# The Formation and Lifetime of Outflows in a Solar Active Region

David H. Brooks College of Science, George Mason University, 4400 University
Drive, Fairfax, VA 22030, USA  Hinode Team, ISAS/JAXA, 3-1-1 Yoshinodai, Chuo-
ku, Sagamihara, Kanagawa 252-5210, Japan  Louise Harra PMOD/WRC, Dorfstrasse
33 CH-7260 Davos Dorf, Switzerland  ETH-Zurich, Hönggerberg campus, HIT
building, Zürich, Switzerland  Stuart D. Bale Physics Department and Space
Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA
Krzysztof Barczynski PMOD/WRC, Dorfstrasse 33 CH-7260 Davos Dorf, Switzerland
ETH-Zurich, Hönggerberg campus, HIT building, Zürich, Switzerland  Cristina
Mandrini Instituto de Astronomía y Física del Espacio (IAFE), CONICET-UBA,
Buenos Aires, Argentina  Vanessa Polito Bay Area Environmental Research
Institute, NASA Research Park, Moffett Field, CA 94035, USA  Lockheed Martin
Solar & Astrophysics Laboratory, Org. A021S, Bldg. 252, 3251 Hanover St., Palo
Alto, CA 94304, USA  Harry P. Warren Space Science Division, Naval Research
Laboratory, Washington, DC 20375, USA

###### Abstract

Active regions are thought to be one contributor to the slow solar wind.
Upflows in EUV coronal spectral lines are routinely osberved at their
boundaries, and provide the most direct way for upflowing material to escape
into the heliosphere. The mechanisms that form and drive these upflows,
however, remain to be fully characterised. It is unclear how quickly they
form, or how long they exist during their lifetimes. They could be initiated
low in the atmosphere during magnetic flux emergence, or as a response to
processes occuring high in the corona when the active region is fully
developed. On 2019, March 31, a simple bipolar active region (AR 12737)
emerged and upflows developed on each side. We used observations from Hinode,
SDO, IRIS, and Parker Solar Probe (PSP) to investigate the formation and
development of the upflows from the eastern side. We used the spectroscopic
data to detect the upflow, and then used the imaging data to try to trace its
signature back to earlier in the active region emergence phase. We find that
the upflow forms quickly, low down in the atmosphere, and that its initiation
appears associated with a small field-opening eruption and the onset of a
radio noise storm detected by PSP. We also confirmed that the upflows existed
for the vast majority of the time the active region was observed. These
results suggest that the contribution to the solar wind occurs even when the
region is small, and continues for most of its lifetime.

Sun: corona–Sun: solar wind–Sun: UV radiation

To be submitted to the Astrophysical Journal

##  1 Introduction

Outflows from the edges of active regions have become a focus of many studies
since they were noted in early observations from Hinode (Kosugi et al., 2007).
These outflows are of great interest because of their likely contribution to
the slow solar wind (Sakao et al., 2007; Harra et al., 2008; Doschek et al.,
2008). Upflows in active regions are most obvious in hot spectral lines formed
around 1–2 MK (Del Zanna, 2008; Warren et al., 2011), and occur in dark areas
where the line intensities are faint, especially at the active region edges
(Doschek et al., 2008). They show bulk plasma motions on the order of tens of
km s-1 (Del Zanna, 2008). A higher speed component, reaching hundreds of km
s-1, is also often present in the blue wing of EUV spectral lines (Bryans et
al., 2010; Peter, 2010; Tian et al., 2011; Brooks & Warren, 2012). Plasma
composition measurements and simple mass flux estimates have strengthened the
idea of a connection to the slow solar wind (Brooks & Warren, 2011; Brooks et
al., 2015; Brooks & Yardley, 2021). Indeed different composition signatures in
the outflows may help explain variability in the slow wind (Brooks et al.,
2020), and the evolution of the upflows has even been linked to radio noise
storms detected close-in to the Sun by Parker Solar Probe (Harra et al.,
2021).

Linking remote sensing and in-situ observations is a key goal of both Parker
Solar Probe (PSP, Fox et al., 2016) and Solar Orbiter (Müller et al., 2020).
Detailed recent reviews focusing on active region outflows are given by Hinode
Review Team et al. (2019) and Tian et al. (2021), while an extensive review
including their contribution to the solar wind was presented by Abbo et al.
(2016).

