Predicting the southward IMF component (the Bz problem)

Develop reliable methods to predict the southward component (Bz) of the interplanetary magnetic field embedded in Earth-directed coronal mass ejections prior to their arrival at Earth, in order to enable accurate forecasting of geomagnetic storm intensity (the "Bz problem").

Background

The geoeffectiveness of a coronal mass ejection depends critically on whether the interplanetary magnetic field has a southward component aligned for reconnection with Earth’s magnetic field. While remote sensing can inform CME structure and propagation, operational forecasts currently rely on in situ measurements near L1 that provide limited lead time.

The paper highlights that, despite progress, the ability to predict the southward Bz component from solar and heliospheric observations remains elusive and is widely recognized in space weather research as the Bz problem.

References

Although we can draw conclusions about the structure and propagation of CMEs in the heliosphere from remote sensing, the prediction of the southward magnetic field component is still an unsolved problem in space weather research, known as the $B_z$ problem \cite<e.g.>[]{vourlidas2019review}.

— First observations of a geomagnetic superstorm with a sub-L1 monitor  (2411.12490 - Weiler et al., 2024) in Introduction (Section 1)

Their value for predicting geomagnetic impact or the magnetic-field orientation at Earth remains a testable statistical question for future catalogs.

— Untangling EIT Waves: What a Measured Speed Actually Traces  (2609.10167 - Podladchikova, 9 Sep 2026) in Section 10, “A compact framework for future measurements”