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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Dear loops friends,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I hope you are well. You might be interested in a new paper just accepted by ApJ:  </span><span style="font-family:"Arial",sans-serif"><a href="https://arxiv.org/abs/2007.15085"><span style="color:blue">https://arxiv.org/abs/2007.15085</span></a></span>.<span style="font-family:"Arial",sans-serif"> 
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Stay safe,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Jim<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><a name="_Hlk38980842"></a><b><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">                                                                       Cross Sections of Coronal
 Loop Flux Tubes<o:p></o:p></span></b></p>
<p class="MsoNormal" style="line-height:115%"><b><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif"><o:p> </o:p></span></b></p>
<p class="MsoNormal" style="line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">                                                                                          James A. Klimchuk<o:p></o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">                                                                             NASA Goddard Space Flight Center<o:p></o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">                                                                                           Craig E. DeForest<o:p></o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">                                                                                 Southwest Research Institute<o:p></o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="line-height:115%"><b><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">                                                                                                 Abstract<o:p></o:p></span></b></p>
<p class="MsoNormal" style="text-align:justify;line-height:115%"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman",serif">            Coronal loops reveal crucial information about the nature of both coronal magnetic fields and coronal
 heating. The shape of the corresponding flux tube cross section and how it varies with position are especially important properties. They are a direct indication of the expansion of the field and of the cross-field spatial distribution of the heating. We have
 studied 20 loops using high spatial resolution observations from the first flight of the Hi-C rocket experiment, measuring the intensity and width as a function of position along the loop axis. We find that intensity and width tend to either be uncorrelated
 or to have a direct dependence, such that they increase or decrease together. This implies that the flux tube cross sections are approximately circular under the assumptions that the tubes have non-negligible twist and that the plasma emissivity is approximately
 uniform along the magnetic field. The shape need not be a perfect circle and the emissivity need not be uniform within the cross section, but sub-resolution patches of emission must be distributed quasi-uniformly within an envelope that has an aspect ratio
 of order unity. This raises questions about the suggestion that flux tubes expand with height, but primarily in the line-of-sight direction so that the corresponding (relatively noticeable) loops appear to have roughly uniform width, a long-standing puzzle.
 It also casts doubt on the idea that most loops correspond to simple warped sheets, although we leave open the possibility of more complex manifold structures.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<span style="font-family:"Arial",sans-serif">********************************************************************************</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">James A. Klimchuk</span><o:p></o:p></p>
<p class="MsoNormalCxSpMiddle" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-add-space:auto">
<span style="font-family:"Arial",sans-serif">NASA Goddard Space Flight Center</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">Solar Physics Lab, Code 671</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">Bldg. 21, Rm. 158</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">Greenbelt, MD  20771</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">USA</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormalCxSpMiddle" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-add-space:auto">
<span style="font-family:"Arial",sans-serif">Phone:  <a href="tel:%28301%29%20286-9060" target="_blank">
<span style="color:blue">1-301-286-9060</span></a></span><o:p></o:p></p>
<p class="MsoNormalCxSpMiddle" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-add-space:auto">
<span style="font-family:"Arial",sans-serif">Fax:      <a href="tel:%28301%29%20286-7194" target="_blank">
<span style="color:blue">1-301-286-7194</span></a></span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">E-mail:  <a href="mailto:James.A.Klimchuk@nasa.gov" target="_blank"><span style="color:blue">James.A.Klimchuk@nasa.gov</span></a>
</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">Homepage:  <a href="http://science.gsfc.nasa.gov/sed/index.cfm?fuseAction=people.jumpBio&&iPhonebookId=15844" target="_blank">
<span style="color:blue">http://science.gsfc.nasa.gov/sed/bio/james.a.klimchuk</span></a></span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">No endorsement by NASA is implied for any correspondence related to my official role in professional organizations.</span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
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<span style="font-family:"Arial",sans-serif">********************************************************************************</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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