Evidence for a correlation between Sun‐aligned arcs and the interplanetary magnetic field direction

Sun‐aligned arcs are long and sometimes narrow optical structures which are oriented in the sun‐earth direction and which occur in the polar cap, generally during intervals of low magnetic activity. Their appearance, as seen by the DMSP and ISIS‐2 satellite photometers, is examined with respect to the north‐south orientation of the interplanetary magnetic field. In the DMSP pictures it is found that they appear when the IMF is directed northward. In 16 out of 18 unambiguous observations of sun‐aligned arcs by ISIS‐2, the IMF was directed northward; for the other two no IMF data was available. These results provide evidence for a strong correlation between the northward direction of the IMF and the occurrence of sun‐aligned arcs in the northern polar cap.