Dissipation of Longitudinal Oscillations in Stratified Nonisothermal Hot Coronal Loops

We investigate the damping of longitudinal (i.e., slow or acoustic) waves in nonisothermal, hot (T≥ 5.0 MK), gravitationally stratified coronal loops. Motivated by SOHO/SUMER and Yohkoh/SXT observations, and by taking into account a range of dissipative mechanisms such as thermal conduction, compressive viscosity, radiative cooling, and heating, the nonlinear governing equations of one-dimensional hydrodynamics are solved numerically for standing-wave oscillations along a magnetic field line. A semicircular shape is chosen to represent the geometry of the coronal loop. It was found that the decay time of standing waves decreases with the increase of the initial temperature, and the periods of oscillations are affected by the different initial footpoint temperatures and loop lengths studied by the numerical experiments. In general, the period of oscillation of standing waves increases and the damping time decreases when the parameter that characterises the temperature at the apex of the loop increases for a fixed footpoint temperature and loop length. A relatively simple second-order scaling polynomial between the damping time and the parameter determining the apex temperature is found. This scaling relation is proposed to be tested observationally. Because of the lack of a larger, statistically relevant number of observational studies of the damping of longitudinal (slow) standing oscillations, it can only be concluded that the numerically predicted decay times are well within the range of values inferred from Doppler shifts observed by SUMER in hot coronal loops.     

Analysis of Two Coronal Loops with Combined TRACE and SOHO/CDS Data

We use an innovative research technique to analyze combined images from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE). We produce a high spatial and temporal resolution simulated CDS raster or “composite” map from TRACE data and use this composite map to jointly analyze data from both instruments. We show some of the advantages of using the “composite” map method for coronal loop studies. We investigate two postflare loop structures. We find cool material (250 000 K) concentrated at the tips or apex of the loops. This material is found to be above its scale height and therefore not in hydrostatic equilibrium. The exposure times of the composite map and TRACE images are used to give an estimate of another loop’s cooling time. The contribution to the emission in the TRACE images for the spectral lines present in its narrow passband is estimated by using the CDS spectral data and CHIANTI to derive synthetic spectra. We obtain cospatial and cotemporal data collected by both instruments in SOHO Joint Observations Program (JOP) 146 and show how the combination of these data can be utilized to obtain more accurate measurements of coronal plasmas than if analyzed individually. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Dissipation of Standing Slow Magnetoacoustic Waves in Hot Coronal Loops

The damping of standing slow waves in hot (T>6 MK) coronal loops of semicircular shape is revisited in both the linear and nonlinear regimes. Dissipation by thermal conduction, compressive viscosity, radiative cooling, and heating are examined for nonstratified and stratified loops. We find that for typical conditions of hot SUMER loops, thermal conduction increases the period of damped oscillations over the sound-crossing time, whereas the decay times are mostly shaped by compressive viscosity. Damping from optically thin radiation is negligible. We also find that thermal conduction alone results in slower damping of the density and velocity waves compared to the observations. Only when compressive viscosity is added do these waves damp out at the same rate as the observed rapidly decaying modes of hot SUMER loop oscillations, in contrast to most current work, which has pointed to thermal conduction as the dominant mechanism. We compare the linear predictions with numerical hydrodynamic calculations. Under the effects of gravity, nonlinear viscous dissipation leads to a reduction of the decay time compared to the homogeneous case. In contrast, the linear results predict that the damping rates are barely affected by gravity.     

Propagation of the Slow Magnetoacoustic Waves in Coronal Loops Above Sunspots

The observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) have revealed the weak dis- turbances (WDs) propagating in the fan-like coronal loops of the active region (AR 11092) at 171 ?A, 193 ?A, and 211 ?A. These WDs seem to be a common phenomenon in this part of the active region. The disturbances originate from the bright loop foot, and propagate along the loops. The observed propagation speed decreases with the increasing temperature, and varies between 40 km/s and 121 km/s, close to and less than the sound speed in coronal loops. Consid- ering the projection effect and the different angles of the loops with respect to the line of sight, this is exactly what the slow-wave model expects. The wavelet analysis shows that the periods of the WDs observed in different wavebands have no signi?cant difference, the two distinct periods, 3 min and more than 10 min, are all detected in the three EUV wavebands. Not only the coronal loops but also the sunspot region in the chromosphere exhibit intensity oscillations with a period of the order of 3 min. This result suggests that the sunspot oscillations can propagate into the corona through the chromosphere and transition region.     

