THE ASTROPHYSICAL JOURNAL, 511:965È975, 1999 February 1 ( The American Astronomical Society. All rights reserved. Printed in U.S.A.

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1 THE ASTROPHYSICAL JOURNAL, 511:965È975, 1999 February 1 ( The American Astronomical Society. All rights reserved. Printed in U.S.A. EVOLUTION OF CHROMOSPHERIC STRUCTURES: HOW CHROMOSPHERIC STRUCTURES CONTRIBUTE TO THE SOLAR He II 30.4 NANOMETER IRRADIANCE AND VARIABILITY JOHN WORDEN,1 THOMAS N. WOODS,2 WERNER M. NEUPERT,3 AND JEAN-PIERRE DELABOUDINIE` RE4 Received 1998 January 14; accepted 1998 September 3 ABSTRACT The bright He II 30.4 nm solar emission is an important energy source for ionization and heating of the EarthÏs upper atmosphere. The analysis of the Solar and Heliospheric Observatory (SOHO) Extreme- Ultraviolet Imaging Telescope (EIT) He II 30.4 nm images provides an improved understanding of how the solar surface structures, i.e., plage, enhanced network (plage remnants), active network, and the quiet chromosphere, contribute to the solar He II 30.4 nm irradiance and its variability. We Ðrst normalize the intensities of each image to the background quiet-chromosphere intensity with a global Ðt that preferentially weights network cell intensities. The resulting quiet-chromosphere intensity scale is stable to within 0.7% (1 p) over the 2 yr data set. The plage, enhanced-network, active-network, and quietchromosphere structures are then identiðed on each EIT He II image with an algorithm that uses criteria of intensity, size, Ðlling factor, and continuity. This decomposition leads to time series of structure area and integrated intensity, their spatial distribution on the solar disk, and their intensity contrast relative to the quiet-chromosphere intensity; thus, these time series show how the solar surface structures contribute to the He II 30.4 nm irradiance. For example, we Ðnd that the active network contributes as much as the plage and enhanced network to the solar He II 30.4 nm irradiance variability during solar minimum. Conversely, the quiet-chromosphere irradiance does not vary during this time period; thus we conclude that long-term He II 30.4 nm irradiance variations can be traced purely to magnetic activity during this time period. We also Ðnd that the plage, enhanced-network, active-network, and quietnetwork intensity contrasts, relative to the quiet chromosphere and averaged over the full area of each structure, are 4.8, 3.3, 2.1, and 1.6, respectively, and these contrasts remain essentially constant with time. Subject headings: solar-terrestrial relations È Sun: activity È Sun: chromosphere È Sun: faculae, plages È Sun: UV radiation 1. INTRODUCTION Understanding solar He II 30.4 nm irradiance variability is very important for understanding the energetics of the terrestrial upper atmosphere. Because the He II (30.4 nm) line is the brightest extreme-ultraviolet (EUV) emission next to H I Lya and because of its short wavelength (Hinteregger, Fukui, & Gilson 1981; Woods et al. 1998), He II 30.4 nm solar radiation is the dominant source of ionizing radiation in the thermosphere (Roble 1995). Thus, the formation of the ionosphere, as well as the heating, density, and expansion of the thermosphere, depend primarily on solar He II 30.4 nm radiation and its variability over the solar cycle. An improved understanding of the solar He II 30.4 nm variability should thus lead to a better understanding of the thermospheric variations. The SOHO EIT experiment provides the necessary images needed to determine the sources of variability for the He II 30.4 nm emission. The SOHO EIT experiment has been taking full-sun images at the 30.4 nm (He II), 28.4 nm (Fe XV), 19.5 nm (Fe XII), and 17.1 nm (Fe IX) wavelengths almost every day since 1996 January, during the solar minimum between solar cycles 22 and 23. According to Delaboudiniere et al. (1995), the primary scientiðc objective of this experiment is 1 National Solar Observatory/National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719; jworden=noao.edu. 2 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO ; Tom.Woods=lasp.colorado.edu. 3 Hughes STX Corporation, Lanham, MD 20706; wneupert=sec.noaa.gov. 4 Institut dïastrophysique Spatiale, Universite Paris XI, Orsay Cedex, France. 965 to study the dynamics of coronal structures over di erent timescales and spatial sizes in order to address problems such as coronal heating and solar wind acceleration. Our research makes use of these He II 30.4 nm images in order to examine the contribution of the di erent solar surface structures, such as plage, enhanced network, active network, and the quiet chromosphere, to the total He II 30.4 nm irradiance and its variability over solar cycle time periods and the 27 day solar rotation period. Our motivation for identifying the sources of the He II irradiance is based on models that concluded that solar ultraviolet irradiance variations could not be adequately described using only the plage and quiet-sun components (e.g., Lean et al. 1982; Skumanich et al. 1984). Worden, White, & Woods (1998, hereafter Paper I), identiðed the plage, enhanced network, and active network on over 1400 National Solar Observatory (NSO) Ca II K spectroheliograms to conðrm that multiple structures are needed to explain Ca II K and other UV irradiance variations over the solar cycle and the 27 day rotation period. Our analysis of He II 30.4 nm spectroheliograms is almost identical to the analysis of Ca II K spectroheliograms in Paper I Previous Solar He II 30.4 nm Measurements Previous measurements of the He II 30.4 nm full-disk irradiance are described by Chapman & Neupert (1974), Hinteregger et al. (1981), Woods, Rottman, & Ucker (1993), and Woods et al. (1998). These authors primarily analyzed the solar cycle evolution and 27 day rotational modulation of the He II 30.4 nm irradiance. Past irradiance measurements suggest that the He II 30.4 nm line varies by about a factor of 2 over the solar cycle, with a solar minimum irra-

