TRACE-derived Temperature and Emission Measure Profiles along Long-lived Coronal Loops: The Role of Filamentation

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T dwarf. These new observations were obtained during commissioning of the near-infrared spectrometer (NIRSPEC), the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10 m telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R approximately 2500 from 1.135 to 1.360 μm (approximately J band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K i) and bands due to iron hydride (FeH) and steam (H2O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen, and the K i lines gradually become weaker but wider because of pressure broadening. An unidentified feature occurs at 1.22 μm that has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 mag brighter in the near-infrared compared with the I band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L dwarf discovered so far by the 2 Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl 229B or SDSS 1624+0029.     

Coronal Temperature Profiles Obtained from Kinetic Models and from Coronal Brightness Measurements Obtained During Solar Eclipses

Coronal density, temperature, and heat-flux distributions for the equatorial and polar corona have been deduced from Saito’s model of averaged coronal white-light (WL) brightness and polarization observations. These distributions are compared with those determined from a kinetic collisionless/exospheric model of the solar corona. This comparison indicates similar distributions at large radial distances (>?7 R_⊙) in the collisionless region. However, rather important differences are found close to the Sun in the acceleration region of the solar wind. The exospheric heat flux is directed away from the Sun, while that inferred from all WL coronal observations is in the opposite direction, i.e. conducting heat from the inner corona toward the chromosphere. This could indicate that the source of coronal heating extends up into the inner corona, where it maximizes at r>1.5 R_⊙, well above the transition region.     

Coronal heating events in high-cadence TRACE data

On March 23rd 1999 a set of TRACE (Transition Region and Coronal Explorer) observations were taken in support of Solar and Heliospheric Observatory (SOHO) / TRACE Joint Observing Program 83. The program is designed to look for coronal heating mechanisms operating at high cadence and to observe their dynamical effects on coronal loop density, temperature and magnetic structure. We present here a study of 27 small, dynamic brightening events seen in this data. These events are seen in the quiet-Sun areas surrounding the active region. The data itself consists of 157 171 Å 512×512 arc sec images at 1 arc sec resolution and 9 second cadence, a previously unavailable combination of cadence, resolution and image size. Three classes of events are introduced: complex, intermediate and simple. All three classes share the property of being dynamic on small time and length scales but differ in the complexity of their behavior. We find that the brightenings across all classes exhibit variations as part of a distribution of time scales (90–400 s) peaked around 228 s. The brightenings are no more than 5 arc sec in diameter. Motions between brightenings occur on time scales from 9 to 90 s and on length scales no greater than 10 arc sec. These motions have velocities estimated to be in the range 89–174 km s^?1. The position of these events in the spectrum of previously observed coronal heating events is discussed.

     

Evolution of a Coronal Loop System

The temporal variation of a loop system that appears to be changing rapidly is examined. The analyzed data were obtained on 15 May 1999, with the Transition Region and Coronal Explorer (TRACE) during an observing campaign and consist of observations in the Fe ix/Fe x 171?Å and Fe xii 195?Å passbands taken at a cadence of ～10 min. The special interest in this loop system is that it looks like one expanding loop; however, careful examination reveals that the loop consists of several strands and that new loop strands become visible successively at higher altitudes and lower loop strands fade out during the one hour of our observations. These strands have different widths, densities, and temperatures and are most probably consisting of, at least, a few unresolved thinner threads. Several geometric and physical parameters are derived for two of the strands and an effort is made to determine their 3D structure based on the extrapolation of the magnetic field lines. Electron density estimates allow us to derive radiative and conductive cooling times and to conclude that these loop strands are cooling by radiation.     

