Titan’s Thermospheric Response to Various Plasma Environments

1. Titan’s Thermospheric Response to Various Plasma Environments J. H. Westlake, J. M. Bell, J. H. Waite, R. E. Johnson, J. G. Luhmann, K. E. Mandt, B. A. Magee, and A. M. Rymer Joseph H. Westlake Doctoral Candidate The University of Texas at San Antonio Southwest Research Institute joseph.westlake@swri.edu jwestlak@gmail.com

2. Observation 10x Difference Goal: To determine the primary driver of the variability in Titan’s thermospheric density structure • Cassini Ion and Neutral Mass Spectrometer (INMS) data in Titan’s thermosphere exhibits large pass to pass variability. Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org

3. Method:Mean Scale Height Global Fit 153.0 ± 1.2 K • Method: • Linear fitting to the logarithm of the nitrogen density from 1050 km to the exobase. • Strengths: • Few assumptions • Isothermal and hydrostatic • Obtains a stable, high quality match to the data. • Weaknesses: • Assumes isothermal conditions within the altitude range studied This is the method chosen to analyze the INMS data set in this study Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Westlake et al. (In Review)

4. Parameter Space • Solar Parameters • Solar Zenith Angle • Sub-Solar Latitude (Season) • Latitude • Local Time • Sun Fixed Longitude • Plasma Parameters • Plasma Environment • Saturn Local Time • Longitude This study assesses each of these parameters independently to determine the controlling process Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Westlake et al. (In Review)

5. Northern hemisphere before southern hemisphere flybys Meridional Dependence? • Prior to October of 2007 INMS only sampled the northern latitudes of Titan • Müller-Wodarg et al. (2008) • The picture of Titan drastically changed when we delved into the southern hemisphere All INMS density points to date Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org

6. Saturn’s Magnetosphere I:Titan’s Local Plasma Configurations • Two studies (Rymer et al., 2009; Simon et al. 2010) have assessed the Cassini Titan encounters, identifying the following configurations: • Plasma Sheet • High energy and density plasmas • Lobe • Similar energies to the plasma sheet flybys but an order of magnitude less in density • Magnetosheath • Lower energies and high fluxes • Bi-Modal • Two different electron populations superimposed Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Rymer et al. (2009)

7. Saturn’s Magnetosphere II:Plasma Influence on the Thermosphere • Ions and electrons penetrate into Titan’s thermosphere depositing their energy. • Ion species include H+, O+, and the pickup ions N2+ and N+ • Solar EUV/UV photons deposit their energy lower in the atmosphere • Solar inputs are balanced by a photochemical feedback system Michael and Johnson (2005), De La Haye et al., (2008), Smith et al., (2009), Shah et al., (2009) Magnetospheric processes exert the greatest influence within the region above 1100 km Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org

8. Plasma Region Dependence Results: 29 K Effective Temperature Difference (Plasma Sheet Vs. Lobe) Largest Observed Systematic Variation in the thermosphere Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Westlake et al. (In Review)

9. Modeled Thermospheric Response • T-GITM = Titan Ionosphere Thermosphere Model (Bell et al., 2010) • Navier-Stokes fluid model which self consistently reproduces globally averaged INMS densities • Run 1 (No Heating) – Only solar • Run 2 (Heating) – Solar + Plasma • H+ (Smith et al., 2009) • Pick up ions (Michael and Johnson, 2005) • O+ (Shah et al., 2009) Results: Using informed heating rates in the upper atmosphere the density variations observed by the INMS are reproduced Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Westlake et al. (In Review)

10. Individual Flybys Results: Plasma sheet flybys exhibit enhanced effective temperatures Lobe flybys show reduced effective temperatures • The mean scale height method was used on each flyby individually Plasma Sheet Average: 144.7 K Δ = 26.5 K Lobe Average: 118.2 K Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Westlake et al. (In Review)

11. Temporal Variations ΔTEff = 29 K ΔTEff = 20 K • The difference in observed effective temperature may deviate more in a temporal fashion than in a spatial fashion • Flybys occurring one Titan day apart with nearly identical trajectories and solar conditions Results: Similarly oriented flybys which are separated by one Titan day (~16 Earth days) show large effective temperature deviations. Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Westlake et al. (In Review)

12. Time Scales? • INMS data indicates that Titan responds on a timescale of less than one Titan day. Thermal Time Constant (Earth Days) Titan’s thermosphere seems to respond to plasma heating on a timescale of about 10 Earth Days Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Bell et al. (Submitted)

13. Simulating Titan’s Plasma Response • Using the T-GITM model we simulate a ½ Titan day heating pulse. • Thermal response with diurnal portion removed • Actual thermal response • Altitude map of thermal response Results • The relaxation time is roughly 10 Earth Days. • Tends to most affect the region above 1000 km. Estimated Recovery Time Pulse Start Simulated Titan Days Pulse Stop Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org Bell et al. (Submitted)

14. Conclusions • During the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may over-ride the expected solar-driven diurnal variation in temperatures in the upper atmosphere. • Temperatures are observed to be enhanced by 29 K on average when Titan is within the plasma sheet over when it is within the lobe regions. • Titan’s thermosphere responds to plasma forcing on timescales less than one Titan day (~10 Earth days) Joseph Westlake (UTSA/SwRI) – joseph.westlake@swri.org

15. Thank You Joseph H. Westlake Doctoral Candidate The University of Texas at San Antonio Southwest Research Institute joseph.westlake@swri.edu jwestlak@gmail.com