Neutrino flavor instabilities intermediate between slow and fast

The neutrino flavor composition in supernovae and neutron star mergers is shaped by flavor instabilities, which are commonly classified into three types: slow (SFI), fast (FFI), and collisional (CFI). The focus here is on mixed-type instabilities combining attributes of the standard varieties. We examine instability growth rates in two-dimensional parameter spaces (``phase diagrams'') whose axes correspond to instabilities of pure type. We find critical curves connecting the pure-type instability onset points. We also exhibit qualitative trends in the growth-rate spectra γ k, emphasizing the occurrence of crossovers and phase transitions in the fastest-growing Fourier mode k max and drawing attention to the distinction between spatial-mean (k = 0) and flavor-wave (k not equal to 0) instabilities. Our paper is a step toward understanding flavor instabilities as they actually occur in nature rather than in idealized limits.