Astron. Astrophys. 351, 1139-1148 (1999)

4. Results

4.1. Coronal hole (CH)

The Northern CH dataset, centered at (0, 910) arc sec, started at 010723UT and ended at 021404UT on 14 July 1996 and had an exposure time of 20s. Since our image is in fact a temporal series for the observational period (1h7min), the total area covered by this dataset was arc sec². The variations of the electron density values for each position along the slit with time is shown in Fig. 5. The allowed range of values for the grey scale is between and cm^-3, with the average electron density for the whole image being cm^-3 (log N_e = ). This average was calculated only for the 37% density values greater than the low density limit. The data in Fig. 5 clearly show several individual density enhancements which are temporal in nature, lasting only a few minutes.

Fig. 5. The O IV electron density variations in the CH dataset.

In Fig. 6, we show the electron density and intensity variations for the O IV 1399 Å line as a function of time for two regions indicated in Fig. 7. As an indication, the typical 1 error in is indicated on each plot, at the positions where the minimum and maximum values for the errors were found. In region A (from 928 to 924 arc sec North), on average we find variations in N_e between consecutive points in the E-W direction comparable to the mean errors in the derived electron density, and in many cases exceeding them. Something similar can be seen in region B (from 919 to 915 arc sec North), where we found variations of up to a factor of three in N_e, while the mean errors were approximately two times smaller.

Fig. 6. Electron density (left panel) and intensity (right panel) values for the CH dataset. These are the values corresponding to locations A and B in the slit image represented in Fig. 7. The typical 1 error in is indicated on each plot, at the positions where the minimum and maximum values for the errors were found.

Fig. 7. A CH image in O IV  1399 Å resulting from the average over the whole observational period. The overploted regions A and B represent areas with large electron density variations (see Fig. 6 for further details).

When checking for some kind of periodicity in the areas of our slit where the electron density could be estimated, we found evidence for periods of 8 and 16 min. For example, in the region between 914.6 to 918.5 arc sec South, we found a period at 8 min and as well as a longer period at 16 min. In this region the electron density ranged between and cm^-3. Between 924.4 to 928.3 arc sec South, a period of 8 min was again visible in N_e (see Fig. 6). The overall distribution of electron density values in this dataset can be seen in Fig. 8.

Fig. 8. Histograms of electron density values for all datasets.

4.2. Active region (AR)

This dataset centered in the active region NOAA 7978, at (630,-200) arc sec on the solar disk started at 073615UT and ended at 084256UT on 10 July 1996. It had an exposure time of 20s and covered an area of approximately arc sec². The variations of the electron density for each position along the slit in the E-W direction is shown in Fig. 9. Here we can see a persistent pattern of variations all along the slit as well as in the E-W direction. The variations along the slit (N-S) are similar in size to those seen along the slit in the CH dataset, i.e. 4-5 arc sec seperated by 10-15 arc sec. In the E-W direction, the variations in Fig. 9 are remarkely similar to those in Fig. 5 despite the fact that it covers a region of 7 arc sec. This therefore tends to indicate that these are mostly temporal in nature rather than 1 arc sec size structures, although such small scale structures probably also exist within Fig. 9.

Fig. 9. The electron density (in cm-3) as derived from O IV for the AR dataset. Saturated areas are those where the electron density is higher than cm-3 and black areas are those with values lower than cm-3.

The electron density values ranged between log N_e= and log N_e=. In Fig. 9 the grey scales were allowed only to range between log N_e9.8-11.2 (), in order to show more clearly the variations in density. The value of the electron density averaged over the total area observed (i.e. arc sec²) is approximately . The distribution of electron density values in this dataset can be seen in Fig. 8, these being significantly larger than those found in the CH.

In Fig. 10 (upper panel) we can see a distinctive variation in the electron density corresponding to each position along the slit. These values are the result of averaging over the 1h7min observation period ( arc sec, E-W direction). This variation in the electron density along the slit is due to individual features of arc sec size. In Fig. 10 (lower panel) we show five different plots together, corresponding to five consecutive regions of 16 arc sec along the slit, in the N-S direction. In each of these plots we show the electron density, averaged over these 16 arc sec (N-S), for each scan position along the E-W direction. For each of these regions of arc sec² we tabulate the average electron density, limited in the E-W direction between 623 to 630 arc sec. The values ranged between log N_e10.670.15 for the region limited in the N-S direction between -233 and -219 arc sec and log N_e10.350.15 between -202 and -188 arc sec. These values are, within the errors (1), similar to the previously mentioned average electron density over the total area covered, . Moreover, the density variations are similar along the slit and the rastered E-W direction for each of these small regions. No spatial variations smaller than arc sec (30 min) are present here. The long time-scale variations present in Fig. 10 (lower and upper panel) are probably due to arc sec scale features passing through during the sit-and-stare nature of the dataset and are distinct from the shorter scale variations mentioned above, that can be seen in Fig. 11. These shorter scale variations (in the E-W direction) seem to be related to the similar temporal variations found in the CH dataset, with periods of approximately 8 and 16 min.

Fig. 10. Top panel: Electron density (with error bars) values corresponding to each position along the slit. These values are the result of an average over the 1h7min observation period ( arc sec, E-W direction). Bottom panel. Density values corresponding to each scan position along the E-W direction for the AR dataset. These values are the result of an average over 16 arc sec, with the exact position range in the N-S direction specified in the top, in each of the small frames. Dashed line: electron density averaged over the total covered area of 782 arc sec2, log Ne = .

