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Draft version July 18, 2016Preprint typeset using LATEX style emulateapj v. 12/16/11
ON THE CONNECTION BETWEEN PROPAGATING CORONAL DISTURBANCES AND CHROMOSPHERICFOOTPOINTS
P. Bryans1, S. W. McIntosh1, I. De Moortel2,1 & B. De Pontieu3,4
Draft version July 18, 2016
ABSTRACT
The Interface Region Imaging Spectrograph (IRIS) provides an unparalleled opportunity to explore the(thermal) interface between the chromosphere, transition region, and the coronal plasma observed bythe Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). The SDO/AIAobservations of a coronal loop footpoint show strong recurring upward propagating signals - “propa-gating coronal disturbances” (PCDs) with apparent speeds of order 120 km/s. That signal has a clearsignature in the slit-jaw images of IRIS in addition to identifiable spectral signatures and diagnosticsin the Mg IIh (2803A) line. In analyzing the Mg IIh line, we are able to observe the presence ofmagnetoacoustic shock waves that are also present in the vicinity of the coronal loop footpoints. Wesee there is enough of a correspondence between the shock propagation in Mg IIh, the evolution of theSi IV line profiles, and the PCD evolution to indicate that these waves are an important ingredientfor PCDs. In addition, the strong flows in the jet-like features in the IRIS Si IV slit-jaw images arealso associated with PCDs, such that waves and flows both appear to be contributing to the signalsobserved.Keywords: Sun: chromosphere — Sun: corona — Sun: oscillations — Sun: transition region — Sun:
magnetic topology
1. INTRODUCTION
Ever since high-resolution, high-cadence observationsof coronal loops became available, propagating inten-sity perturbations have been observed along large, diffusecoronal loops (Schrijver et al. 1999; Berghmans & Clette1999; De Moortel et al. 2000). These quasi-periodic,propagating coronal disturbances (PCDs) are observedto travel at speeds around 100 km/s with characteris-tic periods of the order of minutes (typically 3-10 mins).Based on the propagation speeds, close to the local soundspeed, these travelling intensity (density) perturbationswere generally interpreted as propagating slow magneto-acoustic waves. We refer the interested reader to DeMoortel (2009) for a review of the earlier observations ofPCDs.Various suggestions have been made for the driving
mechanism for these coronal PCDs. Waves with similarperiods to those of PCDs are ubiquitous in the chromo-sphere, which has led to the suggestion that slow modesound waves propagate from the chromosphere into thecorona (De Pontieu et al. 2005). Such chromosphericslow mode waves have been implicated in driving jet-like features known as dynamic fibrils (De Pontieu et al.2007a). In particular, these fibrils or Type I spicules havebeen shown to be driven by slow-mode magnetoacous-tic shocks (Hansteen et al. 2006) that exhibit a distinctsawtooth-like signature in chromospheric emission lines(Langangen et al. 2008). More recently, a transition re-gion responce to dynamic fibrils has been identified in
1 High Altitude Observatory, National Center for AtmosphericResearch, P.O. Box 3000, Boulder, CO 80307, USA
2 School of Mathematics and Statistics, University of St An-drews, St Andrews, Fife, KY16 9SS, UK
3 Lockheed Martin Solar & Astrophysics Lab, Org. A021S,Bldg. 252, 3251 Hanover Street Palo Alto, CA 94304, USA
4 Institute of Theoretical Astrophysics, University of Oslo,Blindern, P.O. Box 1029 Blindern, N-0315 Oslo, Norway
IRIS observations (Skogsrud et al. 2016). However, aclear connection between the shock waves that drive dy-namic fibrils and PCDs has not yet been obtained.The work of De Pontieu et al. (2009) (in addition to
