The solar chromosphere has never been static although it
was often modeled so.Even the quiet-sun internetwork chromosphere
has become thoroughly dynamic with the acoustic shock interpretation
of the Ca II K 2V grains. We concentrate on the latter in this brief review.
Recent analysis of ASP data confirms that their excitation is more likely
set acoustically than magnetically.TRACE imagery permits seeing-free
studies of their occurrence patterns

Berger, T.E.

Lites, Bruce W.

Rutten, R.J.

Shine, R.A.

Utrecht University Repository

“Solar and Stellar Activity: Similarities and Differences”, Eds. C.J. Butler, J.G. Doyle, Astron.Soc. Pac. Conf. Series, 1999. Preprint: http://www.astro.uu.nl/∼rutten/Dynamics of the Quiet Solar ChromosphereRobert J. RuttenSterrekundig Instituut, Utrecht, The NetherlandsBruce W. LitesHigh Altitude Observatory, NCAR, Boulder, USAThomas E. Berger, Richard A. ShineLockheed-Martin Solar and Astrophysics Lab, Palo Alto, USAAbstract. The solar chromosphere has never been static although itwas often modeled so. Even the quiet-sun internetwork chromospherehas become thoroughly dynamic with the acoustic shock interpretationof the Ca II K2V grains. We concentrate on the latter in this brief review.Recent analysis of ASP data confirms that their excitation is more likelyset acoustically than magnetically. TRACE imagery permits seeing-freestudies of their occurrence patterns.1. Introduction: network and internetworkFigure 1 is a paradigmal cartoon taken from Judge & Peter (1998). It depicts avertical slice through the quiet solar photosphere and low chromosphere acrossa single supergranular cell. Clusters of strong-field magnetic elements (“flux-tubes”) sit at the cell borders and constitute the “magnetic network” seen onphotospheric magnetograms. Due to the relative increase of magnetic pressureover gas pressure, their magnetic fields expand higher up and become space-filling in the chromosphere, combining into a “magnetic canopy” overlying thecell interior. Chromospheric heating (dark shading) occurs presumably pref-erentially along the cluster axes since the latter show up as bright 1–3 arcsecgrainy patches on images taken in Ca II H&K, Lyα, and other ultraviolet linesup to considerable ionization temperature. These grainy patches constitute the“chromospheric network”. It coincides with the magnetic network and outlinesthe supergranulation cell boundaries, but very incompletely.The remaining quiet-sun area besides the network grains is called the inter-network. It intermittently contains bright features of its own, in particular theso-called Ca II K2V grains. They have gotten this name because their intensitypeaks dramatically just blueward of the Ca II H&K line centers, but they arealso present, at lower contrast and a slight phase shift, on wider-band Ca II Kfiltergrams such as the ones in Fig. 4 and on wide-band near-UV images from1                                                                                                                                                                        2MmNetwork ChromospherePhotosphereShocksEnhanced EUV emission30Mm SupergranulationGranulation (2Mm), p-modesNot to scale0.3Mm"Field Free,Internetwork" regionFigure 1. Schematic vertical section through a supergranulation cellwith magnetic network at its borders. From Judge & Peter (1998).TRACE such as the ones in Fig. 5 (cf. Rutten et al. 1999). We therefore callthem “internetwork grains” below.2. Fourier and grain signaturesNumerous studies (e.g., Jensen & Orrall 1963, Liu & Sheeley 1971, Lites et al.1993) have shown that the chromospheric network is characterized Fourier-wiseby power at five minutes and longer periodicities, whereas the internetworkmodulation tends to peak between five and two minutes. This fact is currentlybeing rediscovered by SUMER spectroscopists. It is illustrated particularly wellby spatial Fourier charts derived from TRACE image sequences (Fig. 2). Thenetwork stands out bright in single images and in low-frequency modulationmaps, but the contrast with the