LINE INTENSITY RATIOS IN THE EIS RANGE SENSITIVE TO ELECTRON DENSITIES IN 10 7 K PLASMAS

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1 The Astrophysical Journal, 679:843Y847, 2008 May 20 # The American Astronomical Society. All rights reserved. Printed in U.S.A. LINE INTENSITY RATIOS IN THE EIS RANGE SENSITIVE TO ELECTRON DENSITIES IN 10 7 K PLASMAS U. Feldman and E. Landi Artep, Inc., at Space Science Division, Naval Research Laboratory, Washington, DC and G. A. Doschek Space Science Division, Naval Research laboratory, Washington, DC Received 2007 December 12; accepted 2008 February 12 ABSTRACT Electron density variations during the rise, maximum, and decay phases of flaring plasmas at T 10 MK are important quantities to be used to test flare models. To date, electron density values measured in solar flares are, with few exceptions, only lower limits. With the launch of the EUV Imaging Spectrometer (EIS) on Hinode, it has become possible for the first time to measure electron densities and their time evolution during flares. In this paper we discuss electron density diagnostics in the Y10 13 cm 3 range by means of intensity ratios of lines emitted by Ti, Cr, and Mn ions within the Hinode/EIS wavelength range. Subject headinggs: plasmas Sun: corona Sun: flares 1. INTRODUCTION High-resolution X-ray spectrometers designed to study solar flare plasmas from space were launched in 1979 on Solwind/ P 78-1 (Doschek1983), in 1980 on SMM (Acton et al.1980), in 1981 on Hinotori (Tanaka et al.1982), and in 1991 on Yohkoh (Culhane et al.1991). The resulting observations provided a wealth of information on the X-ray time variability of the emitting fluxes, electron temperature, emission measures, and mass motions of solar flare plasmas. Being uncollimated, the derived fluxes and emission measures are the total sum over the entire volume of the flare, while temperatures and mass motions are weighted averages. X-ray imagers with a few arcseconds spatial resolutions launched on Yohkoh (Tsuneta et al. 1991) measured mostly flare shapes and sizes and provided some information on emission measures and motions of the hot flaring regions. Similarly, TRACE has provided EUV images of flares, as well as diagnostics similar to the X-ray imager on Yohkoh, but with much higher spatial resolution ( 1 00 ). However, the density measurements in flares obtained from all these missions are limited, as discussed below. In the present paper we propose intensity ratios of spectral lines expected in the EUV Imaging Spectrometer ( EIS) range (170Y211 and 246Y291 8) that allow time-dependent measurements of electron densities in solar flares. 2. DIAGNOSTIC TECHNIQUES There are three main methods for measuring electron densities in high temperature flaring plasmas. The first and most direct method is based on line intensity ratios. The majority of the prominent spectral lines expected to be present in the 10Y25 MK flaring plasmas are those belonging to the highest Z abundant elements in solar plasmas, and most of them are emitted by highly ionized Fe (Z ¼ 26) and Ni (Z ¼ 28). Although a few line intensity ratios from these ions become sensitive to electron densities in the cm 3 range, their sensitivity is greatest at electron densities of N e > cm 3. The most prominent density sensitive lines from Fe ions result from transitions of the type n ¼ 0(wherenis the principal quantum number) appearing in the EUV (70 < k < 150 8). Unfortunately, only extremely limited EUV solar flare spectra exist. Density- 843 sensitive Fe lines belonging to