Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity

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Abstract

Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity is investigated based on the three-hourly am index and solar wind parameters. When the z component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates is southward, am indices are well correlated with Bs Vx2, where Bs is the southward component of the IMF and Vx is the solar wind velocity in the sun-earth direction. The am-Bs V x2 relationship, however, depends on the range of V x2 the am in higher ranges of Vx2 tends to be larger than am in lower ranges of Vx2 for the same value of Bs Vx2 for both equinoctial and solstitial epochs. Using the data sets of the same Vx2 range, it is shown that distribution of points in the am-Bs V x2 diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each Bs V x2 bin in solstitial epochs are closely consistent with those in equinoctial epochs, if Vx2 for each point at solstices are reduced to Vx2sin2 (Ψ) where Ψ is the geomagnetic colatitude of the sub-solar point. Further, it is shown that monthly averages of the am index in the long period is well correlated with the values of sin2(Ψ) for the middle day of each month. These findings indicate that the factor that contributes to the generation of geomagnetic disturbance is not the velocity of the solar wind, but the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies semi-annually even if the solar wind velocity remains constant, which is considered to be the long-missed key factor causing the equinoctial effect. © Author(s) 2009.

Figures

  • Fig. 1. Plot of geomagnetic am index versus BsV 2x (GSM) for 1998–2007 in velocity ranges of (upper) 100 000<V 2 x <200 000, (middle) 200 000<V 2x <300 000, and (lower) 300 000<V 2 x <400 000, where Vx is the solar wind velocity in the sun-earth direction in km/s and Bs is the magnitude of the southward component of the IMF in nT. (a) Equinoctial epochs (±15 days from equinoxes). (b) Solstitial epochs (±15 days from solstices).
  • Fig. 3. Comparison of average am values for each BsV 2x bin for equinoxes (blue circles) and those for solstices (red circles). Solar wind velocity is reduced by a factor sin (66.5) for solstices.
  • Fig. 2. am index versus BsV 2x at equinoxes (a), and at solstices (b), for 100 000<V 2x <200 000, where Vx is reduced by a factor sin (66.5) for solstices. Average values for severalBsV 2x bins are shown by large blue and red circles, respectively. (c) Superposition of the am-BsV 2x diagrams for equinoxes (blue) and solstices (red).
  • Fig. 4. Relationship between monthly averages of am index and sin2(ψ). (a) Semi-annual variation in monthly am averages for the period 1961–2007. (b) Monthly average of the am indices versus sin2(ψ) at the middle day of each month. (c) Comparison of sin2(ψ) with Svalgaard’s empirical function.

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CITATION STYLE

APA

Yoshida, A. (2009). Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity. Annales Geophysicae, 27(5), 1909–1914. https://doi.org/10.5194/angeo-27-1909-2009

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