physical processes for the creation and destruction of magnetic elements of the quiet Sun are ephemeral regions

THE ASTROPHYSICAL JOURNAL, 509:461È470, 1998 December 10 ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. COMPARISON OF Ha AND He II j304 MACROSPICULES HAIMIN WANG Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314; haimin=solar.njit.edu Received 1998 February 9; accepted 1998 July 17 ABSTRACT Simultaneous observations of limb macrospicules were carried out in Ha by the Big Bear Solar Observatory (BBSO) and in He II j304 by the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. For the Ðrst time, Ha and He II macrospicules are compared, with high spatial and temporal resolution and image enhancement. Data were obtained on 1996 October 6, 7, and 8. The target of the Ðrst and last day was the north pole; on October 7, it was the quiet west limb. BBSO uses a 12 bit digital camera to obtain high-resolution Ha Ðltergrams at [0.65 A, line center, and 0.65 A. The pixel resolution ranges between 0A.17 and 0A.33, and temporal resolution ranges between 30 and 90 s. EIT images have a Ðxed pixel resolution of 2A.5 and temporal resolution between 1 and 7 minutes. We found the following: within the common Ðeld of view of BBSO and EIT, all 53 identiðed He II j304 macrospicules have counterparts in Ha. However, morphologies of He II j304 and Ha macrospicules are completely di erent. He II j304 macrospicules are typically in the form of an elongated ejection, whereas Ha macrospicules are either looplike bright features or much shorter jets. In the polar region, 55 (over 50%) Ha macrospicules do not show any corresponding He II j304 macrospicules. As expected, He II j304 macrospicules occur much more frequently in the polar coronal hole area than in the equatorial region. However, Ha macrospicules occur at an equal rate in the pole and in the equatorial limb. Equatorial Ha macrospicules tend to be conðned because of their looplike structure and the lack of jetlike structure. Based on common properties of di erent events, we propose that the disk counterparts of macrospicules might be Ha [ 1.0 A jets or a subset of explosive events.ïï If this assumption is true, macrospicules are the results of magnetic reconnections, likely due to the network-ephemeral region or network-intranetwork interactions. We propose that magnetic reconnection occurs at about the same rate in the polar coronal hole and in the quiet regions. Ha macrospicules are direct manifestation of magnetic reconnection. He II j304 images detect substantially taller structures that are substantially hotter. Because of dominant vertical line con- Ðguration near the pole, reconnection tends to produce He II j304 macrospicules; in the noncoronal hole regions, He II macrospicules become undetectable because of the inclined magnetic Ðeld conðguration. Subject headings: Sun: chromosphere È Sun: magnetic Ðelds È Sun: UV radiation 1. INTRODUCTION Solar coronal heating and the energy/mass Ñow of solar wind are among the unsolved but very fundamental problems in solar physics. Taking advantage of the ongoing SOHO mission and improved observations from the Big Bear Solar Observatory (BBSO), we are making progress in understanding this problem. One of the steps toward achieving our goal is to understand small-scale ejections, especially the mysterious macrospicules. The term spicule ÏÏ was introduced by Roberts (1945) for Ha macrospicules are much shorter than He II j304 macrospicules. We proposed that high-speed jets observed at Ha [ 1.0 A might be counterparts of limb macrospicules. In the present paper we analyze more comprehensive EIT data and higher resolution Ha data, to compare Ha and He II j304 macrospicules. Evolution of solar magnetic Ðelds is likely the cause of most of solar dynamic phenomenon, including macrospicules. In a