HRTS SPACELAB-2 EXPEIMENT

Flown on Space Shuttle Mission 51-F

July 29, 1985 - August 6, 1985

SCIENTIFIC SIGNIFICANCE OF THE HRTS INVESTIGATIONS

The source of all solar energy is ultimately the result of the thermonuclear reactions taking place in the Sun's core. Heat is transported outward from this core, which has a temperature of about 1.5 x 10⁷ K, to maintain the temperature of the outer layers, which decrease in temperature with distance from the core. The temperature of the photosphere, which is the surface of the Sun visible to the naked eye, is about 5770 K. The amazing thing about the Sun and many other stars is that as distance from the energy generating core increases, the temperature also increases. Just above the photosphere is the chromosphere at 10⁴ K, then the transition zone at 10⁵ K, and finally the corona at 10⁶ K; this corona extends to the Earth and beyond in the form of solar wind.

One of the key challenges in modern solar physics is to understand how energy can be transported and dissipated to form these hotter outer regions. It is clear that solar magnetic fields, which are delineated by the fine scale structures above the photosphere, must play an important role in this process. The emergence and evolution of the magnetic fields are likely responsible for many of the highly transient energetic events observed in solar spectra as well as for the more gradual evolution of the solar structures and the accompanying variability of the overall solar ultraviolet flux. The HRTS instrument is particularly well suited to study these problems because it is able to see spectral lines that are emitted in the three main regions of the Sun-the chromosphere, the transition zone, and the corona-and all at the highest spatial resolution yet available.

HRTS combines the advantages of ground-based solar observatories with those of space instrumentation. First, HRTS is large enough to make measurements equal or superior to those made by powerful telescopes on the ground. Because the shuttle can carry very massive instruments, HRTS is larger than most satellite-borne instruments; it also benefits from the Spacelab's advanced communication system that allows transmission of very high resolution images. Furthermore, the HRTS telescope, unlike ground-based instruments, is not limited by atmospheric effects that filter radiation and distort images, nor is it idled by long nights; aboard Spacelab 2, observations were interrupted only by a brief night portion of each orbit.

Finally, experienced solar physicists working aboard Spacelab 2 as payload specialists were available to interpret HRTS data and aim the telescope at targets of interest. Normally, it takes hours to point unmanned telescopes at specific solar features, and even then the observer may not know precisely where the telescope is aimed.

HRTS INSTRUMENT DESCRIPTION

HRTS consists of a concentric Gregorian telescope with a 30-cm aperture and three focal plane instruments. The telescope is derived from a sounding rocket version that was limited to a 30-cm aperture by the size of the sounding rocket. To adapt the telescope to flux limitations in the ultraviolet, an f :1 5 ratio was selected. The field of view of the Gregorian aperture is 15 arc min by 5 arc min. Only two reflecting surfaces are used in the telescope to reduce reflection losses in the UV, to minimize potential sources of image distortion, and to reduce stray light. A plate scale of 0.02 mm per arc sec is the limiting factor for the UV resolution, which was 0.5 to 0.7 arc sec.

The Focal Plane Instruments

o The tandem Wadsworth spectrograph is the main instrument housed in the focal plane package. It consists of two spherical gratings that form a fully stigmatic spectrum over the wavelength range from 1170 to 1700 Å. The spectrograph has only three reflecting surfaces, one of which is a folding mirror. A film camera that carries 500 ft of Kodak 101 film is located at the 110-mm-long focal plane. Figure 1 illustrates a sample spectrograph full frame. A section of the complete spectrum is shown from an exposure taken with the slit tangential to the solar limb and approximately 60 arc sec inside the limb. The end of the continuum at the top and bottom of the slit can be seen, with chromospheric and transition region line emission continuing to heights above the limb.

Fig. 1 - HRTS spectrograph spectrum, 1500-1650 Å. The slit was located tangentially to, and 60 arc sec inside the solar limb.


Fig. 2 - HRTS spectrograph raster in C IV, 1550 . The raster steps are 1 arc sec apart.

