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What is X-ray Flux?
X-ray flux measures the intensity of X-rays reaching Earth from the Sun, in watts per square meter (W/m²), continuously monitored by GOES satellites. It's the raw measurement behind the familiar A/B/C/M/X solar flare letter scale — each letter represents a tenfold jump in flux.
Why it matters
X-ray flux is the definitive, standardized way solar flares are classified and compared. It's watched continuously by forecasters because a sudden spike is the earliest, most immediate sign of a flare in progress.
Typical values
Background (quiet-sun) flux sits below 10⁻⁷ W/m² (A-class). C-class flares (10⁻⁶-10⁻⁵ W/m²) are common. M-class (10⁻⁵-10⁻⁴) can cause brief blackouts near the poles. X-class (≥10⁻⁴) are the rare, major events.
How scientists measure it
GOES weather satellites carry X-ray sensors (XRS) that continuously measure incoming X-ray intensity across two wavelength channels, with the "long" channel (0.1-0.8nm) serving as the standard measurement for flare classification.
Why it affects Earth
X-rays from a flare travel at the speed of light and reach Earth in about 8 minutes, ionizing the upper atmosphere on the sunlit side and disrupting high-frequency radio communication — this is why strong flares cause near-instant radio blackouts, unlike CMEs which take much longer to arrive.
FAQ
How fast do X-ray flux effects reach Earth?
Essentially instantly (about 8 minutes, the light travel time from the Sun) — much faster than the hours-to-days it takes for an associated CME or proton event to arrive.
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