Abstract
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, theH-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 theH-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.
Highlights
Traditional concepts of the mid- and low-latitude geomagnetic disturbances emphasise that the most important geomagnetic variations are the geomagnetic storms that are caused by the enhanced ring current
Even in midlatitudes the current system that is connected to the auroral oval might occasionally cause prominent geomagnetic variations that measure up to the ring-current storms. They sometimes happen at mid-latitudes, despite such currents are typical for high magnetic latitudes which are the common setting for the auroras
The outline of possible connections with the field aligned currents (FACs) is based on the distribution of net FACs that Shi et al (2008) inferred from the D perturbations observed during the magnetic storm of 25 September 1998
Summary
Traditional concepts of the mid- and low-latitude geomagnetic disturbances emphasise that the most important geomagnetic variations are the geomagnetic storms that are caused by the enhanced ring current. It is known that much more violent geomagnetic field variations than the ring-current storms commonly happen, too These latter variations, called auroral substorms, tend to occur at high magnetic latitudes (e.g., Akasofu & Chapman, 1964). Even in midlatitudes the current system that is connected to the auroral oval might occasionally cause prominent geomagnetic variations that measure up to the ring-current storms They sometimes happen at mid-latitudes, despite such currents (westward, eastward and substorm electrojects and FACs) are typical for high magnetic latitudes which are the common setting for the auroras. The events of Clementinum, Colaba (Carrington storm) and Greenwich occurred in three successive solar cycles, namely 9, 10 and 11 They took place at different MLTs and we believe that they document interesting mid-latitude geomagnetic variations at three different places related to the auroral oval
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