Working Group 3: The Bz Challenge

Society is increasingly dependent on technologies that are vulnerable to the variable output of radiation, energetic particles and magnetized plasma from the Sun. These outputs can drive radical disturbances in the Earth’s environment known as “space weather”. Space weather has become a national priority, driving a requirement for advanced observation-based modeling throughout the Sun-Earth system.

Observations of a pseudostreamer with a magnetic null and non-radial magnetic expansion. Panels a) and b) show extreme ultraviolet images taken by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO/AIA). The pseudostreamer is apparent as the dark dome in Panel a) with characteristic quadrupolar substructure visible in Panel b). Panel c) shows CoMP observations of the fraction of linearly-polarized light, in which the intersection of the two dark lobes identifies the location of a coronal magnetic null (we have marked this position with the yellow star).  Panel d) shows CoMP linear polarization direction both via the green arrows, and through the red/blue color scale indicating radial (black), clockwise-tilting (blue) and counterclockwise-tilting (red).

The so-called “Bz challenge” is a critical–but difficult–problem at the heart of space-weather prediction. It arises because interactions between interplanetary coronal mass ejections (ICMEs) and the Earth's magnetosphere strongly depend on the relative orientations of their magnetic fields.  In particular, ICMEs with large southward directed magnetic field (“Bz”) are likely to result in high space-weather activity.  The “Bz challenge” then is to predict the strength and direction of the ICME magnetic field at the Earth.

In order to predict these properties at the Earth, we first have to be able to predict them at their origin in the Sun’s outer atmosphere, or corona.  This has proved a tricky problem in and of itself. Although there is a long history of observing the magnetic fields at the surface of the Sun, or photosphere, measuring the magnetic fields in the corona itself is only recently becoming practical. For the past several years the Mauna Loa Solar Observatory Coronal Multichannel Polarimeter (MLSO/CoMP) has provided unique measurements of linearly-polarized infrared light, which are directly dependent on the orientation and topology of coronal magnetic fields.

This year, for the first time, CoMP observations were used to identify magnetic “nulls” in the solar corona.  These regions of very low magnetic field strength are important because magnetic reconnection is favored in their vicinity, potentially acting as a trigger for solar eruptions. In addition, CoMP linear-polarization observations represent a new, model-independent measure of magnetic expansion into the solar wind - a key input to space weather predictive models.  Perhaps most compelling, these new observations indicate that the standard models for space-weather prediction may underestimate both the height of the magnetic null, and the magnetic expansion of the solar wind.

CoMP measurements of linearly-polarized light are thus a powerful diagnostic of critical coronal magnetic topologies and the expanding magnetic flux tubes that channel the solar wind. These observations, along with those enabled by future large telescopes such as the Daniel K Inouye Solar Telescope and the proposed COronal Solar Magnetic Observatory, move us ever closer to overcoming the Bz challenge.