Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmgc31b0351s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #GC31B-0351
Physics
7524 Magnetic Fields, 7536 Solar Activity Cycle (2162), 7537 Solar And Stellar Variability (1650), 7538 Solar Irradiance, 7549 Ultraviolet Emissions
Scientific paper
Recent work suggests that the Heliospheric Magnetic Field (HMF) strength, B, at each sunspot minimum varies but little (less than a nT). The variation of B within a solar cycle seems to be due to extra (and likely closed) magnetic flux added by Coronal Mass Ejections (CMEs) riding on top of a "floor" of somewhere between 4 and 5 nT, leading to the conclusion that the open magnetic flux is nearly constant with time, and that, in particular, there is no secular variation of the open flux. B inferred from geomagnetic data back to the 1840s further support this conclusion. In fact, B for the current cycle 23 matches well B for cycle 13, 107 years earlier. The amplitude rY of the diurnal variation of the geomagnetic Y-component is an excellent proxy for the F10.7 radio flux and thus also for the EUV flux (more precisely, the FUV, as the Sq current flows in the E layer). As for the HMF there seems to be a "floor" in rY and hence in F10.7 and hence in the FUV flux, thus the geomagnetic evidence is that there has been no secular change in the background solar minimum EUV (FUV) flux in the past 165 years. Direct measurements (although beset by calibration problems) of the Total Solar Irradiance (TSI) from satellites have only been available for 30 years and indicate that solar irradiance increases with solar activity. Correlating mean annual TSI and sunspot numbers allows one to estimate the part of TSI that varies with the sunspot number. If TSI only depends linearly on the sunspot number then irradiance levels during the Maunder Minimum would be similar to the levels of current solar minima. But TSI is a delicate balance between sunspot darkening and facular brightening, and although both of these increase (in opposite directions) with increasing solar activity, it is not a given that there could not be secular variations in the relative importance of these competing effects. Reconstructions of TSI, all postulate a source of long-term irradiance variability on centennial time scales. Each group of researchers have their own preferred additional source of changes of the "background" TSI, such as evidence from geomagnetic activity, open magnetic flux, ephemeral region occurrence, umbral/penumbral ratios, and the like. The existence of "floors" in HMF and FUV over ~1.6 centuries argues for a lack of secular variations of these parameters on that time scale. I would suggest that the lack of such secular variation undermines the circumstantial evidence for a "hidden" source of irradiance variability and that there therefore also might be a floor in TSI, such that TSI during Grand Minima would simply be that observed at current solar minima. This obviously has implications for solar forcing of terrestrial climate.
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