IRIS Nugget
Welcome to the IRIS Science Nuggets: highlights of recent IRIS scientific results for the solar physics community.
{"id":"pod_polito_vanessa_2021-04-21T16:35:18.398Z","submitter":"","author":"Patrick Antolin","status":"published","creation-date":"2021-04-21T16:35:18.403Z","last-modified-date":"2021-05-14T16:55:37.792Z","credit":"Paolo Pagano, Paola Testa, Antonino Petralia, Fabio Reale","title":"Nanojets of Coronal Heating","contentBlocks":[{"type":"text","text":"Coronal%20heating%20has%20been%20puzzling%20scientists%20for%20decades%20since%20its%20discovery%20more%20than%2080%20years%20ago.%20The%20million-degree%20inner%20corona%2C%20composed%20mainly%20of%20closed%20loop%20structures%2C%20reflects%20a%20mind-blowing%20multi-scale%20problem%20of%20magnetic%20energy%20conversion%20into%20heat.%20Two%20major%20heating%20categories%20have%20been%20proposed.%20MHD%20waves%20%28AC%20mechanisms%29%20constitute%20one%20of%20the%20groups%2C%20while%20the%20other%20is%20based%20on%20the%20slow%20build-up%20of%20magnetic%20stress%20and%20currents%20through%20the%20braiding%20of%20the%20magnetic%20field%2C%20eventually%20released%20through%20magnetic%20reconnection%20%28DC%20mechanisms%29.%20Parker%20et%20al.%201988%20envisioned%20that%20reconnection%20processes%20should%20occur%20continuously%20in%20the%20corona%20once%20critical%20current%20thresholds%20are%20reached%2C%20thereby%20releasing%20magnetic%20energy%20and%20producing%20impulsive%20intensity%20bursts%20known%20as%20nanoflares.%20While%20such%20small-scale%20%28on%20the%20order%20of%20%5Cbegin%7Bequation%7D%2010%5E%7B24%7D%20%5Cend%7Bequation%7D%20ergs%29%20nanoflare-like%20intensity%20bursts%20can%20now%20be%20observed%20with%20current%20instrumentation%20%28e.g.%20Testa%20et%20al.%202013%29%2C%20numerical%20modelling%20has%20shown%20that%20both%20AC%20and%20DC%20mechanisms%20are%20able%20to%20produce%20such%20features%20%28e.g.%20Antolin%20et%20al.%202008%29%2C%20thereby%20preventing%20discrimination%20between%20the%20two%20scenarios."},{"type":"image","file":"","url":"nuggetvideos/2021/04/21/pod_polito_vanessa_2021-04-21T16%3A35%3A18.398Z/fullfov_nanojet.png","hash":"a43c571f5f8c8adc7a77580899db6fbf","mimeType":"image/png","caption":"Fig.%201.%20Discovery%20of%20nanojets.%20%28Left%20panel%29%20Full%20disc%20AIA%20171%20%26Aring%3B%20image%20showing%20the%20IRIS%20field-of-view%20%28red%20square%29%20centred%20on%20an%20off-limb%20prominence%2Fcoronal%20rain%20complex.%20%28Middle%20panel%29%20Zoom-in%20onto%20the%20IRIS%2FSJI%20field-of-view.%20Fast%2C%20bursty%20and%20tiny%20jet-like%20structure%20termed%20%27nanojets%27%20are%20detected%20in%20a%20loop-like%20cool%20structure%20in%20the%20IRIS%2FSJI%201400%20%26Aring%3B%20channel%20%28inner%20zoomed-in%20region%29.%20The%20nanojets%20are%20peculiar%20in%20terms%20of%20their%20transverse%20direction%20with%20respect%20to%20the%20loop.%20They%20occur%20in%20isolation%20or%20in%20clusters.%20%28Right%20panel%29%20An%20image%20sequence%20in%20SJI%201400%20%26Aring%3B%20showing%20a%20nanojet%20cluster%20%28white%20arrows%29%2C%20accompanied%20by%20the%20ejection%20of%20downward-directed%20plasmoids%20%28green%20arrows%29."},{"type":"text","text":"In%20this%20work%20%28Antolin%20et%20al.%202021%29%2C%20the%20IRIS%20spectrograph%27s%20slit%20detected%20a%20unique%20feature%20of%20the%20DC%20heating%20mechanism.%20Observing%20off-limb%20at%20a%20prominence%2Fcoronal%20rain%20complex%2C%20IRIS%20observed%20the%20heating%20to%20multi-million%20degree%20temperatures%20of%20an%20initially%20cool%20loop-like%20structure%20attached%20to%20the%20prominence%20%28Fig.%201%29.%20A%20myriad%20small-scale%2C%20bursty%20events%20with%20nanoflare-like%20energies%20were%20detected.