Interplanetary magnetic field By control of dayside auroras

The effect of the interplanetary magnetic field (IMF) By component on the d ayside auroral oval from Viking UV measurements for March-November 1956 is studied. Observations of dayside auroras from Viking UV images for large po sitive (15 cases) and negative (22 cases) IMF By (\By\ > 4 nT), suggest tha t: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras, (2) for negative IMF By; t he auroral oval tends to be narrow and continuous throughout the noon merid ian without any noon gap or any strong undulation in the auroral distributi on. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of au roral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upwar d field-aligned currents associated with the IMF By in the noon sector. (C) 1999 Elsevier Science Ltd. All rights reserved.