A sudden deviation in the sunlit geomagnetic field (H component) associated with large solar FLARE X-ray emission.
From Guide to the Sun by Kenneth J.H Phillips
Crochets occur in local daylights hours only and the field changes during them are generally an enhancement of the daily variations at the time of their occurence. Crochets are ascribed to increased ionization in the lower ionisphere produced by soft X-ray emission during solar flares. A typical change in the H or Z elements during a crochet is a few tens of nanoteslas.
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