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Showing 1-20 of about 65 results.
Reconnection nanojets in the solar coronaAntolin, PatrickPagano, PaoloTesta, PaolaPetralia, AntoninoReale, FabioDOI: info:10.1038/s41550-020-1199-8v. 554–62
Antolin, Patrick, Pagano, Paolo, Testa, Paola, Petralia, Antonino, and Reale, Fabio. 2021. "Reconnection nanojets in the solar corona." Nature Astronomy 5:54– 62. https://doi.org/10.1038/s41550-020-1199-8
ID: 159353
Type: article
Authors: Antolin, Patrick; Pagano, Paolo; Testa, Paola; Petralia, Antonino; Reale, Fabio
Abstract: The solar corona is shaped and mysteriously heated to millions of degrees by the Sun's magnetic field. It has long been hypothesized that the heating results from a myriad of tiny magnetic energy outbursts called nanoflares, driven by the fundamental process of magnetic reconnection. Misaligned magnetic field lines can break and reconnect, producing nanoflares in avalanche-like processes. However, no direct and unique observations of such nanoflares exist to date, and the lack of a smoking gun has cast doubt on the possibility of solving the coronal heating problem. From coordinated multi-band high-resolution observations, we report on the discovery of very fast and bursty nanojets, the telltale signature of reconnection-based nanoflares resulting in coronal heating. Using state-of-the-art numerical simulations, we demonstrate that the nanojet is a consequence of the slingshot effect from the magnetically tensed, curved magnetic field lines reconnecting at small angles. Nanojets are therefore the key signature of reconnection-based coronal heating in action.
The Drivers of Active Region Outflows into the Slow Solar WindBrooks, David H.Winebarger, Amy R.Savage, SabrinaWarren, Harry P.De Pontieu, BartPeter, HardiCirtain, Jonathan W.Golub, LeonKobayashi, KenMcIntosh, Scott W.McKenzie, DavidMorton, RichardRachmeler, LaurelTesta, PaolaTiwari, SanjivWalsh, RobertDOI: info:10.3847/1538-4357/ab8a4cv. 894144
Brooks, David H., Winebarger, Amy R., Savage, Sabrina, Warren, Harry P., De Pontieu, Bart, Peter, Hardi, Cirtain, Jonathan W., Golub, Leon, Kobayashi, Ken, McIntosh, Scott W., McKenzie, David, Morton, Richard, Rachmeler, Laurel, Testa, Paola, Tiwari, Sanjiv, and Walsh, Robert. 2020. "The Drivers of Active Region Outflows into the Slow Solar Wind." The Astrophysical Journal 894:144. https://doi.org/10.3847/1538-4357/ab8a4c
ID: 156960
Type: article
Authors: Brooks, David H.; Winebarger, Amy R.; Savage, Sabrina; Warren, Harry P.; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub, Leon; Kobayashi, Ken; McIntosh, Scott W.; McKenzie, David; Morton, Richard; Rachmeler, Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert
Abstract: Plasma outflows from the edges of active regions have been suggested as a possible source of the slow solar wind. Spectroscopic measurements show that these outflows have an enhanced elemental composition, which is a distinct signature of the slow wind. Current spectroscopic observations, however, do not have sufficient spatial resolution to distinguish what structures are being measured or determine the driver of the outflows. The High-resolution Coronal Imager (Hi-C) flew on a sounding rocket in 2018 May and observed areas of active region outflow at the highest spatial resolution ever achieved (250 km). Here we use the Hi-C data to disentangle the outflow composition signatures observed with the Hinode satellite during the flight. We show that there are two components to the outflow emission: a substantial contribution from expanded plasma that appears to have been expelled from closed loops in the active region core and a second contribution from dynamic activity in active region plage, with a composition signature that reflects solar photospheric abundances. The two competing drivers of the outflows may explain the variable composition of the slow solar wind.
The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE)De Pontieu, BartMartínez-Sykora, JuanTesta, PaolaWinebarger, Amy R.Daw, AdrianHansteen, ViggoCheung, Mark C. M.Antolin, PatrickDOI: info:10.3847/1538-4357/ab5b03v. 8883
De Pontieu, Bart, Martínez-Sykora, Juan, Testa, Paola, Winebarger, Amy R., Daw, Adrian, Hansteen, Viggo, Cheung, Mark C. M., and Antolin, Patrick. 2020. "The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE)." The Astrophysical Journal 888:3. https://doi.org/10.3847/1538-4357/ab5b03
ID: 155684
Type: article
Authors: De Pontieu, Bart; Martínez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo; Cheung, Mark C. M.; Antolin, Patrick
Abstract: The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed at understanding the physical mechanisms driving the heating of the solar corona and the eruptions that are at the foundation of space weather. MUSE contains two instruments, a multi-slit extreme ultraviolet (EUV) spectrograph and a context imager. It will simultaneously obtain EUV spectra (along 37 slits) and context images with the highest resolution in space (0.″33─0.″4) and time (1─4 s) ever achieved for the transition region (TR) and corona. The MUSE science investigation will exploit major advances in numerical modeling, and observe at the spatial and temporal scales on which competing models make testable and distinguishable predictions, thereby leading to a breakthrough in our understanding of coronal heating and the drivers of space weather. By obtaining spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX 108Å, Fe XXI 108 Å) covering a wide range of TR and coronal temperatures along 37 slits simultaneously, MUSE will be able to "freeze" the evolution of the dynamic coronal plasma. We describe MUSE's multi-slit approach and show that the optimization of the design minimizes the impact of spectral lines from neighboring slits, generally allowing line parameters to be accurately determined. We also describe a Spectral Disambiguation Code to resolve multi-slit ambiguity in locations where secondary lines are bright. We use simulations of the corona and eruptions to perform validation tests and show that the multi-slit disambiguation approach allows accurate determination of MUSE observables in locations where significant multi-slit contamination occurs.
