Observation and Theory of Substorms 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 Second LSAP Workshop, POSTECH, Korea, June 21-22, 2008
Substorm is a global magnetosphere-ionosphere phenomena of energy storage and release process! Solar wind particles enter magnetosphere and are stored in the plasma sheet. Above certain energy threshold, plasma energy is released to cause rapid large scale change of magnetotail configuration and ionosphere aurora.
, Auroral Arcs
, Substorm Auroral Spiral
Substorm Dynamics • Growth Phase (~30 minutes) – Storage of plasma and magnetic energy in plasma sheet • Expansion Phase (~ 30 minutes) – Substorm onset: sudden release of plasma and magnetic energy – Current disruption: reduction of cross-tail current • Recovery Phase (~hours) – Magnetosphere returns to quiet time condition
When plasma energy in the plasma sheet exceeds a threshold, substorms are triggered to release energy: - ~ 10 20 - 10 22 ergs energy is released in 10 2 - 10 3 s - ~ 10 19 - 10 21 ergs energy is dissipated in ionosphere causing awesome auroras due to particle precipitation into ionosphere Auroral substorm is manifestation of magnetospheric substorm ! � When, where and how do substorms occur and evolve? (C. Z. Cheng , Space Science Rev., 2004)
Substorm growth phase, onset & expansion in ionosphere and plasma sheet : When & where? How do they connect? Region Region ? AMPTE/CCE at ~ 8.8 R E Geotail at ~ 10 RE – what are observational features of substorms?
Key Features of Substorms Ionosphere: Magnetosphere: • Growth Phase • Growth Phase – B field thins and becomes tail-like as – Proton and electron aurora region shrinks in width and moves equatorward pressure and cross-tail current increase in near-Earth plasma sheet – “Breakup” arc (with azimuthal mode number of ~ 200-300) appears in proton – ULF instability in Pi 2 frequency precipitation region (poleward side of range (period ~ 60s, Kinetic Ballooning proton aurora) a few minutes prior to Instability) is initiated in a radially onset localized region prior to onset • Onset – “breakup” arc intensifies • Onset – ULF instability grows to large amplitude ( δ B/B ~ 0.5) and brightenting occurs at a local spot initially. at most unstable location. • Expansion Phase – poleward • Expansion Phase – spread of (mainly) and equatorward turbulence region causes pressure profile relaxation � expansion of breakup arc emission and diffuse proton and current disruption, and B electron aurora. dipolarization.
Substorm Observation in Plasma Sheet Observation of substorm magnetic field turbulence, current disruption and dipolarization by AMPTE/CCE located at X ~ 8.8 R E , 23:30 MLT [Cheng and Lui, GRL, 1998]. ULF Instability (Filtered low frequency fluctuation) γ / ω r ∼ 0.2 ω r / ω ci ~ 0.1 • Instability is excited at ~ 23:13:30 UT when β eq ~ 50 >> β C MHD ~ O(1) •Substorm onset occurs at ~ 23:14:20 UT • turbulence, cross-tail current reduction, dipolarization in expansion phase UT
1985 June 1 Event Wavelet Analysis Pi2 instability UT excited
1985 June 1 Magnetospheric Substorm Event • AMPTE/CCE observation at X ~ −8.8 R E , 23:30 MLT • Pressure increases in growth phase • Prior to onset β eq ~ 60 • Pressure decreases and β eq decreases (mainly due to dipolarization) in expansion phase Substorm onsets at t = 0 Lui et al., JGR, 1993.
Low Frequency Instability in 86240 Event
Key Features of Magnetospheric Substorms • 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, γ/ω ~0.1 − 0.2) is initiated in a radially localized region prior to onset • Onset – ULF instability grows to large amplitude ( δ B/B ~ 0.5) at most unstable location. • Expansion Phase – spread of turbulence region causes pressure profile relaxation � crosstail current reduction, and B dipolarization.
