Abstract. An anomalous geomagnetic sudden commencement (SC) occurred on March 24, 1991. It is characterized by an exceptionally large and sharp impulse observed in its initial part along the noon meridian in middle and low latitudes. The analysis of the SC was made by using high time resolution digital data from the 210 ø Meridian Magnetometer Chain in the west Pacific, Sub-Auroral Magnetometer Network (SAMNET) in the United Kingdom and southern Scandinavia, the EISCAT Magnetometer Cross in northern Scandinavia and Svalbard, and Canopus in Canada together with other ground and satellite (GOES 6, GOES 7, CRRES, and GMS) data. The results of the analysis suggest that the pulse observed at lower-latitude ground stations was caused by the propagation of a strong magnetospheric compression of short duration (less than 1 min) which has never been observed before this event. The HF Doppler observation in Kyoto near local noon seems to be consistent with existence of the bipolar electric field associated with the propagating compressional magnetic pulse. The SAMNET stations and CRRES in the early morning also detected positive pulses which delays 30-50 s from the pulses in noon sector. Although the delay in the peak time of the pulse observed on the ground is consistent with ionospheric hydromagnetic wave propagation from the dayside to the nightside with finite speed, the initial onset time of the pulse on the ground was almost simultaneous everywhere suggesting the existence of an "almost instantaneous" propagation mode below the ionosphere. [1993] successfully simulated this event by particle acceleration due to a sharp compressional electromagnetic pulse launched from the magnetopause at 1500 LT in the very initial stage of the SC. The CRRES actually observed an 130 nT monopolar magnetic pulse and an associated bipolar electric pulse with a peak-topeak amplitude of 80 mV/m. The duration of the pulse is about 2 min. Such a large and sharp pulse has never been observed before in this region of the magnetosphere. The SC itself might be produced by a coronal mass ejection (CME) driven interplanetary shock which was presumably related with a 3B optical flare at 2246 UT on March 22, 1991.Since the analyses of this event have mostly been limited to particle data so far, it is necessary to study properties of the electromagnetic pulse observed by CRRES in more detail by analyzing other magnetic data. Although the CRRES observations and the simulation by Li et al. [1993] greatly contributed to understand this peculiar event, we have to recognize that the CRRES observations were made at one point in the magnetosphere. It is difficult therefore to discuss in detail about excitation and propagation of the observed pulse. Analyses of SCs made so far [see Araki, 1994] show that a simple compression of the magnetosphere causes a complex global distribution of amplitude and waveform of SC. The main positive H component increase of SCs is often preceded by a positive or negative impulse of short duration. This preceding impulse ...