Science from our data

Once back in Birmingham the data are processed to extract velocity information from the surface of the Sun. The plot below shows what a typical day’s data looks like from one of our stations. The x-axis gives the time in Universal Time and the y-axis represents the measured intensity of the light received by the detectors in one of our spectrometers. As the day progresses the intensity steadily increases as the sun rises, flattens off around midday and falls away towards sunset. This particular day was from our Sutherland station and appears to have had a fairly clear morning and a cloudy afternoon with sharp drop in intensity at about 15.5 Hours UT.

We are primarily interested in extracting the Doppler information from the solar surface. To do this we need a way of converting from the intensity information above into a velocity measurement. We take the intensity as measured at the blue and red wings of the solar line, IB and IR respectively, and we then construct the following relation:

R = (IB - IR) / (IB + IR)

The (IB + IR) on the bottom acts to normalise the data so that variations in one detector will be reflected in the other. I we produce a plot of R we get something which looks like the following:

We then multiply the ratio by a factor which has the effect of converting the above plot into one with units of velocity. Finally a curve is fitted to the measured velocity ratio and then subtracted off it. What remains are our velocity residuals.