McIntosh & De Pontieu 2009b,a; De Pontieu et al. 2011),presented a new picture of how the pieces of the outersolar atmosphere were connected - of how the structuresobserved could be interpreted as indicators of mass andenergy transport into the corona that originate in themagnetized footpoints of the system. Using SOHO, Hin-ode and SDO observations, they proposed that the coro-nal counterpart of rapidly evolving “Type II” chromo-spheric spicules (De Pontieu et al. 2007b) lay in stronglyblue-shifted components of asymmetric (UV and EUV)line profiles and the quasi-periodic propagating distur-bances that shared common magnetic roots (and veloc-ities Tian et al. 2011b,a). The observed commonalitiesbetween the spicules, the line profile asymmetries, andthe PCDs ultimately led to an alternative suggestion forthe physical interpretation of the PCDs (De Pontieu &McIntosh 2010). De Pontieu & McIntosh (2010) demon-strated that the quasi-periodic nature of the coupled(multi-component) chromospheric-coronal plasma could,in principle, also reproduce the behavior observed in thesingle component analysis of spectral data (e.g., Wanget al. 2009) in addition to the PCD amplitudes andspeeds inferred from broadband imagers (e.g., De Moor-tel & Nakariakov 2012).In addition to the discussion of whether or not these
processes can actually sustain the mass requirements ofthe corona as proposed by De Pontieu et al. (2011), butquestioned by (Klimchuk 2012), this “waves vs flows”debate has sparked a flurry of activity across the com-munity (e.g., Verwichte et al. 2010; De Moortel et al.2015). The outcome of the debate is in the balance. Asa guided slow-mode wave (e.g., De Pontieu et al. 2005),there is no net plasma flow into the corona while, as a
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flow, they are notoriously difficult to measure. In bothcases there is difficulty explaining the apparent consis-tency of the propagation speed (wave or flow) with tem-perature (Judge et al. 2012; Kiddie et al. 2012) withoutthe magnetic field being heavily involved (e.g., De Pon-tieu et al. 2011). The resolution to these debates carriessome significance: Are Type II spicules and PCDs themanifestation of how the lower and outer solar atmo-sphere are connected? Are Type II spicules and PCDsthe means by which mass is loaded into the (open andclosed) heliosphere? There have been suggestions re-cently from both observations (e.g., Nishizuka & Hara2011; Krishna Prasad et al. 2012; Tian et al. 2012) andnumerical modelling (Ofman et al. 2012; Wang et al.2013) that both interpretations might actually be cor-rect, with waves and flows present on the same struc-tures. In this “dual” model, quasi-periodic flows at thevery footpoints of the loops may either generate or begenerated by slow magneto-acoustic waves, which travelfarther along the loops.These questions helped drive the requirements of the
telescope and spectrograph of the Interface Region Imag-ing Spectrograph (IRIS; De Pontieu et al. 2014c). The ini-tial science results of IRIS (e.g., De Pontieu et al. 2014a;Testa et al. 2014; Tian et al. 2014; Peter et al. 2014; DePontieu et al. 2014b) illustrate the abundance of dynamic(thermal and non-thermal) phenomena occurring at thespatio-temporal resolution limit of the instrument, high-lighting the profound complexity of the magneto-thermalstructuring of the solar atmosphere.This Letter presents IRIS observations of the (magne-
tized) footpoints of coronal loops which display strongmulti-thermal PCDs observed by SDO/AIA. Followinga brief discussion of the observations, we analyze thespectral signatures observed at the base of the loop sys-tem and trace the chromospheric variability through thetransition region and into the corona in the spectral andimaging data.
2. OBSERVATIONS
The IRIS spacecraft is in a sun-synchronous low-Earthorbit and carries a 19cm Cassegrain telescope that feedsa dual-range UV spectrograph and slit-jaw imager, with0.16′′ pixels and four 2072x1096 CCDs. The IRIS slit-jaw imager (SJI) includes four passbands: two transitionregion lines (C II 1335A and Si IV 1403A), one chromo-spheric line (Mg II k 2796A) and one photospheric pass-band (2830A), covering a field-of-view of 175′′ x 175′′.The IRIS spectrograph has a 0.33′′ wide and 175′′ longslit that covers FUV passbands from 1332A - 1358A and1390A to 1406A and an NUV passband from 2783A to2834A.We analyse two sets of observations in this Letter: May
15, 2014 between 05:26 and 06:44 UT and September26, 2014 between 00:34 and 01:36 UT. Figure 1 providesIRIS/SJI and SDO/AIA context for the “sit-n-stare”(fixed and tracked slit position on the Sun; IRIS OB-SID 3820109554) observations of IRIS5.Co-spatial and co-temporal SDO/AIA observations
in the 1600A, 131A, 171A and 193A passbands were
5 In addition to SJI and spectral-image movies for these observa-tions, the “level 2” IRIS data for these observations can be foundat the following URLs: raster and “sit-n-stare”.
