Astron. Astrophys. 351, 1139–1148 (1999)
ASTRONOMY
AND
ASTROPHYSICS
Temporal variability in the electron density
at the solar transition region
M.E. Pérez1 , J.G. Doyle1 , E. O’Shea2 , and F.P. Keenan2
1
2
Armagh Observatory, College Hill, Armagh BT61 9DG, Ireland (epp,jgd@star.arm.ac.uk)
Department of Pure and Applied Physics, Queen’s University Belfast, Belfast BT7 1NN, Ireland (E.Oshea,F.Keenan@qub.ac.uk)
Received 23 July 1999 / Accepted 6 October 1999
Abstract. The electron density as measured in the transition
region of a coronal hole, a ‘quiet’ Sun region at disk center plus
an active region shows variations of up to a factor of two at
Te ∼ 1.5 105 K, lasting at most only a few minutes. There is
remarkable agreement between the number of such variations,
their temporal variability and duration in the coronal hole and
‘quiet’ Sun datasets, consistent with an earlier bright point study.
There appears to be evidence of super-granular cells, with the increases in electron density occurring along the network boundaries. At some locations, periodicities of between 8 and 16 min
are visible in the electron density variations. We associate these
variations with the sites of explosive events.
Key words: line: profiles – Sun: activity – Sun: corona – Sun:
transition region – Sun: UV radiation
The Solar Ultraviolet Measurements of Emitted Radiation
(SUMER) instrument (Wilhelm et al. 1997) on SOHO provides
the opportunity to observe the solar atmosphere in the spectral
range from ∼500 to 1600 Å with high spectral and spatial resolution. In first order, the spectral resolution is ∼43mÅ, while
∼22mÅ is achieved in second order. The spatial resolution is approximately 1 arc sec in the E-W direction and 2 arc sec along the
slit (N-S direction). It should be pointed out that only lines separated by less than 40 Å in first order, and 20 Å in second order,
can be observed simultaneously with SUMER due to the size of
the CCD. Surprisingly few lines can be used for density diagnostics, due to blending problems, the weakness of some lines,
and the fact that possible useful lines cannot be observed simultaneously. The line pair most useful for diagnosing the transition
region is probably O iv 1399.8/1401.2 (Wikstol et al. 1997).
2. Theoretical line ratios
1. Introduction
There are abundant references to solar electron density (Ne )
diagnostics in the literature, with e.g. emission lines arising
from transitions in O iv providing accurate determinations of Ne
(Griffiths et al. 1999, Doschek et al. 1998, O’Shea et al. 1998,
Spadaro et al. 1994, Dwivedi & Gupta 1991, Hayes & Shine
1987, Feldman & Doschek 1978). For instance, Hayes & Shine
(1987) used the ratio of Si iv 1402.8 Å and O iv 1401.2 Å, and
found that short-lived bursts typically showed electron density
increases coupled with a small line shift to the red. They suggested this might be caused by ‘ micro-flares ’. Cheng (1980),
analysing coronal loops in Fe xv & Fe xvi lines, found a density enhancement of ∼ 30% in a loop within 7 minutes, plus a
slower variation over a longer time interval. He suggested that
this increase in density could be due to mass ejection from lower
regions, and the associated dissipation of the electric current
associated with the resulting high-density twisted flux strands
(Nakagawa & Stenflo 1979) contributing to the coronal heating.
In this paper we use the O iv 2s2 2p2 P o → 2s2p4 P densitysensitive multiplet around 1400 Å to analyse time-series solar
spectra. More precisely, we use the O iv 1399.8 Å and 1401.2 Å
lines for our analysis.
Send offprint requests to: M.E. Pérez
The model ion for O iv consisted of the 8 energetically lowest
LS states, namely 2s2 2p 2 P; 2s2p2 4 P, 2 D, 2 S, 2 P; 2p3 4 S, 2 D
and 2 P, making a total of 15 fine-structure levels. Energies for
all of these were obtained from Safronova et al. (1996).
Electron impact excitation rates for transitions in O iv were
taken from Zhang et al. (1994). For Einstein A-coefficients, the
calculations of Nussbaumer & Storey (1982), Brage et al. (1996)
and Dankworth & Trefftz (1978) were adopted for the 2s2 2p
2
P1/2 – 2s2 2p 2 P3/2 , 2s2 2p – 2s2p2 and 2s2p2 – 2p3 transitions,
respectively. As noted by, for example Seaton (1964), excitation
by protons may be important for fine-structure transitions. In
the present analysis we have used the proton rates of Foster et
al. (1996, 1997) for transitions within 2s2 2p 2 P and 2s2p2 4 P,
respectively.