Trans-Equatorial Loop System Arising from Coronal Hole Boundaries through Interactions between Active Regions and Coronal Holes

It is not clear how trans-equatorial loop systems (TLSs) are formed, although they have been observed often with Yohkoh/SXT. We focus here on a TLS that appeared on 27 May 1998. Yokoyama and Masuda (Solar Phys. 254, 285, 2009) proposed a new scenario for the formation mechanism of the TLS. In this scenario, they pointed out the importance of magnetic interaction between an active region and a coronal hole to make “strong-seed magnetic fields” before a transient (bright and short-lived) trans-equatorial loop was created. The main aims of this study are to verify the scenario and to make the TLS formation mechanism clear, based on observational data. Yohkoh/SXT images, SOHO/MDI magnetograph data, and Kitt Peak coronal-hole maps were mainly used for our analyses. We investigated the TLS in detail from the time that there were no signatures of the TLS to its clear appearance. The following results are obtained: i) an active region emerged in the vicinity of a coronal-hole boundary, ii) the coronal-hole boundary retreated during the period when the active region was developing, iii) temporal variations of soft X-ray intensities were roughly synchronized between the coronal-hole boundary and a trans-equatorial region, and iv) new closed loops were observed in soft X-rays clearly at the coronal-hole boundary. Since i), ii), iii), and iv) are just what we expect in the scenario of YM2009, the scenario found support. We conclude that the TLS was originating with large-scale magnetic fields of the coronal-hole boundary through magnetic reconnection between the active region and a coronal hole.     

Coronal transients: Loop or bubble?

Many coronal transient exhibit a circular aspect, which has been interpreted up to now as a loop structure. From polarization measurements of the 10 August, 1973 transient observed by the ATM coronagraph, which allows the location of material away from the plane of the sky, we show that this particular transient is more likely to be a 3-dimensional, bubble-shaped structure, than a loop. The radial component of the speed is evaluated. A thin streamer close to the transient is displaced by its passage, both in the plane of the sky and in the direction perpendicular to it.     

Radio Pulsating Structures with Coronal Loop Contraction

We present a multi-wavelength study of a solar eruption event on 20 July 2004, comprising observations in H??, EUV, soft X-rays, and in radio waves with a wide frequency range. The analyzed data show both oscillatory patterns and shock wave signatures during the impulsive phase of the flare. At the same time, large-scale EUV loops located above the active region were observed to contract. Quasi-periodic pulsations with ???10 and ???15 s oscillation periods were detected both in microwave??C?millimeter waves and in decimeter??C?meter waves. Our calculations show that MHD oscillations in the large EUV loops ?C but not likely in the largest contracting loops ?C could have produced the observed periodicity in radio emission, by triggering periodic magnetic reconnection and accelerating particles. As the plasma emission in decimeter??C?meter waves traces the accelerated particle beams and the microwave emission shows a typical gyrosynchrotron flux spectrum (emission created by trapped electrons within the flare loop), we find that the particles responsible for the two different types of emission could have been accelerated in the same process. Radio imaging of the pulsed decimetric??C?metric emission and the shock-generated radio type II burst in the same wavelength range suggest a rather complex scenario for the emission processes and locations. The observed locations cannot be explained by the standard model of flare loops with an erupting plasmoid located above them, driving a shock wave at the CME front.     

EUV Analysis of a Quasi-static Coronal Loop Structure

Decaying active region 10942 is investigated from 4:00?–?16:00 UT on 24 February 2007 using a suite of EUV observing instruments. Results from Hinode/EIS, STEREO and TRACE show that, although the active region has decayed and no sunspot is present, the physical mechanisms that produce distinguishable loop structures, spectral line broadening, and plasma flows still occur. A coronal loop that appears as a blue-shifted structure in Doppler maps is apparent in intensity images of log(T)=6.0?–?6.3 ions. The loop structure is found to be anti-correlated with spectral line broadening generally attributed to non-thermal velocities. This coronal loop structure is investigated physically (temperature, density, geometry) and temporally. Light curves created from imaging instruments show brightening and dimming of the loop structure on two different time scales; short pulses of 10?–?20?min and long duration dimming of two?–?four hours until its disappearance. The coronal loop structure, formed from relatively blue-shifted material that is anti-correlated with spectral line broadening, shows a density of 10¹⁰ to 10^9.3?cm^?3 and is visible for longer than characteristic cooling times. The maximum non-thermal spectral line broadenings are found to be adjacent to the footpoint of the coronal loop structure.     

