A seven‐month solar cycle observed with the Langmuir probe on Pioneer Venus Orbiter

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Abstract
An approximately 7‐month solar cycle has been observed in the photoelectron current measured by the Langmuir probe on Pioneer Venus Orbiter over the time period from 1979 through 1987. The probe photoelectron current, Ipe, is obtained when the spacecraft is outside the Venus ionosphere, and the measured current is due to photoelectron emission caused by EUV solar radiation in the wavelength range from about 30 nm to 121.6 nm. About one half of the Ipe is due to solar Lyman alpha emission. The Ipe data from mid‐1984 through 1987 are dominated by a 7‐month or 216‐day cycle. Spectral analysis of the 1980–1988 data shows that this cycle dominates the spectrum for periods less than 300 days; the second most dominant cycle is at 155 days, a 5‐month period. The Ipe data were spectrally analyzed with three different methods. The 2800‐MHz radio flux, observed from Earth over the same time range, exhibits a similar 7‐month cycle at about 234 days which is stronger than a 5‐month (158‐day) cycle. Solar Mesosphere Explorer (SME) Lyman alpha observations for the time period mid‐1981 through mid‐1988 also have cycles near 5 and 7 months. Since the 7‐month and 5‐month cycles are observed from Venus (Ipe) and from Earth (2800‐MHz radio flux and SME Lyman alpha), the observations suggest that they are intrinsic frequencies, independent of the orbital frequency of the observer. The 7‐ and 5‐month cycles are believed to be caused by long‐lived flux enhancements from nonlinear interactions of global oscillation modes in the Sun's convective envelope (r modes) and radiative interior (g modes).