The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO is being used to measure precise coronal HI Lyα spectral line profiles out to several Doppler half widths. These observations cover heliocentric heights from 1.75 to 3.5 solar radii. Such observations can be used to reveal the proton velocity distribution along the line-of-sight. This paper reports progress toward measuring departures from a Maxwellian distribution that are believed to be needed for the acceleration of solar energetic particles (SEPs) by coronal mass ejection (CME) shocks. The measurements utilize the HI Lyα channel of UVCS, which is equipped with a holographically ruled diffraction grating. Precise characterization of the instrument is required to measure the far line wings and infer the fraction of protons with velocities above the so-called injection velocity. Such velocities are believed to be needed in order for a given proton to participate in the process of coupled turbulent wave excitation and shock acceleration.
展开▼
