Geoeffective interplanetary magnetic field (IMF) from in situ data: realistic versus idealized spiral IMF

Abstract

AbstractThe geoeffective, southward IMF ($$B_\text {s}$$Bs) given in the GSM reference frame as nature presents is compared with that based on idealized, spiral IMF. We obtained$$B_\text {s}$$Bsand$$B_\text {s}$$Bssorted by the IMF polarity ($$B_\text {s}$$Bsfields) from in situ data at a high 16-second resolution. Idealized IMF is derived by omitting the fluctuation of the IMF in the GSEQ Z-direction. Results are: the absolute value of realistic$$B_\text {s}$$Bsis larger than the one from idealized IMF; realistic$$B_\text {s}$$Bspolarity fields exist in all seasons, while those from idealized IMF exist only around spring/fall when the IMF points toward/away from the Sun; idealized$$B_\text {s}$$Bsfields match the predictions of the Russell–McPherron (RM) model almost ideally. The present study has resolved the issue related to the patterns and absolute values of the observed$$B_\text {s}$$Bsfields and those from the RM model that assumes an idealized IMF. It confirms that$$B_{z,\text {GSEQ}}$$Bz,GSEQplays a crucial role for$$B_\text {s}$$Bs. Finally, it paves a way to properly link the variations seen in geomagnetic activity with the pattern of the measured$$B_\text {s}$$Bsfields.