The THEMIS all-sky imaging array -: system design and initial results from the prototype imager

Time History of Events and Macroscale Interactions during Substorms (THEMIS) is a NASA MIDEX mission scheduled for launch in 2006. THEMIS will consist of five magnetospheric satellites in equatorial orbits. Three of the spacecraft will have apogees around 12 Re, while the fourth and fifth will have apogees at similar to 20 and similar to 30 Re. The 12, 20, and 30 Re apogee orbits will have periods of one, two, and four sidereal days, respectively, meaning that all five spacecraft will be at or near apogee in the same meridian every four sidereal days. Furthermore, these conjunctions will always occur over central Canada throughout the mission duration. The five THEMIS satellites will be instrumented with particle and field detectors for measuring relevant plasma parameters, fields, and bulk velocities in the central plasma sheet (CPS). The THEMIS constellation will bracket the current disruption (CD) and near-earth neutral line (NENL) regions and will provide for the first time an opportunity for unambiguous identification of the radial position in the CPS where the substorm process initiates. The primary scientific objective for THEMIS is to determine which of these processes is responsible for substorm onset. THEMIS cannot close this question without complementary ground-based observations in North America. To this end, THEMIS requires the deployment of 20 white light all-sky imagers (ASIs) in a continent-wide array. These ASIs will operate with a cadence of at least one image every 5 s, and will provide mission critical onset and early expansive phase information. In this paper, we present observations from the prototype THEMIS ASI for one substorm event. This image data demonstrates that the THEMIS ASI has the temporal and spatial resolution necessary to meet the mission requirements. Further, in this event we find that the growth phase arc shows wavelike azimuthal structuring and a brightening that occurs virtually simultaneously along the entire length of the arc that is within the ASI field of view. We attribute this wavelike structure to structure in the CPS. We anticipate that the THEMIS ASI array and in situ data will allow for the elucidation of the CPS process that generates this azimuthal structure. (C) 2006 Published by Elsevier Ltd.