Fig. 11. Some of the electron density and intensity values for the AR dataset. These are the values corresponding to locations A, B, C and D in the slit image represented in Fig. 12. The exact range in arc sec is given here in brackets for each of these locations. The typical 1 error in is indicated on each plot, at the positions where the minimum and maximum values for the errors were found.

In Fig. 11 we locate some representative sections along the slit image (plotted as A, B, C & D, see Fig. 12) whose density and line intensity variations in time are shown in more detail. The intensity values plotted in this figure correspond to the O IV 1399 Å line. As an indication, the typical 1 error in is indicated on each plot, at the positions where the minimum and maximum values for the errors were found. Regions A (from -173 to -177 arc sec South) and B (from -215 to -219 arc sec South) correspond to the higher density areas in Fig. 9. In region A we found the biggest density variations between and arc sec West, in particular at position -175 arc sec South (dotted line) we found a logarithmic variation in density of between and , while at position -176 arc sec South (dashed line) there was a variation of between and . Between and arc sec West, the variations were between and at -173 arc sec South (continuous line). In region B, between and arc sec West, we found a logarithmic variation in density of between and at -216 (dotted line) and -217 (dashed line) arc sec South. For this same area, between and arc sec West, this variation was between the logaritmic values and at -215 arc sec South (continuous line). On average we found a variation of a factor of 1.5 in N_e between consecutive positions E-W, that is over 0.44 arc sec, while the mean errors in the electron density were a factor of two less.

Fig. 12. O IV  1399 Å corresponding to the AR dataset. This image results from an average over the whole observational period. The over-plotted regions A, B, C and D in this slit image represent areas with peculiar density variations (see Fig. 11 for further details).

In region C (from -227 to -230 arc sec South), on average we found a variation of a factor of 1.3 between consecutive positions E-W. For instance, we found from -227 to -230 arc sec South variations of a factor of two in N_e between and arc sec West, i.e. within 1 arc sec, thus suggesting that these are temporal in nature. In region D (from -242 to -246 arc sec South), we find three areas in the E-W direction with variations in the electron density greater than a factor of two; namely between and arc sec West, between and arc sec West, and between and arc sec West. Again, these are temporal in nature due to the small area covered in the E-W direction.

When checking for some kind of periodicity we found that, while for some regions along the slit there was no appreciable periodicity, for others there was evidence for approximately 0.8, 1.1 and 1.6 arc sec periodicities which corresponds to 8, 11 and 16 min period. Our binning on the E-W direction was 4 min which corresponds to 0.4 arc sec. The longer periods appear mainly in the northern half of the image, that is the less intense part of the AR, although the electron densities are higher in this region. These were in areas of five arc sec (the running mean for this analysis) at around -195 and -177 arc sec South. From -221 to -230.5 arc sec South there was evidence for periodicities of 0.8 arc sec (8 min) and 1.1 arc sec (11 min) in the density, that extended to approximately -245 arc sec (see also Fig. 11).

4.3. `Quiet' Sun (QS1 & QS2)

Like the CH dataset, the low signal-to-noise in the `quiet' Sun datasets make it very difficult to obtain a reliable estimate for some positions in our raster/temporal image. In fact, a large fraction of the area is in the low density limit. From Fig. 8, we can see that the overall distribution of electron densities is intermediate between that of the CH and AR.

Both datasets were centered at (3, 0) arc sec, i.e. disk center. The first, QS1, started at 170942UT and ended at 181642UT on 10 July '96. This dataset covered an area of arc sec². The corresponding variations of the electron density values for each position along the slit with position along the E-W direction is shown in Fig. 13. The density values ranged between log N_e and log N_e. The average electron density for the whole image was . This average was calculated only for the 30% density values over the minimum density value, log N_e = 9.56.

Fig. 13. The O IV electron density variations in the QS1 dataset.

In this dataset we found periodicities of and arc sec (13 & 16 min period) around 55 arc sec North. We found a period of 1.5 arc sec (10 min) in N_e in an area of five arc sec (running mean) around 49 arc sec North. Our binning on the E-W direction was 4 min which corresponds to arc sec.

The second dataset, that we called QS2, started at 223246UT and ended at 000009UT on 10 July '96, and covered an area of arc sec². The variations of the electron density values for each position is shown in Fig. 14. The allowed range of values for the grey scale is the same as in Fig. 13, log N_e9.56-10.5. The density values ranged between log N_e and log N_e. The average electron density for the whole image was . This average was calculated only for the 32% density values larger than the low density limit for this dataset.

Fig. 14. The O IV electron density variations for the QS2 dataset.

In this dataset we found periodicities of and arc sec (12 & 16 min period) around 9 arc sec North and -20 arc sec South. A period of arc sec was present in N_e between -31 and -35 arc sec South. Our binning on the E-W direction was 4 min which corresponds to 0.6 arc sec. Again, some of the variations in Figs. 13 & 14 are temporal in nature due to their sub arc sec variability while others could be spatial in origin.

For both datasets (see Figs. 13 & 14) there appears to be evidence of super-granular cells, with the increases in electron density occurring along the network boundaries. For example in Fig. 13 there is one from -40 arc sec to 0 arc sec in the N-S direction and another from 0-55 arc sec again in the N-S direction.

© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
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