Mount Wilson synoptic magnetic fields: Improved instrumentation, calibration, and analysis applied to the 2000 July 14 flare and to the evolution of the dipole field

摘要

This paper describes the current status of the 150 foot solar tower telescope program of synoptic observations with an emphasis on the magnetic field data. A newly installed 24-channel system permits routine intercomparison of magnetic fields measured by the lambda676.8 nm line used by the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory and the lambda525.0 nm line used by the 150 foot tower. Two important calibration procedures for treatment of saturation and zero-point offset are described. It is demonstrated that solar rotation can be used to extract the east-west component of the slowly evolving, large-scale magnetic field in a stable fashion. This same analysis produces maps of the neutral line configurations that are well correlated with the positions of quiescent prominences. The analysis is applied to the 2000 July 14 are and shown to demonstrate that the field was sheared due to the westward-moving intrusion of a region just north of the neutral line along which the flaring occurred. A new method for preparing synoptic charts by averaging without differential rotation smearing is presented. These synoptic charts are combined into a new format termed a supersynoptic chart, which makes possible the identification of systematic long-term trends in the magnetic field evolution. Based on these charts, distinct large-scale events of magnetic field bias opposing the old-cycle dipole field are seen. A statistical method using the skewness in the distribution of the polarity bias as a function of longitude is developed. The coincidence between pulses in this skewness and times of rapid change in the Sun's dipole moment is consistent with the idea that a tilt in the orientation of bipolar magnetic regions is responsible for the dipole field reversal. The pulses in skewness are large and limited in number, suggesting the operation of a large-scale instability such as the kink instability. [References: 68]