Flare induced geomagnetic activity and orientation of the photospheric magnetic field

Abstract

Attempts to determine which solar flares produce geomagnetic storms have focused attention on the importance of the direction of the magnetic field at the flare sites. Pudovkin et al.^1–3 have claimed that solar flares that occur above regions in which the photospheric magnetic fields have southward components produce geomagnetic storms whilst flares associated with northward magnetic fields do not. However, other studies have shown that geomagnetic storms are also highly correlated with energetic flares⁴, especially those accompanied by radio spectral type IV bursts^5–8. McNamara⁸ suggested that these two sets of results were not reconcilable because they implied that nearly all type IV bursts were associated with flares with southward fields. Although this implication seemed unreasonable, recent evidence^9,10 lends it some support. We have investigated the relationships between geomagnetic disturbances, flare energies and flare-site magnetic field directions for all large flares that occurred between 1968 and 1979. Contrary to the results of Dodson et al.¹⁰ we find no significant differences between the average energies of flares with northward, southward or east–west magnetic fields. Furthermore, although the inferred direction of the field at the site of a flare appears to control the geoefficiency of the flare to some extent, we find, contrary to Pudovkin et al.¹, that the relationship between geoefficiency and field direction is unclear.