Using the atomic data discussed above in conjunction with
the statistical equilibrium code of Dufton (1977), relative O iv
level populations and hence emission line strengths were calculated for a range of electron temperatures and densities. Details
of the procedures involved and approximations made may be
found in Dufton (1977) and Dufton et al. (1978).
In Fig. 1 we plot the theoretical ratio R = I(1399.8 Å)/
I(1401.2 Å) as a function of electron density at the electron temperature of maximum O iv fractional abundance in ionisation
equilibrium, log Te = 5.2 (Mazzotta et al. 1998). Given errors
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M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
Table 1. Description of observational data
Date
Start UT
End UT
Pointing: X,Y
Slit hni (arc sec2 )
Location
X Width/Y Width
Exposure time
10 July 1996
07:36:15
08:42:56
(630,-200)
h6i 0.3 × 120
AR
∼7 × 82
20 s
17:09:42
18:16:42
(3,0)
h4i 1.0 × 120
QS
∼10 × 112
20 s
14 July 1996
22:32:46
00:00:09
(3,0)
h4i 1.0 × 120
QS
∼10 × 85
20 s
Fig. 1. Theoretical O iv I(1399.8 Å)/I(1401.2 Å) line ratios, calculated
at an electron temperature of log Te = 5.2. The present results are shown
with a continuous line, and those from the CHIANTI database with a
dashed line.
of typically ±10% in the adopted atomic data (see references
above), we would expect the theoretical values of R to be accurate to better than ±15%. We note that R is very insensitive
to variations in the adopted electron temperature. For example,
varying Te by 0.2 dex leads to a <1% change in the theoretical
R ratio.
Also shown for comparison in Fig. 1 are the values of R
obtained from the CHIANTI database (Landi et al. 1999). An
inspection of the figure shows that the current diagnostic calculations are quite similar to those from CHIANTI. We therefore
adopt the former in the present analysis, but note that use of the
CHIANTI ratios would not significantly affect our results nor
discussion.
3. SUMER observations and data reduction
3.1. Data
The data used here were obtained with SUMER on-board SOHO
on 10 and 14 July 1996 (see Table 1). These datasets were taken
in order to look for variations in electron density in the solar
transition region, using the density sensitive line ratio of O iv
1399/1401. The pointing for our observations were centered on
different regions in the Sun: one extended active region (AR),
two ‘quiet’ Sun regions (henceforth QS1 and QS2) and one
01:07:23
02:14:04
(0,910)
h4i 1.0 × 120
Northern CH
∼1 × 112
20 s
Fig. 2. A SOHO EIT image obtained in Fe xv 284 Å on 14 July 1996 at
01:30 (courtesy of the EIT consortium). The SUMER temporal series
for O iv were centered 910 arc sec from disk center, i.e., in the Northern
CH shown in this zoom image.
region in the Northern coronal hole (CH). We used slit number
six for the AR dataset (0.3 × 120 arc sec2 ) and slit number four
for the other datasets (1.0 × 120 arc sec2 ). All the datasets were
taken with a 20 s exposure time, and each region was observed
over a period of approximately one hour and seven minutes.
These observations were taken in a sit-and-stare mode with the
rotational compensation turned off. This meant that for the CH
an area of approximately 1.5×120 arc sec2 was observed, since
the rotational velocity in this region of the Sun is very low (∼1.5
arc sec in 67 minutes, see Fig. 2)1 . An area of 10 × 120 arc sec2
was covered over the observation period for the QS datasets at
disk centre, and ∼7×120 arc sec2 for the AR dataset, (see Fig. 3
& Fig. 4).
Detector A was used for all the datasets and the observations
were taken in first order. Due to very low signal-to-noise or
problems with detector sensitivity at the ends of the slit image,
some positions at the top and/or the bottom of the slit where
clipped out. For the AR dataset thirty positions at the Northern
end and four positions at the Southern end were clipped out,
so that the final dimensions are ∼7 × 82 arc sec2 . For the CH
dataset the final dimensions are ∼1×112 arc sec2 after clipping
low signal-to-noise pixels. For the QS the clipping depended on
the dataset, and it was due to low signal-to-noise since the slit is
centered in the detector. Four positions at the Southern end were
clipped out for both datasets, so that for QS1 the dimensions
1
see http://star.arm.ac.uk/∼ambn/preprints.html for color plots
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
1141
value. Nevertheless, areas with measurable densities were found
and they are discussed in Sect. 4.
