Presentation Open Access

The Prevalence of O IV Density Diagnostics in UV Burst Spectra

Bacon, Amanda


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    <subfield code="a">This work is supported by the NSF-REU Solar Physics program at SAO, grant number AGS-1560313.</subfield>
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    <subfield code="a">The Prevalence of O IV Density Diagnostics in UV Burst Spectra</subfield>
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    <subfield code="a">&lt;p&gt;We present a study of O IV density diagnostics in the far ultraviolet&amp;nbsp;(FUV) spectra of UV bursts. UV bursts appear as compact, short-lived&amp;nbsp;brightenings observed in active regions. The FUV spectra of UV bursts&amp;nbsp;exhibit peculiar properties that suggest they are signatures of magnetic reconnection deeply embedded in the cool solar chromosphere; these include&amp;nbsp;intensification and broadening/splitting of emission lines, the presence of&amp;nbsp;optically thin Si IV 1393.8/1402.8 &amp;Aring;&amp;nbsp;emission lines that form at transition region temperatures (&lt;span class="math-tex"&gt;\(\ge\)&lt;/span&gt; 80,000 K), and the presence of absorption&amp;nbsp;features from cool metallic ions (Ni II 1393.3 &amp;Aring;&amp;nbsp;and Fe II 1392.8 &amp;Aring;). Improving our understanding of these bursts will give us insight into how&amp;nbsp;they contribute to the heating of the lower solar atmosphere. In particular, it is important to constrain the formation altitudes of these events,&amp;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&amp;nbsp;the forbidden O IV 1401.2 &amp;Aring;&amp;nbsp;line appears in UV burst spectra due to its&amp;nbsp;potential to extinguish at high electron densities as a result of collisional&amp;nbsp;de-excitation. This work will determine how often this critically important spectral feature arises in UV burst events. We locate UV bursts in&amp;nbsp;AR11850 over nine observations from 24-27 September 2013 using data&amp;nbsp;from the Interface Region Imaging Spectrograph (IRIS), which provides&amp;nbsp;simultaneous imaging and spectroscopic data of the upper chromosphere&amp;nbsp;and transition region in the far and near ultraviolet (NUV). We detect&amp;nbsp;UV bursts by applying a four-parameter single-Gaussian fit to the Si IV&amp;nbsp;1393.8 &amp;Aring;&amp;nbsp;emission line. We perform cuts in the 4-D parameter space to&amp;nbsp;isolate the UV burst population, then we manually inspect the remaining&amp;nbsp;spectra for signs of Ni II 1393.3 &amp;Aring;&amp;nbsp;absorption. We use the resulting sample to look for instances of the O IV 1401.2 &amp;Aring;&amp;nbsp;emission line. With the&amp;nbsp;intent of obtaining a distribution of its statistical signi cance, we measure&amp;nbsp;the total integrated intensities and their uncertainties for each O IV line&amp;nbsp;associated with a UV burst. We find that 33.62% of the sampled O IV&amp;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&amp;nbsp;UV bursts.&lt;/p&gt;</subfield>
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