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An electron beam entering an accelerating cavity excites a wakefield. Thiswakefield can be decomposed into a series of multi-poles or modes. The dominantcomponent of the transverse wakefield is dipole. This report summarizes thehigher order mode (HOM) signals of the third harmonic cavities of FLASHmeasured at various stages: transmission measurements in the single cavity teststand at Fermilab, at CMTB (Cryo-Module Test Bench) and at FLASH, andbeam-excited measurements at FLASH. Modes in the first two dipole bands and thefifth dipole band have been identified using a global Lorentzian fit technique.The beam-pipe modes at approximately 4 GHz and some modes in the fifth dipoleband have been observed as localized modes, while the first two dipole bands,containing some strong coupling cavity modes, propagate. This report alsopresents the dependence of the localized dipole modes on the transverse beamposition. Linear dependence for various modes has been observed. This makesthem suitable for beam position diagnostics. These modes, together with somepropagating, strong coupling modes, have been considered in the design of adedicated electronics for beam diagnostics with HOMs for the third harmoniccavities.

Higher order mode spectra and the dependence of localized dipole modes  on the transverse beam position in third harmonic superconducting cavities at  FLASH