Use ofMartian Magnetic Field

Topologyas an Indicator of

the Influence of Crustal Sources on Atmospheric Loss

D.A. Brain, D.L. Mitchell, R. Lillis, R. Lin

UC Berkeley Space Sciences LabContact: brain@ssl.berkeley.edu

AGU Fall Meeting - Monday, 13 December 2004

SA13A-1119

• Localized crustal magnetic fields form “mini-magnetospheres” at Mars.

• Magnetic field lines can have 3 topologies: open, closed, and unconnected.

• Closed field regions shield the atmosphere from the solar wind

• Open field lines connect the solar wind to the upper atmosphere and ionosphere.

We use Mars Global Surveyor observations to address two important questions:

• What is the topology of magnetic field lines at Mars?

• What parameters control the variability in topology?

Motivation

See poster #P11A-0945 by Lillis et al., today!

• Use MGS MAG/ER electron data

• Identify topology using pitch angle distributions

(pitch angle = angle between e- velocity and magnetic field)

– Treat 0-90° and 90-180° pitch angle ranges independently

– Classify each distribution as flat, loss cone, plasma void, or source cone (additional “butterfly” category not discussed here)

– Identify as unconnected, open, or closed(with respect to atmosphere, not necessarily surface)

Approach

Data Set

• Mapping orbit data( ~ 400 km altitude, 2am/2pm orbit )

• 01 July 1999 - 14 September 2004( ~ 43 million distributions over 5+ years)

• 115 eV energy channel

• 2, 4, or 8 s time resolution

• Grid by longitude / latitude (1° 1°) and by dayside / terminator / shadow( ~250 distributions per bin )

• Calculate percentage of observations in each bin having a given topology

General Topology

Open Field Lines - Dayside

• Field lines with fewer electrons coming from planet defined as “open”• Open field lines observed away from crustal field regions 50-90% of the time• Open near crustal sources 15-50% of the time less access for solar wind• Some regions near strong crustal sources never/rarely open• Solar wind has much more access to atmosphere in northern hemisphere

Closed Field Lines - Dayside• On closed field lines, field-aligned mirroring electrons should be absorbed by the atmosphere, forming two-sided loss distributions

BUT

• Ionospheric photoelectrons are observable in electron energy spectra only when solar wind electrons are not on the field line (i.e. on closed field lines)

SO

• There must be a source of electrons at 115 eV that acts on timescales shorter than the timescale for formation of two-sided loss distributions

• We proceed using detection of ionospheric photoelectrons as an indication of closed field lines on the dayside

Open Field Lines - Shadow

• Open field lines away from crustal fields most of the time• Southern regions lacking crustal fields open less often than in north• Many open field regions (“cusps”) near crustal sources• Some regions near strong crustal sources never/rarely open• Solar wind often has access to much of the night side upper atmosphere

• Closed field lines found mostly near crustal sources• Open field lines near crustal sources surround closed field regions• Some regions above crustal sources always closed on night side at 400 km• Other regions closed 30-80 % of the time The field line topology near Mars is dynamic

Closed Field Lines - Shadow

Plasma Voids

Types of Closed Field Line - ShadowTwo-sided loss cones

• Plasma voids contain no significant e-

Have been closed longer than timescale for e- loss

• Two-sided loss cones contain sufficient e- to identify loss distribution

• Observed at same local time (2 am) as plasma voids

These field lines are recently closed ore- have been added to closed field lines

Variation with IMF Draping Direction

Determining IMF Draping Direction

• Draping direction successfully determined for 22,214 orbits • Use dayside MAG data from 50-60 North latitude

• Use local horizontal component of magnetic field

• Take mean azimuth angle ( defined with 0° eastward, 90° northward )

as proxy for clock angle of upstream IMF

• Directions clustered from 210-270° in azimuth angle ( southwest direction )

• Two year variation due to LS variation of Mars’ orientation with respect to Sun

• Intermittence of draping direction on ~13 day timescale

• Separate topology dataset into 150-330º draping direction (69%) and 330-150º (31%)

330-150º Draping

150-330º Draping

0°

90°

180°

270°

Intermittence in IMF Direction• Intermittence in IMF direction has 25-27 day period

• Solar rotation period (synodic and sidereal) is 25-27 days

• IMF draping direction clusters near 250° for ~13 days at a time, regardless of orientation of crustal sources with respect to Sun

• IMF direction less clustered rest of the time

OpenField Lines

Dayside

150-330º Draping

330-150º Draping• Some crustal sources more likely to

be open for certain IMF directions (e.g. 230 E, 75 S and 150 E, 50 S)

• MANY more open field lines observed over regions lacking crustal fields for one draping direction possible mass-loading effect?

