Observations of ULF Pulsations During TRINNI Events

Two magnetospheric phenomena are commonly observed in both SuperDARN (Dual Auroral Radar Network) and ground‐based magnetometer network data. These are ULF pulsations driven by magnetohydrodynamic waves, and flow bursts in the high latitude ionosphere driven by magnetic reconnection. Possible connections between these two phenomena are investigated. TRINNI (Tail Reconnection during IMF Northward, Non‐substorm Intervals) events, high‐speed ionospheric flows resulting from magnetotail reconnection during IMF Bz‐positive but By‐dominant periods, have been investigated by many researchers. These events produce measurable enhancements of the cross polar cap potential. Four previously reported TRINNI events have been analyzed using SuperDARN data, together with high‐latitude magnetometer data which show simultaneous ULF pulsations. Fourier analysis of the cross polar cap potential measured by SuperDARN and two magnetic field components revealed common spectral peaks in the 1–5 mHz range. The signals were narrow‐band filtered and complex demodulation was performed to construct the analytic signals, allowing determination of amplitude and phase. In all four events the magnetic field and potential signals showed similar wave packet structure with large‐amplitude oscillations at a common frequency for periods of the order of hours. During these periods the phase differences between the field and potential remained constant. It is argued that this phase locking suggests a causal link between the two oscillations, so that the TRINNIs could be the source of the magnetic pulsations, or they at least share a common driver. Key Points Magnetic field and cross polar cap potential (CPCP) signals showed some degree of coincidence in periods of large‐amplitude oscillations Examined events show nearly constant phase differences during the large‐amplitude oscillation periods, while they fluctuate at other times The constant phase differences indicate that the oscillations in magnetic field components and CPCP are phase locked