the upper thermosphere and at equatorial latitudes, the observations reveal a dominance of the fundamental diurnal tide, and show a close relationship between the measured zonal winds and ion drift velocities. This raises questions about the relative importance of the E- and F-region dynamos and the role of the ions in accelerating the neutral atmosphere. To interpret the measurements, we discuss results from a multiconstitutent model (including O, N 2 and O 2), describing in self-consistent form the interactions between the neutral winds, dynamo electric fields, and ion drifts. Hough modes and spherical harmonics are used to describe the fundamental diurnal tides generated in the lower atmosphere and thermosphere, respectively. The largest contribution to the dynamo electric field is associated with the spherical harmonic component P 1 1 generated primarily by the absorbed e.u.v. radiation. The dynamo electric fields associated with the Hough modes S 1 −2 and S 1 1, generated by the absorbed u.v. and visible radiation in the lower atmosphere, are relatively small. The computed zonal velocities for the neutrals and ions, with the higher order spherical harmonics P 1 3 and P 1 5 included, are synthesized at low latitudes and compared with the observations. The model reproduces reasonably well the salient features : the magnitudes of the neutral winds and ion drift velocities and the ratio between the two (only the computed phase difference is somewhat larger than observed). We analyze the electric potential in terms of different cut-off altitudes below which the (generating) winds are forced to zero. This leads us to conclude that, contrary to common perception, the dynamo electric fields are generated primarily in the F-region not in the E-region. Moreover, the results show that the dynamo interaction causes the neutral winds to increase by about 50%. Since the dynamo electric field is static, this increase occurs primarily in the curl field of the velocity, and the effect on the temperature and density variations is relatively small.