  Macroscopic superconducting components are an important building block of
various quantum circuits. Since several of the envisioned applications require
exposure to magnetic fields, it is of utmost importance to explore the impact
of magnetic fields on their performance. Here we explore the complex pattern of
magnetic field penetration and identify its impact on the resonance frequency
of NbTiN superconducting resonators by combining magneto-optical imaging and
high-frequency measurements. At temperatures below approximately half of the
superconducting critical temperature, the development of magnetic flux
avalanches manifests itself as a noisy response in the field-dependent
resonance frequency. Magneto-optical imaging reveals different regimes and
distinguishes the impact of avalanches in the ground plane and resonator. Our
findings demonstrate that superconducting resonators represent a valuable tool
to investigate magnetic flux dynamics. Moreover, the current blooming of
niobium-based superconducting radio-frequency devices makes this report timely
by unveiling the severe implications of magnetic flux dynamics.