quiet region, magnetic Ðelds can be generally divided into two categories: network Ðelds and intranetwork Ha jets at the limb. On the disk, spicules were identiðed by (IN) Ðelds (Livingston & Harvey 1975). Important Cragg, Howard, & Zirin (1963) as dark jets extending limbward from elements of the chromospheric network. There has been great interest in spicules as a possible source of coronal heating (for a review, see Beckers 1972; Gaizauskas physical processes for the creation and destruction of magnetic elements of the quiet Sun are ephemeral regions (Harvey & Martin 1973) and cancellation (Martin 1983; Livi, Wang, & Martin 1985; Martin, Livi, & Wang 1985), 1985). MacrospiculesÈlong jets following the polar respectively. We have applied a local correlation tracking plumesèwere discovered in Skylab He II j304 EUV images (Bohlin et al. 1975). Moore et al. (1977) showed that Ha macrospicules are connected with tiny Ha limb Ñares in ephemeral regions and are associated with UV macrospicules. LaBonte (1979) tried to Ðnd disk counterparts of macrospicules; however, he mistakenly included many small Ðlament eruptions as macrospicules. Comparison of Ha and He II j304 macrospicules has been a difficult task. Recent work by Delannee et al. (1997) indicated difficulty in Ðnding corresponding Ha and He II j304 macrospicules. In our previous paper (Wang et al. 1998), we attempted to technique to long-integration magnetograms and conðrmed that IN Ðelds follow supergranular Ñow and are swept into the network boundaries. However, IN Ñuxes do not contribute to the formation of network Ðelds because of their bipolar nature (Wang et al. 1996). More importantly, we estimated that interaction between network and IN Ðelds (by cancellation of Ñuxes with opposite magnetic polarities) can produce at least 1.2 ] 1028 ergs s~1 of energy, which is comparable to the energy required to heat the corona. It is reasonable to anticipate that the appearance and evolution of these quiet-sun magnetic features are responsible for the compare BBSO Ha and SOHO full-disk Extreme- dynamics above the photosphere, most likely by a very frequent Ultraviolet Imaging Telescope (EIT) data and found that magnetic reconnection process. 461

462 WANG TABLE 1 OVERVIEW OF SOHO EIT OBSERVING SETUP Wavelength Pixel Size Image Size Observing Duration Cadencea Date Target (A ) (arcsec) (pixels) (UT) (minutes) 1996 Oct 6... North pole He II 304 2.5 192 ] 128 16:00È19:00 7/1/7 1996 Oct 7... West limb He II 304 2.5 128 ] 192 16:00È19:00 7/1/7 1996 Oct 8... North pole He II 304 2.5 192 ] 128 16:00È19:00 7/1/7 a EIT Cadence: 15:00 to 16:00 UT, 7 minutes; 16:00 to 17:00 UT, 1 minute; 17:00 to 18:00 UT, 7 minutes. In a series of papers by Dere and his colleagues at NRL, several di erent chromospheric and transition region phenomena were observed by high-resolution telescope spectrometer (HRTS) Ñights (Dere 1994; Dere, Bartoe, & Bruekner 1983, 1984, 1986, 1989a, 1989b, 1991; Cook, Rutten, & Hoekzema 1996). C I 1561 A observation show chromospheric jets,ïï and they later found that they are actually not jets ÏÏ but possibly associated with IN Ca II K brightenings or may also be associated with chromospheric oscillations (Hoekzema, Rutten, & Cook 1997). In their C IV, OIV, and Si IV observations, they showed transition region explosive events, which are located in chromospheric network. In a more recent work by Chae et al. (1998), using coordinated SUMER/BBSO observations, they found that explosive events are likely due to interactions between network and IN Ðelds, probably by reconnection. Such an idea was proposed by Innes et al. (1997) as well. Wang et al. (1998) used Ha [ 1.0 A movies to identify higher speed jets that are not classiðed as regular spicules because of their birthrate and shape. Those jets were found to be likely associated with converging