11institutetext: Università di Napoli “Federico II”, C.U. Monte Sant’Angelo,
Via Cinthia, I-80126 Naples, Italy
11email<EMAIL_ADDRESS>22institutetext: INAF - Osservatorio Astronomico
di Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy 33institutetext:
DAMTP, University of Cambridge, Wilberforce Rd, Cambridge CB3 0WA, UK

# Dark Halos around Solar Active Regions. I. Emission properties of the Dark
Halo around NOAA 12706.

S.M. Lezzi  0000-0002-8506-1819 1122 V. Andretta  0000-0003-1962-9741 11 M.
Murabito  0000-0002-0144-2252 11 G. Del Zanna  0000-0002-4125-0204 33

(Accepted 19 September 2023)

###### Abstract

Context. Dark areas around active regions (ARs) have been first observed in
chromospheric lines more than a century ago and are now associated to the
H$\alpha$ fibril vortex around ARs. Nowadays, large areas surrounding ARs with
reduced emission relative to the Quiet Sun (QS) are also observed in spectral
lines emitted in the transition-region (TR) and low-corona. For example, they
are clearly seen in the SDO/AIA 171 Å images. We name these chromospheric and
TR/coronal dark regions as Dark Halos (DHs). Coronal DHs are poorly studied
and, because their origin is still unknown, to date it is not clear if they
are related to the chromospheric fibrillar ones. Furthermore, they are often
mistaken for Coronal Holes (CHs).

Aims. Our goal is to characterize the emission properties of a DH by
combining, for the first time, chromospheric, TR and coronal observations in
order to provide observational constraints for future studies on the origin of
DHs. This study also aims at investigating the different properties of DHs and
CHs and at providing a quick-look recipe to distinguish between them.

Methods. We study the DH around AR NOAA 12706 and the southern CH, that were
on the disk on 2018 April 22, by analyzing IRIS full-disk mosaics and SDO/AIA
filtergrams to evaluate their average intensities normalized to the QS. In
addition, we use the AIA images to derive the DH and CH emission measure (EM)
and the IRIS Si IV 1393.7 Å line to estimate the non-thermal velocities of
plasma in the TR. We also employ SDO/HMI magnetograms to study the average
magnetic field strength inside the DH and the CH.

Results. Fibrils are observed all around the AR core in the chromospheric Mg
II h&k IRIS mosaics, most clearly in the h3 and k3 features. The TR emission
in the DH is much lower compared to QS area, unlike in the CH. Moreover, the
DH is much more extended in the low-corona than in the chromospheric Mg II h3
and k3 images. Finally, the intensities, EM, spectral profile, non-thermal
velocity and average magnetic field strength measurements clearly show that
DHs and CHs exhibit different characteristics and therefore should be
considered as distinct types of structures on the Sun.

###### Key Words.:

Sun: atmosphere – Sun: chromosphere – Sun: corona – Sun: transition region –
Sun: UV radiation

##  1 Introduction

Figure 1: From left to right: IRIS Mg II h core full-disk mosaic, AIA 171 Å
and 193 Å images for 13 April 2019 (top panels) and 05 May 2019 (bottom
panels), when the ARs NOAA 12738 and 12740 respectively were on the disk. This
two examples show the appearance of the DHs at the disk center and at the
solar limb, respectively. DHs are dark areas best seen in TR/low-corona lines,
such as Fe IX in the AIA 171 Å waveband. In the chromosphere, such as in Mg II
h&k line cores, in correspondence of their inner part fibrils are often
observed.

Active Regions (hereafter: ARs) on the Sun are typically surrounded by areas
that in some wavelengths appear darker than the average Quiet Sun (hereafter:
QS). They were first identified in the chromosphere as faint dark areas
surrounding plages in the Ca II K-line (Hale & Ellerman 1903; St. John 1911),
and later also in the Ca II 8542 Å line (D’Azumbuja, 1930). Deslanders (1930)
called them “circumfacules”. (1965) found a spatial correlation between Ca II
K-line circumfacules and the H$\alpha$ fibril vortex around ARs and proposed
that the Ca II circumfacular regions are composed of dark H$\alpha$ fibrils.
This suggestion was supported by many authors over the years (e.g. Veeder &
Zirin 1970; Foukal 1971a; Foukal 1971b; Harvey 2006; Rutten et al. 2007;
Cauzzi 2008; Reardon et al. 2009; Pietarila et al. 2009).

Today, thanks to ultraviolet (UV) and extreme-ultraviolet (EUV) observations,
it is possible to observe regions around ARs of reduced emission also in a
wide range of spectral lines originating in Transition Region (hereafter: TR)
and low corona. For instance, areas of reduced emission around ARs can be seen
in the Solar Ultraviolet Measurements of Emitted Radiation (SUMER; Wilhelm al.
1995) O V at 630 Å and S VI at 933 Å images111See, for instance, 07 June 1996
and 12 May 1996 images on SUMER atlas at
https://ads.harvard.edu/books/2003isua.book/., as pointed out by Andretta &
Del Zanna (2014). These authors refer to such structures as “dark halos”
(hereafter: DHs) as they were extended dark areas as seen in full-Sun SOHO/CDS
transition-region lines. We adopt this name in this work from now on.

DHs are also clearly visible in the 171 Å images taken by the Atmospheric
Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics
Observatory (SDO; Pesnell et al. 2012), as noted by Wang et al. (2011), who
instead name them “dark canopies”. These latter authors studied the
relationship between the 171 Å DHs and the evolving photospheric field under
the assumption that the 171 Å DH is the counterpart of the H$\alpha$ fibrils,
i.e. consists of EUV-absorbing chromospheric material (neutral hydrogen or
helium) tracing out horizontal magnetic fields. However, as already pointed
out by Andretta & Del Zanna (2014), inspection of DHs using SUMER observations
reveals that these regions are seen darker even in lines at wavelengths longer
than the edge of the Lyman continuum at 912 Å. Therefore, the interpretation
based on absorption by neutral hydrogen cannot fully explain their low
emission.
On the other hand, more recently, Singh et al. (2021) argued that the
existence of DHs, that they call instead “dark moats”, is related to the
magnetic pressure coming from the strong magnetic field that splays out from
the AR, pressing down and flattening to a low altitude the underlying magnetic
loops. Those loops, which would normally emit the bulk of the 171 Å emission,
are restricted to heights above the surface that are too low to have 171 Å
emitting plasma sustained in them, according to Antiochos & Noci (1986).
According to this model, these flattened loops are expected to have
temperatures less than $\sim$5 $\times$ 104 K. Therefore they should emit into
the emission passband of the 304 Å and appear bright in 304 Å images. However,
DHs are generally visible as darker areas in 304 Å images by visual
inspection, which means that this model is not entirely consistent with
observations.