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Figure 1. : Context images for the PCD observations ofMay 15 2014 and September 26 2014. Panels B and Dshow sample 1330A slit-jaw images of the “sit-n-stare”timeseries for the two observations. The red dashed lineindicates the position of the IRIS slit in the May observa-tions. Panels A and C show SDO/AIA 171A sub-fieldswith the same fields of view as the IRIS observations.The solid red curves indicate the locations of the PCDswe analyse in detail in Sec. 3.
prepped, (internally) coaligned, and normalized usingthe SolarSoft aia prep routine. These data were thende-rotated to the start time of the IRIS observations andinterpolated in time and space to match the IRIS/SJItemporal cadence (9.35 s) and spatial resolution (0.33′′).Finally, the IRIS and SDO/AIA datasets were coalignedusing the first images in the IRIS/SJI (Si IV) 1330A andAIA 1600A datasets.As described in Section 1, chromospheric shocks are
manifest in the position of the core reversal of the opti-cally thick Mg IIh (2803A) line. Following the analysisof Leenaarts et al. (2013a,b) we capture the complex-ity of these line profiles using a double-Gaussian fittingscheme: one Gaussian - the “wing” fit - describes theextended wings of the h-line core while the other - the“core” fit - describes the central reversal. The Mg IIhprofile fit is then the sum of these parts. For the re-mainder of this work we will concentrate on the centroidof the central reversal, “h3”. The position of h3 corre-lates with the line-of-sight velocity at optical depth unity(Leenaarts et al. 2013b).
3. RESULTS
Figure 2 shows a cut-out of the September 26, 2014observations where we have identified a PCD – corre-sponding to the solid red lines in panels C and D ofFig. 1. We show three snapshots in the print versionand encourage the reader to view an animated versionof this figure in the online version of this Letter. The
IRIS PCD Footpoints 3
red boxes highlight a Type II spicule in the IRIS/SJIimages (top row) and a PCD in the AIA running differ-ence images (second row). The bottom two rows showtime-distance plots in running difference along the pathof the PCD/spicule. The red dotted line indicates a prop-agation speed of 99.8 km/s. The PCD and the Type IIspicule appear to share a common origin, with the PCDextending 5-10 arcseconds farther than the spicule. Thisis consistent with the results of De Pontieu et al. (2011)and Samanta et al. (2015), who studied the connectionbetween spicules and PCDs at the solar limb. Our ob-servations show this clear connection on the disk wherethere is much less ambiguity than at the limb. In ad-dition, since Type II spicules are associated with flowsin chromospheric and TR bandpasses (Rouppe van derVoort et al. 2015), the observed association with PCDssuggests that flows play a role in at least a subset ofPCDs.For the September 26, 2014 observations, the IRIS
slit did not coincide with the location of the PCDs andspicules so there is no spectral information for this event.To explore the spectral signature of spicules and PCDs,we turn to the observations of May 16, 2014. In this case,as shown in panel A of Fig. 1, the IRIS slit crossed di-rectly over bright fan-like loops where PCDs are present.To further study the connection between SDO/AIA andIRIS data, we have chosen a location on the IRIS slitwhere the PCDs seen in AIA data appear to originate –a solar Y location of −376′′ and indicated by where thered lines of panels A and B of Fig. 1 intersect the IRISslit (shown in dashed red). Figure 3 shows space-timeplots of the SDO/AIA and IRIS/SJI running-differenceevolution for this location. In this case we use a 62asgari-targhi12 second running difference. The long (diagonal)streaks visible in the 193 and 171A AIA channels (pan-els A and B) are the indicators of PCDs originating atthis footpoint. The inclined (blue dashed) reference linesshown in panels A and B indicate a propagation speed of120km/s. The green circles and lines indicate four PCDsthat have a strong correlation in the different channels ofAIA and in the IRIS SJI. The cooler 131A channel (C)has much poorer signal-to-noise than those in A and B,but we can clearly see the consistent timescales of activityat the bottom of the coronal structure. In the SDO/AIA304A (D) and IRIS/SJI (E) space-time plots we see in-creasing complexity, with much shorter structures, butagain there is a consistency between the patterns andtimescales seen in those panels with those of the coronalemission (see also Pereira et al. 2014).To further explore the signature of PCDs lower in the
