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We present a statistical study of four years of Clustercrossings of the mid-altitude cusp. In this first part of the study, westart by introducing the method we have used a) to define the cuspproperties, b) to sort the interplanetary magnetic field (IMF) conditions orbehaviors into classes, c) to determine the proper time delay between the solarwind monitors and Cluster. Out of the 920 passes that we have analyzed, only 261fulfill our criteria and are considered as cusp crossings. We look at thesize, location and dynamics of the mid-altitude cusp under various IMForientations and solar wind conditions. For southward IMF, Bz rules thelatitudinal dynamics, whereas By governs the zonal dynamics, confirmingprevious works. We show that when |By| is larger than|Bz|, the cusp widens and its location decorrelates fromBy. We interpret this feature in terms of component reconnectionoccurring under By-dominated IMF. For northward IMF, we demonstratethat the location of the cusp depends primarily upon the solar wind dynamicpressure and upon the Y-component of the IMF. Also, the multipointcapability of Cluster allows us to conclude that the cusp needs typicallymore than ~20 min to fully adjust its location and size inresponse to changes in external conditions, and its speed is correlated tovariations in the amplitude of IMF-Bz. Indeed, the velocity in °ILAT/minof the cusp appears to be proportional to the variation in Bzin nT: Vcusp=0.024 ΔBz. Finally, we observe differences inthe behavior of the cusp in the two hemispheres. Those differences suggestthat the cusp moves and widens more freely in the summer hemisphere

Cluster survey of the mid-altitude cusp: 1. size, location, and dynamics