The coronal source of extreme-ultraviolet line profile asymmetries in solar active region outflows

High resolution spectra from the Hinode EUV Imaging Spectrometer (EIS) have revealed that coronal spectral line profiles are sometimes asymmetric, with a faint enhancement in the blue wing on the order of 100 km/s. These asymmetries could be important since they may be subtle, yet diagnostically useful signatures of coronal heating or solar wind acceleration processes. It has also been suggested that they are signatures of chromospheric jets supplying mass and energy to the corona. Until now, however, there have been no studies of the physical properties of the plasma producing the asymmetries. Here we identify regions of asymmetric profiles in the outflows of AR 10978 using an asymmetric Gaussian function and extract the intensities of the faint component using multiple Gaussian fits. We then derive the temperature structure and chemical composition of the plasma producing the asymmetries. We find that the asymmetries are dependent on temperature, and are clearer and stronger in coronal lines. The temperature distribution peaks around 1.4-1.8 MK with an emission measure at least an order of magnitude larger than that at 0.6 MK. The first ionization potential bias is found to be 3-5, implying that the high speed component of the outflows may also contribute to the slow speed wind. Observations and models indicate that it takes time for plasma to evolve to a coronal composition, suggesting that the material is trapped on closed loops before escaping, perhaps by interchange reconnection. The results, therefore, identify the plasma producing the asymmetries as having a coronal origin.Comment: ApJ Letters (in press

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 blue-shifts 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 EIS and confirm their 1 MK nature. We also find that the Fe XII 195.119 A blue shifted 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 A in those regions and find no sign of the recurrent outward propagating disturbances with velocities of 40 - 130 km/s seen in Fe XII. We do observe downward trends (15 - 20 km/s) consistent with the characteristic red-shifts 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.Comment: Movies are available at http://tcrb.nrl.navy.mil/~iuu/out/papers/2010_flows/. To be submitted to Ap

Determining heating time scales in solar active region cores from AIA/SDO Fe XVIII images

We present a study of the frequency of transient brightenings in the core of solar active regions as observed in the Fe XVIII line component of AIA/SDO 94 A filter images. The Fe XVIII emission is isolated using an empirical correction to remove the contribution of "warm" emission to this channel. Comparing with simultaneous observations from EIS/Hinode, we find that the variability observed in Fe XVIII is strongly correlated with the emission from lines formed at similar temperatures. We examine the evolution of loops in the cores of active regions at various stages of evolution. Using a newly developed event detection algorithm we characterize the distribution of event frequency, duration, and magnitude in these active regions. These distributions are similar for regions of similar age and show a consistent pattern as the regions age. This suggests that these characteristics are important constraints for models of solar active regions. We find that the typical frequency of the intensity fluctuations is about 1400s for any given line-of-sight, i.e. about 2-3 events per hour. Using the EBTEL 0D hydrodynamic model, however, we show that this only sets a lower limit on the heating frequency along that line-of-sight.Comment: Submitted to Ap

Measurements of Non-Thermal Line Widths in Solar Active Regions

Spectral line widths are often observed to be larger than can be accounted for by thermal and instrumental broadening alone. This excess broadening is a key observational constraint for both nanoflare and wave dissipation models of coronal heating. Here we present a survey of non-thermal velocities measured in the high temperature loops (1--5MK) often found in the cores of solar active regions. This survey of $\textit{Hinode}$ Extreme Ultraviolet Imaging Spectrometer (EIS) observations covers 15 non-flaring active regions that span a wide range of solar conditions. We find relatively small non-thermal velocities, with a mean value of 17km s$^{-1}$, and no significant trend with temperature or active region magnetic flux. These measurements appear to be inconsistent with those expected from reconnection jets in the corona, chromospheric evaporation induced by coronal nanoflares, and Alfv\'en wave turbulence models. Furthermore, because the observed non-thermal widths are generally small their measurements are difficult and susceptible to systematic effects.Comment: This is the revised version to be published in Ap

The Multi-Instrument (EVE-RHESSI) DEM for Solar Flares, and Implications for Non-Thermal Emission

Solar flare X-ray spectra are typically dominated by thermal bremsstrahlung emission in the soft X-ray ($\lesssim$10 keV) energy range; for hard X-ray energies ($\gtrsim$30 keV), emission is typically non-thermal from beams of electrons. The low-energy extent of non-thermal emission has only been loosely quantified. It has been difficult to obtain a lower limit for a possible non-thermal cutoff energy due to the significantly dominant thermal emission. Here we use solar flare data from the EUV Variability Experiment (EVE) on-board the Solar Dynamics Observatory (SDO) and X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) to calculate the Differential Emission Measure (DEM). This improvement over the isothermal approximation and any single-instrument DEM helps to resolve ambiguities in the range where thermal and non-thermal emission overlap, and to provide constraints on the low-energy cutoff. In the model, thermal emission is from a DEM that is parametrized as multiple gaussians in $Log(T)$. Non-thermal emission results from a photon spectrum obtained using a thick-target emission model. Spectra for both instruments are fit simultaneously in a self-consistent manner. Our results have been obtained using a sample of 52 large (GOES X- and M-class) solar flares observed between February 2011 and February 2013. It turns out that it is often possible to determine low-energy cutoffs early (in the first two minutes) during large flares. Cutoff energies are typically low, less than 10 keV, with most values of the lower limits in the 5--7 keV range.Comment: 10 pages, 12 figures, 2 tables; accepted for publication in Astrophysical Journal; then withdrawn; now 19 pages, 24 figure

