Decreased Sun-Earth energy-coupling efficiency starting from 2006

Masatoshi Yamauchi

doi:10.1186/s40623-015-0211-5

Abstract

The geoeffect of extremely low solar (sunspot) activity starting from the last solar minimum is one of the major space science issues. The present study examines long-term changes of geomagnetic responses seen as Dst, Kp, and AL indices to the same solar wind conditions (density, velocity, magnetic field, and their products) using the NASA OMNI hourly values up to August 2014. Both decadal averages (1965 to 1974, 1975 to 1984, 1985 to 1994, 1995 to 2004 that approximately correspond to solar cycles #20 to 23, respectively, and from 2005) and annual averages indicate that the geomagnetic activity for a given solar wind condition, namely the Sun-Earth coupling efficiency, decreased quantitatively from around 2006 until now compared to the previous four decades. The decrease remains even after the EUV flux (using F10.7 index) is considered and is more obvious in the low-latitude geomagnetic disturbances (Dst) than in the high-latitude geomagnetic disturbances (AL). The results cannot be explained by existing explanations including the ionospheric conductivity effect in the magnetosphere-ionosphere coupling system.

Highlights

The sunspot cycle varies from cycle to cycle over many decades in terms of its peak amplitude, average amplitude during the entire cycle, length of the solar cycle, and the length and depth of the solar minimum defined by the sunspot numbers (Siscoe 1980; Feynman and Fougere 1984; Friis-Christensen and Lassen 1991; Usoskin et al 2007; Svalgaard 2009; Abreu et al 2012)
Short-term Sun-Earth coupling through the plasma and magnetic field has long been studied in the context of space weather
After Akasofu and his co-worker succeeded in quantitatively predicting instantaneous geomagnetic activity from the solar wind input (Perreault and Akasofu 1978; Akasofu 1981), many works have been developed to refine it in obtaining the Sun-Earth coupling function (e.g., Newell et al 2007; Svalgaard 2009)

Summary

Introduction

The sunspot cycle (the so-called 11-year solar cycle) varies from cycle to cycle over many decades in terms of its peak amplitude (e.g., peak sunspot number and number of X-type largest flares), average amplitude during the entire cycle (e.g., average coronal magnetic field and average solar wind velocity), length of the solar cycle, and the length and depth of the solar minimum defined by the sunspot numbers (Siscoe 1980; Feynman and Fougere 1984; Friis-Christensen and Lassen 1991; Usoskin et al 2007; Svalgaard 2009; Abreu et al 2012). There was no sunspot maximum for nearly 70 years around the late 17th century (Maunder Minimum), but the 11-year cycle restarted and repeated for many centuries Such long-term solar variations may affect the Earth through different interplanetary parameters, such as the solar wind, heliospheric magnetic field, solar energetic. After Akasofu and his co-worker succeeded in quantitatively predicting instantaneous (less than 1-h resolution) geomagnetic activity from the solar wind input (Perreault and Akasofu 1978; Akasofu 1981), many works have been developed to refine it in obtaining the Sun-Earth coupling function (e.g., Newell et al 2007; Svalgaard 2009).

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Conclusion