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 measure distribution in 15 active region 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 94Å 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.

It is now apparent that there are at least two heating mechanisms in the Sun's outer atmosphere, or corona. Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1-3). The active corona needs additional heating to reach 2,000,000-4,000,000 K; this heat has been theoretically proposed to come from the reconnection and unravelling of magnetic 'braids'. Evidence favouring that process has been inferred, but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought to have an angular width of about 0.2 arc seconds have not been resolved. Fine-scale braiding has been seen in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.

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We present observations of high temperature emission in the core of a solar active region using instruments on Hinode and SDO. These multi-instrument observations allow us to determine the distribution of plasma temperatures and follow the evolution of emission at different temperatures. We find that at the apex of the high temperature loops the emission measure distribution is strongly peaked near 4 MK and falls off sharply at both higher and lower temperatures. Perhaps most significantly, the emission measure at 0.5 MK is reduced by more than two orders of magnitude from the peak at 4 MK. We also find that the temporal evolution in broad-band soft X-ray images is relatively constant over about 6 hours of observing. Observations in the cooler SDO /AIA bandpasses generally do not show cooling loops in the core of the active region, consistent with the steady emission observed at high temperatures. These observations suggest that the high temperature loops observed in the core of an active region are close to equilibrium. We find that it is possible to reproduce the relative intensities of high temperature emission lines with a simple, high-frequency heating scenario where heating events occur on time scales much less than a cooling time. In contrast, lowfrequency heating scenarios, which are commonly invoked to describe nanoflare models of coronal heating, do not reproduce the relative intensities of high temperature emission lines and predict low-temperature emission that is approximately an order of magnitude too large. We also present an initial look at images from the SDO /AIA 94Å channel, which is sensitive to Fe XVIII.Subject headings: Sun: corona IntroductionUnderstanding the origin of high temperature plasma in the solar corona is one of the central problems in solar physics. The nanoflare concept represents one of the more popular theories for describing how energy stored in the Sun's magnetic field is converted into thermal energy (e.g., Parker

Aims. We use new data from the High-resolution Coronal Imager (Hi-C) with its unprecedented spatial resolution of the solar corona to investigate the structure of coronal loops down to 0.2 . Methods. During a rocket flight, Hi-C provided images of the solar corona in a wavelength band around 193 Å that is dominated by emission from Fe  showing plasma at temperatures around 1.5 MK. We analyze part of the Hi-C field-of-view to study the smallest coronal loops observed so far and search for the possible substructuring of larger loops. Results. We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal loops. Their coronal segments above the chromosphere have a length of only about 1 Mm and a thickness of less than 200 km. They could be interpreted as the coronal signature of small flux tubes breaking through the photosphere with a footpoint distance corresponding to the diameter of a cell of granulation. We find that loops that are longer than 50 Mm have diameters of about 2 or 1.5 Mm, which is consistent with previous observations. However, Hi-C really resolves these loops with some 20 pixels across the loop. Even at this greatly improved spatial resolution, the large loops seem to have no visible substructure. Instead they show a smooth variation in cross-section. Conclusions. That the large coronal loops do not show a substructure on the spatial scale of 0.1 per pixel implies that either the densities and temperatures are smoothly varying across these loops or it places an upper limit on the diameter of the strands the loops might be composed of. We estimate that strands that compose the 2 thick loop would have to be thinner than 15 km. The miniature loops we find for the first time pose a challenge to be properly understood through modeling.