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The Institute of Space and Astronautical Science Report

6. NIGHTSIDE IONOSPHERE

In spite of long Venus night (58 earth days), there is a remarkable abundance of ions on the nightside featuring the same constituents as on the dayside, though with much lower concentration levels (Taylor et al., 1980b). There is a strong solar activity effect which was earlier seen in the radio occultation measure-ments (Knudsen et al., 1987) and has been reconfirmed from the PVO pre-entry measurernents from all the aeronomy experiments (Theis and Brace, 1993; Kar et al., 1994; Spenner et al., 1995, 1996). Figure 17 shows a comparison of nighttime electron densities and temperatures obtained by the OETP during the pre-entry period and during the solar maximum. While the densities decreased during the entry, the temperatures increased (Theis and Brace, 1993). Further, a large variability in the densities has been observed and Figure.18 highlights this variability. It can be noted that the densities can vary by two orders of magnitude during the nightside.

Fig.17:Comparison of elaction densities and temparatures measured by
the OETP instrument during the pre-entry period(FF10.7 120) and solar maximum(F10.7 200).
While the densities decreased, the temperatures increased during the pre-entry period
(from Theis and Brace, 1993).

Fig.18:Nightside slectron density values measured during the entry period by ORPA.
Large variability in the density can be noted (from Spenner et al., 1995)

The existence of the nightside ionosphere has been explained on the basis oftransterminator flow of 0⁺ ions from the dayside (e.g. Knudsen et al., 1980b, Fox and Kliore, 1997 and references therein). This flow occurs because of the large plasma pressure gradient between the day and the nightside and has been confirmed by the ORPA instrument which has measured anti-sunward velocities of about 5 km/s at high altitudes in the terminator region (Miller and Whitten, 1991 and references therein). Figure 19 gives the average ionospheric velocity field as measured by the ORPA. The measured velocities very successfully explain the existence of the nightside ionosphere. Contribution of electron precipitation in the maintenance of the nightside ionosphere is estimated to be in the range 20 to 30% during solar maxirnum. According to Fox and Kliore (1997) the solar cycle response, the solar zenith angle behavior and the overall variability can not be accounted for by electron precipitation as the major ion source. Spenner et al. (1996) conclude equal contributions by transterminator flow and electron precipitation for the maintenance of the nightside ionosphere during medium solar activity. Kar et al. (1994), on the other hand, have concluded electron precipitation as the major ion source during the solar medium and thus the solar minimum too.

Fig.19:Average O⁺ velocity as measured by the ORPA instrument.
Large velocities at the terminators are responible for the maintenance of
the nightside ionosphere(from Miller and Whitten, 1991).



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