Studies of the radial gradient of rotation near the solar surface
by Atefeh Barekat
Date of Examination:2017-03-24
Date of issue:2018-02-16
Advisor:Prof. Dr. Laurent Gizon
Referee:Prof. Dr. Laurent Gizon
Referee:PD Dr. Olga Shishkina
Persistent Address:
http://dx.doi.org/10.53846/goediss-6732
Files in this item
Name:Doctoralthesis_AtefehBarekat_2017.pdf
Size:3.55Mb
Format:PDF
The following license files are associated with this item:
Abstract
English
Continuous observations of solar oscillations over the last twenty years have provided an opportunity to investigate interior properties of the Sun and their variations over the last two solar cycles. In this thesis, I focus on measuring the radial gradient of the rotation rate close to the surface of the Sun and its variations during solar cycle 23 and 24. For this study, I used for the first time f mode frequency splitting data obtained from 360-day time series of Medium-l data from both the Michelson Doppler Imager (MDI: 1996-2010) on board the Solar and Heliospheric Observatory and the Helioseismic and Magnetic Imager (HMI: 2010-2015) on board the Solar Dynamics Observatory to study the near-surface shear layer. Furthermore, I used reprocessed Medium-l MDI and HMI data obtained from 72-day time series that contain less systematic errors than those used in previous studies. In contrast to previous studies, I found that the logarithmic radial gradient of the rotation rate does not change sign at 55◦ latitude. Rather, I found that the value of the radial gradient stays negative and varies between approximately −1 and −0.9 from the equator up to about 50◦ latitude. Above 50◦ latitude the measurements were found to be unreliable based on the discrepancies between the results obtained from different data sets. These include differences between the results obtained from modes in common between 72-day and 360-day time series from each instrument separately. Moreover, the mentioned comparison of HMI data showed that the data obtained from 72-day time series are likely the ones that suffer from systematic errors that are currently not understood. Furthermore, I measured the radial gradient obtained from recently released MDI full-disk data which have been processed differently than Medium-l data. The results showed that the values of the radial gradient at low latitudes are the same as previous measurements to within 1σ. These measurements are in agreement with those predicted by the Kitchatinov and Rüdiger (2005) differential rotation model. By comparing the gradient relative to its nineteen year (1996-2015) time averaged value, I found that it varies by about ±10% and closely follows the solar activity. The change in the radial gradient with solar cycle may provide a means of measuring the subsurface magnetic field as suggested by Kitchatinov (2016).
Keywords: helioseismology – Sun: interior – Sun: rotation