{"id":"pod_polito_vanessa_2021-12-13T18:32:29.569Z","submitter":"Paola Testa (SAO)","author":"Paola Testa [1], Vanessa Polito [2], Bart De Pontieu [3,4,5]","status":"published","creation-date":"2021-12-13T18:32:29.572Z","last-modified-date":"2022-01-07T18:16:30.292Z","credit":"[1] Harvard-Smithsonian Center for Astrophysics; [2] Bay Area Environmental Research Institute; [3] Lockheed Martin Solar & Astrophysics Laboratory; [4] Rosseland Center for Solar Physics, Univ. of Oslo; [5] Institute of Theoretical Astrophysics, Univ. of Oslo","title":"Insights into the heating of the hot non-flaring corona: IRIS observations of nanoflares and loop modeling","contentBlocks":[{"type":"text","text":"The%20heating%20of%20the%20solar%20corona%20is%20one%20of%20the%20main%20open%20issues%20in%20astrophysics.%20The%20heating%20mechanisms%20likely%20operate%20on%20small%20spatial%20and%20temporal%20scales%2C%20and%20it%20is%20quite%20challenging%20to%20detect%20single%20coronal%20heating%20events%2C%20especially%20in%20the%20corona%20where%20efficient%20thermal%20conduction%20smears%20the%20heating%20signatures.%0AThe%20core%20of%20solar%20Active%20Regions%20%28ARs%29%20is%20where%20the%20hottest%20%28%5Cbegin%7Bequation%7D%20%5Cgtrsim%20%5Cend%7Bequation%7D5MK%29%20coronal%20plasma%20is%20observed%20outside%20flares.%20This%20hot%20emission%20is%20typically%20transient%2C%20and%20the%20signatures%20of%20single%20heating%20events%20can%20sometimes%20be%20detected%20as%20short-lived%20brightenings%20of%20the%20footpoint%20emission%2C%20as%20shown%20in%20two%20examples%20in%20Figure%201.%20The%20unprecedented%20high%20spatial%20and%20temporal%20resolution%20of%20IRIS%20is%20crucial%20to%20observe%20this%20highly%20variable%20emission."},{"type":"image","file":"","url":"nuggetvideos/2021/12/13/pod_polito_vanessa_2021-12-13T18%3A32%3A29.569Z/iris_nugget_fig1.png","hash":"db0ac610af7e4c5892921bb252ec982","mimeType":"image/png","caption":"Figure%201%3A%20IRIS%201400%20%26Aring%3B%20slit-jaw%20images%20%28left%29%20and%20AIA%2094%20%26Aring%3B%20images%20%28right%29%20for%20two%20events%20showing%20rapid%20moss%20brightenings%20at%20the%20loop%20footpoints%20%28observed%20in%20the%20Si%20IV%20%E2%88%BC1400%20%26Aring%3B%20transition%20region%20emission%29%20followed%20%28note%20the%20different%20times%20of%20the%20two%20images%29%20by%20transient%20brightenings%20of%20the%20overlying%20hot%20%28%7E5%E2%80%938%20MK%29%20loops%20%28observed%20in%20the%20AIA%2094%20%26Aring%3B%2C%20which%20is%20here%20dominated%20by%20FeXVIII%20emission%29.%20%28Figure%20adapted%20from%20Fig.1%20of%20Testa%2C%20Polito%20%26amp%3B%20De%20Pontieu%202020%29"},{"type":"text","text":"IRIS%20spectral%20observations%20of%20the%20response%20of%20the%20lower%20atmosphere%20%28transition%20region%20and%20chromosphere%29%20to%20coronal%20heating%20events%20provides%20%28Testa%20et%20al.%202014%2C%202020%3B%20Polito%20et%20al.%202018%29%20tight%20constraints%20on%20the%20properties%20of%20the%20heating%20events%20%28e.g.%2C%20duration%2C%20total%20energy%29%20and%20on%20the%20mechanisms%20of%20energy%20transport%20%28e.g.%2C%20non-thermal%20particles%20accelerated%20during%20magnetic%20reconnection%20events%2C%20thermal%20conduction%29.%20Early%20analysis%20of%20IRIS%20observations%20of%20this%20type%20of%20events%20showed%20the%20diagnostic%20potential%20of%20the%20Doppler%20shift%20of%20the%20Si%20IV%20emission%2C%20in%20particular%20with%20sometimes%20observed%20blueshift%20providing%20a%20strong%20indication%20of%20the%20presence%20of%20non-thermal%20particles%2C%20and%20even%20providing%20constraints%20on%20the%20parameters%20of%20their%20distribution%20%28Testa%20et%20al.