Various Local Heating Events in the Earliest Phase of Flux Emergence

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by  Shin Toriumi

Draftversion January9,2017
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
VARIOUS LOCAL HEATING EVENTS IN THE EARLIEST PHASE OF FLUX EMERGENCE
Shin Toriumi1,2, Yukio Katsukawa1, Mark C. M. Cheung1,3
1NationalAstronomicalObservatoryofJapan,2-21-1Osawa,Mitaka,Tokyo181-8588,Japan
3Lockheed MartinSolarandAstrophysicsLaboratory, 3251Hanover Street,Building/252,PaloAlto,CA94304,USA
7 2
[email protected]
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0
ABSTRACT
2
Emergingflux regions(EFRs) are knownto exhibit various sporadiclocal heating events in the lower
n
a atmosphere. To investigate the characteristics of these events, especially to link the photospheric
J magnetic fields and atmospheric dynamics, we analyze Hinode, IRIS, and SDO data of a new EFR

5 in NOAA AR 12401. Out of 151 bright points (BPs) identified in Hinode/SOT Ca images, 29 are

overlapped by an SOT/SP scan. Seven BPs in the EFR center possess mixed-polarity magnetic
]
R backgroundsin the photosphere. Their IRIS UV spectra (e.g., SiIV 1402.8 ˚A) are strongly enhanced
S and red- or blue-shifted with tails reaching ±150 km s−1, which is highly suggestive of bi-directional
. jets, and each brightening lasts for 10 – 15 minutes leaving flare-like light curves. Most of this group
h
p showbaldpatches,the U-shapedphotosphericmagneticloops. Another10BPsarefoundinunipolar
- regionsattheEFRedges. TheyaregenerallyweakerinUVintensitiesandexhibitsystematicredshifts
o with Doppler speeds up to 40 km s−1, which could exceed the local sound speed in the transition
r
t region. Both types of BPs show signs ofstrong temperature increasein the low chromosphere. These
s
a observational results support the physical picture that heating events in the EFR center are due
[
to magnetic reconnection within cancelling undular fields like Ellerman bombs, while the peripheral

1 heating eventsare due to shocksorstrong compressionscausedby fastdownflowsalongthe overlying
v arch filament system.
6
4 Keywords: Sun: chromosphere – Sun: corona – Sun: magnetic fields – Sun: photosphere – (Sun:)
4 sunspots
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0
. 1. INTRODUCTION with a lifetime of . 10 minutes (e.g., Georgoulis et al.
1
2002). As shown by radiative transfer models (e.g.,
0 Magnetic flux emergence and the formation of ac-
7 Kitai 1983), the confinement of the intensity enhance-
tive regions (ARs) is one of the most fascinating
1 ment to the wings of the Hα suggests that Ellerman
phenomena of the Sun (Zwaan 1985; Cheung & Isobe
:
v 2014; Schmieder et al. 2014). Apart from catas- bombs are heating events in the photosphere, while
i high-resolutionobservations revealedthat the Ellerman
X trophic energy-releasing phenomena such as flares and
bombs are located at the sites of magnetic reconnec-
r CMEs (Shibata & Magara 2011) and X-ray jets ob-
a tion in the photosphere with upright flame-like struc-
served higher in the corona (Shibata et al. 1992),
tures(Watanabe et al.2011;Vissers et al.2013,2015b).
a wide variety of smaller-scale events are found in
Recent observations by Interface Region Imaging Spec-
ARs. Repeated brightenings and jet ejections oc-
trograph (IRIS; De Pontieu et al. 2014) show that the
cur in light bridge structures (Roy 1973; Asai et al.
location of the UV bursts (IRIS bombs) are coincident
2001; Toriumi et al. 2015a,b), whereas jet-like features
withlocationswhereopposite-polaritymagneticpatches
are detected in sunspot penumbrae (Katsukawa et al.
in an emerging flux meet (Peter et al. 2014; Tian et al.
2007; Vissers et al. 2015a). Coupling of magnetic
2016). Independently, numerical models of AR forma-
fields and local convection may drive such activity
tionfollowingfluxemergencepointtotheimportanceof
(Spruit & Scharmer 2006).
reconnectioninopposite-polaritypatches(Cheung et al.
Even at the earliest stage of flux emergence, various
2010). Such local heating events occur anywhere in the
small-scale events are observed. A commonly reported
EFRs with dynamic temporal variations.
type of small-scale events is Ellerman bombs, brighten-
Therefore,inordertorevealthenatureoftheseevents,
ings in the Hα line wings (Ellerman1917; see review by
Rutten et al. 2013). They have a typical size of . 1′′ it is crucially important to focus on its earliestphase of
2 Toriumi et al.
the flux emergence (within a few hours from the initial having a pixel size of 0′.′33×0′.′33, FOV of 119′′×118′′,
appearance), detect as many events as possible with a andcadenceof20.6s. Weusedthelevel2data,inwhich
wide field-of-view (FOV) as well as sufficient temporal the dark-current subtraction, flat fielding, and geomet-
and spatial resolutions, and analyze them in a statisti- rical and orbital variation corrections were taken into
cal way. Furthermore, to investigate the physical con- account1.