3.2. Data reduction and calculation of errors
Fig. 3. A SOHO EIT image obtained in Fe xii 195 Å on 10 July 1996
at 20:38 (courtesy of the EIT consortium). The SUMER datasets for
O iv were centered at (3,0) arc sec in the disk center, i.e., in the QS
shown in this zoom image. The SUMER rastered area(s) of ∼10 × 120
are over-plotted with a white rectangle.
Fig. 4. A SOHO EIT image obtained in Fe xii 195 Å on 10 July 1996
at 20:38 (courtesy of the EIT consortium). The SUMER datasets for
O iv were centered at (630,−200) arc sec, i.e., in the AR shown in this
zoom image. The SUMER rastered area of ∼7 × 82 is over-plotted
with a white rectangle.
where reduced to 10 × 112 arc sec2 , but for QS2 the dimensions
where reduced further to 10 × 85 arc sec2 after clipping out
twenty-seven positions in the Northern end of the slit.
The O iv 1401.16 Å line is blended with the S i 1401.51 Å
transition (see Judge et al. 1998, for reference wavelengths), although in most areas in the Sun the S i feature is considerably
weaker than O iv. The S i line was appreciable only in the ‘quiet’
Sun datasets. The O iv 1407/1401 ratio is also available from
our data, but the O iv 1407.38 Å line is blended with the second
order O iii doublet at 703.85 Å, and some preliminary analysis with this ratio showed that unblending the two features was
difficult.
Since the O iv lines we use here are not strong lines we
used a binning in time of four minutes, plus a running mean
along the slit of five pixels, to decrease the noise level of our
data without losing a desirable spatial/time resolution. The low
signal-to-noise of our data in the QS and CH regions made a
reliable estimation of the electron density very difficult for some
positions in our raster/temporal images. This, combined with the
fact that the O iv 1399/1401 density-sensitive ratio is in the low
density limit for a large fraction of the ‘quiet’ Sun and coronal
hole spectra, were the main reasons why for these regions a large
part of our density estimates were set to the minimum theoretical
For the SUMER instrument, the process of data reduction involves three main steps: flat-fielding, de-stretching and radiometric calibration. Our dataset were automatically flat-field corrected on board. The de-stretching process is necessary in particular for the data located towards the edges of the detector due
to various wavelength and spatial distortions (see Siegmund et
al. 1994, Wilhelm et al. 1997). Other non-linearity effects that
ought to be corrected in SUMER are dead-time effects and local
gain depression. Dead-time effects of the detectors become significant for high total detector counts rates, for instance higher
than 50 000 counts s−1 . The local gain depression is critical for
intense lines with more than 10 counts s−1 pixel−1 . Detector
noise is partly reduced by the flat field correction which corrects the readout noise and pixel-to-pixel variations.
The line fitting has been carried out using the CFIT BLOCK
subroutine (Haughan 1997). For all the datasets, only one Gaussian was used to fit either the O iv 1399 Å line or the O iv 1401 Å
line. In the case of O iv 1401 Å, which has the weak line S i
1401.514 Å present in the QS and CH datasets, we checked using two Gaussians but found that the results were more reliable
using only one. For the above corrections the basic IDL routines
can be found from within the SUMER software tree.2
The other source of noise in our data is the photon-related
statistical noise, which obeys a Poisson distribution. Poisson
noise in the data is calculated as the square root of the number
of counts per pixel. For the estimation of the errors that affect
our final results we have to include errors in the line fitting parameters and the propagation of these errors into the line ratio.
Finally, the 1σ uncertainty in the calculated values of the electron density are estimated from the theoretical curve (Fig. 1),
by considering the corresponding 1σ variation in the observed
ratio.
The analysis of periodicities presented in Sect. 4 was carried
out using the PERIODOGRAM.PRO routine given in the CDS
software tree. This routine uses the method of Horne & Baliuna
(1986) to calculate the periodogram.
4. Results
4.1. Coronal hole (CH)
The Northern CH dataset, centered at (0, 910) arc sec, started
at 01:07:23UT and ended at 02:14:04UT 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 ∼1.5 × 112 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 3.6 109 and 2.5 1010 cm−3 , with the average electron density for the whole image being 6.8 109 cm−3
2
sohowww.nascom.nasa.gov/instruments.html
1142
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
Fig. 5. The O iv electron density variations in the CH dataset.
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 1σ error in log Ne is indicated on each plot, at the positions where the minimum and
maximum values for the errors were found.