2 966 WORDEN ET AL. Vol. 511 diance somewhere between 3.0 ] 109 photons s~1 cm~2 (Woods et al. 1998) and 6 ] 109 photons s~1 cm~2 (Hinteregger et al. 1981). The accuracies for these prior measurements are typically about 30%. However, a recent sounding rocket Ñown by T. Woods is expected to measure the He II 30.4 nm irradiance with much improved accuracy. Thompson et al. (1993) discusses partial solar images taken by the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS-3). They found that the plage shape is well correlated with the same plage regions measured on the ground at the He ( nm) absorption line, the Ha absorption line, and the Ca II (854.2) nm line. In addition, they Ðnd that Ðlaments appear darker and that the network has a higher contrast in the He II nm line than it does in He II 30.4 nm. He II (30.4 nm) spectroheliogram Ðlm measurements were also taken aboard Skylab. Mango & Bohlin (1978) analyzed two of these spectroheliograms and found that the quiet Sun shows limb brightening and that the network-to-cell intensity contrasts are between 1.4 and 1.6. Harvey & Sheeley (1977) also analyzed a Skylab He II spectroheliogram and found that the He I nm intensity contrast is larger than that of He II 30.4 nm. To summarize, very few accurate measurements have been made of the solar He II 30.4 nm irradiance variability or of the solar surface structures that emit radiation at this wavelength. Although the Atmospheric Explorer-E (AE-E) measurements described by Hinteregger et al. (1981) were taken between solar minimum and solar maximum, questions remain about the calibration of this instrument and corrections for in-ñight degradation (Lean & Skumanich 1983; Lean 1987). Because we use a large data set of spacebased He II spectroheliograms from the same instrument, we can calibrate ÏÏ each image, i.e., place each image on the same intensity scale, by normalizing each image to its average background quiet-sun intensity. We estimate that variations in the resulting residual intensity scale are 0.7% over the data set and that these variations are likely from instrument changes. This 0.7% uncertainty is better than even the best satellite measurements of UV irradiances (Woods et al. 1996), because there is little variation in the quiet-sun intensity, because the dark count is well described over the mission (Delaboudiniere et al. 1995), and because space-based images do not su er from the daily variations of atmospheric distortion. By analyzing the spatial distribution and the evolution of the structures that are primarily responsible for the variability of the He II 30.4 nm line, we expect to understand the variability of the He II 30.4 nm irradiance over solar cycle timescales and the solar 27 day rotation period Solar Surface Structure DeÐnitions The primary sources of solar He II 30.4 nm radiation and also its solar cycle variability are plage, enhanced network, active network, and the quiet chromosphere. Coronal holes could also contribute to the solar variability of the He II 30.4 nm line. However, a study by Harvey & Livingston (1994) suggests that coronal holes had a negligible e ect on the variability of the He I nm line. Because Harvey & Sheeley (1977) found larger network intensity contrasts in the He I nm line than in He II 30.4 nm, we expect that coronal holes should also contribute negligibly to the variability of the He II 30.4 nm line. In addition, because we do not speciðcally identify coronal holes on these images, irradiance variations due to coronal holes appear within the quiet-chromosphere irradiance variations. As discussed in 5.3, we Ðnd that the quiet-chromosphere irradiance varies negligibly over this time period, and thus it is safe to neglect the contribution to the irradiance variability from coronal holes. The terms plage,ïï enhanced network,ïï active network,ïï and quiet chromosphere ÏÏ are taken from Lean & Skumanich (1983), Zwaan (1987), and Paper I for deðning the surface structures seen in solar magnetograms and chromospheric intensity images. As used in this analysis, these structure deðnitions essentially represent a downward progression of size and intensity, from bright, dense plages down to small, dispersed networks. Following is a brief description of these structures. A plage is the strong magnetic Ðeld that surrounds a coexisting sunspot within an active region. However, plages are bright in intensity images, whereas sunspots are dark. Because sunspots are not easily identiðed in He II 30.4 nm, we must use the spatial and intensity properties of a plage to separate it from the other surface structures seen in the EIT He II 30.4 nm images. Typically, a plage has a brighter intensity, larger size, and higher Ðlling factor (density of bright pixels) than the other surface structures. Although Ðlling factor and intensity are parameters that are di erent for plage regions at di erent wavelengths, the size of a plage can range from 44A to 200A (Harvey & Zwaan 1993), where the Sun has a mean diameter of about 1980A. According to Zwaan (1987), as a plage disperses and fragments it is gradually replaced by an enhanced network. The enhanced network continues to disperse until it is lost in quiet network. Our research deðnes two structures on the He II 30.4 nm images that could qualify as enhanced networks as deðned by Zwaan (1987): (1) the enhanced network and (2) the active network. This subdivision is very useful for modeling the chromospheric and transition region irradiance variability. We deðne enhanced networks as larger, spatially coherent