A. Falchi

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.

Helium Line Formation and Abundance during a C-Class Flare

During a coordinated campaign that took place in 2001 May, a C-class flare was observed both with SOHO instruments and with the Dunn Solar Telescope of the National Solar Observatory at Sacramento Peak. In two previous papers we described the observations and discussed some dynamical aspects of the earlier phases of the flare, as well as the helium line formation in the active region prior to the event. Here we extend the analysis of the helium line formation to the later phases of the flare in two different locations of the flaring area. We have devised a new technique, exploiting all available information from various SOHO instruments, to determine the spectral distribution of the photoionizing EUV radiation produced by the corona overlying the two target regions. In order to find semiempirical models matching all of our observables, we analyzed the effect on the calculated helium spectrum, both of AHe (the He abundance) and of the uncertainties in the incident EUV radiation (level and spectral distribution). We found that the abundance has in most cases (but not in all) a larger effect than the coronal back-radiation. The result of our analysis is that, considering the error of the measured lines and adopting our best estimate for the coronal EUV illumination, the value AHe = 0.075 ± 0.010 in the chromosphere (for T > 6300 K) and transition region yields reasonably good matches for all the observed lines. This value is marginally consistent with the most commonly accepted photospheric value, AHe = 0.085.

Rhessi Images and Spectra of Two Small Flares

We studied the evolution of two small flares (GOES class C2 and C1) that developed in the same active region with different morphological characteristics: one is extended and the other is compact. We analyzed the accuracy and the consistency of different algorithms implemented in Reuven Ramaty High-Energy Spectroscopic Imager (RHESSI) software to reconstruct the image of the emitting sources, for energies between 3 and 12 keV. We found that all tested algorithms give consistent results for the peak position, while the other parameters can differ at most by a factor 2. Pixon and Forward-fit generally converge to similar results but Pixon is more reliable for reconstructing a complex source. We investigated the spectral characteristics of the two flares during their evolution in the 3–25 keV energy band. We found that a single thermal model of the photon spectrum is inadequate to fit the observations and we needed to add either a non-thermal model or a hot thermal one. The non-thermal and the double thermal fits are comparable. If we assume a non-thermal model, the non-thermal energy is always higher than the thermal one. Only during the very final decay phase a single thermal model fits the observed spectrum fairly well.

One dimensional aperture synthesis observations at 2.8 cm of the brightness distribution over the solar equator

The brightness distribution of the equatorial region of the Sun has been investigated at 2.8 cm with an east-west resolution of 16.1″, for the following days: 30 June, 1 July, 3 July and 4 July, 1972. The results confirm the existence of very intense cores inside active regions with typical sizes of the order of 10–30″ and brightness temperatures in the range of 105 K, with possible peaks up to 6 × 106 K. The relationship of these features to the Hα structure is also discussed.

IBIS Observations of Quiet Sun Photosphere - Velocity Structure from Fe I 7090.4 Å

In our contribution we introduce the new Interferometric BIdimensional Spectrometer (IBIS) and present the first results on bisector velocities of two dimensional spectral scans in FeI 7090.4 A comparing granules and intergranular regions. Motivation The comparision of spatially resolved solar lines with the results of simulations is a major task, that is complicated by several factors: The need to obtain observations of sufficient spectral purity, combined with the high spatial resolution necessary to resolve the small scales present in the computations, short acquisition times to avoid the evolution of the observed solar structures, an extended field of view to properly distinguish surface structures, and spectral stability for long sequences of data to evaluate the effects of oscillations and obtain proper time-averages. The new IBIS (Interferometric BIdimensional Spectrometer) instrument, installed at the 76 cm Dunn Solar Telescope, Sacramento Peak Observatory, and fed with an adaptive optics corrected beam, is capable of satisfying these constraints in producing data with highest resolution, regarding space, time and wavelengths. IBIS Characteristics IBIS provides a useful wavelength range of 5800 8600 A , with spectral channels in NaD1 5896 A , Fe I 6302 A , Fe I 7090 A , Fe II 7224 A and Ca II 8542 A (2.0 A channel widths in 5800 7500 A, 3.5 A in 7500 8600 A). The spectral resolving power λ/∆λ equals 212000 274000. The maximum radial wavelength shift is 60 90 mA , the wavelength drifts less than 10m/s in 10 h. Parasitic light is 2.4 0.5%. The field of view measures 80” in diameter with an image scale of 0.085” per pixel. The peak transparency is 15 20% and the transmission profile has a FWHM of 20 40 mA . Exposure times are 10 100 ms, while 20 ms are needed for wavelength tuning. The acquisition rate is 4 frames/s with 512×512 pixels, or 2.5 frames/s with 1024×1024 pixels. In October 2004 a polarimeter will be installed at IBIS. See www.arcetri.astro.it/science/solare/IBIS.

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.

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〉.