Fe XVII X-ray lines in solar coronal plasmas

Calculated intensities of the Fe X-ray lines due to transitions 2p⁶ − 2p⁵3d lines (near 15 Å) and 2p⁶ − 2p⁵3s lines (near 17 Å) are compared with measured line intensities in solar and tokamak spectra. For the solar spectra, temperature T_e is obtained from the ratio of the Fe 16.776 Å line to a nearby Fe line. We find excellent agreement for all the major Fe line features in the 15–17 Å region except the Fe 15.015 Å line, the observed flux of which is less than the theoretical by a factor f. We find that f strongly depends on the heliocentric angle θ of the emitting region, being smallest (0.2) when the region is nearest Sun centre, but nearly 1 near the limb. Attributing this to resonance scattering, we are able to deduce the path length and electron density from the observations. Possible application to stellar active regions is given.     

Chase observations from spacelab 2 — the chase team

The Coronal Helium Abundance Spacelab Experiment (CHASE) was designed and built in the UK by groups at the Rutherford Appleton Laboratory and at the Mullard Space Science Laboratory. The main objective of the experiment was to improve the measurement of the abundance of helium in the Sun, which currently is uncertain by a factor of approximately three. Since most of this helium must be of primordial origin, being formed within a few minutes of the big bang, such a measurement would also have significant cosmological implications.The instrument consisted of a grazing incidence telescope feeding a 1200 lines mm⁻¹ diffraction grating. Parts of the dispersed spectrum were then observed around the Rowland circle with channel electron multipliers and a channel multiplier array plate. During the flight, good observations were made of the Lyman-α lines of hydrogen at 1216 A and of ionised helium at 304 A, both in the corona, where the lines are formed by resonance scattering, and on the solar disc where excitation of the lines is much more complex.The spectrometer was also able to observe many other transition region and coronal lines in ions of O, S and Fe. Images in these lines clearly show how the magnetic fields in active regions constrain the material in arch-shaped loops with the hot material towards the top.     

Ultraviolet spectroscopy of the chromosphere and transition zone at high spatial and temporal resolution

The third rocket flight of the NRL High Resolution Telescope and Spectrograph (HRTS) recorded UV spectra of the quiet Sun in a 10″ × 800″ region. By rastering the slit in 2′' increments, a time series with 20s resolution of two-dimensional spectra with 1′' spatial resolution was obtained. The spectrum includes strong chromospheric diagnostic lines of C I, Si I, and Fe II, transition zone lines of C IV and the continuum which is produced in the temperature minimum. Images of the network show that Fe II emission is well-correlated with dark Hα mottles, that the transition zone is produced in extended structures which are apparently continuations of the mottles, and that the strongest continuum emission is from near the base of the mottles. Time series of C I profiles show little variation in most features aside from the chromospheric jets which develop rapidly over timescales of 30s. The C IV profiles also show only gradual variations over most of the slit except for the numerous small explosive events.     

The chromosphere-corona transition zone above an active region

Intensities and profiles of ion emission lines between 1170 A and 1700 A from an active region on the Sun are measured from spectra obtained with the Naval Research Laboratory's High Resolution Telescope and Spectrograph - HRTS. The measurements provide simultaneous determination of line intensities, wavelength shifts and Doppler widths at 50 separate positions in the active region, with spatial resolution of 1 arc second and spectral resolution 0.07 A. Fine structure variation of intensities and gas flow velocities in the temperature range 20,000–200,000 K are determined. The density sensitive line pair I(1486 N IV)/I(1548 C IV) has been used to measure electron pressures. Derived emission measures imply filling factors of 0.05–0.1 to balance the divergence of conductive flux width radiative losses above 60,000 K.     

Spectral and temporal studies of various late-type stars

The RS CVn stars Capella and σ² CrB have been measured with EXOSAT in soft and medium X-rays for about 24 hours each and the less active late-type star Procyon for about 6.5 hours. In addition, the RS CVn star γ. And was twice observed about one month apart for a total of about 7 hours, with the ME and the LE in the photometer mode only. All three RS CVn stars were detected with the ME-detector. The star σ² CrB showed a flare both in LE and ME with a rise time of about twelve minutes and a decay time of three hours. The active late-type stars σ² CrB and Capella show in the spectral region between 90 and 140 A lines from Fe XVIII to Fe XXIII, which can be resolved with the moderate resolution (Δγ ≈ 5 A) of the spectrometer. These lines are indicative of the presence of hot (≈ 10 MK) plasma like that in a Solar flare. In contrast, the spectrum of the cooler corona of the star Procyon does not show the hot Fe XXII and Fe XXIII lines but instead a blend at 175 A of Fe IX, X and XI lines that are formed in a typically quiet corona of a temperature around 1.5 MK. From the spectral intensities and the additional results of the simultaneous multi-color photometry coronal temperatures and emission measures are derived. There are indications in the spectra that the emission should be interpreted in terms of differential emission measure distribution models.     

