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Astron. Astrophys. 348, 261-270 (1999)

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2. Observations

2.1. EUV observations

The EUV spectra in the coronal hole were taken with the Coronal Diagnostic Spectrometer (CDS) on board of the SOHO satellite. The CDS is a spectrograph aimed to produce spectra of selected regions of the solar surface in six spectral windows of the extreme ultraviolet from 150 Å to 785 Å (Harrison et al. 1995). CDS is composed of two different instruments, namely the Grazing Incidence Spectrometer (GIS) and the Normal Incidence Spectrometer (NIS).

In the present work we have used only the data of the NIS instrument. The NIS operates in two spectral windows covering the 307-379 Å and 513-633 Å spectral range. Of the whole NIS spectral band, only selected portions have been observed, thus reducing the total observing time. The selected spectral windows allow to detect lines formed in a broad range of temperatures, from 104 K to 106 K, thus covering the whole range of temperature between chromosphere and corona. Also, the presence of density sensitive line pairs of [FORMULA] and [FORMULA] allows to measure the electron density of the emitting plasma at transition region and coronal temperatures.

The data files are s5368r00 and s5369r00: the field of view of the s5368r00 file is located north of the other one, along the solar meridian, with only a partial overlapping. The field of view of each file is [FORMULA], giving a total field of view of 122[FORMULA] [FORMULA] 450[FORMULA], covering the central portion of the coronal hole.

The raw data have been cleaned from cosmic rays and calibrated using the standard routines available as part of the CDS software. A combined map of the observed region from the two data files at the wavelength of the [FORMULA] 624.94 Å line, one of the hottest in the present dataset (the temperature of formation is [FORMULA] K) is displayed in Fig. 1. The structure of the coronal hole is evident at the center of the image as a dark lane sorrounded by the brighter quiet Sun plasma. The bright region visible at the bottom is the bright point visible also in in the EIT image shown in Fig. 2.

[FIGURE] Fig. 1. Map of the coronal hole as seen in the [FORMULA] 624.94 Å line. The contour plot indicates the portion of the image considered in the present study.

[FIGURE] Fig. 2. Contours of radio brightness temperature at 410 MHz superimposed to the EIT image taken in the 195 Å filter at 21:00 UT; the small disalignement between the two images is due to the time difference between the observations ([FORMULA]).

does The hole area has been identified with the region where [FORMULA] phot cm-2 s- 1 arcsec-2 and the intensity of each line has been averaged within this area. The contour plot of the selected area is marked in Fig. 1.

The observed lines are listed in Table 1, together with their average intensity, [FORMULA], measured in units of phot cm-2 s- 1 arcsec-2, their uncertainty [FORMULA] and the temperature of formation [FORMULA] defined, consistently with Landi & Landini 1997, as:



Table 1. EUV spectral line intensities observed by CDS. Intensities are in phot cm-2 s- 1 arcsec-2. [FORMULA] represents the experimental uncertainty.

2.2. Radio observations

The Nançay Radioheliograph (NRH) consists of 19 antennas with a minimum baseline of 50 m in the E-W direction and 24 antennas with a minimum baseline of 54.3 m in the N-S direction. The maximum baselines are 3200 m and 1248 m respectively (Kerdraon & Delouis 1997). This instrument supplies two dimensional maps of the Sun at five frequencies between 150 and 450 MHz. The frequencies at which the coronal hole of October 19, 1996 was observed are: 164 MHz, 236 MHz, 327 MHz and 410 MHz.

For details on the instrument performances and on the calibration techniques used see Kerdraon & Delouis 1997.

The time resolution of the NRH can be as high as one images every 0.1 seconds at each frequency and the angular resolution in the N-S and in the E-W directions depends, besides frequency, on the period of the year; moreover, during the day, the beam pattern changes size and rotates in the sky.

Since coronal holes are very stable structures, we have averaged the observations over several hours around the time when the major axis of the beam was roughly parallel to the axis of the southern portion of the coronal hole: this minimizes the contribution of surrounding regions to the hole brightness temperature. The considered radio brightness temperature will be in fact averaged over this wider portion of the coronal hole.

In Table 2 are reported, for the time of our observations, the major and minor axis (M.A. and m.A. respectively) of the ellipse representing the antenna beam at each frequency.


Table 2. Antenna beam width.

In Fig. 2 the radio contour plots at 410 MHz are overlaid on the EIT image of [FORMULA] line at 195 Å: the shape of the hole in the two ranges of wavelengths is very similar and the small disalignement is due to the time difference ([FORMULA]) between the two observations.

The average width of the hole, between points where [FORMULA] on the EIT image shown in Fig. 2 are: [FORMULA] in the northern part and [FORMULA] in the southern one, where the hole axis is tilted of about [FORMULA] with respect to the N-S direction.

If we compare the width of the hole with the minor axis of the beam area shown in Table 2, we find that the northern portion of the hole is narrower than the beam at all frequencies except at 410 MHz. In the southern part, where the hole is larger, its width is still smaller than the beam at 164 MHz, it has about the same size of the beam at 236 MHz and is definitly larger at 327 and 410 MHz. At these two latter frequencies two minima are clearly seen in the radio maps separated by a region of higher [FORMULA] which corresponds to the bright point in the EIT images, very well visible in Fig. 1. At 236 and 164 MHz only one minimum is observed.

The [FORMULA] estimate in the hole has been therefore performed in the following way: we have first traced on each map several E-W scans across the disk: at all frequencies the brightness temperature shows a more or less pronounced minimum in the scans crossing the southern part of the hole. According to what previously said, at the lowest frequency, [FORMULA] MHz, we have considered the deepest of these minima as an upper limit of the [FORMULA] in the hole. At the two highest frequencies, where the southern part of the hole is resolved, we have calculated the average value of the minimum temperature of all scans across this part except that corresponding to the bright point. Finally, at 236 MHz, we have measured both the lowest minimum and the average one. The [FORMULA] values obtained in this way are plotted in Figs. 4 and 5.

The reliability of the [FORMULA] measurement at 164 MHz has been checked in the following way (Delouis, private comunication). A rectangular model of the hole has been assumed taking its width from the 410 MHz map and leaving the brightness temperature [FORMULA] at the bottom as a free parameter. This model has been convolved with the 164 MHz beam and the bottom [FORMULA] value which best reproduced the 164 MHz observations has been evaluated, obtaining [FORMULA] K. This value must be compared with the upper limit [FORMULA] K previously determined.

In general the calibration techniques described in Kerdraon & Delouis 1997 and the method for [FORMULA] determination used in the present work allow to measure radio brightness temperatures with an accuracy better than [FORMULA] K.

It must be pointed out that the radio brightness temperature has been averaged over the whole southern portion of the hole, mostly located south-east of the bright point, while EUV line intensities have been averaged only over the portion located north of the bright point (see Fig. 1). However a comparison between the two minima in the 410 MHz map, shown in Fig. 2 (the frequency where the maximum resolution is achieved) indicates that the difference of the two [FORMULA] is of the order of 15%, the southern minimum being deeper that the northern one.

The real difference between the two values is probably even smaller due to a possible contribution of the surrounding regions to the northern portion of the coronal hole.

The set of data obtained with this procedure are rather different from those previously presented by Chiuderi Drago et al. 1999, due to a better accuracy in handling the radio data.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999