plage remnants, and active networks as smaller, longitudinally dispersed remnants; thus, the enhanced networks contribute strongly to the 27 day rotation modulation, while the active networks do not. In addition, the active-network sizes and intensities overlap more with the background quiet chromosphere, whereas enhanced-network properties do not overlap signiðcantly with quiet-chromosphere properties. Both structures contribute strongly to the solar cycle irradiance variability of the He II 30.4 nm emission and other UV lines. The quiet chromosphere is comprised of the quiet network and the supergranular cell interiors. This is a composition of two remarkably di erent structures that probably have very di erent heating mechanisms (e.g., Carlsson & Stein 1995; Carlsson, Judge, & Wilhelm 1997; Judge, Carlsson, & Wilhelm 1997). Our analysis and previous analyses (e.g., Skumanich et al. 1975; Paper I) show that the average chromospheric image intensity is a combination of intensities from the brighter cell intensities and the fainter quiet network. Thus, cell intensities are typically dimmer than the average image intensity, and quiet-network intensities are typically brighter. A very useful property of the quiet chromosphere is that the average intensity from this dual structure does not vary signiðcantly over the solar cycle; this property was suggested by White & Livingston (1978, 1981), Paper I, and Lean et al. (1998) for the Ca II K emission, and it is conðrmed to within 0.7% (1 p) for the He II 30.4 nm line by this analysis. Therefore, the average

3 No. 2, 1999 SOLAR He II 30.4 NANOMETER IRRADIANCE 967 quiet-chromosphere intensity can be used as a reference intensity for all the He II 30.4 nm images. Finally, we note that the actual identiðcation of these structures is somewhat subjective. Our goal is to conform to the structure deðnitions of Lean & Skumanich (1983), Zwaan (1987), and Paper I. It is somewhat easy to consistently identify the plages on the solar disk, because they are large, bright, and compact structures. However, there is an overlap of properties between the enhanced-network and active-network structures, and between the active network and the quiet network. However, as in Paper I, our results on the intensity distributions of these structures and our analysis of their solar cycle variability and 27 day rotation modulation suggest that our method adequately di erentiates between these structure types. We will comment further on the e ectiveness of our structure identiðcations in 4 and THE SOHO EIT EXPERIMENT Delaboudiniere et al. (1995) describes the SOHO EIT experiment in detail. BrieÑy, the EIT is comprised of a multilayer-coated telescope that images the Sun onto a 1024 ] 1024 pixel CCD image sensor. A Ðlter wheel allows a wavelength selection with bandpass centers at 17.1, 19.5, 28.4, and 30.4 nm. Although the bandpass around 30.4 nm overlaps other strong lines, such as Fe XV (28.4 nm) and Si XII (near 30.4 nm), we estimate that these lines contribute only about 7% of the radiation, with 93% coming from the He II 30.4 nm emission. This estimate is derived by convolving the detector efficiency given by Delaboudiniere et al. (1995) with the Hinteregger solar minimum 21 reference spectrum (Hinteregger et al. 1981). The CCD is periodically baked out to remove ice that has accumulated on its surface. This bakeout has the immediate e ect of increasing the instrument sensitivity. After bakeout of the CCD, the instrument response resumes degradation, which is due to redeposition of water vapor on the surface of the CCD and to continuing EUV-induced radiation damage within the CCD (DeÐse et al. 1997). 3. PREPROCESSING OF EIT He II IMAGES 3.1. EIT Preprocessing Preprocessing of the raw EIT He II 30.4 nm images includes background subtraction, degridding, and Ñat- Ðelding. These initial tasks are performed using the solar software ÏÏ package provided by the SOHO consortium.5 Afterward, the image, in units of counts, is technically proportional to intensity. solar cycle (White & Livingston 1978, 1981; Paper I). We validated this assumption in Paper I by comparing the total signal from the Ca II K images, after they have been normalized to the quiet chromosphere, with the NSO Kitt Peak full-disk Ca II K 0.05 nm index. Following is a description for normalizing the intensities of each image to the same quiet-chromosphere residual intensity scale by Ðtting a series of polynomials to the quiet-chromosphere network and cell intensity variations Quiet-Chromosphere Fit Our method for deriving the background quietchromosphere image is similar to the method described in Paper I for deriving the quiet chromosphere of Ca II K images. A series of Ðfth-order polynomials are Ðtted across each row and column of the image. Pixels with intensities that are 4 p brighter than the average image brightness are weighted to zero; this weighting removes the brighter pixels associated with the active structures. An intensity threshold is adequate for removing the e ects of plages and enhanced networks on the Ðt because the quiet chromosphere predominantly covers the solar disk. In addition, we preferentially weight the cell interior intensities by using the inverse of each pixelïs count as its weight in the Ðt. The two Ðts are then averaged; the resulting image describes the quiet chromosphere, the center-to-limb variation, and any intensity variations due to spatial gradients in the instrument sensitivity. After the quiet-chromosphere image is derived, it is subtracted from the original image to obtain a residual ÏÏ image. Then, as discussed in Paper I, this residual image is divided by the integration of the quiet-chromosphere image as the normalization factor. Subtracting the quietchromosphere image from the original