Articles | Volume 34, issue 11
Ann. Geophys., 34, 943–959, 2016
Ann. Geophys., 34, 943–959, 2016

Regular paper 04 Nov 2016

Regular paper | 04 Nov 2016

Evidence for transient, local ion foreshocks caused by dayside magnetopause reconnection

Yann Pfau-Kempf1,2, Heli Hietala3, Steve E. Milan4, Liisa Juusola1, Sanni Hoilijoki1,2, Urs Ganse5,1, Sebastian von Alfthan6, and Minna Palmroth1 Yann Pfau-Kempf et al.
  • 1Earth Observation, Finnish Meteorological Institute, Helsinki, Finland
  • 2Department of Physics, University of Helsinki, Helsinki, Finland
  • 3Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA
  • 4Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 5Department of Physics and Astronomy, University of Turku, Turku, Finland
  • 6CSC, IT Center for Science, Espoo, Finland

Abstract. We present a scenario resulting in time-dependent behaviour of the bow shock and transient, local ion reflection under unchanging solar wind conditions. Dayside magnetopause reconnection produces flux transfer events driving fast-mode wave fronts in the magnetosheath. These fronts push out the bow shock surface due to their increased downstream pressure. The resulting bow shock deformations lead to a configuration favourable to localized ion reflection and thus the formation of transient, travelling foreshock-like field-aligned ion beams. This is identified in two-dimensional global magnetospheric hybrid-Vlasov simulations of the Earth's magnetosphere performed using the Vlasiator model ( We also present observational data showing the occurrence of dayside reconnection and flux transfer events at the same time as Geotail observations of transient foreshock-like field-aligned ion beams. The spacecraft is located well upstream of the foreshock edge and the bow shock, during a steady southward interplanetary magnetic field and in the absence of any solar wind or interplanetary magnetic field perturbations. This indicates the formation of such localized ion foreshocks.

Short summary
We have simulated the interaction of the solar wind – the charged particles and magnetic fields emitted by the Sun into space – with the magnetic field of the Earth. The solar wind flows supersonically and creates a shock when it encounters the obstacle formed by the geomagnetic field. We have identified a new chain of events which causes phenomena in the downstream region to eventually cause perturbations at the shock and even upstream. This is confirmed by ground and satellite observations.