Ignacio Ugarte-Urra

Jets in Coronal Holes: Hinode Observations and Three-dimensional Computer Modeling

Recent observations of coronal hole areas with the XRT and EIS instruments on board the Hinode satellite have shown with unprecedented detail the launching of fast, hot jets away from the solar surface. In some cases these events coincide with episodes of flux emergence from beneath the photosphere. In this Letter we show results of a three-dimensional numerical experiment of flux emergence from the solar interior into a coronal hole and compare them with simultaneous XRT and EIS observations of a jet-launching event that accompanied the appearance of a bipolar region in MDI magnetograms. The magnetic skeleton and topology that result in the experiment bear a strong resemblance to linear force-free extrapolations of the SOHO/MDI magnetograms. A thin current sheet is formed at the boundary of the emerging plasma. A jet is launched upward along the open reconnected field lines with values of temperature, density, and velocity in agreement with the XRT and EIS observations. Below the jet, a split-vault structure results with two chambers: a shrinking one containing the emerged field loops and a growing one with loops produced by the reconnection. The ongoing reconnection leads to a horizontal drift of the vault-and-jet structure. The timescales, velocities, and other plasma properties in the experiment are consistent with recent statistical studies of this type of event made with Hinode data.

The Magnetic Topology of Coronal Mass Ejection Sources

In an attempt to test current initiation models of coronal mass ejections (CMEs), with an emphasis on the magnetic breakout model, we inspect the magnetic topology of the sources of 26 CME events in the context of their chromospheric and coronal response in an interval of approximately 9 hr around the eruption onset. First we perform current-free (potential) extrapolations of photospheric magnetograms to retrieve the key topological ingredients, such as coronal magnetic null points. Then we compare the reconnection signatures observed in the high-cadence and high spatial resolution Transition Region and Coronal Explorer (TRACE) images with the location of the relevant topological features. The comparison reveals that only seven events can be interpreted in terms of the breakout model, which requires a multipolar topology with preeruption reconnection at a coronal null. We find, however, that a larger number of events (12) cannot be interpreted in those terms. No magnetic null is found in six of them. Seven other cases remain difficult to interpret. We also show that there are no systematic differences between the CME speed and flare energies of events under different interpretations.

ACTIVE REGION TRANSITION REGION LOOP POPULATIONS AND THEIR RELATIONSHIP TO THE CORONA

The relationships among coronal loop structures at different temperatures are not settled. Previous studies have suggested that coronal loops in the core of an active region (AR) are not seen cooling through lower temperatures and therefore are steadily heated. If loops were cooling, the transition region would be an ideal temperature regime to look for a signature of their evolution. The Extreme-ultraviolet Imaging Spectrometer on Hinode provides monochromatic images of the solar transition region and corona at an unprecedented cadence and spatial resolution, making it an ideal instrument to shed light on this issue. Analysis of observations of AR 10978 taken in 2007 December 8-19 indicates that there are two dominant loop populations in the AR: (1) core multitemperature loops that undergo a continuous process of heating and cooling in the full observed temperature range 0.4-2.5 MK and even higher as shown by the X-Ray Telescope and (2) peripheral loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops at transition region temperatures can reach heights of 150 Mm in the corona above the limb and develop downflows with velocities in the range of 39-105 km s–1.

SOLAR CORONAL LOOPS RESOLVED BY HINODE AND THE SOLAR DYNAMICS OBSERVATORY

Despite decades of studying the Sun, the coronal heating problem remains unsolved. One fundamental issue is that we do not know the spatial scale of the coronal heating mechanism. At a spatial resolution of 1000 km or more, it is likely that most observations represent superpositions of multiple unresolved structures. In this Letter, we use a combination of spectroscopic data from the Hinode EUV Imaging Spectrometer and high-resolution images from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory to determine the spatial scales of coronal loops. We use density measurements to construct multi-thread models of the observed loops and confirm these models using the higher spatial resolution imaging data. The results allow us to set constraints on the number of threads needed to reproduce a particular loop structure. We demonstrate that in several cases million degree loops are revealed to be single monolithic structures that are fully spatially resolved by current instruments. The majority of loops, however, must be composed of a number of finer, unresolved threads, but the models suggest that even for these loops the number of threads could be small, implying that they are also close to being resolved. These results challenge heating models of loops based on the reconnection of braided magnetic fields in the corona.

An Investigation into the Variability of Heating in a Solar Active Region

Previous studies have indicated that both steady and impulsive heating mechanisms play a role in active region heating. In this paper, we present a study of 20 hours of soft X-ray and EUVobservations of solar active region NOAA AR 8731. We examine the evolution of six representative loop structures that brighten and fade first from X-ray images and subsequently from the EUV images. We determine their lifetime and the delay between their appearance in the different filters. We find that the lifetime in the EUV filters is much longer than expected for a single cooling loop. We also notice that the delay in the loops’ appearance in the X-ray and EUV filters is proportionaltothelooplength.Wemodeloneoftheloopsusingahydrodynamicmodelwithbothimpulsiveandquasisteady heating functions and find that neither of these simple heating functions can well reproduce the observed loop characteristics in both the X-ray and EUVimages. Hence, although this active region is dominated by variable emission and the characteristics oftheobservedloopsarequalitatively consistentwith acooling loop, the timescale of the heating in this active region remains unknown. Subject headingg Sun: corona Online material: mpeg animations

