The BZ Challenge (WG3)

Space weather arising from interactions between interplanetary coronal mass ejections (ICMEs) and the Earth's magnetosphere strongly depends on the relative orientations of their magnetic fields.  In particular, ICMEs with large southward components correlate with high space-weather activity. It is thus critical to predict the strength and direction of the ICME magnetic field at the Earth.

HAO Strategic Working Group #3 ("The Bz Challenge") focuses on just this issue, and has the overarching goal of overcoming roadblocks in the path to full capability for predicting magnetic field direction and other solar-wind properties at the Earth.  As a critical first step, it is necessary to quantify magnetism back at the solar corona, where CMEs originate.

The BZ challenge

One known CME source region is the so-called "pseudostreamer", a bright dome of coronal plasma surrounded by magnetic fields open to the solar wind.  By adding a twisted magnetic field, or "flux rope", beneath the pseudostreamer dome, Karna et al. (Astrophysical Journal, submitted, 2018) demonstrated that it is possible to improve models of coronal magnetic fields.  In particular, when the flux rope is added to the upper lobe of the pseudostreamer where a dark, elliptical "cavity" resides (see Figure), the dome of the pseudostreamer is pushed upward—resulting in a better match to observations including measurements of linear polarization magnitude and azimuth angle measured by NCAR’s MLSO/CoMP telescope.

Many space-weather prediction models draw upon correlations between solar wind speeds and “potential” coronal magnetic models which assume that no currents are present.  This work demonstrates that coronal currents - as are contained in the magnetic flux rope - significantly affect coronal magnetic fields and by extension solar wind speeds.  Finally, this work is important from the point of view of understanding CME sources, since the continued energization of this magnetic flux rope may be the cause of an observed CME from this region.