Source of Pc4 pulsations observed on the nightside

<jats:p>We have studied magnetic pulsations in the 5‐ to 20‐mHz (Pc4) band observed on 7 January 1997 from ∼1400 UT to ∼1600 UT at ground stations located on the nightside. At low latitudes (<jats:italic>L</jats:italic> < 2) the spectral content of the pulsations did not differ much from the Pi2 pulsations that were observed adjacent to the above interval. However, during the Pc4 event the auroral luminosity measured by the Ultra Violet Imager instrument on the Polar spacecraft was steadily declining toward a very low level. Also, the Geotail spacecraft, located in the plasmasheet at a geocentric distance of ∼30 <jats:italic>R</jats:italic><jats:sub><jats:italic>E</jats:italic></jats:sub> did not detect strong plasma flows. Looking for clues of possible dayside sources for the pulsations, we find that the interplanetary magnetic field measured by the IMP‐8 spacecraft made a small (<30°) angle with the Sun‐Earth axis. This condition is favorable for generation of ultra‐low‐frequency (ULF) waves in the region upstream of the bow shock, and it is known that the upstream waves are a major source of Pc4 pulsations observed on the dayside. Pointing to a common upstream energy source, strong low‐latitude Pc4 pulsations were observed on the dayside during the period of the nightside Pc4 pulsations. However, the spectrum of the upstream magnetic field oscillations at IMP‐8 was characterized by broadband power below 20 mHz instead of a strong peak at the frequency of the observed ground Pc4 pulsations. Plasmaspheric cavity mode resonance may have played a role in filtering the Pc4 pulsations from the broadband input to the magnetosphere.</jats:p>