Abstract
After being turned off in 1972 the OSO-5 satellite was reactivated during the summer of 1974 for one year. The University of Paris experiment designed to monitor the solar Lα flux operated almost perfectly during that period which occurred near a minimum in solar activity. This new set of data is presented here, showing that neither the total Lα flux nor the flux emitted at the center of the line correlate with the solar activity indices in the same manner as previously found at higher levels of activity.
These new observations confirm that the solar Lα flux varies approximately by a factor of two from solar maximum to solar minimum.
Furthermore, the lower boundary of the transition region seems to be strongly perturbated near solar minimum, since the flux variations observed at the center of the Lα line are drastically different from all those previously reported. This seems to be related to the presence of large coronal holes over the Sun.
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Vidal-Madjar, A., Phissamay, B. The solar Lα flux near solar minimum. Sol Phys 66, 259–271 (1980). https://doi.org/10.1007/BF00150583
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DOI: https://doi.org/10.1007/BF00150583