Stare auroral radar observations of Pc 5 geomagnetic pulsations

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Abstract
The Stare (Scandinavian twin auroral radar experiment) auroral radar system has been used to measure ionospheric electric fields associated with Pc 5 geomagnetic pulsations. With this system, electric fields are derived from the drift velocity of radar auroral irregularities. The spatial resolution is 20 km over a 200,000‐km² grid, and the temporal resolution is 20 s. It has been found that the oscillating electric field associated with a hydromagnetic field line resonance produces poleward moving, bandlike regions of radar aurora, which are aligned in the east‐west direction. The drift of the irregularities within these bands is alternately eastward and westward. The Stare electric field data have been used in conjunction with the Biot‐Savart Law and an assumed height‐integrated conductivity of 8–10 Ω−1 to calculate the ground magnetic disturbance. It has been found that the H and Z are well predicted, whereas D is generally underestimated. These results are consistent with a 90° rotation of the magnetic polarization ellipse in the ionosphere. By Fourier analyses of the Stare data it is found that the half‐power latitudinal width of the field line resonance is typically 100 km in the ionosphere. Moreover, the north‐south electric field undergoes a 180° phase shift about the resonance as predicted by theory. The data have been used to estimate equatorial plasma densities for 6 < L < 7, and values of the order of 20 cm−3 have been obtained. However, these determinations are strongly affected by distortions of the geomagnetic field from a dipolar geometry. In summary, (1) the experimental results strongly support the hydromagnetic field line resonance theory of pulsations; (2) the magnetic polarization ellipse is indeed rotated through 90° by the ionosphere; (3) the phenomena previously observed by auroral radar workers in association with Pc 5 pulsations were related to the electric field of the hydromagnetic wave near resonance; and (4) auroral radar measurements can be used to estimate the equatorial magnetospheric plasma density in the region 5 < L < 8.