Using LASCO-C1 spectroscopy for coronal diagnostics

The LASCO-C1 telescope was designed to perform spectral analysis of coronal structures by means of a tunable Fabry–Pérot interferometer acquiring images at different wavelengths. Results from spectral scans of the Fe xiv 5303 Å green coronal emission line are presented. Physical quantities like the ion temperature (line widths), and the flow velocity along the line of sight (Doppler shifts) are obtained over the entire corona.     

太阳宁静区色球磁场观测

利用中国科学院北京天文台怀柔太阳磁场望远镜,对日面中心宁静区光球和色球磁场进行了长时间的积分观测。通过对光球、色球以及色球不同层次的长时间积分的观测发现,网络磁元从光球到色球扩展不大,并且部分内网络磁元升到了色球。这些结果对描述太阳磁场的两大模型提出了挑战。     

The dynamic chromosphere: Pushing the boundaries of observations and models

The interface between the bright solar surface and the million-degree corona continues to hold the key to many unsolved problems in solar physics. Advances in instrumentation now allow us to observe the dynamic structures of the solar chromosphere down to less than $0_{.}^{″}1$ with cadences of just a few seconds and in multiple polarisation states. Such observational progress has been matched by the ever-increasing sophistication of numerical models, which have become necessary to interpret the complex observations. With an emphasis on the quiet Sun, I will review recent progress in the observation and modelling of the chromosphere. Models have come a long way from 1D static atmospheres, but their predictions still fail to reproduce several key observed features. Nevertheless, they have given us invaluable insight into the physical processes that energise the atmosphere. With more physics being added to models, the gap between predictions and observations is narrowing. With the next generation of solar observatories just around the corner, the big question is: will they close the gap?     

Where is the base of the Transition Region? Evidence from TRACE,SDO, IRIS and ALMA observations

Classic solar atmospheric models put the Chromosphere-Corona Transition Region (CCTR) at $～2$ Mm above the ${\tau }_{5000}=1$ level, whereas radiative MHD (rMHD) models place the CCTR in a wider range of heights. However, observational verification is scarce. In this work we review and discuss recent results from various instruments and spectral domains. In SDO and TRACE images spicules appear in emission in the 1600, 1700 and 304 Å bands and in absorption in the EUV bands; the latter is due to photo-ionization of H i and He i, which increases with wavelength. At the shortest available AIA wavelength and taking into account that the photospheric limb is $～0.34$ Mm above the ${\tau }_{5000}=1$ level, we found that CCTR emission starts at $～3.7$ Mm; extrapolating to $\lambda =0$, where there is no chromospheric absorption, we deduced a height of $3.0\pm 0.5$ Mm, which is above the value of 2.14 Mm of the Avrett and Loeser model. Another indicator of the extent of the chromosphere is the height of the network structures. Height differences produce a limbward shift of features with respect to the position of their counterparts in magnetograms. Using this approach, we measured heights of $0.14\pm 0.04$ Mm (at 1700 Å), $0.31\pm 0.09$ Mm (at 1600 Å) and $3.31\pm 0.18$ Mm (at 304 Å) for the center of the solar disk. A previously reported possible solar cycle variation is not confirmed. A third indicator is the position of the limb in the UV, where IRIS observations of the Mg ii triplet lines show that they extend up to $～2.1$ Mm above the 2832 Å limb, while AIA/SDO images give a limb height of $1.4\pm 0.2$ Mm (1600 Å) and $5.7\pm 0.2$ Mm (304 Å). Finally, ALMA mm-$\lambda$ full-disk images provide useful diagnostics, though not very accurate, due to their relatively low resolution; values of $2.4\pm 0.7$ Mm at 1.26 mm and $4.2\pm 2.5$ Mm at 3 mm were obtained. Putting everything together, we conclude that the average chromosphere extends higher than homogeneous models predict, but within the range of rMHD models..     

EUV coronal pattern of complexes of solar activity

Studying of the coronal plasma associated with long-lived complexes of the solar activity is important for understanding a relationship between the magnetic activity and the solar corona changing during the solar cycle.

In the present paper, two long-lived complexes of the solar activity at the beginning of the current solar cycle 23 are investigated by using the Extreme-Ultraviolet data (EUV) from SOHO/EIT. For this purpose the EIT limb synoptic maps during the CR1916–CR1919 (11 November 1996–1 March 1997) are obtained.

