A statistical approach for identifying the ionospheric footprint of magnetospheric boundaries from SuperDARN observations

Identifying and tracking the projection of magnetospheric regions on the high-latitude ionosphere is of primary importance for studying the Solar Wind-Magnetosphere-Ionosphere system and for space weather applications. By its unique spatial coverage and temporal resolution, the Super Dual Auroral Radar Network (SuperDARN) provides key parameters, such as the Doppler spectral width, which allows the monitoring of the ionospheric footprint of some magnetospheric boundaries in near real-time. In this study, we present the first results of a statistical approach for monitoring these magnetospheric boundaries. The singular value decomposition is used as a data reduction tool to describe the backscattered echoes with a small set of parameters. One of these is strongly correlated with the Doppler spectral width, and can thus be used as a proxy for it. Based on this, we propose a Bayesian classifier for identifying the spectral width boundary, which is classically associated with the Polar Cap boundary. The results are in good agreement with previous studies. Two advantages of the method are: the possibility to apply it in near real-time, and its capacity to select the appropriate threshold level for the boundary detection

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

Lower hybrid resonances stimulated by the four CLUSTER relaxation sounders deep inside the plasmasphere: observations and inferred plasma characteristics

International audienceThe frequency range of the WHISPER relaxation sounder instrument on board CLUSTER, 4–80 kHz, has been chosen so as to encompass the electron gyro-frequency, F ce , and the electron plasma frequency, F p , in most regions to be explored. Measurement of those frequencies, which are triggered as resonances by the sounder, provides a direct estimation of in situ fundamental plasma characteristics: electron density and magnetic field intensity. In the late mission phase, CLUSTER penetrated regions deep inside the plas-masphere where F ce and F p are much higher than the upper frequency of the sounder's range. However, they are of the right order of magnitude as to place the lower hybrid frequency , F lh , in the 4–15 kHz band. This characteristic frequency , placed at a resonance of the medium, is triggered by the sounder's transmitter and shows up as an isolated peak in the received spectrum, not present in spectra of naturally occuring VLF waves. This paper illustrates, from analysis of case events, how measured F lh values give access to a plasma diagnostic novel of its kind. CLUSTER, travelling along its orbit, encounters favourable conditions where F ce is increasing and F p decreasing, such that F ce /F p increases from values below unity to values above unity. Measured F lh values thus give access, in turn, to the effective mass, M eff , indicative of plasma ion composition, and to the core plasma-sphere electron density value, a parameter difficult to measure. The analysed case events indicate that the estimated quantities (M eff in the 1.0–1.4 range, N e in the 5 × 10 2 – 10 4 cm −3 range) are varying with external factors (altitude, L value, geomagnetic activity) in a plausible way. Although covering only a restricted region (mid-latitude, low altitude inner plasmasphere), these measurements are available, since Correspondence to: S. Kougblénou ([email protected]) late 2009, for all CLUSTER perigee passes not affected by eclipses (on average, roughly a third of a total of ∼200 passes per year) and offer multipoint observations previously unavailable in this region

Characteristics and conditions of production of transient luminous events observed over a maritime storm

International audienceOn the night of 15/16 November 2007, cameras in southern France detected 30 transient luminous events (TLEs) over a storm located in the Corsican region (France). Among these TLEs, 19 were sprites, 6 were halos, and 5 were elves. For 26 of them, a positive “parent” cloud‐to‐ground lightning (P+CG) flash was identified. The peak current of the P+CG flashes for the sprites had an average value of 63 kA and had a maximum value of 125 kA. The flashes for the halos and the elves had average values of 272 and 351 kA, respectively, and they had maximum values of 312 and 384 kA, respectively. No TLEs were detected after negative CG flashes with very large peak currents. Among the 26 P+CG flashes, 23 were located in a stratiform region with reflectivity values lower than 45 dBZ. The CG flashes in this region were classified into two groups according to the time interval separating them from the following flash: one group with values less than 2 s and one with values greater than 2 s. About 79% of all CGs were produced in a sequence of at least two flashes less than 2 s apart. For 65.5% of the sequences, the first flash was positive with an average peak current of 73 kA, while the later +CG flashes in a sequence had much lower peak currents. Several triangulated sprites were found to be shifted from their P+CG flashes by about 10 to 50 km and preferentially downstream. The observations suggest that the P+CG flashes can initiate both sprites and other CG flashes in a storm

Raisonnement spatial et gestion durable de l'espace : ... du laboratoire au terrain

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Open magnetic flux tubes in the corona and the transport of solar energetic particles

