Thursday, July 17, 2008

Lowest Solar Flux Ever Measured?

Scientists can't decide if this depressing solar cycle just recorded the most depressingly low solar radio flux ever measured.

July 16's reading was a staggeringly low 64.2 sfu (solar flux units), according to the Canadian observatory which makes this measurement. If this holds up, it will be the lowest since this data series began around 1950.

The "Solar Flux" we talk about is an uncorrected measurement of solar radio noise taken three times daily, on a standard frequency of 2800 MHz (10.7 cm). This is an important frequency in radio astronomy, as it comes from a certain hyperfine quantum transition in hydrogen gas excited by solar magnetic flux that also causes "sunspots." It is a much newer data series than the "sunspot number" which is derived from a number of amateur observers worldwide who literally count spots on the sun's disk.

They haven't had much to count recently, as the sun is still in a very deep minimum at the end of a very long cycle (#23).

The most commonly used formula relating solar flux to sunspot numbers in an average case does not allow values lower than around 67, which is considered a completely quiet sun. However, lower values do occur in the daily uncorrected observations. Before this summer's reading, the record low was a 64.7 observed on July 18, 1996.

What's up here? Well, note that both lows were in mid-July, when the Earth is at aphelion (the farthest point from the sun). This wouldn't be an issue if the Earth's orbit were completely circular, but of course like most orbits it's slightly elliptical. Therefore the greatest downward deviation from the average quiet sun case is in summer - and that's when we read these low uncorrected solar fluxes. It's also why the solar flux is corrected to take the Earth's orbit into account.

Presumably when the corrected flux comes out, it'll be a little less depressing. Not a lot, but just a little.