Width Variations along Coronal Loops Observed by TRACE

We have measured width variations along coronal loops observed by TRACE in the 171, 195, and 284 Å bandpasses. The loops are not significantly thicker in the middle compared to near the footpoints, and there is no correlation between the footpoint-to-midpoint expansion and the loop length. This applies to both post-flare and non-flare loops. The observations conflict with our present understanding of active region magnetic fields, and they have important implications for the structure and heating of the corona.     

Time Variability of EUV Brightenings in Coronal Loops Observed with TRACE

Solar Physics - We analyze coronal loops in active region 8272, observed with TRACE on 23 July 1998 during a 70-min interval with a cadence of 1.5&;nbsp;min, in the temperature range of...     

A Research on the Physical Features of Coronal Loop Oscillations

Observations of the solar full-disk were carried out by the Atmo- spheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) with the Fe IX 171 Å line on 16th October 2010. The obtained high-quality data permit us to elaborate on the coronal loop oscillations. It is found that a major flare of GOES (Geostationary Operational Environmental Satellite) class M_2.9 occurred in the active region NOAA 1112 during this period, which triggered a number of coronal loops on the solar surface to oscillate. Among them, there are two coronal loops exhibiting oscillations with different physical features. The oscillation of the coronal loop located at W₄₉₂/S₁₇₀ is a simple harmonic oscillation with a period of 385_s, which abides by the oscillating equation of x = 2.2 sin[2π/385(t–768)], while the other located at W559/S142 is a damping oscillation with a period of 449s, and the oscillating equation is expressed by x = 24.8e - 2π/343 t sin[2π/449(t–1128)], where t is the observational time in units of second.     

冕环振荡的物理特征研究

太阳动力学观测站(Solar Dynamics Observatory,SDO)上的太阳大气成像仪(Atmospheric Imaging Assembly,AIA)利用谱线Fe IX 171 (A)在2010年10月16日对整个日面进行了连续的高分辨率观测,获得了高质量的数据.这些高质量的数据提供了仔细研究冕环振荡的样本.通过分析这份数据,发现活动区NOAA 1112在此期间爆发了一个M2.9级的耀斑.该耀斑触发了太阳表面的多个冕环产生强烈振荡.其中最为明显的两个振荡冕环呈现出截然不同的振荡特征.位于西492 Mm,南170 Mm(简称W492/S170,后面出现的坐标位置均采取这种标注)处的冕环做周期为pA0 =385 s的简谐振荡,其振荡方程为x=2.2sin[2π55(t-768)],其中t为时间,单位为s;而位于W559/S142处的冕环则是一种典型的阻尼振荡,其阻尼振荡周期为Pf =449 s,相应的振荡方程可表示为x=24.8e-2π/342tsin[2π/449(t-1128)].     

On Coronal Loop Heating by Torsional Alfvén Waves

Heating of coronal loops by linear resonant Alfvén waves, excited by the footpoints motions in the photosphere, has been studied. The analysis of single-layer heating is extended to multilayer heating, in semiempirical treatment. Heating and nonthermal velocities in different layers of loops in X-ray bright points, active regions, and large-scale structures are estimated. The average value of velocity is found to be in agreement with the observations.     

