SOHO and Chandra EPHIN (quiet time) daily averaged data

The SOHO and Chandra EPHIN quiet-time products are derived from the mission-wide files all_soho.csv and all_chandra.csv. For both data sets, the time information is converted into calendar datetime, the accumulation time is interpreted in minutes, and only records fulfilling the selected accumulation-time condition are retained. The output is reduced to the common columns datetime, acc, b00, f0, and ef0, so that SOHO and Chandra can be compared directly. The files are written both as yearly products and optionally as combined mission-wide files, with metadata headers documenting the source file, time reconstruction, and selection criteria. The quiet-time selection follows the section-10 method: the yearly B00 distribution is fitted with a Gaussian, and the quiet-time threshold is defined as the fitted mean plus four standard deviations, with mission-dependent upper limits. Records below this threshold are treated as quiet-time candidates, while enhanced B00 intervals are rejected.