Direct Observations of Plasma Upflows and Condensation in a Catastrophically Cooling Solar Transition Region Loop

4.2 MB·English

by  N. B. Orange

Draft version January 13, 2016
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
DIRECT OBSERVATIONS OF PLASMA UPFLOWS AND CONDENSATION IN A CATASTROPHICALLY
COOLING SOLAR TRANSITION REGION LOOP
N. B. Orange, D. L. Chesny, H. M. Oluseyi, K. Hesterly, M. Patel, and P. R. Champey
DepartmentofPhysics&SpaceSciences,FloridaInstituteofTechnology,Melbourne,FL32901
Draft version January 13, 2016
ABSTRACT
Minimal observational evidence exists for fast transition region (TR) upflows in the presence of cool
loops. Observations of such occurrences challenge notions of standard solar atmospheric heating
6 models, as well as their description of bright TR emission. Using the EUV Imaging Spectrometer
1 (EIS) onboard Hinode, we observe fast upflows (v ≤−10 km s−1) over multiple TR temperatures
λ
0 (5.8≤logT ≤ 6.0) at the footpoint sites of a cool loop (logT ≤6.0). Prior to cool loop energizing,
2 asymmetric flows of +5 km s−1 and −60 km s−1 are observed at footpoint sites. These flows speeds

n and patterns occur simultaneously with both magnetic flux cancellation (at site of upflows only)
a derived from the Solar Dynamics Observatory’s (SDOs) Helioseismic Magnetic Imager’s (HMI) line-
J of-sightmagnetogramimages,anda30%massin-fluxatcoronalheights. Theincurrednon-equilibrium

2 structure of the cool loop leads to a catastrophic cooling event, with subsequent plasma evaporation
1 indicating the TR as the heating site. From the magnetic flux evolution we conclude that magnetic
reconnectionbetweenthefootpoint andbackgroundfieldareresponsibleforobservedfastTRplasma

] upflows.
R
Keywords: corona, transition region, upflows
S
.
h
1. INTRODUCTION a response to cooling coronal plasma which was impul-
p
sively heated. The pervasively observed TR redshifts
- Aparamountchallengeinsolarphysicsstillremains—
o (Hansteen et al. 1996) and observations that cool loops
thatisdefinitivelysolvingtheproblemofhowuppersolar
r arecharacterizedbyplasmadownflows,atfootpointsand
t atmospheric structures [i.e., transition region (TR) and
s coronal]areheatedandmaintained(Tripathietal.2012). alongtheloopstructures(e.g.,DelZanna2008;Tripathi
a et al. 2009), have provided significant support to such
Studiesofplasmaloops,theprimarycomponentsofeach
[ models. However, observational evidence is emerging
levelofthesolaratmosphere(Hansonetal.1980;Walker
(to our knowledge only those reported by Tripathi et al.
2 et al. 1993a,b; Golub et al. 1999; Oluseyi et al. 1999a,b),
v have vastly improved and influenced our understanding 2012)thatrevealstheexistenceoffastTRupflowsinthe
9 of solar atmospheric heating, most notably that of the presence of cool loops. The significance of such results
9 corona (Aschwanden & Nightingale 2005; Warren et al. is the introduction of challenges to standard solar atmo-
9 2008). spheric heating models. Particularly these are their de-
4 scriptionsontheoriginofbrightTRemissionandheights
These basic building blocks of the solar corona, i.e.,
0 at which coronal heating occurs. Furthermore, emerg-
plasmaloops,arecommonlyclassifiedviatheirpeaktem-
. ing observational evidence for fast TR upflows, mainly
1 perature (Chitta et al. 2013). Hot loops (logT >6.0)
associated with explosive events (Beckers 1968a,b) and
0 havebeenextensivelystudiedandmodeled(e.g.,Mackay
spicules (De Pontieu et al. 2007; Langangen et al. 2008),
5 etal.2010;Aschwanden&Schrijver2002;Spadaroetal.