internetwork modulation amplitudes reversesat higher frequencies.It is likely that the slow network modulation reflects footpoint buffetingof the magnetic elements by granular dynamics (e.g., Kneer & von Uexku¨ll1986, Wellstein et al. 1998) rather than an intrinsically oscillatory phenomenon,although long-period waves may play a role at the level of the chromosphere(Lites et al. 1993). In the photosphere, the buffeting is vividly demonstratedby the bright point dynamics in the high-resolution G-band image sequence ofBerger & Title (1996) and indeed produces characteristic splitting and mergingtimes per magnetic element of five to seven minutes (Berger et al. 1998).2Figure 2. Fourier modulation maps constructed from a TRACE1600 A˚ image sequence of AR8210 obtained during UT 08:30–11:58on May 1, 1998. The tick marks are 10 arcsec apart. The first panelcontains a sample image. The other panels display relative intensitypower per pixel in the indicated frequency bin. A surge occurred dur-ing the sequence, producing relatively high power at all frequencies.Penumbral waves cause enhanced power in the second panel. Plageand network have larger power than present in the internetwork at lowfrequency, but the power contrast flips at higher frequencies where theinternetwork modulation gains dominance.3Figure 3. Left: high-resolution Ca II K filtergram taken with theSwedish Vacuum Solar Telescope at La Palma (Loefdahl et al. 1998).Right: 70-min temporal average of the same area, demonstrating therapid variation of the internetwork brightness pattern (lower box) byshowing considerable suppression of its time-averaged contrast relativeto the network. From Lites et al. (1999).For the internetwork modulation, commonly called the “chromospheric three-minute” oscillation, the intrinsic oscillatory nature is much more outspoken. Thebest demonstration is obtained by downloading a TRACE near-UV image se-quence (for example the data taken May 12, 1998 UT 14:31–16:00, of which asmall part is shown in Fig. 5) and to watch the rapid internetwork morphologychanges with time. If such a movie is played fast, the network grains appear asislands of stability in a sea of motion made up of rapidly evolving, spidery pat-terns that often display apparent supersonic motion and that are very suggestiveof wave interference.Figures 3–5 illustrate the fast internetwork morphology changes and theappearance of internetwork grains as localized pattern enhancements that flashon and off a few times at 2–4 min intervals, frequently in pairs (Fig. 4). In ad-dition to these ubiquitous enhancements, “persistent flashers” may occasionallymigrate through an internetwork area. They probably represent the chromo-spheric signature of a newly-emerged strong-field ephemeral region on its wayto become part of the network (Brandt et al. 1992, 1994).3. Internetwork grain formation and excitationThe acoustic shock simulations of Carlsson & Stein (1997) have crowned a longsequence of efforts to model chromospheric oscillations, in particular the spec-tral formation of K2V grains (Athay 1970, Cram 1972, Liu & Skumanich 1974,Mein et al. 1987, Leibacher et al. 1982; Rammacher & Ulmschneider 1992; Fleck& Schmitz 1991b, 1993; Kalkofen et al. 1994; Sutmann & Ulmschneider 1995a,4Figure 4. Selected frames from the La Palma Ca II K filtergram se-quence of Loefdahl et al. (1998). The area corresponds to the internet-work region outlined by the lower box in Fig. 3. The markers in thefirst two frames point at a brightening which travels along a patternridge with apparent supersonic motion (70 km/s). The two markersin the other panels (at fixed locations) identify two concurrent recur-rent K2V grains which brightened, in phase, a few times at 2–3 minintervals. From Lites et al. (1999).1995b). The extensive literature on K2V grains has been reviewed by Rutten& Uitenbroek (1991) and Rutten (1994, 1995, 1996). Carlsson and Stein’s de-tailed reproduction of observed spectrally-resolved H2V grain development hasascertained beyond doubt that the grains are caused by weak