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On the height of the 24 February 2011 white-light flare

On the height of the 24 February 2011 white-light flare. Juan Carlos Martinez, Hugh Hudson, Säm Krucker, Gordon Hurford , Charles Lindsey, Sebastien Couvidat and Jesper Schou. Outline. WL and HXR emissions The 24 February 2001 event Stereo observations Some conclusions.

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On the height of the 24 February 2011 white-light flare

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  1. On the height of the 24 February 2011 white-light flare Juan Carlos Martinez, Hugh Hudson, Säm Krucker, Gordon Hurford, Charles Lindsey, Sebastien Couvidat and Jesper Schou

  2. Outline • WL and HXR emissions • The 24 February 2001 event • Stereo observations • Some conclusions 12th RHESSI workshop, Nanjing, China

  3. WL and HXR emissions • White-light continuum is usually associated with the impulsive phase of a solar flare. • This could imply that high-energy non-thermal particles have penetrated deep into the lower solar atmosphere (Najita& Orrall 1970; Švestka1970) • The association with particle acceleration has always suggested >10 keV electrons in particular (Hudson 1972; Rust & Hegwer 1975; Hudson et al. 1992; Neidig & Kane 1993). • These particles can be observed indirectly via their hard X-ray bremsstrahlung emission (Peterson & Winckler 1959). • The bremsstrahlung signature may also result from acceleration directly in the lower atmosphere, rather than in the corona (Fletcher & Hudson 2008). 12th RHESSI workshop, Nanjing, China

  4. WL and HXR emissions ? • Is widely accepted that the HXR and the WL emission associated with it are generated in the chromosphere. • The WL photospheric signatures could be generated by radiative recombination or by backwarming. Thick target WL source Phostospheric WL source HXR (>10 keV) 12th RHESSI workshop, Nanjing, China

  5. GOES M2.0 event in NOAA active region11081, located approximately at N22W45 (Martinez Oliveros et al., 2011) Are we observing the photospheric (backwarming) or the chromospheric component? 12th RHESSI workshop, Nanjing, China

  6. The 24 February 2001 event • The we flare studied was a GOES M3.5 event hosted by NOAA active region 11163. • The flare occurred close to the East limb, but not so far as to occult the footpoints. • Both hard X-rays and white light appear to lie significantly on the front side of the limb. 12th RHESSI workshop, Nanjing, China

  7. In this study we used data from: • SDO/HMI (base difference images @ 6173 A shown here) • RHESSI • STEREO-B 12th RHESSI workshop, Nanjing, China

  8. Martinez-Oliveros et al. in preparation Time series for SOL2011-02-24, showing the GOES 1-8Åsoft X-rays in gray, the RHESSI 30-80 keV flux in black in units normalized to the white light. There is a gradual separation in time of the two footpoint light curves (red and blue), due to background fluctuations. 12th RHESSI workshop, Nanjing, China

  9. Martinez-Oliveros et al. in preparation The emission sources of SOL2011-02-24 as viewed in X-rays (RHESSI ) and white light (SDO /HMI). The left and right panels both show hard X-ray images (30-80 keV) made with the CLEAN technique for the interval 07:30:50.9 – 07:31:35.9 UT, exactly that of the HMI integration. These images were made with RHESSI subcollimators 1-4, with uniform weighting, giving an angular resolution (FWHM) of ~3.1”. 12th RHESSI workshop, Nanjing, China

  10. Schematic view of the radial coordinate of the flare footpoints, in hard X-rays and white light, reducing the information to the essential parts. Martinez-Oliveros et al. in preparation 12th RHESSI workshop, Nanjing, China

  11. STEREO observations Martinez-Oliveros et al. in preparation • This flare was well observed, near disk center, by STEREO-B, near quadrature (space-craft helio-longitude −94.5◦). • One EUV (195 A) image was taken during the integration time of the WL. • The STEREO-B observa-tionsprove that the flare was not occulted. 12th RHESSI workshop, Nanjing, China

  12. STEREO observations The heliographic coordinates of the brightest point of the flare, as interpolated within the saturation region, are (E84.3±1.0◦, N15.5±0.2◦) for the STEREO-B image at 07:31:01 UT, corresponding to µ = 0.076 ± .018 in the geocentric view. An ad hoc uncertainty was taken by assuming a range of ± 10 EUVI pixels (pixel size 1.59”) in the EW direction. Martinez-Oliveros et al. in preparation 12th RHESSI workshop, Nanjing, China

  13. Source height • To determine the absolute source height, we use the limb as a reference by applying a standard inflection-point method. • By comparison with the STEREO-B image, we find an absolute source height of 0.7 ± 1.1 Mm. • There is a systematic error of ∼0.2 Mm, because we have made no correction for albedo. • The dominant source of error is our uncertainty in the STEREO flare position, due to the detector saturation. Martinez-Oliveros et al. in preparation 12th RHESSI workshop, Nanjing, China

  14. Some Conclusions • We find, within errors, that the centroid of the WL emission source coincides with that of the HXR source, in the solar radial direction, to within uncertainties of a fraction of a Mm. • This conclusion differs from that of Battaglia& Kontar (2011), as the result of a different interpretation of the image metadata. The overlay shown here is correct (M. Battaglia & E. Kontar, personal communication 2011). • We found that the height of formation of the WL emission was 0.7 ± 1.1 Mm. This is formally consistent with a thick-target model. • We note that an improved understanding of the STEREO imaging would yield a much better result. 12th RHESSI workshop, Nanjing, China

  15. BUT! 12th RHESSI workshop, Nanjing, China

  16. Yes, that's right! The relative size is the issue. For HMI we can measure the displacement directly from the limb, so the height measurement is quite solid. For RHESSI we must basically measure the distance from Sun center by RAS, so it depends explicitly on the RAS plate scale. How precisely do we know the pixel size? In principle we can calibrate this by comparing with HMI. Thus this comparison, just estimates the systematic effect of aperture size on the inflection-point limb location, in the presence of limb darkening. 12th RHESSI workshop, Nanjing, China

  17. One interesting question is, to what extent does the angular resolution of each telescope displace the limb? The reason for this displacement would be the limb-darkening function. This function depend quite sensitively on the wavelength of the observation (SAS and HMI are 617.3 and 670 nm). Modeling the function we found that a relative displacement of about 0.4”. This suggests that the X-ray source should move down by 0.4 (±0.1 or so) arc sec relative to the WL source. Conclusion (beta): Extreme care with metrology and systematic error is needed draw conclusions at sub-arc sec image scales. 12th RHESSI workshop, Nanjing, China

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