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Decrease in heliospheric magnetic flux in this solar minimum: Recent Ulysses magnetic field observations
Author(s) -
Smith Edward J.,
Balogh Andre
Publication year - 2008
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2008gl035345
Subject(s) - physics , solar wind , ecliptic , polar , latitude , solar minimum , flux (metallurgy) , astrophysics , heliosphere , heliospheric current sheet , solar maximum , magnetic field , equator , interplanetary magnetic field , solar cycle , astronomy , materials science , quantum mechanics , metallurgy
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, B R , multiplied by the square of the radial distance, r, is independent of latitude at both solar minimum and maximum. Measurements of r 2 B R 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, B P , has decreased by a factor of about two between the previous and present latitude scans. Ulysses measurements show that the average value of r 2 B R 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 B R 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 B R 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 B R returns to the same value of ≈ 3 nT at solar minimum. The ratio of B P to the expansion factor, f E , is proportional to the measured open flux and observed and assumed values of B P are used to determine the corresponding values of f E . Another property of the fast solar wind is that it is highly turbulent compared to lower latitudes. To determine if the decrease in r 2 B R and B P 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.