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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 25, issue 1
Ann. Geophys., 25, 191–205, 2007
https://doi.org/10.5194/angeo-25-191-2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 25, 191–205, 2007
https://doi.org/10.5194/angeo-25-191-2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.

  01 Feb 2007

01 Feb 2007

The magnetosphere under weak solar wind forcing

C. J. Farrugia1, A. Grocott2, P. E. Sandholt3, S. W. H. Cowley2, Y. Miyoshi4, F. J. Rich5, V. K. Jordanova6, R. B. Torbert1, and A. Sharma3 C. J. Farrugia et al.
  • 1Space Science Center and Department of Physics, University of New Hampshire, NH, USA
  • 2Department of Physics and Astronomy, University of Leicester, Leicester LE2 4TG, UK
  • 3Department of Physics, University of Oslo, Oslo, Norway
  • 4Solar-Terrestrial Environment Laboratory, Nagoya University, Japan
  • 5Air Force Research Lab, Hanscom AFB, MA, USA
  • 6Los Alamos National Laboratory, Los Alamos, NM, USA

Abstract. The Earth's magnetosphere was very strongly disturbed during the passage of the strong shock and the following interacting ejecta on 21–25 October 2001. These disturbances included two intense storms (Dst*≈−250 and −180 nT, respectively). The cessation of this activity at the start of 24 October ushered in a peculiar state of the magnetosphere which lasted for about 28 h and which we discuss in this paper. The interplanetary field was dominated by the sunward component [B=(4.29±0.77, −0.30±0.71, 0.49±0.45) nT]. We analyze global indicators of geomagnetic disturbances, polar cap precipitation, ground magnetometer records, and ionospheric convection as obtained from SuperDARN radars. The state of the magnetosphere is characterized by the following features: (i) generally weak and patchy (in time) low-latitude dayside reconnection or reconnection poleward of the cusps; (ii) absence of substorms; (iii) a monotonic recovery from the previous storm activity (Dst corrected for magnetopause currents decreasing from ~−65 to ~−35 nT), giving an unforced decreased of ~1.1 nT/h; (iv) the probable absence of viscous-type interaction originating from the Kelvin-Helmholtz (KH) instability; (v) a cross-polar cap potential of just 20–30 kV; (vi) a persistent, polar cap region containing (vii) very weak, and sometimes absent, electron precipitation and no systematic inter-hemisphere asymmetry. Whereas we therefore infer the presence of a moderate amount of open flux, the convection is generally weak and patchy, which we ascribe to the lack of solar wind driver. This magnetospheric state approaches that predicted by Cowley and Lockwood (1992) but has never yet been observed.

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