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Using Challenging Minisatellite Payload (CHAMP) observations, responses of ionospheric radial current (IRC) in F‐layer to sawtooth substorms in different magnetic local times are investigated. The zonal wind effect alone cannot entirely explain the variability of the substorm time disturbance IRC. When substorms commence amid stable southward interplanetary magnetic field (IMF), it induces an eastward (westward) equatorial electric field in the daytime (nighttime). This electric field induces an equatorward (poleward) Hall current at low latitudes, consequently generating an upward (downward) perturbation in IRC. Conversely, substorms with varying IMF Bz, where IMF is southward but with a reduced magnitude or turned north after the onset, induce a westward (eastward) equatorial electric field in the daytime (nighttime). This electric field induces a poleward (equatorward) Hall current at low latitudes, consequently generating a downward (upward) disturbed IRC. Notably, this effect is primarily attributed to the varying IMF Bz rather than solely to substorm onset. Plain Language Summary: Substorms represent intense and dynamic disturbances within the Earth's magnetosphere, characterized by sudden bursts in auroral activity. The interplanetary magnetic field (IMF) stands out as an important factor in triggering substorms. Our work unveils for the first time that substorm accompanied by varying IMF Bz, where IMF is southward but with a reduced magnitude or turned north after the onset, and those during stable southward IMF periods, elicit diametrically contrasting effects on the ionospheric radial current (IRC) across all local time sectors. The IRC assumes a pivotal role in facilitating the energy exchange between the ionospheric E‐F layers, thereby exerting a profound influence on the ionospheric dynamics. Our work underscores a compelling correlation between the disturbed IRC and the penetration of the substorm electric field into the equatorial ionospheric E region under different IMF conditions. The study demonstrates the significant impact of different solar wind IMF conditions on the propagation of energy in Earth's space environment. Key Points: Initial observations of F‐layer radial current responses to substorms under different IMF conditionsSubstorm induces upward (downward) radial currents during the day (night) owing to Hall currents caused by disturbed electric fieldDaytime downward (nighttime upward) radial currents occur only in substorms with varying IMF Bz, due to overshielding electric field [ABSTRACT FROM AUTHOR]