Are Chromospheric Nanoflares a Primary Source of Coronal Plasma?

It has been suggested that the hot plasma of the solar corona comes primarily from impulsive heating events, or nanoflares, that occur in the lower atmosphere, either in the upper part of the ordinary chromosphere or at the tips of type II spicules. We test this idea with a series of hydrodynamic simulations. We find that synthetic Fe XII (195) and Fe XIV (274) line profiles generated from the simulations disagree dramatically with actual observations. The integrated line intensities are much too faint; the blue shifts are much too fast; the blue-red asymmetries are much too large; and the emission is confined to low altitudes. We conclude that chromospheric nanoflares are not a primary source of hot coronal plasma. Such events may play an important role in producing the chromosphere and powering its intense radiation, but they do not, in general, raise the temperature of the plasma to coronal values. Those cases where coronal temperatures are reached must be relatively uncommon. The observed profiles of Fe XII and Fe XIV come primarily from plasma that is heated in the corona itself, either by coronal nanoflares or a quasi-steady coronal heating process. Chromospheric nanoflares might play a role in generating waves that provide this coronal heating.Comment: 14 pages, 6 figures, accepted by Astrophysical Journa

The Cooling of Coronal Plasmas. iv: Catastrophic Cooling of Loops

We examine the radiative cooling of coronal loops and demonstrate that the recently identified catastrophic cooling (Reale and Landi, 2012) is due to the inability of a loop to sustain radiative / enthalpy cooling below a critical temperature, which can be > 1 MK in flares, 0.5 - 1 MK in active regions and 0.1 MK in long tenuous loops. Catastrophic cooling is characterised by a rapid fall in coronal temperature while the coronal density changes by a small amount. Analytic expressions for the critical temperature are derived and show good agreement with numerical results. This effect limits very considerably the lifetime of coronal plasmas below the critical temperature

Combination of molecular similarity measures using data fusion

Many different measures of structural similarity have been suggested for matching chemical structures, each such measure focusing upon some particular type of molecular characteristic. The multi-faceted nature of biological activity suggests that an appropriate similarity measure should encompass many different types of characteristic, and this article discusses the use of data fusion methods to combine the results of searches based on multiple similarity measures. Experiments with several different types of dataset and activity suggest that data fusion provides a simple, but effective, approach to the combination of individual similarity measures. The best results were generally obtained with a fusion rule that sums the rank positions achieved by each molecule in searches using individual measures

Can Big Media do "Big Society"?: A Critical Case Study of Commercial, Convergent Hyperlocal News

The UK Government is committed to helping ‘nurture a new generation of local media companies’. Changes to local media ownership rules allowing companies to follow their customers from platform to platform are supposed to assist in this by encouraging economies of scale. This paper provides a timely case study examining a UK-based commercial local news network owned by Daily Mail & General Trust that leverages economies of scale: Northcliffe Media’s network of 154 Local People websites. The study evaluates the level of audience engagement with the Local People sites through a user survey, and by looking at the numbers of active users, their contributions and their connections with other users. Interviews with ten of the ‘community publishers’ who oversee each site on the ground were conducted, along with a content survey. Although the study reveals a demand for community content, particularly of a practical nature, the results question the extent to which this type of ‘big media’ local news website can succeed as a local social network, reinvigorate political engagement, or encourage citizen reporting. The Government hopes that communities, especially rural ones, will increasingly use the Internet to access local news and information, thereby supporting new, profitable local media companies, who will nurture a sense of local identity and hold locally-elected politicians to account. This case study highlights the difficulties inherent in achieving such outcomes, even using the Government’s preferred convergent, commercial model

Diagnosing the time-dependence of active region core heating from the emission measure: I. Low-frequency nanoflares

Observational measurements of active region emission measures contain clues to the time-dependence of the underlying heating mechanism. A strongly non-linear scaling of the emission measure with temperature indicates a large amount of hot plasma relative to warm plasma. A weakly non-linear (or linear) scaling of the emission measure indicates a relatively large amount of warm plasma, suggesting that the hot active region plasma is allowed to cool and so the heating is impulsive with a long repeat time. This case is called {\it low-frequency} nanoflare heating and we investigate its feasibility as an active region heating scenario here. We explore a parameter space of heating and coronal loop properties with a hydrodynamic model. For each model run, we calculate the slope $\alpha$ of the emission measure distribution $EM(T) \propto T^\alpha$. Our conclusions are: (1) low-frequency nanoflare heating is consistent with about 36% of observed active region cores when uncertainties in the atomic data are not accounted for; (2) proper consideration of uncertainties yields a range in which as many as 77% of observed active regions are consistent with low-frequency nanoflare heating and as few as zero; (3) low-frequency nanoflare heating cannot explain observed slopes greater than 3; (4) the upper limit to the volumetric energy release is in the region of 50 erg cm$^{-3}$ to avoid unphysical magnetic field strengths; (5) the heating timescale may be short for loops of total length less than 40 Mm to be consistent with the observed range of slopes; (6) predicted slopes are consistently steeper for longer loops