The near-Earth plasma originates from either the ionosphere or solar wind. While which of the two is the dominant source remains a subject of significant debate, within the ionospheric sources, there are two major pathways through which the cold dense plasma is loaded from the ionosphere into the magnetosphere; (1) outflow; (2) plasmasphere-ionosphere coupling (i.e., refilling). However, it is not clearly understood where all the cold plasma population is distributed throughout the magnetosphere due to various processes and how the distribution evolves with magnetic activities. The overarching objectives of our studies are to quantify the second pathway, plasmasphere-ionosphere coupling effect, during magnetically active periods. This seminar will focus on the following two specific questions: (1) Where does the cold dense plasma near the dayside magnetopause originate from?; (2) What are the physical mechanisms to explain the observed extreme plasmaspheric erosion for the Sep. 2017 storm? A physics-based model of the ionosphere and plasmasphere, the IPE (Ionosphere-Plasmasphere-Electrodynamics) model is used to address these questions and to help interpret observations. The presentation is closed by identifying outstanding questions, toward improving our understanding of cold plasma coupling between the magnetosphere, plasmasphere, and ionosphere.