Statistical analysis of the dependence of large-scale Birkeland currents on solar wind parameters

Abstract

Abstract. The spatial distributions of large-scale fieldaligned Birkeland currents have been derived using magnetic field data obtained from the Iridium constellation of satellites from February 1999 to December 2007. From this database, we selected intervals that had at least 45% overlap in the large-scale currents between successive hours. The consistency in the current distributions is taken to indicate stability of the large-scale magnetosphere-ionosphere system to within the spatial and temporal resolution of the Iridium observations. The resulting data set of about 1500 two-hour intervals (4% of the data) was sorted first by the interplanetary magnetic field (IMF) GSM clock angle (arctan(B y /B z )) since this governs the spatial morphology of the currents. The Birkeland current densities were then corrected for variations in EUV-produced ionospheric conductance by normalizing the current densities to those occurring for 0 • dipole tilt. To determine the dependence of the currents on other solar wind variables for a given IMF clock angle, the data were then sorted sequentially by the following parameters: the solar wind electric field in the plane normal to the Earth-Sun line, E yz ; the solar wind ram pressure; and the solar wind Alfvén Mach number. The solar wind electric field is the dominant factor determining the Birkeland current intensities. The currents shift toward noon and expand equatorward with increasing solar wind electric field. The total current increases by 0.8 MA per mV m −1 increase in E yz for southward IMF, while for northward IMF it is nearly independent of the electric field, increasing by only 0.1 MA per mV m −1 increase in E yz . The dependence on solar wind pressure is comparatively modest. After correcting for the solar dynamo dependencies in intensity and distribution, the total current intensity increases with solar wind dynamic pressure by 0.4 MA/nPa for southward IMF. Normalizing the BirkeCorrespondence to: H. Korth (haje.korth@jhuapl.edu) land current densities to both the median solar wind electric field and dynamic pressure effects, we find no significant dependence of the Birkeland currents on solar wind Alfvén Mach number.

Characteristics of the terrestrial field-aligned current system

“…At first glance this is a bit surprising since the spatial morphology of the dayside FAC system is well known to be dependent on the IMF conditions (e.g. Anderson et al, 2008;Korth et al, 2010). However, we must keep in mind that our analysis addresses the spatiotemporal characteristics of the FAC system (i.e.…”

Editorial retraction: “Estimation of ground magnetic field variations using solar wind parameters and magnetic indices,” by Claire M. Delides

“…Magnetosphere-ionosphere coupling by the large scale Birkeland currents generates ionosphere Hall currents that perturb the ground magnetic field. The Birkeland currents appear to be sensitive to IMF electric field magnitude and ram pressure [Korth et al, 2010]. Therefore, a starting point for developing a model for describing ground magnetic field data is to investigate the correlation with solar wind parameters.…”

“…Lower latitude data and southern hemisphere data could also have been included. However, this study is focused on the mid-to high latitude currents that appear to be more directly driven by the solar wind [Korth et al, 2010] and also captures auroral dynamics. The northern hemisphere was chosen due to the abundance of magnetometers compared with the southern hemisphere.…”

Solar wind reconstruction from magnetosheath data using an adjoint approach

“…Analogously to the previous studies, the reconstruction can be used to provide solar wind information if in situ solar wind observations are not available. This determines the solar wind parameters, which are the drivers of the magnetospheric current system and are usually required in magnetospheric and ionospheric models (e.g., Nagata et al, 2008;Korth et al, 2010;Zhang et al, 2013;Baker et al, 2013) We reconstruct the solar wind conditions during a spacecraft magnetosheath passage using the magnetosheath model introduced by Nabert et al (2013). The solar wind parameters are boundary conditions of the magnetosheath model and they are varied until the model predictions match the magnetosheath data.…”