  The nature of subproton scale fluctuations in the solar wind is an open
question, partly because two similar types of electromagnetic turbulence can
occur: kinetic Alfven turbulence and whistler turbulence. These two
possibilities, however, have one key qualitative difference: whistler
turbulence, unlike kinetic Alfven turbulence, has negligible power in density
fluctuations. In this Letter, we present new observational data, as well as
analytical and numerical results, to investigate this difference. The results
show, for the first time, that the fluctuations well below the proton scale are
predominantly kinetic Alfven turbulence, and, if present at all, the whistler
fluctuations make up only a small fraction of the total energy.