Figure 1:  AIA 193 Å images showing the emergence and growth of AR 12737 from
00 UT on March 31 to 06 UT on April 1. The white box shows the FOV of the IRIS
observations in Figure 4.

There remain several outstanding issues with our understanding of the
upflows/outflows, and these have significant implications for their
contribution to the slow solar wind. In particular, it is not clear how early
they form in the emergence phase of an active region, nor how long they
persist during its lifetime. Do upflows become outflows and contribute to the
slow wind all the time they exist, or just for a shorter fraction of the
active region lifetime? Clearly active regions are a more significant
contributor to the slow wind if 1) the upflows exist longer, and 2) even small
regions have upflows.

One of the difficulties in studying the early formation phase is that the
spectroscopic instruments used to measure plasma flows around active regions
have small fields-of-view (FOV) and slow slit scanning times. This makes it
challenging to catch active regions as, or soon after, they emerge. Harra et
al. (2010) present observations of emerging flux within an already developed
active region that was observed by the EUV Imaging Spectrometer (EIS, Culhane
et al., 2007) on Hinode. The interaction between newly emerging and pre-
existing opposite polarity magnetic field formed a ring of strong upflows at
the active region edge. This formed quickly - within 12 hours - but we should
note that the magnetic topological conditions were very favourable since large
scale upflows were already present in the overlying developed active region.
These may already have opened the field surrounding the active region. It is
unclear if upflows form as quickly in an isolated active region.

These ideas are closely connected with the upflow formation mechanism itself,
which is another unresolved issue. Perhaps the most popular picture is that
closed field loops in the active region core are opened by interchange
reconnection with the open fields in its surroundings. This can occur at
quasi-separatrix layers (Baker et al., 2009; Mandrini et al., 2015), where
there are strong gradients in magnetic connectivity, and may be driven by the
active region emergence and expansion (Murray et al., 2010; Del Zanna et al.,
2011). This process provides a mechanism to transfer the closed (solar wind-
like) composition of the hot core loops onto open magnetic fields.

Again, a key question is when and where? If emergence is the driver, this
could happen quickly and low down in the atmosphere. Conversely, it could be
that even after upflow formation, the parent active region needs to expand and
interact with high lying magnetic field before the upflow opens to the
heliosphere. Some studies have shown that the upflow magnetic field is not
always open (Edwards et al., 2016), while others suggest that some fraction of
the upflow mass flux also flows through connections to distant active regions
Boutry et al. (2012).

Another possibility is the direct injection of mass and energy into the
upflows by chromospheric jets (De Pontieu et al., 2009). Recently Polito et
al. (2020) found signatures of the upflows in chromospheric spectral lines
observed by the Interface Region Imaging Spectrometer (IRIS, De Pontieu et
al., 2014). Several chromospheric and transition region lines showed different
behavior in the upflows compared to the cores of two active regions. It is
still unclear, however, if these signatures are evidence of the driving
mechanism of the upflows operating at low heights. It is also possible that
they are revealing a chromospheric response to the changed coronal environment
of open-field regions. Based on the multi-wavelength analysis of Hinode/EIS
and IRIS data, Barczynski et al. (2021) argue that at least three parallel
mechanisms generate the plasma upflow, and that these mechanisms are localized
in the chromosphere, transition region, and corona.

AR 12737 emerged on the Earth facing solar disk on 2019, March 31, during the
2nd PSP encounter, and was targetted 1–2 days later by EIS and IRIS. From a
case study of AR 12737, we will show evidence that 1) the upflows in this
region are formed low down, in the early emergence phase, when it is still
relatively small, and 2) once formed, the upflows exist for the entire
observed lifetime of the region.

##  2 Observations

In this work we use several datasets from the Atmospheric Imaging Assembly
(AIA, Lemen et al., 2012) on board the Solar Dynamics Observatory (SDO,
Pesnell et al., 2012). These were downloaded via the web-based interface to
the Joint Science Operations Center (JSOC) at Stanford, are calibrated and
correspond to level 1.5. We retrieved 193 Å data for two long duration time-
periods at 156 s and 600 s cadence, and a high cadence (12 s) multi-wavelength
(304 Å, 171 Å, 193 Å, and 211 Å) dataset around the time of the upflow onset.

