It is shown that proton streams reflected from a bow shock structure that partially organizes their distribution in gyrophase in such a way that the reflected beam distribution function has the formFb(υ⊥², υ∥, ϕ), where ϕ is the phase angle for motions perpendicular to the solar wind fieldBo, are capable of driving a left‐hand (Alfvén) wave at rest frame frequencies ω<Ωı. The driven wave amplitude becomes a sharply peaked function of ω/Ωı=x, and the locationx=xoand peak amplitude of the waveB(xo) depend critically on the ratio of beam speedVbto the Alfvén speedCAand on the value of solar wind β. Driven wave amplitudes are estimated to be up to several gammas for beam speeds (in the rest frame) ofVb∼ 2VSW. The observed frequency and polarization depend upon Doppler shifting due to solar wind flow, and observed polarization could be either left‐hand or right‐hand depending upon the vector properties of the solar wind vel