1. Observations of Open and Closed Magnetic Field Lines at Mars: Implications for the Upper Atmosphere D.A. Brain, D.L. Mitchell, R. Lillis, R. Lin UC Berkeley Space Sciences Lab Contact: brain@ssl.berkeley.edu DPS Meeting - November 8-12, 2004

2. 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

3. 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 (also “M” and “W” categories) –Identify as unconnected, open, or closed (with respect to exobase, not necessarily surface) Approach

4. 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

5. Open Field Lines - Dayside Open field lines observed away from crustal field regions 30-70% 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

6. Closed Field Lines - Dayside Closed field lines mostly found near crustal sources Some parts of atmosphere shielded from solar wind 10-25% of the time (at 400km) Additional regions near strongest crustal sources do not have loss cones, but are likely closed (mirroring population?). Future work will investigate.

7. Open Field Lines - Shadow Open field lines away from crustal fields ~30-70% of the time Many open field regions near crustal sources Some regions near strong crustal sources never/rarely open Solar wind often has access to much of the nightside upper atmosphere

8. 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 nightside at 400 km Other regions closed 30-80 % of the time  The field line topolgy near Mars is dynamic Closed Field Lines - Shadow

9. 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 2pm nightside orbit track at 400 km  loss timescale > ~ 7 hours Two-sided loss cones contain sufficient e - to identify loss distribution Observed at same local time as plasma voids  These field lines are recently closed or e - have been added to closed field lines

10. Determining IMF Draping Direction Draping direction successfully determined for 22,214 orbits Use dayside MAG data on dayside 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 L S variation of Mars’ orientation with respect to Sun Unexplained intermittence of draping direction (12-15 day timescale) Separate topology dataset into 150-330º draping direction (69%) and 330-150º (31%)

11. Open Field 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 300 E, 0N) MANY more open field lines observed over regions lacking crustal fields for one draping direction  convection electric field / mass loading effect? IMF clock angle dramatically influences field topology

12. Closed Field Lines Dayside 150-330º Draping 330-150º Draping Data noisier for 330-150° draping direction, probably due to combinations of low signal, fewer orbits and larger spread in draping directions Closed field lines occur near crustal sources in both cases

13. Open Field Lines Shadow 150-330º Draping 330-150º Draping More solar wind access to nightside atmosphere for 150-330° draping direction Some crustal sources more likely to be open for one draping direction (“blinking”) Some open field regions are larger for one draping direction (“breathing”) Effects appear to be more pronounced near weaker crustal fields

14. 150-330º Draping 330-150º 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

15. Types of Closed Field Line - Shadow Blinking and breathing near weak crustal fields Breathing near strong crustal fields Plasma VoidsTwo-sided Loss Cones 150-330º Draping 330-150º Draping IMF direction affects which side of a weak crustal source is “recently closed”

16. Summary Magnetic field topology near Mars is dynamic IMF orientation relative to Mars contributes to variability in topology IMF orientation affects topology over weak sources more than strong sources The IMF draping direction is not uniformly distributed in time Dayside at 400 km: –Solar wind magnetically connected to atmosphere 50% of the time in some areas –Solar wind shielded from atmosphere at least 15-20% of the time in some areas of strong crustal field –Possible convection electric field effect over regions lacking crustal fields Nightside at 400 km: –Solar wind often magnetically connected to upper atmosphere –Solar wind usually shielded from atmosphere over significant fraction of planet –Either topology is dynamic on the night side or new electrons have access to closed field lines

17. Future Work Explore topology and variability in terminator region Explore variability in topology with solar wind pressure and with L S Explore location and variability of source cone distributions Analyze test cases for specific regions (over strong and weak crustal fields) and for specific periods (during periods of high activity) This research was funded by MDAP Grant NNG04GL35G-05/06

18. General Topology IMF Direction and Topology