For Doppler velocity maps of the active region corona we use EIS measurements.
EIS is a dual spectrograph that observes in the 171–211 Å and 245–291 Å
wavelength ranges with a spectral resolution of 22.3 mÅ. To account for
instrumental effects (defective pixels, dark current, cosmic ray hits) we
reduced the data using the eis_prep SolarSoftware routine. The observations we
analyze are 261${}^{\prime\prime}\times 512^{\prime\prime}$ field-of-view
(FOV) rasters of AR 12737 using the 2′′ slit with coarse scan steps of 3′′.
The exposure time was 40 s.

To obtain the Doppler velocity maps we fit the strong Fe XII 195.119 Å
spectral line. We used a double Gaussian function to take account of the
density sensitive Fe XII blend at 195.179 Å. Measured line centroids were
converted to velocities after first correcting for spectral motion across the
CCD, due to instrument structure temperature variations around the Hinode
orbit, using the artificial neural network model of Kamio et al. (2010). We
then calibrated the velocities to a reference wavelength obtained by averaging
the top part of the FOV, and finally removed a residual orbital variation that
was present following the strategy of Brooks et al. (2020). This last step was
necessary since the neural network model uses data from early in the mission
that are less applicable to recent observations. The accuracy is on the order
of 4–5 km s-1, though we do not use any actual values in this study.

Figure 2:  EIS intensity and Doppler velocity maps of AR 12737 between April 1
and 9. The maps were derived from the Fe xii 195.119 Å spectral line. The
colour bar shows the velocity range.  Figure 3:  Magnetic field model for AR
12737 on April 1. The extrapolated field lines are overlaid in red on the EIS
Doppler velocity map obtained at 15:51 UT and already shown in Figure 2. The
axes show the model scale in Mm with the origin set at the active region
center. The magenta and cyan contours show the line-of-sight magnetic field
for positive and negative values of $\pm$50–500 G. Field lines ending with a
circle leave the computational box and are potentially open.

For observations of chromospheric structure and velocities we used IRIS. The
IRIS instrument observes two wavelength bands in the far and near-ultraviolet
(FUV & NUV) covering 1332–1407 Å and 2783–2835 Å. IRIS has a slit-jaw imager
and spectrograph. Here we focus on observations in the Si IV 1393 Å, C II 1335
Å, and Mg II 2796 Å spectral lines. These cover the transition region, upper
chromosphere, and mid to upper chromosphere, respectively. We use level-2
data, and these are processed to account for instrumental effects (dark
current, geometric effects, flat-field, orbital wavelength variation). The
observations we analyze are 129${}^{\prime\prime}\times 126^{\prime\prime}$
FOV rasters constructed from coarse (2′′ step scans) at a spatial resolution
of 0.33–0.4′′. The exposure time was 15 s.

Doppler velocity maps of the upper chromosphere and transition region were
derived from spectral fits to the C II 1335 Å and Si IV 1393 Å lines. The Si
IV 1393 Å velocities are derived from the line peak of a single Gaussian fit.
The C II 1335 Å line profiles are complex and can be singly or doubly peaked.
The velocities here are derived from the peak if the profile has a single
peak, but are computed from the line reversal position if the profile is
double peaked. We used the algorithm of Rathore et al. (2015) to identify the
profile peaks. We also constructed a map of the Mg II 2796 Å $k2$ asymmetry
i.e. the difference in intensity between the peaks in the red and blue wings
of the line profile. A positive asymmetry can imply upflows due to increased
absorption in the blue wing, while a negative asymmetry can imply downflows
due to increased absorption in the red wing. We analyzed the Mg II spectral
profiles using the iris_get_mg_features_lev2 procedure available in the IRIS
branch of SolarSoftware (Freeland & Handy, 1998).

We also compare our analysis of AIA images with type III radio noise data
recorded by the FIELDS (Bale et al., 2016) Radio Frequency Spectrometer (RFS,
Pulupa et al., 2017) on PSP. The RFS obtains full Stokes parameters using low-
and high-frequency receivers covering a wide range from 10.5 kHz to 19.17 MHz.
The usual spectral cadence is 7 s. We only use a subset of the RFS encounter 2
data, described in detail by Harra et al. (2021), for illustration. We focus
only on the frequency of maximum Stokes intensity. The data were reduced as
described in Harra et al. (2021).

Figure 4:  IRIS Si IV 1393 Å and C II 1335 Å intensity and velocity maps AR
12737 on April 2. The last two panels show the Mg II 2796 Å $k3$ intensity and
asymmetry between the $k2_{R}$ and $k2_{v}$ peaks. The FOV is shown by the
white box in Figure 1.

