1. Magnetoseismology: Pi 2 Travel Time Peter Chi (IGPP/UCLA) 2008 GEM Summer Workshop, Miday, Utah 1998-Jul-22 ▲Maximum deviation in  H-  D plane Pi 2 arrival time: Time of maximum amplitude in the first wave cycle

2. Tamao’s Model of Impulse Propagation via MHD Waves Tamao [1964] Chi and Russell [2005] Compressional Mode: Propagates in all directions Suffers geometrical attenuation (~ 1/r) Alfven Mode: Propagates along magnetic field line Wave energy conserved Compressional Alfven “Tamao travel path” : preserves most wave energy “Timao travel time” : travel time along the Tamao travel path

3. Travel Time Modeling and Inversion Fast mode Alfvén Tamao travel path Travel Time Source at X = -25 Re Source at X = -15 Re Source at X = -20 Re Source at X = -10 Re Inversion results for 2008-Jan-29 Substorm: Pi 2 onset time: 7:13:24 UT (  10 sec) Pi 2 source location: X =  17.9 R E n sh = 1.52 a.m.u./cm  3; T sh = 3.15 keV

4. Time History of Events mm:ss Both substorms show time history of events consistent with the NENL model. t Pi 2 starts in the tail Auroral breakup Conventional Pi2 onset time (ground) 0 (=07:13:00 UT) :301:001:302:002:30 mm:ss 1. THEMIS Substorm 2008-Jan-29 t Pi 2 starts in the tail Auroral breakup Conventional Pi2 onset time (ground) 0 (=06:53:30 UT) :301:001:302:002:30 2. Substorm 1998-Jul-22

5. Pi 2 Arrival Time and Peak Amplitude THEMIS Event 2008-Jan-29

6. Travel-Time Magnetoseismology Inversion provides solutions for start time, source locations, and plasma density and temperature parameters. Density Function Inversion (t 0 : start time) Tamao provides a model of MHD wave travel paths: a b c d e MHD Speeds Magnetic Field: Tsyganenko Model (Assuming a dipolar B & power-law density fcn) Temperature Function

7. MHD Speeds in the Nightside

8. Observation II: Pi 2 ▲Maximum deviation in  H-  D plane 1998-jul-22

9. More Examples of Pi 2 Arrival Time 1998-jul-22

10. Observations of Pi 2 Arrival Time: Indication of Plasmapause Location Reference: Arrival time of preliminary impulses Alfvén speed PP

11. Inversion Results Pi 2LsLs N sh [cm -3 ]T sh [keV] t0t0 Event 117.11.043.8404:03:40 Event 218.70.231.7305:13:20 Event 3 (substorm onset) 22.30.233.8506:53:40 Event 420.20.302.9907:23:20 Source distanceStart time All observations are made by ground magnetometers Only stations with L  15 are used in inversion; One station at L = 29 is not used.

12. Photometer Observations AU AL(nT) Pi2-inferred start time

13. Event Timeline t Current DisruptionNear-Earth-Neutral-Line Substorm Models (Figures taken from THEMIS web site)  Pi 2 starts in the tail Auroral breakup Conventional Pi2 onset time (ground) Start time of putative current disruption 0 (=06:53:30 UT) :301:001:302:002:30 mm:ss