The general idea (and final formula) is given in the following figure. The extraction of the tangential velocity requires a determination of the associated Doppler shift of the Na line. The redundancy allows lots of averaging to improve the data quality. The information in each curve is actually redundant allowing, in this case, 22 determinations of the tangential velocity in the equatorial latitude zone. The figure above shows intensity variation curves plotted for the latitude = zero (equatorial) zone for 9 out of the 22 frames in the spectral series. The (linear) variation of east-west intensity arises from the Doppler effect on the Na D2 line and allows the determination of the line-of-sight (tangential) velocities at points on the east and west solar limbs using the difference in east-west intensity (determined by the fits). Some example intensity variations are shown in the next figure along with linear fits. I then averaged over the vertical (latitude) direction in each zone to map out the horizontal (east-west) intensity profiles for each zone and for each frame in the spectral series.
#Measure differential rotation of sun jhelioviewer series#
The next figure shows one frame (#06) of the spectral series divided up into latitude zones of 13 degrees width each. In fact, if you plot this average disk intensity, it looks like this:Ī neat Na D2 line profile is mapped out, fit quite nicely with a Lorentzian lineshape. The average intensity over the disk clearly varies as you move through the Na D2 absorption line. The numbering identifies the various frames in the spectral series (16 of 22 frames are shown). Such a spectral series of spectroheliograms is shown here: Spectroheliograms were generated at a series of wavelengths through the Na D2 line, stepping through 0.033 Angstrom increments. Here is a spectrum taken with the spectroheliograph (SHG): The present work uses spectroheliograms generated around the Na D2 line at 5890 Angstroms. Here, I'm using a variation of the spectroscopic technique that occurred to me when I noticed a systematic east-west intensity gradient in spectroheliograms measured through a spectral line. Another technique uses spectroscopy to measure photospheric velocities via the Doppler effect on spectral lines observed at the east (approaching) and west (receding) solar limbs. One technique used to measure this is the observation of sunspot motions over a longer term period as a function of solar latitude. A result of this differential motion is that the period of solar rotation varies (gets longer) with increasing solar latitude or, in other words, the angular velocity of a point on the photospheric surface decreases with increasing latitude. Here are the results of an experiment to measure the differential rotation of the Sun.
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