##  3 Analysis results

###  3.1 Evolution of the upflows

AR 12737 was a quiescent active region. Despite its emergence the GOES X-ray
flux remained below B-class while it was the sole region on disk. Figure 1
shows AIA 193 Å images to give an overview of its emergence phase. It appears
to have evolved from a cusp-shaped loop arcade around 00 UT on March 31. By 10
UT it is clear that a new active region is forming, and this development phase
lasts several hours. By 20 UT the typical structure of an active region has
formed. In Figure 1 we can see a hot core loop arcade, high lying million
degree loops, and dark channels from the active region edge, which often show
propagating motions in imaging data. These dark areas at the active region
edge typically show upflow signatures when observed by EIS (Doschek et al.,
2008). In section 3.2 we attempt to estimate these time periods more
quantitatively.

EIS moved to observe AR 12737 from 15:51 UT on April 1, and tracked the region
across the solar disk, making a final slit scan at 06:39 UT on April 9. Figure
2 shows an overview of these observations. EIS spectroscopically confirms the
existence of upflows in the dark channels on both the east and west sides of
AR 12737 as early as 16 UT on April 1.

Harra et al. (2021) used linear force-free field models to establish the
global coronal structure of AR 12737, and discussed the expansion and
development of the eastern upflow after April 1 in detail. They show that the
area of the blue-shifted upflow expands by a factor of 10 between April 1 and
April 4, that the region is associated with large scale magnetic field lines
in their model, and that the area associated with these large scale field
lines increases as the AR expands. This seems to be driven by the expansion of
closed loops to the south east of the AR, which appear as red-shifted in the
EIS velocity map of April 1 (Figure 2 left panel). We show an example of their
magnetic field model in Figure 3, overlaid on an EIS Doppler velocity map to
highlight the associations between the extrapolated field lines and the active
region outflows. Large scale field lines are clearly seen rooted in the
positive polarities at the base of the blue-shifted outflow on the east side
(around coordinates [-30,0]). The field lines are a composite from different
models and the details are described in Harra et al. (2021).

Figure 2 also shows that the eastern upflow is still clearly visible on April
9. Projection effects, however, mean that we can no longer detect blue-shifted
emission in the western upflow.

IRIS scanned AR 12737 from 05:17–05:36 UT on April 2 ($\sim$13.5 hours after
the first EIS scan). Figure 4 shows intensity and velocity diagnostics from
the transition region (Si IV 1393 Å) down through the upper (C II 1335 Å) and
mid- to upper chromosphere (Mg II 2796 Å). Polito et al. (2020) previously
reported chromospheric signatures of coronal upflows in IRIS observations of
two active regions. They found that the average redshift in the Si IV 1393 Å
line was reduced, the C II 1335 Å line was slightly blueshifted, and that Mg
II k2 asymmetries were present, in the upflows compared to the active region
core. Figure 4 indicates that these characteristics are present in the eastern
upflow of AR 12737. Note, for example, the strong redshifts in the core of the
region in the Si IV 1393 Å velocity map, and the reduced redshift (occasional
blueshift) to the east (second panel of Figure 4.

These observations provide another example of chromospheric signatures of the
upflows, but they appear to be reduced in AR 12737 compared to the active
regions analyzed by Polito et al. (2020). Visual inspection of the locations
compared to the EIS data suggests that the spatial correlation is not as
clear, especially as the morphology of the upflow has already evolved in the
13 hours between the EIS and IRIS observations. The weaker signatures are
probably due to the fact that AR 12737 was not yet fully developed on April 2,
so the spectral lines were also weaker, and the upflow region had not yet
expanded to the extent observed on April 4. These results do indicate,
however, that chromospheric signatures of the upflows have already appeared by
two days into the lifetime of the active region.

###  3.2 Formation of the upflows

We have attempted to pinpoint the transition from an emerging flux region to a
formed active region with upflows using a combination of time-slice intensity
tracking, cross-correlation analysis of images, and simple visual inspection.
Without spectroscopic data of the very early emergence phase this is, of
course, difficult to confirm, but here we discuss evidence that the upflows
may have formed as early as 12–16 UT on March 31.

Figure 5:  AIA 304 Å, 171 Å, 193 Å, and 211 Å images showing the small
eruption at 10 UT on March 31. The blue arrow on the 304 Å image points out
the eruption from the solar east side of AR 12737. This image is linked to a
30 min animation in the online version of the manuscript.  Figure 6:  Space-
time analysis showing the mini-eruption in the 304 Å data. The space-time plot
corresponds to the thin blue line in Figure 5. The data were taken at a high
cadence of 12 s between 10–12 UT on March 31. The sky blue arrow points out
the start of the small eruption discussed in the text.

