Abstract and keywords
Abstract (English):
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.

ULF waves, magnetopause, open-closed boundary, MHD surface modes, Alfven resonator
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The open-closed field line boundary (OCB) is the boundary that separates magnetospheric field lines that are dragged by the interplanetary magnetic field from those that are closed within the magnetosphere. The ability to monitor the OCB location allows us to study the electrodynamics of magnetosphere–ionosphere coupling, in particular to estimate energy storage and release in the magnetosphere. There are several methods for discriminating the ionospheric footprint of the dayside OCB, but each of them has some merits and drawbacks.

The open/closed separatrix can be identified using particle precipitation boundaries observed by low-altitude spacecraft [Newell, Meng, 1988; Oksavik et al., 2000]. The poleward edge of high-energy (>10 keV) electron precipitation (‘‘trapping boundary’’) corresponds to particles trapped on closed field lines. Field lines poleward of this region must therefore be open. However, the measurements can be made with cadence about 90 min and along orbit only.

Precipitating soft electrons produce plasma irregularities, and strong coherent backscatter from these irregularities can be monitored by Super Dual Auroral Radar Network (SuperDARN) radars. The spectral width of the Doppler velocity of measured backscattered signal can be interpreted as a manifestation of plasma turbulence of the scattering region. The transition from narrow to broad spectral widths has been utilized to investigate the location of ionospheric boundaries [Baker et al., 1995]. Many studies have revealed that the high-latitude broad Doppler spectral width can be used as an indicator for the cusp region [Moen et al., 2001], even though the interpretation of the broad spectral width is still elusive. However, ionospheric regions with elevated turbulence level can be associated not only with the cusp proper, but with the low-latitude boundary layer; hence, this method may provide ambiguous results.

A demarcation of the cusp soft electron precipitation can be determined by comparing the equatorward boundary of the red (630.0 nm) auroral emission along the photometer meridian scanning [Lorentzen et al., 1996]. Precipitation of soft electrons with 0.1–1 keV energies is a good indicator of open magnetic field lines and corresponds to the cusp aurora dominated by the red emission (Rayleigh intensity ratio 630.0/557.7>1) [Lorentzen, Moen, 2000; Johnsen, Lorentzen, 2012]. However, dayside auroral optical measurements can be made during winter months only.

The co-location of the equatorward edge of the HF radar cusp and the cusp auroral emission has been extensively studied [Yeoman et al., 1997]. Rodger et al. [1995] showed that the radar cusp signatures were located ~0.5° equatorward of the optical cusp, whereas Milan et al. [1999] found that the equatorward boundaries of the optical cusp were, on average, located at slightly lower latitudes than the radar cusp. Comparing the spectral width method to the optical method, the radar OCB proxy was seen to be on average 0.5–1.6° (56–170 km) poleward of the optical OCB [Chen et al., 2015].

A new discriminator of OCB was suggested on the basis of ground magnetometer observations of high-latitude ultra-low-frequency (ULF) activity. The background of this tool is described in the following section.



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