Global auroral responses to magnetospheric compressions by shocks in the solar wind - Two case studies

Abstract : Two case studies are presented of the global auroral responses to shocks in the solar wind at Earth. The z component of the interplanetary magnetic field, Bz, is negative ahead and behind the first shock and positive for the second case. A sudden-commencement geomagnetic storm develops in each case, with maximum D sub st somewhat 190 nT. An immediate auroral response is detected at all longitudes around the auroral oval, in which auroral luminosities increase by a factor of 2-3 with the first samples after each sudden commencement. The time delay in obtaining the first sample varies with local time from about 1 to 18 minutes. No other significant variations in the aurora are associated with the immediate response. Beginning about 30 minutes after each sudden commencement, the aurora becomes active and displays significant variations in its luminosity and spatial distribution. For Bz 0 an intense substorm develops. A sun-aligned transpolar arc forms when Bz 0, appearing first at local midnight as a polar arc and then lengthening sunward from the auroral oval across the polar cap to noon at an average speed of about 1 km/sec.