Suppression of Hydrogen Emission in an X-class White-light Solar Flare

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Figure 1: A schematic diagram of the optical setup. The image selector is placed in the focal plane of the horizontal solar coelostat (HSFA 2, Ondrejov Observatory, Czech Republic). The context camera shows an image of the solar chromosphere in H alpha together with the position of the selected diaphragm. The light is focused onto an optical fibre feeding the HR4000 1D spectrometer.]]
Figure 1: A schematic diagram of the optical setup. The image selector is placed in the focal plane of the horizontal solar coelostat (HSFA 2, Ondrejov Observatory, Czech Republic). The context camera shows an image of the solar chromosphere in H alpha together with the position of the selected diaphragm. The light is focused onto an optical fibre feeding the HR4000 1D spectrometer.]]
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Figure 2 shows NOAA 12087 near the limb on 2014 June 11. This active region produced two X-class flare shortly after its appearance on the  east limb on 2014 June 10. The six snapshots obtained between 8:07 and 9:09 UT show the M3.0 (first row) and X1.0 (forth - sixth rows) flares with significantly different spectral response. The second flare had a clear white-light counterpart also visible in HMI [5].
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Figure 2 shows NOAA 12087 near the limb on 2014 June 11. This active region produced two X-class flares shortly after its appearance on the  east limb on 2014 June 10. The six snapshots obtained between 8:07 and 9:09 UT show the M3.0 (first row) and X1.0 (forth - sixth rows) flares with significantly different spectral response. The second flare had a clear white-light counterpart also visible in HMI [5].
[[File:296f2.png|700px|thumb|center|  
[[File:296f2.png|700px|thumb|center|  

Revision as of 12:40, 3 March 2017


Nugget
Number: 296
1st Author: Ondřej Procházka
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Published: 20 March 2017
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Contents

Introduction

Although we have been observing white-light solar flares (WLF) since 1859, it is still not clear, what their emission mechanism is. Potential scenarios include optically thin hydrogen emission from chromosphere (so-called type I) and optically thick blackbody radiation from deeper layers (type II) [1]. Spectroscopic observations in the NUV/visible become very important in order to distinguish between the two scenarios. There is currently a lack of WLF spectra with most of those currently available obtained more than 30 years ago [2]. Such spectra are routinely observed in M-dwarf stars due to the very high contrast and flare frequency offered by these objects [3].

Spectral observations of solar flares are difficult to obtain due to the difficulty in predicting where the white-light emission emerge and the low contrast. To maximise the probability of capturing a WLF with a slit-based spectrograph we constructed a post-focus instrument (schematic in Figure 1) with a circular diaphragm delimiting only part of the solar disc (typically an active region), over which the light is integrated [4]. The setup allows us to obtain a spectrum of that region (350 nm - 440 nm) with a spectral resolution of 0.03 nm/pixel but without any spatial information within the region. A co-temporal context image and a reflection from the diaphragm are also obtained.

Figure 1: A schematic diagram of the optical setup. The image selector is placed in the focal plane of the horizontal solar coelostat (HSFA 2, Ondrejov Observatory, Czech Republic). The context camera shows an image of the solar chromosphere in H alpha together with the position of the selected diaphragm. The light is focused onto an optical fibre feeding the HR4000 1D spectrometer.

Figure 2 shows NOAA 12087 near the limb on 2014 June 11. This active region produced two X-class flares shortly after its appearance on the east limb on 2014 June 10. The six snapshots obtained between 8:07 and 9:09 UT show the M3.0 (first row) and X1.0 (forth - sixth rows) flares with significantly different spectral response. The second flare had a clear white-light counterpart also visible in HMI [5].

Figure 2: Observations of M3.0 and X1.0 solar flares. The context images in the left column show the solar chromosphere in H alpha with a bright circle (reflection from the delimiting diaphragm) overlaid. Only light inside the circle enters the spectrometer. The middle column shows the flare excess, while the right column shows the excess normalized to quiet Sun.

The spectrum of the X1.0 flare shows an absence of emission in the higher order Balmer lines, which are commonly observed in these events. The lack of hydrogen emission was also observed by SDO/EVE MEGS-B spectrometer in waverange 35 - 105 nm, that covers higher order Lyman lines and GOES/EUVS (shown in Figure 3).

Figure 3: Lightcurves of Lyman continuum recorded by EVE/SDO and X-rays recorded by GOES (upper panel), Lyman lines (middle panel), recorded by EVE/SDO and Lyα (bottom panel), recorded by GOES. The vertical dashed line marks the maximum in RHESSI 100–300 keV flux.

Radiative modeling

In an effort to interpret the lack of hydrogen emission we performed two models using the RADYN code [6]. The `conventional’ approach employs electron beams with parameters estimated from RHESSI observations (F=3E11 erg/cm2/s, E(cut-off)=20 keV, delta=3). The type II WLF are expected to originate in the deeper layers, so in the second model we deposited the same energy flux directly into the temperature minimum region (heights < 600 km). The resulting spectra in Figure 4 are qualitatively different. While the beam heating produces clear and strong emission in Balmer lines, a deposition of the same energy into deeper layers causes a massive increase in the continuum, while the lines have absorption cores with elevated wing emission.

Figure 4: Simulated spectra in the vicinity of the higher Balmer lines for (left) an electron beams (F = 3E11 erg/cm2/s, Ec = 20 keV, d = 3) (right) a response of the atmosphere on direct temperature minimum region heating (F = 3E11 erg/cm2/s).

Conclusions

RADYN simulations provide some very important insights into the origin of the X1.0 WLF emission. In order to generate the strong continuum without substantial Balmer emission the energy must be dissipated deep into the atmosphere (< 500 km). Our context imaging shows that only a small fraction of the measured area was affected by the flare (4 - 7 %). A more comparison of the spectra would require a composite spectrum that will include both flaring and quiet signal corrected for the corresponding areas. Additional simulation work suggests that if we deposit the energy sufficiently deep in the atmosphere the emission wings will disappear and we get clear absorption profiles. More detail on this research were published in Astrophysical Journal

References

  1. Machado et al., White light flares and atmospheric modeling
  2. Hudson et al., The white-light continuum in the impulsive phase of a solar flare
  3. Kowalski et al., Time-resolved Properties and Global Trends in dMe Flares from Simultaneous Photometry and Spectra
  4. Kotrč et al., New Observations of Balmer Continuum Flux in Solar Flares. Instrument Description and First Results
  5. Procházka et al., Suppression of Hydrogen Emission in an X-class White-light Solar Flare
  6. Allred et al., Radiative Hydrodynamic Models of the Optical and Ultraviolet Emission from Solar Flares
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