Solar Physics Division Meeting 2000, June 19-22
Session 4. Magnetic Dynamo
Oral, Chair: J. T. Hoeksema, Monday, June 19, 2000, 10:30am-12:05pm, Forum

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[4.06] The Solar Magnetic Field is Not Increasing

E. Hildner (NOAA/Space Environment Center), C. N. Arge (University of Colorado/CIRES & NOAA/SEC), V. J. Pizzo (NOAA/Space Environment Center)

Lockwood, Stamper, and Wild (hereafter, LSW) conclude (Nature 399, 1999; see also Lockwood et al., Astronomy and Geophysics 40, 1999) that ``the total 'source' magnetic flux in the Sun's atmosphere has risen by 41% since 1964" and by 130% since 1901. This remarkable deduction is based upon: the observed secular increase (with superposed ripples due to solar cycles) of the aa geomagnetic index; the observed correlation between geomagnetic activity and interplanetary magnetic field strength; and, crucially, inferences that 1) interplanetary measurements in Earth's vicinity can be used to characterize the entire heliosphere, and 2) stronger interplanetary fields imply stronger solar fields. LSW ignore transient events. Line-of-sight photospheric field measurements from both Mt. Wilson (MWO) and Wilcox Solar (WSO) Observatories are available for approximately the last three and two solar cycles, respectively. (Data from MWO are available from ~1967 to present and WSO data from ~1976 to present). The data from both observatories agree well with one another during the ~24 year interval for which there are mutual observations. Our analysis of these data show no increase in the solar magnetic flux over the last three solar cycles in contradiction to that deduced by LWS. Nor does the Sun's open flux, the magnetic field escaping into interplanetary space, show a secular increase over the last three cycles, according to calculations of the flux reaching a spherical coronal ``source surface" at a height of 2.5 Rsun. Like LSW, we do not explicitly take transient events into account. We believe that the Sun's average coronal magnetic flux is not rapidly increasing now. We further believe that near-Earth interplanetary measurements can only poorly, if at all, be generalized to characterize the entire heliosphere, and that the strength of the interplanetary magnetic field is not simply related to the overall solar magnetic field strength.

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