Imaging the solar corona in the EUV

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 Å images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 10⁴ K; the Fe IX–X filter at 171 Å images the corona at a temperature of 1.3 × 10⁶ K; the Fe XII filter at 195 Å images the quiet corona outside coronal holes at a temperature of 1.6 × 10⁶ K; and the Fe XV filter at 284 Å images active regions with a temperature of 2.0 × 10⁶ K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.     

Spectral analysis of 3D MHD models of coronal structures

We study extreme-ultraviolet emission line spectra derived from three-dimensional magnetohydrodynamic models of structures in the corona. In order to investigate the effects of increased magnetic activity at photospheric levels in a numerical experiment, a much higher magnetic flux density is applied at the photosphere as compared to the Sun. Thus, we can expect our results to highlight the differences between the Sun and more active, but still solar-like stars. We discuss signatures seen in extreme-ultraviolet emission lines synthesized from these models and compare them to observed signatures in the spatial distribution and temporal evolution of Doppler shifts in lines formed in the transition region and corona. This is of major interest to test the quality of the underlying magnetohydrodynamic model to heat the corona, i.e. currents in the corona driven by photospheric motions (flux braiding).     

A highly-ionized absorber as a new explanation for the spectral changes during dips from X-ray binaries

We report the XMM-Newton detection of narrow Fe xxv and Fe xxvi X-ray absorption lines at 7 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) 4U 1323−62. Such features have now been reported in a growing number of LMXBs seen almost edge-on, indicating that the highly-ionized plasma probably originates in an accretion disk atmosphere or wind. During dipping intervals of 4U 1323−62, the strength of the Fe xxv feature increases while that of the Fe xxvi feature decreases, consistent with the presence of less strongly ionized material in the line-of-sight. As observed previously, the changes in the X-ray spectrum during dips are inconsistent with a simple increase in absorption by cool material. However, we show that the changes in both the narrow absorption features and the continuum can be modeled self-consistently by variations in the properties of an ionized absorber. From persistent to deep dipping the photo-ionization parameter decreases while the equivalent hydrogen column density of the ionized absorber increases. No partial covering of any component of the spectrum, and hence no extended corona, are required. Since highly-ionized absorption features are seen from many other dip sources, this mechanism may also explain the overall changes in X-ray spectrum observed during dipping intervals from these systems.     

Identification and characteristics of source flares emitting 3-He or Fe enriched cosmic rays in the period September to December 1977

During the observation of solar cosmic rays on the Prognoz 6 and Helios 1 and 2 spacecrafts, several events with anomalous composition of accelerated particles (higher abundance of 3-He or Fe nuclei) occurred. We found seven such events from the period September to December 1977 for which data from the Prognoz 6 solar X-ray photometer are available. This material together with published optical and radio data from terrestrial observatories enabled us to identify more reliably the source flares and describe their characteristics. It turned out that the character of X-ray emission accompanying the emission of accelerated particles with anomalous composition shows no pronounced difference from other flares. No correlation has been found among the ratio 3-He/4-He and the angular distance between the field lines connected with the source flare and the place of observation. If a solar flare with anomalous ratio 3-He/4-He appears in a given active region, this region will probably produce other anomalous events.     

Upper limit of the total magnetic flux in an active region according to the photometric-magnetic dynamical model

The photometric-magnetic dynamical model handles the evolution of an individual sunspot as an autonomous nonlinear, though integrable, dynamical system. One of its consequences is the prediction of an upper limit of the sunspot areas. This upper limit is analytically expressed by the model parameters, while its calculated value is verified by the observational data. In addition, an upper limit for the magnetic strength inside the sunspot is also predicted, and then, we obtain the following significant result: The upper limit of the total magnetic flux in an active region is found to be of about 7.23 × 10²³ Mx, namely, phenomenologically equal to the magnetic flux concentrated in the totality of the granules of the quiet Sun, having a typical maximum magnetic strength of about 12G. Therefore, the magnetic flux concentrated in an active region cannot exceed the magnetic flux concentrated in the photosphere as a whole.     