(log Ne = 9.83+0.20
−0.17 ). 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.
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 1σ error in log Ne 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 Ne 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 Ne , while the
mean errors were approximately two times smaller.
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 5 109 and
2 1010 cm−3 . Between 924.4 to 928.3 arc sec South, a period of 8
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
1143
Fig. 8. Histograms of electron density values for all datasets.
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).
min was again visible in Ne (see Fig. 6). The overall distribution
of electron density values in this dataset can be seen in Fig. 8.
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 07:36:15UT and
ended at 08:42:56UT on 10 July 1996. It had an exposure time
of 20s and covered an area of approximately 7 × 82 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.
The electron density values ranged between log Ne =
+0.23
9.56+0.28
−0.25 and log Ne = 11.45−0.18 . In Fig. 9 the grey scales
were allowed only to range between log Ne =9.8–11.2 (6.3 109 −
1.6 1011 ), in order to show more clearly the variations in den-
sity. The value of the electron density averaged over the total
area observed (i.e. 7×82 arc sec2 ) is approximately 10.52+0.11
−0.10 .
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 (∼ 7 arc sec, E-W direction). This variation
in the electron density along the slit is due to individual features
of ∼ 5–10 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 7 × 16 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 Ne =10.67±0.15
for the region limited in the N-S direction between −233 and
−219 arc sec and log Ne =10.35±0.15 between −202 and −188
arc sec. These values are, within the errors (1σ), similar to the
previously mentioned average electron density over the total
area covered, 10.52+0.11
−0.10 . 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 ∼ 3–4
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-andstare 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.
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
1144
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
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 1.6 1011 cm−3 and black areas are those with values lower than 6.3 109 cm−3 .
Fig. 10. Top panel: Electron density (with 1σ error bars) values corresponding to each position along the slit. These values are the result of an
average over the 1h7min observation period (∼ 7 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 7×82 arc sec2 ,
log Ne = 10.52+0.11
−0.10 .
detail. The intensity values plotted in this figure correspond to
the O iv 1399 Å line. As an indication, the typical 1σ error in
log Ne 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 ∼ 625 and ∼ 625.7 arc sec West, in particular at position
−175 arc sec South (dotted line) we found a logarithmic varia+0.18
tion in density of between 10.54+0.13
−0.16 and 11.02−0.15 , while at
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
1145
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 1σ error in
log Ne is indicated on each plot, at the positions where the minimum and maximum values for the errors were found.
position −176 arc sec South (dashed line) there was a variation
+0.12
of between 10.46+0.13
−0.15 and 10.89−0.12 . Between ∼ 626.7 and
∼ 627.5 arc sec West, the variations were between 10.92+0.12
−0.11
and 11.44+0.30
−0.22 at −173 arc sec South (continuous line). In region B, between ∼ 623.6 and ∼ 624.5 arc sec West, we found
a logarithmic variation in density of between 10.94+0.08
−0.08 and
+0.23
11.45−0.18 at −216 (dotted line) and −217 (dashed line) arc
sec South. For this same area, between ∼ 626.8 and ∼ 627.5
arc sec West, this variation was between the logaritmic values
+0.06
10.34+0.07
−0.08 and 10.76−0.06 at −215 arc sec South (continuous
line). On average we found a variation of a factor of 1.5 in Ne
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.
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 Ne between ∼ 628.6 and
∼ 629.7 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 ∼ 624.2 and ∼ 624.8 arc sec West, between
∼ 626.4 and ∼ 627.1 arc sec West, and between ∼ 628.5 and
∼ 629.2 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).
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M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
ered an area of ∼ 10 × 85 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 Ne =9.56–10.5. The density values ranged between
+0.23
log Ne =9.56+0.28
−0.28 and log Ne =10.90−0.22 . The average electron density for the whole image was 10.06+0.25
−0.25 . This average
was calculated only for the ∼32% density values larger than the
low density limit for this dataset.
In this dataset we found periodicities of ∼ 1.8 and ∼ 2.5 arc
sec (∼12 & 16 min period) around 9 arc sec North and −20 arc
sec South. A period of ∼ 2 arc sec was present in Ne 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 NS direction and another from 0–55 arc sec again in the N-S
direction.
5. Discussion
Fig. 12. O iv 1399 Å corresponding to the AR dataset. This image
results from an average over the whole observational period. The overplotted regions A, B, C and D in this slit image represent areas with
peculiar density variations (see Fig. 11 for further details).