We investigate how magnetic fields guide energetic particles through the corona into interplanetary space and eventually to a spacecraft near the Earth. A set of seven simple particle events is studied, where energetic electrons (30–500 keV; Wind spacecraft) or protons (5–55 MeV; SoHO) were released together with low-energy electron beams producing metric-to-kilometric type III emission. Imaging of the coronal (metre-wave) part of this emission with the Nançay Radioheliograph is used to identify the open flux tubes that guide these electrons – and by inference all particles detected at 1 AU. Open coronal field lines are also computed using potential magnetic field extrapolations, constrained by a source surface and by SoHO/MDI measurements in the photosphere (code by Schrijver and DeRosa). We find that in all events the type III radio sources lie in open flux tubes in the potential magnetic field extrapolations. The open flux tubes are rooted in small parts of the parent active region, covering a heliocentric angle of a few degrees in the photosphere. But they expand rapidly above the neighbouring closed magnetic structures and cover several tens of degrees in longitude on the source surface. Some of these open field lines are found to connect the parent active region to the footpoint of the nominal Parker spiral on the source surface, within the uncertainty of about ±10° inherent to the evaluation of its connection longitude. This is so even when the parent active region is as far as 50° away. In two cases where the coronal flux tubes point to high heliolatitudes, the detection of Langmuir waves at the Wind spacecraft in the ecliptic plane suggests that the interplanetary field lines curve down to the ecliptic before reaching 1 AU. We conclude that non-radial open flux tubes in the corona can transport particles over several tens of degrees in longitude even in simple impulsive particle events. In all events we studied, potential magnetic field models give an adequate description of these structures

Lessons learned from WHISPER relaxation sounder in ten years of CLUSTER mission

International audienceThe four WHISPER instruments, part of the Wave Experiment Consortium on board the multi satellite CLUSTER mission, include each a relaxation sounder aimed to measure with a good precision and reliability the frequency positions of F p , electron plasma frequency, and of F ce , electron gyro-frequency. Those quantities give access to electron density and intensity of magnetic field, key parameters defining local plasma regime. WHISPER sounder's design and realization, based on the expertise provided by successful missions operated in late seventies and eighties (GEOS, ISEE, Viking), is characterized by use of a processor devoted to on board frequency analysis via FFT. This choice makes those relaxation sounders unique of their kind, allowing in particular a complete plasma diagnostic to be made in only 1.5 s, almost one order of magnitude faster than in the past. The CLUSTER mission itself is unique, by its multi-point capability, and by its orbit, which along the mission allowed exploring a large variety of key regions of the magnetosphere. We present in this paper a few examples of WHISPER instrument behaviour and results, exploring in particular magnetosheath, polar cap, outer and inner plasmasphere. In the latter region, F p and F ce frequency values are far above the working frequency range of the instrument. This range, however, includes the lower hybrid frequency, F lh , which shows up as a clear resonance triggered by the sounder, and allows measurement of Ne whenever F p /F ce <; 1

Refilling process in the plasmasphere: a 3-D statistical characterization based on Cluster density observations

The Cluster mission offers an excellent opportunity to investigate the evolution of the plasma population in a large part of the inner magnetosphere, explored near its orbit's perigee, over a complete solar cycle. The WHISPER sounder, on board each satellite of the mission, is particularly suitable to study the electron density in this region, between 0.2 and 80 cm⁻³. Compiling WHISPER observations during 1339 perigee passes distributed over more than three years of the Cluster mission, we present first results of a statistical analysis dedicated to the study of the electron density morphology and dynamics along and across magnetic field lines between <I>L</I> = 2 and <I>L</I> = 10. In this study, we examine a specific topic: the refilling of the plasmasphere and trough regions during extended periods of quiet magnetic conditions. To do so, we survey the evolution of the ap index during the days preceding each perigee crossing and sort out electron density profiles along the orbit according to three classes, namely after respectively less than 2 days, between 2 and 4 days, and more than 4 days of quiet magnetic conditions (ap &le; 15 nT) following an active episode (ap > 15 nT). This leads to three independent data subsets. Comparisons between density distributions in the 3-D plasmasphere and trough regions at the three stages of quiet magnetosphere provide novel views about the distribution of matter inside the inner magnetosphere during several days of low activity. Clear signatures of a refilling process inside an expended plasmasphere in formation are noted. A plasmapause-like boundary, at <I>L</I> ~ 6 for all MLT sectors, is formed after 3 to 4 days and expends somewhat further after that. In the outer part of the plasmasphere (<I>L</I> ~ 8), latitudinal profiles of median density values vary essentially according to the MLT sector considered rather than according to the refilling duration. The shape of these density profiles indicates that magnetic flux tubes are not fully replenished after 6 days of quiet conditions. In addition, the outer plasmasphere in the night and dawn sectors (22:00 to 10:00 MLT range) maintains an overall clear deficit of ionospheric population, when compared to the situation in the noon and dusk sectors (10:00 to 22:00 MLT range)