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The 3rd ERG Mission Science Workshop
September 5-8, 2017
ASIAA, Taipei / NCKU, Tainan

Oral Presentation

Study of Substorm Physics in ERG Mission

C. Z. Cheng (Institute of Space and Plasma Sciences, National Cheng Kung University, Taiwan); T. F. Chang (Institute for Space-Earth Environmental Research, Nagoya University, Japan)

Substorm occurs frequently and is a significant energy storage and release process in the magnetosphere-ionosphere. During the substorm growth phase, the plasma pressure increases in the central plasma sheet and the magnetic field stretches outward to become tail-like over a few tens of minutes. Both the plasma energy and the magnetic energy per unit of flux volume increase dramatically in the night side with the plasma beta reaching ~40-60 in the central plasma sheet at ~ 8-10 Earth radii [Zaharia and Cheng, 2003; Cheng, 2004]. For such plasma-magnetic field configurations, high azimuthal mode number kinetic ballooning instabilities (KBIs) [Cheng and Lui, 1998] can be excited to produce Pi2 waves (with ~1 min period) and their parallel electric field accelerates electrons into the ionosphere to produce auroral arcs with bead-like structure along the arc [Chang et al., 2012; Chang and Cheng, 2015]. As KBIs grow to large amplitude, higher frequency (in Pi1 range) instabilities are also excited and together they form turbulence and cause plasma transport in the plasma sheet for a few tens of minutes, which is the substorm expansion phase. During the expansion phase the auroral arcs break up and form vortex structure and expand poleward, earthward and in the east-west directions. In the magnetosphere, the plasma pressure profile relaxes in the near-Earth plasma sheet, and the magnetic field increases and becomes more dipole-like (called the dipolarization), and the cross-tail current is greatly reduced (called current disruption), so do the field-aligned currents. The dynamical dipolarization region expand both earthward and tailward. Associated with the magnetic field dipolarization is the dispersionless energetic particle injection into the inner magnetosphere [Zaharia et al., 2000], which has been observed at the geosynchronous orbit and in the outer radiation belt. Observations and theories will be presented. Suggestion on how ERG satellite measurements can contribute to substorm physics will be discussed.

Chang, T. F. et al., Behavior of Substorm Auroral Arcs and Pi2 Waves: Implication for the Kinetic Ballooning Instability, Ann. Geophysicae, 30, 911 (2012)
Chang, T. F. , C. Z. Cheng, Relationship between wave-like auroral arcs and Pi2 pulsations in plasma sheet prior to substorms, Earth, Planet, Space 67:168 (2015)
Cheng, C. Z., and A. T. Y. Lui, Kinetic Ballooning Instability for Substorm Onset and Current Disruption Observed by AMPTE/CCE, GRL 25, 4091 (1998)
Cheng, C. Z., Physics of Substorm Growth Phase, Onset, and Dipolarization, Space Science Reviews 113, 207 (2004)
Zaharia, S., C. Z. Cheng, J. R. Johnson, Particle Transport and Energization Associated with Disturbed Magnetospheric Events, JGR 105, 18741 (2000)
Zaharia, S., C. Z. Cheng, Near-Earth Thin Current Sheets and Birkeland Currents During Substorm Growth Phase, GRL 30, 1883-1886 (2003)