image removes the minor limb-brightening seen in He II 30.4 nm and any instrument gradients. Dividing by the normalization factor removes temporal changes in the instrument response. The normalization factor (in units of counts) is shown in Figure 1. The sudden jumps ÏÏ and subsequent decay are due to the periodic baking of the EIT. Spikes in sensitivity not due to bakeout events are due to poor-quality images or use of di erent Ðlters. The units of the residual images describe the intensity contrast per pixel relative to the quiet chromo Intensity Normalization In order to identify solar structures such as plages and networks on each image and on a daily basis, it is necessary to have the same intensity scale for each image and for each pixel on the image. Therefore, the small center-to-limb variation and instrument intensity gradients must be removed from each image, and it is necessary to account for instrument sensitivity changes. We perform these tasks by normalizing each image to their background quietchromosphere intensity. Our assumptions are that the quiet-chromosphere intensity does not change with the 5 Available from the SOHO web site at soho. FIG. 1.ÈSums of each quiet-chromosphere surface Ðt. Each sum is normalized to the image integration time. This Ðgure shows how the EIT sensitivity decays with time.

4 968 WORDEN ET AL. Vol. 511 sphere. Multiplying these intensities by the total image area gives the intensity contrast of each pixel relative to the quiet chromosphere minus 1. We use this quiet-chromosphere residual intensity scale as the preferred intensity scale throughout this paper unless otherwise stated Quiet-Chromosphere Residual Intensity Scale and Its Uncertainty Our assumption for normalizing each image to the same intensity scale is that the average quiet-chromosphere intensity changes negligibly with time. Therefore, we can compute the uncertainty of the quiet-chromosphere residual intensity scale by examining variations in the cell intensity contrasts; we examine variations in the cell intensity contrasts because the cell intensities are not likely inñuenced by the dispersal of the active network, whereas there could be some quiet-network intensity variation due to the inñuence of active network. If the quiet-chromosphere intensity changes by some nonproportional amount over the cell and network, or if the dark count/scattered light is incorrectly computed, then the intensity contrast of the supergranular cells would vary from image to image. Figure 2a presents the intensity distributions for 20 of the He II 30.4 nm residual images taken at equal intervals over the 2 yr data set. The left side of the intensity distribution is primarily a distribution of the cell intensities and the edges of the quiet network. Di erences between the intensity distributions result because of changes in area of the active structures. We compute a measure of the cell intensity contrast with the left width at FIG. 2.È(a) Intensity distributions for 20 residual images taken at even intervals from the data set. The LWHM stands for left width at halfmaximum and is computed using a polynomial Ðt to the central part of the intensity distribution. (b) The LWHM for each of the EIT images used in this analysis. The LWHM describes the distribution of supergranule cell intensities. half-maximum (LWHM) of the intensity distribution; this is calculated using a polynomial Ðt to the central part of the intensity distribution. Figure 2b presents the LWHM for each of the EIT images used in this analysis. Although the LWHM shows some variability over the 2 years, much of this variation appears to occur after bakeout events and also after the shutter was left open in 1996 July, which caused a sudden change in CCD sensitivity that was not Ðxed until a bakeout. The average of the LWHM is ^ The rms variation is our estimate of the 1 p variation of the quiet-chromosphere residual intensity scale. Because this intensity scale is proportional to the average quiet-chromosphere value of 1 (not to the average of the LWHM), the intensity scale should be accurate to within 1 ^ , or about 0.7% over the mission. As discussed in 5, this error estimate of the residual intensity scale is consistent with the variations in intensity contrast and irradiance contributions of the quiet networks and the quiet chromosphere. 4. SUMMARY OF DECOMPOSITION ALGORITHM Paper I gives complete details on how the plage, enhanced network, active network, and quiet chromosphere are identiðed on Ca II K 0.05 nm spectroheliograms. The identiðcation method is similar to that used here to identify these same structures on EIT He II images, but the spatial and intensity parameters used in the algorithms are di erent and the border-ðnding techniques are di erent for the active network. We therefore only summarize the process here. Classifying each pixel on the solar disk as within a plage, enhanced-network, active-network, or quiet-chromosphere structure is a multiple-step process that repetitively applies the same basic identiðcation scheme to progressively fainter and smaller structures. Our criteria for identifying a feature as a plage, enhanced network, or active network are intensity, size, and Ðlling factor. These parameters decrease as the algorithm successively identiðes the plage, enhanced networks, and active networks. This image decomposition ÏÏ process does not initially identify the entire feature; only a few points within each feature are tagged correctly. Once a point has been identiðed as being within a plage, enhancednetwork, or active-network feature, a nearest neighbor search using a lower intensity threshold Ðlls out the feature. A Ðnal step for identifying the plages and enhanced networks is to Ðnd the intensity minimum bordering each