Geophysical Research Letters

Decrease in heliospheric magnetic flux in this solar minimum: Recent Ulysses magnetic field observations

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Abstract

[1] The Ulysses spacecraft has traveled from the solar equator at 1.3 and 5.3 AU to above the polar caps at 2.2 AU three times during the last 17 years and has provided measurements of the solar–heliospheric magnetic field. The open magnetic flux, i.e., the radial component, BR, multiplied by the square of the radial distance, r, is independent of latitude at both solar minimum and maximum. Measurements of r2 BR contain information about the average polar cap field strength when allowance is made for the non-radial expansion of the magnetic field and solar wind near the Sun that eliminates the latitude gradient in magnetic pressure. Recent Earth-based magnetograph observations indicate that the Sun's polar cap field strength, BP, has decreased by a factor of about two between the previous and present latitude scans. Ulysses measurements show that the average value of r2 BR has decreased from 3.6 nT (AU)2 in 1993.5 to 1995.0 to 2.3 nT (AU)2 in 2006.1 to 2007.4, a decrease by 0.64. The two Ulysses scans are not precisely at solar minimum. However, in-ecliptic BR is highly correlated with the Ulysses measurements at all latitudes and can be used to determine the open flux at the two solar minima. Averages of BR at the two solar minima are 2.82 and 2.45 nT. This decrease is contrary to the suggestion based on previous solar cycles that BR returns to the same value of ≈ 3 nT at solar minimum. The ratio of BP to the expansion factor, fE, is proportional to the measured open flux and observed and assumed values of BP are used to determine the corresponding values of fE. Another property of the fast solar wind is that it is highly turbulent compared to lower latitudes. To determine if the decrease in r2 BR and BP has affected the intensity of the magnetic fluctuations, the total variances in the magnetic field fluctuations are derived and found to decrease by a factor of 0.75.

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