%20Each%20event%20was%20characterized%20by%20a%20tiny%20jet-like%20feature%20named%20%27nanojet%27%2C%20with%20lengths%20of%20%7E1500%20km%2C%20widths%20of%20%7E500%20km%20and%20durations%20of%2015%20s%20or%20less%2C%20perpendicular%20to%20the%20main%20flow%20of%20the%20plasma%20%28magnetic%20field%20direction%29%2C%20and%20with%20plane-of-the-sky%20and%20line-of-sight%20speeds%20up%20to%20200%20km%2Fs%20or%20more%20%28Fig.%202%29.%20The%20nanojets%20were%20best%20observed%20in%20the%20Si%20IV%201402%20%26Aring%3B%20spectral%20line%20%28and%20the%20SJI%201400%20%26Aring%3B%20channel%29%2C%20while%20there%20was%20almost%20%20no%20trace%20of%20the%20nanojets%20in%20the%20cooler%20Mg%20II%20k%20%28SJI%202796%20%26Aring%3B%29%20and%20Ca%20II%20H%20%28Hinode%2FSOT%2C%20observing%20simultaneously%29%20lines%2C%20indicating%20a%20very%20rapid%20heating%20process.%20The%20DEM%20and%20EUV%20absorption%20analysis%20provided%20temperatures%20and%20number%20densities%20estimates%20of%202%20-%205%20MK%20and%20%5Cbegin%7Bequation%7D%2010%5E%7B10%7D%20-%2010%5E%7B11%7D%20cm%5E%7B-3%7D%20%5Cend%7Bequation%7D%20for%20the%20nanojets%20respectively.%20The%20largest%20events%20were%20also%20accompanied%20with%20plasmoids%20ejected%20along%20the%20axis%20of%20the%20jets%20%28Fig.%201%29.%20Besides%20the%20perpendicular%20trajectory%20to%20the%20magnetic%20field%2C%20two%20other%20highly%20peculiar%20features%20were%20observed.%20First%2C%20the%20nanojets%20occurred%20both%20in%20clusters%20or%20in%20isolation%2C%20suggesting%20an%20impulsive%20and%20episodic%20process%20behind%20the%20jets.%20%20Second%2C%20the%20nanojets%20were%20almost%20all%20pointing%20inward%20with%20respect%20to%20the%20curvature%20of%20the%20loop%2C%20without%20any%20secondary%20jets%20pointing%20in%20the%20opposite%20direction%2C%20as%20expected%20from%20reconnection%20theory."},{"type":"image","file":"","url":"nuggetvideos/2021/04/21/pod_polito_vanessa_2021-04-21T16%3A35%3A18.398Z/SIIV-MgII-spectra-SJI.png","hash":"c320641c252ee35305858136ceed8b8b","mimeType":"image/png","caption":"Fig.%202.%20Spectra%20of%20nanojets.%20%28Top%20row%29%20A%20subset%20of%20the%20IRIS%20field-of-view%20in%20the%20SJI%202796%20%26Aring%3B%20%28left%29%20and%201400%20%26Aring%3B%20%28right%29%20channels%2C%20showing%20snapshots%20of%20two%20nanojets%20%28localized%20brightening%20events%20in%20the%20SJI%201400%20%26Aring%3B%20image%29.%20The%20IRIS%2FSG%20slit%20%28dashed%20line%29%20captures%20the%20spectra%20of%20the%20right-most%20nanojet.%20%28Bottom%20row%29%20The%20Mg%20II%20k%20%28left%29%20and%20Si%20IV%201402%20%26Aring%3B%20%28right%29%20spectra%20along%20the%20slit.%20The%20Si%20IV%20nanojet%20spectra%20overlaid%20over%20the%20image%20%28corresponding%20to%20the%20location%20indicated%20by%20the%20red%20line%29%20is%20characterized%20by%20enhanced%20intensity%20and%20a%20large%20blueshift%20%28up%20to%20100%20km%2Fs%20or%20more%29.%20A%20double-Gaussian%20fit%20to%20the%20spectra%20%28blue%20dashed%20curve%29%20reveals%20the%20strong%20blue%20shifted%20component."},{"type":"image","file":"","url":"nuggetvideos/2021/04/21/pod_polito_vanessa_2021-04-21T16%3A35%3A18.398Z/sketch-common_vs_nano.png","hash":"7c1b9421d55fd9f0e4404c2f96a588a4","mimeType":"image/png","caption":"Fig.%203.%20Nanojet%20interpretation.%20The%20nanojet%20is%20interpreted%20as%20the%20product%20of%20component%20magnetic%20reconnection%20%28small-angle%20reconnection%29%20due%20to%20the%20braiding%20and%20accumulated%20stress%20in%20the%20field%20lines.%20The%20localised%20heating%20combined%20with%20the%20fast%20sideways%20advection%20of%20the%20reconnected%20field%20lines%20from%20magnetic%20tension%20produces%20the%20nanojets."},{"type":"text","text":"The%20nanojets%20are%20interpreted%20as%20the%20signature%20of%20magnetic%20reconnection%20at%20small%20angles%20%28component%20reconnection%29.%20In%20this%20scenario%2C%20the%20nanojet%20is%20the%20result%20of%20the%20sideways%20advection%20of%20the%20magnetic%20field%20lines%2C%20driven%20by%20magnetic%20tension%20and%20pushing%20the%20heated%20plasma%20sideways%20at%20Alfv%26eacute%3Bnic%20speeds%20%28Fig.%203%29.