1 2006), while to a much lesser extent diffuse cool (TR) are providing increasing support that atmospheric heat-
: loops (logT ≤6.0; e.g., Chitta et al. 2013; Tripathi et al. ing is not confined to the corona.
v Though observational evidence exists indicating heat-
2012;Chesnyetal.2013;Mu¨lleretal.2003,2004;Oluseyi
i
X et al. 1999a,b). Recent observational and theoretical ad- ing occurs in cooler regions of the solar atmosphere; the
heights, timescales, and mechanisms responsible remain
r vancesontheheatingofplasmaconfinedwithinhotloop
a structures have revealed that both steady-state (e.g., unclear (Tripathi et al. 2012). Moreover, a current topic
of hot debate is whether fast TR upflows provide signif-
Winebarger et al. 2011; Warren et al. 2010) and impul-
icant mass-influx to coronal heights, as well as the role
siveheating(e.g.,Viall&Klimchuk2012;Tripathietal.
they play in the generation of observed coronal phenom-
2010) processes are consistent with their observed tem-
ena(DePontieuetal.2009;Langangenetal.2008;Klim-
perature and intensity structures. Impulsive heating has
chuk 2012). It is also emphasized that minimal investi-
been found to explain the properties of cool loops (e.g.,
gations have been carried out on how plasma conden-
Spadaroetal.2003),thatarenotinequilibriumandcon-
sation relates to non-thermal equilibrium states of cool
stantlyevolving(e.g.,Ugarte-Urraetal.2009). However,
loops(Mu¨lleretal.2003,2004), whiletheroleoftheun-
what remains unknown is the role plasma condensation
derlying magnetic field plays in such scenarios remains
plays in such processes (Chitta et al. 2013).
unquantified (Chitta et al. 2013).
Impulsive heating events, bundles of nanoflare heated
In relation to the above discussion, we have on hand a
loopstrands,occuratcoronalheightsandresultinchro-
uniquedata-setderivedfromobservationstakenwiththe
mosphericevaporation(e.g.,,Klimchuketal.2008;Klim-
EUV Image Spectrometer (EIS; Culhane et al. 2007) on-
chuk 2009). Bright TR emission is widely considered
boardHinodethatprovidesdirectobservationalevidence
2 Orange et al.
Table 1
ListofEISobservedemissionlineswithformatof: Ion,
wavelength(˚A),andlogarithmicelectrontemperature.
Ion Wavelength(˚A) logT
Heii 256.32 4.9
Ov 192.90 5.4
Feviii 185.21 5.8
Feix 197.86 5.9
Fex 184.54 6.0
Fexii 186.88 6.2
Fexii 195.12 6.2
Fexiv 274.20 6.3
Fexv 284.16 6.4
of high-speed upflows at multiple TR temperatures oc-
curring at the footpoint sites of a catastrophically cool-
ing loop. We compliment these data with line-of-sight
(LOS) magnetogram observations from the Heliosemic
and Magnetic Imager (HMI; Schou et al. 2012) onboard
SolarDynamicsObservatory(SDO)toinvestigatetheef-
fects of magnetic flux evolution on both plasma upflows
and the runaway cooling event.
Inthatrespecttherestofthepaperisasfollows: obser-
vationaldataitsprocessingandanalysisarepresentedin
the next section, § 3 presents the measurement results,
the discussion of these results and our conclusions are
provided in §’s 4 and 5, respectively.
2. OBSERVATIONS,PROCESSING,ANDANALYSIS
Observational data was obtained from EIS during the
following time frame: 14:02:36 UT to 15:19:55 UT at
≈20 min time intervals on 18 October 2011. The data
consistsofrasterscanswitha2(cid:48)(cid:48) slitwidthusing1(cid:48)(cid:48) steps
resulting in a final field-of-view (FOV) of 100(cid:48)(cid:48)×130(cid:48)(cid:48)
(Figure 1). Nine emission lines providing tempera-
turecoveragefromthechromosphere(logT ≈4.9)tothe
corona(logT ≈6.4)wereused. InTable1weprovidethe
emittingions,theirrespectivewavelengths,andpeakfor-
mation temperatures.
Imagepre-processingofEISlevel-0datawasperformed
with standard Solar SoftWare (SSW) to obtain flux cal-
ibrated data. Additional corrections were made for the
spatialoffsetoccurringbetweenitsshort(171–212˚A)and
long (250–290˚A) wavelength bands (Young & Gallagher
2008), instrument and orbital jitter variations (Shimizu
et al. 2007), CCD spectrum drift (Mariska et al. 2007),
and tilt of the emission on the detector. The resultant
EIS level-1 image data then possesses an absolute wave- Figure 1. Topandmiddle: EIS2(cid:48)(cid:48)intensityimages(ergcm−2s−1
length calibration of ±4.4 km s−1 (Kamio et al. 2010). sr−1),observed18October2011at14:59:21UT,ofOv(192.90˚A,
logT≈5.4) and Feviii (185.20˚A, logT≈5.8), respectively, with
The standard SSW routine eis auto fit.pro (Young
green contours denoting the loop and boxed regions denoting it’s
2010) was used to derive integrated spectral line in- footpoints,respectively. Bottom: HMILOSmagnetogram(G)dis-
tensities and their respective Doppler shifts, as well as played over the FOV of the EIS observation observed 18 October
buildbothintensityandLOSvelocityimagesoftheloop 2011at14:59:21UTwithfootpointregionsagainidentified.
