Magnetoseismology: Pi 2 Travel Time Peter Chi (IGPP/UCLA)

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Magnetoseismology Pi 2 Travel TimePeter Chi
(IGPP/UCLA)
Pi 2 arrival time Time of maximum amplitude in
the first wave cycle
?Maximum deviation in ?H- ?D plane
2008 GEM Summer Workshop, Miday, Utah
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Tamaos Model of Impulse Propagation via MHD Waves
Alfven

• Compressional Mode
• Propagates in all directions
• Suffers geometrical attenuation ( 1/r)
• Alfven Mode
• Propagates along magnetic field line
• Wave energy conserved

Compressional
Tamao travel path preserves most wave
energy Timao travel time travel time along
the Tamao travel path
Tamao 1964 Chi and Russell 2005
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Travel Time Modeling and Inversion
Fast mode Alfvén
Tamao travel path

• Inversion results for 2008-Jan-29 Substorm
• Pi 2 onset time 71324 UT (?10 sec)
• Pi 2 source location X ?17.9 RE
• nsh 1.52 a.m.u./cm?3 Tsh 3.15 keV

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Time History of Events
mmss
Both substorms show time history of events
consistent with the NENL model.
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Pi 2 Arrival Time and Peak Amplitude
THEMIS Event 2008-Jan-29
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Travel-Time Magnetoseismology
Tamao provides a model of MHD wave travel paths
Density Function
MHD Speeds
Temperature Function
(Assuming a dipolar B power-law density fcn)
Magnetic Field Tsyganenko Model
Inversion
d
e
a
b
c
(t0 start time)
Inversion provides solutions for start time,
source locations, and plasma density and
temperature parameters.
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MHD Speeds in the Nightside
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Observation II Pi 2
1998-jul-22
?Maximum deviation in ?H- ?D plane
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More Examples of Pi 2 Arrival Time
1998-jul-22
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Observations of Pi 2 Arrival TimeIndication of
Plasmapause Location
Reference Arrival time of preliminary impulses
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Inversion Results
Pi 2 Ls Nshcm-3 Tsh keV t0
Event 1 17.1 1.04 3.84 040340
Event 2 18.7 0.23 1.73 051320
Event 3 (substorm onset) 22.3 0.23 3.85 065340
Event 4 20.2 0.30 2.99 072320
Source distance
Start 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.

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Photometer Observations
Pi2-inferred start time
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Event Timeline
Start time of putative current disruption
Conventional Pi2 onset time (ground)
Pi 2 starts in the tail
Auroral breakup
t
mmss
0 (065330 UT)
30
100
130
200
230
?