Auroral optical emissions during the solar magnetic cloud event of October 1995

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Abstract
The solar magnetic cloud of October 18, 1995, swept by the Earth's magnetosphere, starting at about 2200 UT. This event triggered prolonged auroral displays in the terrestrial atmosphere. The optical emissions from these auroras were recorded with a CCD spectrograph, operating at the radar site in Sondrestromfjord, Greenland. Continuous spectroscopic measurements were made for about 10 hours, from 2200 UT on October 18, 1995, until local dawn set in at 0800 UT on October 19, 1995. These measurements show markedly different spectral characteristics from those of normal auroras observed at other times in various sectors of the auroral oval and in the polar cap region. The differences are most pronounced in the relative brightnesses of the ion and neutral line emissions from the thermospheric oxygen atoms as well as in the O2 At band emissions. Other differences between the normal auroras and the magnetic cloud induced auroras relate to the temperature of the thermospheric height regions where the N2 1PG and the N2+ M band emissions peak in these auroras. Least squares synthetic profiles fit to the rotational distributions of the N2 1PG and the N2+ M bands observed during the solar magnetic cloud event, coupled with auroral model calculations constrained to reproduce the measured emission brightnesses, indicate a higher than normal height for the emissions from the auroras associated with this event. Further evidence for the higher emission altitude is derived from the detailed comparison of the spectral distribution observed during this period with various other measurements of the relatively high‐altitude auroral emissions. All these observations point to the precipitation in the polar terrestrial atmosphere of electrons with an average energy of about 500eV ± 100eV during the interaction of the solar magnetic cloud with the near‐Earth space environment. Such an auroral electron precipitation event is unusual and occurs infrequently; it is similar to the rare type A global red aurora.