Space Climate 7 Meeting Abstract
Solar activity parameters from the sunspot observations by Horrebow
Senthamizh Pavai Valliappan (Leibniz Institute for Astrophysics Potsdam)
Rainer Arlt [1], Christoffer Karoff [2,3], Carsten Sønderskov Jørgensen [3] Affiliation: [1] Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany, [2] Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000, Aarhus C, Denmark, [3] Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000, Aarhus C, Denmark
The sunspot observations carried out by Christian Horrebow at Copenhagen observatory are currently being analyzed as part of reconstructing solar activity parameters for the historical solar cycles before the 20th century. Horrebow led a team of observers during 1761 – 1777 and their sunspot observations are recorded as tables containing positional measurements and as sunspot drawings, which are included often but not for all observations. The recorded sunspot observations cover the solar cycles 1 (maximum), 2, and 3 (beginning) and have been already analyzed by Thiele (1859), Wolf (1873), and Hoyt & Schatten (1995) but to study only numbers of spots or groups. Our studies concentrate on the extraction of heliographic sunspot positions and areas and on the calculation of sunspot group tilt angles. The sunspot observation tables contain the sidereal transit time of sunspots for the right ascension axis and a number of instrumental screw turns for the declination axis, which were recorded subsequently. We assimilated those sunspot positions into a data file, converted them into heliographic coordinates, and the butterfly diagram was constructed. However, the sunspot areas are not available in the observational record. The nature of sunspot groups (bipolar or unipolar) was determined manually, through the analysis of the evolution of sunspot groups over days, and then the tilt angle of sunspot groups are calculated for the probable bipolar groups. With the sunspot areas, area-weighted tilt angles can also be calculated. The calculated mean tilt angle for the solar cycle 2 is consistent with the mean tilt angle obtained for the same solar cycle from another sunspot record by Staudacher. The acquired sunspot positions, areas, and sunspot group tilt angles will be helpful in the long term solar activity studies and their applications.
Mode of presentation: poster