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Dorville N.,University Paris - Sud | Haaland S.,University of Bergen | Haaland S.,Max Planck Institute for Solar Systems Research | Anekallu C.,University College London | And 2 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

Determining the direction normal to the magnetopause layer is a key step for any study of this boundary. Various techniques have been developed for this purpose. We focus here on generic residue analysis (GRA) methods, which are based on conservation laws, and the new iterative BV method, where B represents the magnetic field and V refers to the ion velocity. This method relies on a fit of the magnetic field hodogram against a modeled geometrical shape and on the way this hodogram is described in time. These two methods have different underlying model assumptions and validity ranges. We compare here magnetopause normals predicted by BV and GRA methods to better understand the sensitivity of each method on small departures from its own physical hypotheses. This comparison is carried out first on artificial data with magnetopause-like noise. Then a statistical study is carried out using a list of 149 flank and dayside magnetopause crossings from Cluster data where the BV method is applicable, i.e., where the magnetopause involves a single-layer current sheet, with a crudely C-shaped magnetic hodogram. These two comparisons validate the quality of the BV method for all these cases where it is applicable. The method provides quite reliable normal directions in all these cases, even when the boundary is moving with a varying velocity, which distorts noticeably the results of most of the other methods. Key Points The BV technique is benchmarked with respect to other single-spacecraft methods It is less sensitive to noise than most of the other methods on simulated data A statistical study is made on 149 Cluster magnetopause crossings ©2015. American Geophysical Union. All Rights Reserved. Source


Maes L.,Belgian Institute for Space Aeronomy | Maggiolo R.,Belgian Institute for Space Aeronomy | De Keyser J.,Belgian Institute for Space Aeronomy | Dandouras I.,CNRS Institute for research in astrophysics and planetology | And 5 more authors.
Geophysical Research Letters | Year: 2015

We measure the flux density, composition, and energy of outflowing ions above the polar cap, accelerated by quasi-static electric fields parallel to the magnetic field and associated with polar cap arcs, using Cluster. Mapping the spacecraft position to its ionospheric foot point, we analyze the dependence of these parameters on the solar zenith angle (SZA). We find a clear transition at SZA between ∼94° and ∼107°, with the O+ flux higher above the sunlit ionosphere. This dependence on the illumination of the local ionosphere indicates that significant O+ upflow occurs locally above the polar ionosphere. The same is found for H+, but to a lesser extent. This effect can result in a seasonal variation of the total ion upflow from the polar ionosphere. Furthermore, we show that low-magnitude field-aligned potential drops are preferentially observed above the sunlit ionosphere, suggesting a feedback effect of ionospheric conductivity. © 2015. The Authors. Source


Munteanu C.,Romanian Space Science Institute | Munteanu C.,University of Oulu | Munteanu C.,University of Bucharest | Haaland S.,University of Bergen | And 5 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

We present a statistical study of the performance of three methods used to predict the propagation delay of solar wind structures. These methods are based on boundary normal estimations between the Advanced Composition Explorer (ACE) spacecraft orbiting the L1 libration point and the Cluster spacecraft near the Earth's magnetopause. The boundary normal estimation methods tested are the cross product method (CP), the minimum variance analysis of the magnetic field (MVAB), and the constrained minimum variance analysis (MVAB0). The estimated delay times are compared with the observed ones to obtain a quantitative measure of each method's accuracy. Boundary normal estimations of magnetic field structures embedded in the solar wind are known to be sensitive to small-scale fluctuations. Our study uses wavelet denoising to reduce the effect of these fluctuations. The influence of wavelet denoising on the performance of the three methods is also analyzed. We find that the free parameters of the three methods have to be adapted to each event in order to obtain accurate propagation delays. We also find that by using denoising parameters optimized to each event, 88% of our database of 356 events are estimated to arrive within ±2 min from the observed time delay with MVAB, 74% with CP, and 69% with the MVAB0 method. Our results show that wavelet denoising significantly improves the predictions of the propagation time delay of solar wind discontinuities. © 2013. American Geophysical Union. All Rights Reserved. Source


Haaland S.,University of Bergen | Haaland S.,Max Planck Institute for Solar Systems Research | Eriksson A.,Swedish Institute of Space Physics | Andre M.,Swedish Institute of Space Physics | And 12 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

Low-energy ions of ionospheric origin constitute a significant contributor to the magnetospheric plasma population. Measuring cold ions is difficult though. Observations have to be done at sufficiently high altitudes and typically in regions of space where spacecraft attain a positive charge due to solar illumination. Cold ions are therefore shielded from the satellite particle detectors. Furthermore, spacecraft can only cover key regions of ion outflow during segments of their orbit, so additional complications arise if continuous longtime observations, such as during a geomagnetic storm, are needed. In this paper we suggest a new approach, based on a combination of synoptic observations and a novel technique to estimate the flux and total outflow during the various phases of geomagnetic storms. Our results indicate large variations in both outflow rates and transport throughout the storm. Prior to the storm main phase, outflow rates are moderate, and the cold ions are mainly emanating from moderately sized polar cap regions. Throughout the main phase of the storm, outflow rates increase and the polar cap source regions expand. Furthermore, faster transport, resulting from enhanced convection, leads to a much larger supply of cold ions to the near-Earth region during geomagnetic storms. ©2015. The Authors. Source


Bjoland L.M.,University of Tromso | Chen X.,University Center in Svalbard | Chen X.,University of Oslo | Chen X.,University of Bergen | And 10 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

Joule heating in the ionosphere takes place through collisions between ions and neutrals. Statistical maps of F region Joule heating in the Northern Hemisphere polar ionosphere are derived from satellite measurements of thermospheric wind and radar measurements of ionospheric ion convection. Persistent mesoscale heating is observed near postnoon and postmidnight magnetic local time and centered around 70 magnetic latitude in regions of strong relative ion and neutral drift. The magnitude of the Joule heating is found to be largest during solar maximum and for a southeast oriented interplanetary magnetic field. These conditions are consistent with stronger ion convection producing a larger relative flow between ions and neutrals. The global-scale Joule heating maps quantify persistent (in location) regions of heating that may be used to provide a broader context compared to small-scale studies of the coupling between the thermosphere and ionosphere. ©2015. American Geophysical Union. All Rights Reserved. Source

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