The 500 ft of film allows 1350 spectra, as seen in Fig. 1, to be taken. Also, a mask can be moved into the film plane that restricts the film view to 15 Å at any position between 1170 and 1700 Å. Therefore, 30 small frames replace I full frame, with the potential of 40,500 frames taken in the small frame mode.

The spectrograph can be operated in several modes; full or small frame spectra can be taken at a single position of the 0.5 arc-sec-wide and 15 arc-min-long slit, or the slit can be moved along the direction of dispersion. This allows rastering of the solar surface with any desired raster width. The single slit mode and the raster mode allow time sequences to be constructed. Exposure times vary from 0.3 s for H Lya to several hundred seconds for weak coronal lines. A typical C IV 1550 Å exposure time is 5 s. Therefore in C IV, a raster of 10 steps has a time resolution of 1 min. Figure 2 shows a small frame raster sequence. The intensity modulation in the continuum and in the C IV lines is seen to change from one step to the next. This indicates considerable variation in solar structures over one raster step which, in this case, is 1 arc sec-approximately the resolution of the telescope.

The wide variety of spectral lines and continua in the 1170 to 1700 Å wavelength range is used to diagnose physical conditions such as temperature, pressure, density, and velocity that characterize the different heights in the atmosphere. The continuum, produced by free ionized electrons, originates in the temperature minimum region. Neutral ions of hydrogen, carbon, oxygen, silicon, and sulphur exist in the chromosphere at 10⁴ K where they are excited by the hot electrons to produce spectral lines. At 10⁵ K in the transition zone, the electrons are energetic enough to ionize these elements and excite them to higher energy states to emit spectral lines. At the corona (2 x 10⁶ K) and in flares (10⁷ K), most atoms are completely stripped of electrons and only such highly charged ions as iron, with 26 protons, can maintain a number of bound electrons to produce the spectral lines characteristic of the corona and flares

The broadband UV spectroheliograph is the second instrument of the focal plane package. This uses a zero dispersion, reversed tandem Wadsworth optical system that consists of two concave gratings that form an image of the slit jaws at 1550 Å with a bandwidth of 90 Å. The first grating disperses the light and the second grating acts as a wavelength selector. The two halves of the spectrograph slit jaws, from which the solar image is reflected to the spectroheliograph, have different reflectivities to facilitate simultaneous observations of bright structures on the disk and faint structures above the limb. One slit jaw is coated with Al + MgF₂ for high UV reflectivity to be used above the limb for recording large but weak C IV structures high in the corona. The second slit jaw has an Al + SiO₂ coating that reduces stray light from the disk, but also allows reasonable exposure times (0.3 s) on the disk; coronal exposure times vary from 0.9 to 2.7 s. This spectroheliograph arrangement images a mixture of continua and emission lines on the disk; but high in the corona above 5000 km - C IV emission dominates the images. Figure 3 shows an example of a disk observation. The lighter and darker sides arise from coatings of different reflectivity on the two spectrograph slit jaw plates from which the image is taken. The left, less reflectant side is meant for disk observations. Three fiducial wires cross the slit at top, middle, and bottom. The disk morphology seen in Fig. 3 differs from the bright point structure seen in HRTS spectroheliograms from a previous rocket flight; the spectroheliogram from this flight was tuned to 1600 Å with a narrower passband, and it predominantly imaged the temperature minimum continuum [1]. In Fig. 3, only active region (notice the sunspot) and quiet network elements are seen as extended continuous structures. This difference from the bright point structure seen in temperature minimum images is partly due to a greater contribution of flux lines. Scientists may be observing hotter plasmas where the confined bright points in network and active region temperature minimum structures have expanded with the diverging magnetic fields to form more extended features.

Fig. 3 - HRTS spectroheliograph disk image. The left slit jaw is coated with Al + SiO₂ and has about one tenth the reflectivity of the right slit jaw, which is coated with Al + MgF₂.


Fig. 4 - Limb observation with the HRTS spectroheliograph.

