You are an expert at summarizing long articles. Proceed to summarize the following text: 
the validity of the optically thin approximation for coronal plasma has been discussed in the literature since the beginnings of solar x - ray and euv spectroscopy (  pottasch 1963 , and references therein ) . resonance scattering of @xmath015.01 ( @xmath1 , known as `` 3c '' ) , in particular , has been the subject of a long - standing controversy . observations of the ratio of the 3c flux to that of @xmath015.26 ( @xmath2 , known as `` 3d '' ) gave ratios in the range 1.6 to 2.3 ( rugge & mckenzie 1985 ; schmelz  1997 ; saba  1999 ) , whereas collisional radiative models predicted a ratio of about four ( smith  1985 ; loulergue & nussbaumer 1973 ; bhatia & doschek 1992 ; cornille  1994 ) . schmelz  and saba  also found , using the flat crystal spectrometer ( fcs ) on the _ solar maximum mission _ , that the lowest values were preferentially at the solar limb . this center - to - limb effect bolstered the argument for resonance scattering of @xmath015.01 , given that the limb photons traverse a longer path ( _ cf , _ phillips  1996 ) . significant new results have recently come to light : laboratory measurements from the electron beam ion trap ( ebit ) at lawrence livermore national laboratory found the 3c/3d flux ratio to be 3.04 @xmath3 0.12 ( brown et al . 1998 ) . using the national institute of standards and technology ebit , laming et al . ( 2000 ) confirmed the value to be close to three . individual measurements at different beam energies from the two groups span a range from 2.50 to 3.15 . these measurements imply that the amount of resonance scattering was overestimated in the solar analyses described above . furthermore , new theoretical models are converging toward a ratio closer to three ( e.g. doron & behar 2002 ; chen & pradhan 2002 ; gu 2003 ) , though all published models continue to exceed the measurements by at least 10% . new dirac r - matrix calculation show excellent agreement with the ebit measurements ( chen 2005 ; chen & pradhan 2005 ) . brown  ( 2001 ) have also reported on experiments in which a steady stream of neutral iron was injected into the ebit , producing an underionized plasma with both and . they found 3c/3d flux ratios as low as 1.9 @xmath3 0.11 , and argued that contamination of @xmath015.26 by the inner - shell line ( @xmath4 ) could account for the discrepancy between the laboratory ratio for a pure fe xvii plasma and the solar spectra . blending as an explanation for the solar results presumably implies that the center - to - limb effect in the solar data is due to chance . new observations of stellar coronae have not confirmed the solar line ratios . spectra for many stars observed with _ chandra _ and _ xmm - newton _ yield a 3c/3d ratio of about three ( ness et al . 2003 ; audard et al . 2004 ; gdel et al . 2004 ) , suggesting that stellar coronae do _ not _ generally show resonance scattering ( but see testa  2004 and matranga  2005 ) . full - star observations can not rule out resonance scattering in individual active regions , however , since the number of photons scattered out of the line of sight could be offset by a similar number of photons scattered into the line of sight . furthermore , the sample of stars does not include stars with coronae in the solar coronal temperature range ( 24 mk ) , such as @xmath5 centauri or procyon , for which blending with might be expected . these new experimental , theoretical , and observational results motivate us to re - investigate solar observations from the flat crystal spectrometer on the _ solar maximum mission . _ the reanalysis of these data , using new atomic data , is presented in  2 . in 3 we consider the implications of our results . in particular , we find that resonance scattering is not responsible for the fuzziness seen in the solar images obtained with the 284    passband of the _ transition region and coronal explorer _ ( ) and the euv imaging telescope ( eit ) on the _ solar and heliospheric observatory _ ( ) . the data analyzed here were obtained with the fcs ( acton  1980 ) and are discussed in detail in earlier papers ( schmelz  1997 ; saba  1999 ) . the instrument had a 15 arcsec field - of - view and could scan the soft x - ray resonance lines of prominent ions in the range of 1.5    to 20.0    with a spectral resolution of 0.015    at 15  . in this letter , we reanalyze the lines from 31 of the 33 spectral scans from quiescent active regions ( i @xmath014.21 could not be measured in two of these spectra see below ) . figure 1 shows the portion of a typical fcs spectrum containing the lines of interest . spectra where plasma conditions were changing significantly with time were excluded from the sample . the top panel of figure  2 shows the observed 3c/3d line ratio as a function of temperature . the flux ratio of i @xmath014.21 to @xmath016.78 provides a good temperature diagnostic , with its high signal - to - noise ratio and abundance - insensitivity . calculations using the astrophysical plasma emission code ( apec ) version 1.3 ( smith et al . 