An observational test for the ionospheric or magnetospheric origin of night-time geomagnetic positive bays at low and mid latitudes

Abstract

The night-time positive bay-type perturbations ( ΔH) in the horizontal component ( H) at groundlevel geomagnetic field are compared for three locations: Trivandrum (1.2°S gm. lat.), Alibag (9.5°N gm. lat.) and Tashkent (32.3°N gm. lat.) in the same local time sector. If the bay disturbances originate predominantly in the ionospheric dynamo region currents, then the ΔH values at Trivandrum, an equatorial electrojet station, should be at least twice as large as those at Alibag which is a low-latitude station well outside the electrojet latitudes. For none of the 31 events analyzed does the ratio of ΔH at Trivandrum to that at Alibag exceed 1.3 and it is mostly below 1.0. The values of ΔH at Tashkent, which is a mid-latitude station, are larger by about 50% or more than those at Alibag or Trivandrum. Based on the above results, it is concluded that (a) the night-time positive bays at low and middle geomagnetic latitudes do not originate, in general, in the overhead ionospheric dynamo region currents and (b) the positive bays seem to be caused primarily by magnetic field-aligned currents. It is shown that the method presented in this paper is useful in general for finding out the relative contributions of ionospheric and magnetospheric currents to different types of ground-level geomagnetic field perturbations at low and middle latitudes.