transitions of the type n ¼ 1, which appear in the X-ray region, are also available. Such lines are usually intrinsically fainter than those associated with n ¼ 0 transitions and are often blended. Nevertheless, in a few cases they were successfully measured and provided electron density information. Using intensity ratios from the moderately collimated SMM flat crystal soft X-ray spectrometer, Phillips et al. (1996) reported an electron density of cm 3 in 10 MK flare plasmas. At 10Y25 MK plasmas, elements with Z 20 are mostly in the H-like, He-like, and Li-like ionization stages. However, the reasonably intense flare lines from ions belonging to these isoelectronic sequences are not sensitive to densities expected in the 10Y25 MK flaring plasmas. Some He-like line intensity ratios do become sensitive to flare densities, but they are emitted by ions mostly formed at much lower temperatures than flares. For example, Doschek et al. (1981) reported electron densities as high as cm 3 in an M flare using intensity ratios of two O vii (He-like) lines at 22 8 recorded by the soft X-ray spectrometer on Solwind/P Since the temperature of maximum abundance of O vii is only 2 MK, it is not obvious that the reported result is also valid for 10Y25 MK plasmas. A second method for deriving the electron density is based on the ratio of emission measure (EM ¼ Ne 2 V) and the emitting region volume (V ), assuming a value of the filling factor f (where 0 < f 1). In this method, the electron density is obtained from the relationship p N e ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi EM=fV: ð1þ The accuracy of the electron density depends on the uncertainty in the estimation of the filling factor, a very difficult quantity to estimate, and on the accuracy of the estimated plasma volume. Peak electron density ratios based on this method are on the order of cm 3. Examples of the application of this method are found in Cheng & Widing (1975). A third method uses the cooling time of rapidly decaying flares. To derive the electron density by this method, the temperature decrease with time needs to be well known. We also need to assume that no energy is supplied to the flare once the decay starts, and for simplicity we also assume that energy losses

2 844 FELDMAN, LANDI, & DOSCHEK Vol. 679 TABLE 1 Sources for A Values and Collision Rates for the Ions Providing Density Diagnostic Line Intensity Ratios Ion A Values Collision Rates Ti xvii... Zhang & Sampson (1996) Zhang & Sampson (1996) Galavis et al. (1997) Ti xviii... Zhang & Sampson (1994) Zhang & Sampson (1994) Cr xx... Zhang & Sampson (1994) Zhang & Sampson (1994) Mn xxi... Zhang & Sampson (1994) Zhang & Sampson (1994) Fe xxii... Landi & Gu (2006) Badnell et al. (2001) Landi & Gu (2006) are by radiation only (see discussion in Feldman et al.1982). From the measured rate of decrease of the temperature, the electron density can be measured using the relationship N e ¼ 3kT t rad RT ð Þ ; ð2þ where k is the Boltzmann constant, RT ð Þ is the radiative power loss function, and T is the temperature decrease that occurred during the time interval t rad. Clearly, this technique can be applied to flares where the rate of decrease of temperature can be measured easily, that is, either very short duration flares, or flares observed continuously for a long time (for examples, see Feldman et al. 1982, 1994). This technique is also heavily dependent on assumptions about plasma heating during the decay phase. Using the three techniques mentioned above, it was found that the electron density in