Figure 2: IRIS Mg II h3 full-disk mosaic and AIA full-disk images showing the
DH surrounding AR NOAA 12706 in the northern hemisphere on 2018 April 22. The
three images are center-to-limb corrected. Left panel: the three fibrillar DH
contours are shown in shades of green and the outer AR-core edge is shown red.
Middle and right panels: AIA 171 and 193 Å filtergrams, respectively, taken at
19:24 UT. The three coronal DH contours are shown in shades of violet in the
AIA 171 Å image, while the QS box and the three southern CH contours are shown
in white and in shades of blue, respectively, in the AIA 193 Å image, as
described in Section 3. The contours are obtained through intensity thresholds
from the non center-to-limb corrected images.

Consequently, at the moment the physical mechanism responsible for the DH
reduced emission remains poorly understood. Moreover, it is unclear whether
the coronal EUV-observed DH is related to the chromospheric fibrillar one, as
there are no previous studies analyzing them with chromospheric, TR and
coronal observations together. DHs are very common solar features visible
around ARs independently from the solar cycle and at all distances from the
disk center, as shown for instance in Fig. 1. DH could sometimes be mistaken
for filament channels, which can be usually observed in the H$\alpha$ images
(Makarov et al. 1982). We have verified that for the ARs shown in Fig. 1 and
for AR NOAA 12706 that we have studied in this work no nearby filament
channels are present in the H$\alpha$ images (see Fig. 10), and therefore
these DHs are not filaments channels.
DHs have a spatial extent comparable to that of the associated ARs and
therefore they may influence the irradiance of the Sun, especially during the
maximum of the solar activity, when several ARs are present on the solar disk.

Furthermore, DHs are sometimes mistaken for Coronal Holes (hereafter: CHs),
and up to now there has not been a quantitative characterization that allows a
clear distinction between these two solar features. Andretta & Del Zanna
(2014) noted that these halos are easily seen in TR lines, while CHs are not.
Singh et al. (2021) also observed that CHs tend to be dark across the six
hotter AIA EUV wavelength passbands, in contrast with DHs, which are dark
primarily in 171 Å, but less so or not at all in other wavelengths, such as
193 Å, as also shown in Fig. 1.

Here we use a combination of imaging, spectroscopic, and magnetic field
observations to comprehensively characterize the DH around AR NOAA 12706. This
multi-wavelength approach represents the first study of a DH through all the
layers of the solar atmosphere, from the chromosphere up to the low corona.
The aim of the work is to provide a quantitative characterization of the
emission properties of a DH by using the Interface Region Imgaging
Spectrograph (IRIS; 2014) full-disk mosaics and SDO/AIA (Lemen et al. 2012)
images, in order to provide empirical constraints to the various hypotheses on
the origin of DHs. In addition, our results show fundamental differences
between the DH and the CH under study and support the thesis that DH and CH
are two different types of structures on the Sun. Eventually, we provide a
quick inspection method that can be used to distinguish a DH from a CH.

##  2 Observations

We select AR NOAA 12706 for its suitable location on the disk in the IRIS
mosaics: its intermediate position between the limb and the disk center makes
it possible to look under the coronal loops when looking at AIA images. AR
NOAA 12706 displays a $\beta$ magnetic configuration characterized by one
sunspot of negative polarity and a more diffuse positive magnetic
concentration as little dark pores. It was visible on the disk from 20 April
to 28 April 2018. On April 20, a “light bridge” (see e.g. van Driel-Gesztelyi
& Green 2015, Falco et al. 2016, Felipe et al. 2016 and reference therein)
begins to form within the negative polarity sunspot, indicating the onset of
its fragmentation. Indeed, the spot completely vanishes on April 27. This AR
was observed in a temporal interval of $\sim$ 4 hours by IRIS on 2018 April 22
during the full-disk observation mode, which started on April 22 10:41:50 UT
and finished on April 23 04:05:45 UT. Southwest of NOAA 12706 is NOAA 12707
(see e.g. Figs. 2 and 5), a smaller decaying active region that disappears
from the disk in about two days. Given its small size, we assume that its
presence does not influence the area around AR 12706 and therefore we do not
include it in the analysis. The same IRIS Mg II mosaics have been studied
previously by (2020) and the AR NOAA 12706 is one of the seven ARs studied by
Singh et al. (2021). We will discuss our results in relation to those of these
authors in Sect. 5.

###  2.1 IRIS full-disk mosaics

Primarily used for quasi-regular monitoring of changes in instrument
sensitivity, IRIS full-disk mosaics have proven to be very useful in the study
of the solar atmosphere, as they have been employed in a variety of ways (see
e.g. Schmit et al. 2015; Vial et al. 2019; 2020; Ayres et al. 2021; Koza et
al. 2022). IRIS full-disk mosaics consist of spectral maps of the entire solar
disk built through an observing sequence (currently run approximately once per
month, when IRIS is not in eclipse season) that takes a series of successive
rasters at 184 different pointing locations. Rasters’ spectral windows include
the six strongest spectral lines, i.e. the resonance lines Mg II h and k
($T\sim 8\times 10^{3}$ K), C II 1335 and 1334 Å ($\sim 10^{4}$ K), and Si IV
1393 and 1403 Å ($\sim 6\times 10^{4}$ K). All 184 positions take $\sim 18$
hr, including $\sim 10$ hr of time taken to repoint the spacecraft (Schmit et
al. 2015). Each individual observation consists of a 64-step raster with 2′′
steps and 2 s exposure time at each slit position. The spectra along the slit
have been binned to a resolution of 0.33′′. Mg II mosaics have a spectral
scale of 35 mÅ px-1, while C II and Si IV have a spectral scale of 25 mÅ px-1.
Thus the spectral range of 3.5 Å for Mg II lines and of 1 Å for C II and Si IV
lines is sampled by 101 and 41 spectral points, respectively.