(Figures 1 and 2). This routine fits a single Gaussian
fits were consistent with the report of Del Zanna (2012)
to each pixel forming the raster scan with the well-
thatinon-diskquietSunregionsitcontributesover80%
known mpfit.pro algorithm and propagates uncertain-
of the observed intensity. Further support of this notion
ties from 1σ fit uncertainties. However we note, during
isfoundinadirectcomparisonofdistinctbrightnetwork
this process multiple Gaussian fits were applied to both regionsoftheFexandHeiiintensityimagesinFigure3.
the Heii256.32˚A and Ov192.90˚A spectrums due to Ov spectrum clearly indicated, and supported notions,
their blending with coronal emission lines. Particulary, thatinmostregionstheFexi192.83˚Adominates(Young
Heii256.32˚A is blended with that of the Six256.37˚A et al. 2007a), which is also observed in the similarity of
and Fex256.41˚A lines, while Ov192.90˚A is blended Ov intensity images of Figure 3 to those of the corona.
with that of the Fexi192.83˚A line (Young et al. 2007b; However, in the cool loop regions (i.e., such as those
Brown et al. 2008). Visual inspection of Heii256.32˚A denoted by contours and boxes on Figure 1) enhanced
Catastrophic Cooling in Solar TR Loops 3
Figure 2. Topandbottom: LOSvelocityin(kms−1)andelectrondensity(incm−3)images,bothobservedat14:41:17UTon18October
2011,derivedfromtheintensityimageoftheFeix(197.86˚A,logT ≈5.9),andGaussianintensityratioofFexii186˚A/195˚A,respectively.
Onbothpanelsgreencontoursareintensitylevelsof80%–90%ofthepeakintensityderivedfromFeixemissionlineobservedat14:59:21
UTdenotingthecompletefilledcoolloop.
Ov192.90˚A emission was observed, and as such allowed ground/foreground emission is removed from the loop
it’s spectrum to be resolved and fitted. Moreover, we structure by applying custom written software to im-
point out Young et al. (2007a) observed similar physi- age thumbnails with the loop at their center and back-
calcharacteristicsinthespectrumaround192.83˚Afora ground/foreground emission surrounding it. The tech-
study of an active region TR brightening. niqueobtainsabackgroundestimateby: first,applyinga
FulldiskLOSmagnetogramsfromHMIareutilizedto low-passfiltertoremovehigh-frequencynoisesuchasbad
investigate the magnetic structure of the loops. HMI’s pixels;then,usesahistogramofimagefluxtoisolatethe
magnetogramswereobtainedwithaspatialresolutionof lowest10%;andfinally,appliesaweightedaveragetothe
≈0(cid:48).(cid:48)5atacadenceof≈45sandcorrespondedtotheap- lowest 98% of the isolated flux. It is noted, care is taken
proximatelyonehourofEISobservations. Magnetogram to reduce background overestimation (i.e., generation of
data was pre-processed using standard SSW techniques negative pixel values) since previous studies have shown
incorporating per-pixel noise subtraction (Brown et al. that coronal intensities of loop structures are ≈10% –
2011) and then averaged over ≈2.5 min intervals to in- 20% higher than background/foreground emission (e.g.,
crease the signal-to-noise ratio with additional pointing Viall & Klimchuk 2012; Del Zanna & Mason 2003). The
correctionsusingthetechniquesofOrangeetal.(2013a). result is a background subtracted image where pixel val-
Magnetograms were co-aligned to EIS scans, with obser- ues are reduced by (cid:46)10%. We then measure Fλ(s) by
vational time differences of (cid:46)three minutes, using the averaging over cross-sections of loop width at each point
SSW routine drot map.pro. A resultant alignment er- alongitsspine. Lightcurvesaregeneratedoftotalradia-
ror of (cid:46)2(cid:48)(cid:48) was measured by cross-correlating visually tive flux for each respective region (i.e., loop core, NFP,
brightcoronalstructurestostrongmagnetogramregions. and SFP) by integrating their 3σ brightest flux.
