Edge Localized Modes are an intermittent instability affecting the plasma edge in tokamaks. They can cause a sudden loss between 5 to 30 % of the plasma stored energy. This outburst of heat and particles across the separatrix is preceded by a variety of turbulent and MHD instabilities localized in the narrow pedestal region determining the transport in the edge. Instabilities appear in the form of inter-ELM modes, and precursors, both having a distinct signature regarding toroidal and poloidal structure, amplitude, location, and velocity. To understand the transport caused by the inter-ELM modes as well as the ELM triggering mechanisms, the characterization of these inter-ELM modes as well as the conditions under which they develop, are essential. The work presented in this thesis focuses on the exploration of the inter-ELM modes appearing in the low-frequency part of the spectrum (∼ 10 kHz). At the ASDEX Upgrade tokamak, a variety of edge diagnostics are exploited to access the characteristics of the mode. The particular focus lies on the Electron Cyclotron Emission Imaging (ECEI) diagnostic that measures the electron temperature fluctuations. It delivers information on the spatial structure and mode velocity due to its excellent poloidal resolution. The final upgrade and commissioning of the ECEI diagnostic has been performed during this thesis work.