Browsing by Author "Hansteen, Viggo H."

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Detection of Supersonic Downflows and Associated Heating Events in the Transition Region above Sunspots
(2014-07) Kleint, Lucia; Antolin, P.; Tian, Hui; Judge, P.; Testa, Paola; De Pontieu, Bart; Martínez-Sykora, J.; Reeves, Kathy K.; Wülser, Jean-Pierre; McKillop, Sean; Saar, Steven; Carlsson, Mats; Boerner, P.; Hurlburt, Neal E.; Lemen, James; Tarbell, Ted D.; Title, Alan M.; Golub, Leon; Hansteen, Viggo H.; Jaeggli, Sarah; Kankelborg, Charles
Interface Region Imaging Spectrograph data allow us to study the solar transition region (TR) with an unprecedented spatial resolution of 0.''33. On 2013 August 30, we observed bursts of high Doppler shifts suggesting strong supersonic downflows of up to 200 km s–1 and weaker, slightly slower upflows in the spectral lines Mg II h and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated with brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout the 2 hr observation, with average burst durations of about 20 s. The locations of these shortlived events appear to be the umbral and penumbral footpoints of EUV loops. Fast apparent downflows are observed along these loops in the SJIs and in the Atmospheric Imaging Assembly, suggesting that the loops are thermally unstable. We interpret the observations as cool material falling from coronal heights, and especially coronal rain produced along the thermally unstable loops, which leads to an increase of intensity at the loop footpoints, probably indicating an increase of density and temperature in the TR. The rain speeds are on the higher end of previously reported speeds for this phenomenon, and possibly higher than the freefall velocity along the loops. On other observing days, similar bright dots are sometimes aligned into ribbons, resembling small flare ribbons. These observations provide a first insight into smallscale heating events in sunspots in the TR.
Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares
(2014-10) Testa, Paola; De Pontieu, Bart; Allred, J.; Carlsson, Mats; Reale, F.; Daw, A.; Hansteen, Viggo H.; Martinez-Sykora, J.; Liu, W.; DeLuca, E.E.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Tian, Hui; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Kleint, Lucia; Kankelborg, Charles; Jaeggli, Sarah
The physical processes causing energy exchange between the Sun’s hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High-resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to 60 seconds) of intensity and velocity on small spatial scales (≲500 kilometers) at the footpoints of hot and dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of nonthermal electrons, which are generated in small impulsive (≲30 seconds) heating events called “coronal nanoflares.” The accelerated electrons deposit a sizable fraction of their energy (≲1025 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona.
High‐resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph
(2014-05) Tian, Hui; DeLuca, E.E.; Reeves, Kathy K.; McKillop, Sean; De Pontieu, Bart; Martínez-Sykora, J.; Carlsson, Mats; Hansteen, Viggo H.; Kleint, Lucia; Cheung, M.; Golub, Leon; Saar, Steven; Testa, Paola; Weber, Mark A.; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wülser, Jean-Pierre; Kankelborg, Charles; Jaeggli, Sarah; McIntosh, Scott W.
We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slitjaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the sunspot. The intensity change is ~30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of ~10 km s1 in Si IV. The Si IV oscillation lags those of C II and Mg II by ~3 and ~12 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si IV, whereas the intensity enhancement slightly precedes the maximum blueshift in Mg II. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves.
Homologous Helical Jets: Observations By IRIS, SDO, and Hinode and Magnetic Modeling With Data‐Driven
(2015-03-05) Cheung, Mark C.M.; De Pontieu, B.; Tarbell, Ted D.; Fu, Y.; Tian, Hui; Testa, Paola; Reeves, Kathy K.; Martínez-Sykora, J.; Boerner, B.; Wülser, J.P.; Lemen, James; Title, Alan M.; Hurlburt, Neal E.; Kleint, Lucia; Kankelborg, Charles; Jaeggli, Sarah; Golub, Leon; McKillop, Sean; Saar, Steven; Carlsson, Mats; Hansteen, Viggo H.
We report on observations of recurrent jets by instruments on board the Interface Region Imaging Spectrograph, Solar Dynamics Observatory (SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21, recurrent coronal jets were observed to emanate from NOAA Active Region 11793. Far-ultraviolet spectra probing plasma at transition region temperatures show evidence of oppositely directed flows with components reaching Doppler velocities of ±100 km s−1. Raster Doppler maps using a Si iv transition region line show all four jets to have helical motion of the same sense. Simultaneous observations of the region by SDO and Hinode show that the jets emanate from a source region comprising a pore embedded in the interior of a supergranule. The parasitic pore has opposite polarity flux compared to the surrounding network field. This leads to a spine-fan magnetic topology in the coronal field that is amenable to jet formation. Time-dependent data-driven simulations are used to investigate the underlying drivers for the jets. These numerical experiments show that the emergence of current-carrying magnetic field in the vicinity of the pore supplies the magnetic twist needed for recurrent helical jet formation.
Hot Explosions in the Cool Atmoshere of the Sun
(2014-10) Peter, H.; Tian, Hui; Curdt, W.; Schmidt, D.; Innes, D.; De Pontieu, Bart; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Martinez-Sykora, J.; Kleint, Lucia; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Kankelborg, Charles; Jaeggli, Sarah; Carlsson, Mats; Hansteen, Viggo H.