For this work, mosaics represent a unique resource to investigate the
morphological features of DHs, which have a spatial extent comparable to that
of the associated ARs and could not be properly studied through normal
rasters, given their usually relatively small FoV. In addition, the available
spectral windows allow a view in both chromosphere and TR, thanks to the
chromospheric Mg II h&k lines and the C II and Si IV TR doublets.

###  2.2 SDO data

In order to characterize the emission properties of the DH in the corona, we
use AIA observations for their full-Sun coverage. DHs are relatively stable
structures that evolve in timescales of the order of the AR evolution time
($\sim$ days, see e.g. van Driel-Gesztelyi & Green 2015). Therefore, rather
than analysing all the AIA images in the four-hour period taken by IRIS to
entirely cover the AR by rasters, we study only the AIA image taken at 19:24
UT, which is approximately at the middle of the $\sim$ 4 hr temporal window.

To study the evolution of the magnetic flux along the line of sight, we also
use a series of Helioseismic and Magnetic Imager (HMI; Schou et al. 2012) LOS
magnetograms taken in the Fe I 617.3 nm line with a resolution of 1$"$. They
cover $\sim$ 30 mins of observations, starting from the 19:07:30 UT to
19:36:45 UT with a cadence of 45 s.

Figure 3: C II 1334 & 1335 Å (upper panels) and Si IV 1393 & 1403 Å (lower
panels) average profiles for QS, fibrillar and coronal DH and CH, in red,
green, violet and cyan respectively, with the corresponding normalized
histograms of the integrated intensities. The vertical dotted lines in the
profiles represent the QS reference wavelengths. The yellow and gray bands
identify the spectral intervals used for C II and Si IV intensity and
background evaluation, respectively. In the lower left panels, the dashed
curves are the Si IV 1393 Å fits.  Figure 4: Close-up view of NOAA 12706 and
its surroundings. From top to bottom: IRIS Mg II h3, Si IV 1393 Å and C II
1334 Å. North is up and west is to the right. FoV has dimensions 440” $\times$
410”. In the Mg II h3 it is possible to observe around the AR core the
chromospheric DH made up of fibrils, that have maximum length of approximately
the AR half size in the longitudinal direction. In the C II and Si IV mosaics,
instead, it is still recognizable a halo of reduced emission around the AR,
but less clearly and without the evident presence of fibrillar structures.
Figure 5: AIA close-up views of NOAA 12706 and its surroundings. North is up
and west is to the right. FoV has dimensions 700” $\times$ 620”.

##  3 Analysis

To quantitatively characterize the emission properties of the DH in the
different layers of the solar atmosphere and compare them to the QS and the CH
properties, we define four regions of interest (hereafter: RoIs), described
below. For the DH, the starting point of the analysis is the annular region
whose emission is reduced compared to the QS seen by eye in the IRIS Mg II h&k
core full-disk mosaics (e.g. see left panel of Fig. 2 and in top panel of Fig.
4). In particular, fibrils are clearly observed in the Mg II h&k cores (i.e.
50th spectral point for both lines, see right panel of Fig. 11). Fibrils are
chromospheric features visible mainly in nearline-core wavelengths of the
chromospheric lines (see e.g. Kianfar et al. 2020). Indeed, also in IRIS Mg II
mosaics it is possible to notice that they become less visible as the selected
wavelength approaches the line wings, where they become fainter and eventually
undetectable (see Fig. 11), meaning that they are upper chromospheric
features.
Because the fibrils around the AR are most visible at the Mg II h&k cores, we
use in our analysis only the h3 and k3 images222We use the well-known
nomenclature to define $h_{3}$ as the central depression of the line, $h_{2r}$
as the red emission peak and the $h_{1r}$ as the minimum on the red side of
the central emission peak. These positions are shown as example in the Mg II h
full-disk mean profile of Fig. 11 (right panel)., found through a double
gaussian fit of the Mg II h&k lines. We use the defroi.pro IDL routine for
manually drawing, in the Mg II h3 mosaic, the outer boundary of the DH,
visually following the fibrils, and the outer edge of the AR core that is the
inner DH contour. These two contours are shown in the left panel of Fig. 2 in
green and red respectively. We will refer to the DH identified by the these
two contours as “chromospheric fibrillar DH”. We point out that the close NOAA
12707, on the contrary, does not exhibit a chromospheric DH made up of fibrils
extending radially from the AR core, but it shows few dark filamentous
structures surrounding it.

Looking at AIA filtergrams (see Fig. 5), we notice that the DH is most visible
in the AIA 171 Å image, as also shown by Wang et al. (2011) and Singh et al.
(2021). In this waveband, the DH spatial extent is larger than in the Mg II
mosaics, with coronal loops immediately close to the AR core partially
covering and hiding the dark emission beneath them. Therefore, we decide to
use a second contour for the coronal 171 Å DH, which we call “coronal DH”,
located on the west side of the AR. We define it through an intensity
threshold equal to $55\%$ of the 171 Å average disk emission. We use this
particular value because the area falling inside the corresponding contour is
close to the dark region identifiable by eye. This new contour, used to
evaluate the coronal DH average intensity, is shown in violet in middle panel
of Fig. 2. We point out that in this image projection effects prevents the
visibility of the east side of the coronal DH, which is instead well observed
following the AR during its transit on the solar disk.

