ИСТИНА

MONITORING THE DAYSIDE OPEN-CLOSED FIELD LINE BOUNDARY BY GROUND ULF GEOMAGNETIC PULSATIONS O.V. Kozyreva1, V.A. Pilipenko2, D.A. Lorentzen3, and L.J. Baddeley3 1Institute of Physics of the Earth, Moscow, Russia, 2Geophysical Center, Moscow, Russia, 3Kjell Henriksen Observatory, Svalbard, Norway Geomagnetic pulsations in the Pc5-6 band (periods about 3-15 min) are a persistent feature of dayside high latitudes. A mixture of broadband Irregular Pulsations at Cusp Latitudes (IPCL) and narrowband Pс5 waves is commonly observed. The mechanism and origin of IPCL have not been firmly established. Magnetopause surface eigenmodes were suggested as a potential source of dayside high-latitude ULF waves with frequencies less than 2 mHz. A ground response to these modes is expected to be beneath the ionospheric projection of the open-closed field line boundary (OCB). The dayside open-closed field line boundary proxy is commonly determined either from the SuperDARN radar data as the equatorward boundary of enhanced width of the return radio signal or as the equatorward edge of the auroral red emission from the scanning photometer data. To unambiguously resolve a possible association of IPCL with the magnetopause surface modes and OCB, multi-instrument observation data from Longyearbyen (Svalbard) have been analyzed. The latitudinal structure of high-latitude Pc5-6 pulsations recorded by magnetometers covering near-cusp latitudes was compared with the instant location of the equatorward and poleward boundaries of dayside aurora, using data from the meridian scanning photometer. The comparison has shown that the latitudinal maximum of the broadband near-noon IPCL maximizes about 2o-3o deeper in the magnetosphere than the OCB optical proxy. Therefore, these pulsations cannot be associated with the ground image of the magnetopause surface modes. At the same time, we examined transient geomagnetic and auroral response to sudden changes of IMF to southward. The observed rapid shift of the equatorward red aurora boundary to lower latitudes is accompanied by a burst of geomagnetic pulsations with frequency around 1.5 mHz. The comparison of the local latitudinal structure of these pulsations with the instant location of the equatorward boundary of the cusp aurora, has shown that the latitudinal maximum of the transient ULF response tends to be located near the OCB optical proxy. Additionally, the dayside OCB proxy was determined simultaneously both from the SuperDARN radar and scanning photometer data. In analyzed events the "epicenter" of the Pc5-6 power was around the optical OCB proxy. Thus, the recorded transient pulsations may be associated with the ground image of the magnetopause surface modes. Acknowledgement: We thank the support of Russian Science Foundation grant 16-17-00121.