feature. These intensity minima deðne the complete boundaries of the plages and enhanced networks. All pixels inside adjacent minima are identiðed as belonging to a structure type. We use a di erent method for identifying the activenetwork borders. In Paper I we used only the nearest neighbor search to identify the active-network borders, because the average Ca II K active-network intensity strongly overlaps the intensity distribution of the quiet chromosphere, and therefore too much quiet chromosphere entered into the active-network area if we identiðed the borders by searching for intensity minima. However, the He II 30.4 nm active network has a much larger residual intensity contrast, which better separates the active network from the quiet chromosphere. Unfortunately, use of an intensity minimum still introduces too much noise ÏÏ from the background quiet chromosphere. Therefore, we identify the active-network borders by using the Sobel edge enhancement ÏÏ operator to compute a type of two-

5 No. 2, 1999 SOLAR He II 30.4 NANOMETER IRRADIANCE 969 dimensional derivative of the image. The active-network borders are then determined by searching for the surrounding maxima of this two-dimensional derivative. This pixelby-pixel identiðcation of the plage, enhanced network, and active network results in a structure mask ÏÏ that classiðes each pixel in the He II 30.4 nm image as belonging to a structure class. The size parameters, intensity parameters, and Ðlling factor method used to identify the di erent structures are derived by trial and error. We adjust the size and intensity parameters so that we identify the same plage and enhanced-network regions as identiðed in Ca II K in Paper I. However, the Ðlling factor criteria used to help separate the plage, enhanced network, active network, and quiet chromosphere are the same factors as used in Paper I. Although we can mostly identify the same plage regions on Ca II K and He II, it is harder to consistently identify the same enhanced-network structures on Ca II K and He II 30.4 nm, and the similarities between our identiðcations of active networks at the two wavelengths are even more reduced. These di erences likely result because of the higher intensity contrasts of the solar surface structures seen in He II 30.4 nm, because the formation mechanisms for He II 30.4 nm line are di erent from those for Ca II K, and because the He II 30.4 nm line is formed higher in the solar atmosphere than the Ca II K (e.g., Vernazza et al. 1981; Mariska 1992). An additional di erence between our analysis here and that in Paper I is that we now attempt to identify the quiet network in order to examine their irradiance variability. We use similar intensity and size criteria to identify the active network and quiet network. However, in order to separate the two structures, we then apply the Ðlling factor criteria of the same type that was used to separate the plages from the brighter enhanced network. Thus, the Ðlling factor is our primary criterion for identifying the active network di erently than quiet network. 5. IMAGE ANALYSIS RESULTS The results from our image decomposition analysis include (1) masks of each structure for each image as seen in He II 30.4 nm, (2) intensity distributions of the solar surface structures at the He II 30.4 nm wavelength, (3) time variations of the areas and integrated residual intensities of the plage, enhanced network, active network, and quiet chromosphere, (4) total integrated residual intensity of the He II 30.4 nm line, which is comparable to the He II 30.4 nm irradiance, and (5) He II 30.4 nm intensity contrasts for plage, enhanced network, active network, and quiet network. These results are presented next Structure Masks The structure masks are used to assign each pixel on the He II 30.4 nm spectroheliograms as belonging to one of the surface structures. Thus, useful information about the intensity distributions of the di erent surface structures and of the solar cycle evolution of these structures can be inferred from the numerical masks and He II 30.4 nm images. A Ðrst step in validating consistent identiðcation of the same features on consecutive images is to compare masks on consecutive days. As an example of this validation, Figure 3 presents the He II 30.4 nm images and corresponding masks for 1996 May 11, 13, and 15. As can be seen from the structure masks, the same regions are typically identiðed as plage and enhanced network in all three images. However, one of the enhanced-network structures on the May 11 mask has fragmented into an active network on the May 15 mask Distribution Functions for Solar Structures Using the numerical masks and the He II 30.4 nm residual images, we derive distribution functions for each structure type on each image. The total image, quiet-chromosphere, and active-network intensity distributions are plotted in Figure 4a for 1996 May 13. Adding a value of 1 to these intensity distributions gives the intensity contrast relative to the average quiet-chromosphere intensity. The activenetwork, enhanced-network, and plage intensity distributions are plotted in Figure 4b. These 1996 May 13 distributions are typical for other days. An important property of these distributions is that they overlap, i.e., pixels from di erent structures overlap in intensity. Furthermore, the distribution functions are typically asymmetric and also have distinct widths and locations of the peak intensity. Using these intensity distribution functions or the masks in conjunction with the Ñattened images, we can derive time series of the structure areas, integrated intensities, and contrasts How Chromospheric Structures Contribute to the He II 30.4 nm Irradiance V ariations during Solar Minimum In this section, we examine in detail how the solar