The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by the Interface Region Imaging Spectrograph (IRIS) reveal that it is difficult to determine what is up and down, even in the cool 6000-kelvin photosphere just above the solar surface: This region hosts pockets of hot plasma transiently heated to almost 100,000 kelvin. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection.
An Interface Region Imaging Spectrograph First View on Solar Spicules
(2014-09) Pereira, Tiago M. D.; De Pontieu, Bart; Carlsson, Mats; Hansteen, Viggo H.; Tarbell, Ted D.; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Wülser, J.P.
Solar spicules have eluded modelers and observers for decades. Since the discovery of the more energetic type II, spicules have become a heated topic but their contribution to the energy balance of the low solar atmosphere remains unknown. Here we give a first glimpse of what quietSun spicules look like when observed with NASA's recently launched Interface Region Imaging Spectrograph (IRIS). Using IRIS spectra and filtergrams that sample the chromosphere and transition region, we compare the properties and evolution of spicules as observed in a coordinated campaign with Hinode and the Atmospheric Imaging Assembly. Our IRIS observations allow us to follow the thermal evolution of type II spicules and finally confirm that the fading of Ca II H spicules appears to be caused by rapid heating to higher temperatures. The IRIS spicules do not fade but continue evolving, reaching higher and falling back down after 500800 s. Ca II H type II spicules are thus the initial stages of violent and hotter events that mostly remain invisible in Ca II H filtergrams. These events have very different properties from type I spicules, which show lower velocities and no fading from chromospheric passbands. The IRIS spectra of spicules show the same signature as their proposed disk counterparts, reinforcing earlier work. Spectroheliograms from spectral rasters also confirm that quietSun spicules originate in bushes from the magnetic network. Our results suggest that type II spicules are indeed the site of vigorous heating (to at least transition region temperatures) along extensive parts of the upward moving spicular plasma.
Internetwork Chromospheric Bright Grains Observed with IRIS and SST
(2015-04) Martínez-Sykora, Juan; van der Voort, Luc Rouppe; Carlsson, Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, P.; Hurlburt, Neal E.; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan M.; Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli, Sarah; Kankelborg, Charles
The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet Sun. These bright grains are seen with the IRIS 1330, 1400, and 2796 Ã… slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1 m Solar Telescope (SST) which allows us to have chromospheric (Ca ii 8542 Ã…, Ca ii H 3968 Ã…, HÎ±, and Mg ii k 2796 Ã…) and transition region (C ii 1334 Ã…, Si iv 1403 Ã…) spectral imaging, and single-wavelength Stokes maps in Fe i 6302 Ã… at high spatial ($0\\buildrel{\\prime\\prime}\\over{.} 33$), temporal, and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images (SJIs) with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 Ã… slit-jaw filter comes from both the Mg ii k core and wings. The signal in the C ii and Si iv lines is too weak to explain the presence of grains in the 1300 and 1400 Ã… SJIs and we conclude that the grain signal in these passbands comes mostly from the continuum. Although weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C ii spectra. For some grains, a spectral signature can be found in IRIS Si iv. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region.
On the prevalence of small-scale twist in the solar chromosphere and transition region
(De Pontieu, B., L. Rouppe van der Voort, S. W. McIntosh, T. M. D. Pereira, M. Carlsson, V. Hansteen, H. Skogsrud, et al. “On the Prevalence of Small-Scale Twist in the Solar Chromosphere and Transition Region.” Science 346, no. 6207 (October 16, 2014): 1255732–1255732. doi:10.1126/science.1255732., 2014-10) De Pontieu, Bart; Rouppe van der Voort, L.; McIntosh, Scott W.; Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo H.; Skogsrud, H.; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; DeLuca, E.E.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Kankelborg, Charles; Jaeggli, Sarah; Kleint, Lucia; Martinez-Sykora, J.
The solar chromosphere and transition region (TR) form an interface between the Sun’s surface and its hot outer atmosphere. There, most of the nonthermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33–arc second) observations with NASA’s Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub–arc second scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-meter Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere.
Prevalence of small-scale jets from the networks of the solar transition region and chromosphere
(2014-10) Tian, Hui; DeLuca, E.E.; Cranmer, S.R.; De Pontieu, Bart; Peter, H.; Martinez-Sykora, J.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Miralles, M.P.; McCauley, P.; Saar, Steven; Testa, Paola; Weber, Mark A.; Murphy, N.; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Kleint, Lucia; Kankelborg, Charles; Jaeggli, Sarah; Carlsson, Mats; Hansteen, Viggo H.; McIntosh, Scott W.
As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.
The Unresolved Fine Structure Resolved: IRIS Observations of the Solar Transition Region
(2014-10) Hansteen, Viggo H.; De Pontieu, B.; Carlsson, Mats; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Pereira, Tiago M. D.; De Luca, E.E.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Kankelborg, Charles; Jaeggli, Sarah; Kleint, Lucia; Martínez-Sykora, J.
The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a longstanding problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, lowlying loops or loop segments at transitionregion temperatures that vary rapidly, on the time scales of minutes. We argue that the existence of these loops solves a longstanding observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle.