Solar EUV index for aeronomical studies at Earth from Langmuir probe photoelectron measurements 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, Ipe, is mainly due to the impact of solar EUV photons with energies above the work function of the probe. Ipe thus measures the total EUV flux over the wavelength interval from about 30 nm to Lyman alpha; on Venus this solar flux, VEUV, was defined by Brace et al. (1988). When translated to the solar longitude of Earth, for Earth use, it is called EEUV. To examine the potential of EEUV as a solar EUV flux index, we study the behavior of ionospheric parameters ƒ0E, ƒ0F1, and ƒ0F2 at mid‐latitude stations and compare their relationship with EEUV and with the 10.7‐cm solar radio flux. We find ƒ0F1 and ƒ0F2 to be better correlated with EEUV than with the 10.7‐cm flux. However, ƒ0E 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, EEUV, 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.