nectionbetweenphotosphericmagneticfieldsandatmo- We also analyzed the data taken by the Solar Dy-
spheric dynamics, we need simultaneous photospheric namics Observatory (SDO; Pesnell et al. 2012). The
spectropolarimetry and chromospheric and transition- photospheric evolution was monitored by the Helioseis-
region spectrograph on the EFRs, which can only be mic and Magnetic Imager (HMI; Scherrer et al. 2012;
achieved through the coordinated observation by Hin- Schou et al. 2012), while the response in the higher at-
ode (Kosugi et al. 2007) and IRIS. mospherewascapturedbytheAtmosphericImagingAs-
Althoughthis kindofobservationis difficult andthus sembly(AIA;Lemen et al.2012). Thepixelsizesforthe
is still rare, here we report on the analysis of a data set HMIandAIAimagesare0′.′5and0′.′6,respectively. The
that satisfies all the afore-mentioned conditions. The timecadenceis45sfortheHMIimagesand24sforthe
rest of this paper is organized as follows. First, in Sec- AIA’s two ultraviolet images (1600 and 1700 ˚A). The
tion2,wedescribetheobservations. Then,inSection3, center-to-limb variation was first subtracted from the
wepresenttheanalysisresults,whicharesummarizedin AIA images by applying the method in Toriumi et al.
Section4. Finally, in Section5, we discuss the observed (2014). All the observational data were co-aligned by
phenomena and possible physical pictures. using the AIA images as a reference.
2. OBSERVATIONS 3. RESULTS
ThetargetEFRofthisstudyappearedwithinthepre- 3.1. Overall Evolution
existingARNOAA12401. ThisARhadasimplebipole Figure1 showsthe overviewofthe targetEFRinthis
structure,andthetargetEFRstartedemergencearound study: seemovieforthetemporalevolution. Inthecen-
06:00UT,2015August19,atthe centeroftheAR.The ter ofthe 30′′×30′′ FOV,the HMI magnetogramshows
magnetic flux increased rapidly from around 09:00 UT, thatanumberoftinymagneticelementsofpositiveand
which was captured by Hinode and IRIS (coordinated negative polarities (a few arcsec) emerge at the surface
observation). At this time, the target EFR was located andseparatedeachotherinthenortheast-southwestori-
around(320′′,−270′′) onthe solardisk, i.e., atan angle entation. Eventually, the magnetic elements merge to-
◦
of ∼26 from the disk center (DC). gether to form pores at the edges of the EFR (see the
The spectropolarimeter (SP; Lites et al. 2013) of the HMI intensitygram: pore size ∼5′′).
Hinode/Solar Optical Telescope (SOT; Tsuneta et al. Atthesametime,inthechromosphere,variousbright-
2008) obtained a single raster scan from 10:35 to 11:00 eningeventsareseenindifferentfiltergrams(1400,1600,
UT in the Fe I lines at 6301.5 and 6302.5 ˚A. The SP and 1700˚A). They are located mostly in the central re-
scanhasapixelsizealongtheslitandastepsizeof0′.′32 gion with mixed magnetic polarities but also in the pe-
and 0′.′30, respectively, and the FOV is 113′.′5×122′.′9. ripheralunipolarregions. Thebrightstructuresareseen
Also, the broadband filter imager sequentially shot the tobelargerin1600and1700˚Aimagesnotonlybecause
CaIIH(3968.5˚A)imageswithatemporalgapbetween of the large pixel sizes but also of the scattering effects
08:29 and 10:34 UT. The CaII data have a pixel size of (see, e.g., Rutten 2016). Other filtergrams (e.g., 1330
0′.′22×0′.′22,FOVof223′.′2×111′.′6,andcadenceof63s. and 2796 ˚A) exhibit similar brightening structures (not
We applied standard procedures for prepping the SOT shown).
data sets. The photospheric vectormagnetic fields were In the map of the Mg II k absorption line core (k3),
obtained from the level 2 SP data by using MERLIN anarchfilamentsystem(AFS)isclearlyseen,whichhas
(Lites et al. 2007) and AZAM (Lites et al. 1995). an area of ∼30′′×20′′ with a typical individual length
The IRIS data were made between 07:48 and 10:54 of ∼ 20′′ and a width of ∼ 2′′. The Doppler velocity
UT. This observation sequence (OBS 3860106092)con- mapobtainedfromthe SiIVspectrumrevealsageneral
sists of large sparse 64-step rasters and slit-jaw images trend that the EFR center (crests of the AFS) shows
(SJIs). During this period, 34 raster scans were made upflowsandtheperipheries(footpointsoftheAFS)have
for CII 1334.5and 1335.7 ˚A, SiIV 1402.8˚A, and MgII downflows. These observational results are consistent
h 2803.5 and k 2796.4 ˚A, which have a step size, step withtheHαobservationsreportedbyBruzek(1967)and
cadence, and raster cadence of 1′.′0, 5.2 s, and 330 s, re-
spectively. The pixel size is 0′.′33 along the slit and the
FOV of each scan is 63′.′9×118′.′8. The SJIs are com- 1Absolutewavelengthcalibrationwasdoneusingthecold(pho-
posed of 1330,1400,2796,and 2832˚A filtergrams,each tospheric)NiI2799.5˚Aline.