The Temperature Dependence of Solar Active Region Outflows

Spectroscopic observations with the EUV Imaging Spectrometer (EIS) on Hinode have revealed large areas of high-speed outflows at the periphery of many solar active regions. These outflows are of interest because they may connect to the heliosphere and contribute to the solar wind. In this paper, we use slit rasters from EIS in combination with narrowband slot imaging to study the temperature dependence and morphology of an outflow region and show that it is more complicated than previously thought. Outflows are observed primarily in emission lines from Fe XI to Fe XV. Observations at lower temperatures (Si VII), in contrast, show bright fan-like structures that are dominated by inflows. These data also indicate that the morphology of the outflows and the fans is different, outflows are observed in regions where there is no emission in Si VII. This suggests that the fans, which are often associated with outflows in studies involving imaging data, are not directly related to the active region outflows.

High Spatial Resolution Observations of Loops in the Solar Corona

Understanding how the solar corona is structured is of fundamental importance to determine how the Suns upper atmosphere is heated to high temperatures. Recent spectroscopic studies have suggested that an instrument with a spatial resolution of 200 km or better is necessary to resolve coronal loops. The High Resolution Coronal Imager (Hi-C) achieved this performance on a rocket flight in 2012 July. We use Hi-C data to measure the Gaussian widths of 91 loops observed in the solar corona and find a distribution that peaks at about 270 km. We also use Atmospheric Imaging Assembly data for a subset of these loops and find temperature distributions that are generally very narrow. These observations provide further evidence that loops in the solar corona are often structured at a scale of several hundred kilometers, well above the spatial scale of many proposed physical mechanisms.

Temporal Variability of Active Region Outflows

Recent observations from the Extreme-ultraviolet Imaging Spectrometer (EIS) on board Hinode have shown that low-density areas on the periphery of active regions are characterized by strong blueshifts in the emission of spectral lines formed at 1?MK. These Doppler shifts have been associated with outward propagating disturbances observed with extreme-ultraviolet and soft X-ray imagers. Since these instruments can have broad temperature responses, we investigate these intensity fluctuations using the monochromatic imaging capabilities of the EIS wide slit (slot) and confirm their 1?MK nature. We also look into their spectral temporal variability using narrow slit observations and present the first Doppler movies of the outflow regions. We find that the Fe XII 195.119?? blueshifted spectral profiles at their footpoints exhibit transient blue wing enhancements on timescales as short as the 5 minute cadence. We have also looked at the fan peripheral loops observed at 0.6?MK in Si VII 275.368?? in those regions and find no sign of the recurrent outward propagating disturbances with velocities of 40-130?km s?1 seen in Fe XII. We do observe downward trends (15-20?km s?1) consistent with the characteristic redshifts measured at their footpoints. We, therefore, find no evidence that the structures at these two temperatures and the intensity fluctuations they exhibit are related to one another.

Full-Sun observations for identifying the source of the slow solar wind.

Fast (>700 km s−1) and slow (~400 km s−1) winds stream from the Sun, permeate the heliosphere and influence the near-Earth environment. While the fast wind is known to emanate primarily from polar coronal holes, the source of the slow wind remains unknown. Here we identify possible sites of origin using a slow solar wind source map of the entire Sun, which we construct from specially designed, full-disk observations from the Hinode satellite, and a magnetic field model. Our map provides a full-Sun observation that combines three key ingredients for identifying the sources: velocity, plasma composition and magnetic topology and shows them as solar wind composition plasma outflowing on open magnetic field lines. The area coverage of the identified sources is large enough that the sum of their mass contributions can explain a significant fraction of the mass loss rate of the solar wind.

The X17.2 flare occurred in NOAA 10486: an example of filament destabilization caused by a domino effect

Context. It is now possible to distinguish between two main models describing the mechanisms responsible for eruptive flares : the standard model, which assumes that most of the energy is released, by magnetic reconnection, in the region hosting the core of a sheared magnetic field, and the breakout model, which assumes reconnection occurs at first in a magnetic arcade overlaying the eruptive features. Aims. We analyze the phenomena observed in NOAA 10486 before and during an X17.2 flare that occurred on 2003 October 28, to study the relationship between the pre-flare and flare phases and determine which model is the most suitable for interpreting this event. Methods. We performed an analysis of multiwavelength data set available for the event using radio data (0.8–4.5 GHz), images in the visible range (WL and Hα), EUV images (1600 and 195 A), and X-ray data, as well as MDI longitudinal magnetograms. We determined the temporal sequence of events occurring before and during the X17.2 flare and the magnetic field configuration in the linear force-free field approximation. Results. The active region was characterized by a multiple arcade configuration and the X17.2 flare was preceded, by ∼ 2h , by the partial eruption of one filament. This eruption caused reconnection at null points located in the low atmosphere and a decrease in magnetic tension in the coronal field lines overlaying other filaments present in the active region. As a consequence, these filaments were destabilized and the X17.2 flare occurred. Conclusions. The phenomena observed in NOAA 10486 before and during the X17.2 flare cannot be explained by a simple scenario such as the standard or breakout model, but instead in terms of a so-called domino effect, involving a sequence of destabilizing processes that triggered the flare.