Dispersion relation (yellow line) of ion-acoustic mode for T_e = 20 eV, T_e/T_i = 4, and n = 15 cm⁻³ in the plasma rest frame. The red line shows the upper limit of Doppler-shifted frequencies of ion-acoustic waves in the spacecraft frame for the waves propagating parallel or anti-parallel with the solar wind direction with |V_sw| = 1000 km s⁻¹. The grey area presents typical frequencies of ion-acoustic waves observed in the spacecraft frame. Darker grey delimits a region where ion-acoustic waves start to be strongly damped (kλ_D > 1). The local proton plasma frequency is shown by the green dashed line.