Title: Non Periodic Variations of the Solar Diameter
Author: Andrei, Alexandre Humberto
Affil: Observatorio Nacional, Brazil
Email: oat1@on.br
Authors: Penna, Jucira L.; Reis Neto, Eugenio; D'Avila, Victor A.; de Almeida, Waldenio G. and Jilinski, Evgueni G.
Abstract: Solar diameter variation results, covering the years 1998 to 2000, are presented. In the program, observations are taken in morning and afternoon sessions, each of them with about 16 independent measurements. The observations are made with a CCD Solar Astrolabe, thus timing the limbs crossing trough a fixed zenith distance. The crossing is recorded in 46 frames, dated to the tenth of milli second. The accuracy of a single measurement typically is 100mas.
An instrumental correction was derived for drifts of the observational zenith distance. Likewise, an empirical correction describes variations on the observational conditions. Both type of corrections were found as small as a few mas. This indicates that the systematics are much smaller than random errors affecting the results. It is also to be noticed that the corrections have been obtained independently for the morning and afternoon sessions, giving similar coefficients. The series of results were obtained with three different criteria of relaxation for the points adjusting the solar limb. The results are correlated to better than 0.9. Finally, flat field corrections were obtained from sky pointings, solar pointings, and white screen pointings. The flat field correction was applied to 200 observations (evenly divided between morning and afternoon). The new values obtained show the standard deviation diminished by 20mas, without noticeable changing on the diameter variations.
The resulting series reveals a large coherence for the morning and afternoon sessions. Non periodic variations can be asserted, spanning up to 1 year and with amplitude up to 100mas. A quasi-linear increasing of about 50mas is also present in the data. Periodic variations commensurable with the solar rotation and with the sunspot life characteristic time are also seen, but with amplitude one magnitude smaller. A departure from the figure of a regular disc at mid latitudes can be fitted with amplitude of a few mas.