Geomagnetic Disturbances with Sudden and Gradual Commencements in 1869–1954 According to Observatory Data and Catalogs of Storms

Abstract

Observations of variations in the geomagnetic field and the magnetic activity indices based on these observations are the only source of information about “geomagnetic weather” in the late 19th—first half of the 20th century. The aa indices, calculated from 1868, allow us to distinguish periods of magnetic storms of different magnitude and duration. Information on disturbances can be obtained directly from observational data at observatories, as well as from storm catalogs compiled from local observations on some of them. To understand how the geomagnetic activity changed during eight 11-year solar cycles (1869–1954), we use the aa-indices, the magnetograms and the catalogs of storms. On the basis of aa-indices we select periods during which the maximum value of aa-index (aa_max) exceeds 30 (K_{pa_max} > 3) for more than three hours. These periods are considered disturbed, and it is taken into account that within themselves they could have quiet intervals with a duration of up to 18 h. Depending on the values of aa_max, each event is assigned a corresponding rank. Data on sudden commencement (SC) storms help to distinguish storms with SC among selected disturbances. The resulting sampling divides the disturbances by magnitude and by type of commencement—sudden or gradual (GC). A comparison of the obtained list with the catalogs data shows that the latter, as a rule, includes disturbances with aa_max > 95 (K_{pa_max} > 5) only; smaller events are not included. Some storms in the directory are wrongly attributed to disturbances with GC, not SC. This is indicated by the data of the endorsed list of SCs (http://www.obsebre.es/en/rapid) and our review of the magnetograms of the observatories of St. Petersburg and Pavlovsk; they show almost all SCs from the above list. Our catalog includes 4619 storms: 1340 with SC and 3279 with GC. Using it, we have found out how the number of storms of different magnitude (with SC and GC) changed depending on the number of sunspots and at different phases of solar cycles. It is shown, in particular, that during the period considered there was a faster growth of storms with GC than with SC.