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

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 quasi-stationary 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. <br><br> 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. <br><br> 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

Polar cap ion beams during periods of northward IMF: Cluster statistical results

International audienceAbove the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static field-aligned electric field that can extend to altitudes higher than 7 RE (Maggiolo et al., 2006; Teste et al., 2007). Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas, we have been able to extract about 200 events of accelerated upgoing ion beams above the polar caps from the Cluster database. Most of these observations are taken at altitudes lower than 7 RE and in the Northern Hemisphere. We investigate the statistical properties of these ion beams. We analyze their geometry, the properties of the plasma populations and of the electric field inside and around the beams, as well as their dependence on solar wind and IMF conditions. We show that ~40 % of the ion beams are collocated with a relatively hot and isotropic plasma population. The density and temperature of the isotropic population are highly variable but suggest that this plasma originates from the plasma sheet. The ion beam properties do not change significantly when the isotropic, hot background population is present. Furthermore, during one single polar cap crossing by Cluster it is possible to detect upgoing ion beams both with and without an accompanying isotropic component. The analysis of the variation of the IMF BZ component prior to the detection of the beams indicates that the delay between a northward/southward turning of IMF and the appearance/disappearance of the beams is respectively ~2 h and 20 min. The observed electrodynamic characteristics of high altitude polar cap ion beams suggest that they are closely connected to polar cap auroral arcs. We discuss the implications of these Cluster observations above the polar cap on the magnetospheric dynamics and configuration during prolonged periods of northward IMF

Kinetic simulations of solar wind plasma irregularities crossing the Hermean magnetopause

Context. The physical mechanisms that favor the access of solar wind plasma into the magnetosphere have not been entirely elucidated to date. Studying the transport of finite-sized magnetosheath plasma irregularities across the magnetopause is fundamentally important for characterizing the Hermean environment (of Mercury) as well as for other planetary magnetic and plasma environments. Aims. We investigate the kinetic effects and their role on the penetration and transport of localized solar wind or magnetosheath plasma irregularities within the Hermean magnetosphere under the northward orientation of the interplanetary magnetic field. Methods. We used three-dimensional (3D) particle-in-cell (PIC) simulations adapted to the interaction between plasma elements (irregularities or jets) of a finite spatial extent and the typical magnetic field of Mercury’s magnetosphere. Results. Our simulations reveal the transport of solar wind plasma across the Hermean magnetopause and entry inside the magnetosphere. The 3D plasma elements are braked and deflected in the equatorial plane. The entry process is controlled by the magnetic field gradient at the magnetopause. For reduced jumps of the magnetic field (i.e., for larger values of the interplanetary magnetic field), the magnetospheric penetration is enhanced. The equatorial dynamics of the plasma element is characterized by a dawn-dusk asymmetry generated by first-order guiding center drift effects. More plasma penetrates into the dusk flank and advances deeper inside the magnetosphere than in the dawn flank. Conclusions. The simulated solar wind or magnetosheath plasma jets can cross the Hermean magnetopause and enter into the magnetosphere, as described by the impulsive penetration mechanism

Statistical study of ionospheric ion beams observed by CLUSTER above the polar caps

International audienceAbove the polar caps and during prolonged periods of Northward IMF, the Cluster spacecraft detect accelerated ion beams with energies up to a few keV. They are associated with downward precipitating electrons and converging electric field structures indicating that the acceleration is caused by a quasi-static field aligned electric field that can extend to altitudes up to 5 RE (Maggiolo et al. 2006, Teste et al. 2007). Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas (CDPP, http://cdpp.cesr.fr), we have been able to extract from the Cluster ion detectors dataset the time periods when Cluster encounters polar cap local ion beams. 6 years of data have been mined with this tool. Almost 200 events have been found giving new insight on these structures. After a description of the method used for the automatic detection of the beams, we will discuss their statistical properties. We analyze their relation to solar wind and IMF. In particular, we estimate the delay between a Northward/Southward turning of the IMF and the appearance/disappearance of these beams. The characteristics of the particles detected inside these structures as well as their size, orientation and location are also presented. We show that these ion beams are located on magnetic field lines mapping close to the high latitude magnetopause and in the central part of the lobes and that 40 % of them are detected together with hot isotropic ions. These results will be discussed in term of magnetotail configuration during prolonged periods of Northward IMF

Field-aligned acceleration above the polar caps during prolonged periods of Northward IMF: 1- Ion outflows

