Small-scale structure of electron density in the D region during onset phases of an auroral absorption substorm

Small-scale structure of the electron density in the D region during the preceding and onset phases of auroral absorption substorms classified as unloading processes was studied on the bases of imaging riometer measurements made at Tjornes, Iceland, and broad-beam riometer data collected in the Nordic Countries. The precipitation during the onset phase may be localized, covering only 10 km north-south and east-west directions. After the substorm onset ionospheric absorption was observed to pulsate with a period of 1-12 minutes. Possibly this pulsation is related to the magnetohydrodynamic, field line resonances in the nightside magnetosphere and to the near-Earth neutral line substorm model. (C) 1997 COSPAR.