First, while viewing AIA movies of the emerging active region, we noticed a
small eruption from the east side around 10 UT. Figure 5 shows multi-
wavelength AIA images of the mini-eruption. The figure is linked to a 30 min
animation in the same format. The eruption is best seen in the 304 Å images,
and in temperatures below $\sim$2 MK (formation temperature of the 211 Å
band). Prior to the eruption the region is bipolar with a closed field cusp
shaped arcade – as was shown in Figure 1 left panel. After the eruption, the
loop arcade appears to spread open and rapidly develop. Loops appear to draw
back from the location where the upflows are later observed, and start to
interact with the closed field to the south east: a process that seems to be
integral to the expansion of the upflows after April 1. Even as early as
shortly after the eruption on March 31 it appears that large scale, open, or
distantly connecting long loops develop. The field is predominantly positive
in the core of the AR at this time, with only weak scattered negative flux to
the north so that field lines from the south east can only connect far from
the region i.e. the negative polarities that counterbalance the positive field
are not nearby. This is the picture we also get from a magnetic field
extrapolation we attempted, though unfortunately the AR is too close to the
east limb for the model to be convincing and reliable - so we do not include
it here.

The features of the small eruption can be seen in the space-time intensity
plot of Figure 6, which shows high cadence (12 s) 304 Å data obtained between
10–12 UT on March 31. The plot is made by stacking intensities extracted along
the thin blue line shown in Figure 5. Relatively stable structures, such as
the active region itself, appear as broad horizontal trails in the plot,
whereas dynamic features that move rapidly along the line appear as streaks.
The sky blue arrow $P0$ points out the streak at the start of the small
eruption just after 10 UT (around 60′′ along the slice-line). The fuzzier ‘W’
shaped streaks following $P0$ are a result of the loops spreading open and
then retracting.

To quantitatively show when the active region emerged and infer when the
upflows developed, we examined space-time intensity cuts of the AIA 193 Å
data. Figure 7 shows a context image with two slice-lines overlaid in sky blue
and red. We examined several slice-lines but these two illustrate the behavior
of the eastern upflow. Note that they are not optimised for the upflows on the
western side. The sky blue line runs across the eastern dark lane where EIS
later detects upflows. The idea here is to try to trace back, from when we
know the dark emission is associated with upflows detected by EIS, to as early
as we can in the active region development. The red line runs across the
central loop arcade to try to trace the dark emission farther back in time
when AR 12737 was small, and the upflows, if formed, would be closer to the
core. As the lines run across the upflow and bright core we expect that when
they are stacked in time we should see a bright trail across the space-time
plot that represents the core region, and a dark trail below it that
represents the area where the upflow is later detected. We used 193 Å data at
156 s cadence for this analysis.

Figure 8 shows the results. The top left panel is the space-time plot for the
sky blue line in Figure 7. We can see the emergence of the active region from
$\sim$10–11 UT and the development of the bright loop arcade $\sim$14 UT in
the center of the space-time plot, as expected. The loop arcade further
brightens and expands thereafter, trailing left to right across the space-time
plot (approximately between the 91′′ and 182′′ tick marks on the Y-axis). We
point out a dark trail below the bright trail with the sky blue arrows $P1$
and $P2$ (crossing left to right and centered approximately on the 91′′ tick
mark). This is the trace of the dark areas at the eastern edge of the active
region where we expect upflows should be observed. To reiterate, this is
simply because of the usual correlation between low intensity and blue-shifts
observed previously in many active regions (Doschek et al., 2008). For ease of
identification we have also added a dotted line in the dark trail. As
discussed, EIS does indeed later detect upflows in the Eastern region. It
seems that the dark region can be traced back as far as the first $P1$ arrow
$\sim$16 UT.

The top right panel shows the space-time plot for the slice-line closer in to
the active region core (red in Figure 7). Being closer to the location of flux
emergence, this slice-line shows the appearance of the active region earlier
(before 10 UT). Since the region is emerging at this time, any upflows that
are forming are also more likely to be obscured by the growing loop arcade. We
can see this in the figure. The dark trail, again crossing left to right and
here approximately centered on the 97′′ Y-axis tick mark, brightens and fades
due to the line-of-sight intereference by the bright loop arcade.
Nevertheless, the sky blue arrow, $P3$, points out that the dark trail of the
eastern upflow can be traced back to around $\sim$12–13 UT. We again added a
dotted line for ease of identification.

