What is the Nighttime Light Interaction I ndex? Validations at Yangtze River Delta Urban Agglomerations

Urban spatial interaction serves as an indicative measure for estimating the intensity and character of interurban linkages and relationships. The previous studies have utilized intercity relational data (e.g., population migration, goods trade, and information exchange) to build up urban connections directly. Besides, the nighttime light (NTL) data have also been adopted to simulate a dynamic urban intercity flow. However, the relevant studies have not clearly defined the urban spatial interaction based on the NTL data. To answer this question, we used trial-and-error testing to define the NTL-based interaction. First, five traditional urban interactions were selected as the potential definitions, including population migration, transfer of innovation, information flow, financial flow, and urban composite interaction. Second, as usual, the NTL-based urban interaction, named the Nighttime Light Interaction (NTLI) index, was simulated based on the NPP-VIIRS-like NTL data and the radiation model. Taking the Yangtze River Delta Urban Agglomerations (YRDUA) as an example, we found that the NTL-based urban interaction is more like the population migration at the urban agglomeration scale and the provincial scale with R 2 of 0.71 and 0.59, respectively. In addition to this, the NTLI index has a weak correlation with the Transfer of Patent (TP) index, Information Flow (IF) index, Economic Interaction (EI) index, and Composite Interaction (CI) index. To sum up, the interaction network from NTL data can be an adequate proxy of urban population interaction, rather than the knowledge network or economic network. This study provides a new thought for urban network simulation and urban population mobility research.