Figure 4 illustrates a spectroheliograph image of the solar limb. The slit is placed tangentially to the limb with the more highly reflectant slit jaw above the limb and the less reflectant half covering the disk, reducing the disk scattered light which otherwise would obscure fine structure. Above the limb, continuum emission is absent, and the spicular structures seen are from transition region and chromospheric line emission. Jets and spicular emission can be followed as they evolve over approximately 15 min of good pointing by the Spacelab 2 Instrument Pointing System (IPS) during a 30-min observing sequence.

o An H-alpha system is the third focal plane instrument. This system also uses a reflected image from the slit jaw mirrors. A mica Fabry-Perot filter, whose passband is temperature-controlled, produces an Ha image sent both to a Westinghouse SIT TV camera and to a film camera. The Ha system was used for telescope focusing and target selection by the Spacelab 2 crew observing live TV images at their work station. The Ha images could be sent to the ground if communications allowed, recorded on video tape by the crew, or recorded on film as part of the execution of observing sequences.

SPACELAB 2 OPERATIONS

Eighteen different instrument-observing sequences were executed, some many times, over 23 orbits devoted to solar observing. Essentially all of the available flight film was exposed roughly 600 full frame and 18,000 short frame spectrograph exposures, 1330 spectroheliograph exposures, and 510 Ha exposures. An additional 8 hours on video cassettes were recorded from the Ha system both by the crew and from downlink.

Several factors affected to some extent the scientific data obtained by HRTS during the Spacelab 2 mission. These included:

o IPS related limitations. The strong point of the HRTS instrument is its potential sub-arc-second spatial resolution. In previous rocket flights a best performance of 0.8 arc sec was achieved. As was fully expected, the basic limitation on spatial resolution during the Spacelab 2 mission was the Spacelab IPS jitter. Exposures less than approximately 5 s appear to reach spatial resolution of 1 to 2 arc sec, while longer exposures do not show the finest detail. For the next Spacelab flight of HRTS, an internal image motion compensation system will eliminate jitter as a problem.

o Heating problems. The HRTS instrument experienced heating caused by sunlight reflected from the shuttle bay at much higher levels than was expected. Periods of instrumental power down were required to keep experiment computer and film temperatures below critical limits. Temperature problems affected the passband of the Ha filter, which sometimes drifted into the line wing.

o Film background level. The flight film shows a background clear plate level which increased during the course of the mission. The effect on the data is to somewhat decrease the signal-to-noise ratio by pushing the background level from the toe of the film characteristic curve up onto the linear portion. The time development of the background makes individual film characteristic curves necessary for quantitative photometry. Digital image processing of the microdensitometer tracings can improve the image quality.

o Instrument sensitivity degradation. The spectrograph began to lose sensitivity below 1400 Å by the second orbit of HRTS operations. It became progressively ' worse, and H Lya, at 1216 Å, was down in sensitivity by a factor of 10 to 100 for most of the mission. Above 1400 Å the basic sensitivity remained more constants

These factors affected to some extent the scientific data acquired by HRTS, but by no means were severely limiting. Despite any mission or instrumental difficulties, the scientific return from HRTS was very encouraging.

SCIENTIFIC RESULTS

Years from now scientists will be able to look back on the Spacelab 2 flight and more clearly pick out the most important new results obtained. Today they have only begun analysis of the data and can see a number of observations that seem exciting. A few representative examples are:

o Spicules at the limb. There are spectrograph, spectroheliograph, and Ha observations of spicules at the limb, and the combination of these three provides new results on the structure and evolution of transition region spicules and their relation to Ha spicules.

o Small, high-velocity events. These are well known from previous rocket flights [2], but Spacelab 2 provides a much greater data base and allows links with fine structure features, observed from ground-based instruments, to be established. It is also possible that the spectroheliograph limb observations may throw new light on the C IV events seen in slit spectra.

o Survey program. A HRTS spectral survey program obtained raster spectra that cover about 25 % of the solar disk. This program was meant to establish statistical global properties of the fine structure features previously observed during HRTS rocket flights.

