Excess open solar magnetic flux from satellite data:2. A survey of kinematic effects

摘要

We investigate the "flux excess" effect, whereby open solar flux estimates fromspacecraft increase with increasing heliocentric distance. We analyze the kinematiceffect on these open solar flux estimates of large-scale longitudinal structure, in the solarwind flow, with particular emphasis on correcting estimates made using data from near-Earth satellites. We show that scatter, but no net bias, is introduced by the kinematic"bunching effect" on sampling and that this is true for both compression and rarefactionregions. The observed flux excesses, as a function of heliocentric distance, are shown to beconsistent with open solar flux estimates from solar magnetograms made using thepotential field source surface method and are well explained by the kinematic effect ofsolar wind speed variations on the frozen-in heliospheric field. Applying this kinematiccorrection to the Omni-2 interplanetary data set shows that the open solar flux at solarminimum fell from an annual mean of 3.82 x 1016 Wb in 1987 to, close to half thatvalue (1.98 x 1016 Wb) in 2007, making the fall in the minimum value over the last twosolar cycles considerably faster than the rise inferred from geomagnetic activityobservations over four solar cycles in the first half of the 20th century.