Suppression of the dayside magnetopause surface modes

Abstract

Abstract. Magnetopause surface eigenmodes were suggested as a potential source of dayside high-latitude broadband pulsations in the Pc5-6 band (frequency about 1-2 mHz). However, the search for a ground signature of these modes has not provided encouraging results. The comparison of multi-instrument data from Svalbard with the latitudinal structure of Pc5-6 pulsations, recorded by magnetometers covering near-cusp latitudes, has shown that often the latitudinal maximum of pulsation power occurs about 2-3° deeper in the magnetosphere than the dayside open-closed field line boundary (OCB). The OCB proxy was determined from SuperDARN radar data as the equatorward boundary of enhanced width of a return radio signal. The OCB-ULF correspondence is further examined by comparing the latitudinal profile of the near-noon pulsation power with the equatorward edge of the auroral red emission from the meridian scanning photometer. In most analyzed events, the "epicenter" of Pc5-6 power is at 1-2° lower latitude than the optical OCB proxy. Therefore, the dayside Pc5-6 pulsations cannot be associated with the ground image of the magnetopause surface modes or with oscillations of the last field line. A lack of ground response to these modes beneath the ionospheric projection of OCB seems puzzling. As a possible explanation, we suggest that a high variability of the outer magnetosphere near the magnetopause region may suppress the excitation efficiency. To quantify this hypothesis, we consider a driven field line resonator terminated by conjugate ionospheres with stochastic fluctuations of its eigenfrequency. A solution of this problem predicts a substantial deterioration of resonant properties of MHD resonator even under a relatively low level of background fluctuations. This effect may explain why there is no ground response to magnetopause surface modes or oscillations of the last field line at the OCB latitude, but it can be seen at somewhat lower latitudes with more regular and stable magnetic and plasma structure.

Auroral, Ionospheric and Ground Magnetic Signatures of Magnetopause Surface Modes

“…(2019) suggest that short‐lived quasi‐periodic motions of the OCB in auroral keograms and ground magnetic oscillations near the OCB with large latitudinal scales and similar periodicities across L ‐shells might distinguish magnetopause surface modes from the Alfvén continuum, presenting potential case studies. However, Pc5‐6 band (periods ∼3–15 min) oscillations in ground magnetometer data have been shown to peak systematically equatorward of optical and ionospheric proxies for the cusp OCB by 1–3° (O. Kozyreva et al., 2019; V. A. Pilipenko et al., 2017, 2018). In the absence of conjugate space‐based observations, conclusions have been mixed over whether these results relate to MSE and what the implications are for its excitation efficiency.…”

“…However, Pc5-6 band (periods ∼ 3-15 min) oscillations in ground magnetometer data have been shown to peak systematically equatorward of optical and ionospheric proxies for the cusp OCB by 1-3 • (V. A. Pilipenko et al, 2017Pilipenko et al, , 2018O. Kozyreva et al, 2019).…”