large flares at the time of maximum emission varies between 5 ; and cm 3. The largest value was obtained from the line intensity ratio of 2lY3l 0 Fe xxii lines in the k < 17 8 range, recorded by an uncollimated X-ray spectrometer (Phillips et al.1996). To our knowledge, with the exception of the above-mentioned O vii measurements, no measurements of the electron density temporal evolution during the rise, peak, and decay of solar flaring plasmas have yet been made. 3. EIS DENSITY DIAGNOSTICS IN FLARES Here we report on useful density sensitive line ratios in the EIS range. The lines we discuss in this paper belong to highly ionized Ti, Cr, and Mn ions. We calculate their line emissivities and emissivity ratios using version 5.2 of the CHIANTI database (Dere et al. 1997; Landi et al. 2006), which includes proton excitation rates for all the ions whose ratios we consider: Ti xvii, Ti xviii, Cr xx, Mn xxi, and Fe xxii. References to the original collision excitation rates and A values are reported in Table 1. The accuracy of these transition rates is difficult to assess; it is normally thought that A values are accurate to 10%Y15%, and the collision rates to up to 30%. However, this accuracy may be severely limited by two additional important factors: the size of the atomic model (because of radiative cascades), and the inclusion of resonant excitation in the collision rate. This will be discussed later for the B-like ions (Ti xviii,cr xx,mn xxi,and Fe xxii). The emissivity ratios have been calculated at the temperature of maximum abundance T max of each ion: log T max ¼ 6:8(Tixvii, xviii), 7.0 (Cr xx,mnxxi) and 7.1 (Fe xxii), according to Mazzotta et al. (1998). The emissivity ratios for the Ti, Cr, and Mn ions are, however, also temperature sensitive, changing by a factor up to 1.5 in the range log T max 0:2 log T log T max þ 0:2. In order to use such ratios, it is necessary to measure the temperature of the emitting plasma as a preliminary step. Although Ti, Cr and Mn are less abundant than Fe by factors of 60 to 360 (Table 3), it is expected that the high sensitivity of EIS, as well as the brightness of flare plasmas, will allow the measurement of the intensities of these lines in solar flare spectra with sufficient statistical significance to be used to measure detailed time-dependent electron densities. The temperature of the largest solar flares could approach 30 MK, and the emission measure could be as high as cm 3 (Feldman et al. 1996, 1995). In typical X-class flares, the emission measure is on the order of cm 3, and the temperature at maximum is 20Y25 MK. The emission measure of M class flares is approximately an order of magnitude lower, and the electron temperature on average is in the 15 MK range. EIS is a high-resolution stigmatic instrument operating in the 170Y211 8 and 246Y291 8 ranges. This well-calibrated instrument was launched into space aboard the Hinode observatory in 2006 November. For details on EIS properties and calibration, see Culhane et al. (2007), Korendyke et al. (2006), and Lang et al. (2006). Using the calibration properties of EIS, the averaged emission measure from an M2 class flare, an electron density in the cm 3 range, and typical coronal elemental abundances, the number of photons per second from a 1 00 ; 1 00 area in lines expected in the EIS range were calculated by J. T. Mariska (2007, private communication). The calculated number of photons impinging per second on the CCD and the resulting data numbers (DN) for Fe xviiyxxiv lines are listed in Table 2. As indicated in the