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. 2017 Jun 27:8:15905.
doi: 10.1038/ncomms15905.

Beam electrons as a source of Hα flare ribbons

Malcolm Druett  1 , Eamon Scullion  1 , Valentina Zharkova  1 , Sarah Matthews  2 , Sergei Zharkov  3 , Luc Rouppe Van der Voort  4
Affiliations

Beam electrons as a source of Hα flare ribbons

Malcolm Druett et al. Nat Commun. .

Abstract

The observations of solar flare onsets show rapid increase of hard and soft X-rays, ultra-violet emission with large Doppler blue shifts associated with plasma upflows, and Hα hydrogen emission with red shifts up to 1-4 Å. Modern radiative hydrodynamic models account well for blue-shifted emission, but struggle to reproduce closely the red-shifted Hα lines. Here we present a joint hydrodynamic and radiative model showing that during the first seconds of beam injection the effects caused by beam electrons can reproduce Hα line profiles with large red-shifts closely matching those observed in a C1.5 flare by the Swedish Solar Telescope. The model also accounts closely for timing and magnitude of upward motion to the corona observed 29 s after the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. The active region topology and hard X-ray emission.
(a) The GOES X-Ray light curves of the flare in the 1–8 Å (black) and 0.5–4.0 Å (magenta) channels. The vertical dashed lines correspond to the time interval of the RHESSI spectrum in event 2. (b) RHESSI photon flux spectrum for event 2 with residuals derived with CLEAN in the 20 s interval around the time of Hα emission for thermal (green line) plus single power-law (yellow line) components, giving the total (magenta line). Hard X-ray emission is mostly of thermal nature with a small non-thermal component (see for details the current section and 'Methods section: Reduction of Hα line emission') with the parameters: spectral index about 3.8 and initial energy flux can be a factor (0.7–3) of F0=1010  erg cm−2 s−1. (c) The hard X-ray emission contours appearing in event 1 (top) and event 2 (Bottom, blue contour) coinciding with the times of the observations of Hα kernels with red-shifts in the ribbon (09:16 UT). These are overlaid onto the HMI magnetogram. The response in the 5–12 keV channel is shown using red contours, and the response in the 12–25 keV channel with blue. (d) Hard X-ray emission overlaid on the HMI magnetogram appearing with the event 3 occurring ∼4 min later (09:20 UT), during the maximum in GOES light curve.
Figure 2
Figure 2. Observations with AIA.
(a) A context image for the observation in AIA 193 Å overlaid with the CRISP FOV outlined in green within AR11778. Inset left: The co-temporal (09:16:09 UT) CRISP image in the Hα line far red wing reveals bright flare ribbons point to by the white arrow for event 2 that are co-spatial within RHESSI imaging contours in 6–12 keV (green) and 12–25 keV (purple). Inset right: The Hα dopplergram for the 33 pt. spectral scan per pixel, containing blue/red-shifted motions marked by the relevant colour presented in the range of ±20 km s−1. The blue boxes in the insets (a) highlight the section of the ribbon formation in event 2, which is displayed in b. (b) The image sequence describing evolution of the ribbon in the AIA 94, 171 and 304 Å channels from top to bottom, respectively. These are co-spatial and co-temporal with the bright ribbon features (contoured in red), in the Hα far red-wing images of +1.3 Å. The 171 Å channel reveals a bright jet-like protrusion (within the blue boxed region) that appears to form between the time frames 09:15:54 UT +15 s and +29 s (corresponding to 93 km s−1) in the direction of the blue arrow and disappears by the time frame +49 s.
Figure 3
Figure 3. Hα line profile observations using SST.
