Abstract
Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100–240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008–2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.
Highlights
One of the important phenomena of the substorm auroral arcs observed by THEMIS Ground-Based Observatory (GBO), which consists of a network of all-sky imagers (ASIs), magnetometers, etc. (Mende et al 2008), is that the substorm onset arcs are characterized by wave-like bright spot structure prior to substorm onset
We show that the azimuthal mode number of the substorm onset arc wave-like structure is similar to the azimuthal mode number of the Pi2 δBy disturbances in the plasma sheet
Because the most unstable kinetic ballooning instability in the plasma sheet has high azimuthal mode number and low frequency in the Pi2 range, it is a candidate to explain the Pi2 disturbance observed in the plasma sheet
Summary
The launch of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission (Angelopoulos 2008) has provided researchers with data of simultaneous ground, ionospheric, and magnetospheric observations to investigate the substorm onset phenomena which typically manifest within the crucial tens of seconds around substorm onset time.One of the important phenomena of the substorm auroral arcs observed by THEMIS Ground-Based Observatory (GBO), which consists of a network of all-sky imagers (ASIs), magnetometers, etc. (Mende et al 2008), is that the substorm onset arcs are characterized by wave-like bright spot structure prior to substorm onset (e.g., Donovan et al 2006; Liang et al 2007; Sakaguchi et al.2009; Uritsky et al 2009; Rae et al 2010; Chang et al 2012), and the intensity of substorm onset arc is found to grow exponentially during the pre-onset period of time (e.g., Voronkov et al 1999, 2003; Donovan et al 2008; Liang et al 2008; Rae et al 2010; Chang et al 2012). The launch of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission (Angelopoulos 2008) has provided researchers with data of simultaneous ground, ionospheric, and magnetospheric observations to investigate the substorm onset phenomena which typically manifest within the crucial tens of seconds around substorm onset time. One of the important phenomena of the substorm auroral arcs observed by THEMIS Ground-Based Observatory (GBO), which consists of a network of all-sky imagers (ASIs), magnetometers, etc. It is generally suggested that the wave-like arc brightness structure is related to some instability excited in the inner plasma sheet. Pi2 disturbances appear prior to the magnetic field dipolarization onset in the plasma sheet and Pi1 disturbances are commonly observed at or after the substorm onset
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