o Sunspots. A variety of observations were obtained of a sunspot in active region 4682 and of a smaller sunspot in active region 4680. Interestingly, the larger sunspot shows velocity flows connected with a light bridge that are similar to although not as dramatic as flows seen in two sunspot observations from earlier HRTS rocket flights.

o Spectroheliograph disk observations. We must first understand the cause of an observed difference in morphology between the Spacelab 2 spectroheliograph disk observations, with the passband centered at C IV, and earlier HRTS rocket observations with a narrower passband centered at 1600 Å - the temperature minimum region of the continuum showing small discrete bright points [1]. We can study structure and evolution of disk images from programs lasting up to 30 min.

o Filaments and prominences. Observations were obtained both of a disk filament, which in the course of the mission lifted and disappeared, and also of a large limb prominence.

The next sections discuss in more detail four representative topics from the HRTS Spacelab 2 data.

UV Spicules at the Solar Limb

Since the HRTS II rocket flight in 1978 [1], broadband spectroheliograms flown on HRTS payloads have obtained UV observations above the solar limb of increasing resolution and clarity. Technical improvements culminated in the Spacelab 2 spectroheliograms, which for the first time gave high-resolution images as well as accompanying Ha observations during a 15-min time series.

Fig. 5 - Time series of HRTS spectroheliograms showing temporal evolution of UV spicules at the limb. Exposures are at relative times of 0, 4, 7, 10, and 12 min (top to bottom).

Figure 5 illustrates a time series of spectroheliograph images of the solar limb. The slit was placed tangentially to the limb with the more highly reflectant slit jaw above the limb and the less reflectant side covering the disk, reducing the disk-scattered light that otherwise would obscure fine structure. The sharp boundary in exposure at the limb marks the slit position. Three fiducial wires cross the slit at the left, center, and right.

The spectroheliograph was tuned to 1550 Å, the wavelength of the C IV resonance doublet at 1548 and 1550 Å. Above the limb, continuum emission is absent, and the spicular structures seen arise only from transition region (predominantly C IV) and chromospheric line emission within the spectroheliograph passband. These images are from a 30-min long observing sequence designed for limb study. During this period the IPS jitter was sufficiently small to see the finest spatial resolution (1 to 2 arc sec) only in the last 15 min. These exposures show the morphology and temporal evolution of UV spicules. This emission appears to have two components. The first component consists of ordinary UV spicules, extending to a height of 10 arc see, with a lifetime of probably 10 to 15 min (although this is difficult to obtain since it is also the period of good pointing stability). These also probably appear as fingerlike mottles extending from network elements seen on the disk close to the limb. This component, ubiquitous along the limb, is the UV analog of Ha spicules. The second component consists of more transient, fainter (3 to 10 times fainter than the UV spicules) superspicules, which extend up to a height of 20 arc sec, and which can form, develop, and disappear on a shorter timescale-as short as 3 min. These are much less common than the UV spicules; they may have a connection with the small C IV high-velocity events seen on the disk in slit spectra. With the available temporal resolution - 1 min with a brief period at the beginning of 20 s - these superspicules appear more to form in place than to rise as ejecta; they can appear curved, as in a section of a loop, but can also flex in the course of development.

Fig. 6 - Comparison of HRTS spectroheliogram (upper, in white) and HRTS Ha image (lower, in black) showing spicules at the limb

Ha observations from the same sequence have good quality images of Ha spicules. Indeed these images, because of the short, 0.3-s exposure time in relation to the pointing jitter, may give the highest spatial resolution of any film exposures in the mission. As shown in Fig. 6, Ha and UV spicules occur at similar locations and appear to have a reasonable degree of correspondence. But the UV superspicules do not appear to have an Ha counterpart of sufficient brightness to appear on even the longer exposures.