The middle panel of Figure 8 shows results from our image-to-image correlation
analysis. The idea here is to establish when the active region has formed its
basic structure (likely with upflows). We compute the linear Pearson
correlation coefficient, $r$, between successive images for two time periods
(08 UT March 31–08 UT April 1 and 00 UT March 31–00 UT April 2) at two
different cadences (156 s and 600 s) inside the boxed region shown in Figure
7. When there are large changes between successive images, $r$ will decrease,
and when there are minimal changes, $r$ will increase. We use the higher
cadence data to ensure we capture any rapid changes. As we can see from the
figure, the two datasets show essentially the same result. The correlation
coefficient between images is high, $r$ above 0.95 for most of the time-
series, as we would expect given that AR 12737 was a quiescent region. Both
time series also show that flux emerged rapidly between $\sim$10–13 UT.
Dynamic activity during emergence caused $r$ to fall as low as $\sim$0.85, but
changes began to reduce after this time and $r$ returned to its typical value
by 14–16 UT. These timescales and periods are in line with what we discerned
from the AIA movies and space-time plots. AR 12737 does not appear to alter
significantly from this time until EIS observes the upflows.

Figure 7:  AIA 193 Å context for the space-time and correlation analysis,
taken at 07 UT on April 1. The sky blue line corresponds to the space-time
plot shown in the left panel of Fig. 8. The red line corresponds to the space-
time plot shown in the right panel of Fig. 8. The white box shows the region
used for the correlation analysis in the lower panel of Fig. 8.

Figure 8:  Space-time and correlation analysis. Top left: space-time plot
corresponding to the sky blue line in Fig. 7. Top right: space-time plot
corresponding to the red line in Fig. 7. Middle panel: image-to-image
correlation at 156 s (blue dots) and 600 s (sky blue dots) cadence for the
region shown by the white box in Figure 7. The sky blue arrows in the top
panels indicate features discussed in the text. Bottom panel: RFS data showing
the onset of the type III radio storm detected by PSP. The plot shows the
frequency of the peak normalized intensity (red) during the same time-interval
as the correlation analysis. Data points with frequency above 6$\times$106 Hz
are increased in size to visually highlight the noise storm.

The lower panel of Figure 8 shows data from FIELDS/RFS for the same time-
period as the image-to-image correlation analysis. The data are derived from
radio Stokes $I$ intensities detected by RFS in its two highest frequency bins
(18.28–19.17 MHz). The plot shows the frequencies of the maximum of the mode
normalized (over 40 s windows) Stokes $I$ intensities. As discussed by Harra
et al. (2021), the radio noise storm detected at PSP appears most likely
associated with the eastern upflow as it expands from April 1 to April 4. The
drift to lower frequencies with time also suggests the emission height, or
lateral expansion, is increasing. Looking here at the time period closer to
active region emergence, sporadic indications of the start of the high
frequency ($>$10 MHz) radio emission are seen as early as 8–10 UT, while the
noise storm is clearly visible from 13 UT onwards. If the radio noise storm is
associated with the upflows, then the RFS data independently support the
conclusion that the upflows formed in the 10–13 UT period as the active region
developed following the small eruption. The radio emission drift to lower
frequencies is already visible on March 31 – April 1.

##  4 Summary and Discussion

We have studied the formation and lifetime of the eastern upflow from AR 12737
using EUV spectroscopic observations from EIS, NUV & FUV spectroscopic data
from IRIS, images from AIA, and radio data from FIELDS/RFS. Our goal was to
understand where and how quickly upflows form in an active region, and how
long they might contribute to the slow solar wind during the observed
lifetime.

These observations establish the following timeline. The active region emerged
and developed from a cusp shaped loop arcade from 8 UT on March 31. A small
eruption occurred $\sim$10 UT, and the typical structure of an active region
with bright core loops and peripheral dark features (usually associated with
upflows) formed from 12–16 UT. Space-time intensity plots and image-to-image
correlation analysis support this picture and show that the active region did
not alter appreciably until EIS observed it at 15:50 UT on April 1. At this
time, EIS confirmed the presence of blue-shifted upflows associated with
large-scale field lines connecting out of the AR, from both the east and west
edges, in force-free field models (Figure 3). IRIS observed the region
$\sim$13.5 hours later, and detected signatures of the upflow in the
chromosphere and transition region. The active region grew and the upflow area
expanded between April 1–4. The magnetic field modeling is consistent with
this expansion, and there is an associated increase in the number of large-
scale (potentially open) field lines (Harra et al., 2021). The eastern upflow
was still present when the region was last observed by EIS at 06:39 UT on
April 9. The region was observed for 9 days.

