Nature of Subproton Scale Turbulence in the Solar Wind

The nature of subproton scale fluctuations in the solar wind is an important question, partly because two similar types of strong electromagnetic turbulence can occur: kinetic Alfvén turbulence and whistler turbulence. These two possibilities, however, have one key qualitative difference: whistler turbulence, unlike kinetic Alfvén turbulence, has negligible power in density fluctuations. We present new observational data, as well as analytical and numerical results, to investigate this difference. These results show, for the first time, that the fluctuations well below the proton scale are predominantly kinetic Alfvén turbulence, and, if present at all, the whistler fluctuations make up only a small fraction of the total energy. Implications for solar wind heating will be discussed. Reference: Chen et al., Phys. Rev. Lett. 110, 225002 (2013).