The coronal temperature structures derived from the three lines 171A (Fe IX,X), 195A (Fe XII)and 284A (Fe XV) are investigated by applying an algorithm developed by Zhang et al. [Zhang, J., White, S.M., Kundu, M.R. ApJ 527, 977, 1999]. Standard EIT software are used for the temperature estimation from the ratio of two lines of Fe IX,X and Fe XII.

The method of the rotational tomography with a correction for an inclination of the Earth’s orbit (B-angle) to the helioequator is applied to obtain the three-dimensional (3-D) coronal structure of the complex of the solar activity. The results reveal difference in temperature structures related to multi-poles magnetic structures of the complex of solar activity and to the typical, the bipolar activity complex.     

Results of XUV full sun imaging spectroscopy for eruptive and transient events by the spirit spectroheliograph on the CORONAS-F mission

SPIRIT (SPectroheliograph Ic soft X-Ray Imaging Telescope) is the current experiment on board theCORONAS-F satellite launched on July 31, 2001 (Oraevskii & Sobelman, 2002). The main goal of this experiment is to study a structure and dynamics of the solar atmosphere in the wide scale of heights (from the chromosphere to a far corona) and of temperatures (from ten thousands through thirty millions Kelvins) by means of the XUV imaging spectroscopy. Since the launch of the CORONAS-F satellite more than three hundred thousands of images and spectroheliograms have been recorded. For the first time continuous series of monochromatic full Sun images in MgXII lines at 8.42 Å (doublet: 8.418 and 8.423 Å) were obtained. These series include long-term continuous observations of duration up to 10 days with the cadence of 100 sec as well as temporal sequences with duration of a few minutes and high resolution of 7 sec, synchronized with flares. The spectroheliograms for the whole disk and off-limb regions are also recorded in the spectral bands 177 – 207 and 285 – 335 Å providing spectra with high resolution of various coronal structures including eruptive and transient events. This paper presents preliminary results of quick look analysis of some observational data obtained by means of the SPIRIT spectroheliographs.     

CMEs and active regions on the sun

The relationship between active regions (ARs) and coronal mass ejections (CMEs) is studied. For this purpose a statistical analysis of 694 CMEs associated with ARs was carried out. We considered the relationship between properties of the CMEs and ARs characterized using the McIntosh classification. We demonstrated that CMEs are likely to be launched from ARs in the mature phase of their evolution when they have complex magnetic field. The fastest and halo CMEs can be ejected only from the most complex ARs (when an AR is a bipolar group of spots with large asymmetric penumbras around the main spot with many smaller spots in the group). We also showed that the wider events have a tendency to originate from uncomplicated magnetic structures. This tendency was used for estimation of the real angular widths of the halo CMEs. The probability of launching of fast CMEs increases together with increase of the complexity and size of ARs. The widest, but slow, CMEs originate from the simplest magnetic structure which are still able to produce CMEs. Our results could be useful for forecasting of space weather.     

The damping of small-amplitude oscillations in quiescent prominences

The presence of small-amplitude oscillations in prominences is well-known from long time ago. These oscillations, whose exciters are still unknown, seem to be of local nature and are interpreted in terms of magnetohydrodynamic (MHD) waves. During last years, observational evidence about the damping of these oscillations has grown and several mechanisms able to damp these oscillations have been the subject of intense theoretical modelling. Among them, the most efficient seem to be radiative cooling and ion-neutral collisions. Radiative cooling is able to damp slow MHD waves efficiently, while ion-neutral collisions, in partially ionised plasmas like those of solar prominences, can also damp fast MHD waves. In this paper, we plan to summarize our current knowledge about the time and spatial damping of small-amplitude oscillations in prominences.     

Model of a fluxtube with a twisted magnetic field in the stratified solar atmosphere

We build a single vertical straight magnetic fluxtube spanning the solar photosphere and the transition region which does not expand with height. We assume that the fluxtube containing twisted magnetic fields is in magnetohydrostatic equilibrium within a realistic stratified atmosphere subject to solar gravity. Incorporating specific forms of current density and gas pressure in the Grad–Shafranov equation, we solve the magnetic flux function, and find it to be separable with a Coulomb wave function in radial direction while the vertical part of the solution decreases exponentially. We employ improved fluxtube boundary conditions and take a realistic ambient external pressure for the photosphere to transition region, to derive a family of solutions for reasonable values of the fluxtube radius and magnetic field strength at the base of the axis that are the free parameters in our model. We find that our model estimates are consistent with the magnetic field strength and the radii of Magnetic bright points (MBPs) as estimated from observations. We also derive thermodynamic quantities inside the fluxtube.     

