Abstract
Abstract. In this paper we present two substorm events with coordinated ground-based and in-situ THEMIS observations, and focus our interest on the wave activities in Pi1 and Pi2 bands from minutes before the substorm expansion phase (EP) onset to minutes after the local current disruption (CD). We find that Pi2 band (40–100 s) wave appears 1–2 min before the substorm onset and last over the entire EP interval, while higher-frequency wave within Pi1 band (10–30 s) emerges within few tens of seconds after the EP onset, intensifies during the local CD, and fades afterwards. The pre-onset Pi2 waves are attributed to a ballooning mode which acts as the seed perturbation to the substorm EP onset process. The azimuthal wavenumber estimated from the Doppler shift nature of the ballooning mode is consistent with the longitudinal "wavelength" inferred from the onset auroral structures. The Pi1 waves appearing within few tens of seconds after the EP onset are interpreted as supportive of a two-fluid instability mode of thin current sheet investigated in an accompanying paper (Liu and Liang, 2009). During the local CD, broadband wave activities from Pi2 band to well above the ion gyrofrequency are observed, suggesting the coexistence of various plasma instabilities featuring different frequency ranges.
Highlights
Ultra-low frequency (ULF) waves have long been recognized as an essential element in magnetospheric physics
We revealed that a Pi2 band wave (40–100 s) usually appears 1–2 min before the substorm expansion phase (EP) onset, while higher-frequency wave, nominally at Pi1 band (10–30 s), emerge within few tens of seconds after the EP onset and intensifies during the local current disruption (CD)
From our survey of THEMIS events we find that the ion temperature drop (ITD) minutes prior to the local CD onset is a common feature when the probe is at the current sheet boundary
Summary
While the primary objective of THEMIS mission is a timing between the mid-tail disturbance (e.g., fast flows) and the near-Earth disturbance (e.g., CD) in a substorm sequence, THEMIS has another remarkable feature in that it provides an unprecedented opportunity of coordinated in-situ and ground-based observations of the seconds-scale dynamics of substorm process, and is suited for the study of ULF wave activity. Satellite observations may yield the temporal characteristic such as wave frequency and growth rate (in terms of instability), while ground-based auroral observations with 3-s cadence provide the most accurate way of EP onset timing to establish a reference epoch for the wave activity, and have unmatched capability to provide the spatial structures, such as the longitudinal wavelength, of substorm intensifications (Liang et al, 2008).
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