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 17:09:42UT and ended at 18:16:42UT
on 10 July ’96. This dataset covered an area of ∼10 × 112
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
+0.53
between log Ne =9.56+0.28
−0.25 and log Ne =11.09−0.33 . The average electron density for the whole image was 10.05+0.22
−0.23 . This
average was calculated only for the 30% density values over the
minimum density value, log Ne =9.56.
In this dataset we found periodicities of ∼ 2 and ∼ 2.5 arc
sec (∼13 & 16 min period) around 55 arc sec North. We found
a period of ∼1.5 arc sec (∼10 min) in Ne 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 ∼ 0.6 arc sec.
The second dataset, that we called QS2, started at
22:32:46UT and ended at 00:00:09UT on 10 July ’96, and cov-
Datasets taken in a coronal hole, a‘quiet’ Sun region at disk center plus an active region show variations in the electron density
in the transition region over time periods of a few minutes. Such
variations can be as large as a factor of two in ∼5 minutes, but
unfortunately the time resolution of our datasets do not permit
us to detect faster variability. Electron density enhancements
due to spatial structures of 5–10 arc sec are also clearly visible.
In each dataset there are a few locations where the electron density showed periodicities of between 8 and 16 min. There is a
remarkable agreement between the scale and temporal variability in the coronal hole and ‘quiet’ Sun datasets, in agreement
with a bright point study by Habbal et al. (1990) who found
that these were indistinguishable. The above study also showed
that bright point detection had two maxima, one at coronal temperatures and the other at 1–2 105 K, i.e. around the formation
temperature of O iv.
Numerous studies (e.g. the statistical analyses of the HRTS3 mission by Dere et al. 1983 or SOHO Chae et al., 1998, Pérez
et al. 1999), have shown ultraviolet explosive events occurring
in a burst-type manner in the solar transition region. Explosive
events have been connected to magnetic reconnection occurring
on time-scales of minutes over regions with sizes of few arc sec.
The distribution of density increases along the network boundaries, as reported in our present work, is consistent with the
predominant location of explosive events as already observed
by Dere (1991) and recently by Chae et al. (1998). Moreover,
these density enhancements are in good agreement with numerical simulations of explosive events by Sarro et al. (1999), who
found increases of a factor of two or three at these temperatures.
In the CH dataset we calculate a birthrate of 6 10−21 cm−2 s−1
for these density enhancements, in excellent agreement with that
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
1147
Fig. 13. The O iv electron density variations in the QS1 dataset.
Fig. 14. The O iv electron density variations for the QS2 dataset.
derived by Dere et al. (1983) for the explosive event birthrate
as observed in C iv.
Judge et al. (1998) found evidence in support of the
‘ nanoflare ’ picture of coronal heating, that would explain his
observations of predominantly downward-propagating compressive waves in the solar transition region. Judge et al. do
not rule out other heating mechanisms such as resonant absorption of Alfvén waves as described by Ofman et al. (1998).
This mechanism would be consistent as well with a density
structure showing filamentary and closely spaced density enhancements up to a factor of two, varying on a time-scale of
minutes. Thus, in principle, this could also be the cause of the
downward-propagating compressive waves. Nevertheless, other
work by Peter & Judge (1999) and Teriaca et al. (1999) recently
suggested that nano-flares predominantly occurring around the
O vi formation temperature (3 105 K) could account for the redshift observed in the low and middle transition region and for
the blue-shift seen in the upper transition region and coronal
lines. This idea of nano-flares/explosive events occurring in the
high transition region is also in agreement with the present detected electron density enhancements. From this point of view,
the larger range of density values detected for the active region
could be explained in terms of higher frequency of occurrence
and/or energy releases in the active region with respect to the
‘quiet’ Sun or coronal holes. Nevertheless, a preliminary analysis of the line widths variations for the data presented here has
not been conclusive. Further numerical work based on this type
of model is required.
1148
M.E. Pérez et al.: Temporal variability in the electron density at the solar transition region
Acknowledgements. Research at Armagh Observatory is grant-aided
by the Department of Education for N. Ireland while partial support
for software and hardware is provided by the STARLINK Project
which is funded by the UK PPARC. This work was supported by
PPARC grants GR/K43315 and GR/L57449. We would like to thank
the SUMER and EIT teams at Goddard Space Flight Center for
their help in obtaining the data. The SUMER project is financially
supported by DLR, CNES, NASA, and PRODEX. SUMER is
part of SOHO, the Solar and Heliospheric Observatory of ESA and
NASA. MEP is supported via a studentship from Armagh Observatory.
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