Sunlight is responsible for most the ionization of the Earth's ionosphere. A nighttime, the only ionizing influences are cosmic rays. So, there is a distinct shape to a 24-hour SID data graph, with unique "signatures" or shapes of the graph appearing at sunrise and sunset. Here is an example of a 24 hour period without any solar flares:
Day and Night in SID Data
We added the colors and labels to the graph to help you understand it.
During the nighttime, cosmic rays and thunderstorms and other natural phenomena affect the ionosphere. Hence the data is usuall very wiggly or "noisy". During the day, the Sun's influence overpowers the cosmic rays and other forces, so the data is usually "quiet."
Although the SID monitors are designed to detect Sudden Ionospheric Disturbances (SIDs, caused by solar flares), they also detect the normal influence of solar X-rays and UV light as well as cosmic rays at nighttime.