We also note, the image’s vicinity to solar disk center Resultant spectral intensities of the Fexii emission
((cid:46)±150(cid:48)(cid:48) in both the solar x and y directions) result in lines(Table1)wereusedtogenerateelectrondensityim-
negligible projection effects. ages of the loop (Figure 2) via the techniques discussed
The loop plus minimal background emission was de- byYoung(2011). Itisnoted,thougheachFexiiemission
fined by using a semi-supervised tracing algorithm ap- lineisblended,previousstudieshavesuggestedfordensi-
plied at each observational time step to Ov192.9˚A ties<1010 cm−3,consistentwithresultsherein,blending
(logT ≈5.4) intensity images (Figure 1). The resul- effects to the Fe xii195˚A line are not important (Dere
tant region was then used to isolate the loop in all 2008). Moreover, Dere et al. (2007) have suggested a
other EUV images. We assigned the loop a set coor- root-mean-squareerrorof≈1.6fordeterminationofden-
dinates s, corresponding to pixels along its spine, and sities in the quiet Sun when this line ratio is utilized in
segmentedthemintothreedistinctregionsofcore,north, theaforementionedconditions. Then,LOSvelocities(vλ;
and south footpoints (NFP and SFP, respectively; Fig- km s−1) and electron density (N ; cm−3) are measured
e
ure1). Segmentationwasperformedusingloopfootpoint as a function of loop length by the techniques described
regionsidentifiedinFeviii185.2˚A(logT ≈5.8)intensity above. Light curves of these parameters as a function of
images at each time step. It is noted, the footpoint re- loopregionareobtainedbysmoothingoverthecoreand
gionswerevisuallyverifiedincorrespondingHeii256.3˚A footpoint regions as function of time.
Theloop’sfootpointregionsareusedtoaggregatemag-
(logT ≈4.9).
neticfielddataateachtimestep(Figure1). Thesemag-
Prior to measuring radiative flux (F ; arbitrary units)
λ
netogram data cubes are used to measure the magnetic
as a function of loop length s, we apply a rigorous back-
fluxdensity,i.e.,thenumberofpositiveandnegativepo-
ground subtraction method. In this method, solar back-
4 Orange et al.
Figure 3. EIS intensity images (erg cm−2 s−1 sr−1), covering electron temperatures in the range of logT≈4.9–6.2 (left to right,
respectively),observedon18October2011from14:20:40UTto15:19:55UT(toptobottom,respectively)showingboththecool(logT≤6.0)
and hot loop (logT>6.0) flux evolution. Note, an arrow identifies the filled cool loop corresponding to the time of peak emission in the
coreregion.
larity elements above and below a threshold value of 20 ShowninthetoprowofFigure3andcorrespondingto
G, respectively. This threshold value is consistent with the14:20:40UT,temperatures≤1MKarecharacterized
the average field-of-view strength of our magnetic field by visually bright footpoints and diffuse partially to not
imagery. It is noted, analyzing magnetic field imagery filledloops. Thecoolloopcanthenbeseenbeginningto
in this manner provides information on flux likely con- fill from the SFP to the NFP at logT ≈5.8 and 14:41:17
tributing to reconnection events given the notion that UT (i.e., 2nd row from top of Figure 3). The loop is
stronger flux (i.e., > threshold) reconnects while weaker completelyfilled(logT ≤6.0),withatypicallength≈40
flux (i.e., < threshold) is “scattered” (Sakai et al. 1997; Mm,by14:59:21UT(i.e.,3rd rowfromtopFigure3and
Chesny 2013, private communication). identified by arrow on Feviii185.2,˚A image). In the last
Toexaminethetemperaturestructureasafunctionof
observation time, 15:19:55 UT, the loop is returning to
loop region, we use the aforementioned measurements of
equilibriumandhascooledsufficientlygiventheobserved
total radiative flux to execute an emission measure EM
decreases in visually bright plasma.
locianalysisviathetechniquesdiscussedbyOrangeetal.
