Which of these emission modes is best for auroral propagation?

Explication

The auroral curtain moves and its ionization is turbulent, so the scattered signal picks up heavy Doppler spread that turns a clean carrier into a rasping, buzzy note; CW survives because its information is carried by the presence or absence of a tone rather than by the tone's purity. SSB is often distorted to the edge of intelligibility, and FM and RTTY need far more spectral coherence than an auroral path preserves.