Use of Martian Magnetic Field Topology as 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 Lab Contact: 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 ° 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 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 % of the time  The field line topology near Mars is dynamic Closed Field Lines - Shadow

Plasma Voids Types of Closed Field Line - Shadow Two-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 or e - 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 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 ° in azimuth angle ( southwest direction ) Two year variation due to L S variation of Mars’ orientation with respect to Sun Intermittence of draping direction on ~13 day timescale Separate topology dataset into º draping direction (69%) and º (31%) º Draping º Draping 0°0° 90° 180° 270°

Intermittence in IMF Direction Intermittence in IMF direction has day period Solar rotation period (synodic and sidereal) is 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

Open Field Lines Dayside º Draping º 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

Closed Field Lines Dayside º Draping º Draping Data noisier for ° 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 )

Open Field Lines Shadow º Draping º 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

º Draping º Draping Closed Field 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

º Draping º 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 B 0 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 LowHigh

Open Field 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

Closed Field 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

Open Field 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

Closed Field 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 spectrum plasma void 2-sided loss cones open 1-sided loss cones 1-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 Angle Day SideNight Side Open Closed 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 L S 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

Variation with Solar Wind Pressure

Open Field Lines - Dayside

Closed Field Lines - Dayside

Open Field Lines - Shadow

Closed Field Lines - Shadow