Dayside Magnetic Depression Following Interplanetary Shock Arrivals During the February 1958 and July 1959 Superstorms

The present study investigates mid- and low-latitude ground magnetic disturbances observed following the arrival of three interplanetary (IP) shocks during the super-geomagnetic storms of February 1958 and July 1959. One may expect that after IP shocks, the H (northward) magnetic component increases globally but especially on the dayside. However, in each event, the H component was depressed sharply for 1-2 hr in the dawn-to-noon sector, whereas it increased in other local time (LT) sectors. Observed magnetic deflections suggest that there existed field-aligned currents (FACs) flowing into and out of the auroral zone around the western and eastern edges of the LT sector of the dayside H depression. These features strongly suggests that the observed H depression was a remote effect of a R1-sense FAC system. It was previously reported that similar ground magnetic disturbances were observed after the SSC of the 2003 Halloween storm, which reveals striking similarities to the well-known H depression observed at Colaba during the 1859 Carrington storm. It is therefore suggested that the external driving behind IP shocks, especially those associated with major storms, is most optimum for the sharp reduction of the dayside H component through the formation and intensification of the dayside FAC system. Associated magnetic disturbances are considered to be larger in magnitude with increasing magnetic latitude, and oriented azimuthally as well as meridionally. Such magnetic disturbances in dayside midlatitudes may not be discussed very often as a target of space weather, but their potential impacts on ground infrastructures probably require closer attention. Geomagnetic storms of February 1958 and July 1959 were historically intense. After the arrival of interplanetary (IP) shocks during these storms, the horizontal component of midlatitude magnetic field was sharply depressed at prenoon, but at other local times (LTs) it enhanced significantly as generally expected from solar wind compression. This LT distribution of magnetic disturbances can be explained by a dayside wedge-shaped current circuit, which connects the magnetosphere with the ionosphere. Similar ground magnetic disturbances were reported previously for the October 2003 (Halloween) storm, and the extraordinary H depression observed at Colaba during the 1859 Carrington storm was also observed when Colaba was at prenoon. It is therefore suggested that the driver of this dayside current system, probably southward interplanetary magnetic field, often becomes extraordinarily large behind IP shocks, especially those of major storms, and intensifies this current system to unusual levels. The associated magnetic disturbance is expected to be larger at higher latitudes, and oriented in any direction. These features exhibit a distinct contrast to magnetic disturbances caused by other magnetospheric current systems (e.g., the ring current). Dayside ground magnetic disturbances are rarely discussed as a potential cause of hazardous space weather events, but probably deserve closer attention. The H magnetic component was depressed sharply in dayside mid- and low latitudes following IP shock arrivals during the 1958 and 1959 storms The H depression was confined from dawn to noon, and the direction of magnetic deflection depended on local time as expected for an R1 field-aligned current (FAC) system For extreme solar wind driving, this dayside R1 FAC system can be a potential cause of geomagnetic hazards especially in the subauroral zone