For the CH definition, we use another intensity threshold, equal to the $35\%$
of the 193 Å average disk emission. Indeed, tools based on intensity threshold
of coronal EUV images have been largely used for the detection of CHs in the
past (e.g. Krista & Gallagher 2009; Reiss et al. 2015; Heinemann et al. 2019).
The CH contour is shown in blue in Fig. 2. For the QS we use a box of
$250"\times 337"$ located outside CH and AR areas (see right panel in Fig. 2)
and chosen such that its radial distance from the center is approximately
similar to that of the AR, in order to minimize center-to-limb variations. In
summary, we have defined four RoIs, shown in Fig. 2:

  * •

chromospheric fibrillar DH;

  * •

upper coronal DH;

  * •

southern CH;

  * •

QS.

###  3.1 Data processing

The AIA images have been processed by first deconvolving the psf with
aia_deconvolve_richardsonlucy.pro IDL routine. All IRIS mosaics are first of
all processed using the new_spike.pro IDL routine in order to remove spikes.
In the C II and Si IV mosaics an anomalous background is present and it is
thought to be a calibration issue, likely related to incomplete compensation
for stray light contamination (see Ayres et al. 2021 for further details). For
this reason a background has been calculated and subtracted, as also done by
Ayres et al. (2021). In particular, for every spatial pixel, we evaluate the
background $I_{bkg}$ as follow:

$\centering
I_{bkg}=0.5\cdot\Biggl{(}\sum_{i_{red}=1}^{8}I_{i_{red}}+\sum_{i_{blue}=1}^{8}I_{i_{blue}}\Biggr{)}\@add@centering$
(1)

where
$i_{red}\in[\lambda_{ref}+12\Delta\lambda,\lambda_{ref}+19\Delta\lambda]$,
$i_{blue}\in[\lambda_{ref}-19\Delta\lambda,\lambda_{ref}-12\Delta\lambda]$,
and where $\lambda_{ref}$ represents the QS reference wavelength and
$\Delta\lambda=0.025$ Å corresponds to the spectral scale of C II and Si IV.
The C II and Si IV background spectral intervals are shown as gray bands in
Fig. 3.

Then we correct the line intensities for the center-to-limb variation, as
described in Appendix. In Fig. 3 we show the center-to-limb corrected average
line profiles for C II and Si IV doublets inside the four RoIs, i.e. QS,
fibrillar DH, coronal DH and CH, with the corresponding normalized histograms
of the integrated intensities.

For C II and Si IV, we use the mosaics after the above processing computed
integrating the lines on the spectral intervals shown as yellow bands in Fig.
3. In Fig. 4 we show a close-up view of AR NOAA 12706 as observed, from top to
bottom, in Mg II h3 and in the background-removed and peak-integrated Si IV
1393 Å and C II 1334 Å center-to-limb corrected mosaics. These zooms are
representative also for the Mg II k3, Si IV 1403 Å and C II 1335 Å,
respectively, that we do not show because they appear remarkably similar.

###  3.2 Average intensity measurements

In order to quantitatively characterize the emission properties of the
fibrillar and the coronal DHs, we evaluate the average intensity (in DN)
inside the four RoIs. We apply the contours defined before as follows:

  * •

the annular Mg II h3 fibrillar DH contour is applied to all IRIS mosaics;

  * •

the AIA 171 Å coronal DH contour is applied to all AIA filtergrams;

  * •

the same AIA 193 Å CH contour is applied to all IRIS mosaics and AIA
filtergrams;

  * •

the same QS box is applied to all IRIS mosaics and AIA filtergrams.

Then we normalize the fibrillar DH, the coronal DH and the CH average
intensities to the QS, by computing the ratio defined as:

$\mathrm{ratio}=\dfrac{I_{av,RoI}}{I_{av,QS}},$ (2)

where the RoIs are the fibrillar DH, the coronal DH and the CH.

The uncertainties on the ratios defined in the above Eq. 2 are in principle
the result of the propagation of the uncertainties attached to each step of
the analysis, i.e. the definition of the RoI, the calculation of the average
intensity within the RoI, and the center-to-limb correction. In the case of
intensities from IRIS spectra, we considered in addition the uncertainty due
to the definition of the spectral intervals used for evaluating the background
and the total intensity, and the contribution due to the noise in each
spectral pixel of the profile. We evaluated each one of these sources of
uncertainties, and found that by far the largest contribution is due to the
RoI selection criteria.

In order to estimate the uncertainties on the ratio given by Eq. 2, we then
proceed as follows:

  * •

For the fibrillar DH we drew by hand two additionally contours by being more
and less restricted on the fibril-edge definition. These contours are shown in
different shades of green in the left panel of Fig. 2.

  * •

For the coronal DH and CH, we changed the value of the intensity thresholds
defining the RoIs; in particular, we considered a 2% variation in the
thresholds defining the coronal DH RoI, i.e. we considered two additional
contours at 55$\%$ and 59$\%$ of the mean disk value, and a 10% variation for
the CH RoI, i.e. at 25$\%$ and 45$\%$ of the mean disk value. We thus obtained
for each of those RoI two additional regions of slightly smaller and larger
areas, respectively, than the reference RoI. The resulting three coronal DH
contours are shown in violet in the middle panel of Fig. 2; the three CH
contours are shown in blue in the right panel of Fig. 2.

The uncertainties of the ratios of Eq. 2 are then obtained by computing the
difference of the mean intensities in these additional regions with respect to
the value obtained in the reference RoI.

