During the flyby of the spacecraft Giotto at comet p/Halley Poynting vectors and Elsasser variables have been determined to study wave propagation directions. The observed wave properties are compared with the theoretically predicted RH-, LH+ and LH- wave modes. Between 10:36 and 19:11 SCET on March 13, 1986 the predicted dominating energy propagation direction mostly corresponds with the observed one. Only between 12:11 and 13:44 SCET there is no agreement. In this region the excitation of waves is dominated by pick-up ions which are not locally implanted ions in the solar wind but have been picked-up outside the plasma regime studied, and whose free energy is not already used up.¿ 1997 American Geophysical Union