太阳黑子自动识别与特征参量自动提取

日面上黑子数目反映了太阳活动水平的高低.黑子形态的复杂性和磁场的非势性与太阳活动爆发密切相关.随着高时空精度的太阳观测数据量的急剧增长，快速准确地自动识别日面上的黑子以及对黑子群特征自动提取已成为太阳活动预报的现实需求.本文针对SDO/HMI的活动区白光数据，利用数学形态法开展黑子自动识别研究，并在黑子识别基础上对黑子群的面积和黑子数进行了计算.通过对利用2011-2017年HMI活动区数据计算得到的黑子群面积和黑子数与NOAA/SWPC发布的活动区相应参量进行比较，发现本文计算结果与SWPC发布数据的变化趋势基本一致，相关性较好.其中黑子群面积的相关系数为0.77，黑子数的相关系数为0.79.研究结果表明，利用本文方法对SDO/HMI数据进行处理，能够得到高时间分辨率的黑子群特征参量，可为太阳活动预报提供及时准确的输入.      

Active galaxies observed by IUE

IUE has extended the grasp of ultraviolet astronomy to cover active galaxies and quasars fainter than the sixteenth magnitude. These observations have:(i) provided a diagnostic for the source of ionization in active galaxies,(ii) cast light on the excitation mechanism of the Fe II lines,(iii) shown the broad hydrogen lines in Seyferts and quasars are not in their recombination ratios,(iv) demonstrated the absence of gas in BL Lac objects,(v) supported the gravitational lens explanation of the double quasar,(vi) demonstrated the presence of a hot (30 000 K) black body in active nuclei,(vii) discovered stratification of the ionization conditions in the Broad Line Regions of active nuclei.     

The HRS GTO program to study the neutral hydrogen column density and D/H ratio in the local interstellar medium

Early in the HST mission the HRS Team will observe the Lyman alpha line at 100,000 spectral resolution toward 7 late-type local stars. The purpose is to derive the hydrogen and deuterium column densities and D/H ratios along lines of sight toward nearby stars. Here we present theoretical line profiles that demonstrate why 10⁵ spectral resolution and high signal-to-noise are needed to derive accurate column densities from spectral lines that lie close to the flat part of the curve of growth and may contain multiple velocity components. The aim of the HRS program is to obtain column densities in the hydrogen and deuterium Lyman alpha lines along a variety of lines of sight within and extending beyond the local cloudlet in which the Sun is located near an edge. The broad chromospheric Lyman alpha emission lines of late-type stars are used as background sources against which to measure the interstellar absorption features. We will also obtain profiles of interstellar absorption features in Mg II and Fe II lines to derive the broadening parameter and/or identify possible multiple velocity components in the lines of sight.     

The thermal X-ray spectrum of the 2003 April 26 solar flare

Observations and their analysis of the thermal X-ray spectrum of the M2 flare on 2003 April 26 are described. The spectrum observed by the RHESSI mission cover the energy range from ∼5 to ∼50 keV. With its ∼1-keV spectral resolution, intensities and equivalent widths of two line complexes, the Fe line group at 6.7 keV (mostly due to Fe xxv lines and Fe xxiv satellites) and the Fe/Ni line group at 8 keV (mostly due to higher-excitation Fe xxv lines and Ni xxvii lines) were obtained as a function of time through a number of flares. The abundance of Fe can also be determined from RHESSI spectra; it appears to be consistent with a coronal value for at least some times during the flare. Comparisons of RHESSI spectra with those from the RESIK crystal spectrometer on CORONAS-F show very satisfactory agreement, giving much confidence in the intensity calibration of both instruments.     

Solar and late-type dwarfs

The Einstein Observatory and the IUE satellite have provided the observational basis for a major restructuring in theories of coronal formation for late-type stars. For the first time, coronal and transition region emission from a large sample of low mass (1 M_o) dwarf stars has been directly observed, with the unexpected result that essentially all such stars are x-ray emitters. The Sun, which was previously assumed to be typical, is now known to be at the low end of the x-ray luminosity function for solar-type stars. K- and M-dwarfs are observed to have nearly the same luminosity distributions as G-dwarfs and all of these spectral types have a large spread in x-ray luminosity.Observationally, there is a strong correlation between the strength of coronal emission in stars with outer convective zones and the rotation rates of these stars. At the present time we have only the beginnings of a satisfactory theoretical explanation for this correlation; although we are beginning to understand the connection between coronal emission strength and the magnetic field, we do not yet understand the stellar dynamo which generates the magnetic field. Studies of the coronal emission of stars may lead to a better understanding of stellar dynamos.     

