Astrophysical context

Fundamental processes in the plasma universe are often organized by magnetic fields and accompanied by energetic particles. Examples range from planetary aurorae and solar activity to astrophysical shocks and pulsar magnetospheres (Fig. 1). Magnetic coupling works across very different plasma regimes and may yield complex interaction patterns. An ideal test-bed for studying this type of fundamental plasma coupling is the geospace environment where the collisionless magnetospheric plasma interacts with the collisional polar ionosphere through exchange of energetic particles, electromagnetic fields and currents. While the magnetosphere-ionosphere (M-I) coupling in the morning and evening sectors is often rather steady and can be well described by simplified current systems and electromagnetic fields, the transition region in the midnight sector (Fig. 2), known as the Harang region (HR), is much more dynamic. The current and field configuration is complex and essentially three-dimensional, and the HR is believed to play an important role in the substorm cycle. The auroral M-I system is typically far from equilibrium and the substorm phases correspond to different conditions of the large scale energy flux through this system, associated with loading / unloading of magnetic energy. Even if a direct connection is difficult to establish at present, similar systems may occur quite generally in magnetized astrophysical plasmas.

Hubble image of the planetary nebula M2-9, whose structure could be explained by a combined magnetic field-aligned plasma outflow and an equatorial expansion such as that in solar CMEs. From Lundin (2001). Images from THEMIS ground based observatories illustrating the spatial and temporal variability of the aurora near the midnight sector. The collage shows a snapshot of a highly dynamic aurora over northern Canada and Alaska. From Mende et al. (2009).

Project objectives

The project aims to investigate the M-I coupling modes in the HR, by exploring the 3-D configuration and temporal evolution of the system during the various substorm phases. Specific issues to be examined are the configuration of the auroral current circuit, the plasma convection and electric field, the energy conversion and transfer between magnetosphere and ionosphere. Due to a unique constellation of spacecraft missions and ground facilities, it is possible at present to probe the plasma and electromagnetic field in all the key regions of the M-I coupling chain. Data from the THEMIS mission in the inner plasma sheet, from the Cluster spacecraft at the top side of the auroral acceleration region (AAR), from low altitude satellites like FAST, REIMEI, or DMSP below the AAR, and from ground based observatories, enable a comprehensive exploration, with emphasis on conjugate events. ISSI provides optimum conditions for the work of an international team holding the required expertise. The project will include three ISSI workshops devoted to: a) the collisional, ionospheric end of the M-I system; b) the collisionless, magnetospheric end of the M-I system; c) investigation of major conjunction events, with data available from all the key regions. The project, to be executed by a team of 10 people, is expected to materialize in case study papers, discussing HR specific M-I coupling features, as well as one concluding paper, providing a comprehensive view over the M-I coupling in the HR during the substorm cycle.