It is shown that proton steams 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 form Fb(&ngr;⊥2, &ngr;, ϕ), where ϕ is the phase angle for motions perpendicular to the solar wind field B0, are capable of driving a left-hand (Alfv¿n) wave at rest frame frequencies &ohgr;<&OHgr;i. The driven wave amplitude becomes a sharply peaked function of &ohgr;/&OHgr;i=&khgr;, and the location &khgr;=&khgr;0 and peak amplitude of the wave B(&khgr;0) depend critically on the ratio of beam speed Vb to the Alfv¿n speed Ca and on the value of solar wind &bgr;. Driven wave amplitudes are estimated to be up to several gammas for beam speeds (in the rest frame) of Vb~2Vsw. The observed frequency and polarization depend upon Doppler shifting due to solar wind flow, and observed polarization could be either left-hand depending upon the vector properties of the solar wind velocity and magnetic field.