Global Coronal Seismology

Following the observation and analysis of large-scale coronal-wave-like disturbances, we discuss the theoretical progress made in the field of global coronal seismology. Using simple mathematical techniques we determine average values for the magnetic field together with a magnetic map of the quiet Sun. The interaction between global coronal waves and coronal loops allows us to study loop oscillations in a much wider context, i.e. we connect global and local coronal oscillations.     

Effect of Gravity on the Fast Modes of a Horizontal Coronal Slab

Theoretical studies of the normal modes of a coronal slab often neglect gravity, as in Edwin and Roberts (Solar Phys. 71, 239, 1982). Here we study analytically the effect of gravity acting on a horizontal slab as a first step away from a homogeneous medium. Because of the inclusion of gravity, the symmetry of a homogeneous slab is lost, so the normal modes cannot be classified into kink and sausage modes. The presence of gravity also modifies the oscillatory frequencies of the slab, as well as the lower cutoff frequency, resulting in the possible transition between surface and body modes. For general coronal parameters, the dimensionless gravity term turns out to be small, so these effects are also small. A.J. Díaz’s current affiliation: Universitat de les Illes Balears, Palma, E-07122, Spain.     

An Impulsive Heating Model for the Evolution of Coronal Loops

It was suggested by Parker that the solar corona is heated by many small energy release events generally called microflares or nanoflares. More and more observations showed flows and intensity variations in nonflaring loops. Both theories and observations have indicated that the heating of coronal loops should actually be unsteady. Using SOLFTM (Solar Flux Tube Model), we investigate the hydrodynamics of coronal loops undergoing different manners of impulsive heating with the same total energy deposition. The half length of the loops is 110 Mm, a typical length of active region loops. We divide the loops into two categories: loops that experience catastrophic cooling and loops that do not. It is found that when the nanoflare heating sources are in the coronal part, the loops are in non-catastrophic-cooling state and their evolutions are similar. When the heating is localized below the transition region, the loops evolve in quite different ways. It is shown that with increasing number of heating pulses and inter-pulse time, the catastrophic cooling is weakened, delayed, or even disappears altogether.     

Coronal mass ejections of solar cycle 23

I summarize the statistical, physical, and morphological properties of coronal mass ejections (CMEs) of solar cycle 23, as observed by the Solar and Heliospheric Observatory (SOHO) mission. The SOHO data is by far the most extensive data, which made it possible to fully establish the properties of CMEs as a phenomenon of utmost importance to Sun-Earth connection as well as to the heliosphere. I also discuss various subsets of CMEs that are of primary importance for their impact on Earth.     

Transequatorial Filament Eruption and Its Link to a Coronal Mass Ejection

We revisit the Bastille Day flare/CME Event of 2000 July 14, and demonstrate that this flare/CME event is not related to only one single active region (AR). Activation and eruption of a huge transequatorial filament are seen to precede the simultaneous filament eruption and flare in the source active region, NOAA AR 9077, and the full halo-CME in the high corona. Evidence of reconfiguration of large-scale magnetic structures related to the event is illustrated by SOHO EIT and Yohkoh SXT observations, as well as, the reconstructed 3D magnetic lines of force based on the force-free assumption. We suggest that the AR filament in AR9077 was connected to the transequatorial filament. The large-scale magnetic composition related to the transequatorial filament and its sheared magnetic arcade appears to be an essential part of the CME parent magnetic structure. Estimations show that the filament-arcade system has enough magnetic helicity to account for the helicity carried by the related CMEs. In addition, rather global magnetic connectivity, covering almost all the visible range in longitude and a huge span in latitude on the Sun, is implied by the Nancay Radioheliograph (NRH) observations. The analysis of the Bastille Day event suggests that although the triggering of a global CME might take place in an AR, a much larger scale magnetic composition seems to be the source of the ejected magnetic flux, helicity and plasma. The Bastille Day event is the first described example in the literature, in which a transequatorial filament activity appears to play a key role in a global CME. Many tens of halo-CME are found to be associated with transequatorial filaments and their magnetic environment.     

