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The interaction of interplanetary coronal mass ejections (ICME) with each other and with co-rotating interaction regions (CIR) changes their configuration, dynamics, magnetic field and plasma characteristics and can make space weather forecasting difficult. During the period of March 20–25, 2011, the Solar Terrestrial Relation Observatory ( STEREO  B) encountered a compound stream containing several interacting structures. Our analysis suggests that the stream consists of two ICMEs followed by an embedded ICME/CIR. The sudden appearance of the third ICME within the fast wind side of the CIR causes the proton temperature(   $T_{p} $    T  p) to drop suddenly to its lowest level in about 1.2 hours, from    $3.89 \times 10^{5}$   3.89  ×    10    5 K to    $1.07 \times 10^{4}$   1.07  ×    10    4 K (by a factor of ≈36). The fast wind which follows the CIR influences not only the third ICME’s temperature but also its proton beta (   $\upbeta_{\mathrm{p}}$    β  p). In addition, the third ICME impacts the CIR through expansion and deceleration. A forward pressure wave penetrates ICME1. It’s source is either the second ICME, the merged third ICME and CIR, or it may be a remnant shock. The compression causes an increase of the    $T_{p}$    T  pand    $\upbeta_{\mathrm{p}}$    β  pof the second and part of the first ICME. Despite the signatures of four large-scale interacting structures within the compound stream, it is difficult to reconcile the  in-situ  sequence with other remote sensing observations of ejecta close to the Sun because of the large system of coronal expanding loops above the active region. Compound streams therefore remain difficult to interpret, and further understanding of the subject will depend on the future study of similar events.

The compound stream event of March 20-25, 2011 as measured by the STEREO B spacecraft