Decreased Sun-Earth energy-coupling efficiency starting from 2006

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.

Diminishing activity of recent solar cycles (22–24) and their impact on geospace

“…A study by Yamauchi (2015) decreased reports the decrease in Sun-Earth coupling efficiency during recent past (i.e., 2005-2014) than those during the previous four decades (i.e., 1965-2004). Their study conclude that this observed decrease cannot be explained by existing magnetosphereionosphere coupling knowledge but rather it might be related to the unidentified mechanism connected to the long-term solar activity.…”