New Insights From the 2003 Halloween Storm Into the Colaba 1600 nT Magnetic Depression During the 1859 Carrington Storm

The present study investigates the cause of a sharp horizontal (H) magnetic depression observed on the dayside during the 2003 Halloween storm, and discusses if the same process could cause the 1,600 nT H depression observed at Colaba during the 1859 Carrington storm. For the Halloween H depression, it is found that (a) it developed in correlation with southward interplanetary magnetic field (IMF) BZ in the sheath region of a coronal mass ejection; (b) its magnitude decreased significantly with decreasing magnetic latitude; (c) it was highly correlated with westward and eastward sub‐auroral zone magnetic deflections at earlier and later local times, respectively; and (d) the westward auroral electrojet (AEJ) enhanced simultaneously in the entire nightside auroral zone, whereas the enhancement of the dayside eastward AEJ was unclear. These features suggest that the dayside R1‐sense wedge current system, which was driven by dayside magnetic reconnection, was the cause of the H depression, and the associated field‐aligned currents closed primarily with the westward AEJ through the nightside. The Colaba H depression also took place on the dayside and lasted for ∼2 hr. Furthermore, it took place within a few hours after the storm commencement, and the westward AEJ enhanced simultaneously in the dawn sector. These similarities suggest that the dayside wedge current system was also the cause of this historical event. The sharp recovery of the Colaba H depression, which has been a challenge to explain, may be attributed to the decay of this current system due to a northward IMF turning. Plain Language Summary During the 1859 Carrington storm, the horizontal magnetic field decreased by ∼1,600 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 particles 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 tha