SPRG Seminars
September 7, 2010:
"Estimates of solar coronal rotation by different observing techniques"
Dr. Hari Vats, PRL (India)
Though solar rotation is known for the last four centuries, yet it has many unsolved issues still. The rotation has been measured by three methods, namely, (1) *Tracers: *using quasi permanent features on the solar surface e.g. sunspots, filaments etc. (2) *Spectroscopy:* wherein one measures Doppler shift of emission lines and (3)* Flux Modulation:* this is the method that we have used extensively on the following observations: * The disk integrated radio flux at 2.8 GHz for the period of six solar cycles. This provided long term variability. This showed that there are three components in the variability of solar rotation period thus estimated, namely, 11 year period (corresponding to solar activity cycle), 22 year period (corresponding to Hale cycle) and an irregular component.
* Simultaneous multi-frequency (11) radio emissions over 26 months. This showed that there is a differential rotation as a function altitude in the solar corona. The disk integrated measurements give no information on the latitude variation.
* Radio images at 17 GHz for the period 1999-2005 gives very interesting information about the differential rotation of the solar corona at the height of these emission. The differential profile is shallower than those for photosphere and chromospheres. There is also an asymmetry in the rotation of northern and southern hemispheres of the Sun
* YOHKOH SXT images for the period 1992 -2001 show an yearly variability of the solar rotation as a function of latitude. The average profile is even shallower than that obtained by radio images at 17 GHz. The north-south asymmetry is also weaker.
In this seminar I will present these results and discuss their similarities and differences with other estimates.