The Solar Radio Microwave Flux and the Sunspot Number

Details The Solar Radio Microwave Flux and the Sunspot Number The Solar Radio Microwave Flux and the Sunspot Number

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh13c..03s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SH13C-03

Astronomy and Astrophysics

Astrophysics

[7524] Solar Physics, Astrophysics, And Astronomy / Magnetic Fields, [7534] Solar Physics, Astrophysics, And Astronomy / Radio Emissions, [7536] Solar Physics, Astrophysics, And Astronomy / Solar Activity Cycle, [7537] Solar Physics, Astrophysics, And Astronomy / Solar And Stellar Variability

Scientific paper

Since 1947 the flux of microwaves from the Sun at wavelengths between 3 and 30 cm [frequencies between 10 and 1 GHz] has been routinely measured. This emission comes from both thromosphere and the corona and has two main sources: thermal bremsstrahlung (free-free emission) and thermal gyroradiation. These mechanisms give rise to enhanced radiation when the density and magnetic field increase, so the microwave radiation is a good measure of general solar activity. Strong magnetic fields occur in the network and can persist for weeks or longer; hence there is a strong rotational signal in the emission superposed on a solar cycle variation of the background coronal signal. The radio flux measurements can be calibrated absolutely and are not very sensitive to observing conditions, and in principle have no personal equation. They may thus be the most objective measure of solar activity, and our many decades-long flux record could throw light on the important issue of the long-term variation of solar activity. The longest series of observations F10.7, begun by Covington in Ottawa, Canada in April 1947 and maintained to this day. Other observatories also have long and continuing series of measurements of the microwave flux. One can now ask how this measure of solar activity compares to other measures, in particular the sunspot number. We correlate the sunspot number against the F10.7 flux for the interval 1951-1988, and obtain a good polynomial fit (R^2 = 0.977) up until ~1989.0 after which time the observed sunspot number falls progressively below the fitted number. Three obvious hypotheses present themselves: 1) The sunspot counting procedure or observers have changed, with resulting artificial changes of the sunspot number as they have in the past. 2) Physical changes in the corona or chromosphere have occurred. 3) Livingston & Penn’s observations that the sunspots are getting warmer during the last decade, leading to a decreased contrast with the surrounding photosphere and hence lessened visibility, possibly resulting in an undercount of sunspots. The near constancy of the flux at minima since 1954 argues against a change of the physical conditions at the source locations, leaving the exciting possibility that Livingston & Penn may be correct.

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