Conjugate asymmetries in sudden commencement absorption and the sudden commencement absorption event of February 28, 1969

A balloon observation of sudden commencement X rays (up to 2 times background) was made near Macquarie Island (λ = −61°) at 0423 UT on February 28, 1969. A synoptic study of this event, using riometer data, reveals that in the morning sector of the auroral zone the pattern of sudden commencement absorption (sca) is similar to that inferred from previous statistical studies. The situation in the afternoon sector is not clear. The sca at College was 10 times greater than that at Macquarie Island, and thus an asymmetry in sca between stations occupying positions of near conjugacy is indicated. A restricted period of data from the conjugate stations Great Whale River and Byrd reveals two other asymmetrical sca events. It is suggested that asymmetries in conjugate sca, and perhaps variations in the longitudinal distribution of sca, occur when a solar wind shock normal makes a large angle with the geomagnetic equatorial plane or the sun-earth line.