Polar cap arcs from the magnetosphere to the ionosphere: kinetic modelling and observations by Cluster and TIMED

Abstract

Abstract. On 1 April 2004 the GUVI imager onboard the TIMED spacecraft spots an isolated and elongated polar cap arc. About 20 min later, the Cluster satellites detect an isolated upflowing ion beam above the polar cap. Cluster observations show that the ions are accelerated upward by a quasistationary electric field. The field-aligned potential drop is estimated to about 700 V and the upflowing ions are accompanied by a tenuous population of isotropic protons with a temperature of about 500 eV.The magnetic footpoints of the ion outflows observed by Cluster are situated in the prolongation of the polar cap arc observed by TIMED GUVI. The upflowing ion beam and the polar cap arc may be different signatures of the same phenomenon, as suggested by a recent statistical study of polar cap ion beams using Cluster data.We use Cluster observations at high altitude as input to a quasi-stationary magnetosphere-ionosphere (MI) coupling model. Using a Knight-type current-voltage relationship and the current continuity at the topside ionosphere, the model computes the energy spectrum of precipitating electrons at the top of the ionosphere corresponding to the generator electric field observed by Cluster. The MI coupling model provides a field-aligned potential drop in agreement with Cluster observations of upflowing ions and a spatial scale of the polar cap arc consistent with the optical observations by TIMED. The computed energy spectrum of the precipitating electrons is used as input to the Trans4 ionospheric transport code. This 1-D model, based on Boltzmann's kinetic formalism, takes into account ionospheric processes such as photoionization and electron/proton precipitation, and computes the optical and UV emissions due to precipitating electrons. The emission rates provided by the Trans4 code are compared to the optical observations by TIMED. They are similar in size and intensity. Data and modelling results are consistent with the scenario of quasi-static acceleration of electrons that generate a polar cap arc as they precipitate in the ionosphere. The detailed observations of the acceleration region by Cluster and the large scale image of the polar cap arc provided by TIMED are two different features of the same phenomenon. Combined together, they bring new light on the configuration of the high-latitude magnetosphere during prolonged periods of Northward IMF. Possible implications of the modelling results for optical observations of polar cap arcs are also discussed.

The Earth: Plasma Sources, Losses, and Transport Processes

“…The simultaneous acceleration of the precipitating electron beams demonstrates that the potential structure must extend to altitudes higher than the spacecraft, whereas the acceleration region is assumed to be confined at the topside of the ionosphere in the auroral zone (Maggiolo et al 2006;Teste et al 2007). A case study with a good conjunction between Cluster observations of precipitating electrons and ion outflows at high altitude, and optical observations of an arc by the TIMED spacecraft, confirmed the relationship between polar cap arcs and accelerated ion outflows with typical shape of inverted "V"s (Maggiolo et al 2012). A statistical study of ion outflows showed that they form elongated and sun-aligned structures with widths typically of the order of 30 km mapped to the ionospheric level.…”

“…A statistical study of ion outflows showed that they form elongated and sun-aligned structures with widths typically of the order of 30 km mapped to the ionospheric level. Their temperature is of the order of tens of eVs and they are accelerated to average energies of about 400-500 eV, with highest values up to 1-2 keV (Maggiolo et al 2011). Periods of northward or weak IMF are not favorable for low-latitude reconnection processes at the magnetopause.…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…They are probably associated with polar cap arcs (Maggiolo et al 2012). It is an intermittent source of O + ions active only during prolonged periods of northward IMF and the total outflow flux associated with polar cap ion beams is low compared to other sources.…”

Dayside and nightside segments of a polar arc: The particle characteristics

“…Precipitating protons also exist in a polar arc [e.g., Peterson and Shelley , 1984], but their contribution to the polar arc's energy budget is significantly smaller than that of electrons [e.g., Hubert et al , 2004]. Polar arcs can be accompanied by upward ion beams at high altitudes (several Earth's radii above the ground), which originate from the quasi‐static field‐aligned acceleration driving ions and electrons upward and downward, respectively [e.g., Maggiolo et al , 2012, and references therein]. Plasma convection around the polar arcs exhibits a large variability with multiple reversals [e.g., Newell et al , 2009, and references therein].…”