Authors: Kristopher McGlinchey and Nicolas Labrosse
SUPA, School of Physics and Astronomy, University of Glasgow.
Theory predicts that when a solar prominence erupts into the corona, the intensity of the He II line at 304 Å will decrease as a function of the radial velocity of the plasma [1,2]. This predicted change in the radiation output is due to the so-called Doppler dimming effect. Doppler dimming is the decrease in intensity of an atomic resonance line that is pumped by external radiation, when the plasma in which it forms moves so that the pump line is Doppler shifted out of resonance (in the moving frame). Doppler dimming is widely used to diagnose the solar wind speed, and in this nugget we look for the Doppler dimming signature during prominence eruptions. If found, this could provide a diagnostic tool to probe the thermodynamic conditions of the plasma in eruptive prominences. We use He II images from the AIA on board the Solar Dynamics Observatory.
We studied 4 events on 13th June 2010 (a movie of which is shown in Figure 1), 8th September 2010, 19th March 2011 and 10th June 2011. An image cadence of 1 minute was used, apart from for the slowly-evolving 13th June 2010 event, for which an image cadence of 15 minutes was adequate. For each event, a feature in the prominence plasma was chosen that could be tracked confidently through many images. For the 10-06-11 event, two separate parts of the prominence were tracked. The intensities shown in Figure 2 are the nearest-neighbour averaged intensities, corrected for the exposure time and then normalised by the intensity corresponding to the lowest velocity (which is not necessarily at the beginning of the tracking). These normalised intensities can then be compared to those calculated from non-LTE models of the process (i.e. radiative transfer calculations out of local thermodynamic equilibrium).
It was found that for three of the cases studied, the intensity does indeed decrease as velocity increases, as set out by theoretical models. However, for one of the events the opposite is found to be true, in that there is an increase in He II line intensity as velocity increases.
Earlier modeling [1,2] assumed that plasma parameters stays constant during an eruption, but we can expect that the plasma conditions change during an eruption. So we have carried out new theoretical modelling, in which the plasma parameters (such as temperature, column mass, pressure) are allowed to vary. 100 new models were computed, each having varying input plasma parameters. The model corresponding to the lowest velocity was chosen as a reference model and the line intensities for all other models were normalised to the line intensity obtained with that reference model. With this, we can investigate whether the models reproduce the observed intensity behaviour.
As illustrated in Figure 3, it was found that when the plasma parameters are allowed to vary, there were roughly the same number of models where the He II line intensity increases with radial velocity as models where the intensity decreases with radial velocity. We also found that for the higher intensity models, 79% have a larger column mass than the reference model, and for the dimmer models 67% have a smaller column mass than the reference model. These calculations therefore give an idea of how variations of the physical conditions in the prominence plasma can affect the radiative output of He II resonance lines. It turns out that the main parameters that will determine whether the intensity of the He II line increases or decreases with radial velocity are the evolution of the column mass, and also the evolution of the temperature inside the prominence.
The observational results do not contradict the initial theoretical calculations that prompted this investigation, rather they highlight the need for more detailed calculations such as the ones considered in this study. The results show that investigating eruptive prominences in this way could provide a diagnostic tool for probing conditions in the plasma. We are now studying how we can improve the technique by adding another line such as the hydrogen Lyman alpha line, which should be observed by the EUI imager on Solar Orbiter.
More information on the data analysis and the modelling procedure can be found in .
Special thanks go to the Nuffield Foundation for funding part of this research.