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Astron. Astrophys. 342, 563-574 (1999)

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1. Introduction

Achieving high temporal and spatial resolution in solar X-ray observations is one of the main challenges of solar coronal physics. There is evidence that the solar corona is structured down to sub-arcsec dimensions (Gomez et al. 1993). On the other hand, plasma theory applied to the corona suggests that the magnetized plasma is subject of significant filamentation (van den Oord 1992, Litwin & Rosner 1993, Velli 1995) Such a filamentation has also been invoked as a means to generate and distribute efficiently heat released by instabilities in magnetized plasma (Parker 1988).

The SAO Normal Incidence X-ray Telescope (NIXT, Golub & Herant 1989) represents one of the fundamental steps toward obtaining the sub-arcsec spatial resolution required to observe the very small scales. For an exhaustive analysis and interpretation of the observations made by the NIXT it is important to obtain information about the physical conditions of the emitting plasma. Such a task is not straightforward with NIXT, since its single and narrow spectral band practically inhibits the application of temperature diagnostic methods, and in particular the filter ratio method used by wide band telescopes such as the Soft X-ray Telescope (SXT) on board the Yohkoh satellite.

This work is devoted to analyze and interpret the observation of selected coronal loop structures made simultaneously by NIXT and Yohkoh/SXT on April 12, 1993 (see also Yoshida et al. 1995), in order to obtain a detailed and accurate scenario taking advantage of the different insight provided by the two instruments.

Our analysis is enriched by the determination of the pressure of the plasma emitting in the NIXT band, made possible by the application of a method developed previously by Peres et al. (1994, hereafter Paper I). This method is based on extensive loop modeling and allows us to evaluate the plasma pressure directly from the analysis of the NIXT brightness distribution along the loops.

Our models show that the pressure, in a hydrostatic loop of given length, determines the temperature distribution along the loop of the confined plasma. If the plasma is observed through a narrow spectral band, such as the NIXT band, including very few spectral lines with specific formation temperatures, the brightness distribution along the loop will be strongly influenced by the plasma temperature.

On the basis of the loop emission in the NIXT pass-band, synthesized from the results of hydrostatic loop models, it has been shown in Paper I that the shape of the brightness distribution changes with pressure, yielding bright footpoints in high pressure loops and a more uniform brightness in lower pressure loops. Here we present the first quantitative application of this pressure diagnostics, which can be applied, after proper calibration, to any imaging instrument with a narrow pass-band around [FORMULA] K.

As an implication of the pressure determination, by comparing the observed NIXT loop total luminosity to that predicted from hydrostatic models, we are able to estimate the volume filling factor of the plasma emitting in the NIXT band and the level of filamentation in the observed loops.

In order to obtain a scenario as complete as possible we perform the same kind of analysis on Yohkoh/SXT data, for which direct temperature diagnostics are possible and thereby obtain information complementary to that of NIXT.

NIXT and Yohkoh/SXT are characterized by different optics and detectors, which result in significant differences in their response to the plasma temperature. NIXT is based on multi-layer normal-incidence mirrors which yield high reflectivity and a high spatial resolution (1.2") in a narrow spectral band centered on 63.5 Å and 1.4 Å wide (Golub et al. 1990). This band contains the intense MgX at 63.5 Å and Fe XVI at 63.7 Å lines, which determine a maximum instrument sensitivity to plasma radiating between 1 and 3 MK. Yohkoh/SXT (Tsuneta et al. 1991) instead is based on grazing-incidence optics, has a lower spatial resolution (2.5"), a much wider spectral band, with the possibility of using various filters, and is more sensitive to higher temperatures (roughly speaking above 2.5 MK).

Given the sensitivity to plasma at different temperatures, we expect a priori significant differences in the detectable plasma and therefore both in the morphological appearance of the coronal structure and in the physical conditions of the detected plasma. Indeed, the sensitivity to different temperatures has been invoked by Yoshida et al. (1995) to explain the significantly different appearance of some coronal structures, different from those selected by us, detected in the two simultaneous observations made on April 12, 1993.

Our analysis of the two observations, and their combination and comparison, allow us to go beyond the simple confirmation that the two instruments look at plasmas in different thermal conditions, and provide us with a very rich scenario of the loop structuring and conditions, as we will illustrate in the following.

In Sect. 2, we select and describe the loop structures as observed with NIXT and Yohkoh; in Sect. 3 we analyze the NIXT data, and evaluate the plasma pressure and the volume filling factor; in Sect. 4 we analyze the corresponding structures in SXT images. In Sect. 5 we discuss the results and draw our conclusions.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999
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