Thanks for the links guys. Still digesting/thinking on both. Been really busy this week, but I'll be back this weekend more
Found a good blurb from solar scientist Leif Svalgaard in the solarcycle24.com guestbook... it's got some really good findings, I just
to post it for future references.
Definitely check out his site @ www.leif.org/research - and I recommend keeping wikipedia open in a different internet browser tab for word/acronym consultation.
from Leif Svalgaard, post #466 @ http://www.a-free-guestbook.com/guestbook....me=solarcycle24There are five lines of evidence forming the argument that the sun has not been any dimmer in the past. And by ’past’ I mean ’recent past’ [thousands, not billions of years - when the Earth was formed, the sun was 25% dimmer; it has slowly increased in brightness and will continue to do so over billions of years].
Line 1;
The Total solar Irradiance (TSI) does have several sources. The first and most important is simply the temperature in the photosphere. The hotter the sun, the higher the TSI. Some spectral lines are VERY sensitive to even minute changes in temperature. Livingston et al. has very carefully measured the line depth of such temperature-sensitive lines over more than 30 years spanning three solar cycles [Sun-as-a-Star Spectrum Variations 1974-2006, W. Livingston, L. Wallace, O. R. White, M. S. Giampapa, The Astrophysical Journal, Volume 657, Issue 2, pp. 1137-1149, 2007, DOI; 10.1086/511127]. They report [and I apologize for the somewhat technical turn my argument is taking, but if you really want to know, there is no avoiding this], "that both Ca II K and C I 5380A intensities are constant, indicating that the basal quiet atmosphere is unaffected by cycle magnetism within our observational error. A lower limit to the Ca II K central intensity atmosphere is 0.040. This possibly represents conditions as they were during the Maunder Minimum [their words, remember]. Within our capability to measure it using the C I 5380A line the global (Full Disk) and basal (Center Disk) photospheric temperature is constant over the activity cycles 21, 22, and 23". I have known Bill Livingston [and White] for over 35 years and he is a very careful and competent observer, so I trust his result fully.
Line 2;
Since the 1960 we have known that the sun’s surface oscillates up and down [with typical periods of ~5 minutes]. These oscillations are waves very much like seismic waves in the Earth [caused by earthquakes] and can just as earthquakes seismic waves can be used to probe the interior of the Earth, be used to probe the solar interior. There are millions of such solar waves at any given time and there are different kinds (called ’modes’) of waves. The solar p-modes are acoustic [sound waves] normal modes. You one can imagine a frequency increase with an increasing magnetic field, due to the increase in magnetic pressure raising the local speed of sound near the surface where it is cooler and where the p-modes spend most of their time. Of course one can also imagine higher frequencies may result from an induced shrinking of the sound cavity and/or an isobaric warming of the cavity. Another kind is the solar f-modes that are the eigenmodes of the sun having no radial null points [i.e. asymptotically surface waves; again I apologize for the technical mumbo-jumbo]. From the solar cycle variations of p- and f-modes [and we have now enough data from the SOHO spacecraft to make such a study] we now have an internally consistent picture of the origin of these frequency
changes that implies a sun that is coolest at activity maximum when it is most irradiant. Now, how can that be? How can a cooler [overall, including the cooler sunspots, for instance, as the temperature of the non-magnetic areas of the sun didn’t change {see line 1 above}] sun radiate more? It can do that, if it is bigger! The change in the radius of the Sun from minimum to maximum is about 1 km. Goode and Dziembowski (Sunshine, Earthshine and Climate Change I. Origin of, and Limits on Solar Variability, by Goode, Philip R. & Dziembowski, W. A., Journal of the Korean Astronomical Society, vol. 36, S1, pp. S75-S81, 2003) used the helioseismic data to determine the shape changes in the Sun with rising activity. They calculated the so-called shape asymmetries from the seismic data and found each coefficient was essentially zero at activity minimum and rose in precise spatial correlation with rising surface activity, as measured using Ca II K data from Big Bear Solar Observatory. From this one can conclude that there is a rising corrugation of the solar surface due to rising activity, implying a cooler and smaller active sun, whose increased irradiance is totally due to activity induced corrugation, that makes the sun a better radiator [that is why radiators have ribs]. This interpretation has been recently observationally verified by Berger et al. (Berger, T.E., van der Voort, L., Rouppe, Loefdahl, M., Contrast analysis of Solar faculae and magnetic bright points. Astrophysical Journal, vol. 661, p.1272, 2007) using the new Swedish Solar Telescope. They have directly observed these corrugations. Goode & Dziembowski conclude that the Sun cannot have been any dimmer, on the time steps of solar evolution, than it is now at activity minimum.
