University of Kansas Titan Studies

Draft Titan's Ionosphere: Model Comparisons with Casini Ta Data T. E. Cravens et al.

Image courtesy of NASA/JPL-Caltech.

Results

The photochemical model was used to generate vertical ion density profiles for solar zenith angles appropriate for the Cassini Orbiter trajectory at Ta. Only the calculated electron densities are shown in this paper.

First, we present results from model runs with ionization only from incident magnetospheric electrons (i.e., solar radiation is turned off). Time histories of n_e along the spacecraft track are shown for four such cases (Figure 2). The Cassini CAPS measurements in the outer magnetosphere favor magnetospheric electrons with n_e = 0.1 and T_e = 100 eV [CAPS paper in preparation, 2005]. The model electron density maxima for all these cases are located near (i.e., for the 200 eV case) or above the spacecraft CA altitude (1174 km). The peak density on the outbound track for both the RPWS measurements and the 100 eV model results is located 130 s after CA.

Figure 2. Time histories of the electron density along the Cassini Orbiter trajectory are shown for the 4 magnetospheric cases listed in Figure 1. Solar zenith angle and altitude along the track are also shown, but no ionization from solar radiation was included. Electron densities measured by the RPWS experiment [Wahlund et al., 2005] are also shown.

Calculated electron densities for only solar ionization inputs (no magnetospheric electrons, but photoelectrons are included) and with RPWS electron temperatures are shown as vertical profiles (Figure 3) and as a time history (Figure 4). The inbound leg of the trajectory is on the dayside (negative times from CA) and the outbound leg is on the nightside (positive times from CA). The maximum density along the track for both the RPWS data and the model is at t = -80 s (altitude z approx.= 1210 km; SZA approx.= 83 deg.). These densities are n_e approx.= 3300 cm^-3 and 4000 cm^-3 for the solar only model and the RPWS data, respectively (also see Table 1). The actual ionospheric peak in the model is located below the spacecraft for the dayside, although at CA (z = 1174 km; SZA = 91.1 deg.), the track is near the model density peak. The RPWS data shows a second maximum on the outbound track (t approx.= +130 s), but the solar only model does not.

Figure 3. Calculated radial/vertical electron density profiles for solar ionization only (no magnetospheric electron contribution) and at several times (in units of seconds) and solar zenith angles for the Cassini Orbiter. The solar zenith angle varies from 58.9 deg. (for t = -400 s) to 108.6 deg. (t = +200 s). The solar zenith angle for closest approach (time t = 0 s) was 91.1 deg. The line and open circles indicate the relevant densities for the spacecraft track.

Figure 4. The same as Figure 2 but the model electron densities are for calculations for two cases: (1) ionization only by solar radiation (Solar Flux 1) and (2) ionization from both solar radiation (Solar Flux 1) and from incident magnetospheric electrons with a temperature of 100 eV.

The model was also run for both solar and magnetospheric inputs with Solar Flux 1 and the RPWS electron temperature profile (Figure 4). Overall, the model and the data are in agreement, particularly near the main maximum near t = -100 s. However, the model densities for times -20 s to +70 s somewhat exceed the measured densities, and the model densities for t approx.= -150 s are somewhat too small. Furthermore, where the data shows a second maximum near t = +130 s (on the nightside), the model density only shows a ledge. The data also exhibits considerable small-scale structure at higher altitudes on the nightside and the model does not (discussed later).

Table 1 tabulates electron densities for t = -100 s for a variety of model cases (including some not shown in the figures) and for RPWS. Model densities for the enhanced solar flux case (Solar Flux 2) are obviously higher than the Solar Flux 1 densities. And the densities calculated with the lower electron temperatures from Gan et al. [1993] are somewhat lower than the densities calculated using the higher RPWS temperatures. For the 3 cases listed for which both solar and magnetospheric inputs were included, the model density values in the table range from 3670 cm^-3 to 4520 cm^-3; these values are within 15 percent of the measured density.

Table 1. Electron densities at t = -100 s (altitude of 1218 km and SZA = 82.2 deg.)

¹Wahlund et al. [2005]
²Gan et al. [1993]

Next: Discussion and Conclusions

References

Return to abstract and contents page for this Titan draft manuscript.
Return to Titan Studies main page.
Return to Space Physics home page.

Last modified August 17, 2005 Tizby Hunt-Ward tizby@ku.edu