Local Heating Events in Flux Emergence 3
Bruzek (1969). The intensity of the Mg II triplet line regionsoutsidetheEFR(nineBPs). Thegeneraltrends
between MgII h and k is enhanced at the footpoints of of these events are similar to those of the limb-side and
the AFS. DC-side unipolar events.
In this section, we introduce the spectral and polari-
3.2. Selection of Brightening Events metricfeaturesofthemixed-polarity,limb-sideunipolar,
For surveying the variety of local heating events, we andDC-sideunipolareventswithshowingtherepresen-
identifiedbrightpoints(BPs)fromtheSOTCaIIHim- tative BPs.
ages. Theintensity thresholdforpicking upsuchevents
3.3.1. Mixed-polarity Events
is the 5σ or higher above the mean of the AIA 1700
˚A quiet-Sun values, which is suggested by Rutten et al. The mixed-polarity BPs are basically located in the
(2013). central region of the EFR. A typical example of the
Figure 2(a) shows a scatter diagram of 1700 ˚A inten- mixed-polarity event is shown in Figure 3. This figure
sity and Ca II H intensity. By fitting a straight line to consists of three filtergrams and a circular polarization
this doubly logarithmic plot, we obtained the relation map of a BP located at (X,Y)=(336′′,−274′′) in Fig-
betweenthe twodatasetsandfoundthatthe 5σ+mean ure 2(b) plus the IRIS spectra and SOT SP Stokes-V/I
criterion of the 1700 ˚A images, which is 4232.1 DN in profiles for the three different locations within the BP.
the present data set, is equivalent to 1005.1 DN in the Typically, in magnetized regions, Stokes-V/I has an
Caimages. WeadoptedthisvaluefordetectingtheBPs antisymmetricprofilewithrespecttothelinecenterwith
in the Ca images and this criterion led to a total of 151 two lobes of opposite signs in the red and blue wings.
BPs from the sequential Ca data series in the period The three Stokes-V/I profiles in Figure 3 show a re-
from 10:34 to 10:59 UT (25 frames). versal of the sign, which indicates the transition of the
Out of this 151 BPs, we found that 29 BPs were cov- LOSmagneticfieldsbetweenpositive(coloredblue)and
ered by the SOT SP scan. Figure 2(b) shows the dis- negative (red) polarities. At the central position of the
tribution of the 29 BPs on the SOT SP circular po- BP located between the opposite polarities, the purple
larization map (Stokes-V/I signal at 6301.43 ˚A), which Stokes-V/Iprofileshowsanasymmetric,irregularshape.
representstheline-of-sight(LOS)componentofthepho- Such shapes are often interpreted as a co-existence of
tospheric magnetic fields. Here, the BPs have a size of closely neighboring two different magnetic components,
sub-arcsec to a few arcsec and are mostly seen in the e.g., positive and negative polarities, within a single
magnetized regions. pixel (Cheung et al. 2008), or as a complex stratifica-
We then checked the magnetic context of tionofmagnetizedatmospherealongtheLOSdirection.
the 29 BPs using the degree of mixed polarity One of the clearest characteristics of the mixed-
(Katsukawa & Tsuneta 2005), polarity BPs is the strong enhancements of the line in-
(cid:12) (cid:12) tensities and spectral widths. In Figure 3, the spectral
pmix =1−(cid:12)(cid:12)|Φ+|−|Φ−|(cid:12)(cid:12), (1) lines have their maximum intensities at location indi-
(cid:12)|Φ+|+|Φ−|(cid:12) cated by the purple cross, i.e., at the BP center (see
whereΦ±isthetotalLOSfluxofthepositive(+)orneg- filtergrams and spectra). Among others, the peak in-
ative (−) polarities within each BP patch. If pmix = 0 tensity of Si IV spectrum is as much as 50 times higher
theBPhaseitherapositiveornegativepolarity(unipo- than its quiet-Sun value. The purple profile at the BP
lar), while pmix 6=0 indicates that the BP has both po- center shows extended line wings of Doppler speeds of
laritieswithintheCaintensitycontour(mixed-polarity). upto±100km s−1. Morphologically,this BPhasasize
Asaresult,wefoundthattherearesevenmixed-polarity of about 3′′×3′′ with a point-like bright core.
