Efficient Calculation of Signal Propagation Times for Radio-Based Neutrino Detection

The Askaryan effect in dense dielectric media is the leading method for detecting ultrahigh energy (>10 PeV) neutrinos via their radio emissions. Neutrinos point directly back to their source and are therefore ideal candidates for multimessenger astronomy, but observing these low-flux particles requires large dielectric volumes such as polar ice sheets. In-ice detectors reconstruct radio signal arrival direction by looking up pre-calculated propagation times from possible interaction locations. Solving for these “travel-times” through glacial ice means simulating wave propagation through a smoothly varying index of refraction. Current methods use numerical minimization to find paths between n pairs of points with complexity O(n²). Applying the Fast Marching Method (FMM) reduces complexity to O(nlog(n)), thereby enabling efficient calculation of travel-times over large volumes. However, the FMM only returns earliest travel-time solutions, resulting in information loss: in heterogeneous media, waves change their propagation direction due to reflection and refraction, leading to multiple travel-time solutions at each point. In this talk, we introduce methods for extending the FMM to multi-valued travel-times and discuss first applications of these methods to data from Summit Station, Greenland.