In Figure 4 we have provided the light curves of the
(2013b). We employ the physical assumptions used by
NFP, core, and SFP. In the NFP, the flux decreases un-
Orange et al. (2013b), with exception of N which is de-
e til the cool loop begins to fill (14:41 UT) and increases
rivedfromourmeasurementsdiscussedabove. Note,loci
thereafter, with exception of logT ≥6.2 which continu-
curves represent the EM as it originates from isothermal
ally decrease. The core region of the loop experiences
plasmaatagiventemperaturetherebyrevealingisother-
similar trends in flux evolution, as function of tempera-
mal plasmas where all curves meet (Kamio et al. 2011).
ture,asthatoftheNFP,withexceptionlogT ≥6.2which
exhibit a triangular shape. SFP light curves for temper-
3. RESULTS
Catastrophic Cooling in Solar TR Loops 5
Figure 4. FluxF (arbitraryunits)vstime(14:02UT–15:19UT)oftheloop’sNFP,core,andSFPregions(lefttoright,respectively)
λ
displayedfromtoptobottomasafunctionofincreasingtemperaturefortheemissionlinesofTable1,respectively.
atures over 5.8≤logT ≤6.0 peak in irradiance at 14:41 logT ≈5.9 the plasma upflow speed peaked at ≈-60 km
UT,whilebothcoolerandhotterregimespeak≈20min s−1,withupflowingplasmastilloccurringoveratemper-
later (14:59 UT). aturerangeof5.8≤logT ≤6.2. However,attheSFPsite
In relation to the cool loop, logT ≤6.0, irradiance and this same temperature range plasma was falling at
peaks are consistent with the structural evolution, ob- a typical rate of (cid:46)5 km s−1 (Figure 5). Upon complete
served in Figure 3 (i.e., consistent with previous results fillingofthecoolloop(i.e.,14:59UT)theplasmaflowdi-
that loop fills from the SFP to the NFP). In terms of rectionsversustemperaturehadreturnedtotheirtypical
the temporal evolution these flux peaks occur as follows: form,thatisupflowingplasmaintheupperTRandlower
first SFP at 14:41 UT, then core at 14:59 UT, and fi- corona (5.8≤logT ≤6.2) with falling plasma at cooler
nally NFP at 15:19 UT. Inspection of Figure 4 indicates andhottertemperatures. Wepointout,maximizedNFP
that the cool loop is not a result of cooling coronal ma- plasma upflows, particularly over 5.8≤logT ≤6.0 tem-
terial,basedoncomparisonsbetweenpeakTRandcoro- peratures,correspondedwiththetransitionfromupflows
nal fluxes at the SFP site where said TR flux peaks pre- todownflows,atsimilartemperatures,intheSFPregion.
cedethoseofhottertemperatures. Furthermore,theSFP
peak in TR EUV flux transverses the loop back to the The coronal (logT ≈6.2) electron density evolution in
NFP while hotter regions peak in the NFP and progress theNFPwasroughlyconstantoverthetimeframestud-
towards the SFP site. ied here (Figure 6). However, in the SFP region sig-
At the NFP prior to the appearance of the cool loop nificant density fluctuations occurred and are described
(14:20UT;≈40min),temperaturesover5.8≤logT ≤6.2 as follows (Figure 6). During the first ≈20 mins (14:20
were characterized by plasma upflow speeds of ≈8.0 UT–14:41 UT) a mass in-flux of ≈30% is found. Next,
– 40 km s−1 (Figure 5) with plasma falling at hotter mass-loss of ≈40% is witnessed over the next ≈20 mins
and cooler temperatures. In the NFP at 14:41 UT and (14:41UT–14:59UT).Finally,duringthetimeinwhich
6 Orange et al.
Figure 6. Normalizedelectrondensity(logNe,atlogT≈6.2)as
a function of loop position (s ). The NFP and SFP regions are
λ
denoted on the plot, while the observation times of 14:20:40 UT,
14:41:17UT,14:59:21UT,and15:19:55UTarerepresentedbyx’s,
squares,triangles,anddiamonds,respectively.
as well as the approximately constant TR EM distribu-
tion,aresuggestiveofunresolvedstructure(Brooksetal.
2012) and are expected given the varying visual nature
observed in emission lines formed at these temperatures
(Figure 3).
The EM loci analysis also indicates the SFP is the
site of condensation, based on observed differences be-
tween EM’s of the two footpoint regions (logT’s≤6.0;
Figure 7). This notion is expected when a single foot-
point acts as the dominate energization site (Craig &
McClymont 1986; McClymont & Craig 1987). These re-
sults are further consistent with previous observations
that the cool loop is filling from the SFP to the NFP.
Moreover, they are consistent with both the evolution of
flux and velocity observed in each of the loop regions as
a function of temperature.
