Geoeffective Interplanetary Magnetic Field (IMF) from in Situ Data: Realistic Versus Idealized Spiral IMF

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