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Some recent studies point out that currents related to the auroral oval, electrojets and field aligned currents (FACs), are serious candidates for the mechanism of the intense mid-latitude magnetic storms. It is interesting to re-analyse historical data under the light of this modern knowledge. In this aim, we analysed two intense magnetic storms that were recorded by observatories Clementinum (Prague) and Greenwich on 17 November 1848 and 4 February 1872, respectively. The latter has been marked as an extraordinary event by several authors, in particular in connection with auroras. The former, however, has been little known in the space weather community. Both these events possessed swift and extensive variations of the horizontal (H) component (>400 nT and >500 nT, respectively) and were accompanied by auroras sighted at very low magnetic latitudes. This implies that the auroral oval on the north hemisphere was vastly extended southward. The variations of the magnetic declination also indicate that during these events the auroral oval was situated at magnetic latitudes lower than those of the observatories. The storms studied in this paper occurred at different magnetic local times (MLTs), ~23 MLT and ~19 MLT. Therefore, they might represent mid-latitude events related to different parts of the auroral oval. In this paper, the H-variation recorded at Clementinum in 1848 is interpreted to be a substorm due to the ionospheric substorm electrojet. The Greenwich event registered in 1872 then seems to be a combination of the ring-current storm with a positive variation of the H-component caused by the eastward electrojet. Both the events of 1848 and 1872 appear to exemplify phenomena that are common in high magnetic latitudes but which may occasionally happen also at mid-latitudes.

Magnetic observations at the Clementinum Observatory were started in July 1839 and data up to 1917 were published in yearbooks. In addition to regular daily observations with a typical 2-hr time step, two categories of data with a higher cadence were provided in the first decade. The first category 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 organized 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 were published. The declination was observed using the magnetic compass principle, while a bifilar apparatus was used to measure the horizontal intensity. When converting the data to the physical units of the SI, we have achieved the consistency of these high cadence data with previously published regular hourly observations of the geomagnetic field. This requirement was also met by considering the temperature dependence of measurements by the bifilar apparatus in determining the absolute values of horizontal intensity. Revealing the historical observations of the geomagnetic field can serve as a valuable material for studying the space weather in the past and may also contribute to the refinement of global field modelling. [ABSTRACT FROM AUTHOR]