magnetic bipoles. They might be candidates of macrospicules on the disk. Later in the paper, we will further discuss the possible relationship between macrospicules and disk events like explosive events ÏÏ and Ha [ 1.0 A jets. 2. OBSERVATIONS From 1996 October 6È8, We carried out coordinated observations between BBSO and EIT on board SOHO. On October 6 and 8, the target was the north pole, and on October 7 it was the quiet-sun west limb. Each day, EIT obtained 3 hr of He II j304 data from 16:00 to 19:00 UT. The EIT pixel size was 2A.5, the image dimension parallel to the limb was 192 pixels, and that perpendicular to the limb was 128 pixels. In the Ðrst and third hour, EIT cadence was 7 minutes, and in the second hour it was 1 minute. Several full-disk images were obtained immediately following the high-resolution run, which facilitates comparisons with BBSO data. The EIT observing setup is summarized in Table 1 for clarity. BBSO obtained Ha images during the EIT observing periods. Full-disk center-line Ha images were obtained with the 2K ] 2K Kodak camera. The pixel resolution is 1A.0. The primary usage of full-disk data in this paper is for image alignment of high-resolution images. The key component of Ha observation was the Orbiting Solar Laboratory (OSL) camera-zeiss Ðlter system on the 26 inch (0.74 m) telescope. The bandpass of the tunable Zeiss Ðlter is 0.25 A. The Ðeld of view of the observation was 170A ] 170A. The setup of BBSO high-resolution Ha observations is summarized in Table 2. Note that BBSO Ðeld of view (limbdimension) is 1 of that of EIT. 3 3. DATA REDUCTION Obviously, the most important but difficult task is to accurately align BBSO Ha images with EIT He II j304 images. Full-disk Ha and He II j304 images played a critical role in achieving this. The high-resolution Ha image scale can be accurately determined with respect to full-disk Ha images. We Ðrst align Ha full-disk with EIT full-disk images. Then we align Ha high-resolution images with fulldisk Ha images, and EIT partial image with EIT full-disk images. Finally, high-resolution Ha images and partialframe EIT images are aligned. We are conðdent that the accuracy of our alignment is of the order of 1 EIT pixel (2A.5). For each Ha and He II j304 image in the sequence, a median Ðlter was performed to obtain the background; each raw image was then subtracted by its background image. The contrast of Ha limb structures is signiðcantly enhanced because of the removal of the limb darkening. The He II j304 image is enhanced only slightly because of the lack of limb darkening in the raw data. We select an optimized kernel size of the median Ðltering of 50A for He II images and 20A for Ha images. The artifacts produced by the median Ðltering technique are dark envelopes surrounding bright features, which do not a ect the identiðcation of bright macrospicules. The wavelength dependence of Ha macrospicules will not be discussed in this paper, because it is a very difficult problem and morphologies of macrospicules are somehow similar in all three wavelengths for the purpose of identiðcation of events. The example of such similarity is demonstrated in Figure 1, which compares Ha macrospicules at [0.65 A, line center, and 0.65 A. We believe that some variation from one wavelength to the other is due to time evolution of features, since images are taken in a span of 76 s. The di erence between the second and third image is TABLE 2 OVERVIEW OF BBSO OSL CAMERA Ha OBSERVING SETUP Pixel Size Field of View Filter Bandpass Cadence Wavelength (arcsec) Image Size (arcsec) (A ) (s) Ha LC, ^0.65 A... 0.17È0.33 512 ] 512 or 1024 ] 1024 170 ] 170 0.25 30È90

FIG. 1.ÈComparison of Ha centerline, blue and red wing images, 1996 October 6, at the north pole. All the images in all the Ðgures are after image enhancements, as discussed in the text.

