Analysis of a multi-wavelength time-resolved observation of a coronal loop

Several items on the diagnostics and interpretation of coronal loop observations are under debate. In this work, we analyze a well-defined loop system detected in a time-resolved observation in several spectral bands. The dataset includes simultaneous images in the TRACE 171 A, 195 A and 284 A bands, and Yohkoh/SXT, and two rasters taken with SoHO/CDS in twelve relevant lines. The loop is initially best visible in the TRACE 195 A filter band, and later in the 171 A filter band, with correspondence with the CDS raster images at log T \~ 6.0-6.1. We have taken as pixel-by-pixel background the latest TRACE, Yohkoh and CDS images where the loop has faded out. We examine the loop morphology evolution, the light curves, the TRACE filter ratio distribution and evolution, the images and emission measure from the CDS spectral lines. Our analysis detects that, after background subtraction, the emission along the loop and its evolution are non-uniform, especially in the 171 A filter band, and that the TRACE 195/171 filter ratio has a moderately non-uniform distribution along the loop and evolves in time. Both the light curves and the filter ratio evolution indicate a globally cooling loop. Relatively hot plasma may be present at the beginning while, during the first CDS raster, the data indicate a rather moderate thermal structuring of the loop. Our data analysis supports a coherent scenario across the different bands and instruments, points out difficulties in diagnostic methods and puts quantitative basis for detailed forward modeling.Comment: 20 pages, 13 figs, refereed, in pres

Dynamic Temperature Structure of the Corona

The solar corona is heated to million degrees and information about the temperature structure is a key to understand the heating mechanisms. Although it is not easy to measure, the temperature looks to be remarkably steady in the solar corona and in active regions outside of transient events, like flares. On the other hand, there is strong evidence of multi-thermal structures, out of equilibrium for most of the time. Is there a way to obtain a coherent scenario? The secret might be in the fine structuring of the corona, and SDO is providing new and important information on this issue

Coronal structure and dynamics

Recent spatial solar missions, such as Hinode and Solar Dynamics Observatory, reveal a more and more highly structured and dynamic corona, with an increasing importance and debate. The connection of structure and dynamics to coronal heating is fundamental. Observations and evidence of fine coronal structuring, e.g. moss and emission measure distributions, and dynamics, e.g. Doppler shifts, spicules, are discussed and compared to current vision and models. Open questions and future perspectives are outlined to conclude

Coronal Loops: Observations and Modeling of Confined Plasma

Coronal loops are the building blocks of the X-ray bright solar corona. They owe their brightness to the dense confined plasma, and this review focuses on loops mostly as structures confining plasma. After a brief historical overview, the review is divided into two separate but not independent sections: the first illustrates the observational framework, the second reviews the theoretical knowledge. Quiescent loops and their confined plasma are considered, and therefore topics such as loop oscillations and flaring loops (except for non-solar ones which provide information on stellar loops) are not specifically addressed here. The observational section discusses loop classification and populations, and then describes the morphology of coronal loops, its relationship with the magnetic field, and the concept of loops as multi-stranded structures. The following part of this section is devoted to the characteristics of the loop plasma, and of its thermal structure in particular, according to the classification into hot, warm and cool loops. Then, temporal analyses of loops and the observations of plasma dynamics and flows are illustrated. In the modeling section starts some basics of loop physics are provided, supplying some fundamental scaling laws and timescales, a useful tool for consultation. The concept of loop modeling is introduced, and models are distinguished between those treating loops as monolithic and static, and those resolving loops into thin and dynamic strands. Then more specific discussions address modeling the loop fine structure, and the plasma flowing along the loops. Special attention is devoted to the question of loop heating, with separate discussion of wave (AC) and impulsive (DC) heating. Finally, a brief discussion about stellar X-ray emitting structures related to coronal loops is included and followed by conclusions and open questions.Comment: 75 pages, 17 figures, to be published in Living Reviews in Solar Physic

A min-cut approach to functional regionalization, with a case study of the Italian local labour market areas

In several economical, statistical and geographical applications, a territory must be subdivided into functional regions. Such regions are not fixed and politically delimited, but should be identified by analyzing the interactions among all its constituent localities. This is a very delicate and important task, that often turns out to be computationally difficult. In this work we propose an innovative approach to this problem based on the solution of minimum cut problems over an undirected graph called here transitions graph. The proposed procedure guarantees that the obtained regions satisfy all the statistical conditions required when considering this type of problems. Results on real-world instances show the effectiveness of the proposed approach

Non-equilibrium of Ionization and the Detection of Hot Plasma in Nanoflare-heated Coronal Loops

Impulsive nanoflares are expected to transiently heat the plasma confined in coronal loops to temperatures of the order of 10 MK. Such hot plasma is hardly detected in quiet and active regions, outside flares. During rapid and short heat pulses in rarified loops the plasma can be highly out of equilibrium of ionization. Here we investigate the effects of the non-equilibrium of ionization (NEI) on the detection of hot plasma in coronal loops. Time-dependent loop hydrodynamic simulations are specifically devoted to this task, including saturated thermal conduction, and coupled to the detailed solution of the equations of ionization rate for several abundant elements. In our simulations, initially cool and rarified magnetic flux tubes are heated to 10 MK by nanoflares deposited either at the footpoints or at the loop apex. We test for different pulse durations, and find that, due to NEI effects, the loop plasma may never be detected at temperatures above ~5 MK for heat pulses shorter than about 1 min. We discuss some implications in the framework of multi-stranded nanoflare-heated coronal loops.Comment: 22 pages, 7 figures, accepted for publicatio

The early B-type star Rho Oph A is an X-ray lighthouse

We present the results of a 140 ks XMM-Newton observation of the B2 star $\rho$ Ophiuchi A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely corresponds to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK ($kT\sim5$ keV). From the analysis of its rise, we infer a magnetic field of $\ge300$ G and a size of the flaring region of $\sim1.4-1.9\times10^{11}$ cm, which corresponds to $\sim25\%-30\%$ of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of $\ge$2.5 years, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems and $\rho$ Ophiuchi would constitute an extreme system that is worthy of further investigation.Comment: 10 pages, 7 figures, 2 tables, A&A accepted, this is the version after the language editor correction

X-ray flare modeling in the single giant HR 9024

We analyze a Chandra-HETGS observation of the single G-type giant HR 9024. The high flux allows us to examine spectral line and continuum diagnostics at high temporal resolution, to derive plasma parameters (thermal distribution, abundances, temperature, ...). A time-dependent 1D hydrodynamic loop model with semi-length 10$^{12}$cm ($\sim R_{\star}$), and impulsive footpoint heating triggering the flare, satisfactorily reproduces the observed evolution of temperature and emission measure, derived from the analysis of the strong continuum emission. The observed characteristics of the flare appear to be common features in very large flares in active stars (also pre-main sequence stars), possibly indicating some fundamental physics for these very dynamic and extreme phenomena in stellar coronae.Comment: 2 pages, 4 figures. To be included in the proceedings of the 'X-ray universe 2005 meeting' held in San Lorenzo de El Escorial (Spain), 26-30 Sep 200