The imaging of the UV spicules, their clear correspondence with Ha spicules from simultaneous observations, and the superspicules are interesting results from Spacelab 2. But even more work on spicules will be possible from the slit spectra. Spectra should allow study of the intensity-height distribution and most interestingly, of velocities, especially a search for signatures of rotation such as were suggested by spicule-tilted spectral features seen in the HRTS IV rocket data [3].

Statistical Properties of Small, High-Velocity Events in the Transition Region

Fig. 7 - Example of a C IV small frame raster from one execution (the seventh) of the survey sequence. Raster steps are 1 arc sec apart. A complete raster covers 60 slit positions. Individual high velocity events are marked and lettered A to H.

Probably the most interesting discovery of the HRTS program has been the spectrographic observation of small (1 to 8 arc sec) transition region events, seen best in the strong C IV, 1548 and 1550 Å lines, which have profiles broadened to blue, to red, or to both (see Fig. 7). These broadenings are typically about 100 km/s but have been observed on the HRTS R rocket flight at up to 400 km/s in jets - those events with a pronounced blueward profile only. Their physical significance lies in the contribution in mass, momentum, and energy flux which they might make to the upper solar atmosphere. Even though they are of small spatial scale, the dependence of the several fluxes on powers of velocity can make these events energetically important globally if they are sufficiently common [2]. It is just the statistical properties of jets that are most difficult to extrapolate from the limited observations of a 4-min rocket flight.

For Spacelab 2, a special survey program was designed to cover approximately 30% of the flat disk surface in spatial rasters. A basic sequence was nominally to be executed 16 times over four consecutive orbits, establishing global statistical properties at this epoch in the solar cycle. It may be possible to compare results from this flight with results from a similar survey during a future mission at a different epoch in the solar cycle.

In practice, 13 executions of the survey sequence were accomplished during orbits 80 to 83 of the Spacelab 2 mission over 3 August 21:27 UT to 4 August 02:53 UT, covering approximately 25% of the flat disk. The basic survey sequence, lasting 8 min, consisted of a 60 slit position raster of the 920-s-long slit using a 1 arc sec step width, performed in a 14 Å interval around C IV 1548 and 1550 Å with 5-s exposures. This basic survey sequence was followed by a coarser 3 slit position raster with a 30 arc sec step width over the same area, performed over the entire 1190 to 1680 Å region with exposure times of 20 s, 8 s, and 3 s in order to record the full range of intensity.

Fig. 8 - Distributions of velocity, size, and lifetime of high-velocity events. (a) The distribution of the velocity of the leading edge of high-velocity events. The number of events is binned in 16.7 km/s intervals down to a cutoff near 50 km/s. jets or bullets (blue), red-shifted events (red), and explosive or turbulent events (explosive) are separately treated. (b) The size distribution (along the spectrograph slit) of high-velocity events. All three classes are combined. © The lifetime distribution of high-velocity events. All three classes are combined.

In this first analysis, a visual inspection of photographic prints of the fine rasters in C IV was made to find small (typically 1 to 8 arc sec) events showing velocity signatures of at least 50 km/s to blue, to red, or to both. We found at least 549 such features, with 35% of these predominantly blue-shifted (jets or bullets), 25% predominantly red-shifted, and 40% both red- and blue-shifted events (turbulent or explosive events). The distribution of the number of events vs the leading edge velocity for these events (Fig. 8(a)) is roughly Gaussian, with all three classes of events peaking in number near 80 km/s; one blue-shifted event is as high as 200 km/s. There must be approximately 2000 of these events on the visible hemisphere, or 4000 on the whole Sun, at any time in this epoch.

Figure 8(b) shows the distribution of the event size along the slit. The distribution of the lifetime of events has been studied from a e sequence executed in a later orbit which repeatedly rastered a 27 arc-sec-wide area over 17 min with 1-min temporal resolution. The average lifetime for all three properties of these events (Fig. 8(c)) is 90 s. Thus the average global birth rate of the combined features is around 44/s.

