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 2104 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 B 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) B (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 Onset will be seen across all latitudes on the ground within ~10 sec (without the need of Earth-ionosphere 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 tTamao shortest path to f ield line Density Function ds v f (r ) f ield-aligned m Lmp RE 1 nmp r n m Lpp RE 2 n pp r ds v A (r ) if L Lpp , if L Lpp (Lpp: plamapause L-value) Nonlinear optimization χ2 i t i ,observed t 0 t i ,Tamao i 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 E Z [R ] 3 0 Alfven Speed [km/s] 2 5 800 600 E -5 -40 Z [R ] 1 -35 -30 -25 -20 -15 X [RE] Ion Density (log10 cm-3) -10 -5 0 5 0 400 -5 -40 2.5 200 -35 -30 -25 -15 -10 -5 0 E Z [R ] 2 -20 X [RE] 0 1.5 1 0.5 -5 -40 Magnetosonic Speed [km/s] 5 -35 -30 -25 -20 X [RE] -15 -10 -5 800 0 600 E Z [R ] Temperature [eV] 5 0 400 E Z [R ] 3000 2000 0 -5 -40 200 -35 -30 -25 -20 X [RE] -15 -10 -5 0 1000 -5 -40 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 True Value -10 -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