The Solar Oblateness at Solar Minimum as Observed by RHESSI/SAS

The RHESSI solar aspect sensor (SAS) has provided oblateness measurements of the optical solar disk with unprecedented precision. SAS measures the optical solar profile at 670 nm in the red continuum. SAS consists of three spacially uniformly distributed linear CCDs mounted on a rotating spacecraft; a crucial ingredient in access of calibration parameters. From the SAS data, differential radius measurements can be derived at about 100 Hz including about 1000 full CCD readouts per day for calibration purposes. For a three month period during the active phase of the solar cycle in 2004, the shape of the solar disk has been measured discovering an apparent excess oblateness which we attributed to the enhanced network. In order to avoid confusion between magnetic activity and a correlated brightness enhancement in the SAS signal at 670 nm, the SAS data has been masked using the SOHO/EIT284A data. The measured oblateness as function of the masking level is then extrapolated for a value of the underlaying, presumably non-magnetic sun. Here, we present the analysis of the RHESSI/SAS data during the solar minimum with the inferred interpretation for the oblateness signal.