The evolution of the normalized magnetic flux density
at both footpoint sites is shown in Figure 8. We note,
theNFPandSFPsitescorrespondedtothepositiveand
negative polarity magnetic flux, respectively. The posi-
tive magnetic flux density evolution, for intensities >20
Figure 5. LOS velocity (km s−1) versus electron temperature G, is described in detail as follows (i.e., NFP). The flux
(logT) for the NFP (asterisks) and SFP (pluses) regions derived densitywasdecreasingatarateof≈1%min−1 untilthe
from observational data on 18 October 2011 at the observational coolloopstartedtofill(14:41UT).Magneticfluxdensity
times of14:20:40 UT–14:59:21 UT(top to bottom, respectively).
was increasing at a similar rate during 14:41 UT–14:59
Note, v <0 and > indicate upflows and downflows, respectively,
λ
whilev =0isdenotedbydashedline. UT,thuscorrelatingwiththecompletefillingofthecool
λ
loop. While the cool loop drained and returned to equi-
theloopcooledcompletely(14:59UT–15:19UT)itcon- librium the magnetic flux density was again decreasing
tinued to lose mass with a total loss of ≈30%. at≈1%min−1. FortheSFPsite,themagneticfluxden-
In Figure 7 we have provided EM loci curves for both sity, intensities <−20 G, were approximately constant
footpoints as a function of observation time. It is ob- with minimal fluctuations, ≈±5%, during the observa-
served,adistinctiveisothermalcomponentatlogT ≈6.2 tiontimeframestudied(Figure8). InFigure8amagne-
(i.e., NFP and SFP; Figure 7). We note, though not togramsequencecenteredontheNFPhasbeenprovided
shownhere,theseresultsareindicativeoftheloopcore’s toshowthemajorfluxelementscontributedtomagnetic
EM loci analysis as well. The isothermal component is fluxdensitymeasurementsarecontainedanddonotdrift
expected given the hot loop’s consistent visually bright out of the FOV.
nature both throughout our observational sequence and
emission lines with such formation temperatures (Fig- 4. DISCUSSION
ure 3). Furthermore, the EM loci analysis indicates that We have presented observations of a cool loop
thecoolloopanditsfootpoints,peakformationtempera- (logT ≤6.0)directlybelowathermallyisolatedhotcoro-
tures≤1.0MK,werenon-isothermalduringouranalysis nal loop (logT ≈6.2) recorded on 18 October 2011 by
givenitsbroaddistributionoflocicurves. Theseresults, EIS near solar center (≤100(cid:48)(cid:48) in both the solar x and y
Catastrophic Cooling in Solar TR Loops 7
Figure 7. EMlocicurves(blueandgreenrepresentemissionlineswithpeakformationtemperaturesintheTRandcorona,respectively)
for observation times of 14:20:40 UT–14:59:21 UT (top to bottom, respectively) of the NFP and SFP regions, left and right columns,
respectively,derivedfromEISemissionlineintensities(Table1).
directions). Our study, to best of our knowledge, pro- ure 7). This result is expected in the presence of impul-
vides the first observational evidence of plasma upflows siveheatingtypeevents(Brooksetal.2012). Weexplain
in the presence of cool loops in quiet Sun regions. HMI these results by first noting that at the cool loop onset
LOS magnetic field observations were utilized to investi- (i.e., initial filling) non-steady symmetrical flows indi-
gate the relationship between EUV flux evolutions and cate an asymmetric loop structure (Mariska et al. 1982;
plasma motion compared to that of the underlying mag- Craig & McClymont 1986; McClymont & Craig 1987),
netic field. The intensity and velocity structure of the while simultaneously plasma condensation is occurring
cool loop showed characteristics similar to those pub- at the SFP site (Figure 7). The runaway cooling ob-
lished by Tripathi et al. (2012) for active region loops, served ≈20 min later, 14:59 UT, in lower coronal and
andcontrastthosemosttypicallyfoundinthepresenceof upper TR EUV images is then indicative of the move-
suchstructures,e.g.,redshiftsalongandatthefootpoints ment of the condensation region to the less heated foot-
of these structures (e.g., Del Zanna 2008; Tripathi et al. point (i.e., NFP). Craig & McClymont (1986) noted the
2009;Chesnyetal.2013). Bothfootpointswerepredom- temperature gradient of a condensing loop leg (i.e., our
inately blueshifted throughout the upper TR and lower SFP region) is shallower than that of the evaporating
corona (5.8≤logT ≤6.2) during the cool loop’s lifetime, leg (i.e., our NFP region), and as such is characterized
except when filling began (14:41 UT). At this time a by larger EMs. Therefore, using the minima of our foot-
symmetrical flow was observed that corresponded with pointEMdistributions,overlogT=5.8-6.0(Figure7),
maximal upflow speeds in the NFP region (v ≈60 km a heating rate asymmetry of ≈2% existed between the
λ
s−1)peakingatlogT ≈5.9anddecreasedwithincreasing NFPandSFPregions. Wepointout,Mu¨lleretal.(2003)
temperature. reported a 1% energy asymmetry between loop legs dic-
As discussed in § 1, cool loops have often been consid- tates the draining direction which supports our observa-
eredtobearesultofcoolingandcondensingcoronalma- tions of the condensation being driven from the SFP to
terialwhichwasheatedimpulsivelyovermanystrandsat the NFP. These results provide significant evidence that
coronalheights. However,likethesuggestionsofTripathi the catastrophic cooling event occurred from the loop’s
et al. (2012), our observations of plasma motions do not non-equilibrium state. Moreover, the non-equilibrium
support models which predict upper TR and lower coro- state formed when plasma condensation began in a sin-
nalemissionlinesthataredominatedbyredshiftedemis- gle footpoint. Below we hypothesize on the mechanism
sion. Our EM loci analysis indicates the presence of un- responsible for initiating the condensation event.
