Abstract
Abstract. Semiannual variation of geomagnetic activity and ap-index in particular is supposed to consist of heliospheric factor (axial hypothesis and Russell-McPherron effect) and magnetospheric/ionospheric factor (equinoctical hypothesis). In our investigation we express ap-index as a magnetospheric response function to solar wind and IMF input. Seasonal variation in ap-index on average (1963–2003) is ~4 nT and consists of ~2.1–2.3 nT of magnetospheric/ionospheric part, 0.6–1.3 nT of heliospheric part (including 0.2–0.3 nT of R-M effect), 0.1–0.4 nT is due to the non-linear term. 90% confidence range of all estimates is ~0.1–0.25 nT. While autumn/spring magnetospheric response functions are almost identical, there is substantial difference between winter and summer functions. The increase of solar wind input in autumn and spring is also different by a factor of two.
Highlights
Semiannual variation (SAV) of geomagnetic activity with maxima near equinoxes appears in several forms, such as a periodic wave in geomagnetic indices like ap (Russell and McPherron, 1973; Svalgaard, 1977; Cliver et al, 2000; Le Mouel et al, 2004), or a tendency of strong storms to occur during spring/autumn (Cliver et al, 2004; Svalgaard et al, 2002)
(2) Cortie effect or axial hypothesis (Cortie, 1912; Bohlin, 1977) attributes SAV to increase of heliographic latitude around equinoxes, placing Earth closer to more geoefficient fast solar wind streams from mid-latitude coronal holes
We investigate its composition in ap index with a carefully selected quantitative model of solar wind driving
Summary
Semiannual variation (SAV) of geomagnetic activity with maxima near equinoxes appears in several forms, such as a periodic wave in geomagnetic indices like ap (Russell and McPherron, 1973; Svalgaard, 1977; Cliver et al, 2000; Le Mouel et al, 2004), or a tendency of strong storms to occur during spring/autumn (Cliver et al, 2004; Svalgaard et al, 2002). With a recent investigation of SAV phases it was shown that 65–75% of this variation is due to equinoctial effect, axial effect contributes of the order of 20%, while the RM effect is the smallest with ∼10% (Cliver et al, 2000, 2002). Consistent analysis of SAV, representing an index as an explicit function of solar wind input and magnetospheric response was not performed. This is the main task of our investigation
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