Abstract
Type II white-light flares (WLFs) are rare impulsive events in solar atmosphere observed across the electromagnetic spectrum, but without the characteristic emission in hydrogen lines. A unique set of multi-instrument observations is presented with broad wavelength range spectra in the visible and UV range. Data from the RHESSI and Fermi satellites provide evidence of both non-thermal electron and proton beams during the studied flare. The observations put important constraints on the beam parameters and allow a deeper investigation using radiative hydrodynamic modelling.Radiative hydrodynamic simulations show that the observed flare signatures cannot be
explained with ‘standard’ electron beams, since such beams deposit at least part of their energy in the upper chromosphere where hydrogen lines are formed. Instead, the models suggest that the beams penetrate through the upper chromosphere and deliver their energy directly into the lower chromosphere. Such beams can be dominated by both electrons and/or protons, but electron beams require rather exotic parameters. On the other hand, proton beams penetrate easily through the upper chromosphere and deliver enough energy to power the white-light emission while their energy flux can remain
relatively low.
The models show that the previously reported time lag between the X-ray emission and WL emission is not a type II WLFs’ feature, instead these events do not show radiative losses via Lyman emission originating in the top chromosphere. The WL continuum is dominated by free-bound hydrogen emission originating in the mid-lower chromosphere.
| Date of Award | 2019 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Mihalis Mathioudakis (Supervisor) |