Daytime Pc5 Diffuse Auroral Pulsations And Their Association With Outer Magnetospheric Ulf Waves

South Pole Station, Antarctica (SPA, magnetic latitude = -74.5 degrees, magnetic local time (MLT) = UT-3.5 h), is a unique observatory which can capture daytime auroral forms throughout austral winter season. We have studied the properties and origin of ultralow-frequency (ULF) range modulation of daytime diffuse aurora, using data acquired on June 23, 2017 by multi-instrument measurements at SPA and in situ measurements in the dayside outer magnetosphere. At 1500-1600 UT, monochromatic Pc5-range pulsations (period similar to 10 min) emerged in the midday diffuse auroral region. The sequential 2-D images reveal that the auroral pulsations result from the repetitive formation of faint, diffuse auroral patches, propagating poleward at a speed of similar to 1.5 km s(-1). Interestingly, no obviously similar magnetic pulsations were found at SPA. The results differ fundamentally from the ground optical and magnetic signatures expected for a standing field line resonance. On the other hand, the co-located riometer and VLF receiver observed clearly synchronized pulsations, suggesting that tens-of-keV electrons interact with modulated chorus waves and then are scattered down to the auroral pulsation region. During the same interval, the THEMIS-D spacecraft detected corresponding Pc5 oscillations in the dayside outer magnetosphere (9-10 R-E and similar to 15 MLT). The compressional component of the magnetospheric Pc5 waves, presumably driven by an external source, exhibited a good correspondence to the daytime Pc5 auroral pulsations. The simultaneous SPA-THEMIS observations highlight the role of compressional Pc5 pulsations in the dayside outer magnetosphere in determining the periodicity of daytime high-latitude diffuse auroral pulsations.