IRIS Observations of the Low-atmosphere Counterparts of Active Region OutflowsPolito, VanessaDe Pontieu, BartTesta, PaolaBrooks, David H.Hansteen, ViggoDOI: info:10.3847/1538-4357/abba1dv. 90368
Polito, Vanessa, De Pontieu, Bart, Testa, Paola, Brooks, David H., and Hansteen, Viggo. 2020. "IRIS Observations of the Low-atmosphere Counterparts of Active Region Outflows." The Astrophysical Journal 903:68. https://doi.org/10.3847/1538-4357/abba1d
ID: 158828
Type: article
Authors: Polito, Vanessa; De Pontieu, Bart; Testa, Paola; Brooks, David H.; Hansteen, Viggo
Abstract: Active region (AR) outflows have been studied in detail since the launch of Hinode/EIS and are believed to provide a possible source of mass and energy to the slow solar wind. In this work, we investigate the lower atmospheric counterpart of AR outflows using observations from the Interface Region Imaging Spectrograph (IRIS). We find that the IRIS Si IV, C II> and Mg II transition region (TR) and chromospheric lines exhibit different spectral features in the outflows as compared to neighboring regions at the footpoints ("moss") of hot AR loops. The average redshift of Si IV in the outflow region (?5.5 km s-1) is smaller than typical moss (?12-13 km s-1) and quiet Sun (?7.5 km s-1) values, while the C II line is blueshifted (?-1.1-1.5 km s-1), in contrast to the moss where it is observed to be redshifted by about ?2.5 km s-1. Further, we observe that the low atmosphere underneath the coronal outflows is highly structured, with the presence of blueshifts in Si IV and positive Mg II k2 asymmetries (which can be interpreted as signatures of chromospheric upflows) which are mostly not observed in the moss. These observations show a clear correlation between the coronal outflows and the chromosphere and TR underneath, which has not been shown before. Our work strongly suggests that these regions are not separate environments and should be treated together, and that current leading theories of AR outflows, such as the interchange reconnection model, need to take into account the dynamics of the low atmosphere.
IRIS Observations of Short-term Variability in Moss Associated with Transient Hot Coronal LoopsTesta, PaolaPolito, VanessaPontieu, Bart DeDOI: info:10.3847/1538-4357/ab63cfv. 889124
Testa, Paola, Polito, Vanessa, and Pontieu, Bart De. 2020. "IRIS Observations of Short-term Variability in Moss Associated with Transient Hot Coronal Loops." The Astrophysical Journal 889:124. https://doi.org/10.3847/1538-4357/ab63cf
ID: 155816
Type: article
Authors: Testa, Paola; Polito, Vanessa; Pontieu, Bart De
Abstract: We observed rapid variability (≲60 s) at the footpoints of transient, hot (∼8─10 MK) coronal loops in active region cores, with the Interface Region Imaging Spectrograph (IRIS). The high spatial (∼0"33) and temporal (≲5─10 s) resolution of IRIS is often crucial for the detection of this variability. We show how, in combination with 1D RADYN loop modeling, these IRIS spectral observations of the transition region (TR) and chromosphere provide powerful diagnostics of the properties of coronal heating and energy transport (thermal conduction or nonthermal electrons, NTEs). Our simulations of nanoflare-heated loops indicate that emission in the Mg ii triplet can be used as a sensitive diagnostic for nonthermal particles. In our events, we observe a large variety of IRIS spectral properties (intensity, Doppler shifts, broadening, chromospheric/TR line ratios, Mg ii triplet emission) even for different footpoints of the same coronal events. In several events, we find spectroscopic evidence for NTEs (e.g., TR blueshifts and Mg ii triplet emission), suggesting that particle acceleration can occur even for very small magnetic reconnection events, which are generally below the detection threshold of hard X-ray instruments that provide direct detection of emission of nonthermal particles.
On the Coronal Temperature in Solar MicroflaresTesta, PaolaReale, FabioDOI: info:10.3847/1538-4357/abb36ev. 90231
Testa, Paola and Reale, Fabio. 2020. "On the Coronal Temperature in Solar Microflares." The Astrophysical Journal 902:31. https://doi.org/10.3847/1538-4357/abb36e
ID: 157475
Type: article
Authors: Testa, Paola; Reale, Fabio
Abstract: We present a study of solar imaging and spectral observations of a microflare, focusing on the temperature diagnostics provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, and the Extreme-Ultraviolet Imaging Spectrometer (EIS) on board Hinode. Our data analysis, in particular from the emission in the 131 and 94 Å channels, indicates that the heated plasma reaches temperatures of ≳10 MK, at odds with a previous analysis of the same event, and we discuss the reason for the discrepancy. A particularly interesting aspect is the likely presence of the Fe XXIII 263.76 Å line, though weak, in EIS spectra in the early phases of the event, supporting the presence of high temperature plasma. Hydrodynamic 1D modeling of a single loop heated with a 3 minute pulse to 12-15 MK reproduces well most observed features along one of the brightening loops, including intensities in the AIA hot channels and their temporal variability, as well as the intensity, Doppler shift, and line width of the EIS Fe XXIII line, and its timing relative to the AIA emission. Overall, we obtain a coherent scenario of a typical microflaring loop system and provide constraints on the intensity of the energy release as well as its spatial and temporal distribution, both along and across the loop.