atmosphere we turn to the spectral information fromIRIS. As outlined in Sections 1 and 2, dynamic fibrils orType I spicules can be identified by the wavelength of thecore reversal of the optically thick Mg IIh line measuredby IRIS. In Fig. 4, we compare the temporal evolution ofthe h3 wavelength with the PCD signature in AIA 171A.The top panel shows the running difference AIA 171 Aimage (same as panel B of Fig. 3). The red dashed lineshows the position of the IRIS slit. The second and thirdpanels of Fig. 4 show the spectral evolution of the IRISdata for Si IV 1403 A and Mg IIh 2803 A, respectively,for the same Y position of −376′′. One immediately no-tices periodic sawtooth-like patterns in the Mg IIh data,and corresponding blue shifts in the Si IV data. There
also appears to be a correspondence between these fea-tures and the PCDs in the AIA running difference image.This connection is best visualised by plotting the param-eters shown in the lower panel as a function of time. Theblack curve is the wavelength of h3 and the red curve isthe negative of the AIA 171 A intensity. The AIA in-tensity here has been smoothed over the time series andnormalized to the h3 wavelength. Plotting the negativeAIA intensity helps to compare the timing of the PCDsand the h3 wavelength swing.The correlation between PCDs in the corona and Type
I spicules is evident for the entire time series at the loca-tion shown in Fig. 4. The h3 wavelength is clearly seento swing back and forth, symptomatic of the magneto-acoustic shocks that drive dynamic fibrils or Type Ispicules. Peaks in the AIA intensity regularly occur atthe beginning of these shocks, i.e., where the h3 wave-length swings to the blue. After ∼ 50 min of the ob-servation the periodicity of the PCDs changes from ∼ 5to ∼ 3 min. The magnetoacoustic shocks show a similarchange in period at this time, strengthening the argu-ment that these phenomena are related.
4. DISCUSSION
The IRIS and SDO observations shown above indicatethere is a connection between chromospheric activity andPCDs, but observing that connection is complicated bythe geometry and temporal evolution of the interface. DePontieu et al. (2011) provided a tentative link betweenchromospheric activity, line profile asymmetries (via Hin-ode/EIS raster maps) and PCDs. Exploiting the im-provements in spectral, spatial, and temporal samplingof the chromosphere and transition region with IRIS overHinode/EIS, we develop an improved view of the rootsof the coronal PCDs.Demonstrating the connection between chromospheric
activity and PCDs is hindered by the difficulty in simul-taneously measuring PCDs and their footpoints in thechromosphere. For the example of this Letter, the IRISslit crosses a region of fan-like loops where PCDs are ev-ident over several tens of arcseconds. However, most ofthe observation covers PCDs that have already propa-gated higher into the corona, away from their footpoints.Given the geometry of this particular active region, andthe nature of the sit-n-stare operational mode of IRIS,we are only able to identify a single location where PCDsand chromospheric footpoints were observed simultane-ously. For this location, however, we can identify a con-nection between PCDs and Type I spicules.A further difficulty in connecting chromospheric activ-
ity with PCDs in the observation of May 16, 2014 is thatPCDs are ubiquitous over the region of coronal loops andhappening periodically. Identifying Type II spicules thatcorrespond with a particular PCD is thus extremely diffi-cult because of the crowded nature of these events. Usinga different observation (September 26, 2014), where wecan isolate a single PCD, does allow us to connect PCDsto Type II spicules.Despite the improved resolution and cadence of IRIS,
establishing a clear one-to-one correspondence remainsdifficult due to the effects of line-of-sight superpositionand the geometry of the viewing angle. We have bothmagneto-acoustic shock (Type I) and jet (Type II) re-lated phenomena, which occur in close vicinity to each
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Figure 2. : The upper 6 panels show snapshots from a movie of a Type II spicule and PCD (see online version for thefull animation). The top row shows the IRIS/SJI images and the second row shows the SDO/AIA images. The redboxes highlight the location of the spicule and PCD. The lower two panels show time-distance plots of the same regionin running difference.
