Measurements of Solar Oblateness during the SDO Mission
Abstract
Beginning in April 2010, the Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory (SDO) spacecraft has been making periodic measurements of the solar shape. The primary observations are 4096 by 4096 pixel full Sun images taken in the continuum of the 617.3 nm Fe I absorption line in 4 linear polarizations. It is necessary to determine the instrument optical distortion in order to extract the solar shape from the full Sun images. This is accomplished during a roll maneuver of the SDO spacecraft in which the spacecraft is rotated 360 degrees around the Sun-spacecraft line while taking a series of images at 32 uniformly spaced roll angles. Measurements of the solar oblateness are typically obtained twice per year, and eighteen roll maneuvers have been performed by the SDO spacecraft to date. Initially these observations were taken in April and October from 2011 to 2014. During the April 2015 roll, however, the spacecraft maneuver was aborted due to a pointing anomaly. This error condition was identified, but subsequent roll maneuvers were shifted to January and July of the following years. The mean equator to pole radius difference over the nine years of observations is 6.0 +/- 1.0 milli-arcseconds. The higher order (hexadecapole) term is consistent with 0. The long term trend of the solar oblateness does not show a correlation with the current solar sunspot cycle. Details of the measurements and trending will be discussed.
- Publication:
-
Catalyzing Solar Connections
- Pub Date:
- November 2018
- Bibcode:
- 2018csc..confE...5B