%202014%29.%20Hard%20X-ray%20observations%20of%20non-thermal%20particles%20indicate%20that%20their%20emission%20is%20generally%20compatible%20with%20power-law%20distribution.%20One%20of%20the%20main%20parameters%20of%20these%20power-laws%20is%20the%20low-energy%20cutoff%20%28%5Cbegin%7Bequation%7D%20E_C%20%5Cend%7Bequation%7D%29%20which%20is%20crucial%20to%20determine%20the%20total%20energy%20in%20the%20accelerated%20electrons.%20%5Cbegin%7Bequation%7D%20E_C%20%5Cend%7Bequation%7D%20%20is%20often%20difficult%20to%20determine%20from%20hard%20X-ray%20observations%20because%20%20of%20the%20overall%20of%20thermal%20and%20non-thermal%20spectra.%20The%20IRIS%20emission%20observed%20at%20the%20footpoint%20of%20AR%20hot%20loops%20heated%20by%20nanoflares%20are%20highly%20sensitive%20to%20the%20energy%20of%20the%20non-thermal%20electrons%2C%20providing%20excellent%20diagnostics%20of%20%5Cbegin%7Bequation%7D%20E_C%20%5Cend%7Bequation%7D%29%20for%20very%20small%20heating%20events%20typically%20below%20detection%20thresholds%20of%20hard%20X-ray%20observatories.%0A%0AIn%20order%20to%20diagnose%20the%20properties%20of%20the%20heating%2C%20and%20the%20presence%20and%20properties%20of%20the%20non-thermal%20particles%2C%20modeling%20of%20the%20atmospheric%20plasma%20response%20to%20nanoflares%20is%20needed.%20The%20%20RADYN%20code%20%28Carlsson%20%26amp%3B%20Stein%201997%2C%20Allred%20et%20al.%202005%2C%202015%29%20is%20well%20suited%20to%20these%20studies%20as%20it%20includes%20non-Local%20Thermodynamic%20Equilibrium%20%28non-LTE%29%20radiative%20transfer%2C%20which%20is%20necessary%20to%20model%20the%20chromospheric%20emission%2C%20and%20it%20allows%20to%20model%20heating%20also%20by%20non-thermal%20electrons.%20In%20Testa%20et%20al.%20%282014%29%20we%20conducted%20an%20initial%20exploration%20of%20the%20parameter%20space%20of%201D%20loop%20models%2C%20which%20showed%20the%20potential%20of%20diagnostics%20of%20the%20IRIS%20Si%20IV%20spectral%20properties%2C%20and%20in%20follow-up%20papers%20%28Polito%20et%20al.%202018%2C%20Testa%20et%20al.%202020%29%20we%20extended%20the%20modeling%20efforts%20to%20include%20IRIS%20chromospheric%20diagnostics%20such%20as%20the%20C%20II%2C%20Mg%20II%2C%20and%20Mg%20II%20triplet%20lines%2C%20and%20to%20include%20hybrid%20models%20with%20heating%20by%20thermal%20conduction%20AND%20non-thermal%20particles.%0AIn%20Testa%20et%20al.%20%282020%29%20we%20presented%20an%20analysis%20of%20several%20IRIS%20spectral%20observations%20of%20the%20footpoint%20brightening%20associated%20with%20heating%20of%20hot%20coronal%20loops%2C%20and%20carried%20out%20a%20qualitative%20comparison%20with%20the%20models%20to%20infer%20the%20typical%20properties%20of%20these%20heating%20events.%0AIn%20Figure%202%20we%20show%20an%20example%20of%20the%20IRIS%20spectral%20observables%20for%20one%20of%20the%20heating%20events%20of%20Figure%201."},{"type":"image","file":"","url":"nuggetvideos/2021/12/13/pod_polito_vanessa_2021-12-13T18%3A32%3A29.569Z/iris_nugget_fig2.png","hash":"93f4c53f018a963014783958f6edd5e1","mimeType":"image/png","caption":"Figure%202%3A%20Temporal%20evolution%20of%20IRIS%20spectral%20observables%20for%20one%20of%20the%20events%20shown%20in%20Fig.1%20%28top%20row%29.%20We%20show%20the%20light%20curves%20%28left%20panel%29%20in%20IRIS%20transition%20region%20%28Si%20IV%29%20and%20chromospheric%20%28C%20II%2C%20Mg%20II%2C%20and%20Mg%20II%20triplet%29%20lines%2C%20and%20the%20spectral%20line%20profiles%20%28right%20panels%29%2C%20as%20a%20function%20of%20velocity%2C%20for%20each%20time%20step%2C%20color%20coded%20as%20indicated%20in%20the%20color%20bar%20on%20the%20right.%20%28Figure%20adapted%20from%20Fig.