International audienceThe Cluster spacecraft reveal the presence of successive current sheets of opposite polarity above the polar caps during extended periods of northward IMF. At Cluster altitude (5-7 RE), the upward part of this current system consists of ion beams accelerated by quasi-static electric fields and associated with precipitating electrons. They are surrounded by low energy upflowing electron beams carrying a downward current. In this paper, we focus on Cluster observations of upflowing ion beams in the upward current region. These Polar Cap Ion Beams (PCIB) are accelerated by quasi-static electric fields. A recent statistical study shed new light on these structures. The PCIB have lifetimes that can exceed 20 minutes, they appear after northward turning of the IMF and disappear when it turns southward with a delay estimated to respectively ~2 hours and 20 minutes. Interestingly, about 40% of PCIB are detected together with isotropic plasma clouds located in the magnetospheric lobes. When present, the isotropic plasma population has a strong spatial correlation with PCIB suggesting that it is linked to the observed ion outflows. The temperature and pitch angle distribution of these isotropic ions show that they likely originate from the plasmasheet. The statistical properties of PCIB (size, orientation, IMF dependency...) are similar to those of optical polar cap arcs suggesting that both are different signatures of the same phenomenon. This is further confirmed by a case study combining Cluster in-situ data and TIMED optical observations. Using the high-altitude Cluster data as input to a magnetosphere-ionosphere coupling model we have been able to compute the UV emissions produced by the precipitating electrons associated with the PCIB. The modeled photo-emission is in good agreement with that of a polar cap arc observed by TIMED during the same time period. We'll discuss the new insight these observations give on the interaction between the magnetosphere and ionosphere during prolonged periods of northward IMF

Statistical study of ionospheric ion beams observed by CLUSTER above the polar caps

International audienceAbove the polar caps and during prolonged periods of Northward IMF, the Cluster spacecraft detect accelerated ion beams with energies up to a few keV. They are associated with downward precipitating electrons and converging electric field structures indicating that the acceleration is caused by a quasi-static field aligned electric field that can extend to altitudes up to 5 RE (Maggiolo et al. 2006, Teste et al. 2007). Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas (CDPP, http://cdpp.cesr.fr), we have been able to extract from the Cluster ion detectors dataset the time periods when Cluster encounters polar cap local ion beams. 6 years of data have been mined with this tool. Almost 200 events have been found giving new insight on these structures. After a description of the method used for the automatic detection of the beams, we will discuss their statistical properties. We analyze their relation to solar wind and IMF. In particular, we estimate the delay between a Northward/Southward turning of the IMF and the appearance/disappearance of these beams. The characteristics of the particles detected inside these structures as well as their size, orientation and location are also presented. We show that these ion beams are located on magnetic field lines mapping close to the high latitude magnetopause and in the central part of the lobes and that 40 % of them are detected together with hot isotropic ions. These results will be discussed in term of magnetotail configuration during prolonged periods of Northward IMF

Probing the Magnetospheric Generator of Quiet Electrostatic Auroral Arcs From Ground Based Optical Observations and Magnetosphere‐ionosphere Coupling Modeling

Abstract Observations of a quiet electrostatic auroral arc by the ALIS network on 5 March 2008 are used to infer a two‐dimensional map of the flux of precipitating energy. Among a family of numerical solutions of a stationary magnetosphere—ionosphere coupling model in which the origin of the arc is a magnetospheric generator interface, we find which generator interface properties best fit the observed precipitating energy flux. The procedure finds that the plasma populations in the generator are colder and more rarefied on one side of the interface and warmer and denser on the other side, similar to a transition between plasma trough and plasma sheet plasmas. The increase of the arc's brightness, the decrease of its thickness and its slight spatial undulation may be driven by an increase of plasma sheet electron temperature in the tailward direction, tangential to the interface, and a local spatial indentation in the dawn‐ward direction

Magnetosphere-ionosphere coupling at high latitude during periods of Northward IMF

International audienceDuring periods of northward IMF, auroral features are observed above the polar caps. These polar cap arcs are the signature of magnetosphere-ionosphere coupling processes occurring at high latitudes. Recent observations from the Cluster satellites at high altitude (5-7 RE) and from spacecraft imagers give new insight on these processes. Polar cap arcs can occur on a variety of scales, from large-scale arcs called 'transpolar arcs' or 'theta aurora' to small-scale isolated arcs. There is strong evidence that large-scale transpolar arcs are located on field lines that have been closed by magnetotail reconnection but remain embedded within the lobes. This is further confirmed by recent observations from the IMAGE and Cluster satellites. Small scale structures consisting in a succession of current sheets with opposite polarity are also observed above the polar caps during periods of northward IMF. Cluster and TIMED observations reveal that they are associated with thin polar cap arcs. The upward current is carried by upflowing ionospheric ions and precipitating electrons accelerated by quasi-static electric fields. The precipitating electrons are cold and likely of solar wind origin. However, for half of the events, plasmasheet-like isotropic ions are detected simultaneously. Consequently the magnetic field lines do not neatly fit into either a closed and open configuration. A detailed review of these observations is proposed, which are subsequently used to constrain ideas on magnetosphere-ionosphere interactions and on magnetospheric configuration at high latitudes during prolonged periods of northward IMF