Solar EUV measurements at Venus based on photoelectron emission from the Pioneer Venus Langmuir Probe

Abstract

The photoelectron current from the Pioneer Venus Langmuir probe has provided measurements of the solar extreme ultraviolet flux at Venus since 1979. This current is the product of the photoelectric yield of the collector and the solar spectrum at wavelengths short enough to cause emission. Calculations show that approximately 51% of the emission is due to Lyman α (1216 Å), 46% is produced by wavelengths between 550 and 1100 Å, and less than 3% is due to wavelengths longer than Lyman α. Thus the Langmuir probe provides a direct measure of the total solar EUV flux, including most of the wavelengths that produce the Venus ionosphere and heat and excite neutrals in the thermosphere. The measurement technique is described, and the daily average measurements of photocurrent obtained between 1979 and 1987 are presented. The photocurrents exhibit variations related to the solar cycle and solar rotation, as well as a major 7.2‐month periodicity. We present three indices of EUV based on the measurements: (1) the photoemission current itself, (2) the total EUV flux, and (3) an F10.7‐like solar index. These are compared with related measurements made simultaneously at Earth. Since the measurements are made at Venus, they are more useful for many Pioneer Venus investigations than are Earth or Earth satellite observations of the Sun. These data may also help solar physicists track the intensity of EUV emission regions on the Sun while they are not visible from the Earth. The EUV flux profile of a solar flare event is also illustrated. In the future the method also could be applied on a comet mission to obtain the incident solar EUV flux, to measure the EUV extinction profiles of the cometary atmosphere, and to sample directly the dust and gas environment of the comet through the ionization the dust and gas produce when they impact the collector.