We analytically discuss wave excitation in a homogeneous three component plasma consisting of solar wind protons. electrons and a beam of cometary water ions applied to the plasma environment of comet 67P/Churyumov-Gerasimenko. The resulting dispersion relations are studied in a solar wind rest frame. where a cometary current is solely generated by the water ion beam. https://chefesquipmenters.shop/product-category/measuring-cups/