surface structures, i.e., plage, enhanced network, active network, and the quiet chromosphere, contribute to the He II 30.4 nm irradiance variations. There are two primary timescales noticeable in solar irradiance variations: the solar cycle, or long-term variations, and the 27 day rotation modulation (e.g., Donnelly, Hintenregger, & Heath 1986; Lean 1987). We Ðrst examine how the surface structures contribute to the long-term He II variations and then how they contribute to its 27 day rotation modulation L ong-t erm Variations Figure 5 shows the total integrated residual intensity and the integrated residual intensities for the plage, enhanced network, and active network. The total integrated residual intensity is computed by summing all pixels from each of the residual intensity images (recall, add 1 to these values to obtain the intensity value relative to the average quietchromosphere intensity). Likewise, the integrated residual intensities of the plage and enhanced network are computed by adding all intensities from each pixel identiðed as belonging to a plage or enhanced-network structure, respectively. Figure 6 shows the integrated residual intensities for the quiet networks and the quiet chromosphere. The sum of intensities from the plage, enhanced network, active network, quiet network, and quiet chromosphere equals the total integrated residual intensity. Variations in the He II 30.4 nm total integrated intensity are almost exactly the same as variations in the He II 30.4 nm irradiance; however, there are likely minor 27 day rotation modulation di erences between the true He II 30.4 nm irradiance variations and the integrated intensity variation because of the slight center-to-limb brightening seen in the EIT He II images. The usefulness of Figures 5 and 6 is that they show how each structure contributes to the variability of the He II 30.4 nm irradiance during solar minimum activity levels.

6 FIG.3a FIG.3b FIG.3c FIG.3d FIG.3e FIG.3f FIG. 3.È(a), (c), and (e) are the residual EIT He II.4 nm images for 1996 May 11, May 13, and May 15, respectively, and (b), (d), and ( f ) are their corresponding structure masks. The plages are the horizontally lined gray-structures. In the structure masks, the enhanced networks are the white-ðlled structures. The active networks are the gray-ðlled structures. The quiet networks are the black Ðlled structures, and the rest of the solar disk is the quiet chromosphere.

7 SOLAR He II 30.4 NANOMETER IRRADIANCE 971 FIG. 4.È(a) Intensity distributions from the 1996 May 13 He II residual image for the total image (solid line), quiet chromosphere (dotted line), and quiet network (dashed line). (b) The intensity distributions for the active network (solid line), enhanced network (dotted line), and plage (dashed line). Figure 5a shows a local activity minimum in the He II integrated residual intensity from 1996 February through April; this is consistent with the results of Paper I, which show that the plage, enhanced network, and active network reach a local activity minimum in 1996 May. Around 1996 May 3, a large plage region appears on the solar disk; after this time period, the He II 30.4 nm integrated intensity shows a steady increase. The long-term change in the residual intensity of these time series for the plage, enhanced network, active network, quiet network, and quiet chromosphere are approximately 0.03, 0.03, 0.04, 0.007, and [0.007, respectively. Thus, it is the variation in the active network that is most responsible for long-term variations in the He II total integrated intensity during this solar minimum time period. It is interesting that the plage do not appear to contribute to long-term He II variations until the second half of The increase in plage activity in late 1997 August indicates the advent of solar cycle 23. After the increase in plage activity there are subsequent increases in the enhanced and active networks Quiet-Chromosphere Irradiance Variations An interesting aspect of this analysis is that we Ðnd essentially no variation in the quiet-chromosphere/quietnetwork irradiance over this 2 yr data set. Variations that are seen in the quiet-chromosphere and quiet-network residual intensities appear to correspond either to bakeouts or to the 1996 July shutter incident,ïï in which the shutter was accidentally left open for several hours. One could argue that because we normalize the image intensities to the quiet chromosphere, we cannot detect changes in the quiet- FIG. 5.È(a) Time series of the total integrated residual intensity. The total is computed by summing the residual intensity of each pixel on each image. (b) The plage integrated residual intensity. (c) The enhancednetwork integrated residual intensity. (d) The active-network residual intensity. chromosphere intensity. However, any change in the quietchromosphere intensity that is not proportional across the cell and network intensities would be seen as a changing quiet-chromosphere contrast. As discussed in 3.2.2, we found that the quiet-chromosphere cell intensity contrast remained essentially constant over this data set. In fact, cell intensity contrast variations track variations in the quietnetwork residual intensity, which in turn appear to track instrumental events. In addition, the rms variation of the cell intensity contrast, quiet-network residual intensity, and quiet-chromosphere residual intensity have similar values. We therefore conclude that cell and quiet-chromosphere intensity changes are not responsible for irradiance variations seen in He II 30.4 nm. Thus, solar cycle variations can be traced purely to magnetic features, in agreement with Lean et al. (1998). However, our result does not rule out that quiet-photosphere intensity changes could a ect visible light variations, because photospheric visible light variations are on the order of 0.1% over the solar cycle (Willson & Hudson 1991) and we cannot detect quiet-chromosphere intensity variations of less than the 0.7% residual intensity uncertainty over this 2 yr data set Solar Minimum Irradiance for Di erent Solar Cycles Because the active- and quiet-network integrated intensities are nonzero during minimum solar activity, the solar minimum ÏÏ irradiance, deðned as the minimum measured