table, the DN s 1 values for the Fe xviiyxxiv lines are in the 3,000Y100,000 range. In X-class flares, which are an order of magnitude brighter, the DN s 1 numbers will most likely also be an order of magnitude TABLE 2 Estimated Number of Photons and Data Numbers Expected from a 1 00 ; 1 00 Area of the Sun during Peak Emission from an M2 Class Flare Ion k (8) Transition log T Incident Photons (s 1 ) DN (s 1 ) Fe xxi s 2 2p 21 D 2 Y2s2p 3 3 D Fe xxiv s 2 S 1/2 Y2p 2 P 3/ Fe xx s 2 2p 3 2 P 3/2 Y2s2p 44 P 3/ Fe xvii s 2 2p 5 3s 1 P 1 Y2s 2 2p 5 3p 1 S Fe xxii s 2 2p 2 P 3/2 Y2s2p 24 P 5/ Fe xvii s 2 2p 5 3s 3 P 2 Y2s 2 2p 5 3p 1 S Fe xxiv s 2 S 1/2 Y2p 2 P 1/ Fe xxiii s 21 S 0 Y2s2p 3 P Fe xvii s 2 2p 5 3p 3 D 2 Y2s 2 2p 5 3d 3 F Fe xxi s 2 2p 2 3 P 2 Y2s2p 3 5 S

3 No. 1, 2008 FLARE DENSITIES WITH EIS 845 TABLE 3 Photospheric and Coronal Abundances of Ti, Cr, Mn, and Fe Relative to H loga el Element Photospheric Coronal A el /A Fe Ti Cr Mn Fe Note. The abundance of H was defined to be log A H ¼ 12. larger, i.e., in the 30,000Y1,000,000 range. Since the abundances of Ti, Cr, and Mn are 60Y360 times lower than the Fe abundance (Table 3), and the lines from the elements we consider are similar in nature to those emitted by the Fe ions, it is expected that during a 10 s exposure, highly ionized Ti, Cr and Mn lines from an X-class flare will have DN values in the thousands, and can be easily observed by EIS. 4. B-LIKE AND C-LIKE DENSITY-SENSITIVE LINE INTENSITY RATIOS Table 4 lists Ti xvii, Tixviii, Tixx, Crxix, Crxx, Crxxii, Mn xxi,andfexxii lines in the EIS range that belong to the Li-, B-like, and C-like isoelectronic sequences and are expected to be visible in EIS spectra during solar flares. Intensity ratios from several of the lines listed in Table 4, which belong to the B-like and C-like sequences, are sensitive to electron densities expected in the 10Y25 MK flaring plasmas. Table 4 also reports the transitions from other ions and elements that are found very close to the flare ions we are interested in, and that might not be resolved by EIS. It is difficult to estimate the importance of the contributions of these blending lines to the flare lines, but they most likely will need to be accounted for when using intensity ratios from the ions in Table 4 observed during flares. However, the distinction between the hot and cold line contribution should not be too difficult to make, because flare emission will be localized in small areas along the slit, while that of colder ions not participating in the flares will be more uniformly spread along the slit. We can then interpolate the latter and remove it from the former. The 2s 2 2p ground configuration of B-like ions consists of the 2 P 1/2 and 2 P 3/2 levels. Under very low electron density conditions, practically the entire ion population is in the 2 P 1/2 ground level. As a result of electron collisions, the preferred excitation channel is into the J ¼ 1/2 and J ¼ 3/2 levels of the first excited configuration 2s2p 2. Under these conditions, J ¼ 5/2 levels from the 2s2p 2 configuration, for which the main excitation channel is from 2s 2 2p 2 P 3/2, will be underpopulated. As the electron density increases, the population of the ground 2 P 3/2 level also increases, so that the relative population between the J ¼ 5/2 levels and the J ¼ 1/2 and J ¼ 3/2 levels in the excited configuration also starts increasing. Figure 1 is a plot of the 2s 2 2p 2 P 3/2 Y2s2p 24 P 5 =2/ 2s 2 2p 2 P 1/2 Y2s2p 24 P 1 =2 intensity line ratios vs. electron density of Ti xviii ( / ), Cr xx ( / ), Mn xxi ( / ), and Fe xxii ( / ). Although the Ti xviii lines are outside the EIS range, we included the ratio to demonstrate the progression in the electron density sensitivity as a function of the atomic number (element). Figure 2 shows the 2s 2 2p 2 P 3/2 Y2s2p 22 D 5 =2/2s 2 2p 2 P 1/2 Y2s2p 22 D 3 =2 intensity ratio vs. electron density in Ti xviii ( / ) and the 2s 2 2p 2 P 3/2 Y2s2p 22 D 5 =2/2s 2 2p 2 P 3/2 Y2s2p 22 D 3 =2) intensity ratio vs. electron density in Cr xx ( / ). Clearly, the electron sensitivity range of the Ti xviii, Cr xx, and Mn xxi intensity ratios are ideal for diagnosing the 10 MK flaring plasmas. The 2s 2 2p 2 ground configuration of C-like ions consists of five levels: 3 P 0;1;2, 1 D 2, and 1 S 0. Under very low electron density conditions, practically the entire ion population is in the 3 P 0 level. Thus, as a result of electron collisions, the preferred excitation TABLE 4 Ti, Cr, Mn, and Fe Lines in the EIS Range Ion Transition k (8) log T max ( K) Blends Ti xvii s 2 2p 2 3 P 0 Y2s2p 3 3 D Ti xviii s 2 2p 2 P 1/2 Y2s2p 2 2 D 3/ Ti xvii s 2 2p 2 3 P 1 Y2s2p 3 3 D Ti xvii s 2 2p 2 3 P 1 Y2s2p 3 3 D Fe xi Ti xvii s 2 2p 2 3 P 2 Y2s2p 3 3 D Fe xi Ti xvii s 2 2p 2 3 P 2 Y2s2p 3 3 D Fe x Ti xvii s 2 2p 2 3 P 2 Y2s2p 3 3 D Fe xxii Ti xviii s 2 2p 2 P 3/2 Y2s2p 2 2 D 5/ Fe xiii Ti xviii s 2 2p 2 P 3/2 Y2s2p 2 2 D 3/ Ti xx s 2 S 1/2 Y2p 2 P 3/ Cr xx s 2 2p 2 P 3/2 Y2s2p 2 2 D 5/ Cr xx s 2 2p 2 P 3/2 Y2s2p 2 2 D 3/ Cr xxii s 2 S 1/2 Y2p 2 P 1/ Cr xix s 2 2p 2 3 P 1 Y2s2p 35 S S xi Cr xx s 2 2p 2 P 1/2 Y2s2p 24 P 1/ Cr xx s 2 2p 2 P 3/2 Y2s2p 24 P 5/ Mn xxi s 2 2p 2 P 3/2 Y2s2p 2 2 D 3/ Mn xxi s 2 2p 2 P 1/2 Y2s2p 24 P 1/ Mn xxi s 2 2p 2 P 3/2 Y2s2p 24 P 5/ Fe xxii s 2 2p 2 P 1/2 Y2s2p 24 P 1/ S xi Fe xxii s 2 2p 2 P 3/2 Y2s2p 24 P 5/ Notes. T max is the temperature of maximum ion fractional abundance. Wavelengths marked with an asterisk ( ) have been previously observed and reported in the literature. The emissivity is given by emission measure EM = cm 3, N e ¼ cm 3, and solar coronal abundances.

4 846 FELDMAN, LANDI, & DOSCHEK Vol. 679 Fig. 1. The 2s 2 2p 2 P 3/2 Y2s2p 24 P 5/2 /2s 2 2p 2 P 1/2 Y2s2p 24 P 1/2 line intensity ratios vs. electron density in Ti xviii ( / ), Cr xx ( / ), Mn xxi ( / ), and Fe xxii ( / ). Ratios have been calculated at the temperature of maximum abundance of each ion: log T ¼ 7:0 (Cr xx and Mn xxi) and 7.1 ( Fe xxii). Fig. 3. Ti xvii line intensity ratios versus electron density: / (2s 2 2p 23 P 1 Y2s2p 33 D 2 /2s 2 2p 23 P 0 Y2s2p 33 D 1 ), / (2s 2 2p 2 3 P 2 Y2s2p 33 D 3 /2s 2 2p 23 P 0 Y2s2p 33 D 1 ) and / (2s 2 2p 23 P 2 Y 2s2p 33 D 3 /2s 2 2p 23 P 1 Y2s2p 33 D 2 ). Ratios have been calculated at the temperature of maximum abundance log T ¼ 6:8. channel will be to the first excited configuration 2s2p 3 levels with J ¼ 1. Levels from the excited configuration with J ¼ 2and J ¼ 3, mainly excited from the 2s 2 2p 23 P 1 and 3 P 2 levels, will be under populated when compared to those with J ¼ 0 and J ¼ 1. As the electron density increases, at first the relative population between the 2s 2 2p 2, 3 P 1,and 3 P 0 will start to increase, and will increase the population of the J ¼ 2 levels in the excited configuration 2s2p 3. With a further increase in the electron density, the population of the 2s 2 2p 23 P 2 will also start increasing, thus enhancing the 2s2p 33 D 3 level population. Figure 3 is a plot of the intensity ratios vs. electron density of the following Ti xvii lines: / (2s 2 2p 23 P 1 Y2s2p 33 D 2 /2s 2 2p 2 3 P 0 Y2s2p 33 D 1 ), / (2s 2 2p 23 P 2 