Orbit Design for the THEMIS Mission

Abstract

THEMIS, NASA's fifth Medium Class Explorer (MIDEX) mission will monitor the onset and macro-scale evolution of magnetospheric substorms. It is a fleet of 5 small satellites (probes) measuring in situ the magnetospheric particles and fields while a network of 20 ground based observatories (GBOs) monitor auroral brightening over Northern America. Three inner probes (∼1 day period, 10 R E apogee) monitor current disruption and two outer probes (∼2 day and ∼4 day period, 20 R E and 30 R E apogees respectively) monitor lobe flux dissipation. In order to time and localize substorm onsets, THEMIS utilizes SunEarth aligned conjunctions between the probes when the ground-based observatories are on the nightside. To maintain high recurrence of conjunctions the outer orbits have to be actively adjusted during each observation season. Orbit maintenance is required to rearrange the inner probes for dayside observations and also inject the probes into their science orbits after near-simultaneous release from a common launch vehicle.We present an overview of the orbit strategy, which is primarily driven by the scientific goals of the mission but also represents a compromise between the probe thermal constraints and fuel capabilities. We outline the process of orbit design, describe the mission profile and explain how mission requirements are targeted and evaluated. Mission-specific tools, based on high-fidelity orbit prediction and common magnetospheric models, are also presented. The planning results have been verified by in-flight data from launch through the end of the first primary science seasons and have been used for mission adjustments subject to the early scientific results from the coast phase and first tail season.

The THEMIS Mission

“…Given a 3-6 hr recurrence time for substorms (Borovsky et al 1993), this necessitates 30 hrs of useful data in each year of conjunctions. THEMIS's orbit strategy accounts for >260 hrs of conjunctions in each year (Frey et al 2008). Clear evidence that tail-aligned spacecraft equipped with THEMIS-like instrumentation can indeed monitor the progression of the incoming flows despite their δY ∼ 1-3 R E localization comes from fortuitous ISTP conjunctions during north-south arcs at late substorm recovery (Henderson et al 1998;Sergeev et al 2000).…”

“…The THEMIS mission design is described in Sect. 3 and expanded upon in Frey et al (2008). Figure 1 shows the orbital configurations during the first year of the baseline THEMIS mission.…”

THEMIS Science Objectives and Mission Phases

“…Based on current orbit predictions (Frey et al 2008), the THEMIS probes will accumulate more than 250 h/year of tail-aligned conjunctions (no spacecraft separated by more than 2 R E in the cross-tail or Y-direction from P1). With alignments lasting ∼12 hours, and substorms recurring every ∼3 hours, each alignment should result in 3-4 substorms for a mission total of ∼80 substorms.…”

Low-frequency magnetic field fluctuations in Earth's plasma environment observed by THEMIS

“…NASA's current satellite mission to study geomagnetic substorms, THEMIS (Angelopoulos, 2008;Sibeck and Angelopoulos, 2008), operates in the near Earth plasma environment and provides simultaneous measurements of five spacecraft. The satellites cover large areas of Earth's solar wind interaction region (Frey et al, 2008). Therefore, the THEMIS data are very suitable for a statistical investigation and the magnetic field data set provides a unique possibility to study globally magnetic field fluctuations in Earth's plasma environment under solar minimum conditions.…”