Line 3;
Foukal et al. (Foukal, P., North, G., Wigley, T., A stellar view on solar variations and climate. Science, vol. 306, p. 68, 2004) point out the Sun’s web-like chromospheric magnetic network (an easily visible solar structure seen through a Ca II K filter) would have looked very different a century ago, if there had been a significant change in the magnetic field of the sun supposedly increasing TSI. However, there is a century of Mt. Wilson Solar Observatory Ca II K data which reveal that the early 20th century network is indistinguishable from that of today.
Line 4;
Svalgaard & Cliver have recently (A Floor in the Solar Wind Magnetic Field, by L. Svalgaard and E. W. Cliver, The Astrophysical Journal, vol. 661, L203–L206, 2007 June 1, 2007)
shown that long-term (∼130 years) reconstruction of the interplanetary magnetic field (IMF) based on geomagnetic indices indicates that the solar wind magnetic field strength [and thus that of the sun itself, from which the IMF originates] has a “floor,” a baseline value in annual averages that it approaches at each 11 yr solar minimum. In the ecliptic plane at 1 AU [at the Earth], the IMF floor is ∼4.6 nT, a value substantiated by direct solar wind measurements and cosmogenic nuclei data. We identify the floor with a constant (over centuries) baseline open magnetic flux at 1 AU of ~4x10^14 Weber, corresponding to a constant strength (∼10^11 Ampere) of the heliospheric current. Solar cycle variations of the IMF strength ride on top of the floor. They point out that such a floor has implications for (1) the solar wind during grand minima — we are given a glimpse of Maunder minimum conditions at every 11 yr minimum; (2) current models of the solar wind — both source surface and MHD models are based on the assumption, invalidated by Ulysses, that the largest scale fields determine the magnitude of the IMF; consequently, these models are unable to reproduce the high-latitude observations; and (3) the use of geomagnetic input data for precursor-type predictions of the coming sunspot maximum — this common practice is rendered doubtful by the observed disconnect between solar polar field strength and heliospheric field strength [the wrong prediction by the NASA panel for cycle 23 was based on this, and the prediction {of a high cycle} by one half of panel for cycle 24 is also partly based on this]. The constancy of the IMF also has implications for the interpretation of the Galactic cosmic ray flux.
Line 5;
But maybe it is the Ultraviolet flux that varies and affects the stratospheric ozone concentration and thereby influences the climate. I have earlier in (Calibrating the Sunspot Number using the “Magnetic Needle”, L. Svalgaard; CAWSES News, 4(1), 6.5, 2007] pointed out that the amplitude 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). There is a weak trend in the amplitude of 10% since the 1840s that can be understood as being due to an increase of ionospheric conductance resulting from the 10% decrease of the Earth’s main field. Correcting for and removing this trend then leads to the conclusion that (as for the IMF) 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. Several earlier reconstructions of TSI, reviewed in Fröhlich, C. & J. Lean (Solar Radiative Output and its Variability; Evidence and Mechanisms, Astron..& Astrophys. Rev., 12(4), 273, 2004, Doi;10.1007/s00159-004-0024-1.[6] 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 IMF and FUV over ~1.6 centuries argues for a lack of secular variations of these parameters on that time scale. The five lines of evidence discussed above 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.