A subspace flow field is a naturally-occurring phenomenon which can result in spatial distortion fields. The phenomenon is similar to a wormhole in that it can produce neutrinos, ionized hydrogen, and theta-band radiation. Unlike a wormhole, however, it would not be unusual to find no signs whatsoever of quantum-level fluctuations.
In 2373, the Nyrian scientist Rislan attempted to convince B'Elanna Torres that a subspace flow field was responsible for the systematic replacement of the crew of the USS Voyager with Nyrians, but Torres realized that such a field was not responsible upon analyzing the spatial harmonics, which were not compatible with a subspace flow field. When Rislan realized that Torres was on the verge of discovering their plot to take over the ship, he had her translocated to a self-contained biosphere, where the Nyrians were imprisoning the Voyager crew. (VOY: "Displaced")