3.3 10.7 cm solar flux
The 10.7 cm Solar Flux is the disk integrated emission from the Sun at the radio wavelength of
10.7 cm (2800 MHz) (cf. Tapping and Charrois, 1994). This measure of solar activity has
advantages over sunspot numbers and areas in that it is completely objective and can be made
under virtually all weather conditions. Measurements of this flux have been taken daily by the
Canadian Solar Radio Monitoring Programme since 1946. Several measurements are taken each
day and care is taken to avoid reporting values influenced by flaring activity. Observations
were made in the Ottawa area from 1946 to 1990. In 1990 a new flux monitor was installed at
Penticton, British Columbia and run in parallel with the Ottawa monitor for six months before
moving the Ottawa monitor itself to Penticton as a back-up. Measurements are provided daily
The relationship between the 10.7 cm radio flux and the International Sunspot Number is somewhat
more complicated than that for sunspot area. First of all the 10.7 cm radio flux has a base level of
about 67 solar flux units. Secondly, the slope of the relationship changes as the sunspot number
increases up to about 30. This is captured in a formula given by Holland and Vaughn (1984) as:
In addition to this slightly nonlinear relationship there is evidence that the 10.7 cm radio flux lags behind
the sunspot number by about 1-month (Bachmann and White, 1994).
Figure 9: 10.7cm Radio Flux vs. International Sunspot Number for the period of August 1947 to
March 2009. Data obtained prior to cycle 23 are shown with filled dots while data obtained during
cycle 23 are shown with open circles. The Holland and Vaughn formula relating the radio flux to the
sunspot number is shown with the solid line. These two quantities are correlated at the 99.7% level.
Figure 9 shows the relationship between the 10.7 cm radio flux and the International Sunspot Number.
The two measures are highly correlated (r = 0.995, r2 = 0.990). The Holland and Vaughn formula fits the
early data quite well. However, the data for cycle 23 after about 1998 lies systematically higher than the
levels given by the Holland and Vaughn formula.