Y2s2p 33 D 3 / 2s 2 2p 2 3 P 0 Y2s2p 33 D 1 ), and / (2s 2 2p 23 P 2 Y 2s2p 33 D 3 /2s 2 2p 23 P 1 Y2s2p 33 D 2 ). As expected, the line originating from the 2s2p 33 D 3 level becomes sensitive at somewhat higher electron density than the line originating from the 2s2p 33 D 2 level. As demonstrated in the figure, the Ti xvii intensity ratios are effective indicators for densities of 1 ; < N e < 1 ; cm 3. To facilitate the use of the results displayed in Figures 1Y3, the intensity ratios R have been fitted using the formula N e R ¼ a þ b ; N e þ N c where the parameters a and b, and the critical density N c, are listed in Table 5. The parameter a indicates the value of R in the low-density limit, while a þ b is the value of the ratio R in the high-density limit. The fit is excellent for all ratios. The Ti xvii / ratio has not been fitted with the above formula, as its dependence on the electron density is shallower. 5. ACCURACY OF ATOMIC DATA The accuracy of the atomic data and the completeness of the atomic models are crucial for the reliability of the density measurements. Most of the ratios discussed in the previous section are emitted by ions with low abundance in the solar atmosphere: although important for tokamak plasmas, these ions have been traditionally neglected by the atomic physics community, and only a few calculations of the atomic data and collision rates necessary to evaluate their emissivities have been carried out. It is therefore important to try to assess the accuracy of their emissivities. Figure 1 indicates that the low-density limit of the Fe xxii ratio does not follow the decreasing trend shown by the B-like Ti, Cr, ð3þ TABLE 5 Parameters of the Fit of the Intensity Ratios Ion Ratio a b log N c Fig. 2. The 2s 2 2p 2 P 3/2 Y2s2p 22 D 5/2 /2s 2 2p 2 P 1/2 Y2s2p 22 D 3/2 intensity ratio vs. electron density in Ti xviii ( / ) and the 2s 2 2p 2 P 3/2 Y 2s2p 22 D 5/2 /2s 2 2p 2 P 3/2 Y2s2p 22 D 3/2 intensity ratio vs. electron density in Cr xx ( / ). Ratios have been calculated at the temperature of maximum abundance of each ion: log T ¼ 6:8 (Ti xvii) and 7.0 (Cr xx). Cr xx / Mn xxi / Fe xxii / Cr xx / Ti xviii / Ti xvii / Ti xvii / Note. As displayed in Figs. 1Y3, according to the formula in eq. (3).

5 No. 1, 2008 FLARE DENSITIES WITH EIS 847 and Mn ratios, whose low-density limit value decreases as the atomic number weight Z increases along the isoelectronic sequence. There are several reasons for this behavior: ionization and recombination into excited levels, included only in Fe xxii; different size of the atomic model; and resonant excitation in the collision rates. We have used CHIANTI to assess the importance of these factors, and we have found that the effect of ionization and recombination is rather modest (within 25%). On the other hand, the number of levels in the atomic model (and hence the importance of radiative cascades) and resonant excitation are very important. The atomic model of Fe xxii includes 513 levels, whereas the model for B-like Ti, Cr, and Mn ions only includes 15 levels. Fe xxii collision rates come from a very extensive calculation by Badnell et al. (2001) that includes resonant excitation, while the other B-like ions neglect it. To test the combination of the importance of these two factors, we have recalculated the Fe xxii ratio using the data in version 1 of the CHIANTI database, where the Fe xxii atomic model included only 125 levels and no resonant excitation was considered: the value if the low-density limit decreases to 0.5, much closer to the other ions. A lower limit of 0.363, in line with the results of the other ions in the sequence, is obtained if the atomic model of Fe xxii is decreased to the same 15 levels as the other ions, still omitting resonance excitation. These results cast some cloud on the accuracy of the data for the B-like Ti, Cr, and Mn ions, because the Fe xxii atomic model and collision data are much more extensive and accurate than for the former ions. This demonstrates the need for more extensive calculations of collision rates for many more levels for B-like ions. 6. SUMMARY AND CONCLUSIONS In the present work we have investigated the presence of line intensity pairs from highly ionized Ti, Cr, Mn, and Fe in the EIS range that can be used to measure the electron density in flares as a function of time. As discussed in x 4 and displayed in Figures 1Y3, a number of intensity ratios of lines listed in Table 4, which belong to Ti xvii, Ti xviii, Crxx, and Mn xxi, are excellent indicators of electron densities for the Y10 14 cm 3 range expected to be present in solar flaring plasmas. We believe that the B-like and C-like ions from Ti, Cr, and Mn are among the best available electron density indicators for flaring plasmas in the EIS wavelength range. Some of the lines used in these ratios might be blended with colder quiet-sun and active region lines, whose contribution to the total observed intensities might be nonnegligible. However, the stigmatic properties of EIS should allow us to evaluate the effects of blending in many cases. We have also shown that many of the ions providing these ratios need improved collision rates that include resonant excitation, as well as more extensive atomic models that include radiative cascades following collisional excitation. We recommend that such calculations be performed. The authors acknowledge support from the Hinode NASA Phase E funding. The work of E. L. is supported by the NNG06EA14I, NNH06CD24C, and other NASA grants. Acton, L. W., et al. 1980, Sol. Phys., 65, 53 Badnell, N. R., Griffin, D. C., & Mitnik, D. M. 2001, J. Phys. B, 34, 5071 Bhatia, A. K., Feldman, U., & Seely, J. F. 1986, At. Data Nucl. Data Tables, 35, 319 Cheng, C.-C. and Widing, K. G. 1975, ApJ, 201, 735 Culhane, J. L., et al. 1991, Sol. Phys., 136, , Sol. Phys., 243, 19 Dere, K. P., Landi, E., Mason, H. E., Monsignori Fossi, B. C., & Young, P. R. 1997, A&AS, 125, 149 Doschek, G. A. 1983, Sol. Phys., 86, 9 Doschek, G. A., Feldman, U., Landecker, P. B., & McKenzie, D. L. 1981, ApJ, 249, 372 Feldman, U., Doschek, G. A., Behring, W. E., & Phillips, K. J. H. 1996, ApJ, 460, 1034 Feldman, U., Doschek, G. A., & Kreplin, R. W. 1982, ApJ, 260, 885 Feldman, U., Hiei, E., Phillips, K. J. H., Brown, C. M., & Lang, J. 1994, ApJ, 421, 843 REFERENCES Feldman, U., Laming, J. M., & Doschek, G. A. 1995, ApJ, 451, L79 Galavis, M. E., Mendoza,C., & Zeippen, C. J. 1997, A&AS, 123, 159 Korendyke, CM., Brown, C. M., Thomas, R. J., et al. 2006, Appl. Opt., 45, 8674 Landi, E., & Gu, M. F. 2006, ApJ, 640, 1171 Landi, E., Del Zanna, G., Young, P. R., Dere, K. P., Mason, H. E., & Landini, M. 2006, ApJS, 162, 261 Lang, J., Kent, B. J., Paustian, W., et al. 2006, Appl. Opt., 45, 8689 Mazzotta, P., Mazzitelli, G., Colafrancesco, S., & Vittorio, N. 1998, A&AS, 133, 403 Phillips, J. J. H., Bhatia, A. K., Mason, H. E., & Zarro, D. M. 1996, ApJ, 466, 549 Tanaka, K., Watanabe, T., Nishi, K., & Akita, K. 1982, ApJ, 254, L59 Tsuneta, S., et al. 1991, Sol. Phys., 136, 37 Zhang, H. L., & Sampson, D. H. 1994, At. Data Nucl. Data Tables, 56, , At. Data Nucl. Data Tables, 63, 275