%202%20of%20Testa%2C%20Polito%20%26amp%3B%20De%20Pontieu%202020%29"},{"type":"text","text":"One%20of%20the%20most%20interesting%20new%20results%20of%20our%20modeling%20efforts%20is%20that%20the%20strong%20emission%20in%20the%20Mg%20II%20triplet%2C%20which%20is%20observed%20in%20several%20of%20these%20transient%20events%20%28including%20some%20of%20the%20smallest%20ones%2C%20like%20the%20one%20shown%20in%20Figure%202%29%2C%20is%20present%20in%20our%20simulations%20only%20when%20non-thermal%20particles%20are%20present.%0A%0AHistograms%20of%20the%20observed%20IRIS%20spectral%20properties%2C%20compared%20with%20predictions%20of%20a%20selected%20subset%20of%20models%20covering%20most%20of%20the%20explored%20parameter%20space%2C%20are%20shown%20in%20Figure%203."},{"type":"image","file":"","url":"nuggetvideos/2021/12/13/pod_polito_vanessa_2021-12-13T18%3A32%3A29.569Z/iris_nugget_fig3.png","hash":"35bd04b993f0fd55d68cdc6a82097efe","mimeType":"image/png","caption":"Figure%203%3A%20Histograms%20of%20observed%20properties%20of%20footpoint%20brightenings%20observed%20by%20IRIS%20and%20associated%20with%20coronal%20heating%20events.%20Top%20row%3A%20Si%20IV%20Doppler%20velocity%2C%20Si%20IV%20nonthermal%20line%20broadening%2C%20duration%20of%20the%20moss%20brightening%20in%20Si%20IV%3B%20Bottom%20row%3A%20ratio%20of%20intensity%20maxima%20for%20C%20II%2FSi%20IV%20and%20Mg%20II%2FSi%20IV.%20The%20colored%20symbols%20show%20the%20corresponding%20predictions%20of%20the%20different%20RADYN%20models%20%28each%20symbol%20corresponds%20to%20one%20RADYN%20simulation%20as%20described%20in%20the%20legend%20of%20the%20top%20left%20panel%29.%20%28Adapted%20from%20Figure%206%20of%20Testa%2C%20Polito%20%26amp%3B%20De%20Pontieu%202020%29"},{"type":"text","text":"These%20results%20indicate%20that%20the%20brightenings%20are%20characterized%20by%3A%20%281%29%20short%20duration%20of%20the%20events%20%28%5Cbegin%7Bequation%7D%20%5Cgtrsim%20%5Cend%7Bequation%7D30s%29%2C%20%282%29%20broad%20distributions%20of%20Si%20IV%20Doppler%20shifts%2C%20%283%29%20typically%20modest%20increase%20of%20non-thermal%20broadening%2C%20%284%29%20similar%20behavior%20of%20Si%20IV%20and%20C%20II%20emission%2C%20whereas%20the%20increase%20of%20Mg%20II%20intensity%20appears%20less%20closely%20tied%20to%20the%20Si%20IV%20emission.%0AWe%20note%20that%20a%20large%20variety%20of%20IRIS%20spectral%20properties%20is%20observed%20even%20for%20different%20footpoints%20in%20the%20same%20coronal%20events.%20Also%2C%20Mg%20II%20triplet%20emission%20is%20observed%20for%20several%20events%2C%20strongly%20suggesting%20the%20presence%20of%20non-thermal%20particles%20in%20these%20nanoflares%2C%20and%2C%20in%20turn%2C%20the%20similarities%20large%20flares%20and%20these%20smaller%20%28clusters%20of%20nanoflare-size%29%20events%20heating%20the%20AR%20cores."}],"references":["<a href=\"https://ui.adsabs.harvard.edu/abs/2020ApJ...889..124T/abstract\">Testa, Polito & De Pontieu, ApJ 889, 124 (2020)</a>","<a href=\"https://ui.adsabs.harvard.edu/abs/2014Sci...346B.315T/abstract\">Testa, et al. Science, Volume 346, 315 (2014)</a>","<a href=\"âÂÂÂÂÂÂÂÂÂÂÂÂhttps://ui.adsabs.harvard.edu/abs/2018ApJ...856..178P/abstract"\"> Polito et al., ApJ, 856, 178 (2018)</a>","<a href=\"âÂÂÂÂÂÂÂÂÂÂhttps://ui.adsabs.harvard.edu/abs/1997LNP...489..159C/abstract"\"> Carlsson & Stein LNP, 489, 159, (1997)</a>","<a href=\"âÂÂÂÂÂÂÂÂÂÂhttps://ui.adsabs.harvard.edu/abs/2005ApJ...630..573A/abstract"\"> Allred et al., ApJ, 630, 573 (2005)</a>","<a href=\"âÂÂÂÂÂÂÂÂÂÂhttps://ui.adsabs.harvard.edu/abs/2015ApJ...809..104A/abstract"\"> Allred et al., ApJ, 809, 104, (2015)</a>","","","",""],"pubDate":"2022-01-10T21:28:50.841Z"}