Nighttime equatorial thermospheric meridional winds have been derived using h′F data from two equatorial stations nearly on the same magnetic meridian. It is shown that the winds derived are quite consistent with earlier observations.
Abstract. Night-time F-region vertical electrodynamic drifts at the magnetic equatorial station, Trivandrum are obtained for a period of 2 years, 1989 and 1990 (corresponding to solar cycle maximum epoch), using ionosonde h'F data. The seasonal variation of the vertical drift is found to be associated with the longitudinal gradients of the thermospheric zonal wind. Further, the seasonal variation of the prereversal enhancement of the vertical drift is associated with the time difference between the sunset times of the conjugate E-regions (magnetic field line linked to F-region) which is indicative of the longitudinal gradients of the conductivity (of the E-region). The vertical drifts and the causative zonal electric fields at Trivandrum are compared with those at Jicamarca and F-region zonal electric field models. It is seen that the night-time downward drift (as also the causative westward electric field) at Jicamarca is greater than that at Trivandrum. The prereversal enhancement of the drift is greater at Jicamarca than at Trivandrum during the summer and the equinoxes, whereas during the winter the opposite is the case.
Two-dimensional CFD simulations were performed on a fullsection numerical model of an as-built slot virtual impactor prototype and its completely symmetric ideal counterpart. The simulations reproduce the trends of the experimentally observed performance including verification of a third region in the transmission efficiency curve, which is a drop-in transmission efficiency for large particle sizes. Visualization of simulated particle tracks show this decrease is attributed to a crossing trajectory phenomenon, whereby larger particles that acquire enough inertia in a chamfered acceleration nozzle, crossover the vertical mid-plane and impact on the opposite-side wall, particularly on the wall of the receiver section. Some experimental data presented in the literature for rectangular slot and round-nozzle virtual impactors with chamfered 45• half-angle acceleration nozzles (similar to the geometry tested herein), show a similar drop-in transmission efficiency that commences at a particle Stokes numbers of about 6. However, many studies do not demonstrate the drop in transmission efficiency because wall losses are not taken into account. The Reynolds number, based on the acceleration nozzle size and velocity, does not noticeably affect the onset of the phenomenon. The crossing trajectory phenomenon can severely restrict the size range over which a virtual impactor can be used as an efficient particle concentrator. Geometrical asymmetry from dimensional tolerance considerations in the construction of a virtual impactor exacerbates the impact of the crossing trajectory phenomenon.
The E and F region eastward electric fields in the postsunset period obtained from VHF backscatter radar and ionosonde observations respectively at the magnetic equatorial station at Trivandrum are compared. It is found that the F region eastward electric field is greater than the E region electric field indicating latitudinal gradients in the E region electric field.
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