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The Prevalence of O IV Density Diagnostics in UV Burst Spectra

Bacon, Amanda

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  "description": "<p>We present a study of O IV density diagnostics in the far ultraviolet&nbsp;(FUV) spectra of UV bursts. UV bursts appear as compact, short-lived&nbsp;brightenings observed in active regions. The FUV spectra of UV bursts&nbsp;exhibit peculiar properties that suggest they are signatures of magnetic reconnection deeply embedded in the cool solar chromosphere; these include&nbsp;intensification and broadening/splitting of emission lines, the presence of&nbsp;optically thin Si IV 1393.8/1402.8 &Aring;&nbsp;emission lines that form at transition region temperatures (<span class=\"math-tex\">\\(\\ge\\)</span> 80,000 K), and the presence of absorption&nbsp;features from cool metallic ions (Ni II 1393.3 &Aring;&nbsp;and Fe II 1392.8 &Aring;). Improving our understanding of these bursts will give us insight into how&nbsp;they contribute to the heating of the lower solar atmosphere. In particular, it is important to constrain the formation altitudes of these events,&nbsp;which can be estimated via lower-bound measurements of electron densities using O IV and Si IV emission lines. However, it is unclear how often&nbsp;the forbidden O IV 1401.2 &Aring;&nbsp;line appears in UV burst spectra due to its&nbsp;potential to extinguish at high electron densities as a result of collisional&nbsp;de-excitation. This work will determine how often this critically important spectral feature arises in UV burst events. We locate UV bursts in&nbsp;AR11850 over nine observations from 24-27 September 2013 using data&nbsp;from the Interface Region Imaging Spectrograph (IRIS), which provides&nbsp;simultaneous imaging and spectroscopic data of the upper chromosphere&nbsp;and transition region in the far and near ultraviolet (NUV). We detect&nbsp;UV bursts by applying a four-parameter single-Gaussian fit to the Si IV&nbsp;1393.8 &Aring;&nbsp;emission line. We perform cuts in the 4-D parameter space to&nbsp;isolate the UV burst population, then we manually inspect the remaining&nbsp;spectra for signs of Ni II 1393.3 &Aring;&nbsp;absorption. We use the resulting sample to look for instances of the O IV 1401.2 &Aring;&nbsp;emission line. With the&nbsp;intent of obtaining a distribution of its statistical signi cance, we measure&nbsp;the total integrated intensities and their uncertainties for each O IV line&nbsp;associated with a UV burst. We find that 33.62% of the sampled O IV&nbsp;lines have signal-to-noise ratios above 3.0, which demonstrates that electron density and altitude estimates are possible for a sizable fraction of&nbsp;UV bursts.</p>", 
  "license": "", 
  "creator": [
      "affiliation": "Bennington College", 
      "@type": "Person", 
      "name": "Bacon, Amanda"
  "url": "", 
  "datePublished": "2018-11-13", 
  "keywords": [
    "Solar activity", 
    "Solar chromosphere"
  "@context": "", 
  "identifier": "", 
  "@id": "", 
  "@type": "PresentationDigitalDocument", 
  "name": "The Prevalence of O IV Density Diagnostics in UV Burst Spectra"
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