BPs and 22 unipolar BPs. It is noteworthy here that the Doppler shifts of the
IRIS spectra, especially of the transition-region lines
3.3. Spectral and Polarimetric Features
(Si IV and C II), show a positional dependence. For
The 29 Ca BPs identified in Section 3.2 are classi- example, the spectra indicated by red color, i.e., those
fied into the following categories. The first group is the of the DC side ofthe BP,havered-shifted profiles,indi-
mixed-polarityevents(sevenBPs). Theyaredistributed catingthatthematerialatthetransition-regiontemper-
mostly inthe center ofthe targetEFR.The secondand ature is flowing down. The Doppler velocity measured
thethirdgroupsaretheunipolareventsinthelimb-side, at the Si IV peak is about 20 km s−1. On the other
i.e., the south-western end of the EFR (five BPs) and hand, the blue-coloredspectra,i.e., the limb-wardones,
in the DC-side, i.e., the north-eastern end (five BPs), are clearly blue-shifted, suggesting the upflow with an
respectively. absolute velocity of ∼30 km s−1. Similar Doppler-shift
There are some remaining unipolar events, which are dependencyofthelocation(DC-wardorlimb-ward)was
distributedintheEFRcenter(threeBPs)andtheplage recently reported by Vissers et al. (2015b).
4 Toriumi et al.
TheMgIIhandkspectraoftenexhibitdouble-peaked unipolar nature of this BP is clearly reflected: they do
profileswiththeso-called“self-absorption”cores. These not show a reversalor an irregular shape.
centralreversalsarecausedbytheAFS(seeMgIIk3im- The lineintensities aregenerallyweakerthanthoseof
age of Figure 1), indicating that the Ca BPs are mostly the mixed-polarity events. For example, the maximum
covered by the thick and dense AFS material that ex- intensities of the Si IV and C II lines in Figure 4 are
ertslargeopacity. Similarly,the CIIspectraoftenshow at most five times of the quiet-Sun value, as opposedto
centralreversals,too,whiletheSiIVdonotinthisdata muchmore than10times forthe mixed-polarityevents.
set. Anothercharacteristicofthelimb-sideunipolarevents
ThesubordinateMgIItripletlineissometimesseenin is that the Si IV and C II spectra coherently show red-
emission. In Figure 3, the triplet emission is exception- shifts, i.e., downflows. In Figure 4, the Doppler velocity
ally remarkableatthe center ofthe BP (purple profile). at the peak of the SiIV line reaches 30 km s−1.
According to Pereira et al. (2015), the triplet emission It is also interesting that the Mg II lines do not
israreandindicatesasteeptemperatureincreaseabove show regular double-peaked profiles with central rever-
1500 K in the lower chromosphere. These features are salsbutsingle-peaked(ortop-flat)profiles. Accordingto
also reported by Vissers et al. (2015b) and Tian et al. Carlsson et al. (2015), such single-peaked Mg II k pro-
(2016). files are found above the plage regions and imply the
The circular polarization map in Figure 3 also shows existence of hot and dense chromosphere of about 6500
thelocationofthepolarityinversionlines(PILs),where K and a transition region at a high column mass. In
themagneticfieldhasnoradialcomponent(i.e.,Bz =0) fact, the single-peaked Mg II k profiles in this data set
and whether the magnetic structure atthe PIL is “dip” are seen mostly in the vicinity of or above the devel-
or“baldpatch”,inwhichthe fieldlines haveaconcave- oping pores. Their Doppler shifts are not remarkable
up configuration, compared to those of the transition-region lines (Si IV
B =0 and B·∇B >0, (2) and CII). It should also be noted that the MgII triplet
z z
emission,whichindicatesthelower-chromosphericheat-
or “top”, in which the field lines are concave down, ing, are seen at some locations.
Bz =0 and B·∇Bz <0 (3) 3.3.3. Unipolar Events: DC-side
(Pariat et al. 2004; Watanabe et al. 2008). Since we The second group of the unipolar BPs is located in
haveonlyasingle-layermagnetogram,itwouldbebetter the DC-side, i.e., the north-eastern end, of the target
to explicitly describe the bald patch as EFR.Figure5showsthesampleeventaround(X,Y)=
∂B ∂B (328′′,−266′′) in Figure 2(b).
B =0 and B z +B z >0 (4)
z x y This BP also exists at one footpoint of the AFS seen
∂x ∂y
in the Mg II k3 images (Figure 1). The Stokes-V/I sig-
and the counterpart as
nals in Figure 5 show that this BP has a positive mag-
B =0 and B ∂Bz +B ∂Bz <0. (5) netic polarity with no irregular profiles. The line inten-
z x y
∂x ∂y sities are againweakerthan those of the mixed-polarity
Notethatwehere usethe photosphericvectormagnetic events. Especially in the 1400 ˚A map, the intensity en-
fields (B ,B ,B ), which is transformed to the local hancement is much less marked.
x y z
Cartesianreferenceframe(x,y,z)withzˆ beingthelocal The systematic redshifts are also seen in the Si IV
radialdirection. Itisclearlyseenfromthecircularpolar- and C II profiles. However, the redshift speeds of these
izationmapofFigure3thatthePILpassesthroughthe events are generally smaller than those of the limb-side
centerofthe CaBP andthat the magneticfields havea unipolar events. The maximum Doppler velocityof this
bald-patchconfiguration(yellowdots). It is found that, BP (Figure 5) is 15.6 km s−1.