Observation and Modeling of High-temperature Solar Active Region Emission during the High-resolution Coronal Imager Flight of 2018 May 29Warren, Harry P.Reep, Jeffrey W.Crump, Nicholas A.Ugarte-Urra, IgnacioBrooks, David H.Winebarger, Amy R.Savage, SabrinaDe Pontieu, BartPeter, HardiCirtain, Jonathan W.Golub, LeonKobayashi, KenMcKenzie, DavidMorton, RichardRachmeler, LaurelTesta, PaolaTiwari, SanjivWalsh, RobertDOI: info:10.3847/1538-4357/ab917cv. 89651
Warren, Harry P., Reep, Jeffrey W., Crump, Nicholas A., Ugarte-Urra, Ignacio, Brooks, David H., Winebarger, Amy R., Savage, Sabrina, De Pontieu, Bart, Peter, Hardi, Cirtain, Jonathan W., Golub, Leon, Kobayashi, Ken, McKenzie, David, Morton, Richard, Rachmeler, Laurel, Testa, Paola, Tiwari, Sanjiv, and Walsh, Robert. 2020. "Observation and Modeling of High-temperature Solar Active Region Emission during the High-resolution Coronal Imager Flight of 2018 May 29." The Astrophysical Journal 896:51. https://doi.org/10.3847/1538-4357/ab917c
ID: 157470
Type: article
Authors: Warren, Harry P.; Reep, Jeffrey W.; Crump, Nicholas A.; Ugarte-Urra, Ignacio; Brooks, David H.; Winebarger, Amy R.; Savage, Sabrina; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub, Leon; Kobayashi, Ken; McKenzie, David; Morton, Richard; Rachmeler, Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert
Abstract: Excellent coordinated observations of NOAA active region 12712 were obtained during the flight of the High-resolution Coronal Imager (Hi-C) sounding rocket on 2018 May 29. This region displayed a typical active region core structure with relatively short, high-temperature loops crossing the polarity inversion line and bright "moss" located at the footpoints of these loops. The differential emission measure (DEM) in the active region core is very sharply peaked at about 4 MK. Further, there is little evidence for impulsive heating events in the moss, even at the high spatial resolution and cadence of Hi-C. This suggests that active region core heating is occurring at a high frequency and keeping the loops close to equilibrium. To create a time-dependent simulation of the active region core, we combine nonlinear force-free extrapolations of the measured magnetic field with a heating rate that is dependent on the field strength and loop length and has a Poisson waiting time distribution. We use the approximate solutions to the hydrodynamic loop equations to simulate the full ensemble of active region core loops for a range of heating parameters. In all cases, we find that high-frequency heating provides the best match to the observed DEM. For selected field lines, we solve the full hydrodynamic loop equations, including radiative transfer in the chromosphere, to simulate transition region and chromospheric emission. We find that for heating scenarios consistent with the DEM, classical signatures of energy release, such as transition region brightenings and chromospheric evaporation, are weak, suggesting that they would be difficult to detect.
Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712Williams, ThomasWalsh, Robert W.Winebarger, Amy R.Brooks, David H.Cirtain, Jonathan W.De Pontieu, BartGolub, LeonKobayashi, KenMcKenzie, David E.Morton, Richard J.Peter, HardiRachmeler, Laurel A.Savage, Sabrina L.Testa, PaolaTiwari, Sanjiv K.Warren, Harry P.Watkinson, Benjamin J.DOI: info:10.3847/1538-4357/ab6dcfv. 892134
Williams, Thomas, Walsh, Robert W., Winebarger, Amy R., Brooks, David H., Cirtain, Jonathan W., De Pontieu, Bart, Golub, Leon, Kobayashi, Ken, McKenzie, David E., Morton, Richard J., Peter, Hardi, Rachmeler, Laurel A., Savage, Sabrina L., Testa, Paola, Tiwari, Sanjiv K., Warren, Harry P., and Watkinson, Benjamin J. 2020. "Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712." The Astrophysical Journal 892:134. https://doi.org/10.3847/1538-4357/ab6dcf
ID: 157291
Type: article
Authors: Williams, Thomas; Walsh, Robert W.; Winebarger, Amy R.; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; Golub, Leon; Kobayashi, Ken; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Rachmeler, Laurel A.; Savage, Sabrina L.; Testa, Paola; Tiwari, Sanjiv K.; Warren, Harry P.; Watkinson, Benjamin J.