We conclude that the upflows in AR 12737 formed early in its lifetime (no
later than 32 hours after emergence) and persisted for as long as EIS tracked
it (85% of the observed lifetime). Any contribution to the slow solar wind is
therefore not a short lived phenomenon.

The lack of spectroscopic data within the first 32 hours makes it difficult to
confirm the exact time of upflow formation, but our analysis also suggests
that it occurred earlier. Based on the space-time and correlation analysis,
the eastern upflow can be traced back to when the typical structure of the
active region was formed between 12–16 UT on March 31. That is, the upflow may
have formed as little as 4–8 hours after emergence and persisted for 95% of
the observed lifetime. The small eruption could have opened the magnetic field
on the eastern side before this, and the onset of the radio noise storm
detected by FIELDS/RFS occurred at the start of this period. We should add the
caveat that the upflow might not contribute to the slow wind all the time it
is observed, but magnetic modeling of the region suggests it is associated
with large-scale expanding field lines the whole time that it was tracked, so
in principle the plasma flows can become outflows and escape to the
heliosphere.

The evidence also suggests that the upflow formation occurs low down in the
atmosphere. The mini-eruption ejected from the base of the cusp loop arcade
soon after emergence while the active region was still small. It was also best
observed in the AIA filters associated with cooler temperatures, especially
304 Å. Even if we only consider the spectroscopic data, possible signatures of
the upflows were observed in the chromosphere and transition region by IRIS on
April 2, and the EIS data show that the active region and upflow did not grow
to their full extent until April 4. This implies that the upflow formation was
well underway before AR 12737 had expanded to interact with high lying
magnetic fields. The radio noise storm observed by FIELDS/RFS also showed a
frequency drift that can be interpreted as the emission height forming at
lower altitudes. Future multi-mission observations, of the earliest stages of
active region emergence, will hopefully pin down more accurately some of the
suggestions put forward in this article.

This study benefited from discussions at the meeting of ISSI international
team 463; project title ‘Exploring The Solar Wind In Regions Closer Than Ever
Observed Before’. The work of DHB and HPW was funded by the NASA Hinode
program. VP acknowledges support by NASA contract NNG09FA40C (IRIS) and grant
no. 80NSSC21K0623. CM acknowledges financial support from the Argentine grants
PICT 2016-0221 (ANPCyT) and UBACyT 20020170100611BA (UBA). This work is
supported by the Swiss National Science Foundation - SNF. Hinode is a Japanese
mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and
NASA and STFC (UK) as international partners. It is operated by these agencies
in co-operation with ESA and NSC (Norway). The SDO data are courtesy of
NASA/SDO and the AIA, EVE, and HMI science teams. IRIS is a NASA small
explorer mission developed and operated by LMSAL with mission operations
executed at NASA Ames Research center and major contributions to downlink
communications funded by ESA and the Norwegian Space Centre. We acknowledge
the NASA Parker Solar Probe Mission and FIELDS team led by S.D.Bale for use of
data. The PSP/FIELDS experiment was developed and is operated under NASA
contract NNN06AA01C.