resolved structure and non-isothermal plasma through- Hegglandetal.(2009)suggestobservationsofsolarat-
outthe coolerlayersof theatmosphere (logT ≤6.0; Fig- mospheric bi-directional jets are useful tools for probing
8 Orange et al.
Figure 8. Right: evolution of the normalized magnetic surface flux density (ρΦ±; arbitrary units) for the NFP (asterisks, Φ+ > 20 G)
andSFP(triangles,Φ− <20G)regionsovertheobservationaltimeframestudiedherein. Left: magnetogramtemporalevolutionofNFP
regionovertheobservationaltimeframeof14:20UT–15:19UTfromtoprighttobottomrightinclockwisefashion,respectively.
the heights in which magnetic energy is being converted was cooler regions of the solar atmosphere, particularly
to thermal energy. As such, bi-directional jets provide theupperTR.Moreover,thesenotionssupportfootpoint
uniquediagnostictoolsforconstrainingtheheightsofat- heating scenarios, possibly at higher temperatures than
mospheric heating. Inspecting our velocity versus tem- typically considered, (Tripathi et al. 2012; Aschwanden
perature profiles prior to and at the onset of loop fill- et al. 2007). Finally, our results also point to the fact
ing (Figure 5), a bi-directional jet is occurring between that this whole process can be considered a domino ef-
logT ≈5.4–5.8 at the NFP site. Combining these ob- fectasadirectresultofTRmagneticreconnectioninthe
servationswithsignificantdropsinmagneticsurfaceflux opposing loop leg. Therefore, we have presented strong
density (Figure 8) occurring simultaneously both spa- evidence supporting notions that coronal heating is not
tially with the NFP and temporally with peak plasma confined to only coronal temperatures, while tracing the
upflows, we find support that impulsive magnetic recon- origin of its magnetic energy conversion source.
nection events between the photospheric footpoint and It is worth noting, for logT ≥6.2 light curves indicate
surrounding background field are the source of the jet. footpoint heating of the hot loop in the NFP region,
Consistencies of our observational reports to Heggland which peaked at ≈14:20 UT (Figure 4). Coronal EM
etal.’s(2009)TRsimulatedreconnection,leadustosug- loci provide evidence of condensation occurring simulta-
gestthereconnectioneventpropelledacooldenseblobof neously in time and spatial region (Figure 7). Using the
plasma upwards along the field lines to the region of the techniques discussed previously, a heating asymmetry of
SFP. Thereby, the SFP’s sudden density enhancement ≈5% was measured between the footpoints for tempera-
(Figure 6), and most likely the conversion of magnetic turesoflogT ≈6.2–6.4(Figure7). Theseresultsfurther
waveenergytoheat(Hollweg&Yang1988;Poedts&de support our suggestion of footpoint heating for the hot
Groof 2004), initiated plasma condensation. However, loop(i.e.,NFPregion),aswellasthereportsofTripathi
it cannot be ruled out that a dip in the magnetic field etal.(2012)andAschwandenetal.(2007). Finally,sym-
topology,atorveryneartheSFP,wasresponsibleforthe metricalflowsinthehotloopatlogT ≈6.2and14:20UT
initiation of plasma condensation (Muller et al. 2003). (Figure 5) provide evidence that the heating event was
Thepreviousdiscussionpointstothefactthatthecool occurring at coronal heights with high enough frequency
loop was heated in a single footpoint (i.e., SFP) which to maintain its visually and isothermally stable nature
lead to a runaway cooling based on the non-equilibrium (Figures 3 and 7, respectively).
structure of the loop. The SFP heating event is consid-
eredtobelowfrequencyinnature(e.g.,nanoflare;Chitta 5. CONCLUSION
etal.2013)asthisexplainsourEMlociresults(Figure7)
This work has presented a play-by-play of the life cy-
while remaining consistent with previous notions of cool
cle of a cool loop that emphasized the importance of the
loops (Ugarte-Urra et al. 2009; Spadaro et al. 2003).