Abstract: Following the success of the first mission, the High-Resolution Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1) on 2018 May 29 from the White Sands Missile Range, NM, USA. On this occasion, 329 s of 17.2 nm data of target active region AR 12712 were captured with a cadence of ≈4 s, and a plate scale of 0.129 arcsec pixel-1. Using data captured by Hi-C 2.1 and co-aligned observations from SDO/AIA 17.1 nm, we investigate the widths of 49 coronal strands. We search for evidence of substructure within the strands that is not detected by AIA, and further consider whether these strands are fully resolved by Hi-C 2.1. With the aid of multi-scale Gaussian normalization, strands from a region of low emission that can only be visualized against the contrast of the darker, underlying moss are studied. A comparison is made between these low-emission strands and those from regions of higher emission within the target active region. It is found that Hi-C 2.1 can resolve individual strands as small as ≈202 km, though the more typical strand widths seen are ≈513 km. For coronal strands within the region of low emission, the most likely width is significantly narrower than the high- emission strands at ≈388 km. This places the low-emission coronal strands beneath the resolving capabilities of SDO/AIA, highlighting the need for a permanent solar observatory with the resolving power of Hi-C.
Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent RatesBradshaw, Stephen J.Testa, PaolaDOI: info:10.3847/1538-4357/aafe85v. 872123
Bradshaw, Stephen J. and Testa, Paola. 2019. "Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent Rates." The Astrophysical Journal 872:123. https://doi.org/10.3847/1538-4357/aafe85
ID: 150490
Type: article
Authors: Bradshaw, Stephen J.; Testa, Paola
Abstract: In the work described here, we investigate atomic processes leading to the formation of emission lines within the Interface Region Imaging Spectrograph wavelength range at temperatures near 105 K. We focus on (1) non-equilibrium and (2) density-dependent effects influencing the formation and radiative properties of S IV and O IV. These two effects have significant impacts on spectroscopic diagnostic measurements of quantities associated with the plasma that emission lines from S IV and O IV provide. We demonstrate this by examining nanoflare-based coronal heating to determine what the detectable signatures are in transition region emission. A detailed comparison between predictions from numerical experiments and several sets of observational data is presented to show how one can ascertain when non-equilibrium ionization and/or density-dependent atomic processes are important for diagnosing nanoflare properties, the magnitude of their contribution, and what information can be reliably extracted from the spectral data. Our key findings are the following. (1) The S/O intensity ratio is a powerful diagnostic of non-equilibrium ionization. (2) Non-equilibrium ionization has a strong effect on the observed line intensities even in the case of relatively weak nanoflare heating. (3) The density dependence of atomic rate coefficients is only important when the ion population is out of equilibrium. (4) In the sample of active regions we examined, weak nanoflares coupled with non-equilibrium ionization and density-dependent atomic rates were required to explain the observed properties (e.g., the S/O intensity ratios). (5) Enhanced S/O intensity ratios cannot be due solely to the heating strength and must depend on other processes (e.g., heating frequency, non-Maxwellian distributions).
A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flareCheung, M. C. M.Rempel, M.Chintzoglou, G.Chen, F.Testa, PaolaMartínez-Sykora, J.Sainz Dalda, A.DeRosa, M. L.Malanushenko, A.Hansteen, V.De Pontieu, B.Carlsson, M.Gudiksen, B.McIntosh, S. W.DOI: info:10.1038/s41550-018-0629-3v. 3160–166
Cheung, M. C. M., Rempel, M., Chintzoglou, G., Chen, F., Testa, Paola, Martínez-Sykora, J., Sainz Dalda, A., DeRosa, M. L., Malanushenko, A., Hansteen, V., De Pontieu, B., Carlsson, M., Gudiksen, B., and McIntosh, S. W. 2019. "A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flare." Nature Astronomy 3:160– 166. https://doi.org/10.1038/s41550-018-0629-3
ID: 154590
Type: article
Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.; Testa, Paola; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.; Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen, B.; McIntosh, S. W.
Abstract: Solar and stellar flares are the most intense emitters of X-rays and extreme ultraviolet radiation in planetary systems1,2. On the Sun, strong flares are usually found in newly emerging sunspot regions3. The emergence of these magnetic sunspot groups leads to the accumulation of magnetic energy in the corona. When the magnetic field undergoes abrupt relaxation, the energy released powers coronal mass ejections as well as heating plasma to temperatures beyond tens of millions of kelvins. While recent work has shed light on how magnetic energy and twist accumulate in the corona4 and on how three-dimensional magnetic reconnection allows for rapid energy release5,6, a self-consistent model capturing how such magnetic changes translate into observable diagnostics has remained elusive. Here, we present a comprehensive radiative magnetohydrodynamics simulation of a solar flare capturing the process from emergence to eruption. The simulation has sufficient realism for the synthesis of remote sensing measurements to compare with observations at visible, ultraviolet and X-ray wavelengths. This unifying model allows us to explain a number of well-known features of solar flares7, including the time profile of the X-ray flux during flares, origin and temporal evolution of chromospheric evaporation and condensation, and sweeping of flare ribbons in the lower atmosphere. Furthermore, the model reproduces the apparent non-thermal shape of coronal X-ray spectra, which is the result of the superposition of multi-component super-hot plasmas8 up to and beyond 100 million K.