8 972 WORDEN ET AL. Vol. 511 FIG. 6.ÈIntegrated residual intensities for (a) the quiet network and (b) the quiet chromosphere. The solid line indicates the average of each time series. solar irradiance, could be di erent for di erent solar cycles. For example, a smaller active-network area during this solar minimum would result in a lower value for the He II 30.4 nm solar minimum irradiance represented in Figure 5a Day Rotation Modulation The 27 day rotation modulation of the plages has been well documented and discussed in previous research (e.g., see Swartz & Overbeek 1971; Cook, Brueckner, & Van- Hoosier 1980; Donnelly et al. 1983; Lean 1987; Paper I). However, it is apparent in Figure 5 that the enhanced network also has a strong 27 day rotation modulation. This result is consistent with that of Paper I, i.e., it is primarily the plage and enhanced network that cause the 27 day rotation modulation of the irradiance. While the active network can have a small, intermediate 27 day rotation period, it is too dispersed longitudinally to contribute much to the 27 day period of the irradiance Spatial Distribution of the Solar He II Chromospheric Structures Figure 7 presents time series of the areas of the plage, enhanced network, active network, and quiet network as a fraction of the solar disk area. The time series in Figure 7 show that, as seen in the He II 30.4 nm line during solar minimum conditions, the plage and enhanced network typically each cover about 1% of the Sun. The plage and enhanced-network areas are very consistent with those found in Paper I for Ca II K during this time period; however, this is expected because we used the Ca II K images and structure masks, discussed in Paper I, to determine the intensity and size parameters used to identify plage FIG. 7.ÈTime series of the structure areas, as a fraction of the solar disk, for (a) plage, (b), enhanced network, (c) active network, and (d) quiet network. Data gaps are Ðlled using linear interpolation. and enhanced-network structures on the He II 30.4 nm images. We therefore refer the reader to the discussion in Paper I for how our He II plage areas compare with those of research that uses other types of solar images. We also Ðnd that the active network can cover 3%È10% of the solar disk and that the quiet network typically covers about 17% of the solar disk. The dramatic changes in the active-network area suggest that the active-network area responds quickly to changes in the plage and enhancednetwork area, the likely original source of the active network (Martin 1988). Unfortunately, because the Ca II K network intensity is too close to the average image brightness, we could not adequately determine the Ca II K network areas, and therefore we cannot conðdently compare these network areas with those from Paper I. However, our results are consistent with those of Foukal, Harvey, & Hill (1991), who found that the photospheric magnetic network covers about 15% of the solar disk during solar minimum activity conditions and about 22% during solar maximum activity conditions. Likely their deðnition of the magnetic network combines our quiet network and active network. Thus we would expect a change in network area from 20% during solar minimum to a value larger than 27% during solar maximum. The spatial di erences between the He II quiet and active networks and the photospheric magnetic network likely result because the magnetic network Ðlls a larger area in the transition region than it does in the photosphere (e.g., StenÑo 1994).

9 No. 2, 1999 SOLAR He II 30.4 NANOMETER IRRADIANCE Intensity Contrasts of the He II 30.4 nm Surface Structures Figure 8 presents time series of the intensity contrasts of the plage and enhanced network relative to the quiet chromosphere. Figure 9 shows time series of the activenetwork, quiet-network, and quiet-chromosphere intensity contrasts; we present the quiet-chromosphere contrasts to show the deviations from its expected average of 1. The intensity contrasts are the average intensity of each structure relative to the average quiet-chromosphere intensity (see, e.g., Paper I). The average intensity contrasts of the plage and enhanced networks are 4.77 ^ 0.88 (18.0%) and 3.29 ^ 0.48 (15.0%). The average intensity contrasts of the active and quiet networks are 2.05 ^ 0.04 (1.9%) and 1.60 ^ 0.01 (0.6%), respectively. The intensity contrasts of the plage, enhanced network, and active network are derived using only the nonzero values from Figures 5 and 6. The average contrast ÏÏ of the quiet chromosphere is 1.00 ^ 0.01 (1.0%); the fact that the quiet-chromosphere contrast is very close to its expected value of 1 gives us additional conðdence in our intensity normalization methods and structure identiðcation algorithms. In addition, the uncertainties in the quiet-network and quietchromosphere contrasts are consistent with our uncertainty of the quiet-chromosphere residual intensity scale. As also found in Paper I, we Ðnd no obvious long-term variability in the plage and enhanced-network contrasts. However, we note that these contrasts are averaged over multiple structures; thus, contrasts could vary between individual structures. We also Ðnd that the active- and quietnetwork contrasts are constant to a high degree of accuracy. FIG. 9.