The ratios for the selected RoIs and the associated uncertainties computed as
described above, are shown in Fig. 6. The dispersion of the ratios shown with
shaded green and blue bands is obtained using the additional contours of
slightly different extent to give an estimate of the ratio variability. These
results will be discussed in more details in Sec. 4.

|  $I_{0}$ [DN] |  $\lambda_{0}$ [Å] |  $\sigma$ [Å] |  $I_{1}$ [DN] |  $\xi$ [km/s]
---|---|---|---|---|---
f.DH | 1.18 | 1393.76 | 0.077 | -0.0001 | 22.2$\pm$0.2
c.DH | 1.09 | 1393.78 | 0.088 | -0.003 | 25.7$\pm$0.5
CH | 1.17 | 1393.76 | 0.094 | -0.004 | 27.8$\pm$0.7
QS | 1.34 | 1393.77 | 0.087 | -0.00001 | 25.6$\pm$0.4
Table 1: Si IV 1393 Å gaussian fit parameters inside the fibrillar DH, the
coronal DH, the CH and the QS; $\xi$ is the non-thermal velocity retrieved
using Eq. 3.  Figure 6: DH (in green) and southern CH (in blue) average
intensity ratios in IRIS lines and AIA filters. The shaded areas are the ratio
uncertainties, computed as discussed in Section 3. The chromospheric fibrillar
and the 171 Å coronal DH contours have been employed for the IRIS and AIA
ratios, respectively.

###  3.3 Non-thermal velocity measurements

In the TR the profiles of the spectral lines often exhibits, in addition to
the thermal and instrumental broadenings, excess broadening which is
associated to unresolved plasma motions that can transfer mass and energy
between the chromosphere and the corona. This plasma flows are likely to be
intimately related to the still unknown physical processes occurring in the
TR. We study the non-thermal broadenings inside the four defined RoIs using
the optically thin line Si IV 1393.7 Å. For optically thin lines the spectral
profile is assumed Gaussian and the Gaussian width $\sigma$ can be expressed
as (2018):

$\sigma^{2}=\dfrac{\lambda_{0}^{2}}{2c^{2}}\Biggl{(}\dfrac{2kT_{i}}{M}+\xi^{2}\Biggr{)}+\sigma_{I}^{2},$
(3)

where $c,k,T_{i},\sigma_{I},\xi$ are the speed of light, the Boltzmann
constant, the temperature of the ion, the additional instrumental broadening,
which is 3.9 km s-1 for IRIS (2018, 2014), and the non thermal velocity, which
represents the most probable velocity of the random plasma motions.

We estimate $\sigma$ inside the QS, the fibrillar DH, the coronal DH and the
CH by fitting the Si IV 1393 Å mean line profiles shown in Fig. 3 with a
gaussian and a costant term. We assume that the Si IV formation temperature of
$60\times 10^{3}$ K. This is lower than the temperature of $80\times 10^{3}$ K
obtained in CHIANTI using the zero-density ionization equilibrium (2019), and
is derived with the new ionization equilibrium calculations by Dufresne et al.
2021 that include density-dependent effects and charge transfer. The fits for
the four RoIs are shown in bottom left panel of Fig. 3. The fit parameters and
non-thermal velocities $\xi$ found are reported in Table 1. The fit parameters
$I_{0}$, $\lambda_{0}$, $I_{1}$ and $\sigma$, are the height, the center, the
constant term and the standard deviation of the Gaussian, respectively.

###  3.4 Emission measure

To deduce the temperature distribution of the coronal plasma along the LOS
from the AIA filtergrams, we calculate inside the coronal DH, the QS and the
CH the average total column emission measure function $EM$, defined as

$EM=\int_{h}N_{e}^{2}dh=\int_{T}DEM(T)dT,\quad\quad\quad\quad\quad\quad\quad[cm^{-5}]$
(4)

where $N_{e}$ is the electron number density and $DEM(T)$ is the column
differential emission measure, defined as $DEM(T)=N_{e}^{2}(dT/dh)^{-1}$.

We calculate the $DEM(T)$ function employing the sparse inversion technique by
(2015), which assumes that the intensity in each pixel in an image from the
$j$-th AIA waveband can be written as

$I_{j}=\int_{0}^{\infty}K_{j}(T)DEM(T)dT,$ (5)

where $K_{j}(T)$ is the function which describes the response of the $j$-th
channel with the temperature. This method inverts the previous equation
obtaining $DEM$ solutions after giving as input the EUV images from the AIA
channels 94, 131, 171, 193, 211, and 335 Å and the temperature response
functions of these channels. We implemented this technique through the
routines aia_sparse_em_init.pro and aia_sparse_em_solve.pro available in IDL
SolarSoft. The first one builds up the response functions, while the second
one is the proper $DEM$ solver.

Then we obtained the average emission measures using Eq. 4 by integrating the
$DEM$ over the temperature range from $log$T[K] = 5.6 to $log$T[K] = 6.3. We
do not evaluate the average emission measure inside the fibrillar DH because
AR loops cover that RoI.

Figure 7: Average log EM curves as functions of log T calculated over the QS,
coronal DH and southern CH. The DH and the CH show a similar depletion in the
EM compared to the QS in the 5.6-5.9 log T range. At higher temperatures the
CH exhibits a trend opposite to that of DH and QS, with a strong EM reduction.

###  3.5 HMI magnetic field strengths

We use the time series of HMI magnetograms to investigate the temporal
evolution of the average signed $<B_{LOS}>$ and unsigned $<|B_{LOS}|>$
magnetic field strength inside the four RoIs. The noise level of 45-s
magnetograms exhibits a center-to-limb variation, being lower near disk center
and increasing radially approaching the limb up to $\sim$ 8 G (Yeo et al.
2013). Therefore, we cut all the pixels in the range [-8,8] G. The results of
the average signed and unsigned magnetic field strength measurements, together
with standard deviations, are reported in Fig. 8. We point out that the
magnetic field strength depends on the threshold used in the noise removal.
However, we found that the relative distances between the RoIs remain the same
also when using different thresholds.

Figure 8: Average signed (top panel) and unsigned (bottom panel) magnetic
field strength $\sim$ 30 minutes time evolution inside the four RoIs. The
colored bands are the uncertainties. Figure 9: Averaged QS-subtracted
histograms of the four RoIs. The gray band corresponds to the noise interval
[-8,8] G that has been cut to obtain the magnetic field strength measurements
shown in Fig. 8.