Topological and statistical properties of nonlinear force-free fields

We use our semi-analytic solution of the nonlinear force-free field equation to construct three-dimensional magnetic fields that are applicable to the solar corona and study their statistical properties for estimating the degree of braiding exhibited by these fields. We present a new formula for calculating the winding number and compare it with the formula for the crossing number. The comparison is shown for a toy model of two helices and for realistic cases of nonlinear force-free fields; conceptually the formulae are nearly the same but the resulting distributions calculated for a given topology can be different. We also calculate linkages, which are useful topological quantities that are independent measures of the contribution of magnetic braiding to the total free energy and relative helicity of the field. Finally, we derive new analytical bounds for the free energy and relative helicity for the field configurations in terms of the linking number. These bounds will be of utility in estimating the braided energy available for nano-flares or for eruptions.     

Solar physics with the Square Kilometre Array

The Square Kilometre Array (SKA) will be the largest radio telescope ever built, aiming to provide collecting area larger than 1?km². The SKA will have two independent instruments, SKA-LOW comprising of dipoles organized as aperture arrays in Australia and SKA-MID comprising of dishes in South Africa. Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage and construction is expected to start in 2020. Both SKA1-LOW (frequency range of 50–350?MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350–1050, 950–1760, and 4600–15,300?MHz, respectively) are important for solar observations. In this paper we present SKA’s unique capabilities in terms of spatial, spectral, and temporal resolution, as well as sensitivity and show that they have the potential to provide major new insights in solar physics topics of capital importance including (i) the structure and evolution of the solar corona, (ii) coronal heating, (iii) solar flare dynamics including particle acceleration and transport, (iv) the dynamics and structure of coronal mass ejections, and (v) the solar aspects of space weather. Observations of the Sun jointly with the new generation of ground-based and space-borne instruments promise unprecedented discoveries.     

Synoptic magnetic field in cycle 23 and in the beginning of the cycle 24

The SOHO/MDI data provide the uniform time series of the synoptic magnetic maps which cover the period of the cycle 23 and the beginning of the cycle 24. It is very interesting period because of the long and deep solar minimum between the cycles 23 and 24. Synoptic structure of the solar magnetic field shows variability during solar cycles. It is known that the magnetic activity contributes to the solar irradiance. The axisymmetrical distribution of the magnetic flux (Fig. 3c) is closely associated with the ‘butterfly’ diagram in the EUV emission (Benevolenskaya et al., 2001). And, also, the magnetic field (B_∥) shows the non-uniform distributions of the solar activity with longitude, so-called ‘active zones’, and ‘coronal holes’ in the mid-latitude. Polar coronal holes are forming after the solar maxima and they persist during the solar minima. SOHO/EIT data in the emission of Fe XII (195 Å) could be a proxy for the coronal holes tracking. The active longitudinal zones or active longitude exist due to the reappearance of the activity and it is clearly seen in the synoptic structure of the solar cycle. On the descending branch of the solar cycle 23 active zones are less pronounced comparing with previous cycles 20, 21 and 22. Moreover, the weak polar magnetic field precedes the long and deep solar minimum. In this paper we have discussed the development of solar cycles 23 and 24 in details.     

A MHD-turbulence model for solar corona

The disposition of energy in the solar corona has always been a problem of great interest. It remains an open question how the low temperature photosphere supports the occurence of solar extreme phenomena. In this work, a turbulent heating mechanism for the solar corona through the framework of reduced magnetohydrodynamics (RMHD) is proposed. Two-dimensional incompressible long time simulations of the average energy disposition have been carried out with the aim to reveal the characteristics of the long time statistical behavior of a two-dimensional cross-section of a coronal loop and the importance of the photospheric time scales in the understanding of the underlying mechanisms. It was found that for a slow, shear type photospheric driving the magnetic field in the loop self-organizes at large scales via an inverse MHD cascade. The system undergoes three distinct evolutionary phases. The initial forcing conditions are quickly “forgotten” giving way to an inverse cascade accompanied with and ending up to electric current dissipation. Scaling laws are being proposed in order to quantify the nonlinearity of the system response which seems to become more impulsive for decreasing resistivity. It is also shown that few, if any, qualitative changes in the above results occur by increasing spatial resolution.     