infivemixed-polarityBPsoutofthe seventotalevents, InFigure5,thesingle-peakedMgIIkprofiles(redand
the PIL is dominated by the bald patch: see Appendix purple)arefoundnearthepore,whichisconsistentwith
A for PILs of the six remaining mixed-polarity BPs. the limb-side unipolar events (Figure 4), while the blue
Mg II k profile indicates that this location is probably
3.3.2. Unipolar Events: Limb-side
coveredbytheAFS.Althoughnotclear,Figure5shows
Figure 4 is a typical example of the unipolar BP in an indication of a slight MgII triplet emission.
the limb-side of the target EFR. This BP is centered at
(X,Y)=(350′′,−272′′) in Figure 2(b). This location is 3.3.4. Averaged Profiles
one of the the south-westernfootpoints of the AFS (see Figure6comparestheprofilesoftheSiIV,MgII,CII,
Figure 1) and is surrounded by a large magnetic patch and Stokes-V/I averaged over the areas covered by the
of the negative polarity. In the Stokes-V/I profiles, the seven mixed-polarity BPs, five limb-side unipolar BPs,
Local Heating Events in Flux Emergence 5
and five DC-side unipolar BPs. level. Similar drastic Si IV enhancements of more than
The mixed-polarity events (orange) are characterized 10 times are seen for most of the mixed-polarity events.
by their enhanced and broadened profiles, especially of Another feature to note for the mixed-polarity events
the Si IV and C II lines. The peak intensities of the is that the intensity enhancements are often repetitive
averaged Si IV and C II lines are about 9 times of the at the same locations. For the mixed-polarity BP in
quiet-Sunlevels,while theirwingsexceed±150km s−1. Figure 7, the preceding event occurs about 20 minutes
The Mg II k has a double-peaked profile, which indi- before the main one.
cates that the mixed-polarity BPs are generally located Onthe otherhand,the lightcurvesforbothlimb-side
below the fibrilar canopy of the AFS. The MgII triplet and DC-side unipolar events are generally less marked.
emissionisnomoreseeninthisaveragedprofilebecause TheevolutionsoftypicaleventsinFigure7(middleand
the triplet emission is rare and, if occurs, exists only in bottom)arerathersteadyovertime,andthenormalized
the cores of the BPs. intensities become only a few times, at most 10 times,
The clearest feature of the limb-side unipolar events of the quiet-Sun levels. The most enhanced intensities
(green solid), which shows a negative polarity in the are again of the SiIV spectra.
Stokes-V/I profile, is the redshifts of the SiIV and CII
3.5. Statistical Trends
spectra. These are caused by the systematic downflows
atthefootpointsoftheAFS.Thepeakintensitiesofthe The statistical trends for the analyzed 29 Ca BPs are
Si IV and C II lines are darker, only twice and 4 times shown in Figure 8. Here, the Ca intensities of both
of their quiet-Sun values, respectively. Another obvious typesoftheBPsareclearlydistinct. Themixed-polarity
characteristic is the single-peaked shape of the Mg II k eventsaregenerallybrighterthantheunipolaroneswith
spectrum. Such a profile is created probably because the critical value being ∼1600 DN.
these AFS footpoints are located above or very close to The maximum area of the Ca BPs is positively cor-
the developing pores. The MgII triplet is seen in emis- related with the intensity. Most of the mixed-polarity
sion, indicating the heating of the lower chromosphere. eventshaveareasrangingfrom0.8×106to4.5×106km2,
Comparedtothe limb-sideevents,theredshifts ofthe which is equivalent to the radii of 500 to 1200 km if
Si IV and C II lines of the DC-side, positive unipolar one assumes circular shapes. The unipolar BPs are
events(greendashed)arenotveryprominent,whilethe .2×106km2 inarea,oroftheradiusof.800km. The
peak intensities of these lines show intermediate values Ca lifetime shows, although more than half of the mea-
between the mixed-polarity and the limb-side unipolar: surements are limited by the cadence and the durations
six and eight times the quiet-Sun values. The Mg II k of the Ca observation by SOT, which are 1 minute and
has an asymmetric profile with the central dip, proba- 25 minutes, respectively,a positive relationwith the Ca
bly caused by the mixture of the single-peaked profiles intensity.
near the pores and the double-peaked ones under the The plot for the maximum and minimum Doppler
AFS. Similar to the limb-side events, the Mg II triplet velocities, which is obtained by simply fitting a single
emission is seen. Gaussian function to the Si IV profile, clearly shows
the difference between the two groups. While the
3.4. Temporal Evolutions
mixed-polarity events have a wider velocity range from
Figure 7 plots the light curves of several chromo- −32.4 km s−1 (blueshift) to 20.4 km s−1 (redshift), the
sphericandtransition-regionlinesforthethreerepresen- unipolar ones are remarkably one-sided with a range of
tativeCaBPsintroducedinSection3.3(mixed-polarity, −8.4 to 35.7 km s−1, i.e., the downflows (redshifts) are
limb-side unipolar, and DC-side unipolar events). The dominated. Note that we here use the single-Gaussian
light curves are obtained as time-varying intensities in- fitting,whichprovidestheDopplervelocitynearthecore
tegrated over a 1′′×1′′ box, centered at the middle of of the spectrum and may be affected when the profile
the BPs. In this figure, each light curve is normalized contains of more than two components.