%20The%20nanojet%20is%20therefore%20different%20from%20the%20usual%20reconnection%20jets%20%28such%20as%20chromospheric%20jets%29%20in%20the%20sense%20that%20the%20fastest%20flows%20are%20not%20field-aligned%20and%20reflect%20the%20fast%20reconfiguration%20of%20the%20magnetic%20field%20lines%20post-reconnection.%20This%20interpretation%20is%20supported%20by%203D%20MHD%20numerical%20simulations%20with%20the%20PLUTO%20code%2C%20that%20reproduced%20the%20thermal%20and%20dynamic%20properties%20of%20the%20jets.%20We%20speculate%20that%20the%20peculiar%20unidirectional%20feature%20is%20a%20product%20of%20global%20loop%20curvature%20and%2For%20the%20local%20curvature%20due%20to%20braiding%2C%20leading%20to%20asymmetry%20in%20the%20evolution%20of%20the%20magnetic%20tension%20force%20at%20either%20side%20of%20the%20reconnection%20region."},{"type":"image","file":"","url":"nuggetvideos/2021/04/21/pod_polito_vanessa_2021-04-21T16%3A35%3A18.398Z/hotloop.png","hash":"f0931e86cfb2ac000d8beb070b0a4b23","mimeType":"image/png","caption":"Fig.%204.%20A%20coronal%20loop%20forms.%20Following%20the%20nanojet%20occurrence%20%28roughly%20150%20events%20in%2013%20min%29%2C%20the%20initially%20cool%20loop%20heats%20up%20to%20millions%20of%20degrees%2C%20captured%20in%20various%20AIA%20and%20IRIS%20channels.%20From%20left%20to%20right%3A%20IRIS%201400%20%26Aring%3B%20%2860%2C000%20K%29%2C%20AIA%20304%20%26Aring%3B%20%28100%2C000%20K%29%2C%20AIA%20171%20%26Aring%3B%20%28800%2C000%20K%29%20and%20AIA%20193%20%26Aring%3B%20%281%2C500%2C000%20K%29%20images."},{"type":"text","text":"Globally%2C%20the%20nanojets%20were%20observed%20to%20spread%20across%20and%20along%20the%20loop%20in%20an%20avalanche-like%20progression%20%28%7E150%20events%20over%2013%20min%29.%20Following%20the%20nanojets%2C%20hot%20coronal%20strands%20formed%20starting%20from%20the%20nanojet%20locations%2C%20spanning%20the%20entire%20length%20of%20the%20loop%20%28Fig.%204%29.%20The%20initial%20coronal%20rain%20along%20the%20cool%20structure%2C%20here%20acting%20as%20a%20high-resolution%20tracer%20of%20the%20magnetic%20field%20dynamics%2C%20revealed%20misalignments%20between%20the%20strands%20of%20up%20to%2025%20degrees%2C%20suggestive%20of%20braiding.%20During%20the%20heating%20episodes%20the%20amount%20of%20apparent%20braiding%20and%20misalignment%20was%20reduced%2C%20accompanied%20with%20complex%20internal%20rotations%20and%20signatures%20of%20untwisting%2C%20thereby%20matching%20the%20reconnection%20picture.%20The%20driver%20of%20the%20reconnection%20was%20suspected%20to%20be%20the%20prominence%20at%20the%20apex%20of%20the%20loop%2C%20which%20gave%20indications%20of%20becoming%20partly%20unstable%20through%20the%20unloading%20material%20%28similar%20to%20the%20scenario%20suggested%20in%20Keppens%20%26amp%3B%20Xia%202014%29.%20The%20magnetic%20field%20lines%20released%20from%20the%20material%20would%20expand%20from%20below%2C%20putting%20pressure%20on%20the%20loop%20structure%20above%2C%20anchored%20to%20the%20prominence.%20It%20was%20hypothesised%20that%20such%20nanojets%20should%20be%20independent%20of%20the%20reconnection%20driver%20and%20should%20therefore%20be%20observed%20in%20various%20coronal%20structures%20subject%20to%20braiding."}],"references":["<a href=\"https://ui.adsabs.harvard.edu/abs/2021NatAs...5...54A/abstract\">Antolin, P., Pagano, P., Testa, P., Petralia, A., Reale, F., Nat. Astron. 5, 54 (2021) </a>","<a href=\"https://ui.adsabs.harvard.edu/abs/2008ApJ...688..669A/abstract\"> Antolin, P., et al. ApJ, 688, 669 (2008)</a>","<a href=\"https://ui.adsabs.harvard.edu/abs/2014ApJ...789...22K/abstract\"> Keppens, R. & Xia, C. ApJ 789, 22 (2014)</a>","<a href=\"https://ui.adsabs.harvard.edu/abs/1988ApJ...330..474P/abstract\">Parker, E. N., ApJ 330, 474 (1988)</a>","<a href=\"https://ui.adsabs.harvard.edu/abs/2013ApJ...770L...1T/abstract\"> Testa, P. et al. ApJL 770, L1 (2013)</a>","","","","",""],"pubDate":"2021-05-11T22:43:11.902Z"}