A number of plasma properties of these events have been calculated by using a combination of parameter values from the observations and estimates of other values. This provides some insight into the importance of these small scale features in the energy balance of the upper solar atmosphere, but is sometimes dependent on assumptions that are still open to question. The result for the total global energy contribution of these events from radiation, enthalpy flow, and kinetic energy flow is approximately 1. 5 x 10²⁷ ergs/s, where v = 80 km/s, N_e = 10¹⁰/cm', T = 10⁵ K, size 2 arc sec, and there is a global total of 4000 events.

If the global energy contribution is converted to a surface flux by dividing by the Sun's surface area, we find around 2.5 x 10⁴ ergs/cm²s as the energy flux that could be contributed by the high-velocity events. This should be compared with an energy flux of around 5 x 10⁶ ergs/cm 2 s to heat the quiet chromosphere, or 10⁷ ergs/ cm ² s to heat the corona in an active region. The high-velocity events do not appear to be a viable source for the general heating of the upper atmosphere, but they are still an interesting solar phenomenon. There is every reason to think that these events may be analogous to very small solar flares, and the elucidation of the plasma-magnetic field interaction producing them will be both an advance in plasma physics and a probable hint to a mechanism for flare heating.

Discrete Structural Components of Emission in the Chromosphere and Transition Region

Fig. 9 - Discrete structural components of solar emission in lines of carbon covering a range of chromospheric and transition region temperatures. Profiles show the discrete structural elements along a 250 arc sec section o the HRTS slit. From the left are Cont 1615 Å (30 s exposure), C I 1613 Å (10 s exposure), C I 1656 Å (10 s exposure), C I 1656 Å (3 s exposure), C II 1335 Å (3 s exposure), and C IV 1548 Å (3 s exposure).

Spectra obtained during periods of greatest pointing stability show that a large fraction of the total emission pattern is due to discrete structures along the slit. This is in distinct contrast to the more continuous variation of profiles along the slit during periods of reduced pointing stability. Images of discrete spicular structures in Ha and C IV lead us to identify the discrete emission components as spicules. Spectra of C I 1613 Å, C I 1656 , C 11 1335 Å, C IV 1548 , and the continuum at 1615 Å have been analyzed to determine the relevant physical quantities in these structures. Representative profiles of these lines over a 250 arc sec section of the slit are shown in Fig. 9. The C I line at 1613 Å is an intercombination line with much less optical depth than the C I resonance line at 1656 Å. In the 1615 Å continuum, the average full width at half maximum (FWHM) along the slit is 2200 km; in the C 1 1613 Å intercombination line, the average FWHM is 2400 km; and in the C IV line at 1548 Å,

the average FWHM is 2900 km. This reveals a small but real increase in the average widths of these spicules from the temperature minimum region to the transition region. A familiar characteristic of transition region line profiles is the need to invoke a turbulent broadening to explain their observed line width. It is possible to attribute some of this turbulent width to variations in bulk velocity of multiple structures within the slit. Spectra of discrete structures allow us to measure the purely microturbulent width of these structures. When this is done, a range of microturbulent velocities from 0 to 25 km/s is found with an average microturbulent velocity of 14 km/s. This is significantly less than previously quoted values that tend to lie in the 20 to 25 km/s range. This leads to a decrease in the energy of acoustic or Alfvenic waves often invoked to produce the necessary heating of structures in the upper solar atmosphere. It is also possible to derive the densities in these discrete structures from their line-of-sight differential emission measure N_e²dl determined from the integrated intensity of the C IV profile.

If these are roughly cylindrical structures, then the line-of-sight path length should equal the FWHM, and consequently, Ne = [N_e²dl/FWHM]^1/2. When calculated for each discrete C IV element, a range of electron densities from 10⁸ to 10⁹/cm³ is obtained. This is about a factor of 10 lower than one would expect from commonly derived values of electron pressures in the quiet transition region of 6 to 10 x 10¹⁴/ cm³ K. The other alternative is that these discrete structures are not completely filled with C IV-emitting material. In that case, the fill factor for these discrete structures is I %, and the lateral dimension of these subresolution elements is approximately 30 km or less. The existence of such small subresolution structures implies larger gradients that could enhance the heating rates for these structures through wave dissipation.

