R. Falciani

The solar chromosphere at high resolution with IBIS - I. New insights from the Ca II 854.2 nm line

Context. The chromosphere remains a poorly understood part of the solar atmosphere, as current modeling and observing capabilities are still ill-suited to investigate in depth its fully 3-dim ensional nature. In particular, chromospheric observatio ns that can preserve high spatial and temporal resolution while providing spectral information over extended fields of view are still very sc arce. Aims. In this paper, we seek to establish the suitability of imagin g spectroscopy performed in the Ca II 854.2 nm line as a means to investigate the solar chromosphere at high resolution. Methods. We utilize monochromatic images obtained with the Interferometric BIdimensional Spectrometer (IBIS) at multiple wavelengths within the Ca II 854.2 nm line and over several quiet areas. We analyze both the morphological properties derived from narrow-band monochromatic images and the average spectral properties of distinct solar features such as network point s, internetwork areas and fibrils. Results. The spectral properties derived over quiet-Sun targets are in full agreement with earlier results obtained with fixed-s lit spectrographic observations, highlighting the reliability of the spectral information obtained with IBIS. Furthermore, the very narrowband IBIS imaging reveals with much clarity the dual nature of the Ca II 854.2 nm line: its outer wings gradually sample the solar photosphere, while the core is a purely chromospheric indicator. The latter displays a wealth of fine structures including bri ght points, akin to the Ca II H2V and K2V grains, as well as fibrils originating from even the smallest magnetic elements. The fibrils occupy a large fraction of the observed field of view even in the quiet region s, and clearly outline atmospheric volumes with different dynamical properties, strongly dependent on the local magnetic topology. This highlights the fact that 1-D models stratified alon g the vertical direction can provide only a very limited representation of the actual chromospheric physics. Conclusions. Imaging spectroscopy in the Ca II 854.2 nm line currently represents one of the best observational tools to investigate the highly structured and highly dynamical chromospheric environment. A high performance instrument such as IBIS is crucial in order to achieve the necessary spectral purity and stabilit y, spatial resolution, and temporal cadence.

Helium line formation and abundance in a solar active region

An observing campaign (SOHO JOP 139), coordinated between ground-based and Solar and Heliospheric Observatory (SOHO) instruments, has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines. The chromospheric lines Ca II K, Hα, and Na I D, as well as He I 10830, 5876, 584, and He II 304 A lines have been observed. The EUV radiation in the range λ 1 × 104 K. This region, between the chromosphere and transition region, has been indicated as a good candidate for processes that might be responsible for strong variations of [He]. The set of our observables can still be well reproduced in both cases, changing the atmospheric structure mainly in the low transition region. This implies that, to choose between different values of [He], it is necessary to constrain the transition region with different observables, independent of the He lines.

High temporal and spatial resolution observations of a solar flare on June 7, 1991

Abstract We present some preliminary results on a small flare observed at the NSO-Sacramento Peak with very good spatial resolution (.6″–.7″). The flare occurred on June, 7 1991 at ∼ 13:43 UT in the NOAA region 6659 and has been observed by the COMPTON BATSE experiment in the 25–50 KeV range. A very complex velocity pattern is found for chromospheric and metallic lines: a small region (5″ wide) presents a strong blue-shifts in all these lines, lasting through the flare, while, only in the Ca II-K and H δ lines, we find a zone (again ∼ 5″ wide) of strong red asymmetries contemporary to the hard X-rays spikes. These observations show the importance of having high spatial and temporal resolution measurements in the study of flares.

Variation of the profiles of medium-strong photospheric lines with heliographic latitude

The asymmetric profiles of 11 metallic lines are studied, at μ = 0.3, as functions of ϑ. Their variations cannot be interpreted as due to temperature effects, but might imply a dependence on ϑ of the photospheric velocity field.

Infrared and Submillimeter Diagnostics of Activity and Flares

We give a critical review of the observations of solar activity in the IR and sub-mm range, which are quite scarce, except for the Fe I triplet at 1.56 μm and the Mg I emission lines at 12.32 μm. These, however, are mainly intended for solar magnetic field studies rather than physical diagnostics on activity phenomena. We compute the emission in some continuuum windows and in some detectable Paschen and Brackett lines in two extreme flare models, viz. a “chromospheric” and a white-light flare model. The utility of the Paschen and Brackett lines as diagnostics of the atmospheric state is questionable since more information can be obtained more easily by observing the higher Balmer lines. On the other hand, observations in various continuum windows can be of high scientific value and efficiency. We also discuss possible coordination with simultaneous visible observations, in order to increase the diagnostic efficiency of a prospective observing run.

Needs and constraints for coordinated programs of photospheric and chromospheric studies of flares

Abstract Suggestions for a coordinated observing program of the photospheric and chromospheric components of a flare are given; the maximum efficiency can be reached only if simultaneous measurements of coronal emission component may be obtained. The observing program has to be changed according to the different characteristics of a flare in each phase of its development. During the build-up phase it is of particular interest to establish the correlation between the photospheric velocity field (determined from bidimensional spectroscopic images in the 5576 Fe I line) and the magnetic field. During the transient explosive phase particular emphasys should be given to high time resolution observations (at least 1 sec) in order to correlate the response of chromospheric and photospheric signatures to the energy release producing the impusive phase in the hard X-ray and microwave emission. Observations with different filters in the center and red wing of the H α line, in CN (0-0) band and in some continuous windows are suggested in order to have time and spatial resolution as high as possible. Spectral observations in the region 3400 – 4300 A give the possibility to measure the red shift of the Balmer lines presumably associated to the chromospheric condensation moving downward. When the flare atmosphere may be considered in pressure balance (after tens of seconds) the time resolution is not crucial and the H α and Na-D 2 line profile may be obtained with the Zeiss and UBF filter used in bidimensional spectroscopic mode. H α line gives the possibility to test the effect of the heating mechanism while the Na-D 2 line is useful to monitor the penetration depth of the flare disturbance.

Observational maps of the moments of strong line profiles on the solar disk

Using the method of solar bidimensional spectroscopy based on the Universal Birefringent Filter (UBF), we have determined the bidimensional maps of moments of some chromospheric lines. The observational material, referring to a quiet region on the disk center, have been acquired with the UBF of the NSO at Sacramento Peak on Aug. 27,1985. In this note we present the work in progress and the new observational aspects arising from this diagnostic method.

MULTISPECTRAL OBSERVATIONS OF AN ERUPTIVE FLARE

We analyze the pre-flare and impulsive phase of an eruptive (two-ribbon) flare at several wavelengths. The total energy (mechanical plus radiative) released by the flare is 8 x 1030 erg, about a factor 6 higher than the free magnetic energy (1.3 1030 erg) estimated from the non-potentiality of the magnetic field configuration in the flare area. During the impulsive phase, we find a very good time coincidence between the hard X-ray light curve and the light curves for 2 small areas (≃ 4″ in size) in the red wing of the Hα line and in the He-D3 line center. This temporal coincidence is compatible with the interpretation that hard X-ray emission is produced by bremsstrahlung of accelerated electron beams striking these dense areas. For the other regions of the Hα ribbons we find more gradual light curves, suggesting a different energy transport mechanism such as conduction.

Minor Photospheric and Chromospheric Magnetic Activity and Related Coronal Signatures

Solar activity is associated with the appearance, temporal evolution and spatial modifications of the magnetic field at the solar surface and with its interactions with plasma mass flows.

BIDIMENSIONAL SPECTROSCOPY OF NETWORK BRIGHT POINTS

We develop an automatic, computer controlled procedure to select and to analyze the Network Bright Points (NBPs) on solar images. These have been obtained at the Sac Peak Vacuum Tower Telescope by means of the Universal Birefringent Filter and Zeiss Hα filters, tuned, respectively, along the profiles of the Hβ, Mg-b1, Na-D2, and Hα lines.A structure is identified as an NBP if at the wavelength Hα- 1.5 A its maximum intensity is greater than 〈I〉 + 3σ and its area is greater than 1.5 arc sec2 at 〈I〉 + 1.5σ, where 〈I〉 is the mean value and σ the standard deviation of the intensity distribution on the image. Each detected NBP is then searched and confirmed in all the remaining 31 images at different wavelengths.For each NBP several parameters are measured (position, area, mean and maximum contrast, Dopplergram velocity, compactness, and so on) and some identification constraints are applied.The statistical analysis of the various parameter distributions, for NBPs present within an active region and its surroundings, shows that two types of NBPs can be identified according to the value of their mean contrast Cmin the Hα- 1.5 Å image (Cm ≤ 0.1 → type I, Cm> 0.1 → type II). The type I NBPs (all occurring on the boundaries of the supergranular network) appear to be much more frequent (180/26) than the type II ones.The size 〈A〉 of type I NBPs is less than 1.0 arc sec for Hα/Hβ wings but of the order of 1.2 arc sec for Na-D2 and Mg-bl. The mean contrast Cm is around the value of 10% along the Na-D2 and Mg-bl profiles and of 20% along the Hα/Hβ wings.The Cm - 〈A〉 scatter diagrams show, for the photospheric radiation (h < 100 km), a narrow range of variability for Cmin correspondence with a wide range for 〈A〉. For radiation orginated at higher levels (h > 200 km), the Cm- 〈A〉 scatter diagrams seem to indicate, even if with a large variance, that the highest Cms tend to correspond to the highest 〈A〉 values.The mean Doppler shift is close to zero for Na-D2 and Mg-bl lines but negative (downward motion) for Hα and Hβ lines.The type II NBPs tends to be preferentially located in the neighbourhood of small, compact sunspots and their detectability is almost constant through all the 4 studied line profiles. No conclusions can be derived on the mean size, contrast and Doppler shift values because their distributions are too dispersed. The only positive information is that its Cm- 〈A〉 scatter diagram, in Hα and Hβ wings, indicates a wide range of variability for Cm in correspondence with very narrow range of variability for 〈A〉.