Conservation of open solar magnetic flux and the floor in the heliospheric magnetic field

Abstract

The near‐Earth heliospheric magnetic field intensity, |B|, exhibits a strong solar cycle variation, but returns to the same “floor” value each solar minimum. The current minimum, however, has seen |B| drop below previous minima, bringing in to question the existence of a floor, or at the very least requiring a re‐assessment of its value. In this study we assume heliospheric flux consists of a constant open flux component and a time‐varying contribution from CMEs. In this scenario, the true floor is |B| with zero CME contribution. Using observed CME rates over the solar cycle, we estimate the “no‐CME” |B| floor at ∼4.0 ± 0.3 nT, lower than previous floor estimates and below |B| observed this solar minimum. We speculate that the drop in |B| observed this minimum may be due to a persistently lower CME rate than the previous minimum, though there are large uncertainties in the supporting observational data.