Effect of Variable Background on an Oscillating Hot Coronal Loop

We investigate the effect of a variable, i.e. time-dependent, background on the standing acoustic (i.e. longitudinal) modes generated in a hot coronal loop. A theoretical model of 1D geometry describing the coronal loop is applied. The background temperature is allowed to change as a function of time and undergoes an exponential decay with characteristic cooling times typical for coronal loops. The magnetic field is assumed to be uniform. Thermal conduction is assumed to be the dominant mechanism for damping hot coronal oscillations in the presence of a physically unspecified thermodynamic source that maintains the initial equilibrium. The influence of the rapidly cooling background plasma on the behaviour of standing acoustic (longitudinal) waves is investigated analytically. The temporally evolving dispersion relation and wave amplitude are derived by using the Wenzel–Kramers–Brillouin theory. An analytic solution for the time-dependent amplitude that describes the influence of thermal conduction on the standing longitudinal (acoustic) wave is obtained by exploiting the properties of Sturm–Liouville problems. Next, numerical evaluations further illustrate the behaviour of the standing acoustic waves in a system with a variable, time-dependent background. The results are applied to a number of detected loop oscillations. We find a remarkable agreement between the theoretical predictions and the observations. Despite the emergence of the cooling background plasma in the medium, thermal conduction is found to cause a strong damping for the slow standing magneto–acoustic waves in hot coronal loops in general. In addition to this, the increase in the value of thermal conductivity leads to a strong decay in the amplitude of the longitudinal standing slow MHD waves.     

Numerical Simulations of Vertical Oscillations of a Curved Coronal Loop

We consider an impulsively-started, vertical excitation of a solar coronal loop that is embedded into a potential arcade. The two-dimensional numerical model we implement includes the effects of line curvature and allows us to explore the effect of varying the initial pulse position. The results of the numerical simulations reveal kink mode oscillations with waveperiods that are reasonably close to the observational findings of Wang and Solanki (2004).     

How To Use Magnetic Field Information For Coronal Loop Identification

The structure of the solar corona is dominated by the magnetic field because the magnetic pressure is about four orders of magnitude higher than the plasma pressure. Due to the high conductivity the emitting coronal plasma (visible, e.g., in SOHO/EIT) outlines the magnetic field lines. The gradient of the emitting plasma structures is significantly lower parallel to the magnetic field lines than in the perpendicular direction. Consequently information regarding the coronal magnetic field can be used for the interpretation of coronal plasma structures. We extrapolate the coronal magnetic field from photospheric magnetic field measurements into the corona. The extrapolation method depends on assumptions regarding coronal currents, e.g., potential fields (current-free) or force-free fields (current parallel to magnetic field). As a next step we project the reconstructed 3D magnetic field lines on an EIT-image and compare with the emitting plasma structures. Coronal loops are identified as closed magnetic field lines with a high emissivity in EIT and a small gradient of the emissivity along the magnetic field.     

Note on the Initial Value Problem for Coronal Loop Kink Waves

Simple fluxtube models of coronal loops have previously been found to support leaky oscillations, where energy escapes from the tube laterally, thereby damping the mode. Of particular interest is the Principal Fast Leaky Kink mode (PFLK), which may be implicated in the decay of loop oscillations observed by TRACE. However, recently, M.S. Ruderman and B. Roberts, J. Plarma Phys. (in press), claimed that the PFLK mode, and many other leaky modes of oscillation, are unphysical, based on an initial value calculation. In this note, their arguments are shown to be incorrect.     

Magnetic Relaxation and Particle Acceleration in a Flaring Twisted Coronal Loop

In the present work we aim to study particle acceleration in twisted coronal loops. For this purpose, an MHD model of magnetic reconnection in a linearly unstable twisted magnetic fluxtube is considered. Further, the electric and magnetic fields obtained in the MHD simulations are used to calculate proton and electron trajectories in the guiding-centre approximation. It is shown that particle acceleration in such a model is distributed rather uniformly along the coronal loop and the high-energy population remains generally neutral. It also follows from the model that the horizontal cross-section of the volume occupied by high-energy particles near the loop footpoints increases with time, which can be used as an observational proxy.     

Modeling Coronal Loop Abundances: Effects of Parameter Variations on Observables

Coronal abundance variations arise from and affect solar atmospheric processes such as coronal heating and structural dynamics. Lenz (1999) presented initial results of a numerical theoretical study of abundances and ion heating rates in static, steady-state coronal loops. We present here a closer investigation of two fundamental aspects of the physics of coronal abundances, relating modeling parameters to observables: (1) the effect of varying the abundances on the electron temperature and (2) the effect of varying the ion heating rate on abundances.