Quiet time pattern of auroral arcs for different directions of the interplanetary magnetic field in the Y-Z plane


  • K. Lassen,

  • C. Danielsen


Quiet time arcs observed from the Greenland all-sky camera network have been ordered in corrected geomagnetic latitude/time mass plots according to the values of the Y and Z components of the IMF. Two different patterns of discrete quiet arcs occur. In one system the arcs are ordered along the statistical auroral oval; in the other, which we have called the polar cap pattern of discrete arcs, the arcs are ordered partly in the sun-earth direction over the polar cap, especially on the dawnside of the pole, partly along the 78°–80° corrected geomagnetic latitude curve in the morning to noon and noon to evening hours. The oval arc pattern is most prominent when Bz is negative. It gradually contracts and finally practically vanishes as the steady IMF is shifted toward a northward direction. The contraction takes place differently, depending on the sign of By. The polar cap pattern is dominant when Bz is positive. The sun-aligned arcs of the polar cap pattern gradually disappear as Bz goes toward negative values. The high-latitude arcs in the forenoon sector seem to be present for all values of Bz considered. No dependence on By has been found in the pure polar cap pattern. Apart from periods of extremely great numerical values of Bz, the oval and the polar cap discrete arc patterns are found to coexist, at least in our statistical presentation.