The solar and interplanetary causes of the recent minimum in geomagnetic activity (MGA23): a combination of midlatitude small coronal holes, low IMF IB/IsubZ/sub variances, low solar wind speeds and low solar magnetic fields

摘要

Minima in geomagnetic activity (MGA) at Earth at the ends of SC23 and SC22have been identified. The two MGAs (called MGA23 and MGA22, respectively)were present in 2009 and 1997, delayed from the sunspot number minima in2008 and 1996 by ~1/2–1 years. Part of the solar and interplanetarycauses of the MGAs were exceptionally low solar (and thus lowinterplanetary) magnetic fields. Another important factor in MGA23 was thedisappearance of equatorial and low latitude coronal holes and theappearance of midlatitude coronal holes. The location of the holes relativeto the ecliptic plane led to low solar wind speeds and low IMF (B_z)variances (σ_Bz2) and normalized variances (σ_Bz2/B₀2) at Earth, with concomitant reduced solarwind-magnetospheric energy coupling. One result was the lowest ap indices inthe history of ap recording. The results presented here are used to commenton the possible solar and interplanetary causes of the low geomagneticactivity that occurred during the Maunder Minimum.