464 WANG TABLE 3 PROPERTIES OF HE IIj304 MACROSPICULES Date Target Na T b Rc 1996 Oct 6... North pole 11 ^ 5 13^5 3^1 1996 Oct 7... West limb 0.1 ^ 0.1 15 ^ 9 0.02 ^ 0.02 1996 Oct 8... North pole 13 ^ 5 14^5 3^2 a Number in Ðeld of view. b Mean lifetime (minutes). c Birthrate (events s~1). larger because of the longer time gap. We combine observations in all wavelengths to increase the cadence to study the evolution and lifetime of macrospicules. 4. RESULTS 4.1. Birthrate and L ifetime of He II j304 Macrospicules In order to Ðnd the global birth rate of macrospicules, we need to estimate the e ective surface area over which the features are visible at the limb. This is not an easy task, because such an area is viewed tangentially instead of directly. We adopt the method of Zirin (1988): considering a simple geometric structure, one should Ðnd that if the average height of macrospicules is h and the solar radius is R, then at a distance l \ (2hR)1@2 from the limb, a typical jet will disappear from the limb image. The observations would cover an e ective surface area of w ] l, where w is the image size in the dimension parallel to the limb. Using h \ 10A, R \ 960A, and w \ 475A, we found that l \ 130A and the ratio of the e ective covered area to the whole surface area of the Sun X \ 0.0054. Similar to what we discussed in the previous paper (Wang et al. 1998), the global birthrate of macrospicules is then R \ N/(TX), where N is average number of macrospicules in an observing Ðeld of view and T is their average lifetime. As expected, the global birthrate (R) ofheii j304 macrospicules is much higher inside the polar coronal hole region (2.7 s~1) than the equatorial limb (0.02 s~1), as reñected in Table 3. Finding the He II j304 macrospicule lifetime is straightforwardèby studying the He II j304 time-sequence images. We show a He II j304 sequence in the upper half of Figure 2. The upward ejection and downward returning motions are usually clearly observed, so the lifetime is easily determined. Based on the study of 57 macrospicules, we found that the mean lifetime is 14 ^ 5 minutes. This value is similar to numerous previous studies (e.g., Bohlin et al. 1975). On the other hand, the evolution pattern of Ha macrospicules is not well deðned (see the lower half of Fig. 2) because of substantial changes in morphology from one image to the other. Consequently, the determination of the lifetime of Ha macrospicules would have much larger uncertainty. We will discuss this point later. 4.2. Comparison between Ha and He II j304 Macrospicules In the previous section, based on the lifetime and birthrates of He II j304 macrospicules, we conðrmed that we are discussing the same macrospicules as those of early Skylab observations. In this section, we will show a number of comparisons between simultaneous Ha and He II macrospicules observations. The primary results are summarized in Table 4 and described in detail here. Figure 3 compares near-simultaneous Ha and He II j304 images at 18:00 UT, on 1996 October 8. Image scale and time are marked. Six He II j304 macrospicules are marked by letters AÈF; B and F are the two major ones. Their Ha counterparts are marked in the lower panel. We should point out that in Ha images, only B looks like the traditional Ha (30A long) macrospicule (Moore et al. 1977), which is almost as long as the He II macrospicule. Such long Ha macrospicules do not occur often. During 9 hr of continuous data coverage, we only observed two macrospicules that have similar long, jetlike structure in both Ha and He II j304. Remaining macrospicules have completely di erent structure in Ha and He II j304. There are two kinds of basic Ha activities corresponding to He II j304 macrospicules: (1) 43% of He II j304 macrospicules correspond to short jetlike Ha features, and (2) 57% of He II j304 macrospicules correspond to looplike Ha structure in the base of He II macrospicules. The loop structure in the base of jet B in the Ha image is the most obvious example of a loop. We now call both features macrospicules (although it is not very proper to name features in the second category this way). We note that more than half the Ha macrospicules have no corresponding He II j304 macrospicules; the macrospicules marked by g, h, and i are three examples of Ha macrospicules without He II counterparts. Among 108 Ha candidates studied, only 53 correspond to He II j304 macrospicules. Among the Ha macrospicules without He II counterparts, roughly half are jetlike, and other half are looplike. Karovska & Habbal (1994) applied an image enhancement technique to C III j977 data and demonstrated low-lying arches at the bases of the macrospicules. We failed to demonstrate arch structures clearly with He II j304 observation, although our He II j304 observations have slightly better spatial resolution. In Figure 4, we show four other He II j304 macrospicules observed on 1996 October 6. Again, they are all associated with much shorter Ha counterparts. However, even more Ha macrospicules do not have associated He II j304 events in this Ðeld of view. Note that Ha and He II j304 images were taken only 2 s apart. The most interesting comparison is for the west limb observations on 1996 October 7. In the EIT movie, there is TABLE 4 COMPARISON OF HE IIj304 AND Ha MACROSPICULES Mean Lifetime % Jetlike Percentage Jetlike Percentage Wavelength (minutes) in NPa in WLa Birthrate in NP Birthrate in WL He II j304... 14^5 100 100 3 ^ 1 0.02 ^ 0.02 Ha... 11^8 43 10 8^6 7^5 a NP, north pole; WL, west limb

FIG. 2.ÈTime evolution of a macrospicule as observed in He II j304 and Ha blue wing on October 6. The Ðeld of view is 40A ] 50A.

