Reliability of sunspots as tracers of solar surface rotation
Abstract
NEW methods of measuring solar rotation have produced significantly different rotation speeds to those derived from sunspots1,2. Spectroscopic measurements of Doppler displacements give a so-called plasma rotation velocity of 13.76° d-1 (sidereal) at the solar equator3 as against 14.38 ° d-1 (sidereal) from sunspot proper motion velocity4,5. Other techniques of measuring solar rotation lead to differences in the differential rotation from a strong variation of the solar rotation with latitude for sunspots4 and photospheric plasma3 to hardly any variation at all for coronal holes6. Our understanding of these differences is very poor. It is therefore appropriate to obtain additional accurate solar rotation measurements with as many different techniques as possible and to check the validity of the assumption that the proper motion of tracers, like sunspots, correspond to the actual motion of matter on the solar surface. We report here the result of plasma rotation velocity measurements inside sunspot umbrae and their relation to the rotation velocities derived from spot proper motions. We find that plasmas inside and outside the spots rotate at similar rates, and conclude that sunspots make poor tracers.
- Publication:
-
Nature
- Pub Date:
- March 1976
- DOI:
- 10.1038/260227a0
- Bibcode:
- 1976Natur.260..227B
- Keywords:
-
- Angular Velocity;
- Plasma Dynamics;
- Solar Rotation;
- Sunspots;
- Umbras;
- Line Spectra;
- Photosphere;
- Plasma Spectra;
- Solar Spectra;
- Solar Wind;
- Titanium;
- Tracers;
- Velocity Measurement;
- Solar Physics