Time-dependent Simulations of Solar Wind Evolution from Subsonic to Supersonic Flow with Different Coronal-base Outflow Speeds

Recent studies have suggested that solar wind initialized with a flow speed exceeding the eigenspeed at the coronal inner boundary may develop a discontinuity near the sonic point, where the flow speed surpasses the local sound speed. Beyond this discontinuity, the solar wind becomes fully supersonic and continues to accelerate with increasing radial distance. This scenario contrasts with the classical Parker solar-wind solution, in which only flows that adjust themselves to the eigenspeed at the inner boundary can smoothly pass through the critical (sonic) point and transition to supersonic speeds, whereas solutions with other initial flow speeds are not physically realized. In this work, we examine the viability of this alternative flow regime using time-dependent numerical simulations of solar-wind outflows originating from the coronal base. Our results are consistent with the classical Parker solution and demonstrate that such a discontinuity cannot form at the sonic point within a typical time-dependent framework. Regardless of changes in the initial flow speed at the inner boundary, the flow speed converges to the eigenspeed, with a compensating wave emerging at the boundary, owing to characteristic constraints imposed on the initial conditions at the coronal base.