Evening/forenoon asymmetry in the 27-day oscillation of the low-latitude magnetic field

Abstract

Power spectra of low-latitude horizontal intensity, computed from series derived from observations restricted to different local times, reveal a large evening/forenoon asymmetry in the magnitude of the 27-day oscillation of the field. The asymmetry is observed during periods of declining phase of the solar cycle and is very pronounced during periods of high activity. It is considered that selective data derived from afternoon/evening observations of field in the low latitudes by virtue of their high signal-to-noise ratio are likely to be most useful in the investigations of the 27-day variation and in the study of fine structure of the 27-day line in the spectrums of the field.