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We document the detailed dynamics of thedayside aurora in the sim1200-1600 MLT sector in response to a sharp southwardturning of the interplanetary magnetic field (IMF) under negative IMF Byconditions. Features not documented in previous work are elucidated by using twomeridan scanning photometers (separated by 2 h) and an all-sky auroral imager inNy Ålesund, Svalbard (75.5°MLAT) in combination with magnetogramsfrom stations on Svalbard, covering the latitude range 71°75°MLAT.The initial auroral response may be divided into three phases consisting of: (1)intensification of both the red (630.0 nm) and green (557.7 nm) line emissionsin the cusp aurora near 1200 MLT and ~100 km equatorward shift of itsequatorward boundary, at ~75°MLAT, (2)eastward and poleward expansions of the cusp aurora, reaching the 1430 MLTmeridian after 5-6 min, and (3) east-west expansion of the higher-latitudeaurora (at ~77°78°MLAT)in the postnoon sector. The associated magnetic disturbance is characterized byan initial positive deflection of the X-component at stations located 100-400 kmsouth of the aurora, corresponding to enhanced Sunward return flow associatedwith the merging convection cell in the post-noon sector. The sequence of partlyoverlapping poleward moving auroral forms (PMAFs) during the first 15 min,accompanied by corresponding pulsations in the convection current, was followedby a strong westward contraction of the cusp aurora when the ground magnetogramsindicated a temporary return to the pre-onset level. These observations arediscussed in relation to the Cowley-Lockwood model of ionospheric response topulsed magnetopause reconnection. Key words. Mangetospheric physics (magnetopause · cuspand boundary layers; solar wind-magnetosphere interactions; auroral phenomena

The dynamic cusp aurora on 30 November 1997: response to southward turning of the IMF