acoustic shockswhich propagate upwards through the low chromosphere. The marked H&Kprofile asymmetry (absence of concurrent H2R peaks) results from substantialline-center opacity shifts caused by higher-located matter that falls back afterhaving been kicked up by earlier shock passages.Questions yet open are whether internetwork grains appear preferentiallyat specific places betraying some piston mechanism, to what extent the shocksmay be diagnosed in higher layers, and whether the shocks play some significantrole, for example in chromospheric heating or in the FIP element segregation5Figure 5. Cutouts from a TRACE 1550 A˚ image sequence (May 12,1998 UT 15:00:26–15:10:28) illustrating quiet-sun internetwork behav-ior. The passband contains C IV lines that brighten at magnetic ac-tivity and C I lines that provide internetwork grain signatures. Thenumbers specify elapsed time in seconds.observed in the fast solar wind. The issue of acoustic internetwork heating isaddressed by Ulmschneider and by Cuntz elsewhere in these proceedings. Forthe FIP flip issue see Rutten (1998) and other reviews in Fro¨hlich et al. (1998).The question to what height the shocks penetrate recognizably is a matterof debate in the current literature, mostly on the basis of SUMER spectrometry(e.g., Steffens et al. 1997, Judge et al. 1997, Carlsson et al. 1997, Curdt & Heinzel6Figure 6. Histograms of the occurrence of weak internetworkfields measured with the Advanced Stokes Polarimeter at theNSO/Sacramento Peak Dunn Telescope. The quantity |BASPapp | repre-sents the apparent flux density derived from profile-integrated StokesV measurements of the two Fe I 6302 A˚ lines. The solid curve specifiesthe flux density distribution for all of an internetwork area, whereasthe similar dashed distribution holds for only those space-time loca-tions where a Ca II H2V grain appeared, as measured on simultaneouscospatial Ca II H spectrograms. The dotted curve includes strong-fieldnetwork elements (they do not produce kilogauss apparent flux den-sity because the magnetic elements are not resolved; the differencebetween the measured apparent flux density and the actual intrinsicfield strength B ≈ 1400 Gauss is largely a measure of spatial fillingfactor within the seeing-smeared and instrument-dependent resolutonelement). The narrowest distribution is a noise estimate derived fromthe continuum which has no intrinsic Stokes V signal. Its peak definesthe noise threshold of these data. From Lites et al. (1999).1998). Much of the latter is hampered by insufficient solar area, temporal extent,spectral signal and line formation variety, but it is nevertheless clear that theinternetwork scene is less clear in higher-formed UV lines than it is in H&K.This was already established from rocket data (Hoekzema et al. 1997). Part70{5 5{10 15{20 25{30 35{40 45{5050{55 60{65 70{75 80{85 90{95 95{100Figure 7. Contour maps measuring the amount of spatial correlationbetween the brightness of the granulation pattern and the occurrence oflarge oscillation amplitude, as functions of Fourier frequency (lower ab-scissae; corresponding periods along the tops) and time delay betweenbrightness sampling and Fourier amplitude measurement (ordinate).The time delay is positive when brightness is sampled after Fourieramplitude. The panels divide the granulation brightness range into5% bins, the top-left one being for image pixels in the darkest inter-granular lanes, the bottom-right one for the brightest 5% of all pixels.Contour values exceeding unity (bright) imply that pixels with Fourieramplitude over twice the average, at the given frequency, are preferen-tially coaligned with pixels of the given brightness class. The contourvalue unity (grey) implies absence of systematic coalignment. Valuesbelow unity (dark) imply negative alignment (spatial avoidance). Thebright blob at 2–3 min period in the upper-left panel implies that high-frequency waves are preferentially found at the very darkest features.From Hoekzema et al. (1998).of the confusion may lie along the line of sight. The Ca II H2V and K2V grainemission senses the upcoming shocks just barely, when they are still quite weakand relatively regular, because the H&K line source functions are dominated byscattering in the higher layers where vertical shock interference produces moreirregularity. It is also likely that high-frequency shock sequences interfere withthe three-minute ones to a considerably larger extent than simulated by Carlsson& Stein (Ulmschneider, these proceedings). In addition, shock signatures arevery much different between optically thick and optically thin line formation.Finally, it is likely that the shock trajectories are not radial as is assumed in 1Dmodelling or in pixel-by-pixel time sequence and temporal Fourier analyses. Thefrequent occurrence of K2V grains in synchronized pairs suggests the presenceof horizontally spreading disturbances.8The issue of whether internetwork grains betray specific pistons is yet open.A long-standing debate concerns the claim by Sivaraman & Livingston (1982)that K2V grains correlate one-to-one with the presence of enhanced magneticfield (e.g., Rutten & Uitenbroek 1991, Kneer & Von Uexkull 1993, Von Uexkuell& Kneer 1995, Steffens et al. 1996, Hofmann et al. 1996, Remling et al. 1996,Nindos & Zirin 1998, Wellstein et al. 1998). Recently, this claim has been testeddirectly by Lites et al. (1999) by repeating the measurements of Sivaraman &Livingston (1982) at higher precision and sensitivity. A key result is shown inFig. 6. It indicates that, down to the measurement sensitivity of 3 Mx cm−2 inapparent flux density, there is no correlation between the presence of H2V grainsand enhanced Stokes V amplitude. A similar absence of significant correlationwas found between H2V grains and horizontal internetwork fields diagnosed fromlinear polarization (Lites et al. 1996). Thus, the occurrence of internetworkgrains does not seem to depend on magnetism, apart from the rare persistentflashers.It seems most likely that turbulent convection supplies the pistons that ex-cite shock sequences producing bright internetwork grains. In particular, thegrains may mark locations where “acoustic events” and “intergranular holes”follow on granular collapse (cf. Restaino et al. 1993; Rimmele et al. 1995; Rast1995; Roudier et al. 1997; Hoekzema & Rutten 1998; Hoekzema et al. 1998;Goode et al. 1998). Figure 7 from Hoekzema et al. (1998) demonstrates instatistical fashion that high Fourier amplitudes in the 2–3 min period rangeare found preferentially above the darkest features in the granulation. Thesecorrespondence charts have been derived from white light images and thereforecontain photospheric signal only. Combination of cospatial TRACE white lightand near-UV image sequences will enable occurrence correlation between chro-mospheric internetwork grains and photospheric morphology and flow patternsdown to granular scales, with sufficiently large data sets to assess the signif-icance of any relation. The internetwork may so become the first domain inwhich chromospheric and photospheric dynamics are tied together in detail.ReferencesAthay R. G., 1970, Solar Phys. 11, 347Berger T. E., Loefdahl M. G., Shine R. S., Title A. M., 1998, ApJ 495, 973Berger T. E., Title A. M., 1996, ApJ 463, 365Brandt P. N., Rutten R. J., Shine R. A., Trujillo Bueno J., 1992, in M. S.Giampapa, J. A. Bookbinder (eds.), Cool Stars, Stellar Systems, and theSun, Proc. Seventh Cambridge Workshop, Astron. Soc. Pac. Conf. Series26, p. 161Brandt P. N., Rutten R. J., Shine R. A., Trujillo Bueno J., 1994, in R. J. Rutten,C. J. Schrijver (eds.), Solar Surface Magnetism, NATO ASI Series C433,Kluwer, Dordrecht, p. 251Carlsson M., Judge P. G., Wilhelm K., 1997, ApJ 486, L63Carlsson M., Stein R. F., 1997, ApJ 481, 500Cram L. E., 1972, Solar Phys. 22, 375Curdt W., Heinzel P., 1998, ApJ 503, L959Fleck B., Schmitz F., 1991, A&A 250, 235Fleck B., Schmitz F., 1993, A&A 273, 671Fro¨hlich C., Huber M. C. E., Solanki S., von Steiger R. (eds.), 1998, Solar Com-position and its Evolution – from Core to Corona, Procs. ISSI Workshop,Space