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

A new Method to Constrain the Iron Abundance from Cooling Delays in Coronal Loops

Recent observations with TRACE reveal that the time delay between the appearance of a cooling loop in different EUV temperature filters is proportional to the loop length, dt_12 ~ L. We model this cooling delay in terms of radiative loss and confirm this linear relationship theoretically. We derive an expression that can be used to constrain the coronal iron enhancement alpha_Fe=A_Fe^cor/A_Fe^Ph relative to the photospheric value as function of the cooling delay dt_12, flux F_2, loop width w, and filling factor q_w < 1. With this relation we find upper limits on the iron abundance enhancement of alpha_Fe < 4.8+/-1.7 for 10 small-scale nanoflare loops, and alpha_Fe < 1.4+/-0.4 for 5 large-scale loops, in the temperature range of T~1.0-1.4 MK. This result supports the previous finding that low-FIP elements, including Fe, are enhanced in the corona. The same relation constitutes also a lower limit for the filling factor, which is q_w > 0.2+/-0.1 and q_w > 0.8+/-0.2 for the two groups of coronal loops.Comment: 2 Figure

Multi-Thread Hydrodynamic Modeling of a Solar Flare

Past hydrodynamic simulations have been able to reproduce the high temperatures and densities characteristic of solar flares. These simulations, however, have not been able to account for the slow decay of the observed flare emission or the absence of blueshifts in high spectral resolution line profiles. Recent work has suggested that modeling a flare as an sequence of independently heated threads instead of as a single loop may resolve the discrepancies between the simulations and observations. In this paper we present a method for computing multi-thread, time-dependent hydrodynamic simulations of solar flares and apply it to observations of the Masuda flare of 1992 January 13. We show that it is possible to reproduce the temporal evolution of high temperature thermal flare plasma observed with the instruments on the \textit{GOES} and \textit{Yohkoh} satellites. The results from these simulations suggest that the heating time-scale for a individual thread is on the order of 200 s. Significantly shorter heating time scales (20 s) lead to very high temperatures and are inconsistent with the emission observed by \textit{Yohkoh}.Comment: Submitted to Ap

How to Commission, Operate and Maintain a Large Future Accelerator Complex from Far Remote

A study on future large accelerators [1] has considered a facility, which is designed, built and operated by a worldwide collaboration of equal partner institutions, and which is remote from most of these institutions. The full range of operation was considered including commi-ssioning, machine development, maintenance, trouble shooting and repair. Experience from existing accele-rators confirms that most of these activities are already performed 'remotely'. The large high-energy physics ex-periments and astronomy projects, already involve inter-national collaborations of distant institutions. Based on this experience, the prospects for a machine operated remotely from far sites are encouraging. Experts from each laboratory would remain at their home institution but continue to participate in the operation of the machine after construction. Experts are required to be on site only during initial commissioning and for par-ticularly difficult problems. Repairs require an on-site non-expert maintenance crew. Most of the interventions can be made without an expert and many of the rest resolved with remote assistance. There appears to be no technical obstacle to controlling an accelerator from a distance. The major challenge is to solve the complex management and communication problems.Comment: ICALEPCS 2001 abstract ID No. FRBI001 invited talk submitting author F. Willeke 5 pages, 1 figur

Fast Time Structure During Transient Microwave Brightenings: Evidence for Nonthermal Processes

Transient microwave brightenings (TMBs) are small-scale energy releases from the periphery of sunspot umbrae, with a flux density two orders of magnitude smaller than that from a typical flare. Gopalswamy et al (1994) first reported the detection of the TMBs and it was pointed out that the radio emission implied a region of very high magnetic field so that the emission mechanism has to be gyroresonance or nonthermal gyrosynchrotron, but not free-free emission. It was not possible to decide between gyroresonance and gyrosynchrotron processes because of the low time resolution (30 s) used in the data analysis. We have since performed a detailed analysis of the Very Large Array data with full time resolution (3.3 s) at two wavelengths (2 and 3.6 cm) and we can now adequately address the question of the emission mechanism of the TMBs. We find that nonthermal processes indeed take place during the TMBs. We present evidence for nonthermal emission in the form of temporal and spatial structure of the TMBs. The fast time structure cannot be explained by a thermodynamic cooling time and therefore requires a nonthermal process. Using the physical parameters obtained from X-ray and radio observations, we determine the magnetic field parameters of the loop and estimate the energy released during the TMBs. The impulsive components of TMBs imply an energy release rate of 1.3 x 10^22 erg/s so that the thermal energy content of the TMBs could be less than 10^24 erg.Comment: 15 pages (Latex), 4 figures (eps). ApJ Letters in press (1997

Beam diagnostics for charge and position measurements in ELI-NP GBS

The advanced source of Gamma-ray photons to be built in Bucharest (Romania), as part of the ELI-NP European Research Infrastructure, will generate photons by Compton back-scattering in the collision between a multi-bunch electron beam and a high intensity recirculated laser pulse. An S-Band photoinjector and the following C-band Linac at a maximum energy of 720MeV, under construction by an European consortium (EurogammaS) led by INFN, will operate at 100Hz repetition rate with trains of 32 electron bunches, separated by 16ns and a 250pC nominal charge. The different BPMs and current transformers used to measure transverse beam position and charge along the LINAC are described. Design criteria, production status and bench test results of the charge and position pickups are reported in the paper, together with the related data acquisition systems

Bayesian Estimation of Hardness Ratios: Modeling and Computations

A commonly used measure to summarize the nature of a photon spectrum is the so-called Hardness Ratio, which compares the number of counts observed in different passbands. The hardness ratio is especially useful to distinguish between and categorize weak sources as a proxy for detailed spectral fitting. However, in this regime classical methods of error propagation fail, and the estimates of spectral hardness become unreliable. Here we develop a rigorous statistical treatment of hardness ratios that properly deals with detected photons as independent Poisson random variables and correctly deals with the non-Gaussian nature of the error propagation. The method is Bayesian in nature, and thus can be generalized to carry out a multitude of source-population--based analyses. We verify our method with simulation studies, and compare it with the classical method. We apply this method to real world examples, such as the identification of candidate quiescent Low-mass X-ray binaries in globular clusters, and tracking the time evolution of a flare on a low-mass star.Comment: 43 pages, 10 figures, 3 tables; submitted to Ap

The energetics of the gradual phase

Reseachers compare results with those in the chapter by Moore et al. (1980), who reached five main conclusions about the gradual phase: (1) the typical density of the soft X-ray emitting plasma is between 10 to the 11th power and 10 to the 12th power cm-3 for compact flares and between 10 to the 10th power and 10 to the 11th power cm-3 for a large-area flare; (2) cooling is by conduction and radiation in roughly equal proportions; (3) continual heating is needed in the decay phase of two-ribbon flares; (4) continual heating is probably not needed in compact events; (5) most of the soft-X-ray-emitting plasma results from chromospheric evaporation. The goal was to reexamine these problems with the data from the Solar Maximum Mission (SMM) and other supporting instruments as well as to take advantage of recent theoretical advances. SMM is capable of measuring coronal temperatures more accurately and with a better cadence than has been possible before. The SMM data set is also unique in that the complete transit of an active region was observed, with soft X-ray and UV images being taken every few minutes. Researcher's were therefore able to establish the pre-flare conditions of the region and see whether anything has changed as a result of the flare. The assumptions made in attempting to determine the required plasma parameters are described. The derived parameters for the five prime flares are presented, and the role of numerical simulations is discussed