2001 ) give the temperatures for each of the individual measured flux ratios . the apec emissivities incorporate the ionization balance models of mazzotta et al . models for the other strong lines , @xmath017.05 and @xmath017.10 , are less certain than for @xmath016.78 , since these lines have a larger contribution from dielectronic recombination and resulting cascades ( see gu 2003 ) , and hence are more dependent on the ionization state model . the and g - ratios ( i.e. the ratios of the sum of the forbidden plus intercombination line fluxes to the flux of the resonance line ) are also temperature - dependent , but are of lower signal - to - noise ratio due to the weakness of the intercombination lines . most of the observed 3c/3d ratios are clustered , with significantly less than the average laboratory value of 2.9 ; however , two of the three highest temperature measurements also give higher 3c/3d ratios , within 1@xmath6 of 2.9 . moreover , the best - fit line to these data shows a strong temperature - dependence , inconsistent with calculations ( gu 2003 ) , while the flux ratio of 3c to @xmath016.78 ( middle panel ) shows only modest temperature - dependence as expected from calculations . these results strongly suggest blending of @xmath015.26 .    in the underionized ebit plasma , brown et al . ( 2001 ) also measure a second inner - shell line at 15.21    ( @xmath7 ) . we identify this line for the first time in an active region spectrum , and use it to estimate the strength of the blend at 15.26   . calculations from the hebrew university lawrence livermore atomic code ( hullac ) , which will be available in the next apec release , give a scaling factor of 0.83 ( d. liedahl , private communication ) . we note that this scaling factor is significantly higher than the factor of 0.5 recommended by brown et al . ( 2001 ) based on multiconfigurational dirac - fock calculations , primarily due to a large difference in the branching ratio between radiative decay and autoionization for @xmath015.21   . the bottom panel of figure 2 shows the line ratios that result after subtracting the @xmath015.26 blend , using @xmath015.21 as a proxy to determine its flux . the weighted mean of this distribution is 2.76 @xmath3 0.23 , statistically indistinguishable from the laboratory ratio . while the @xmath015.21 proxy is , to our knowledge , unblended at temperatures below 5  @xmath8  k , its use for hotter plasmas is complicated by the presence of @xmath015.20 ( e.g. the _ chandra _ spectrum of capella , desai et al . 2005 ) . it seems likely that the line tentatively identified in the flare spectrum by phillips et al . ( 1982 ) as may be dominated by as well .    in light of this new analysis , we reconsider other examples of full - sun x - ray spectra which include @xmath015.01 and @xmath015.26 . in particular , we expect active region measurements to show lower line ratios and flare spectra to approach the laboratory value . indeed active region measurements show lower 3c/3d values ( blake  1965 ; evans & pounds 1968 ; walker  1974 ) , whereas flare spectra ( e.g. neupert 1973 ) give higher values . these results are thus consistent with blending at 15.26    in the low - temperature active region spectra . they also suggest that a reduced ratio might be observed also in high signal - to - noise ratio , high resolution spectra of stars with cooler coronae . resonance scattering has been suggested as one of the reasons for the fuzzy appearance of the images obtained from the 284  passband of  and -eit , illustrated in figure  3 . in this section , we discuss possible explanations of the 284-fuzziness and estimate the upper limit for the resonance scattering contribution to this fuzziness . a common explanation for the fuzzy appearance of the  and eit 284    images is instrument scattering ; however , measurements of the 284    point spread function are identical to those of the 171- and 195    passbands , which show much cleaner images ( golub et al . furthermore , prominences seen in absorption have sharp edges in the 284    images ( fig .  