Multi-component Decomposition of Astronomical Spectra by Compressed SensingCheung, Mark C. M.De Pontieu, BartMartínez-Sykora, JuanTesta, PaolaWinebarger, Amy R.Daw, AdrianHansteen, ViggoAntolin, PatrickTarbell, Theodore D.Wuelser, Jean-PierreYoung, PeterMUSE TeamDOI: info:10.3847/1538-4357/ab263dv. 88213
Cheung, Mark C. M., De Pontieu, Bart, Martínez-Sykora, Juan, Testa, Paola, Winebarger, Amy R., Daw, Adrian, Hansteen, Viggo, Antolin, Patrick, Tarbell, Theodore D., Wuelser, Jean-Pierre, Young, Peter, and MUSE Team. 2019. "Multi-component Decomposition of Astronomical Spectra by Compressed Sensing." The Astrophysical Journal 882:13. https://doi.org/10.3847/1538-4357/ab263d
ID: 154397
Type: article
Authors: Cheung, Mark C. M.; De Pontieu, Bart; Martínez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian; Hansteen, Viggo; Antolin, Patrick; Tarbell, Theodore D.; Wuelser, Jean-Pierre; Young, Peter; MUSE Team
Abstract: The signal measured by an astronomical spectrometer may be due to radiation from a multi-component mixture of plasmas with a range of physical properties (e.g., temperature, Doppler velocity). Confusion between multiple components may be exacerbated if the spectrometer sensor is illuminated by overlapping spectra dispersed from different slits, with each slit being exposed to radiation from a different portion of an extended astrophysical object. We use a compressed sensing method to robustly retrieve the different components. This method can be adopted for a variety of spectrometer configurations, including single- slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer mission), and slot spectrometers (which produce overlappograms).
Automated Detection of Rapid Variability of Moss Using SDO/AIA and Its Connection to the Solar CoronaGraham, David R.De Pontieu, BartTesta, PaolaDOI: info:10.3847/2041-8213/ab2f91v. 880L12
Graham, David R., De Pontieu, Bart, and Testa, Paola. 2019. "Automated Detection of Rapid Variability of Moss Using SDO/AIA and Its Connection to the Solar Corona." The Astrophysical Journal 880:L12. https://doi.org/10.3847/2041-8213/ab2f91
ID: 154163
Type: article
Authors: Graham, David R.; De Pontieu, Bart; Testa, Paola
Abstract: Active region moss-the upper transition region of hot loops-was observed exhibiting rapid intensity variability on timescales of order 15 s by Testa et al. in a short time series (̃150 s) data set from Hi-C (High- resolution Coronal Imager). The intensity fluctuations in the subarcsecond 193A images (̃1.5 MK plasma) were uncharacteristic of steadily heated moss and were considered an indication of heating events connected to the corona. Intriguingly, these brightenings displayed a connection to the ends of transient hot loops seen in the corona. Following the same active region, AR11520, for 6 days, we demonstrate an algorithm designed to detect the same temporal variability in lower resolution Atmospheric Imaging Assembly (AIA) data, significantly expanding the number of events detected. Multiple analogous regions to the Hi-C data are successfully detected, showing moss that appears to "sparkle" prior to clear brightening of connected high-temperature loops; this is confirmed by the hot AIA channels and the isolated Fe XVIII emission. The result is illuminating, as the same behavior has recently been shown by Polito et al. while simulating nanoflares with a beam of electrons depositing their energy in the lower atmosphere. Furthermore, the variability is localized mostly to the hot core of the region, hence we reinforce the diagnostic potential of moss variability as the driver of energy release in the corona. The ubiquitous nature of this phenomenon, and the ability to detect it in data with extended time series, and large fields of view, opens a new window into investigating the coronal heating mechanism.
Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network LanesPanesar, Navdeep K.Sterling, Alphonse C.Moore, Ronald L.Winebarger, Amy R.Tiwari, Sanjiv K.Savage, Sabrina L.Golub, Leon E.Rachmeler, Laurel A.Kobayashi, KenBrooks, David H.Cirtain, Jonathan W.De Pontieu, BartMcKenzie, David E.Morton, Richard J.Peter, HardiTesta, PaolaWalsh, Robert W.Warren, Harry P.DOI: info:10.3847/2041-8213/ab594av. 887L8
Panesar, Navdeep K., Sterling, Alphonse C., Moore, Ronald L., Winebarger, Amy R., Tiwari, Sanjiv K., Savage, Sabrina L., Golub, Leon E., Rachmeler, Laurel A., Kobayashi, Ken, Brooks, David H., Cirtain, Jonathan W., De Pontieu, Bart, McKenzie, David E., Morton, Richard J., Peter, Hardi, Testa, Paola, Walsh, Robert W., and Warren, Harry P. 2019. "Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network Lanes." The Astrophysical Journal 887:L8. https://doi.org/10.3847/2041-8213/ab594a
ID: 154519
Type: article
Authors: Panesar, Navdeep K.; Sterling, Alphonse C.; Moore, Ronald L.; Winebarger, Amy R.; Tiwari, Sanjiv K.; Savage, Sabrina L.; Golub, Leon E.; Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Testa, Paola; Walsh, Robert W.; Warren, Harry P.
