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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