Data of magnetic storms and term-day observations from the Prague-Clementinum observatory (1839-1849)

We provide in digital form unique data observed at Prague-Clementinum in years 1839-1849. The data were obtained by digitising and processing the original old records published in the yearbooks of this historical observatory. The data contain two categories of high cadence observations of magnetic declination and horizontal intensity. The first of these categories captures 73 magnetic storms with their magnitude being of at least a moderate level. The second category is the dense observations during the days that had been agreed for joint measurements by the observatories organised in the Göttingen Magnetic Union (GMU), these measurements being known as term-day observations. Whereas four terms per year were set by GMU, many observatories agreed to carry out additional observations in the eight remaining months. The term-day observations also continued being performed for several years after the end of the GMU activities in 1841. Data of 120 term-days from January 1840 to December 1849 are published. The observations started at 10 p.m. of Göttingen Mean Time and lasted 24 hours. The interval between observations was 5 minutes (in April, June and July 1842 exceptionally 6 minutes). All time data were transformed into UT, based on the longitude of the Göttingen observatory, which was 9.950°. The time shift is thus 39'48''. The date in the file name of magnetic storm indicates the start day of the event, whereas the date in the term-day file name indicates the second day which involves substantial part of the observations (22 out of 24 hours).