Solar Coronal Loops Resolved by Hinode and SDO

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 (EIS) and high resolution images from the Atmospheric Imaging Assembly (AIA) 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.Comment: Revised version accepted by ApJ Letter

Observations of Thermal Flare Plasma with the EUV Variability Experiment

One of the defining characteristics of a solar flare is the impulsive formation of very high temperature plasma. The properties of the thermal emission are not well understood, however, and the analysis of solar flare observations is often predicated on the assumption that the flare plasma is isothermal. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of emission lines that span a wide range of temperatures (e.g., Fe XV-Fe XXIV) and allow for thermal flare plasma to be studied in detail. In this paper we describe a method for computing the differential emission measure distribution in a flare using EVE observations and apply it to several representative events. We find that in all phases of the flare the differential emission measure distribution is broad. Comparisons of EVE spectra with calculations based on parameters derived from the GOES soft X-ray fluxes indicate that the isothermal approximation is generally a poor representation of the thermal structure of a flare.Comment: Paper has not been submitted. Comments are welcome

A Systematic Survey of High Temperature Emission in Solar Active Regions

The recent analysis of observations taken with the EIS instrument on Hinode suggests that well constrained measurements of the temperature distribution in solar active regions can finally be made. Such measurements are critical for constraining theories of coronal heating. Past analysis, however, has suffered from limited sample sizes and large uncertainties at temperatures between 5 and 10 MK. Here we present a systematic study of the differential emission cores. We focus on measurements in the "inter-moss" region, that is, the region between the loop footpoints, where the observations are easier to interpret. To reduce the uncertainties at the highest temperatures we present a new method for isolating the Fe XVIII emission in the AIA/SDO 94 channel. The resulting differential emission measure distributions confirm our previous analysis showing that the temperature distribution in an active region core is often strongly peaked near 4 MK. We characterize the properties of the emission distribution as a function of the total unsigned magnetic flux. We find that the amount of high temperature emission in the active region core is correlated with the total unsigned magnetic flux, while the emission at lower temperatures, in contrast, is inversely related. These results provide compelling evidence that high temperature active region emission is often close to equilibrium, although weaker active regions may be dominated by evolving million degree loops in the core.Comment: Figures are degraded to reduce file sizes. Email the first author for a higher resolution version of the pdf. Submitted to ApJ. Minor revisions have been made to the manuscript. The AIA data were reprocessed and the emission measures have changed slightl

Chromospheric Evaporation in an M1.8 Flare Observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode

We discuss observations of chromospheric evaporation for a complex flare that occurred on 9 March 2012 near 03:30 UT obtained from the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft. This was a multiple event with a strong energy input that reached the M1.8 class when observed by EIS. EIS obtained a full-CCD spectrum of the flare. Chromospheric evaporation characterized by 150-200 km/s upflows was observed in multiple locations in multi-million degree spectral lines of flare ions such as Fe XXII, Fe XXIII, and Fe XXIV, with simultaneous 20-60 km/s upflows in million degree coronal lines from ions such as Fe XII - Fe XVI. The behavior of cooler, transition region ions such as O VI, Fe VIII, He II, and Fe X is more complex, but upflows were also observed in Fe VIII and Fe X lines. At a point close to strong energy input in space and time, the flare ions Fe XXII, Fe XXIII, and Fe XXIV reveal an isothermal source with a temperature close to 14 MK and no strong blueshifted components. At this location there is a strong downflow in cooler active region lines from ions such as Fe XIII and Fe XIV. We speculate that this downflow may be evidence of the downward shock produced by reconnection in the current sheet seen in MHD simulations. A sunquake also occurred near this location.Comment: 14 pages, 1 table, 17 figure

Properties and Modeling of Unresolved Fine Structure Loops Observed in the Solar Transition Region by IRIS

Recent observations from the Interface Region Imaging Spectrograph (IRIS) have discovered a new class of numerous low-lying dynamic loop structures, and it has been argued that they are the long-postulated unresolved fine structures (UFS) that dominate the emission of the solar transition region. In this letter, we combine IRIS measurements of the properties of a sample of 108 UFS (intensities, lengths, widths, lifetimes) with 1-D non-equilibrium ionization simulations using the HYDRAD hydrodynamic model to examine whether the UFS are now truly spatially resolved in the sense of being individual structures rather than composed of multiple magnetic threads. We find that a simulation of an impulsively heated single strand can reproduce most of the observed properties suggesting that the UFS may be resolved, and the distribution of UFS widths implies that they are structured on a spatial scale of 133km on average. Spatial scales of a few hundred km appear to be typical for a range of chromospheric and coronal structures, and we conjecture that this could be an important clue to the coronal heating process.Comment: This is the version to be publishe