Spectral Line Selection for HMI: A Comparison of Fe I 6173 and Ni I 6768

We present a study of two spectral lines, Fe I 6173 Angstroms and Ni I 6768 Angstroms, that were candidates to be used in the Helioseismic and Magnetic Imager (HMI) for observing Doppler velocity and the vector magnetic field. The line profiles were studied using the Mt. Wilson Observatory, the Advanced Stokes Polarimeter and the Kitt Peak McMath telescope and one meter Fourier transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua and no blends that threaten instrument performance. The Fe I line is 2% deeper, 15% narrower and has a 6% smaller equivalent width than the Ni I line. The potential of each spectral line to recover pre-assigned solar conditions is tested using a least-squares minimization technique to fit Milne-Eddington models to tens of thousands of line profiles that have been sampled at five spectral positions across the line. Overall, the Fe I line has a better performance than the Ni I line for vector magnetic field retrieval. We selected the Fe I spectral line for use in HMI due to its better performance for magnetic diagnostics while not sacrificing velocity information

Professional learning for distributed leadership:Primary headteachers’ perspectives

This article draws from a small-scale study of headteachers motivated to positively impact on the quality of pupil experience by involving all staff in a distributed perspective on leadership. Each headteacher perceived leadership as involving learned processes requiring support and experience, expending considerable effort in providing a fertile environment for learning about its practice. This perspective developed from their personal experience of challenging established leadership orthodoxies prior to and since appointment to headship. The article explores the impact of formal work-based postgraduate leadership preparation and experiential professional learning on each headteacher’s understandings of distributed leadership and its practice. It then explores the ways in which they supported the professional learning of staff. The article concludes by suggesting that headteachers and staff encounter a range of challenges in developing school practices inherent in distributed leadership and can benefit from ongoing support with informed reflection on practice beyond initial preparation for headship

Spectral line selection for HMI

We present information on two spectral lines, Fe I 6173 Å and Ni I 6768 Å, that were candidates for use in the Helioseismic and Magnetic Imager (HMI) instrument. Both Fe I and Ni I profiles have clean continuum and no blends that threaten performance. The higher Landé factor of Fe I means its operational velocity range in regions of strong magnetic field is smaller than for Ni I. Fe I performs better than Ni I for vector magnetic field retrieval. Inversion results show that Fe I consistently determines field strength and flux more accurately than the Ni I line. Inversions show inclination and azimuthal errors are recovered to ≈ 2° above 600 Mx/cm2 for Fe I and above 1000 Mx/cm2 for Ni I. The Fe I line was recommended, and ultimately chosen, for use in HMI