##  4 Results

The first results we show are the C II 1334 & 1335 Å (upper panels) and Si IV
1393 & 1403 Å (bottom panels) mean line profiles of Fig. 3. The QS, fibrillar
DH, coronal DH and CH profiles are shown in red, green, violet and cyan,
respectively, after the background removal and the center-to-limb correction.
The gaussian Si IV fits are added in the bottom panels of Fig. 3. At the same
time, we show the histograms of the integrated intensity (in DN) inside the
RoIs, normalized to the number of spatial pixels. We do not show the Mg II
profiles because in the analysis we use only the h3 and k3 images.

The Si IV lines have Gaussian profiles, while the C II lines are flatter than
a Gaussian, as also reported in Upendran & Tripathi (2021) for the CH and QS
cases. We also notice a hinted double peak in the C II CH line. In all cases,
the DH presents narrower lines and intensity peaks slightly higher than the CH
(except for the Si IV 1403 Å line). In the four lines the QS is more intense
than the other RoIs, with the only exception of the coronal DH, that is nearly
identical to the QS in the C II lines. Overall, we do not find evident Doppler
shifts in the profiles.

The histogram shapes are unimodal and have a broader right side, probably due
to bright points. The four lines display a similar histogram of intensities
scheme: the DH distribution (green) is shifted towards smaller intensity
counts and has a peak value between those of the QS and the CH.

Regarding the non-thermal velocities obtained with the gaussian Si IV 1393 Å
fit, the results of which are shown in Table 1, we find that the plasma has
non-thermal velocities $\xi$ of $\sim$ 22, 26, 26 and 28 km/s inside the
fibrillar DH, coronal DH, QS and CH, respectively, with the coronal DH having
essentially the same $\xi$ of the QS. The QS value is close to the averaged
values found by Rao et al. (2022).

In Fig. 6 we show the DH (in green) and CH (in blue) ratios for the IRIS lines
and the AIA bands. The IRIS lines are sorted according to the nominal
formation temperature as determined from the standard CHIANTI ionization
equilibrium. The IRIS and AIA DH ratios are referred to the fibrillar and the
coronal DH, respectively. A fibrillar DH is observed in all IRIS lines,
because IRIS ratios are less than one, and it is darkest in the C II lines.
The CH ratios display a similar trend but with higher ratios, closer to 1. In
the AIA side of the plot, the coronal DH shows ratios less than one in all
filters, except for the AIA 94 Å, with a positive trend increasing with
temperature. On the other side, the CH shows evidence of different behaviour:
the CH has a drop in the intensity relative to the QS, which reaches a minimum
in the AIA 193 and 211 Å bands and then increase in the hottest AIA bands.
Therefore, the fibrillar and coronal DHs show approximately constant ratios of
the order of 0.8, which increase approaching the hottest AIA bands. On the
contrary, the CH exhibits an opposite behaviour, being almost not visible in
the chromosphere and TR and becoming evident approaching the corona.

Fig. 7 shows the average EM as function of temperature obtained using the AIA
filtergrams for the QS, the coronal DH and the CH in red, green and cyan
respectively. The plot shows clearly that in the temperature range $log$ T[K]
$=5.6-6.1$ both the coronal DH and the CH have a lower emission measure
compared to the QS, while in the range $log$ T[K] $=6.1-6.3$, corresponding to
the AIA channels 193 and 211 Å, the DH and the CH have again opposite
behaviour. We note that the DH and QS emission measure results in the
temperature range $log$ T[K] $=5.6-6.3$ are comparable with those found by
Singh et al. (2021), even if the RoIs are selected differently and the center-
to-limb effect is not corrected.

In Fig. 8 we plot the average signed and unsigned magnetic field strength in
the four RoIs obtained from the $\sim$ 30-minute sequence of magnetograms. We
find that during the selected time interval the magnetic field strengths
remain constant, and only the fibrillar DH displays a weak time evolution,
probably related to new flux emerging in the RoI. We find that the fibrillar
DH and the QS have similar averaged signed magnetic fields strengths, close to
zero (consistent with the results by Wang et al. (2011)), while the CH is
unipolar, and the coronal DH presents values intermediate between the CH and
the QS. On the other side, the fibrillar and coronal DH show average unsigned
magnetic field strengths higher than the QS, and the CH displays the lowest
values.

In Fig. 9 we also show the 30-minute averaged QS-subtracted normalized
histograms, which have been obtained by first normalizing the 30-minute
averaged histograms to the total number of pixels and then by subtracting the
QS histogram. We find that the fibrillar and coronal DH histograms are
centered on the zero, while the CH histogram peak is shifted towards negative
magnetic field strength values.

##  5 Discussion and conclusions

Dark halos (DHs) have for a long time been observed as “circumfacular regions”
surrounding active regions, but have also been known under different names.
Typically, they appear as regions darker than the quiet Sun in chromospheric
or TR lines. These large-scale structures are normally best studied in full-
Sun imaging or imaging-spectroscopy. We thus inspected the available set of
IRIS full-Sun mosaics (2013 to present), and found that most ARs surroundings
exhibit clear DH signatures in the core of the Mg II h & k lines, appearing as
dark fibrils with almost radial symmetry around the AR core. We therefore
concluded that these structures are very common solar features. Nevertheless,
the literature describing these structures is very sparse, providing only a
partial view in one or two wavelengths only.

In this work, we presented the first quantitative characterization of the
emission properties of the DH associated with active region NOAA 12706, from
the chromosphere to the low corona, using IRIS full-Sun mosaics and SDO/AIA
images. We complement the results with a preliminary analysis of the SDO/HMI
magnetograms.

We found that IRIS scans in the C II and Si IV lines of the area around NOAA
12706 show that the DH seen in those lines coincides with the region covered
by Mg II fibrils, although no clear fibrillar pattern could be detected in
those TR lines.