by its quiet-Sun value. Themaximumintensitiesofthe SiIV,CII,andMgII
For the mixed-polarity event (Figure 7 top), the light triplet lines also show positive relations with the Ca II
curvesshowdrasticenhancementaround10:45UTwith H intensity. However, the dynamic range for the Mg II
a duration of 10 to 15 minutes. They show similar evo- triplet is less than one order of magnitudes, 120 to 690
lutionprofileswitheachother,afastrisefollowedbyan DN, as opposed to the wider ranges of the Si IV and
extended decay, which is highly reminiscent of the light CII, 7.3 to 2000 DN and 19 to 1500 DN, respectively.
curvesofHαandsoftX-raysrecordedinthe solarflares
4. SUMMARY
(e.g., Kane 1974). The most significant enhancement is
of the wavelength-integrated intensity of the Si IV line, Inthis paper,wehaveinvestigatedvariouslocalheat-
which increases up to almost 100 times of its quiet-Sun ing events in the EFRs using the co-observation data
6 Toriumi et al.
obtained by Hinode and IRIS, complemented by SDO. the IRIS andHα spectraof atextbook Ellermanbomb.
Themainobservationalresultsofourstudyaresumma- Recently, Tian et al. (2016) and Grubecka et al. (2016)
rized as follows. demonstrated that the Mg II h and k lines of Ellerman
Themixed-polarityeventsareseeninthecentralpart bombs show intense brightening in the wings and no
of the EFR, which is characterized by enhanced and significant enhancement in the core, similar to the Hα
broadenedchromosphericandtransition-regionspectra. profiles. The Mg II spectra in Figure 3 agree with this
In particular, the Si IV line brightens up significantly trend, which further supports that our events are likely
to 100 times of the quiet-Sun intensity. Sometimes the Ellerman bombs.
SiIVandCIIspectraspreadovertheDopplerspeedsof Recent high-resolution observations indicate that
±150 km s−1. The Doppler velocities at the Si IV line the Ellerman bombs are the photospheric reconnec-
centerhaveapositionaltendencythattheredshiftsofup tion with flame-like structures (Watanabe et al. 2011;
to 20 km s−1 are seen in the DC side and the blueshifts Vissers et al. 2013; Rutten et al. 2013). Such flames
of up to 30 km s−1 are seen in the limb side within the maynotberesolvedinourobservationswithaminimum
BP. The Mg II k spectra often show the central rever- pixel size of 0′.′22 (Ca II H). Also, especially in 1700 ˚A,
sals, which is caused by the overlyingAFS obscuration, they may be blurred by scattering (Rutten 2016).
whilesometimestheMgIItripletisseeninemission,in- The redandblueshiftsobservedinthemixed-polarity
dicating the lower chromosphericheating. These events BPscanbeinterpretedasthebi-directionaljetsfromthe
are mostly found above the PILs with the bald-patch reconnectionsites. The DC- andlimb-warddependence
(i.e., U-shaped) magnetic configurations in the photo- oftheDopplervelocitymaybeduetotheviewingangle.
sphere. They are larger in size, longer in lifetime. The AsthetargetEFRofthisstudyislocatedawayfromthe
light curves show flare-like evolutions with durations of DC by 26◦ – 27◦, the slanted viewing angle may cause
10–15minutesandthebrighteningsrecurwithperiods thesamplingofupflowsinthelimb-side(orupperparts)
of about 20 minutes. and downflows in the DC-side (lower parts). Perhaps
Theunipolareventsaremostlyfoundintheperipheral onemightexpectthesiphonflowalongthesmallΩ-loop
regionsoftheEFR,i.e.,thefootpointsoftheAFS.These forexplainingtheredandblueshifts. However,thedom-
eventsareweakerinintensitieswiththemaximumSiIV inanceofthebald-patchPILsforthemixed-polarityBPs
intensity being at most 10 times of the quiet-Sun level. favors the U-loop cancellation theory (Georgoulis et al.