ÈTime series of (a) the active-network intensity contrasts, (b) the quiet network, and (c) the quiet-chromosphere intensity contrasts (expected to be 1) relative to the average quiet-chromosphere intensity. The diamonds are the intensity contrasts and the solid line is the average of the intensity contrasts. The quiet-chromosphere intensity contrast is the average contrast of all pixels on the solar disk after the plage, enhanced network, and active network are removed. FIG. 8.ÈTime series of (a) the plage intensity contrasts and (b) the enhanced-network intensity contrasts relative to the quiet-chromosphere intensity. The diamonds are the intensity contrasts, and the solid line is the average of the intensity contrasts. This propertyèthat the structure contrasts are constant with timeèis very useful for modeling the He II 30.4 nm irradiance, because structure areas, as measured from the ground using Ca II KorHeI nm images, can be used in conjunction with these intensity contrasts to model the He II 30.4 nm irradiance. 6. COMPARISON OF He II 30.4 NANOMETER INTEGRATED INTENSITY WITH OTHER SOLAR INDICES In this section we compare the He II 30.4 nm integrated residual intensity with the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) Mg core-to-wing ratio, the SOLSTICE He II nm irradiance, and the NSO Kitt Peak He nm equivalent width. These comparisons are meant as an additional, partial validation of our intensity normalization of the He II 30.4 nm images. Ideally, we would compare the He II 30.4 nm integrated intensity with the He II 30.4 nm irradiance in order to completely

10 974 WORDEN ET AL. Vol. 511 validate our image intensity normalization. Unfortunately, daily measurements of the He II 30.4 nm irradiance are unavailable. It is also not realistic to compare with the SOHO Solar EUV Monitor (SEM) measurements, both because coronal radiation contributes about half of the signal to their wide bandpass He II 30.4 nm irradiance measurements (Ogawa et al. 1998) and because questions remain about SEM long-term sensitivity changes. Figure 10 compares the He II 30.4 nm residual intensity with these other solar indices: the Mg II core-to-wing ratio and the He I EW are more reliable than the He II nm irradiance because of SOLSTICE wavelength calibration problems. The solar irradiance, as seen in each of these time series, shows little long-term variability during 1996 and then begins to increase in mid-1997 because of the increase in plage activity. Thus the long-term variations of these indices are consistent with the EIT He II 30.4 nm residual intensity. The correlation, r, is best between the He II 30.4 nm residual intensity and the SOLSTICE Mg II core-towing ratio because the Mg II index has a longer overlapping time series than the He I nm index and because it is not as a ected by the wavelength calibration problems as the He II nm irradiance. SOLSTICE wavelength calibration problems should be corrected in the near future, and updated irradiances are expected to be released in the fall of CONCLUSIONS Our results strongly indicate that multiple structure types contribute to the variability of the He II 30.4 nm line over the solar cycle. Our analysis conðrms the conclusions from past irradiance models (e.g., Lean & Skumanich 1983), which showed that the plage alone could not account for solar irradiance variations on both the 27 day rotation and solar cycle (11 yr) timescales. In Paper I we found that the plage and enhanced network were the primary sources of Ca II K irradiance variability and that there was very little contribution from the active network. Our analysis of the He II 30.4 nm images suggests that the active network can also contribute strongly to He II 30.4 nm long-term irradiance variations. Therefore, it is likely that at least four components (plage, enhanced network, active network, and quiet chromosphere) are needed if one is to accurately model chromospheric/transition region irradiance variations. In addition, we also Ðnd that quiet-chromosphere intensity variations negligibly contribute to solar He II 30.4 nm irradiance variations; He II 30.4 nm irradiance variations can be completely traced to magnetic features. The He II structure areas and intensity contrasts presented here are necessary parameters for computing a multicomponent model of the He II 30.4 nm irradiance that could use ground-based Ca II K structure areas as input. However, we need to extend the Ca II K structure-area time series, as derived in Paper I, before we can compute with good conðdence an accurate He II 30.4 nm irradiance variability model. Extending the Ca II K and He II 0.4 nm structure-area time series and comparing these time series will be the subject of a future manuscript. FIG. 10.È(a) He II 30.4 nm residual intensity as seen in Fig. 5a. (b) SOLSTICE Mg II core-to-wing ratio. (c) SOLSTICE He II nm irradiance in units of 109 photons s~1 cm~2. (d) NSO Kitt Peak He I nm equivalent width. Each of these time series is smoothed with a 5 day Gaussian smoother. The He I EW data are available only through 1997 April. We deeply thank Oran R. White, who provided much insight and advice on this analysis. We also thank Je Newmark for providing help with the EIT software and with questions about the EIT instrument response, and Mark Giampapa for reviewing this paper. We would also like to thank Jack Harvey, Bob Howard, Karen Harvey, and Giulliana detoma for their comments and suggestions. This research is supported by the National Science Foundation grant ATM , interagency agreement S E, NASA grant NAG , and the Office of Naval Research grant N J W. M. N. is supported by the NASA contract NAS with the Hughes STX Corporation. SOHO is a project of international cooperation between ESA and NASA.

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