The mode associated with sā (ā -10.4373) has a much faster time constant than the other modes. If we're interested in the system's behavior over longer time scales, we could ignore this fast mode to get a reduced-order model.
Reason: The fast mode will decay much more quickly than the others, so its effect on the system's long-term behavior will be minimal. The system's response will be dominated by the slower modes (sā and sā) after the initial transient period.
To create a reduced-order model, we would eliminate the fast mode, resulting in a second-order system that approximates the original system's behavior for t >> \tau ā.