Research ArticlePLANETARY SCIENCE

The development of lower-atmosphere turbulence early in a solar flare

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Science Advances  05 Dec 2018:
Vol. 4, no. 12, eaav2794
DOI: 10.1126/sciadv.aav2794
  • Fig. 1 Light curves and the images and line spectra of flare SOL2016-12-06T10:36:58 at 10:37:55 UT.

    (A) Si IV 1402.77 Å (blue), RHESSI 6 to 12 keV (pink), and AIA 131 Å (black) light curves. (B) AIA 131 Å background image with contours (white) showing the IRIS slit position (green) and RHESSI 6 to 12 keV contours (pink). (C) The eastern ribbon using an IRIS slit-jaw 1400 Å image overlaid with AIA 131 Å contours. (D) Si IV spectral lines at seven locations [colored dots in (B) and (C)]; black line, current time; violet solid, previous time. The dashed-dotted line shows the position of the inferred reference wavelength of 1402.775 ± 0.020 Å. A movie showing all times is available (movie S1). DN, data number.

  • Fig. 2 Temporal evolution of Si IV spectral lines during the flare.

    The Si IV lines at six example flare times (black), observed over a 2″ region along the north-south direction. The lines are fitted with a single Gaussian (gray) that provides an estimate of the line centroid position (vertical purple line) and FWHM (horizontal turquoise line). We show the inferred Si IV reference wavelength plus its uncertainty (1402.775 ± 0.020 Å) (lime regions) and the Si IV laboratory wavelength (1402.77 Å) (black dashed lines).

  • Fig. 3 Temporal evolution of the Si IV line properties during the flare.

    (A) The total FWHMs and nonthermal velocities vnth (turquoise) and (B) the centroid positions and bulk velocities v (purple), observed over a 2″ region along the north-south direction. Both the results of the Gaussian fitting and the moment analysis (see legend) give near-identical results. The Si IV integrated intensity (gray) and the RHESSI 6- to 12-keV light curve (black) are displayed. The dashed lines indicate the times of six spectral lines shown in Fig. 2. Top: A sinusoidal function is fitted to the variations in vnth, estimating the period P and amplitude A (see legend). Bottom: Inferred reference wavelength (lime) and uncertainty (red and blue).

  • Fig. 4 A cartoon of the flaring loop and an interpretation of the observations.

    (A) We envisage a situation where multiple velocity fluctuations occur in the coronal loop and in the region emitting Si IV (red) before and during the flare, perpendicular to some guiding field direction z, and at some angle >0° to the line of sight. (B) Left: We set up a wave and pass it through the modeled region, along z, representing the region emitting Si IV. As the wave passes the length of the region (denoted as z/<λ>, where <λ> is the average wavelength of the wave), its amplitude and wavelength vary with time. (C) Left: An observer (i.e., IRIS) sees all motions (velocity fluctuations) integrated along the line of sight, and we determine the resulting bulk velocity (first moment) and nonthermal velocity (second moment). One wave with varying amplitude and wavelength cannot reproduce the IRIS observations; a single wave produces clear oscillations in the bulk velocity. (B, right) – as (B, left) but for a spectrum of 10 different interacting waves (here, representing wave interactions that can lead to turbulent dissipation). This produces a result (C, right) similar to the IRIS observation, helping us to understand the nature of plasma fluctuations in the emission region (see Materials and Methods). Movies for (B) are available (movies S2 and S3).

  • Fig. 5 Temporal evolution of Si IV 1402.77 Å.

    (A) FWHM (nonthermal velocity, vnth) and (B) centroid position (bulk velocity, v) over the spatially integrated region of Y between −114.9″ and −112.9″ for all times observed by IRIS during the observation (gray) and the flare time [turquoise (A) and purple (B)]. In (A), the nonthermal velocity in the region before and after the flare is shown in the legend, calculated for the raster time resolution (black) and a time binned case (orange). In (B), the determined Si IV reference wavelength in the region plus its uncertainty is shown in the legend for the raster time resolution (black) and a time binned case (pink).

  • Fig. 6 Determining Si IV 1402.77 Å line optical depth.

    (A and B) An output from the RADYN simulations showing the flare model temperature (A) and electron number density (B) versus height above the photosphere, produced using the input parameters shown. The gray region shows the quiet Sun model atmosphere [VAL-C; (34)]. Si IV is formed at log T = 4.9, and the simulations indicate that the electron number density at this temperature ranges from log ne = 9.5 to 11.5, particularly at early times. (C) The line center optical depth, determined using τ0 ~ 0.26 f <ne>/1010 (cm−3), versus electron number density ne for different line FWHM and filling factors f. For most cases, Si IV 1402.77 Å should be optically thin (i.e., τ0 < 1) and free of radiative absorption effects.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/12/eaav2794/DC1

    Fig. S1. A context image of the solar flare in active region 12615.

    Fig. S2. Si IV contribution function.

    Fig. S3. A comparison of Si IV 1402.77 Å and Mg II 2796.35 Å centroid positions.

    Fig. S4. Temporal evolution of Si IV 1402.77 Å line properties during the flare (for two individual pixels).

    Movie S1. Light curves and the images and line spectra of flare SOL2016-12-06T10:36:58 at all studied times during the flare rise, peak, and decay (associated with Fig. 1).

    Movie S2. Plasma velocity fluctuations in space and time due to the passage of a single wave (associated with Fig. 4).

    Movie S3. Plasma velocity fluctuations in space and time due to the passage of multiple interacting waves (associated with Fig. 4).

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. A context image of the solar flare in active region 12615.
    • Fig. S2. Si IV contribution function.
    • Fig. S3. A comparison of Si IV 1402.77 Å and Mg II 2796.35 Å centroid positions.
    • Fig. S4. Temporal evolution of Si IV 1402.77 Å line properties during the flare (for two individual pixels).

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Light curves and the images and line spectra of flare SOL2016-12-06T10:36:58 at all studied times during the flare rise, peak, and decay (associated with Fig. 1).
    • Movie S2 (.mp4 format). Plasma velocity fluctuations in space and time due to the passage of a single wave (associated with Fig. 4).
    • Movie S3 (.mp4 format). Plasma velocity fluctuations in space and time due to the passage of multiple interacting waves (associated with Fig. 4).

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