3 , lower panel ) , demonstrating that the instrument can resolve fine structures . a second candidate explanation is contamination of @xmath0304 photons in the passband . this contamination certainly exists for coronal holes , which do not appear dark ( as they do , for example , in the thin aluminum images from the soft x - ray telescope on ) ; however , there should be no significant @xmath0304-contribution in active region areas because of the higher temperatures , or above the solar limb where the scale height of @xmath0304 is too small . resonance scattering has also been suggested as the cause of the  284    fuzziness . the @xmath015.01 result allows us to estimate the contribution of resonance scattering to @xmath0284.2 . using chen s ( 2005 ) calculated 3c/3d ratio of 2.85 at @xmath9 6.4 , and the observed fcs ratio of 2.76 @xmath3 0.21 gives an escape probability of 2.76/2.85 , corresponding to @xmath10 . following acton ( 1978 ) , @xmath11 , where @xmath12 is the oscillator strength , @xmath13 is the ionization fraction ( which is a function of temperature ) , and @xmath0 is the wavelength . solving for @xmath14 , with the ionization balance at @xmath9 6.4 , gives        this corresponds to an escape probability of 92% , which indicates that very few photons are available to contribute to the 284   - image fuzziness . for possible uncertainties of 10 to 15 % in experimental and theoretical 3c/3d ratios , the escape probability ranges from 0.84 to 1.0 , for @xmath17 . using the same argument , with the relative coronal abundances of fludra & schmelz ( 1999 ) , we find that the @xmath018.97 opacity is comparable to that of @xmath015.01 , while the opacities of @xmath013.46 and @xmath09.17 are 2 to 3 times lower , confirming the findings of schmelz et al . ( 1997 ) that these important diagnostics are not contaminated for use in emission measure distribution analysis . the assumed temperature @xmath18 , estimated from the measured ratios of @xmath014.21 to @xmath016.78 , can be further constrained by the laboratory measurements . the experimental 3c/3d ratio was measured as a function of the abundance ratio of to , providing a good fit to the theoretical predictions . the experimental plasma is not in collisional ionization equilibrium , due to the continuous ionization from neutral iron up to the charge states of interest , and moreover , the collisional processes are excited by a narrow beam rather than a maxwellian distribution as assumed for the solar coronal plasma . thus , an interpretation of the experimental line ratio in terms of an ionization equilibrium temperature is not strictly valid , since the resonance excitation contributions will not be identical . nevertheless , this estimate can give a consistency check on the population . for the laboratory value where the populations of and are equal , the 3c/3d ratio is 1.90 @xmath3 0.11 ( brown et al . 2001 ) , slightly below the average of the cluster . thus the diagnostics all appear consistent with each other in the blending scenario .    in collisional ionization equilibrium , such a ratio corresponds to @xmath19 6.32 . for this lower temperature , @xmath20 is about 3 times larger ; however , chen s ( 2005 ) predicted line ratio is about 5% less , such that our average value puts @xmath21 essentially at zero , with an upper limit below 0.2 . meanwhile , loop differential emission measure ( dem ) distributions tend to be fairly flat - topped between @xmath9 6.3 and 6.4 ( e.g. schmelz et al . 2001 ) , such that an intermediate value seems most reliable for estimating the @xmath20 ratio . we then find @xmath22 0.12 , with a plausible range from 0.0 to @xmath23 0.2 . for opacities in this range , the model images of wood & raymond ( 2000 ) still retain the appearance of loops with sharp rather than blurred boundaries . we refer to the last candidate explanation for the 284   - fuzziness as the `` filling - factor '' model , which is related to the observational result that coronal structures appear fuzzy if they are not resolved by the instrument ( see fig . 3 of deluca et al . 2005 ) . it is well established that the dem of active region and quiet sun plasma peaks between 2 and 3 mk . in other words , more higher temperature ( 284   ) plasma exists along the line of sight than cooler ( 171   ) plasma , such that the piling up of structures along the line of sight may contribute to the fuzziness factor . in the 171-  band ( @xmath241  mk ) , the 0.5-arcsec resolution of  revealed substantial substructure that had never been seen before ; in the 284-  band ( @xmath2423  mk ) , the spatial resolution does not appear to be quite sufficient . an additional factor of two improvement in spatial resolution may be adequate , but 0.1-arcsec pixel size is technically feasible and seems preferable . we conclude , therefore , that the filling - factor model provides the most likely explanation for the 284-image fuzziness , and thus future instruments with higher spatial resolution may be able to resolve the active region plasma into its component structures . efforts to understand the coronal heating process should benefit from resolved images closer to the temperature at which the dominant heating occurs . we would