Structure of the Program of Short-term Prediction of Powerful Solar Flares

Input data of the system are two-dimensional images and one-dimensional distributions of total and polarized solar emission at 5.2 cm wavelength obtained with SSRT. Together with photoheliograms, magnetograms, Hα-filtergrams and characteristics of active regions received from other sources, they form the initial database. The first stage includes superimposing the images, identifying microwave sources with active regions, assigning NOAA numbers to the sources, and determining for each active region the heliolatitude, extent, and inclination angle of the group's axis to the equator. These data are used to calculate the boundaries of longitude zones for each active region. A next stage involves determining the brightness temperatures of microwave sources less than the polarization distribution, the degree of polarization, and microwave emission flux, as well as calculating the parameters of microwave sources. Each parameter is assigned its own value of the weight factor, and the sum of values is used to draw the conclusion about the flare occurrence probability in each active region and on the Sun in general.      

The method for predicting solar proton events

Dynamic processes in the interplanetary space have been investigated using time variations in time parameters of the cosmic-ray rigidity spectrum. Change of heliosphere electromagnetic characteristics has been found out to precede sporadic phenomena on the Sun. In particular, it is shown that sporadic phenomena are followed by generation of local polarization electric fields, decrease of the magnetic-field strength in small-scale heliospheric structures, and increase of the potential difference between the pole and the plane of the ecliptic. These features allow prediction of solar proton events in advance (from several hours to several tens of hours) with a high degree of confirmation.     

New measurements of photospheric magnetic fields in late-type stars and emerging trends

We report on recent results of our program to measure photospheric magnetic field strengths and filling factors from the analysis of unpolarized high spectral resolution and S/N line profiles. We have analyzed spectra obtained with the KPNO 4-m FTS, the MMT, and the McMath Reticon Spectrograph. With the latter instrument we now have an extensive data set through our synoptic and survey observing programs. Photospheric magnetic field parameters are obtained by comparison of observed and theoretical line profiles using an LTE code that includes line saturation and the full Zeeman pattern. We determine the nonmagnetic line broadening parameters for a star by fitting profiles of low Landé g lines formed under conditions similar to the high g lines which are used to infer magnetic fields. The effects of line blending are cancelled to first order by fitting profiles of the same high g lines in comparison inactive stars of the same spectral type. We have measured field parameters (outside of starspots) in dwarf stars of spectral type GO to dM3.5e (Ad Leo). We find a trend of increasing field strength with later spectral type, consistent with equal gas and magnetic pressure in the photosphere, and a trend of increasing filling factor with activity. Our data are inconsistent with the hypothesis of a constant number of magnetic flux tubes on active stars.     

Seismological studies of the sun and other stars

Observations of a large number of different oscillation frequencies in the Sun provide an opportunity for detailed testing of the theory of stellar structure and evolution. At present highly significant discrepancies remain between observed and computed frequencies, and so our models of the solar interior have to be modified. With further improvements in the observations it might become possible to make a direct empirical determination of the density structure throughout the Sun.Similar oscillations have so far not been detected in other stars, but attempts to do so are under way. Theoretical estimates indicate that amplitudes somewhat greater than for the Sun might be expected for early F stars on the main sequence, and that the amplitude increases rapidly with decreasing gravity. Observation of such oscillations would enable investigations of the structure of these stars, and would in addition provide valuable information about the excitation mechanism of the oscillations.     

Hard X-ray and high-frequency decimetric radio observations of the 4 April 2002 solar flare

Hard X-ray and high frequency decimetric type III radio bursts have been observed in association with the soft X-raysolar flare (GOES class M 6.1) on 4 April 2002 (1532 UT). The flare apparently occurred 6 degrees behind the east limb of the Sun in the active region NOAA 9898. Hard X-ray spectra and images were obtained by the X-ray imager on RHESSI during the impulsive phase of the flare. The Brazilian Solar Spectroscope and Ondrejov Radio Telescopes recorded type III bursts in 800–1400 MHz range in association with the flare. The images of the 3–6, 6–12, 12–25, and 25–50 keV X-ray sources, obtained simultaneously by RHESSI during the early impulsive phase of the flare, show that all the four X-ray sources were essentially at the same location well above the limb of the Sun. During the early impulsive phase, the X-ray spectrum over 8–30 keV range was consistent with a power law with a negative exponent of 6. The radio spectra show drifting radio structures with emission in a relatively narrow (Δf ≤ 200 MHz) frequency range indicating injection of energetic electrons into a plasmoid which is slowly drifting upwards in the corona.