Abstract
The magneto-hydrodynamic (MHD) description of the solar wind has long been regarded as one of the most successful frameworks for investigating solar wind heating and acceleration. Under a set of simplifying assumptions, it provides a reduced large-scale description that is widely used to study the energetic role of electromagnetic fluctuations in the solar wind energy budget.In practice, applying simplification can result in the loss of some physics compared to an exact set of equations. However, an observational energy budget can help us to verify whether any energetic information has been lost in the process of simplification.
To do so, we directly test the validity of the large-scale non-linear ideal MHD energy budget with solar wind observations from Parker Solar Probe (PSP) and Solar Orbiter (SO). After verifying the reliability of the measurements through comparison with previous studies of similar solar wind properties, we test the conservation of the total energy predicted by the MHD theory. We find that the average total energy increases with radial distance by 49% (+-9%) between 14 and 203 solar radii. The increase in kinetic energy is not sufficiently compensated by the thermal and electromagnetic contributions.
After investigating the major sources of observational uncertainty, we find that they cannot solve the observed energy increase.
These results suggest that the large-scale theoretical description may neglect an energetically significant contribution to solar wind acceleration, or alternatively that part of the underlying theoretical assumptions may not be well suited for describing the observations.