TR as the site of coronal heating. Our results have pro-
Our pervasive blueshifts immediately after catastrophic
videdsignificantinsightontherelationshipbetweennon-
cooling, are indicative of plasma evaporation (Tripathi
equilibriumstructuringandcondensationincoolplasma
et al. 2012), suggest the origin of the nanoflare storm
loops. It has also provided the first observational evi-
Catastrophic Cooling in Solar TR Loops 9
dence,tobestofourknowledge,ofplasmaupflowsinthe —.2012,ArXive-prints
presenceofcoolloopsinquietSunregionswhilesupport- DelZanna,G.,&Mason,H.E.2003,A&A,406,1089
ing the findings of Tripathi et al. (2012) for cool active Dere,K.P.2008,A&A,491,561
Dere,K.P.,Doschek,G.A.,Mariska,J.T.,etal.2007,PASJ,59,
region loops. We have built upon the work of Tripathi
721
et al. (2012) by including complimentary LOS magne-
Golub,L.,Bookbinder,J.,Deluca,E.,etal.1999,Physicsof
togramdataandcoronaldensityevolution. Weconclude Plasmas,6,2205
from the evolution of the underlying magnetic flux that Hanson,J.M.,Roelof,E.C.,&Gold,R.E.1980,NASA
observed TR upflows are an indication of magnetic re- STI/ReconTechnicalReportN,81,28032
Hansteen,V.,Maltby,P.,&Malagoli,A.1996,inAstronomical
connection at similar atmospheric heights.
SocietyofthePacificConferenceSeries,Vol.111,Astronomical
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provide conclusive statements about the specifics of cool J.T.Mariska,116–121
loop heating and the shared relationship between their Heggland,L.,DePontieu,B.,&Hansteen,V.H.2009,ApJ,702,
continuous evolution and plasma condensation, which 1
Hollweg,J.V.,&Yang,G.1988,J.Geophys.Res.,93,5423
is planned for in a forthcoming paper. Moreover, these
Kamio,S.,Curdt,W.,Teriaca,L.,&Innes,D.E.2011,A&A,
are required to better understand the cool loop’s non- 529,A21
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Klimchuk,J.A.,Patsourakos,S.,&Cargill,P.J.2008,ApJ,682,
loop. However, significant coronal mass-influx peaking
1351
simultaneously with TR plasma upflows suggest such a
Langangen,Ø.,DePontieu,B.,Carlsson,M.,etal.2008,ApJ,
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of the mass and energy coupling relationship shared Mackay,D.H.,Karpen,J.T.,Ballester,J.L.,Schmieder,B.,&
betweenthesetwoloopstructures,theTRandcoronain Aulanier,G.2010,SpaceSci.Rev.,151,333
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Young,Jr.,T.R.1982,ApJ,255,783
of the hot loop are a direct result of the conversion of
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The authors greatly appreciate the reviewers con-
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structive comments on the manuscript. This research
289
was supported by National Aeronautics and Space Ad- Oluseyi,H.M.,Walker,II,A.B.C.,Porter,J.,Hoover,R.B.,&
ministration (NASA) grant NNX-07AT01G and Na- Barbee,Jr.,T.W.1999a,ApJ,524,1105
tional Science Foundation (NSF) grant AST-0736479. Oluseyi,H.M.,Walker,II,A.B.C.,Santiago,D.I.,Hoover,
R.B.,&Barbee,Jr.,T.W.1999b,ApJ,527,992
Any opinions, findings and conclusions or recommen-
Orange,N.,Oluseyi,H.,Chesny,D.,etal.2013a,Sol.Phys.
dations expressed in this material are those of the au- —.2013b,Sol.Phys.
thor(s) and do not necessarily reflect the views of the Poedts,S.,&deGroof,A.2004,inESASpecialPublication,Vol.
NSF or NASA. The final publication is available at 575,SOHO15CoronalHeating,ed.R.W.Walsh,J.Ireland,
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