Auroral Substorm Observations • Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) • THEMIS All Sky Imagers • FORMOSAT-2/ISUAL
Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) all sky camera
CANOPUS Observation of Substorm (2/19/96) (Voronkov et al, 2003) Protons Hard electrons Soft electrons – Proton and electron aurora region moves equatorward during growth phase – “Breakup” arc (557.7 nm) appears in proton precipitation region (poleward side of proton aurora) a few minutes prior to onset
FORMOSAT- -2/ISUAL observation 2/ISUAL observation FORMOSAT of substorm substorm auroral auroral breakup arc breakup arc of 2006/12/21 event 2006/12/21 event Trigger Time : 2006/12/21 08:29:20.410 UT Exposure Duration : 1 s SAT. Exposure Interval : 1.4 s Filter : 630.0 nm FOV MCP HV : 700 V
Substorm breakup arc evolution breakup arc evolution Substorm 2006/12/21 MLAT (degree) GLAT (degree) Time (UT) 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.
Arc structure prior to onset of 2006/12/21 substorm auroral breakup 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.
2006/12/21 substorm onset arc is located at Herang discontinuity
Quiet-time and breakup arc structures Event Date Description Quiet time arc; 2007/01/15 m=700 ; Vp = 0 km/s Arc during storm recovery 2004/08/31 phase: m = 360; Vp = 9.3 km/s Substorm breakup arc: 2007/01/18 m=330; Vp = 38 km/s Substorm breakup arc: 2007/01/30 m=260; Vp = - 9 km/s Breakup-arc: 2006/12/21 m=220; Vp = 48 km/s
Key Features of Auroral Substorms • Growth Phase – Proton and electron aurora region shrinks in width and moves equatorward – “Breakup” arc with azimuthal mode number of ~ 200-300 (separation distance between bright spots ~ 100-200 km) appears in proton precipitation region (poleward side of proton aurora) a few minutes prior to onset • Onset – “breakup” arc intensifies and brightenting occurs at a local spot initially and spreads along arc. • Expansion Phase – poleward (mainly) and equatorward expansion of breakup arc emission and diffuse proton and electron aurora.
Key Features of Substorms Ionosphere: Magnetosphere: • Growth Phase • Growth Phase – Proton and electron aurora region – B field thins and becomes tail-like as shrinks in width and moves equatorward pressure and cross-tail current increase in near-Earth plasma sheet – “Breakup” arc (with azimuthal mode number of ~ 200-300) appears in proton – ULF instability in Pi 2 frequency precipitation region (poleward side of range (period ~ 60s, Kinetic Ballooning proton aurora) a few minutes prior to Instability) is initiated in a radially onset localized region prior to onset • Onset – “breakup” arc intensifies • Onset – ULF instability grows to large amplitude ( δ B/B ~ 0.5) and brightenting occurs at a local spot initially. at most unstable location. • Expansion Phase – poleward • Expansion Phase – spread of (mainly) and equatorward turbulence region causes expansion of breakup arc pressure profile relaxation � emission and diffuse proton and current reduction, and B electron aurora. dipolarization.
Critical Physics Issues of Substorm • How does energy build up in the plasma sheet during growth phase? - How does plasma sheet thinning occur? by plasma pressure increase in plasma sheet • What is substorm onset mechanism? - Is it kinetic ballooning instability (KBI)? Yes, breakup arc - How are particles accelerated to produce breakup arc? by E || of KBI • How does plasma sheet evolve during expansion phase? - How does an instability initially localized in near-Earth plasma sheet lead to a global multi-scale turbulence? Not known yet - How does the turbulence cause dipolarization & current reduction? Plasma transport • What is the role of magnetic reconnection? Probably enhances plasma convection in growth phase, but no direct relationship with substorm onset!
Quiet-Time & Growth Phase Magnetospheres: Profiles Along Sun-Earth Axis Assume quasi - static equilibriu m : r r × = ∇ J B P r = ∇ ψ × ∇ α B β = 2 2 / P B Pressure increase • Dashed lines are for typical quiet time values. � in plasma sheet in • Magnetic well at ~ 7.5 R E for growth phase. growth phase
3D Magnetic Field in Quiet Time & Growth Phase Growth Phase Quiet Time
β J φ B in (nA/m2) Growth Phase Magnetosphere J φ . Current sheet thickness ~ 1 R E
P and B in Equatorial Plane Quiet Time Field Disturbed Time Field A local magnetic well at X ' – 8 R E for disturbed time case
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