[P-EM09] Space Weather and Space Climate

[PEM09-14] The Colaba (Mumbai) 1600 nT Magnetic Depression during the 1859 Carrington Storm: Its Cause and Implications into Space Weather

★Invited Papers

*Shinichi Ohtani¹ (1.Johns Hopkins University Applied Physics Laboratory)

Keywords:Geomagentic storm, Solar wind - magnetosphere coupling, Dayside field-aligned current system

During the Carrington storm, which took place in September 1859, the horizontal magnetic field decreased by ~1600 nT at Mumbai (India) on the dayside. Whether its cause was magnetospheric or ionospheric has been controversial. Interestingly, a similar, but smaller, magnetic depression was observed during the 2003 Halloween storm. It is found that the magnitude of this Halloween-storm magnetic depression was smaller closer to the equator, which precludes the possibility that it was caused by the ring current, a magnetospheric current carried by energetic ions drifting around Earth. Instead, the magnetic depression was well correlated with zonal (east-west) magnetic disturbances observed at earlier and later local times, and also with the variation of the southward component of the interplanetary magnetic field, a general measure of the solar wind driving. It is therefore concluded that the Halloween-storm magnetic depression was caused by a dayside field-aligned current (FAC) system driven by the solar wind-magnetosphere interaction; the FAC is a current flowing along magnetic field lines between the magnetosphere and ionosphere. From similarities in higher-latitude magnetic disturbances observed in the two events, it is suggested that this dayside FAC system was also the cause of the Mumbai magnetic depression during the Carrington storm (see the attached Figure). In this presentation I also discuss requirements for predicting similar events in the future.