Abstract: We present high-resolution, high-cadence observations of six, fine-scale, on-disk jet-like events observed by the High-resolution Coronal Imager 2.1 (Hi-C 2.1) during its sounding-rocket flight. We combine the Hi-C 2.1 images with images from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS), and investigate each event's magnetic setting with co-aligned line-of-sight magnetograms from the SDO/Helioseismic and Magnetic Imager (HMI). We find that (i) all six events are jetlet-like (having apparent properties of jetlets), (ii) all six are rooted at edges of magnetic network lanes, (iii) four of the jetlet-like events stem from sites of flux cancelation between majority- polarity network flux and merging minority-polarity flux, and (iv) four of the jetlet-like events show brightenings at their bases reminiscent of the base brightenings in coronal jets. The average spire length of the six jetlet-like events (9000 ± 3000 km) is three times shorter than that for IRIS jetlets (27,000 ± 8000 km). While not ruling out other generation mechanisms, the observations suggest that at least four of these events may be miniature versions of both larger-scale coronal jets that are driven by minifilament eruptions and still-larger-scale solar eruptions that are driven by filament eruptions. Therefore, we propose that our Hi-C events are driven by the eruption of a tiny sheared-field flux rope, and that the flux rope field is built and triggered to erupt by flux cancelation.
Can the Superposition of Evaporative Flows Explain Broad Fe XXI Profiles during Solar Flares?Polito, VanessaTesta, PaolaDe Pontieu, BartDOI: info:10.3847/2041-8213/ab290bv. 879L17
Polito, Vanessa, Testa, Paola, and De Pontieu, Bart. 2019. "Can the Superposition of Evaporative Flows Explain Broad Fe XXI Profiles during Solar Flares?." The Astrophysical Journal 879:L17. https://doi.org/10.3847/2041-8213/ab290b
ID: 154175
Type: article
Authors: Polito, Vanessa; Testa, Paola; De Pontieu, Bart
Abstract: The observation of the high-temperature (≳10 MK) Fe XXI 1354.1 Å line with the Interface Region Imaging Spectrograph has provided significant insights into the chromospheric evaporation process in flares. In particular, the line is often observed to be completely blueshifted, in contrast to previous observations at lower spatial and spectral resolution, and in agreement with predictions from theoretical models. Interestingly, the line is also observed to be mostly symmetric and significantly broader than expected from thermal motions (assuming the peak formation temperature of the ion is in equilibrium). One popular interpretation for the nonthermal broadening is the superposition of flows from different loop strands. In this work, we test this scenario by forward-modeling the Fe XXI line profile assuming different possible observational scenarios using hydrodynamic simulations of multi-thread flare loops with the 1D RADYN code. Our results indicate that the superposition of flows alone cannot easily reproduce both the symmetry and the significant broadening of the line and that some other physical process, such as turbulence, or a much larger ion temperature than previously expected, likely needs to be invoked in order to explain the observed profiles.
The High-Resolution Coronal Imager, Flight 2.1Rachmeler, Laurel A.Winebarger, Amy R.Savage, Sabrina L.Golub, LeonKobayashi, KenVigil, Genevieve D.Brooks, David H.Cirtain, Jonathan W.De Pontieu, BartMcKenzie, David E.Morton, Richard J.Peter, HardiTesta, PaolaTiwari, Sanjiv K.Walsh, Robert W.Warren, Harry P.Alexander, CarolineAnsell, DarrenBeabout, Brent L.Beabout, Dyana L.Bethge, Christian W.Champey, Patrick R.Cheimets, Peter N.Cooper, Mark A.Creel, Helen K.Gates, RichardGomez, CarlosGuillory, AnthonyHaight, HarlanHogue, William D.Holloway, ToddHyde, David W.Kenyon, RichardMarshall, Joseph N.McCracken, Jeff E.McCracken, KennethMitchell, Karen O.Ordway, MarkOwen, TimRanganathan, JaganRobertson, Bryan A.Payne, M. JaniePodgorski, WilliamPryor, JonathanSamra, JennaSloan, Mark D.Soohoo, Howard A.Steele, D. BrandonThompson, Furman V.Thornton, Gary S.Watkinson, BenjaminWindt, DavidDOI: info:10.1007/s11207-019-1551-2v. 294174
Rachmeler, Laurel A., Winebarger, Amy R., Savage, Sabrina L., Golub, Leon, Kobayashi, Ken, Vigil, Genevieve D., Brooks, David H., Cirtain, Jonathan W., De Pontieu, Bart, McKenzie, David E., Morton, Richard J., Peter, Hardi, Testa, Paola, Tiwari, Sanjiv K., Walsh, Robert W., Warren, Harry P., Alexander, Caroline, Ansell, Darren, Beabout, Brent L., Beabout, Dyana L., Bethge, Christian W., Champey, Patrick R., Cheimets, Peter N., Cooper, Mark A., Creel, Helen K. et al. 2019. "The High-Resolution Coronal Imager, Flight 2.1." Solar Physics 294:174. https://doi.org/10.1007/s11207-019-1551-2
ID: 154524
Type: article