Turbulence in an Emerging Flux Region

Fig. 10 - Ha photographs (Hida Observatory) of active region 4682 on 2 August and 3 August 1985 (top). Magnetograms (Okayama Observatory) and potential field calculations (Tokyo Astronomical Observatory) are also shown (bottom). The box in the upper tight-hand image shows the location of the highly turbulent region.

Three large rasters of C IV exposures centered on the sunspot in active region 4682 were obtained. The slit of the spectrograph ran north-south, and the rasters covered an emerging flux region (EFR) at their western edge. The largest area of highly turbulent velocities was seen in the 9 raster steps that covered the EFR. Active region 4682 on 2 August is a typical old active region. Figure 10, left bottom, shows an Okayama Observatory magnetogram taken on 2 August 1985. Also included in the magnetogram are calculated potential field lines (University of Tokyo). At left top, a Hida Observatory Ha picture of the active region is shown. The magnetogram of the next day (3 August) was scanned between 00:53 and 02:22 UT. The Spacelab 2 rasters were produced from 02:18 to 02:35 UT on 3 August. They overlap in space and time with the second magnetogram. This magnetogram shows two small new flux regions of opposite polarity to the north of the sunspot. The eastward region has the same polarity as the sunspot and its surrounding plage, while the westward EFT has opposite polarity.

Fig. 11 - Ha loops connecting the newly emerging flux region with its surroundings on 3 August, 07.06 UT. Note that this Ha picture was taken approximately 5 hours after the spectra were obtained.


Fig. 12 - Turbulent C IV spectra of the EFR in active region 4682. Three rasters are displayed from top to bottom. The raster steps are 2 arc sec in width.

Fig. 13 - A typical turbulent EFR profile compared with an ordinary C IV profile from an adjacent area of the active region

A very bright compact loop system can be seen at the location of the EFR in Fig. 10. Figure 11 shows an enlargement of this area; the box indicates the area of large turbulence. In Fig. 12, the highly turbulent C IV spectra are shown. Wide profiles can be seen on 9 rastered spectra, which corresponds to a width of 18 arc sec; along the slit, the turbulent profiles cover approximately 35 arc sec. This area is well centered over the bright Ha EFR. The three rasters were taken in intervals of 13 min and 4 min respectively. Strong changes can be seen not only from raster step to raster step but also at the same location at different times. A scan of a broad C IV 1548 Å profile is shown in Fig. 13 together with an adjacent regular active region profile.

It is obvious that the field line arrangement shown in the 2 August magnetogram will be altered by appearance of the EFR. As the Ha pictures (Fig. 10) demonstrate, new magnetic connections develop between the EFR and the adjacent opposite polarity plage. These connections are the location of the strong turbulence seen in the C IV spectra. Although there is a realignment of filamentary chromospheric structure surrounding active region 4682, the potential field calculation indicates that the coronal connections are not altered significantly as a result of the appearance of the EFR. As demonstrated before, strong plasma turbulence can be observed everywhere on the Sun, but on a smaller spatial scale. The line broadenings and Doppler shifts, however, are similar to those observed in the EFR. It may be that the broad profiles observed in the quiet Sun are also caused by newly emerging flux on a smaller spatial scale.

Four representative topics from the HRTS Spacelab 2 data have been discussed. There is a wealth of additional observations open to study by guest investigators as well as by the HRTS experimenters that could advance a wide range of topics in solar physics. We would welcome interested readers who wish to examine the HRTS data.

• Go to Spacelab-2 publication,
• Go to HRTS/SKYLAB Database,
• Go to HRTS Project Home Page
  
  
• Go to SUSIM Spacelab-2 Experiment,
• Go to SUSIM ATLAS Home Page,
• Go to SUSIM Home Page,
  
  
• Go to the Solar Physics Branch Home Page or
• You can visit the NRL Space Science Division home page.