太阳活动区冕环若干研究进展

日冕加热是太阳物理中一个基本问题。随着一批高性能仪器（如TRACE、SOHO、Yohkoh）投入观测，作为太阳日冕中一种基本结构的冕环，其观测资料日益丰富。冕环加热是日冕加热的一个重要组成部分，越来越得到人们的重视。在简要介绍冕环最新观测和研究进展后，以其一维模型为基础，着重讨论了现有冕环加热结构和加热机制的研究进展。     

Coronal Flux Rope Equilibria in Closed Magnetic Fields

Using a 2.5-dimensional ideal MHD model in Cartesian coordinates,we investigate the equilibrium properties of coronal magnetic flux ropes in background magnetic fields that are completely closed.The background fields are produced by a dipole,a quadrupole,and an octapole,respectively,located below the photosphere at the same depth.A magnetic flux rope is then launched from below the photo-sphere,and its magnetic properties,i.e,the annular magnetic fluxφp and the axial magnetic fluxφz,are controlled by a single emergence parameter.The whole sys-tem eventually evolves into equilibrium,and the resultant flux rope is characterized by three geometrical parameters:the height of the rope axis,the half-width of the rope,and the length of the vertical current sheet below the rope.It is found that the geometrical parameters increase monotonically and continuously with increasing φp and φz:no catastrophe occurs.Moreover,there exists a steep segment in the profiles of the geometrical parameters versus either φp or φz,and the faster the background field decays with height,the larger both the gradient and the growth amplitude within the steep segment will be.     

A Filament—Associated Halo Coronal Mass Ejection

1 INTRODUCTIONCoronal majss ejections (CMEs) are often seen as spectacular eruptions of matter fromthe Sun which propagate outward through the heliosphere and often interact with the Earth'smagnetosphere (Hundhausen, 1997; Gosling, 1997; and references herein). It is well known thatthese interactions can have substalltial consequences on the geomagnetic environment of theEarth, sometimes resulting in damage to satellites (e.g., McAllister et al., 1996; Berdichevskyet al., 1998). CMEs…     

Magnetic Properties of Metric Noise Storms Associated with Coronal Mass Ejections

Using Nancay Radioheliograph (NRH) imaging observations, combined with SOHO/Michelson Doppler Imager (MDI) magnetogram observations and coronal magnetic field extrapolation, we studied the magnetic nature of metric noise storms that are associated with coronal mass ejections (CMEs). Four events are selected: the events of 2000 July 14, 2001 April 26, 2002 August 16 and 2001 March 28. The identified noise storm sources cover or partially cover the active regions (ARs), but the centers of storm sources are offset from the ARs. Using extrapolated magnetic field lines, we find that the noise storm sources trace the boundary between the open and closed field lines. We demonstrate that the disappearance of noise storm source is followed by the appearance of the burst source. The burst sources spread on the solar disk and their distributions correspond to the extent of the CME in LASCO C2 field of view. All the SOHO/Extreme Ultraviolet Imaging Telescope (EIT) dimmings associ- ated with noise storm sources are located at the periphery of noise storms where the magnetic lines of force were previously closed and low-lying. When the closed field becomes partially or fully open, the basic configurations of noise storm sources are changed, then the noise storm sources are no longer observed. These observations provide the information that the variations of noise storms manifest the restructuring or reconfiguring of the coronal magnetic field.     

一个复杂太阳射电爆发及其微波源和远紫外冕环特征的初步研究

利用太阳射电宽带频谱仪(0．7-7．6GHz)于2001年10月19日观测到的复杂太阳射电大爆发，呈现出许多有趣的特征，结合NoRH(Nobeyama Radio Heliograph)的高空间分辨率射电成像观测及TRACE(Transition Region and Coronal Explorer)在远紫外(EUV)波段的高空间分辨率成像观测资料，分析了该爆发的射电频谱特征和微波射电源的演化以及它们与复杂的EUV日冕环系统的关系，该爆发是一个双带大耀斑的射电表征．前一部分以宽带(从厘米到米波)爆发为主，机制是回旋同步辐射，所对应的是环足源的辐射；后一部分以窄带(分米到米波)分米波爆发为主，机制是等离子体辐射和回旋共振辐射的联合，对应的是环顶源的辐射。     

日冕分米波射电甚短周期脉动的观测证据

利用国家天文台云南天文台“分米波(700—1500MHz)射电频谱仪”和“四波段太阳射电高时间分辨率同步观测系统”分别于2001年6月24日和1990年7月30日观测到了两个稀少事件，前者是一个小射电爆发，其上升相伴随有短周期(约29、40和100毫秒)的脉动，后者是一个射电大爆发，在2840MHz上产生了周期约30毫秒的射电脉动，还着重讨论其甚短周期(如29—40毫秒)的脉动现象，甚短周期脉动可能是归因于起源在日冕深处不稳定区域的哨声波束周期链对射电辐射的调制，或沉降电子束驱动的静电高混杂波，经由波-波非线性相互作用导致甚短周期的射电脉动。     