Authors: Rachmeler, Laurel A.; Winebarger, Amy R.; Savage, Sabrina L.; Golub, Leon; Kobayashi, Ken; Vigil, Genevieve D.; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Testa, Paola; Tiwari, Sanjiv K.; Walsh, Robert W.; Warren, Harry P.; Alexander, Caroline; Ansell, Darren; Beabout, Brent L.; Beabout, Dyana L.; Bethge, Christian W.; Champey, Patrick R.; Cheimets, Peter N.; Cooper, Mark A.; Creel, Helen K.; Gates, Richard; Gomez, Carlos; Guillory, Anthony; Haight, Harlan; Hogue, William D.; Holloway, Todd; Hyde, David W.; Kenyon, Richard; Marshall, Joseph N.; McCracken, Jeff E.; McCracken, Kenneth; Mitchell, Karen O.; Ordway, Mark; Owen, Tim; Ranganathan, Jagan; Robertson, Bryan A.; Payne, M. Janie; Podgorski, William; Pryor, Jonathan; Samra, Jenna; Sloan, Mark D.; Soohoo, Howard A.; Steele, D. Brandon; Thompson, Furman V.; Thornton, Gary S.; Watkinson, Benjamin; Windt, David
Abstract: The third flight of the High-Resolution Coronal Imager (Hi-C 2.1) occurred on May 29, 2018; the Sounding Rocket was launched from White Sands Missile Range in New Mexico. The instrument has been modified from its original configuration (Hi-C 1) to observe the solar corona in a passband that peaks near 172 Å, and uses a new, custom-built low-noise camera. The instrument targeted Active Region 12712, and captured 78 images at a cadence of 4.4 s (18:56:22 - 19:01:57 UT; 5 min and 35 s observing time). The image spatial resolution varies due to quasi- periodic motion blur from the rocket; sharp images contain resolved features of at least 0.47 arcsec. There are coordinated observations from multiple ground- and space-based telescopes providing an unprecedented opportunity to observe the mass and energy coupling between the chromosphere and the corona. Details of the instrument and the data set are presented in this paper.
Impulsive Coronal Heating from Large-scale Magnetic Rearrangements: From IRIS to SDO/AIAReale, FabioTesta, PaolaPetralia, AntoninoGraham, David R.DOI: info:10.3847/1538-4357/ab304fv. 8827
Reale, Fabio, Testa, Paola, Petralia, Antonino, and Graham, David R. 2019. "Impulsive Coronal Heating from Large-scale Magnetic Rearrangements: From IRIS to SDO/AIA." The Astrophysical Journal 882:7. https://doi.org/10.3847/1538-4357/ab304f
ID: 154445
Type: article
Authors: Reale, Fabio; Testa, Paola; Petralia, Antonino; Graham, David R.
Abstract: The Interface Region Imaging Spectrograph (IRIS) has observed bright spots at the transition region footpoints associated with heating in the overlying loops, as observed by coronal imagers. Some of these brightenings show significant blueshifts in the Si IV line at 1402.77 Å ({log}T[{{K}}]≈ 4.9). Such blueshifts cannot be reproduced by coronal loop models assuming heating by thermal conduction only, but are consistent with electron beam heating, highlighting for the first time the possible importance of nonthermal electrons in the heating of nonflaring active regions. Here we report on the coronal counterparts of these brightenings observed in the hot channels of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We show that the IRIS bright spots are the footpoints of very hot and transient coronal loops that clearly experience strong magnetic interactions and rearrangements, thus confirming the impulsive nature of the heating and providing important constraints for their physical interpretation.
Large-amplitude Quasiperiodic Pulsations as Evidence of Impulsive Heating in Hot Transient Loop Systems Detected in the EUV with SDO/AIAReale, FabioTesta, PaolaPetralia, AntoninoKolotkov, Dmitrii Y.DOI: info:10.3847/1538-4357/ab4270v. 884131
Reale, Fabio, Testa, Paola, Petralia, Antonino, and Kolotkov, Dmitrii Y. 2019. "Large-amplitude Quasiperiodic Pulsations as Evidence of Impulsive Heating in Hot Transient Loop Systems Detected in the EUV with SDO/AIA." The Astrophysical Journal 884:131. https://doi.org/10.3847/1538-4357/ab4270
ID: 154711
Type: article
Authors: Reale, Fabio; Testa, Paola; Petralia, Antonino; Kolotkov, Dmitrii Y.
Abstract: Short heat pulses can trigger plasma pressure fronts inside closed magnetic tubes in the corona. The alternation of condensations and rarefactions from the pressure modes drive large-amplitude pulsations in the plasma emission. Here we show the detection of such pulsations along magnetic tubes that brighten transiently in the hot 94 Å EUV channel of the Solar Dynamics Observatory/AIA. The pulsations are consistent with those predicted by hydrodynamic loop modeling, and confirm pulsed heating in the loop system. The comparison of observations and model provides constraints on the heat deposition: a good agreement requires loop twisting and pulses deposited close to the footpoints with a duration of 0.5 minutes in one loop, and deposited in the corona with a duration of 2.5 minutes in another loop of the same loop system.
Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDOTiwari, Sanjiv K.Panesar, Navdeep K.Moore, Ronald L.De Pontieu, BartWinebarger, Amy R.Golub, LeonSavage, Sabrina L.Rachmeler, Laurel A.Kobayashi, KenTesta, PaolaWarren, Harry P.Brooks, David H.Cirtain, Jonathan W.McKenzie, David E.Morton, Richard J.Peter, HardiWalsh, Robert W.DOI: info:10.3847/1538-4357/ab54c1v. 88756
Tiwari, Sanjiv K., Panesar, Navdeep K., Moore, Ronald L., De Pontieu, Bart, Winebarger, Amy R., Golub, Leon, Savage, Sabrina L., Rachmeler, Laurel A., Kobayashi, Ken, Testa, Paola, Warren, Harry P., Brooks, David H., Cirtain, Jonathan W., McKenzie, David E., Morton, Richard J., Peter, Hardi, and Walsh, Robert W. 2019. "Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDO." The Astrophysical Journal 887:56. https://doi.org/10.3847/1538-4357/ab54c1
ID: 154545
Type: article
Authors: Tiwari, Sanjiv K.; Panesar, Navdeep K.; Moore, Ronald L.; De Pontieu, Bart; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.; Rachmeler, Laurel A.; Kobayashi, Ken; Testa, Paola; Warren, Harry P.; Brooks, David H.; Cirtain, Jonathan W.; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Walsh, Robert W.
Abstract: The second Hi-C flight (Hi-C 2.1) provided unprecedentedly high spatial and temporal resolution (∼250 km, 4.4 s) coronal EUV images of Fe IX/X emission at 172 Å of AR 12712 on 2018 May 29, during 18:56:21─19:01:56 UT. Three morphologically different types (I: dot-like; II: loop-like III: surge/jet-like) of fine-scale sudden-brightening events (tiny microflares) are seen within and at the ends of an arch filament system in the core of the AR. Although type Is (not reported before) resemble IRIS bombs (in size, and brightness with respect to surroundings), our dot-like events are apparently much hotter and shorter in span (70 s). We complement the 5 minute duration Hi-C 2.1 data with SDO/HMI magnetograms, SDO/AIA EUV images, and IRIS UV spectra and slit-jaw images to examine, at the sites of these events, brightenings and flows in the transition region and corona and evolution of magnetic flux in the photosphere. Most, if not all, of the events are seated at sites of opposite-polarity magnetic flux convergence (sometimes driven by adjacent flux emergence), implying likely flux cancellation at the microflare's polarity inversion line. In the IRIS spectra and images, we find confirming evidence of field-aligned outflow from brightenings at the ends of loops of the arch filament system. In types I and II the explosion is confined, while in type III the explosion is ejective and drives jet-like outflow. The light curves from Hi-C, AIA, and IRIS peak nearly simultaneously for many of these events, and none of the events display a systematic cooling sequence as seen in typical coronal flares, suggesting that these tiny brightening events have chromospheric/transition region origin.
Guided flows in coronal magnetic flux tubesPetralia, A.Reale, F.Testa, P.DOI: info:10.1051/0004-6361/201731827v. 609A18
Petralia, A., Reale, F., and Testa, P. 2018. "Guided flows in coronal magnetic flux tubes." Astronomy and Astrophysics 609:A18. https://doi.org/10.1051/0004-6361/201731827
ID: 145779
Type: article
Authors: Petralia, A.; Reale, F.; Testa, P.
Abstract: Context. There is evidence that coronal plasma flows break down into fragments and become laminar.
Aims: We investigate this effect by modelling flows confined along magnetic channels.
Methods: We consider a full magnetohydrodynamic (MHD) model of a solar atmosphere box with a dipole magnetic field. We compare the propagation of a cylindrical flow perfectly aligned with the field to that of another flow with a slight misalignment. We assume a flow speed of 200 km s-1 and an ambient magnetic field of 30 G.
Results: We find that although the aligned flow maintains its cylindrical symmetry while it travels along the magnetic tube, the misaligned one is rapidly squashed on one side, becoming laminar and eventually fragmented because of the interaction and back-reaction of the magnetic field. This model could explain an observation made by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory of erupted fragments that fall back onto the solar surface as thin and elongated strands and end up in a hedge-like configuration.
Conclusions: The initial alignment of plasma flow plays an important role in determining the possible laminar structure and fragmentation of flows while they travel along magnetic channels. Movies are available in electronic form at http://www.aanda.org
Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN SimulationsPolito, V.Testa, P.Allred, J.De Pontieu, B.Carlsson, M.Pereira, T. M. D.Gosic, MilanReale, FabioDOI: info:10.3847/1538-4357/aab49ev. 856178
Polito, V., Testa, P., Allred, J., De Pontieu, B., Carlsson, M., Pereira, T. M. D., Gosic, Milan, and Reale, Fabio. 2018. "Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN Simulations." The Astrophysical Journal 856:178. https://doi.org/10.3847/1538-4357/aab49e
ID: 147074
Type: article
Authors: Polito, V.; Testa, P.; Allred, J.; De Pontieu, B.; Carlsson, M.; Pereira, T. M. D.; Gosic, Milan; Reale, Fabio
Abstract: We present the results of 1D hydrodynamic simulations of coronal loops that are subject to nanoflares, caused by either in situ thermal heating or nonthermal electron (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTEs with high enough low-energy cutoff ({E}{{C}}) deposit energy in the lower TR and chromosphere, causing blueshifts (up to ~20 km s-1) in the IRIS Si IV lines, which thermal conduction cannot reproduce. The {E}{{C}} threshold value for the blueshifts depends on the total energy of the events (≈5 keV for 1024 erg, up to 15 keV for 1025 erg). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and {E}{{C}}. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (}}. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (9 cm-3). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94 Å) for single strands is below current detectability and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg II chromospheric lines can help further constrain the properties of the heating mechanisms.