Extending the analysis to SDO/AIA images confirms that DHs are clearly visible
in the 171 Å band, as already noted by other authors (Wang et al., 2011; Singh
et al., 2021). However, we also found that the area of the AIA 171 Å DH seems
to be significantly larger than in IRIS chromospheric and TR lines, although a
direct comparison over the entire region is difficult due to the presence of
bright AR loops in the line of sight in many places. In this work, we
therefore distinguish between the DH seen in Mg II h & k line core and in AIA
171 Å, which we term “fibrillar” and “coronal” DH, respectively. It is not yet
clear if and how the two structures are related, but it seems plausible that
the “coronal” DH encompasses and extends the “fibrillar” DH. This point
clearly deserves further investigation, which we are currently carrying out,
by analysing for instance a larger set of ARs over a larger range of
wavelengths than in the current study.

In this context, it might be worth noting that the AIA 304 Å line seems to
share traits of both kinds of DHs: filtergrams at that wavelength are clearly
darker in the fibrillar DH, but the extended area corresponding to the coronal
DH is also noticeable darker than the average quiet Sun.

DHs are a clear feature in TR lines. Andretta & Del Zanna noted their presence
around all active regions in TR lines observed by the SOHO/CDS and SOHO/SUMER
full-disk scans. Our analysis, in effect, showed that fibrillar DHs are
clearly detectable in TR lines, in particular above the Lyman continuum edge;
it is therefore unlikely that the appearance of these structures could be due
to absorption by cool material, as postulated by Wang et al. (2011). However,
the IRIS C II and Si IV lines are formed at temperatures low enough that the
coronal DH, observed clearly in the AIA 171 $\AA$ band, is not visible.

Considering that DHs seen in the AIA 171 Å waveband appear remarkably similar
to coronal holes (CHs), we also compared in detail the properties of those two
structures, comparing in particular both the “fibrillar” and “coronal” with
the CH seen on the same date at the south pole, using as reference a patch of
quiet Sun. Our analysis took into account the effect of average center-to-limb
variation which is quite apparent in the intensities of most lines and
waveband in the data set.

In addition, we computed average values of the signed and unsigned magnetic
field in the regions under studies, using SDO/HMI data taken during the same
interval as the IRIS raster scans covering the selected regions.

Our analysis clearly indicates that the DH and the CH under study are
different structures, as summarised by the following results:

  * •

fibrils seen in Mg II h & k line core are found only in the area around the
associated AR, i.e. in the fibrillar DH;

  * •

compared to the reference quiet Sun area, the line intensities are different
over a wide range of temperatures; this finding is confirmed by the analysis
of the emission measures;

  * •

the average C II and Si IV line profiles in the two region are different; in
particular the Si IV lines exhibit slightly different non-thermal widths
compare to both quiet Sun and CH;

  * •

both the fibrillar and coronal DHs in average do not appear to be as markedly
unipolar regions as the CH;

  * •

the average signed magnetic field in the fibrillar DH, in particular, seem to
be vanishingly small, as in the reference quiet Sun patch.

We speculate that the above results are valid for all ARs. If this is the
case, some of the differences between DHs and CHs could provide proxies for
distinguishing the two structures. Such proxies could be especially useful in
the case of CHs close to ARs, e.g. in the case of equatorial CHs.

In particular, we propose that the different DH and CH emission
characteristics in the AIA filters can be used as a simple recipe to
distinguish between DHs and CHs. More specifically, as illustrated by Figs. 5
and 6, DHs are clearly outlined in the AIA 171 Å waveband, while they are
nearly indistinguishable from quiescent coronal areas in the AIA 193 Å or AIA
211 Å wavebands. On the contrary, CHs are dark in all AIA coronal bands,
including AIA 193 Å or AIA 211 Å.

An illustrative example of the above recipe is given by the cases of ARs NOAA
12738 and 12740 shown in Fig. 1, which display a $\beta$ and an $\alpha$333An
active region containing a single sunspot or group of sunspots all having the
same magnetic polarity. configuration, respectively. In both cases, fibrils
are observed around the brightest part of the AR in the Mg II h & h line core.
The DH is also clearly detectable around both ARs in the AIA 171 Å images. On
the contrary, the AIA 193 Å filtergrams does not show any significantly darker
area around the AR core. The CH is instead well visible in both AIA 193 and
171 Å wavebands.

IRIS mosaics are taken only once per month. This means that it is not always
possible to check for the presence of fibrils around an AR. However, we
highlight that the AIA 304 Å channel, given its peculiarity of showing the
annular shadow of fibrils around the AR cores, can be used to have an estimate
of the spatial location of the fibrillar DH in the chromosphere when IRIS data
are not available.

DHs are large-scale, long-lasting structures that are clearly associated with
active regions. While they apparently persist and perhaps evolve throughout
the AR lifetime, they are most likely related to the AR emergence process.
There is clearly much less TR plasma connecting the opposite polarities
compared to QS regions. The causes could be related to a different magnetic
flux emergence in DH which leads to a different coronal heating compared to
the quiet Sun areas, or to processes occurring in the corona (or both).
Regarding the latter, it is worth mentioning that dark regions were found
around a few isolated ARs studied by (2011). They were, on large scales,
relatively unipolar which led the authors to propose an interchange
reconnection model between these regions and the hot AR core loops. It is
clear that some interaction must occur between the emerging flux in the cores
of ARs and the surrounding pre-existing corona, but it is unclear if the dark
halos are related to the interchange reconnection process, especially if it is
confirmed that, in terms of magnetic field configurations, they are QS-like
regions of largely mixed polarity.

In conclusion, dark halos remain remarkably common, large, and persistent
solar structures, and yet their characteristics and nature is largely unknown.
With this work we have begun to provide a first characterisation of the
emission properties of such structures. We are carrying out follow-up studies
to better determine the properties of these features. They include the use of
Hinode/EIS (2007) spectroscopic data to better characterise the changes in the
TR/coronal emission, and of Solar Orbiter EUI images (Rochus et al. 2020), to
study DHs at much higher spatial resolution than AIA. We are also carrying out
an analysis of the evolution of DHs in terms of TR/coronal properties and
photospheric magnetic field.

