An Analysis of Trouvelot's Auroral Drawing on 1/2 March 1872: Plausible Evidence for Recurrent Geomagnetic Storms

摘要

This work examines Trouvelot's observations and drawing of an auroral display during the night of 1 March 1872. It is known that the auroral oval moves equatorward to midlatitude and even low latitude during large geomagnetic storms. Trouvelot's graphical record of the great aurora on 1 March 1872 has been often cited as a remarkable example of a midlatitude aurora, although it is puzzling that this apparently occurred on a geomagnetically quiet day. Kataoka et al. (2019, JSWSC, 9, A16) even criticised this as a dating error. Here, we investigate Trouvelot's descriptions and available geomagnetic measurements in detail. Our analysis shows that the original date of Trouvelot's auroral drawing is most probably accurate in local time. Moreover, Trouvelot's descriptions and the observational site show that the auroral visibility fell at the beginning of 2 March 1872 in Greenwich Mean Time (GMT). Consulting simultaneous variations of magnetograms at Helsinki and Greenwich, we found that the nightside aurora specifically coincides with the initial phase of the storm (substorm) and suggests a close association with a substorm triggered by sudden magnetospheric compression. This case study shows that short geomagnetic storms can be overlooked in a daily aa index and they can also cause midlatitude aurorae. Moreover, we found ≈27-day intervals between this storm, the extreme storms on 4-6 February 1872, and another "bright aurora" that was reported on 6 January 1872. Based on their intervals, these midlatitude aurorae have probably resulted from recurrent solar activity.