The Si IV spectra show systematic downflows with the 2002; Pariat et al. 2004; Watanabe et al. 2008).
maximum Doppler speed of ∼ 40 km s−1 at the line The unipolar BPs at the periphery of the EFR may
center. The redshift velocity is generally faster for the be caused by the downflows (Figures 9(c) and (d)).
limb-side events than the DC-side events. If the BP is Bruzek(1969)foundthefastdowndraftsof∼50km s−1
close to or abovea pore,the MgII k spectrum revealsa (in Hα) along the field lines of the AFS at both foot-
single-peakedprofile with no absorptioncore,similarto points. Using magnetohydrodynamic (MHD) simula-
thespectrafoundintheplageregions. TheMgIItriplet tions, Shibata et al. (1989) modeled the EFRs and sug-
isseeninemissionoccasionally. Theyaresmallerinsize, gested that the bright plages near the AFS footpoints
shorter in lifetime, and the evolution is steady. are caused by the shocks,which is produced by the fast
downflows of 30 to 50 km s−1 along the AFS entering
5. DISCUSSION the high-density lower atmosphere. The above observa-
The observational results summarized in Section 4 tions and simulations point to the possibility that the
lend support to the physical picture illustrated in Fig- unipolar BPs in the present study are also due to the
ure 9. In EFRs, the magnetic fields appear at the pho- downflows.
tosphere as Ω-shaped or undular field lines and evolve If we simply assume a CHIANTI-based (i.e., coronal-
through episodes of merging and cancellations of frag- equilibrium) SiIV formationtemperature of logT [K]=
mentary magnetic elements. The AFS is seen above, 4.8 –4.9 (De Pontieu et al.2014; Peter et al.2014), the
which may represent the ascending magnetic fields in local adiabatic sound speed is & 40 km s−1. How-
the higher altitudes. ever, it is more likely that the plasma at the AFS foot-
The observed mixed-polarity Ca BPs may indicate pointsisdenserandthattheSiIVtemperatureiscooler
magnetic reconnection between the cancelling positive than these values. Since Rutten (2016) suggests 15,000
and negative polarities, as shown in Figure 9(b). Al- to 20,000 K for the Si IV temperature for the Eller-
thoughwe have no Hα observationfor our events,these man bombs (the heating of low-chromospheric to mid-
BPs should be closely related to the classical Ellerman photospheric plasma), let us assume, say, 40,000 K as
bombs (Ellerman1917). The striking consistencies may the temperature. Then, we obtain the sound speed of
be found if one compares Figure 3 of this paper and ∼30km s−1. Therefore,thereisagoodchancethatthe
Figure 4 of Vissers et al. (2015b), in which they show downflows observed above the unipolar regions, whose
Local Heating Events in Flux Emergence 7
Dopplervelocityisupto35.7km s−1,arecomparableto transfer and examine the proposed scenarios.
or exceed the local sound speed. Although we did not
directly detect the shock fronts, considering that the
The authorsaregratefulto the anonymousrefereefor
Mg II lines are not strongly Doppler-shifted (Section
improvingthemanuscript. TheauthorsthankISSIBern
3.3.2) and that the typical sound speed in the photo-
for the support to the team “Solar UV bursts — a new
sphereisbyfarsmaller,theremaybeshocks,oratleast
insight to magnetic reconnection.” S.T. would like to
strong compressions and condensations of the material,
thank Dr. Robert Rutten, who visited NAOJ and gave
which locally heat the atmosphere.
a lecture on the solar spectral formation, for fruitful
Thedifferencesoftheobservationalcharacteristicsbe-
discussion. Data are courtesy of the science teams of
tween the limb-side and DC-side events, especially of
Hinode, IRIS, and SDO. Hinode is a Japanese mission
theDopplervelocities,maybeduetotheviewingangle:
developed and launched by ISAS/JAXA, with NAOJ
compare Figures 9(c) and (d).
as domestic partner and NASA and STFC (UK) as in-
Perhapsthisscenariohasarelationwiththetransient
ternational partners. It is operated by these agencies
supersonic downflows and associated heating events
in cooperation with ESA and NSC (Norway). IRIS
found above the sunspots by Kleint et al. (2014) and
is a NASA small explorer mission developed and op-
Tian et al. (2014). However, their downflows are by far
faster, exceeding 200 km s−1, andprobably are the ma- erated by LMSAL with mission operations executed at
NASAAmesResearchcenterandmajorcontributionsto
terial falling higher from the coronal heights such as
downlink communications funded by ESA and the Nor-
coronal rain, as opposed to our steady, much slower
wegian Space Centre. HMI and AIA are instruments
downflows originated from the AFS.
on board SDO, a mission for NASA’s Living With a
The heating mechanisms suggested in this sec-
Star program. This work was carried out on the So-
tion explain the observations without inconsisten-
lar Data Analysis System operated by the Astronomy
cies and are, although not perfectly, supported
Data Center in cooperation with the Hinode Science
by MHD simulations (e.g., Isobe et al. 2007;
Center of NAOJ. This work was supported by JSPS
Tortosa-Andreu& Moreno-Insertis 2009; Cheung et al.
KAKENHI Grant Numbers JP16K17671 (PI: S. Tori-
2010; Shibata et al. 1989). However, especially for the
umi), JP15H05814 (PI: K. Ichimoto), and JP25220703
heatingintheunipolarregions,wecannotruleoutother
(PI: S. Tsuneta). MCMC acknowledges support by
possibilities such as magnetic reconnection that we
NASA contracts NNG09FA40C (IRIS), NNG04EA00C
cannot resolve, or the component reconnection between
(SDO/AIA) and NNM07AA01C (Hinode/SOT), and
the field lines of colliding magnetic patches of the same
grant NNX14AI14G (Heliophysics Grand Challenges
polarity(see, e.g., Figure12c ofGeorgoulis et al. 2002).