Bayesian coronal seismology

In contrast to the situation in a laboratory, the study of the solar atmosphere has to be pursued without direct access to the physical conditions of interest. Information is therefore incomplete and uncertain and inference methods need to be employed to diagnose the physical conditions and processes. One of such methods, solar atmospheric seismology, makes use of observed and theoretically predicted properties of waves to infer plasma and magnetic field properties. A recent development in solar atmospheric seismology consists in the use of inversion and model comparison methods based on Bayesian analysis. In this paper, the philosophy and methodology of Bayesian analysis are first explained. Then, we provide an account of what has been achieved so far from the application of these techniques to solar atmospheric seismology and a prospect of possible future extensions.     

On the nature of coronal loops above the quiet sun network

The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We investigate a model of an active region for two sunspots surrounded by magnetic field patches comparable in magnetic flux to the sunspots. The model results in emission from the model corona being concentrated in loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV and X-ray emission outline the magnetic field, following the general paradigm. However, in our model, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field lines. This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network.     

Phase asynchrony between coronal index and sunspot numbers

In this paper, the phase asynchrony between coronal index and sunspot numbers is investigated. It is found that, (1) the sunspot numbers begin one month earlier than coronal index, which should mathematically lead to phase asynchrony between them but with a slight effect; (2) the 11-year Schwabe cycle is the only one period with statistical significance for coronal index and sunspot numbers, and the difference between the length of the Schwabe cycle of them should also lead to phase asynchrony between them; (3) although coronal index and sunspot numbers are coherent in low-frequency components corresponding to the 11-year Schwabe cycle, they are asynchronous in phase in high-frequency components; (4) their different definitions and physical meanings may be a major reason why there is a phase asynchrony between them.     

太阳宁静区磁漏斗结构的扩展特征

通过对观测的光球纵向磁场进行势场外推, 重构了一个太阳宁静区光球以上的磁场结构. 结果显示, 在20 Mm 以下, 开放磁力线呈现多个明显的小磁漏斗结构, 这些小磁漏斗结构随高度扩展, 并且在20 Mm 左右融合成大的漏斗结构. 通过系统地研究磁漏斗结构横截面积随高度的变化趋势, 发现太阳宁静区磁漏斗结构的截面积随高度近似线性扩展, 磁漏斗结构在较低高度上(<20Mm) 扩展的速度比在较高高度上 (>20Mm) 扩展的速度要快. 这一结果对太阳风起源和磁环中物质流动的二维数值模拟具有重要的意义. 同时还发现, 闭合磁力线的数目随高度以指数函数的形式减少.     

Scientific requirements for future spatially resolved white-light and broad-band high-cadence observations of the Sun

Several important issues are open in the field of solar variability and they wait their solution which up to now was attempted using critical ground-based instrumentations. However, accurate photometric data are attainable only from space. New observational material should be collected with high enough spatial and spectral resolution, covering the whole visible range of the electromagnetic spectrum as well infrared and ultraviolet to reconstruct the total solar irradiance: (1) the absolute contributions of different small-scale structural entities of the solar atmosphere from the white light flares and from micro-flares are still poorly known; (2) we do not know the absolute contributions of different structural elements of the solar atmosphere to the long-term and to the cyclic variations of the solar irradiance, including features of the polar regions of the Sun; (3) the variations of the chromospheric magnetic network are still poorly evaluated; (4) only scarce information is available about the spectral variations of different small-scale features in the high photosphere. Variability of the Sun in white light can be studied with higher spectral, spatial and time resolution using space-born telescopes, which are more appropriate for this purpose than ground based observatories because of better seeing conditions, no interference of the terrestrial atmosphere and a more precise calibration procedure. Scientific requirements for such observations and the possible experimental tools proposed for their solution. Suggested solar studies have broader astrophysical importance.     

Solar wind velocity measurements near the sun using Ulysses radio amplitude correlations at two frequencies

Measurements of solar wind velocity have been derived from simultaneous coronal sounding observations of radio amplitude scintillations at both S-band and X-band during the solar conjunction of the Ulysses spacecraft in August 1991. The signal amplitude was recorded with an averaging time of 1 s. A cross-correlation analysis between S- and X-band amplitude fluctuations shows that the fluctuation signature at S-band appears to be shifted to earlier times with respect to the X-band recording. The time difference is proportional to the coronal separation of the ray paths and inversely proportional to the apparent velocity of plasma inhomogeneities across the ray paths. Preliminary estimates of solar wind speed obtained using model calculations of the differential refraction are found to lie near the expected transition from subsonic to supersonic velocities at solar offset distances of the order of 6–8 R. As a byproduct of the investigation, we find that the transition from weak to saturated scintillation occurs at about 16 R for S-band and 7 R for X-band.