HAO Colloquium - Jie Zhao, HAO

Coronal magnetism spectropolarimetric diagnostics from IR to UV

The magnetic field in the corona is important for understanding solar activity, but is difficult to measure due to the tenuous plasma. Therefore many alternative methods have been adopted to get the 3D magnetic field in the corona, such as extrapolation methods relying on the photospheric magnetograms. Such extrapolations make problematic assumptions about the force-free nature of the photosphere, and are highly sensitive to uncertainties in the photosphere magnetic measurements. Measuring the coronal magnetic field directly is thus to be desired, and linear polarization measurements in the visible/IR are already obtained by the Mauna Loa Solar Observatory CoMP telescope providing information about coronal magnetic direction and topology. However other observations such as circular polarization in the visible/ IR and UV unsaturated Hanle measurements are needed to better observe the 3D coronal field.

Solar coronal heating is a long-lasting but still unresolved problem that relates to the coronal magnetic field, and is also tightly coupled to the question of the acceleration of the solar wind. Various solar wind models have been proposed that motivate the study of in-situ and remotesensing observations in order to choose between them. Some information has been provided in the past, for example by UVCS on board SOHO which showed an anisotropic temperature distribution of oxygen ions in the solar wind. But UV coronal polarimetric observations, which diagnose both the coronal magnetic field and its solar wind properties, have only been measured once as a proof of concept by SUMER on board SOHO.

Until the above observations are routinely available, we turn to the FORWARD suite of SolarSoft codes (Gibson et al. 2016) which enables the simulation of a variety of coronal polarization data from physical state models. In particular, we use the LOWHUND model (Low&Hundhausen, 1995), which constructs a flux rope under a dipole arcade in order to demonstrate how the line-of-sight and plane-of-sky components of the magnetic field modify the linear polarization in both the unsaturated (UV) and saturated (IR) Hanle regimes. We also make use of FORWARD's GIBBAGLOW global model (Gibson et al. 1996), which constructs a global axisymmetric multipolar magnetic field in which open and closed magnetic fields are separated by current sheets, allowing the analysis of solar wind velocity and anisotropy on linear polarization in UV lines. These studies allow us to consider how observations of UV and IR might be used together in future to diagnose the coronal magnetic field and solar wind acceleration.

Date and time: 
Wednesday, August 14, 2019 - 2:00pm to 3:00pm