Unique Multi-spacecraft constellation during active Kelvin-Helmholtz Instability

We present observations of enhanced Kelvin-Helmholtz (KH) wave activity detected on November 27, 2021 between 05:00 - 6:30 UT, during predominantly southward IMF orientation, at the low-latitude magnetopause boundary by THEMIS and CLUSTER, both located on the magnetospheric side and separated by about 10 R_E in the xy plane in the GSM system. Such a constellation of spacecraft and their multi-point measurements provides a unique opportunity to study the propagation of KH waves along the flank magnetopause and shed light on their evolution from the dayside (THEMIS location) to the nightside flank sector (Cluster location). Furthermore, the spacecraft separation enable us to recover information on the extent of the waves and the penetration of magnetosheath plasma into the magnetospheric side. Interestingly, such fluctuations were observed by all three THEMIS A, D and E spacecraft, whereas only two of the CLUSTER spacecraft (C1, C2) clearly observed them. In addition, C1 and C2 observed quite periodic fluctuations in the magnetic field, while THEMIS observed less periodic fluctuations separated by intervals of observation of relatively quiet magnetosheath plasma. These observations suggest a growth and evolution, or interaction between KH waves/vortices as they propagate tailward. Finally, a conjunction with MMS located at the same xy location as Cluster, but in the southern hemisphere, provides a better understanding of the surrounding plasma, as well as the effect of KH waves possibly propagating to the high-latitude magnetosphere.