Declining solar polar fields and their signatures in the solar wind: Implications to near earth space

Abstract

Studies in recent times have reported that solar high latitude fields have been steadily declining since around 1995. The previous solar cycle 23 has also been unusual in that, it experienced one of the deepest solar minima in the past 100 years with over 70% of the days in 2007 and 2008 being entirely spotless. We have made a detailed study of the effect of such a steady and prolonged decline in solar photospheric magnetic fields on the solar wind using interplanetary scintillation (IPS) observations of solar wind micro-turbulence levels. Our study, covering solar cycles 21, 22 and 23, has also shown a steady decline in solar wind micro-turbulence levels, in the distance range 0.2 to 0.8 AU, in sync with the declining solar photospheric fields. These results beg the question as to whether we are headed towards a long period of little or no sunspot activity, in a manner similar to the period between 1645 and 1715 known as the Maunder minimum, when the sun was completely devoid of sunspots. We have estimated the expected sunspot number around the maximum of solar cycle 25 and find that cycle 25 will be much weaker than the current cycle 24 which had a peak sunspot number in November 2013 of around 75. We have also looked for signatures of the declining solar wind micro-turbulence levels in the Earth's ionosphere and our study indicates that the night time ionospheric cut-off could reduce to well below 5 MHz if the decline in the solar magnetic fields continues beyond 2020.