Search

Context. Systematic observations of magnetic field strength and polarity in sunspots began at Mount Wilson Observatory (MWO), USA in early 1917. Except for a few brief interruptions, this historical dataset continues till present. Aims. The sunspot field strength and polarity observations are critical in our project of reconstructing the solar magnetic field over the last hundred years. Here we provide a detailed description of the newly digitized dataset of drawings of sunspot magnetic field observations. Methods. The digitization of MWO drawings is based on a software package develope d by us. It includes a semi-automatic selection of solar limbs and other features of the drawing, and a manual entry of the time of observations, the measured field strength and other notes hand-written on each drawing. The data are preserved in a MySQL database. Results. We provide a brief history of the project and describe the results from digitizing this historical dataset. We also provide a summary of the final dataset, and describe its known limitations. Finally, we compare the sunspot magnetic field measurements with other instruments, and demonstrate that, if needed, the dataset could be continued using modern observations such as, for example, Vector Stokes Magnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) platform., Comment: 14 pages, 12 figures, 4 tables, Astronomy and Astrophysics, accepted

Early telescopic observations of sunspots were conducted with instruments of relatively small aperture. These instruments also suffered from a higher level of scattered light, and the human eye served as a "detector". The eye's ability to resolve small details depends on image contrast, and on average the intensity variations smaller than $\approx$ 3\% contrast relative to background are not detected even if they are resolved by the telescope. Here we study the effect of these three parameters (telescope aperture, scattered light, and detection threshold of human vision) on sunspot number, group number, and area of sunspots. As an "ideal" dataset, we employ white-light (pseudo-continuum) observations from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics Observatory, and we model the appearance of sunspots by degrading the HMI images to corresponding telescope apertures with an added scattered light. We discuss the effects of different parameters on sunspot counts and derive functional dependencies, which could be used to normalize historical observations of sunspot counts to common denominator., Comment: Accepted for publication in MNRAS, 7 pages, 6 figures, 2 tables

We investigate gradient of magnetic field across neutral lines (NLs), and compare their properties for NLs with and without the chromospheric filaments. Our results show that there is a range of preferred magnetic field gradients where the filament formation is enhanced. On the other hand, a horizontal gradient of magnetic field across a NL alone does not appear to be a single factor that determines if a filament will form (or not) in a given location., Comment: 11 pages, 8 figures