Sci. Rev., in pressGoode P. R., Strous L. H., Rimmele T. R., Stebbins R. T., 1998, ApJ 495, L27Hoekzema N. M., Brandt P. N., Rutten R. J., 1998, A&A 333, 322Hoekzema N. M., Rutten R. J., 1998, A&A 329, 725Hoekzema N. M., Rutten R. J., Cook J. W., 1997, ApJ 474, 518Hofmann J., Steffens S., Deubner F. L., 1996, A&A 308, 192Jensen E., Orrall F. Q., 1963, PASP 75, 162Judge P., Carlsson M., Wilhelm K., 1997, ApJ 490, L195Judge P. G., Peter H., 1998, in C. Fro¨hlich, M. C. E. Huber, S. Solanki, R.von Steiger (eds.), Solar Composition and its Evolution – from Core toCorona, Procs. ISSI Workshop, Space Sci. Rev., in pressKalkofen W., Rossi P., Bodo G., Massaglia S., 1994, A&A 284, 976Kneer F., von Uexku¨ll M., 1986, A&A 155, 178Kneer F., Von Uexkull M., 1993, A&A 274, 584Leibacher J., Gouttebroze P., Stein R. F., 1982, ApJ 258, 393Lites B. W., Leka K. D., Skumanich A., Martinez Pillet V., Shimizu T., 1996,ApJ 460, 1019Lites B. W., Rutten R. J., Berger T. E., 1999, ApJ submittedLites B. W., Rutten R. J., Kalkofen W., 1993, ApJ 414, 345Liu S. Y., Sheeley N. R., 1971, Solar Phys. 20, 282Liu S.-Y., Skumanich A., 1974, Solar Phys. 38, 105Loefdahl M. G., Berger T. E., Shine R. S., Title A. M., 1998, ApJ 495, 965Mein P., Mein N., Malherbe J. M., Dame´ L., 1987, A&A 177, 283Nindos A., Zirin H., 1998, Solar Phys. 179, 253Rammacher W., Ulmschneider P., 1992, A&A 253, 586Rast M. P., 1995, ApJ 443, 863Remling B., Deubner F. L., Steffens S., 1996, A&A 316, 196Restaino S. R., Stebbins R. T., Goode P. R., 1993, ApJ 408, L57Rimmele T. R., Goode P. R., Harold E., Stebbins R. T., 1995, ApJ 444, L119Roudier T., Malherbe J. M., November L., Vigneau J., Coupinot G., Lafon M.,Muller R., 1997, A&A 320, 605Rutten R. J., 1994, in M. Carlsson (ed.), Chromospheric Dynamics, Proc. Mini-workshop, Inst. Theor. Astrophys., Oslo, p. 25Rutten R. J., 1995, in J. T. Hoeksema, V. Domingo, B. Fleck, B. Battrick(eds.), Helioseismology, Proc. Fourth SOHOWorkshop, ESA SP–376 Vol.1, ESA Publ. Div., ESTEC, Noordwijk, p. 151Rutten R. J., 1996, in K. G. Strassmeier, J. L. Linsky (eds.), Stellar SurfaceStructure, Procs. Symp. 176 IAU, Kluwer, Dordrecht, p. 38510Rutten R. J., 1998, in C. Fro¨hlich, M. C. E. Huber, S. Solanki, R. von Steiger(eds.), Solar Composition and its Evolution – from Core to Corona, Procs.ISSI Workshop, Space Sci. Rev., in pressRutten R. J., de Pontieu B., Lites B. W., 1999, in T. Rimmele, K. Balasubra-maniam, R. Radick (eds.), High Resolution Solar Physics: Theory, Ob-servations, and Techniques, Procs. 19th NSO/Sacramento Peak SummerWorkshop, Astron. Soc. Pac. Conf. Series, in pressRutten R. J., Uitenbroek H., 1991, Solar Phys. 134, 15Sivaraman K. R., Livingston W. C., 1982, Solar Phys. 80, 227Steffens S., Deubner F.-L., Fleck B., Wilhelm K., Schu¨hle U., Curdt W., Har-rison R., Gurman J., Thompson B. J., Brekke P., Delaboudinie`re J.-P.,Lemaire P., Hessel B., Rutten R. J., 1997, in B. Schmieder, J. C. T. delIniesta, M. Va´zquez (eds.), Advances in the physics of sunspots, Procs.First Adv. in Solar Physics Euroconf., Astron. Soc. Pac. Conf. Series 118,p. 284Steffens S., Hofmann J., Deubner F. L., 1996, A&A 307, 288Sutmann G., Ulmschneider P., 1995a, A&A 294, 232Sutmann G., Ulmschneider P., 1995b, A&A 294, 241Von Uexkuell M., Kneer F., 1995, A&A 294, 252Wellstein S., Kneer F., Von Uexkuell M., 1998, A&A 335, 32311

Dynamics of the Quiet Solar Chromosphere

The solar chromosphere has never been static although it

was often modeled so.Even the quiet-sun internetwork chromosphere

has become thoroughly dynamic with the acoustic shock interpretation

of the Ca II K 2V grains. We concentrate on the latter in this brief review.

Recent analysis of ASP data confirms that their excitation is more likely

set acoustically than magnetically.TRACE imagery permits seeing-free

studies of their occurrence patterns

NARCIS 

http://dspace.library.uu.nl:8080/bitstream/handle/1874/1083/armrev.pdf?sequence=1

Dynamics of the Quiet Solar Chromosphere

Abstract

Similar works

Full text

Available Versions

Utrecht University Repository

NARCIS