Lucia Kleint


Changes of the Magnetic Field during Flares
The photospheric magnetic field has been found to change during flares in many observations. It is however unclear which mechanism transports the required energy through the solar atmosphere. In contrast, chromospheric magnetic fields are very difficult to measure, both because of the lower signal and because of the more complex interpretation due to non-local thermal equilibrium.

We obtained the first high-resolution chromospheric polarimetric flare data and found stepwise chromospheric magnetic field changes during a flare (Kleint, 2017). The chromospheric changes are stronger and occur in a larger area. They are located near footpoints of loops, in contrast to photospheric changes which predominantly occur near neutral lines. A surprising result was a difference in timing, sign, and size of the changes when comparing the two atmospheric layers. Another surprise was that a model of contracting loops is not compatible with the observed direction of changes. Instead, we rather interpret the changes as an untwisting of loops, but future vector magnetic field measurements will need to verify this hypothesis.

Left: Photosphere (background) with color-coded photospheric magnetic field changes and contours of hard X-ray radiation. Right: Chromospheric magnetic field changes. From (Kleint, 2017).

Dissipation of Flare Energy: Continuum Emission
Intensities of several pixels before (left) and during the flare (right). The enhancement is largest in the NUV because of the contribution of the Balmer continuum. A simple blackbody-fit, as is often used, is not a good fit for flare enhancements. From (Kleint et al., 2016)
Continuum emission, sometimes also called white-light emission, is a long-standing puzzle of flare physics. It is unclear how the photosphere can be heated by a few hundred degrees because electron beams are thought to be stopped higher up in the chromosphere. Additionally, continuum emission is not purely photospheric, but also contains contributions from optically thin recombination radiation (Balmer, Paschen, ... continua) forming in the chromosphere.
In a study of the best-observed flare, the first X-flare that was observed by IRIS, we found strong enhancements in the NUV, which we interpreted as the Balmer continuum (Heinzel & Kleint, 2014). By combining spectral points from the UV to the IR we were able to determine that this flare had continuum emission contributions from the photosphere and the chromosphere, which support a scenario of ''backwarming'' where energy is radiatively transported into the lower solar atmosphere (Kleint et al., 2016). The energy contained in the continuum emission is ~10%-20% of the energy of accelerated electrons >20 keV, indicating that other energy dissipation mechanisms need to be explored as well.

The best-ever observed flare
Photosphere (background) and chromosphere (inset) during the X1 flare on 2014-03-29 taken at the DST.
X-flares are the strongest and rarest types of flares. There so far is no way to predict when or where a flare will occur. It is therefore very rare to observe them with high-resolution telescopes, which only see a small percentage of the solar surface. But on March 29, 2014 we achieved something unprecedented: We coordinated a flare-observing campaign at the ground-based Dunn Solar Telescope and caught an X1 flare together with coordinated spacecraft observations whose pointing we had to guess and request a day in advance. The observations are so special that they resulted in a live NASA press conference and several press releases, e.g.: These observations led to several publications and new insights into flare physics. A selection of different publications: