Prominence plasma diagnostics through EUV absorption

In this paper we introduce a new diagnostic technique that uses prominence EUV and UV absorption to determine the prominence plasma electron temperature and column emission measure, as well as He/H relative abundance; if a realistic assumption on the geometry of the absorbing plasma can be made, this technique can also yield the absorbing plasma electron density. This technique capitalizes on the absorption properties of Hydrogen and Helium at different wavelength ranges and temperature regimes. Several cases where this technique can be successfully applied are described. This technique works best when prominence plasmas are hotter than 15,000 K and thus it is ideally suited for rapidly heating erupting prominences observed during the initial phases of coronal mass ejections. An example is made using simulated intensities of 4 channels of the SDO/AIA instrument. This technique can be easily applied to existing observations from almost all space missions devoted to the study of the solar atmosphere, which we list.Comment: 17 pages, 4 figures, submitted to Ap

Post-flare UV light curves explained with thermal instability of loop plasma

In the present work we study the C8 flare occurred on September 26, 2000 at 19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the impulsive phase to well beyond the disappearance in the X-rays. The emission first decayed progressively through equilibrium states until the plasma reached 2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si iii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium conditions), are emitted at the same time and all evolve in a similar way. Here we show that the simultaneous emission of lines with such a different formation temperature is due to thermal instability occurring in the flaring plasma as soon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative start time of the light curves of each line in the correct order with a simple (and standard) model of a single flaring loop. The agreement with the observed light curves is greatly improved, and a slower evolution of the line emission is predicted, if we assume that the model loop consists of an ensemble of subloops or strands heated at slightly different times. Our analysis can be useful for flare observations with SDO/EVE.Comment: 24 pages, 7 figures, accepted for publicatio

Characterization of the Circadian Properties of Runt-related Transcription Factor 2 (Runx2) and its Role in the Suprachiasmatic Nucleus

Circadian rhythms orchestrate physiological, behavioral and cognitive processes in order to anticipate and adapt organisms to key environmental cues. These endogenously driven oscillations are generated by a network of interlocked auto-regulatory transcriptional-translational feedback loops driven forward by the Bmal1/Clock heterodimer transcription factor. Given the ubiquitous and dynamic quality of circadian rhythms, the identification of factors involved in the coordination and regulation of the endogenous oscillations is central in broadening our understanding of biological timing systems. In an examination of gene expression in the mammalian central circadian pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), revealed a previously unreported rhythmic expression of runt-related transcription factor 2 (Runx2). The goal of the research described in the present dissertation was to determine the mechanistic basis of rhythmic Runx2 expression as well as determine whether Runx2 interacts with the core molecular machinery of the circadian clock. First, rhythmic gene expression Runx2 in the SCN, olfactory blub (OB) and paraventricular nucleus (PVN) of adult mice was demonstrated using quantitative real-time polymerase chain reaction and immunohistochemistry served to show rhythmic Runx2 protein expression in the SCN. Second, using SCN tissues from Bmal-/- mice or by transiently silencing BMAL1 expression in vitro it was shown that Runx2 expression is dependent on a functional core molecular clock network, namely the transcriptional activity of Bmal1. Chromatin immunoprecipitation was used to demonstrate that BMAL1 directly interacted with its putative binding sites within the RUNX2 promoter suggesting that the core clock machinery directly regulates its rhythmic gene expression. Next, Runx2 haplodeficient mice and transient silencing of RUNX2 expression in vitro revealed that decreases in Runx2 expression leads to a dampened amplitude of rhythmic Bmal1 expression and a lengthening of the period of molecular as well as running wheel behavioral rhythms. Finally, RUNX2 was found to interact with a putative binding site identified in the BMAL1 promoter suggesting that the influence of RUNX2 on the amplitude of BMAL1 expression was at least in part based on direct regulation at the gene level. Together, these findings serve to establish the foundational framework of the reciprocal relationship between the Runx2 transcription factor and the molecular network underlying circadian rhythms

The role of radiative losses in the late evolution of pulse-heated coronal loops/strands

Radiative losses from optically thin plasma are an important ingredient for modeling plasma confined in the solar corona. Spectral models are continuously updated to include the emission from more spectral lines, with significant effects on radiative losses, especially around 1 MK. We investigate the effect of changing the radiative losses temperature dependence due to upgrading of spectral codes on predictions obtained from modeling plasma confined in the solar corona. The hydrodynamic simulation of a pulse-heated loop strand is revisited comparing results using an old and a recent radiative losses function. We find significant changes in the plasma evolution during the late phases of plasma cooling: when the recent radiative loss curve is used, the plasma cooling rate increases significantly when temperatures reach 1-2 MK. Such more rapid cooling occurs when the plasma density is larger than a threshold value, and therefore in impulsive heating models that cause the loop plasma to become overdense. The fast cooling has the effect of steepening the slope of the emission measure distribution of coronal plasmas with temperature at temperatures lower than ~2 MK. The effects of changes in the radiative losses curves can be important for modeling the late phases of the evolution of pulse-heated coronal loops, and, more in general, of thermally unstable optically thin plasmas.Comment: 16 pages, 7 figures, accepted for publicatio

Bright hot impacts by erupted fragments falling back on the Sun: UV redshifts in stellar accretion

A solar eruption after a flare on 7 Jun 2011 produced EUV-bright impacts of fallbacks far from the eruption site, observed with the Solar Dynamics Observatory. These impacts can be taken as a template for the impact of stellar accretion flows. Broad red-shifted UV lines have been commonly observed in young accreting stars. Here we study the emission from the impacts in the Atmospheric Imaging Assembly's UV channels and compare the inferred velocity distribution to stellar observations. We model the impacts with 2D hydrodynamic simulations. We find that the localised UV 1600A emission and its timing with respect to the EUV emission can be explained by the impact of a cloud of fragments. The first impacts produce strong initial upflows. The following fragments are hit and shocked by these upflows. The UV emission comes mostly from the shocked front shell of the fragments while they are still falling, and is therefore redshifted when observed from above. The EUV emission instead continues from the hot surface layer that is fed by the impacts. Fragmented accretion can therefore explain broad redshifted UV lines (e.g. C IV 1550A) to speeds around 400 km/s observed in accreting young stellar objects.Comment: 12 pages, 4 figures (movies available upon request), accepted for publicatio

La valutazione e i suoi effetti sull'università: una comparazione fra Italia e Francia

The paper presents the results of a comparative study on four universities in France and in Italy, which investigates how evaluation helps to strengthen the university as autonomous and professional organization, able to formulating strategies and/or tactics of adaptation in relation to regulatory changes and context, overcoming the traditional loosely coupled feature. The hypothesis is that the academic organizations show different effects of resilience, linked to intended and unintended changes

How does collegiality survive managerially led universities? Evidence from a European Survey

Today's universities are, accordingly to Clark's entrepreneurial model, sustained by managerialism, whereas collegialism may remain in contrast or work in a different way. More recent literature suggests the clash such as the potential for coexistence between managerialism and collegialism. The study analyses data from a survey of 26 universities in 8 European countries, focusing on middle managers (MMs). The results show that at the level of the individual institutions, there are notable positive correlations between the presence of collegial and of managerial cultures. Multilevel regressions at institutional level are analysed, to ‘predict’ collegiality in light of the universities' managerial culture and other factors affecting organizational change: accountability; distribution of discretional power; funding; impact of quality assurance (QA) and evaluation. The results illustrate that in more managerial universities, collegial culture increases above all when MMs believe that distance-steering tools (QA and evaluation) have had positive impacts. We find that collegiality can indeed thrive, even when ‘managerially led’

Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic fields?

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 days observation of the Orion Nebula Cluster (ONC). Analysis of the flare decay allows to determine the size, peak density and magnetic field of the flaring structure. A total of 32 events (the most powerful 1% of COUP flares), have sufficient statistics for the analysis. A broad range of decay times (from 10 to 400 ks) are present in the sample. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures which are found, are in a number of cases very long, with semi-lengths up to 10^12 cm, implying the presence of magnetic fields of hundreds of G extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures ($ >> R_*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L <= R_*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures which confine the flaring plasma are actually the same type of structures which channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.Comment: Accepted to ApJS, COUP special issu

Temperature distribution of a non-flaring active region from simultaneous Hinode XRT and EIS observations

We analyze coordinated Hinode XRT and EIS observations of a non-flaring active region to investigate the thermal properties of coronal plasma taking advantage of the complementary diagnostics provided by the two instruments. In particular we want to explore the presence of hot plasma in non-flaring regions. Independent temperature analyses from the XRT multi-filter dataset, and the EIS spectra, including the instrument entire wavelength range, provide a cross-check of the different temperature diagnostics techniques applicable to broad-band and spectral data respectively, and insights into cross-calibration of the two instruments. The emission measure distribution, EM(T), we derive from the two datasets have similar width and peak temperature, but show a systematic shift of the absolute values, the EIS EM(T) being smaller than XRT EM(T) by approximately a factor 2. We explore possible causes of this discrepancy, and we discuss the influence of the assumptions for the plasma element abundances. Specifically, we find that the disagreement between the results from the two instruments is significantly mitigated by assuming chemical composition closer to the solar photospheric composition rather than the often adopted "coronal" composition (Feldman 1992). We find that the data do not provide conclusive evidence on the high temperature (log T[K] >~ 6.5) tail of the plasma temperature distribution, however, suggesting its presence to a level in agreement with recent findings for other non-flaring regions.Comment: 14 pages, 15 figures. Accepted for publication in the Astrophysical Journa

Spectroscopy of very hot plasma in non-flaring parts of a solar limb active region: spatial and temporal properties

In this work we investigate the thermal structure of an off-limb active region in various non-flaring areas, as it provides key information on the way these structures are heated. In particular, we concentrate in the very hot component (>3 MK) as it is a crucial element to discriminate between different heating mechanisms. We present an analysis using Fe and Ca emission lines from both SOHO/SUMER and HINODE/EIS. A dataset covering all ionization stages from Fe X to Fe XIX has been used for the thermal analysis (both DEM and EM). Ca XIV is used for the SUMER-EIS radiometric cross-calibration. We show how the very hot plasma is present and persistent almost everywhere in the core of the limb AR. The off-limb AR is clearly structured in Fe XVIII. Almost everywhere, the EM analysis reveals plasma at 10 MK (visible in Fe XIX emission) which is down to 0.1% of EM of the main 3 MK plasma. We estimate the power law index of the hot tail of the EM to be between -8.5 and -4.4. However, we leave an open question on the possible existence of a small minor peak at around 10 MK. The absence in some part of the AR of Fe XIX and Fe XXIII lines (which fall into our spectral range) enables us to determine an upper limit on the EM at such temperatures. Our results include a new Ca XIV 943.59 \AA~ atomic model