Coronal Magnetic Flux Rope Equilibria and Magnetic Helicity

1 INTRODUCTIONObservations show that the magnetic helicity of solar magnetic structures has a predominantsign in each hemisphere of the Sun, positive in the southern hemisphere and negative in thenorthern, regardless of the solar cycle (Rust, 1994). The magnetic helicity is strictly conservedin the frame of ideal MHD (WOltjer, 1958), and approximately conserved in the presence ofresistive dissipation and magnetic reconnection in a highly conductive plajsma (Taylor, 1974;Berger, 1984; H…     

Automatic Detection and Classification of Coronal Holes and Filaments Based on EUV and Magnetogram Observations of the Solar Disk

A new method for the automated detection of coronal holes and filaments on the solar disk is presented. The starting point is coronal images taken by the Extreme Ultraviolet Telescope on the Solar and Heliospheric Observatory (SOHO/EIT) in the Fe ix/x 171 Å, Fe xii 195 Å, and He ii 304 Å extreme ultraviolet (EUV) lines and the corresponding full-disk magnetograms from the Michelson Doppler Imager (SOHO/MDI) from different phases of the solar cycle. The images are processed to enhance their contrast and to enable the automatic detection of the two candidate features, which are visually indistinguishable in these images. Comparisons are made with existing databases, such as the He i 10830 Å NSO/Kitt Peak coronal-hole maps and the Solar Feature Catalog (SFC) from the European Grid of Solar Observations (EGSO), to discriminate between the two features. By mapping the features onto the corresponding magnetograms, distinct magnetic signatures are then derived. Coronal holes are found to have a skewed distribution of magnetic-field intensities, with values often reaching 100?–?200 gauss, and a relative magnetic-flux imbalance. Filaments, in contrast, have a symmetric distribution of field intensity values around zero, have smaller magnetic-field intensity than coronal holes, and lie along a magnetic-field reversal line. The identification of candidate features from the processed images and the determination of their distinct magnetic signatures are then combined to achieve the automated detection of coronal holes and filaments from EUV images of the solar disk. Application of this technique to all three wavelengths does not yield identical results. Furthermore, the best agreement among all three wavelengths and NSO/Kitt Peak coronal-hole maps occurs during the declining phase of solar activity. The He ii data mostly fail to yield the location of filaments at solar minimum and provide only a subset at the declining phase or peak of the solar cycle. However, the Fe ix/x 171 Å and Fe xii 195 Å data yield a larger number of filaments than the Hα data of the SFC.     

On Interpretation of the Radiation of Coronal Suprathermal Streams. II

This is the second part of the series of two articles which consider the effect of Compton scattering of the photospheric radiation on fast electrons of the coronal suprathermal streams. As compared to the previous part, a more realistic height-dependent model problem is treated. The results of numerical calculations for the mean frequency change and the proper cross-section for both the sunward and antisunward directed beams of electrons are given as a function of height and the slope angle. It is concluded that, depending on the angle between directions of the initial outburst and magnetic field, the scattering on the beams moving away the Sun may produce measurable drifts in frequency to shortwave as well as to longwave domains of the spectrum. At the same time, the sunward directed beams result only in an increase in the photon energy.     

On the Excitation of Leaky Modes in Cylindrical Loops

The role of leaky waves in the coronal loop oscillations observed by TRACE is not yet clearly understood. In this work, the excitation of fast waves in solar coronal loops modelled as dense plasma cylindrical tubes in a uniform straight magnetic field is investigated. We study the trapped and especially leaky modes (whose energy escapes from the tube) that result from an initial disturbance by solving the time-dependent problem numerically. We find that the stationary state of the tube motion is given by the trapped normal modes. By contrast, the transient behaviour between the initial and the stationary phase is dominated by wave leakage. The so-called trig leaky modes are clearly identified since the transient behaviour shows periods and damping times that are in agreement with the values calculated from the normal-mode analysis. Consequently, these radiating modes have physical significance. However, we have not found any evidence for the excitation of other types of modes, such as the principal leaky kink mode. J. Andries is postdoctoral Fellow of the National Fund for Scientific Research – Flanders (Belgium) (F.W.O.-Vlaanderen).     

Oscillations in a solar pore

Temporal variations of a solar pore were observed at the ground based Vacuum Tower Telescope (VTT) on Tenerife and with the satellite TRACE. At the VTT Stokes I and V of the iron line at 1.56μm originating in the deep photosphere, was measured. TRACE delivered UV images at 170 nm which show chromospheric continuum. In a part of the pore we find oscillations of the magnetic field in the 5 minute range. Velocities derived from shifts of the Stokes V profiles show 5 minutes everywhere in the pore, but the coherence of magnetic field and velocities is low. The intensity at 170 nm varies with 3 minutes, and for a part of the whole time series additionally with 4 minutes.