Travel-time
Magnetoseismology
Peter J. Chi and C. T. Russell
UCLA/IGPP
Acknowledgments: MHD simulation results are provided by Dong-Hun Lee.
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
1
The Scientific Question
„
„
Earth
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
How do the
impulse-induced
perturbations
propagate in the
magnetosphere?
How can such
knowledge help
us estimate the
state of the
magnetosphere?
2
Three Seismologies, Two Methods
Helioseismology
Magnetoseismology
Normal-mode
Travel-time
Seismology
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
3
Scale Lengths Associated with
Magnetoseismology
Scale Comparison
Dayside impulses
Nightside impulses
Earth & Seismic
Waves
Magnetosphere
& MHD Waves
Radius
6400 km
~64,000 km
(sunward)
Wave
speed
1-14
km/s
300-10,000
km/s
Wavelength
~100 km
104-105 km
Scale
length of
∇V
Typically
≥ 103 km
Typically
≤ 2×104 km
Can’t use ray tracing in
Peter Chi: Travel-time magnetoseismology !!!
2007-1-26 Polar Telecon
magnetoseismology
4
Tamao’s Model of Impulse
Propagation via MHD Waves
Alfven
Tsutomu
Tamao
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
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
Tamao [1964a]
5
Impulse Travel Time Predicted by
Tamao’s Model
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
6
Simulation of Sudden Impulse Propagation via MHD
Waves
δBν
Alfvén speed
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
(Simulation made by
Dong-Hun Lee)
7
Currents Associated with the Compression
MHD waves
Kikuchi and Araki
[1979]; Kikuchi et al.
[1996]
?
Field-aligned
current
Tamao [1964b]
2007-1-26 Polar Telecon
Kikuchi and
Araki [1979]
Ionospheric
Hall current
Peter Chi: Travel-time
magnetoseismology
8
Propagation of Impulse Front
(Onset)
δB
(Noon meridian)
MHD simulation
t
onset arrival
• It is argued that the
Earth-ionosphere
waveguide model is
evidenced by the
simultaneous onset time
seen on the ground across
all latitudes.
2007-1-26 Polar Telecon
Earth
Subsolar point
ÎOnset will be seen across all latitudes on the
ground within ~10 sec (without the need of Earthionosphere wave guide)!
Peter Chi: Travel-time
magnetoseismology
9
Propagation of Impulse
Conventional (static) View
Dynamics View
+
Tamao [1964b]
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
10
MHD Simulation on Ionospheric
Currents due to Impulse
Plasmapause
Latitude
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
11
Tremors (quakes) in space
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
12
Density Model and Inversion
Tamao Model of Wave Propagation
∫
t Tamao ≡
shortest path
to field line
Density
Function
ds
+
v f (r )
∫
field - aligned
m
⎧
⎛ L mp R E ⎞ 1
⎪ n mp ⎜
⎟⎟
⎜
r
⎪
⎝
⎠
n=⎨
m
⎛ L pp R E ⎞ 2
⎪
⎟⎟
⎪ n pp ⎜⎜ r
⎠
⎝
⎩
ds
v A (r )
if L > L pp ,
if L ≤ L pp
(Lpp: plamapause L-value)
Nonlinear
optimization
χ2 =
∑
i
⎛ t i ,observed − t 0 − t i ,Tamao
⎜⎜
σi
⎝
⎞
⎟⎟
⎠
2
e
d
c
b
a
(t0: start time)
Parameters to solve: (t0,) nmp, m1, Lpp, npp, m2
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
13
Density Profile Estimated through
Magnetoseismic Inversion
In situ observations by Akebono also suggest that the
plasmapause Lpp ~ 3.5 prior to the SI event.
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
14
Polar’s Observation of Impulse Time
Polar
48.4 sec
• Estimated Start time: 2344:20 UT
• Calculated travel time from impact location
to Polar: 48.4 sec
• Observed arrival time at Polar: 2345:08 UT
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
15
Nightside Magnetoseismology
5
Tamao’s travel path:
1. From low L to high L
2. From high L to low L
0
-5
-40
-35
-30
-25
-20
-15
-10
-5
0
Magnetic Field (log10 nT)
5
Z [RE]
3
0
2
Alfven Speed [km/s]
5
800
-5
-40
-35
-30
-25
-20
-15
X [RE]
Ion Density (log10 cm-3)
-10
-5
Z [RE]
1
0
5
600
0
400
-5
-40
2.5
200
-35
-30
-25
Z [RE]
2
0
-20
X [RE]
-15
-10
-5
0
1.5
1
0.5
-5
-40
-35
-30
-25
-20
X [RE]
-15
-10
-5
Magnetosonic Speed [km/s]
5
0
Z [RE]
Temperature [eV]
5
800
600
0
400
Z [RE]
3000
2000
0
-5
-40
200
-35
1000
-5
-40
-30
-25
-20
X [RE]
-15
-10
-5
0
0
-35
-30
-25
2007-1-26 Polar Telecon
-20
X [RE]
-15
-10
-5
0
Peter Chi: Travel-time
magnetoseismology
16
Test of Inversion: Using Artificial Data
Artificial Data:
5
• 10 imaginary observations
equally spaced between 6
and 24 in L-value (all
outside the plasmasphere)
0
-5
-40
-35
-30
• Timing uncertainty: 5 sec
• Arrival time consistent
with Tamao travel time
-25
-20
-15
-10
True Value
-5
0
Estimation
t0 [sec]
0.0
0.2 ± 10.0
LS
15.0
15 ± 0.1
• Plasma:
nsh [a.m.u./cm3]
0.60
0.60 ± 0.09
n = nsh/(1 – exp–L/12)
Tsh [eV]
3000
2980 ± 800
Models:
• Magnetic Field: T89
T = Tsh/(1 – 0.5 exp–L/12)
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
17
Polar-THEMIS Joint Observations for
Magnetoseismic Studies
• THEMIS will deploy five satellites near the equatorial plane.
• Magnetoseismic inversion can take advantage of all satellite observations, but it
favors the observations distributed across different regions.
• Polar can provide joint observations at high latitudes, giving critical timing
records for Alfven wave propagation.
Polar
2007-1-26 Polar Telecon
Peter Chi: Travel-time
magnetoseismology
(Modified from the diagram
on the THEMIS web site)
18