## References

  * Abbo et al. (2016) Abbo, L., Ofman, L., Antiochos, S. K., et al. 2016, Space Sci. Rev., 201, 55
  * Baker et al. (2009) Baker, D., van Driel-Gesztelyi, L., Mandrini, C. H., Démoulin, P., & Murray, M. J. 2009, ApJ, 705, 926
  * Bale et al. (2016) Bale, S. D., Goetz, K., Harvey, P. R., et al. 2016, Space Sci. Rev., 204, 49
  * Boutry et al. (2012) Boutry, C., Buchlin, E., Vial, J. C., & Régnier, S. 2012, ApJ, 752, 13
  * Brooks et al. (2015) Brooks, D. H., Ugarte-Urra, I., & Warren, H. P. 2015, Nature Communications, 6, 5947
  * Brooks & Warren (2011) Brooks, D. H., & Warren, H. P. 2011, ApJ, 727, L13
  * Brooks & Warren (2012) —. 2012, ApJ, 760, L5
  * Brooks & Yardley (2021) Brooks, D. H., & Yardley, S. L. 2021, Science Advances, 7, eabf0068
  * Brooks et al. (2020) Brooks, D. H., Winebarger, A. R., Savage, S., et al. 2020, ApJ, 894, 144
  * Bryans et al. (2010) Bryans, P., Young, P. R., & Doschek, G. A. 2010, ApJ, 715, 1012
  * Culhane et al. (2007) Culhane, J. L., Harra, L. K., James, A. M., et al. 2007, Sol. Phys., 243, 19
  * De Pontieu et al. (2009) De Pontieu, B., McIntosh, S. W., Hansteen, V. H., & Schrijver, C. J. 2009, ApJ, 701, L1
  * De Pontieu et al. (2014) De Pontieu, B., Title, A. M., Lemen, J. R., et al. 2014, Sol. Phys., 289, 2733
  * Del Zanna (2008) Del Zanna, G. 2008, A&A, 481, L49
  * Del Zanna et al. (2011) Del Zanna, G., Aulanier, G., Klein, K. L., & Török, T. 2011, A&A, 526, A137
  * Doschek et al. (2008) Doschek, G. A., Warren, H. P., Mariska, J. T., et al. 2008, ApJ, 686, 1362
  * Edwards et al. (2016) Edwards, S. J., Parnell, C. E., Harra, L. K., Culhane, J. L., & Brooks, D. H. 2016, Sol. Phys., 291, 117
  * Fox et al. (2016) Fox, N. J., Velli, M. C., Bale, S. D., et al. 2016, Space Sci. Rev., 204, 7
  * Freeland & Handy (1998) Freeland, S. L., & Handy, B. N. 1998, Sol. Phys., 182, 497
  * Harra et al. (2021) Harra, L., Brooks, D. H., Bale, S. D., et al. 2021, A&A, 650, A7
  * Harra et al. (2010) Harra, L. K., Magara, T., Hara, H., et al. 2010, Sol. Phys., 263, 105
  * Harra et al. (2008) Harra, L. K., Sakao, T., Mandrini, C. H., et al. 2008, ApJ, 676, L147
  * Hinode Review Team et al. (2019) Hinode Review Team, Al-Janabi, K., Antolin, P., et al. 2019, PASJ, 71, R1
  * Kamio et al. (2010) Kamio, S., Hara, H., Watanabe, T., Fredvik, T., & Hansteen, V. H. 2010, Sol. Phys., 266, 209
  * Kosugi et al. (2007) Kosugi, T., Matsuzaki, K., Sakao, T., et al. 2007, Sol. Phys., 243, 3
  * Lemen et al. (2012) Lemen, J. R., Title, A. M., Akin, D. J., et al. 2012, Sol. Phys., 275, 17
  * Mandrini et al. (2015) Mandrini, C. H., Baker, D., Démoulin, P., et al. 2015, ApJ, 809, 73
  * Müller et al. (2020) Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1
  * Murray et al. (2010) Murray, M. J., Baker, D., van Driel-Gesztelyi, L., & Sun, J. 2010, Sol. Phys., 261, 253
  * Pesnell et al. (2012) Pesnell, W. D., Thompson, B. J., & Chamberlin, P. C. 2012, Sol. Phys., 275, 3
  * Peter (2010) Peter, H. 2010, A&A, 521, A51
  * Polito et al. (2020) Polito, V., De Pontieu, B., Testa, P., Brooks, D. H., & Hansteen, V. 2020, ApJ, 903, 68
  * Pulupa et al. (2017) Pulupa, M., Bale, S. D., Bonnell, J. W., et al. 2017, Journal of Geophysical Research (Space Physics), 122, 2836
  * Rathore et al. (2015) Rathore, B., Pereira, T. M. D., Carlsson, M., & De Pontieu, B. 2015, ApJ, 814, 70
  * Sakao et al. (2007) Sakao, T., Kano, R., Narukage, N., et al. 2007, Science, 318, 1585
  * Tian et al. (2011) Tian, H., McIntosh, S. W., De Pontieu, B., et al. 2011, ApJ, 738, 18
  * Warren et al. (2011) Warren, H. P., Ugarte-Urra, I., Young, P. R., & Stenborg, G. 2011, ApJ, 727, 58