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.

The normalized rate of occurrence of dayside Pc 3,4 pulsations from L=2.4 to 4.3 has a strong enhancement for low cone angles of the interplanetary magnetic field. When the angle of the IMF to the earth sun line, θBX, is 15° or less the occurrence rate is 7‐8 times the average rate at L=2.4 to 2.8 and 2.2 to 3.5 times the average rate at L=4 to 4.3. These waves disappear when the IMF is nearly at right angles to the sun‐earth‐line. This absence of pulsations occurs over the widest range of angles at lowest L‐values. These observations are consistent with a source originating in the waves upstream of the subsolar bow shock which are transported by convection to the magnetopause where they couple to oscillations of magnetospheric field lines. Since the index of refraction of the magnetospheric plasma decreases with decreasing radial distance, except at the plasmapause, inward propagating waves should be refracted away from the radial direction. Thus, to reach low L‐values the waves should couple near the stagnation point and propagate nearly radially inwards. Upstream waves should be convected to the stagnation point for only a limited range of θBX. However, to reach higher L‐values the coupling may be at later local times where cross streamline propagation can bring waves from a larger range of θBX. The streamline geometry and its connection to the foreshock region is illustrated for a variety of IMF orientations using a simple approximation to the magnetosheath flow field.

No abstract

No abstract

Rep. Prog. Phys.!1972[35 803-881 limitations. Details of electronic detection systems are not described except where appropriate reference cannot be included.The instruments described are in the following main classifications : magnetometers using nuclear and atomic resonance, saturable cores, suspended magnets, induction coils and superconducting devices.In addition, the magnetometer systems which use artificially created fields to allow vector measurements to be made by total field sensors are described. Their operation performance is assessed and the use of bias and backing-off fields are discussed.The use of the proton gyromagnetic ratio as the absolute standard of geomagnetic field measurement is summarized. There follow-s a discussion of the requirement for routine ground-based geomagnetic data to be supplied to the scientific community in digital form for direct machine read-in, and some speculation about practicable automatically recording observatory systems.