Solar EUV index for aeronomical studies at earth from Langmuir probe photoelectron measurement on the Pioneer Venus Orbiter

Abstract

We present a solar EUV index for aeronomical studies at Earth, obtained from the Langmuir probe measurement of photoelectron current on the Pioneer Venus orbiter (PVO). When the probe is in the solar wind and is negatively biased, the current, I_pe , is mainly due to the impact of solar EUV photons with energies above the work function of the probe. I_pe thus measures the total EUV flux over the wavelength interval from about 30 nm to Lyman alpha; on Venus this solar flux, V _EUV, was defined by Brace et al. (1988). When translated to the solar longitude of Earth, for Earth use, it is called E _EUV. To examine the potential of E _EUV as a solar EUV flux index, we study the behavior of ionospheric parameters f₀E, f₀F 1, and f₀F 2 at mid-latitude stations and compare their relationship with E _EUV and with the 10.7-cm solar radio flux. We find f0F 1 and f0F 2 to be better correlated with E EUV than with the 10.7-cm flux. However, f0E is better correlated with the 10.7-cm flux, because the 10.7-cm flux is also a proxy for soft X rays, which are an important ionizing source in the E region. We also provide a table of the EUV index, E _EUV, for the period February 12, 1979, through most of 1991 which extends in time the table (of Venus values) published earlier by Brace et al. (1988). The EUV index will continue to grow until the end of the PVO mission in late 1992.