Top panel: decrease in the field intensity B (blue) – and anticorrelated density increase (red) – observed towards the end of the day, before the HCS crossing in a region of highly compressible (interval 4 in panel g of Fig. 3). Middle panel: a detail of the fluctuations located in the region where the field is lowest (light purple region in the top panel). This is characterized by fluctuations in B that are of the same order of the background field. Bottom panel: a zoom in the compressible structure (yellow region in the middle panel). These ion-scale fluctuations display anti-correlation between B and n, while there is little power in the other components B_min and B_int; as a consequence the B_max profile overlaps with the field intensity B.