Instability of P-waves just below the transition region in a global solar wind simulation

Context. To progress in the understanding of the solar wind and coronal dynamics, numerical modeling must include the transition region within the simulation domain, and not just as a bottom boundary. Published simulations including a transition region within the domain often do not take into account any modeling of the heat sources and sinks which generate the corona; in the rare self-consistent simulations including it, the transition region itself appears to be chaotic in space and time. Aims. Our aim is to investigate the response to perturbations of the solar atmosphere including transition region and wind, and more specifically how wave propagation is modified by the presence of heat sources and sinks, in the simple 1D, hydrodynamical case, including chromosphere and solar wind. Methods. We integrate the time-dependent hydrodynamic equations of the solar wind with spherical symmetry, including conduction, radiative cooling and a prescribed mechanical heat flux. Once a quasi-stationary wind is established, we study the response of the system to pressure oscillations at the photospheric boundary. We use transparent boundary conditions Results. We find that wavepackets with high enough amplitude propagating upward from the photosphere implode just below the transition region. This implosion is due to the radiative cooling term generating pressure holes close to the wave crests of the wave, which make the wave collapse. In the case where heat sources and sinks are not present in the equations, the wave remains stable whatever the initial wave amplitude, which is compatible with published work. Conclusions. The instability found here is not an instability of the TR itself: on the contrary, instability ceases when the wavepacket enters the TR where conduction is able to balance the cooling. However, the TR as a whole can be destabilized by such implosions, which should be observable when and where the TR is high enough above the optically thick regions.