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

Abstract

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.

Motion of polar cap arcs

“…It is also clearly seen that most of the arcs (≈70 %) were observed when the IMF was directed northward (0°< CA < 90°and 270°< CA < 360°). Early studies by Berkey et al [1976] and Lassen and Danielsen [1978] found good correlation between IMF B z northward conditions and appearance of polar cap arcs. Our statistical results are basically consistent with their earlier finding.…”

A suggestion that two‐dimensional turbulence contributes to polar cap convection for Bz north

“…Velocity shear and the corresponding vorticity are such that upward field-aligned currents and associated precipitating electrons occur for positive vorticity. We believe that polar cap arcs, which fill the polar cap for B z north [Lassen and Danielsen, 1978], are the signature of positive vorticity (W) since W z = (r  V z ) = J || [Kelley, 2009]. Thus, the arcs show the input scales for 2-D turbulence, which, as argued next, contribute to the PCP for B z north.…”

“…[6] The second type of polar cap aurora was reported by Lassen and Danielsen [1978] and is shown in Figure 2. These multiple arcs were synthesized from all-sky camera data over many years.…”

“…Mass plots of arcs in corrected geomagnetic coordinates for different values of B z . Clearly, the polar cap has many arcs for B z north and none for B z south [afterLassen and Danielsen, 1978].…”

IMF control of the Earth's magnetosphere

“…Lassen and Danielsen (1978) have demonstrated that the occurrence of the arcs of this type is associated with the northward polarity of IMF while the arcs extending along the oval are associated with the southward IMF polarity. By examining the energy spectrum offions and electrons observed by AE-D, Burch et al (1979) have found indications that thermal electrons are accelerated in the polar cap to > 1 keV when the IMF polarity is northward by a potential difference generated along magnetic field lines.…”