IRIS PCD Footpoints 5
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Figure 3. : From bottom to top we show the thermal vari-ation of the space-time plots at y = −356′′ for SDO/AIA193A (A), 171A (B), 131A (C), 304A (D), and IRIS SJI1330A(E). In each case the location of the IRIS slit isshown as a thick red dashed line. For reference, panelB shows inclined blue dashed lines which indicate anapparent speed of 120km/s of the propagating coronaldisturbances observed. The green circles and lines aredrawn to indicate particularly strong correspondences inthe observational sequence.
other in the observed regions. The complicated natureof establishing a one-to-one connection is illustrated inFig. 5. This figure shows a schematic representation ofa chromospheric Type I or Type II spicule (blue), evi-dent in the data as slow magneto-acoustic shocks (TypeI), transition region brightenings (orange, Type I) or jets(orange, Type II), and the coronal PCDs (red). The ex-act location of the IRIS slit will result in either a “hit”where all signatures are captured co-spatially and co-temporally or a “miss”, where a one-to-one correspon-dence will not be clear. Clearly the critical position ofthe 1D IRIS slit will result in only occasional “hits” and alot of “misses”. Connecting all three phenomena – TypeI spicules, Type II spicules, and PCDs – in a single ob-servation will require an improved observation sequencewhere, rather than IRIS being in sit-n-stare mode, theslit rasters rapidly across a region of PCDs.In addition to the limitations of the 1D slit, the ge-
ometry and/or magnetic flux of the footpoints appear to
be changing very rapidly - on timescales comparable tothe periods of the PCDs - further complicating the one-to-one correspondence between chromospheric dynamicsand the coronal PCDs. This is a problem that becomesfurther compounded by pushing to higher and higher spa-tial resolution measurements - emphasizing the need toacquire two-dimensional spectral information about theinterface.
5. CONCLUSION
This paper uses IRIS data to identify the chromo-spheric and transition region dynamics associated withpropagating coronal disturbances (PCDs) observed byAIA. Despite the complexity of connecting chromo-spheric, TR and coronal dynamics, our results show thatthe wavelength coverage of IRIS and the improved spatialand temporal resolution allow a one-to-one connection,but only when the viewing geometry allows. In particu-lar, we find nice examples where both shock waves (TypeI spicules or dynamic fibrils) and strong flows (Type IIspicules) are connected with propagating coronal distur-bances. This illustrates that both the waves and flowsinterpretations appear to be present in our observations.
IRIS is a NASA Small Explorer developed and op-erated by LMSAL with mission operations executed atNASA Ames Research center and major contributions todownlink communications funded by ESA and the Nor-wegian Space Center. This work is supported by NASAcontract NNG09FA40C (IRIS) and has benefited fromdiscussions at the International Space Science Institute(ISSI) meeting on “Heating of the Magnetized Chromo-sphere”. The visit of SMC to St Andrews was partlysupported by a Royal Society of Edinburgh Research Vis-itor to Scotland Grant. The research leading to these re-sults has also received funding from the UK Science andTechnology Facilities Council, the European CommissionSeventh Framework Programme (FP7/ 2007-2013) underthe grant agreement SOLSPANET (project No. 269299)and the European Union Horizon 2020 research and in-novation programme (grant agreement No. 647214).
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Figure 4. : The top three rows show (from top to bottom) a running time difference of SDO/AIA 171A and “λ - time”plots of Si IV (1403A) and Mg IIh (2803A). The location of the IRIS slit in the running difference is shown as a reddashed line while the horizontal (black) dashed lines in the other three panels show the zero wavelength positions. Thelowest panel shows the h3 wavelength (black line) and the negative of the AIA 171 A intensity (red line). The AIAintensity has been normalized to the h3 wavelength for ease of comparison.
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IRIS PCD Footpoints 7
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Figure 5. : Cartoon of impact that geometry can have on establishing the chromo-coronal connection via the chro-mospheric spicule (Type I and or Type II; blue), the corresponding transition region brightening or jet (orange) andcoronal PCD (red). From left to right we show the “hit” and “miss” scenarios discussed in the body of the text.