Research).
Therefore, it is of necessity to numerically model these
mechanisms with taking into account the radiative
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Overall Evolution (2015-08-19 10:47 UT)

SJI 1400 Å Mg II k3 Mg II triplet Si IV Doppler

-255
AIA 1600 Å AIA 1700 Å HMI magnetogram HMI intensitygram
-260
c]
se -265
c
ar
Y [ -270
olar- -275
S
-280
325 330 335 340 345 350
Solar-X [arcsec]
Figure 1. Composite of various observations to show the
overallevolutionofthetargetEFR.Thecorrespondingmovie
provides the temporal evolution. From top left to bottom
right, IRIS SJI 1400 ˚A, intensitygram at the Mg II k line
core (k3), intensitygram at the Mg II triplet line, Doppler-
gramproducedfromtheSiIV1403˚Aspectrum(blue,white,
andredcorrespondto−10,0,and+40km s−1,respectively),
SDO/AIA 1600 ˚A, 1700 ˚A, SDO/HMI magnetogram (black
andwhitecorrespondto−200and200G,respectively),and
intensitygram. The white arrow in the top left panel indi-
cates the direction of theDC.
Local Heating Events in Flux Emergence 9
(a) Scatter plot: Ca II H vs. 1700 Å
N]
D
y [ 1005.1
sit 1000
n
e
nt
H i
a II
C
4232.1
100
1000 10000
1700 Å intensity [DN]
(b) Distribution of Ca BPs
−255
SOT/SP CP
−260
C
c] −265
e
s
c
ar
Y [ −270
ar-
ol B
S −275
A
−280
325 330 335 340 345 350
Solar-X [arcsec]
Figure 2. (a) Scatter plot of the SOT Ca II H intensity
versus the AIA 1700 ˚A intensity. Density contours in log-
arithmic scale are plotted for large density regions to avoid
the plot saturation, while the logarithmic distributions for
both parameters are shown at the right and top of the dia-
gram. The vertical line indicates the 5σ+mean level of the
1700 ˚A intensity and the horizontal line shows the equiva-
lent value for the Ca II H intensity. (b) Distribution of the
Ca BPs plotted on the SOT SP circular polarization map,
identifying the 29 BPs analyzed in this study. Red contour
indicates the Ca II H intensity level obtained in panel (a),
while the red arrow at the bottom left shows the direction
of the DC. A, B, and C label the three representative BPs
introduced in Section 3.3.
10 Toriumi et al.
Mixed-polarity event (event A)
x [arcsec] x [arcsec] x [arcsec] x [arcsec]
0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3
1700 Å 1400 Å Ca II H CP
3
]
c
e
s 2
c
r
a
[
y
1
0
Doppler velocity [km s−1] Doppler velocity [km s−1]
−400 −200 0 −400 −200 0 200 400 600 800 1000

Si IV Mg II
800
2000
] ]
N N
D 600 1500 D
[ [
y y
sit 400 1000 sit
n n
e e
Int 200 500 Int
0 0
1401 1402 1403 2792 2794 2796 2798 2800 2802 2804
Wavelength [Å] Wavelength [Å]
Doppler velocity [km s−1]
−400 −200 0 200

1000 C II Stokes-V/I
0.2
800
]
N
D 600 0.1 V/I
nsity [ 400 0 okes-
nte −0.1 St
I 200
−0.2
0
1334 1335 1336 6301 6302 6303
Wavelength [Å] Wavelength [Å]
Figure 3. Spectral and polarimetric profiles of the sam-
ple mixed-polarity Ca BP (event A) located at (X,Y) =
(336′′,−274′′) in Figure 2(b). Four panels at the top are
thefiltergrams of AIA 1700 ˚A, IRISSJI 1400 ˚A, SOTCaII
H, and the SOT SP circular polarization map. The white
contourdelineates theCaBPs, whilethewhitearrowinthe
1700˚AimageshowsthedirectionoftheDC.Thedotsinthe
circular polarization map indicate the locations of the PIL,
where yellow and turquoise show that the magnetic fields
have “dip” (i.e, bald-patch) and “top” structures, respec-
tively (see text for details). Four bottom panels providethe
IRIS spectra of Si IV, C II, and Mg II, and the SOT SP
Stokes-V/I.Theprofilesofdifferentcolors(red,purple,blue)
are sampled at three location indicated by the + signs of
corresponding colors in thetop four images, while theblack
profiles are the averaged quiet-Sun levels obtained from the
same data sets. In the SiIV and CII panels, the quiet-Sun
profiles are multiplied by factors of 10. The vertical lines
showthemainlinesandsomeblends,whichareOIV1401.2
˚A,SiIV 1402.8 ˚A,MgII k 2796.4 ˚A,MgII triplet 2798.8 ˚A,
CII 1335.7 ˚A,FeI 6301.5 ˚A,and FeI 6302.5 ˚A.