###### Acknowledgements.

This study was partly supported by the Italian agreement ASI-INAF 2021-12-HH.0
“Missione Solar-C EUVST—Supporto scientifico di Fase B/C/D.” GDZ acknowledges
financial support from STFC (UK) via the consolidated grants to the atomic
astrophysics group (AAG) at DAMTP, University of Cambridge (ST/T000481/1 and
ST/X001059/1). This study has made use of SAO/NASA Astrophysics Data System’s
bibliographic services. IRIS is a NASA small-explorer mission developed and
operated by LMSAL with mission operations executed at the NASA Ames Research
Center and major contributions to downlink communications funded by the
European Space Agency and the Norwegian Space Center. CHIANTI is a
collaborative project involving George Mason University, the University of
Michigan (USA), University of Cambridge (UK) and NASA Goddard Space Flight
Center (USA).

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##  Appendix A H$\alpha$ GONG images

Figure 10: GONG H$\alpha$ images of ARs NOAA 12740 (left panel), NOAA 12706
(middle panel) and NOAA 12738 (right panel). Around the three ARs the fibrils
observed in the Mg II h3 mosaic are glimpsed. No other structures, such as
filaments or filament channels, are observed. Data were acquired by the GONG
instruments hosted by Learmonth Solar Observatory and Udaipur Solar
Observatory, operated by the National Solar Observatory Integrated Synoptic
Program (NISP).

##  Appendix B Mg II h fibrils

Figure 11: Close-up view of AR NOAA 12706 and its surroundings in the Mg II h
non center-to-limb corrected mosaic at different spectral pixels, i.e., from
left to right, at the 50th, 54th and 63th spectral point, corresponding
respectively to the h3, $h_{2r}$ and $h_{1r}$ in the full-disk mean line
profile (right panel). North is up and west is to the right. Field of view has
dimensions $500"\times 410"$. It is possible to notice a downward trend in
visibility of fibrils as moving away from $h_{3}$.

##  Appendix C Center-to-limb variations

Figure 12: Bi-dimensional histograms vs. $\mu$ are shown for all 2018 April 22
IRIS mosaics (left panels) and AIA filtergrams (right panels) used in the
analysis. Each red point is the mode of the histogram in the interval
$\Delta\mu=0.02$. The blue curve is the fit obtained from the seven QS dates,
as described in Appendix.

In our case, the center-to-limb effects are expected to be relevant in the
average intensity measurements only for the CH and the coronal DH, since the
QS and the fibrillar DH are at a similar radial distance from the disk center.
Different center-to-limb effects are evident in all IRIS mosaics and AIA
filtergrams. Mg II h&k lines are known to present limb darkening (e.g. Morrill
& Korendyke 2008) which has been measured in IRIS mosaics by Gunár et al.
(2021). These authors find that the amplitude of the limb darkening decreases
as the wavelength range used for the integration. As a consequence, we would
not expect to find this effect in the h3 or $k_{3}$ full-disks. However, we
find a non-negligible limb darkening in the h3 and k3 mosaics. All other IRIS
mosaics and AIA filtergrams (except AIA 304 Å) show instead more or less
obvious center-to-limb brightening, with the Si IV mosaics and the hotter AIA
bands being the most conspicuous.
We choose to correct these variations through a standardized empirical method
which take into account the center-to-limb trend of each waveband/wavelength.
The observed intensity I${}_{obs}(\mu$ ) for each line/band, as a function of
$\mu$, i.e. the cosine of the heliocentric angle $\theta$, is a function of
the intensity observed at the disk center I($\mu$ =1 ) hence not modified by
line-of-sight effects. This function is expressed with the factor fλ($\mu$)
which is different for each line or band:

$I_{obs,\lambda}(\mu)=f_{\lambda}(\mu)I_{\lambda}(1).$ (6)

Therefore, knowing $f_{\lambda}(\mu)$ it is possible to obtain the intensity
$I_{\lambda}(\mu=1)$ corrected for the center-to-limb effects.
To retrieve the factor $f_{\lambda}(\mu)$ for each IRIS line and AIA waveband
we use seven dates of minimum of solar activity (2018-03-26, 2018-09-24,
2018-10-23, 2019-09-22, 2019-10-20, 2020-02-23, 2020-03-23) to characterize
the dependence of intensity from radial distance. Following an approach
similar to that reported in Andretta & Del Zanna (2014), we compute the bi-
dimensional histogram of intensities as function of $\mu$ in each annulus of
size $\Delta\mu=0.02$ starting from the disk center. We exclude from the
calculation all pixels with $|Y|<0.85\>R_{\odot}$ in order to minimize the
contribution of polar CHs that would otherwise affect the quiet Sun profile,
above all during the minimum of solar activity. Then we evaluate the peak
intensities averaged on the seven dates for each IRIS line and AIA band.
Then, we fit the logarithm of these peak intensities as function of $\mu$ with
a cubic function:

$log\>I(\mu)=A+B(1-\mu)+C(1-\mu)^{2}+D(1-\mu)^{3},$ (7)

so that the factor $f_{\lambda}(\mu)$ can be simply computed from the
following equation:

$log\>f_{\lambda}(\mu)=logI(\mu)-logI(1)=B(1-\mu)+C(1-\mu)^{2}+D(1-\mu)^{3}.$
(8)

The fits of the seven quiet-Sun day average peak-intensity curves we obtained
for IRIS lines and AIA bands are shown in blue in Fig. 12 overplotted to the
bi-dimensional histograms as function of $\mu$ for the 2018 April 22 date
(i.e. AR NOAA 12706 date). Only AIA 94 Å is not shown because its plot is very
similar to the AIA 335 Å one. The red lines are the measured positions of
intensity peaks of the histograms. The cubic function fits very well the
observed average center-to-limb variation of intensities. The seven QS C II
and Si IV mosaics used to compute $f_{\lambda}(\mu)$ are obtained through line
integral. Then, the center-to-limb correction is applied to each wavelength of
the 2018 April 22 C II and Si IV datacubes under the assumption of optically
thin line, which is confirmed by the limb brightening found and shown in Figs.
12.