PIONEER-VENUS-ORBITER MEASUREMENTS OF SOLAR EUV FLUX DURING SOLAR-CYCLE-21 AND SOLAR-CYCLE-22

The Pioneer Venus Orbiter (PVO) had on board the electron temperature probe experiment which measured temperature and concentration of elect rons in the ionosphere of Venus. When the probe was outside the Venus ionosphere and was in the solar wind, the probe current was entirely d ue to solar photons striking the probe surface. This probe thus measur ed integrated solar EUV flux (Ipe) over a 13-year period from January 1979 to December 1991, thereby covering the declining phase of solar c ycle 21 and the rising phase of solar cycle 22. In this paper, we exam ine the behavior of Ipe translated to the solar longitude of Earth (to be called EIpe) during the two solar cycles. We find that total EUV f lux changed by about 60% during solar cycle 21 and by about 100% in so lar cycle 22. We also compare this flux with other solar activity indi cators such as F-10.7, L alpha, and the solar magnetic field. We find that while the daily values of EIpe are highly correlated with F-10.7 (correlation coefficient 0.87), there is a large scatter in EIpe for a ny value of this Earth-based index. A comparison of EIpe with SME and UARS SOLSTICE L alpha measurements taken during the same period shows that EIpe tracks L alpha quite faithfully with a correlation coefficie nt of 0.94. Similar comparison with the solar magnetic held (Bs) shows that EIpe correlates better with Bs than with F-10.7. We also compare EIpe with total solar irradiance measured during the same period.