Observation and Theory of Substorm Onset C. Z. (Frank) Cheng (1,2), T. F. Chang (2), Sorin Zaharia (3), N. N. Gorelenkov (4) (1)Plasma and Space Science Center, National Cheng Kung University, Taiwan (2) Department of Physics, National Cheng Kung University, Taiwan (3) Los Alamos National Laboratory, USA (4) Princeton Plasma Physics Laboratory, USA

Magnetospheric Physics Questions How do particles, momentum and energy transfer from solar wind across magnetopause into magnetosphere? How are particles heated and stored in the plasma sheet? How are substorms triggered and how is energy released during substorms? How are particles transported and accelerated during storms & substorms?

Solar wind particles entering the magnetosphere are stored in plasma sheet, and when plasma energy (or beta) exceeds a critical limit, substorms are initiated. Solar Wind: n ~ 10cm -3, T ~ 10eV, V ~ 200 – 2000 km/s Particles entering magnetosphere are stored mainly in plasma sheet, form current system

When the plasma energy in the plasma sheet exceeds a threshold, substorms are triggered to release energy: - ~ ergs released in s - ~ ergs dissipated in ionosphere causing awesome auroras  When, where and how do substorms occur and evolve? (Cheng, Space Science Rev., 2004)

, Auroral Arcs

, Substorm Auroral Spiral

– what are observational features of substorm onset? ? Substorm growth phase, onset & expansion in ionosphere and plasma sheet : When & where? How do they connect? AMPTE/CCE at ~ 8.8 R E Geotail at ~ 10 RE Region

Substorm Observation in Plasma Sheet Observation of substorm magnetic field turbulence, current disruption and dipolarization by AMPTE/CCE at X ~ 8.8 R E, 23:30 MLT [Cheng and Lui, GRL, 1998]. UT  /  r  0.2  r /  ci ~ 0.1 ULF Instability (Filtered low frequency fluctuation) Instability is excited at ~ 23:13:30 UT when  eq ~ 50 >>  C MHD ~ O(1) Substorm onset occurs at ~ 23:14:20 UT turbulence, cross-tail current reduction, dipolarization in expansion phase

Key Features of Magnetospheric Substorm Onset Growth Phase –B field thins and becomes tail-like as pressure and cross-tail current increase in near-Earth plasma sheet – ULF instability in Pi 2 frequency range (period ~ 60s, Kinetic Ballooning Instability,  ~0.1  0.2) is initiated in a radially localized region prior to onset Onset – ULF instability grows to large amplitude (  B/B ~ 0.5) at most unstable location.

Trigger Time : 2006/12/21 08:29: UT Exposure Duration : 1 s Exposure Interval : 1.4 s Filter : nm MCP HV : 700 V 2006/12/21 event FOV SAT. FORMOSAT-2/ISUAL observation of onset of substorm auroral breakup

Substorm breakup arc evolution ISUAL successive images with 1 sec exposure were taken every 1.4 second. Breakup arc brightening begins at 08:28:24 UT. Substorm expansion onsets at ~08:29:20 UT. MLAT (degree) GLAT (degree) Time (UT) 2006/12/21

Prior to expansion onset breakup arc appears at ~ at 08:28:24 UT with azimuthal mode number m ~ 200 and westward phase velocity (V p )~ 48 km/s. Arc structure prior to onset of 2006/12/21 substorm auroral breakup

2006/12/21 substorm onset arc is located at Herang discontinuity

Quiet-time and breakup arc structures Event Date Description 2007/01/15 Quiet time arc; m=700 ; Vp = 0 km/s 2004/08/31 Arc during storm recovery phase: m = 360; Vp = 9.3 km/s 2007/01/18 Substorm breakup arc: m=330; Vp = 38 km/s 2007/01/30 Substorm breakup arc: m=260; Vp = - 9 km/s 2006/12/21 Breakup-arc: m=220; Vp = 48 km/s

Growth Phase Magnetosphere JJ  B in (nA/m2) JJ Current sheet thickness ~ 1 R E.

Growth Phase Magnetospheric Equilibrium Field with Current Sheet Pink flux surface: L = 4.1 (  = 60.5 ± ); Blue flux surface: L = 5.2 (  = 64.1 ± )

Cross-Tail Current density in Equatorial plane Birkeland Currents Growth Magnetosphere

Ideal MHD Ballooning Instability Y (R E ) Most unstable ballooning instability is located at tailward side of the cross-tail current sheet ! f 2 (mHz 2 ) in equatorial planef 2 (mHz 2 ) in northern polar region Most unstable ballooning instability is at the transition region between R- 1 and R-2 field-aligned currents !

Kinetic Theory of Ballooning Modes Stabilization of MHD ballooning modes [Cheng and Lui, 1998, Cheng and Gorelenkov, 2004]: - finite ion gyroradius and trapped electron dynamics enhance  E || and produce  J || which enhances stabilizing field line tension and stabilizes MHD type ballooning modes. Destabilization of “resonant” type kinetic ballooning modes: -  di = 0 wave-particle drift resonance creates new “resonant” type KBM instability.

Resonant Kinetic Ballooning Modes Ideal MHD Ballooning ModeResonant KBM Assume n e (X = -6.6 R E, Y=0, Z=0) = 2 cm -3, T i = 11 keV, T e = 5.5 keV (thus n e  P), azimuthal mode number m = 500  A0 = 0.03 s -1 X =  R

Resonant Kinetic Ballooning Modes Assume n e (X = -6.6 R E, Y=0, Z=0) = 2 cm -3, T i = 11 keV, T e = 5.5 keV, azimuthal mode number m = 500  A0 = 0.03 s -1 - Most unstable mode at X ~ -8 R E is m = 250, f ~ 60 mHz,  /  ~ If n e is larger, frequency will be lower. (X =  R) X =  8 R E m

Summary How does plasma sheet thinning occur during growth phase?  Plasma sheet thinning & high  eq in near-Earth plasma sheet is due to plasma pressure buildup. What is substorm onset mechanism?  g lobal Kinetic Ballooning Instability is responsible for aurora breakup arc formation & structure observed in ionosphere & ULF instability (in Pi 2 frequency range) observed in plasma sheet. How does plasma sheet evolve during expansion phase?  understand dynamical auroral breakup process in ionosphere and current disruption & magnetic field dipolarization in plasma sheet due to turbulence & plasma transport (plasma pressure relaxation).