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
10x Difference
• Cassini Ion and Neutral Mass Spectrometer (INMS) data in Titan’s thermosphere exhibits large pass to pass variability.
Joseph Westlake (UTSA/SwRI) – [email protected]
• 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
s
s
s Hdzdn
n11
Global Fit153.0 ± 1.2 K
Westlake et al. (In Review)Joseph Westlake (UTSA/SwRI) – [email protected]
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) – [email protected]
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) – [email protected]
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) – [email protected]
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 systemMichael 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) – [email protected]
Plasma Region Dependence
Region TEff (K)
Global Average 153.0 ± 1.2
Plasma Sheet 160.7 ± 1.0
Lobe 131.7 ± 1.2
Bi-Modal 145.1 ± 1.9
Magnetosheath 144.3 ± 1.8
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) – [email protected]
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 reproducedWestlake et al. (In Review)Joseph Westlake (UTSA/SwRI) – [email protected]
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
Lobe Average: 118.2 K
Δ = 26.5 K
Westlake et al. (In Review)Joseph Westlake (UTSA/SwRI) – [email protected]
Temporal Variations
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
ΔTEff = 29 KΔTEff = 20 K
Westlake et al. (In Review)Joseph Westlake (UTSA/SwRI) – [email protected]
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)
)()()(rQrTr tot
Bell et al. (Submitted)Joseph Westlake (UTSA/SwRI) – [email protected]
Pulse StartEstimated Recovery
TimePulse 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) – [email protected]
Results• The relaxation time is
roughly 10 Earth Days.• Tends to most affect the
region above 1000 km.
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) – [email protected]
Thank You
Joseph H. Westlake Doctoral Candidate
The University of Texas at San Antonio Southwest Research Institute