Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

Abstract

We have carried out the first comprehensive investigation of enhanced line emission from molecular hydrogen, H${}_2$ at 1333.79 Å, observed at flare ribbons in SOL2014-04-18T13:03. The cool H${}_2$ emission is known to be fluorescently excited by Si IV 1402.77 Å UV radiation and provides a unique view of the temperature minimum region (TMR). Strong H${}_2$ emission was observed when the Si IV 1402.77 Å emission was bright during the flare impulsive phase and gradual decay phase, but it dimmed during the GOES peak. H${}_2$ line broadening showed non-thermal speeds in the range 7-18 ${\mathrm{km}\mathrm{s}}^{-1}$, possibly corresponding to turbulent plasma flows. Small red (blue) shifts, up to 1.8 (4.9) ${\mathrm{km}\mathrm{s}}^{-1}$ were measured. The intensity ratio of Si IV 1393.76 Å and Si IV 1402.77 Å confirmed that plasma was optically thin to Si IV (where the ratio = 2) during the impulsive phase of the flare in locations where strong H${}_2$ emission was observed. In contrast, the ratio differs from optically thin value of 2 in parts of ribbons, indicating a role for opacity effects. A strong spatial and temporal correlation between H${}_2$ and Si IV emission was evident supporting the notion that fluorescent excitation is responsible.

Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

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• Imaging and Spectroscopic Diagnostics on the Formation of Two Magnetic Flux Ropes Revealed by SDO/AIA and IRIS
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Cited by in corpus (7)

• Correlated Spatiotemporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-rays in a Solar Flare
• Diagnosing the Optically Thick/Thin Features Using the Intensity Ratio of Si IV Resonance Lines in Solar Flares
• Small-scale magnetic flux emergence preceding a chain of energetic solar atmospheric events
• On the intensity ratio variation of the Si IV 1394/1403 Å lines during solar flares
• Variation in the intensity ratio at each wavelength point of the Si iv 1394/1403 Å lines. Spectral diagnostics of a bifurcated eruption
• Hard X-rays from the deep solar atmosphere. An unusual UV burst with flare properties
• Specialist Discussion Meeting: 3D structure of the flare chromosphere