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

The geoeffective, southward IMF ([Formula: see text] ) given in the GSM reference frame as nature presents is compared with that based on idealized, spiral IMF. We obtained [Formula: see text] and [Formula: see text] sorted by the IMF polarity ([Formula: see text] fields) 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 [Formula: see text] is larger than the one from idealized IMF; realistic [Formula: see text] polarity 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 [Formula: see text] fields 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 [Formula: see text] fields and those from the RM model that assumes an idealized IMF. It confirms that [Formula: see text] plays a crucial role for [Formula: see text] . Finally, it paves a way to properly link the variations seen in geomagnetic activity with the pattern of the measured [Formula: see text] fields.