466 FIG. 3.ÈComparison of He II j304 and Ha blue wing images of 1996 October 8 at the north pole. Capital letters mark macrospicules observed at both wavelengths. Lowercase letters mark macrospicules that appeared in Ha only. The horizontal image scale is 170A.

467 FIG. 4.ÈComparison of He II j304 and Ha blue wing images of 1996 October 6 at the north pole. Capital letters mark macrospicules observed at both wavelengths.

FIG. 5.ÈComparison of He II j304 and Ha line center images of 1996 October 7 at the west limb (the west limb is up). The complicated structure on the right side of Ha image is part of a prominence.

COMPARISON OF Ha AND He II j304 MACROSPICULES 469 no single macrospicule within the BBSO observing Ðeld of view, and Ha macrospicules still appear at a similar rate to that of the polar regions. Figure 5 compares such a pair. Because of telescope pointing error, only half of BBSOÏs Ðeld of view is within EITÏs Ðeld of view. From Figures 3 and 4 (polar region), we found that in a typical image, there are about 10 Ha macrospicules covering 170A. For the west limb data we found that, on average, there are about Ðve macrospicules covering 90AÈresulting in the similar Ha macrospicule birth rate in the north pole and the west limb. Although the birthrate of Ha macrospicules in the west limb is similar to that of the north pole, 90% of the macrospicules are looplike structures. On the polar region, only 57% of Ha macrospicules are looplike. This fact demonstrates the tendency of conðnement of Ha macrospicules in the lower atmosphere outside the polar region. Both Ha and He II j304 macrospicules tend to recur in several Ðxed sites at the limb, a property shared by the disk Ha [ 1.0 A jets (Wang et al. 1998) and the explosive events (Chae et al. 1998). To measure the lifetime of Ha jets and establish an evolutionary correlation between Ha and He II macrospicules is very difficult, because Ha structures vary a lot, even though our observations had about a 30 s cadence. Such a difficulty is demonstrated in Figure 2. The vertical white line in Figure 2 (Ha image at 17:29:08) indicates the location of the Ha macrospicule corresponding to the He II j304 macrospicule. The feature changes signiðcantly from one frame to the other, so it is extremely difficult to identify whether it is a single ejection or there are two ejections that peak at 17:21 and 17:31 UT. We found that the lifetime of Ha macrospicules (11 ^ 8 minutes) is similar to that of He II j304 macrospicules but with much larger uncertainty. Using the same equation as that for He II j304 macrospicules and using N \ 10, w \ 170A, and X \ 0.0019, we found that the global birthrate of Ha macrospicules is R \ 8 ^ 6 events s~1. 5. DISCUSSION Let us Ðrst summarize the key points of our observations: 1. He II j304 and Ha macrospicules are completely di erent in morphology. 2. He II j304 macrospicules tend to occur much more frequently in polar region than in the equatorial region. 3. Ha macrospicules occur at a similar rate in polar regions and equatorial limbs, although in the polar region 57% of them are looplike and in the equatorial region over 90% of them are looplike. 4. Every He II j304 macrospicule has an Ha counterpart; many Ha macrospicules do not show in He II j304. Based on the above results on the comparison of Ha and He II j304 macrospicules as well as on results from a few of our previous papers (Wang 1998; Wang et al. 1998; Chae et al. 1998), let us Ðrst discuss possible disk counterparts of limb macrospicules. What do macrospicules look like if they are observed on the disk? Here we propose two possible disk counterparts, both tending to occur repeatedly in the same sites on the quiet Sun and both having evidence that they are associated with magnetic reconnections: 1. Ha [ 1.0 A jets (Wang et al. 1998): The global birthrate of macrospicules is less than half of Ha [ 1.0 A jets, which can be explained by uncertainties in the method of estimating the global birthrate for limb features like macrospicules; the e ective area calculation discussed in 4.1 is really a rough estimate. 