(a) The CRISP Hα line core image (6,563 Å) with a blue box outlining the part of the flare ribbon under investigation. The green box corresponds to the pixels selected to construct the average quiet Sun spectral profiles, ie, close to the ribbon formation and free of any activity, within the time interval of the ribbon formation. (b) The corresponding FOV for the Hα far red wing intensity at +1.3 Å, with a red box corresponding to the region where the spectral profiles of interest are extracted. (c) The contoured ribbons of the Hα line core image for the blue box region is presented. (d) The averaged and normalized Hα spectral line profiles, determined from the red box pixels, are presented for time intervals corresponding to the 1st (09:15:54 UT: red solid line), the second (+7 s: purple solid line) and the third (+16 s: blue solid line) time frames. The Hα line profiles display exceptionally strong red-shifts. (e) The averaged and normalized Hα spectral line profiles for significantly later time frames corresponding to +29 s (red solid line), +49 s (purple solid line) and +56 s (blue solid line) when there were no longer strong red-shifts but rather core emission with peaks in both blue and red near wings. The black solid lines describes the averaged QS background Hα profile, deduced from the region defined by the green box in a. Intensities were normalized against the background levels using the QS intensity of 9,890 counts per pixel at 6561.7 Å; as a reference.
Figure 4
Figure 4. Simulated hydrodynamic responses.
The simulated hydrodynamic responses of a flaring atmosphere to injection of a beam with the initial flux of 1010 erg cm−2 s−1 (F10 model, left panels) and 3 × 1010 erg cm−2 s−1 (3F10 model, right panels) following Zharkova and Zharkov showing column depth dependencies of: (a,d)—the electron kinetic temperature, K, (b,e)—the plasma macrovelocity, km s−1 and (c,f)—the plasma number density, cm−3 forming a flaring corona, chromosphere and photosphere (see the text for more details).
Figure 5
Figure 5. Simulated and observed Hα line enhancements.
(a) the synthetic Hα line normalized intensity versus a distance (λ−λ0), in Å, from the Hα line central wavelength, λ0 (λ0=6563 Å) taken from the simulation at +5 s after a beam onset for the F10 model (magenta line), the 3F10 model (cyan line) and a model with initial flux 7 × 109 erg cm−2 s−1 (7F9 model, yellow line) (b) the normalized background-subtracted Hα profile observed +7 s after the ribbon onset in the event 2. (c) The Hα line normalized intensity simulated for the F10 model at later times after the beam onset: +30 s (red solid line) and +70 s (blue solid line) and (d) the observed Hα profiles at the similar times of +29 s (red solid line) and +56 s (blue solid line) after the event 2 onset.
Figure 6
Figure 6. The normalized response functions of the AIA channels.
The normalized response functions of the AIA 94 Å (green line), 171 Å (yellow line) and 304 Å (red line) channels plotted against log10 of T (temperature). The AIA 94 Å channel has its largest sensitivity peak close to 10 MK but it is not limited in sensitivity to that specific temperature. It is shown in the green line that the AIA 94 Å channel has a secondary peak with maximum at the temperature of 1–2 MK.
Figure 7
Figure 7. Simulated and observed AIA light curves.
(a) The simulated light curves in the AIA 94 Å (green line) and 171 Å (yellow line) channels for contributions from the flaring corona, transition region and chromosphere. The simulation does not include background from the overlying upper corona or neighbouring corona. Observed values for the 94 Å (green crosses) and 171 Å (yellow crosses) channels including this background are shown. (b) The simulated profiles of the signals in AIA 94 Å (green line) and 171 Å (yellow line) above background. These profiles have been normalized to 1 at their peak values. The AIA 171 Å channel is particularly bright compared to the AIA 94 Å channel at around 30 s. (c) The normalized fractional excess in AIA 171 Å. The normalized light curves in panel b were subtracted to find the relative excess in the 171 Å channel. This excess is plotted as a fraction of the emission in the 171 Å channel at each instant. The full width half maximum (Horizontal black bar) indicates the times at which the jet is particularly bright in AIA 171 Å compared to AIA 94 Å, the vertical bar represents the maximum relative brightness at around 30 s.
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