titan’s thermospheric response to various plasma environments

Titan’s Thermospheric Response to Various Plasma

Environments

Joseph H. Westlake Doctoral Candidate

The University of Texas at San Antonio Southwest Research Institute

[email protected]

[email protected]

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

mailto:[email protected]


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


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


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


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


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


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


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)


Thank You

Joseph H. Westlake Doctoral Candidate

The University of Texas at San Antonio Southwest Research Institute

[email protected]

[email protected]



titan’s thermospheric response to various plasma environments

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