• IMF clock angle dramatically influences field topology

ClosedField Lines

Dayside

150-330º Draping

330-150º Draping• Data noisier for 330-150° draping

direction, likely due to combination of fewer orbits and larger spread in draping directions

• Real differences in the locations of “closed” field regions(e.g. region near 70E, 30N “moves”)

( ask to see animations )

OpenField Lines

Shadow

150-330º Draping

330-150º Draping• Some crustal sources (210 E, 45 S)

more likely to be open for one draping direction (“blinking”)

• Some cusp regions (350 E, 12 N) larger for one draping direction (“breathing”)

• Effects appear to be more pronounced near weaker crustal fields

150-330º Draping

330-150º Draping

ClosedField Lines

Shadow

• Strong crustal sources shield atmosphere from solar wind regardless of IMF direction

• Closed field line regions near weak crustal sources “breathe” and “blink” as IMF direction changes

150-330º Draping

330-150º Draping

Two-sided Loss Cones

Shadow

• “blinking” over some weak sources (e.g. 315 E, 15 N)

• IMF direction affects which side of a closed field region “closed recently”

OR

IMF affects where electrons are able to diffuse into closed field regions

• NEAT!

Variation with Solar Wind Pressure

Determining Upstream Pressure

• Upstream dynamic pressure successfully approximated for 22,486 orbits • Apply technique similar to that of Cider et al., JGR, 2003• Use dayside MAG data with SZA < 110 ° (exclude regions with strong crustal fields)

• Fit orbital field magnitude profile to B0 cos(SZA) function

• Assume upstream pressure converted to magnetic pressure in sheath

• Take estimated |B| at SZA=0 ° as proxy for upstream pressure

• Separate topology dataset into high pressure (44%) and low pressure (56%) orbits

Pressure Proxy Extraction

Low High

OpenField Lines

Dayside

Low Pressure

High Pressure• More open field for high pressure in

northern hemisphere

• Cusp-regions are larger and more frequently observed during high pressure periods

Compressed solar wind interaction region results in increased solar wind access to atmosphere

ClosedField Lines

Dayside

Low Pressure

High Pressure• Fewer closed field regions in northern

hemisphere for high pressure ionosphere less “puffy”

• Fewer closed field lines above crustal sources during high pressure mini-magnetospheres compressed

• Atmosphere is less protected during periods of high pressure

OpenField Lines

Shadow

• Same trends observed as for day side

• High pressure periods result in more solar wind access to the atmosphere, and larger cusps

• Effects more pronounced over weak crustal sources relative to strong crustal sources

High Pressure

Low Pressure

ClosedField Lines

Shadow

• High pressure periods result in smaller closed field regions

• Effects more pronounced over weak crustal sources

• Night side atmosphere less protected during periods of high pressure

High Pressure

Low Pressure

Variation with SZA, Magnetic elevation angle

Variation with SZA

ionospheric spectrumplasma void2-sided loss cones

open1-sided loss cones1-sided source cones

• Solar wind has more access to atmosphere in shadow

• 1-sided source cones especially likely in sunlight, past the terminator

• More 2-sided loss cones and plasma void distributions observed on night side than on dayside

• Ionospheric photoelectron spectra more common at high SZA (PEB flaring)

Variation with Elevation AngleDay Side Night Side

Ope

nC

lose

d

• Elevation angle = angle B makes with local horizontal

• Open field lines (shadow): More when field is radial More during high pressure

• Open field lines (day): More loss cones for radial field More during high pressure Unusual feature for small negative elevation angles

• Closed field lines (shadow): Fewer during high pressure More voids for horizontal field More loss cones for radial field

• Closed field lines (day): More loss cones for radial field More loss cones during high pressure More iono. spectra for radial field Fewer iono. spec. during high pressure Dashed lines = High Pressure Solid lines = Low Pressure

Summary

• Electron measurements provide information about solar wind access to the Martian atmosphere

• Magnetic field topology near Mars is dynamic

• The IMF draping direction affects solar wind access to the atmosphere, moving and expanding open and closed field regions observed at 400 km

• Solar wind pressure affects magnetic field topology, increasing the amount of solar wind access to the atmosphere during periods of high pressure

• Open field regions are more common on the Martian night side, and when the local magnetic field is vertical. Night side atmospheric shielding is more common when the local magnetic field is horizontal

Future Work• Explore topology and variability in terminator region

• Explore variability in topology with LS

• Analyze test cases for specific regions (over strong and weak crustal fields) and for specific periods (during periods of high activity)

• Apply these statistical results to caluclations of the amount of atmospheric shielding caused by crustal fields, and the amount of solar wind access to the atmosphere

This research was funded by MDAP Grant NNG04GL35G-05/06

Variation with IMF Draping Direction

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Variation with Solar Wind Pressure

Open Field Lines - Dayside

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Closed Field Lines - Dayside

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Open Field Lines - Shadow

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Closed Field Lines - Shadow

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Closed Field Lines - Shadow

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Closed Field Lines - Shadow

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