Titan’s Thermospheric Response to Various Plasma Environments Joseph H. Westlake Doctoral Candidate The University of Texas at San Antonio Southwest Research Institute 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

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

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 Method: Mean Scale Height This is the method chosen to analyze the INMS data set in this study Global Fit ± 1.2 K Global Fit ± 1.2 K Westlake et al. (In Review) Joseph Westlake (UTSA/SwRI) –

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 Westlake et al. (In Review) Joseph Westlake (UTSA/SwRI) –

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

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 Rymer et al. (2009) Joseph Westlake (UTSA/SwRI) –

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 N 2 + 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) –

Plasma Region Dependence RegionT Eff (K) Global Average153.0 ± 1.2 Plasma Sheet160.7 ± 1.0 Lobe131.7 ± 1.2 Bi-Modal145.1 ± 1.9 Magnetosheath144.3 ± 1.8 Results: 29 K Effective Temperature Difference (Plasma Sheet Vs. Lobe) Largest Observed Systematic Variation in the thermosphere Results: 29 K Effective Temperature Difference (Plasma Sheet Vs. Lobe) Largest Observed Systematic Variation in the thermosphere Westlake et al. (In Review) Joseph Westlake (UTSA/SwRI) –

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) Westlake et al. (In Review) Joseph Westlake (UTSA/SwRI) –

Individual Flybys Results: Plasma sheet flybys exhibit enhanced effective temperatures Lobe flybys show reduced effective temperatures 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: K Lobe Average: K Δ = 26.5 K Westlake et al. (In Review) Joseph Westlake (UTSA/SwRI) –

Temporal Variations Results: Similarly oriented flybys which are separated by one Titan day (~16 Earth days) show large effective temperature deviations. Results: Similarly oriented flybys which are separated by one Titan day (~16 Earth days) show large effective temperature deviations. 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 ΔT Eff = 29 K ΔT Eff = 20 K Westlake et al. (In Review) Joseph Westlake (UTSA/SwRI) –

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

Pulse Start Estimated Recovery Time Estimated Recovery Time Pulse Stop Simulating Titan’s Plasma Response Using the T-GITM model we simulate a ½ Titan day heating pulse. (A)Thermal response with diurnal portion removed (B)Actual thermal response (C)Altitude map of thermal response Simulated Titan Days Bell et al. (Submitted) Joseph Westlake (UTSA/SwRI) –

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) –

Thank You Joseph H. Westlake Doctoral Candidate The University of Texas at San Antonio Southwest Research Institute