Intensity and time series of extreme solar-terrestrial storm in 1946 March

Abstract

ABSTRACT Major solar eruptions occasionally cause magnetic superstorms on the Earth. Despite their serious consequences, the low frequency of their occurrence provides us with only limited cases through modern instrumental observations, and the intensities of historical storms before the coverage of the Dst index have been only sporadically estimated. Herein, we examine a solar-terrestrial storm that occurred in 1946 March and quantitatively evaluate its parameters. During the ascending phase of Solar Cycle 18, two moderate sunspot groups caused a major flare. The H α flaring area was recorded to be ≥600–1200 millionths of solar hemisphere, suggesting that this was an M- or X-class flare in soft X-ray intensity. Upon this eruption, a rapid interplanetary coronal mass ejection (ICME) with an average speed of ≈1590 km s−1 was launched. Based on measurements in four known mid-latitude and relatively complete magnetograms, the arrival of this extreme ICME caused a magnetic superstorm, which caused an initial phase with the H-component amplitude of ≥80 nT, followed by a main phase whose intensity was reconstructed as ≤−512 nT using most negative Dst* estimates. Meanwhile, the equatorial boundary of the auroral oval extended down to ≤41${^{\circ}_{.}}$8 in invariant latitude and formed a corona aurora in Watheroo, Australia. Interestingly, during this magnetic superstorm, larger magnetic disturbances were recorded at dusk and near the dip equator on the dayside. Its cause may be associated with a strong westward equatorial electrojet and field-aligned current, in addition to the contribution from the storm-time ring current.