The three-spacecraft Swarm mission, launched by ESA in November 2013, contributes to investigations of the near-Earth magnetic field, by quantifying the contribution from internal and external sources, with applications in various fields like ocean circulation, meteorology and climate research, space weather, or geodesy. Swarm consists of three satellites on circular polar orbits, two of which, Swarm A(lpha) and C(harlie), fly side by side at 460 km altitude, while the third one, Swarm B(ravo), flies at 530 km altitude. The Swarm payload, identical on all satellites and consisting of one scalar and one vector magnetometer, an electric field instrument (drift meter), an accelerometer, and GPS receivers, will enable an unprecedented exploration of the spatial and temporal variations of the geomagnetic field.

ALEOS makes use of a comprehensive Swarm data set, with emphasis on the magnetic field and electric field, which are essential quantities for ionospheric electrodynamics. The nominal 50 Hz / 2 Hz sampling frequencies of the magnetic / electric field imply a mapped Nyquist spatial resolution of about 300 m / 7.5 km, appropriate for auroral investigations. Electric field data at a sampling rate of 16 Hz (900 m), providing better access to small scales, are available as well. In addition, accelerometer data may provide useful information on the neutral winds, so far mostly ignored in auroral studies. The ionospheric conductance, required as well, will be obtained either from radar observations, for conjugate events, or by applying techniques formerly used in ground based studies (e.g. Inhester et al., 1992; Amm, 1995) to the Swarm data (Amm et al., 2009).

(a) Gradients along auroral arcs and adjustment of the 1D arc models

Auroral arcs are typically described in terms of 1D stripes of increased ionospheric conductance. While the 1D arc model, often realized in the evening and morning sectors of the auroral oval, was studied extensively in the past (e. g. Marklund, 1984; Sugiura, 1984), real arcs can exhibit 2D features. By using the ALADYN (AuroralL Arc electroDYNamics) technique (Marghitu et al., 2004, 2009, 2011), it is possible, for example, to investigate the actual divergence of the auroral electrojets by using single satellite data. The examination of Fast Auroral SnapshoT Explorer(FAST, Pfaff et al., 2001) data suggested that this divergence can be significant over certain latitude ranges, comparable with the FAC density. However, just by data from one satellite, it is not possible to say whether the electrojet divergence is driven by gradients in the conductance, in the electric field, or both. The two side-by-side Swarm satellites enable for the first time a systematic examination of the gradients along the arc.

1D arc (left), 2D arc (middle), and general 2D aurora (right). The two arc cartoons show the conductance (gray shade, only gradients along the arc are indicated), FAC (circles), ionospheric electric field (thin red arrows), and ionospheric current (hatched arrows, red and green show the Pedersen and Hall components). For the 1D arc, the normal electric field drives a Pedersen current normal to the arc, closed by FAC, and a divergence free Hall electrojet. For the 2D arc, the electrojet is no longer divergence free, because of the gradients in conductance and electric field along the arc whose examination requires at least two satellites. In order to explore the general, 2D aurora, at least three satellites are required. Arc cartoons from Marghitu et al. (2011). Credit photo: James Spann (NASA/GSFC).

(b) 2D auroral electrodynamics

The complex auroral activity associated with substorms (e.g. subtorm onset, westward travelling surge) can be adequately explored only in two dimensions. While such investigations, addressing meso-scale phenomena, have been carried out based on ground observations (e. g. Untiedt and Baumjohann, 1993; Vanhamäki and Amm, 2011), higher resolution studies require satellite data. In order to explore auroral electrodynamics in its 2D generality, at least three satellites are needed. Tools for processing the multi-point Swarm data have already been developed during the preparation phase of the mission (e.g. Ritter et al, 2006), while other techniques of potential interest for Swarm take advantage of the Cluster heritage (e.g. Vogt et al., 2008, 2009; De Keyser, 2008).

(c) Neutral wind influence

By its accelerometer experiment, Swarm will enable systematic investigation of the ionospheric F-region neutral winds and, possibly, indirect information on the E-region neutral winds - which are potentially important for auroral electrodynamics. While typical velocities of the neutral winds in the auroral region are rather small (e.g. Brekke et al., 1994; Nozawa and Brekke, 1995), those velocities can still be significant in regions where the convection electric field is small as well - for example near the plasma convection reversal boundary, where the convection changes direction from sunward to anti-sunward. The latitudinal width of this ‘boundary’ can be occasionally significant, when regions of clearly poleward and equatorward electric field are separated by a wide region of weak electric field (e.g. Gjerloev and Hoffman, 2001).

(d) The I-T role in the coupled M-I-T system

The investigation of ionospheric electrodynamics made possible by Swarm will contribute also to a better understanding of the coupled M-I-T system at auroral latitudes. Studies on the 3D M-I coupling, based on in-situ multi-point observations, have started to become available only recently (e. g. Keiling et al., 2009; Frey et al., 2010), and in this context Swarm has a significant potential to contribute low altitude information. Conjugate events with Cluster (Escoubet et al, 2001) and THEMIS (Angelopoulos, 2008), as well as with MMS, after its upcoming launch in 2015, will provide ideal occasions to observe the structure and development of the auroral current circuit at different altitudes.

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