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Crustal Fields in the Solar Wind:
Implications forAtmospheric Escape
Dave Brain
LASP
University of Colorado
July 24, 2003
Atmospheric Escape to Space
• Evidenceisotope measurementsspacecraft (Phobos, MGS)ionosphere models
• Why do we care?Climate historyAtmospheric chemistry
• Loss estimates.15 - 1 bar CO2 (over history) 1025 - 1026 particles / s ( today )
Atmospheric Escape to Space
• Main issues
#1 Loss over Martian historyAmountTiming
#2 Today’s lossAmountSpecies
Processes
To improve estimates of #1, must improve knowledge in many areas, including #2
MGS Measurements
• Ancient dynamo( early protection for atmosphere )
• Strong crustal sources( affect loss after dynamo turn-off )
this was an animation
Loss Today
Relevant Loss Processes
• Photochemical loss
• Ion pickup • Sputtering
• Bulk removal
neutralion
(Contemporary)
Relevant Loss Processes
• Photochemical loss
• Ion pickup • Sputtering
• Bulk removal
Crustal sources affect these processes through:atmospheric shieldingfield topologyopen field lines
neutralion
(Contemporary)
Atmospheric Shielding
Atmospheric Shielding
Implications of shielding:
1. Reduced ionization charge exchange electron impact
2. Photo-ion motion changed
Atmospheric Shielding
LS = 0 SW Pressure = 5e-9 dynes cm-2
Theoretical Martianpressure balance obstacle
to the solar wind
PSW = Pcrust + Pionosphere
this was an animation
Atmospheric Shielding
With crustal sources
Without crustal sources
The volume of protected atmosphere is larger (by a factor of 2-8) when crustal sources are considered.
How many (more) neutrals are protected?
CO2: < 10% O: 10-60 %
Field Topology
Field Topology
Implication of altered topology:
Modification of charged particle motion
Escape could be enhanced or diminished depending upon orientation of crustal fields relative to solar wind flow
Field Topology
With crustal sources Without crustal sources
Crustal sources severely alter the field topology close to Mars
MHD simulations(courtesy Y. Ma and A. Nagy)
Field Topology
these were animations
Open Field Lines
Open Field Lines
Implications of open field lines:
1. Access to lower atmosphere for SW charged
particles
2. “Escape hatches” for planetary ions
Open Field Lines
Open field lines exist near crustal “cusps” of near-vertical field
Dayside Data
Open Field Lines
Estimates of quantity of open field lines
• Simple models~7% by area at exobase (~200 km) in region of strong crustal
sources
• MAG Data~ 1-2% by area on Martian dayside (2pm local time) at 400 km
Loss over Martian History
Loss over Martian History
Magnetic History
• Mars forms
• Dynamo on
• Surface strongly magnetized
• Dynamo off
• Large impacts - magnetization erased
• Northern resurfacing - magnetization erased
• Relaxation of crustal magnetization to present
Time
Loss over Martian History
Magnetic History
• Mars forms
• Dynamo on
• Surface strongly magnetized
• Dynamo off
• Large impacts - magnetization erased
• Northern resurfacing - magnetization erased
• Relaxation of crustal magnetization to present
Impact
~3.5 Gya
Shielding from global
field
Shielding by crustal sources
Outgassing
Needed Measurements
Concurrent particle and field measurements!
Low-altitude (and surface) measurements of crustal magnetization
Atmospheric/ionospheric density and temperature at high altitude
Measurements of carrier and grain size of magnetization
Solar wind measurements at Mars
Time history of ionization processes at Mars
Coverage in local time, SZA, altitude, and geographic location
Summary
• Atmospheric escape to space has important implications for Mars’ past climate and current atmospheric chemistry
• Crustal magnetic sources might effect present loss rates:• shielding of the atmosphere• alteration of particle trajectories through field topology• particle exchange along open field lines
• Crustal effects have persisted since Mars’ dynamo ceased