The mean field of disturbance of polar geomagnetic storms

Summary —The diurnal variation of the mean disturbance vectors ( S D ) is determined at a number of observatories lying roughly along the geomagnetic meridian of Λ ∼ 120°, from Spitzbergen over Scandinavia and to Potsdam. The values of S D have been obtained for four degrees of geomagnetic activities. The variation of the mean disturbance field across the geomagnetic meridian 120° in the course of the day is drawn up on maps for each of the three components in D , H , and Z . It is shown that this graphical representation of the variations in the mean D ‐field can be regarded as a fairly good approximation to the mean disturbance‐field around the polar cap. It is shown that there are discontinuities in the auroral zone; the zone consists of two separate branches. The oscillations in the position of the D ‐field and of the auroral zone are studied for various degrees of geomagnetic activities and compared with the mean positions of the aurora. The heights and dimensions of the disturbing current‐system along the auroral zone are discussed. It is shown that an assumption of an internal part of the D ‐field of 0.4 of the total field is not in accordance with the observed D ‐field. The characteristics of the mean perturbing vectors at a station (Tromsö) lying underneath the auroral zone are discussed. Examples of records of auroral luminosities are compared with the simultaneous geomagnetic perturbations.