Kk. Mahajan et al., PIONEER-VENUS-ORBITER MEASUREMENTS OF SOLAR EUV FLUX DURING SOLAR-CYCLE-21 AND SOLAR-CYCLE-22, Solar physics, 177(1-2), 1998, pp. 203-216
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.