like to thank j. saba , k. nasraoui , d. liedahl , l. golub , and j. cirtain . solar physics research at the university of memphis is supported by nsf atm-0402729 and nasa nng05ge68 g . n. b. was supported by nasa contract nas8 - 39073 to the smithsonian astrophysical observatory for the chandra x - ray center . acton , l.  w. 1978 , , 225 , 1069 acton , l.  w.  1980 , , 65 , 53 audard , m. , telleschi , a. , gdel , m. , skinner , s.  l. , pallavicini , r. , & mitra - kraev , u. 2004 , , 617 , 531 bhatia , a.  k. & doschek , g.  a. 1992 , atomic data nuclear data , 52 , 1 blake , r.  l. , chubb , t.  a. , friedman , h. , unzicker , a.  e. 1965 , , 142 , 1 brown , g.  v. , beiersdorfer , p. , chen , h. , chen , m.  h. , reed , k.  j. 2001 , , 557 , l75 brown , g.  v. , beiersdorfer , p. , liedahl , d.  a. , widmann , k. , kahn , s.  m. 1998 , , 502 , 1015 chen , g .- x . 2005 , submitted chen , g.  x. , & pradhan , a.  k. 2002 , , 89 , 13202 chen , g.  x. , & pradhan , a.  k. 2005 , mnras , submitted cornille , m. , dubau , j. , faucher , p. , bely - dubau , f. , & blancard , c. 1994 , , 1 , 1 deluca , e.e . , weber , m.a . , sette , a.l . , golub , l. , shibasaki , k. , sakao , t. & kano , r. 2005 , adv . space res . , in press desai , p. et al . 2005 , , 625 , l59 doron , r. & behar , e. 2002 , , 574 , 518 evans , k. & pounds , k.  a. 1968 , , 152 , 319 fludra , a. & schmelz , j.  t. ( 1999 ) , a&a , 348 , 286 golub , l. et al . 1999 , phys . plasmas , 6 , 2205 gu , m .- f . 2003 , , 582 , 1241 gdel , m. , audard , m. , reale , f. , skinner , s.  l. , linsky , j.  l. 2004 , , 416 , 713 laming , j.  m.  2000 , , 545 , l161 loulergue , m. & nussbaumer , h. 1973 , , 45 , 125 matranga , m. , mathioudakis , m. , kay , h.  r.  m. , & keenan , f.  p. 2005 , , 621 , l125 mazzotta , p. , mazzitelli , g. , colafrancesco , s. , & vittorio , n. 1998 , , 133 , 403 ness , j .- u . , schmitt , j. h. m. m. , audard , m.  j. , gdel , m. , & mewe , r. 2003 , , 407 , 347 neupert , w.m . , swartz , m. , kastner , s  o. 1973 , , 31 , 171 phillips , k. j. h. , greer , c. j. , bhatia , a.  k. , keenan , f.  p. 1996 , , 469 , l57 phillips , k. j. h. et al . 1982 , , 256 , 774 pottasch , s. r. 1963 , , 137 , 945 rugge , h. r. & mckenzie , d.  l. 1985 , , 297 , 338 saba , j. l. r. , schmelz , j.  t. , bhatia , a.  k. , & strong , k.  t. 1999 , , 510 , 1064 schmelz , j. t. , saba , j. l. r. , chauvin , j.  c. , & strong , k.  t. 1997 , , 477 , 509 schmelz , j. t. , scopes , r. t. , cirtain , j. w. , winter , h. d. , & allen , j. d. 2001 , , 556 , 896 smith , b. w. , raymond , j. c. , mann , j.  b. , & cowan , r.  d. 1985 , , 298 , 898 smith , r. k. , brickhouse , n. s. , liedahl , d. a. , & raymond , j. c. 2001 , , 556 , l91 testa , p. , drake , j.  j. , peres , g. , & deluca , e.  e. 2004 , , 609 , l79 walker , a. b. c. , rugge , h. r. , weiss , k. 1974 , , 194 , 471 wood , k. , & raymond , j. c. 2000 , , 540 , 563    14.21 to @xmath016.78 ) . the dotted line shows the best flat fit and the dashed line shows the best linear fit ; ( _ upper _ ) 3c/3d ratio with flat ( @xmath25 ) and linear ( @xmath26 ) fits ; ( _ middle _ ) @xmath015.01-to-@xmath016.78 ratio with flat ( @xmath27 ) and linear ( @xmath28 ) fits ; ( _ lower _ ) same as ( _ upper _ ) except that 0.83 times the @xmath015.21 flux has been subtracted from @xmath015.26    flux for all but the three hottest spectra with flat ( @xmath29 ) and linear ( @xmath30 ) fits.,width=288 ]
resonance scattering has often been invoked to explain the disagreement between the observed and predicted line ratios of @xmath015.01 to @xmath015.26 ( the `` 3c/3d '' ratio ) . in this 
 process photons of @xmath015.01 , with its much higher oscillator strength , are preferentially scattered out of the line of sight , thus reducing the observed line ratio . 
 recent laboratory measurements , however , have found significant inner - shell lines at 15.21 and 15.26   , suggesting that the observed 3c/3d ratio results from blending . given our new understanding of the fundamental spectroscopy 
 , we have re - examined the original solar spectra , identifying the @xmath015.21 line and measuring its flux to account for the contribution of to the @xmath015.26 flux . 
 deblending brings the 3c/3d ratio into good agreement with the experimental ratio ; hence , we find no need to invoke resonance scattering . 
 low opacity in @xmath015.01 also implies low opacity for fe  xv @xmath0284.2 , ruling out resonance scattering as the cause of the fuzziness of  and -eit 284  images . 
 the images must , instead , be unresolved due to the large number of structures at this temperature . 
 insignificant resonance scattering implies that future instruments with higher spatial resolution could resolve the active region plasma into its component loop structures .