2. Explosive events (Chae et al. 1998): The explosive events have been also identiðed as sites of magnetic reconnection due to network and IN encounters (Chae et al. 1998). However, the global birthrate of explosive events is 500 s, closer to the birthrate of spicules. If macrospicules are associated with explosive events, they must be associated with a subset of explosive events. We propose that Ha macrospicules show the base part of reconnection, and reconnection rates are about the same in the coronal hole and noncoronal hole regions. So Ha macrospicules would occur at the same rate everywhere. If the disk counterparts of macrospicules are indeed Ha [ 1.0 A jets or a subset of explosive events, as discussed above, then the macrospicules might be due to magnetic reconnections and are most likely due to networkephemeral region or network-in encounters (Wang et al. 1998; Chae et al. 1998). In addition, the looplike Ha structures in the base of ejection and some cases of erupting loops are more direct evidence of the magnetic reconnection. Wang et al. (1996) demonstrated that cancellation of magnetic Ðelds provides at least 1028 ergs s~1 of energy to power reconnection events in the quiet Sun. When macrospicules are observed at the limb, in the polar coronal hole region, Ha macrospicules may also appear as He II j304 macrospicules, because Ðeld lines of most network magnetic elements are vertical there. This reconnection process is similar to that in Shibata et al.ïs (1992) jet model: after a reconnection, plasma is ejected and moves along the dominant open magnetic Ðeld line. Even in the polar region, over 50% of the Ha macrospicules do not have counterparts of He II j304 macrospicules. This could be because (1) Ha macrospicules occur in the sites that do not have a vertical Ðeld component, or (2) the emission measure at He II j304ïs temperature is not high enough for He II j304 emission to appear. In the noncoronal hole region, almost all Ha macrospicules have no corresponding He j304 macrospicules, due to a lack of vertical Ðeld components at the reconnection sites. Ha macrospicules are more conðned there because of inclined Ðeld lines. Even in the polar region, morphologies of macrospicules in Ha and He II j304 are very di erent, maybe because of the following reason: Ha shows the plasma component with a temperature below 104 K, while the He II j304 line is a result of radiative de-excitation followed by collisional excitation. The temperature range is (5È8) ] 104 K. There are some additional difficulties in comparing those two lines: the Ha line covers a huge range of formation heights from lower chromosphere and upper chromosphere. Because of the broad wavelength range (150È350 A ) of the EIT Ðlter, the temperature range of EIT observations is also broad. There are two peaks, one at 8 ] 104 K, the other at 106 K (Delaboudiniere et al. 1995). The second peak is due to contamination of Si XI at 303 A. Such a high-temperature component should not contribute signiðcantly to macrospicules, because macrospicules are absent at other hightemperature lines such as Fe IX/X 170 A, which peaks at 1.3 ] 106 K. From this study, we cannot establish the relationship between spicules and macrospicules. It is likely that macro-

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