A Brief Scientific History of the NRL HRTS Program


Introduction

Since its first flight in 1975, the NRL High Resolution Telescope and Spectrograph (HRTS) has now recorded high quality ultraviolet spectra of the sun on 8 rocket flights and during extended operations on the Space Shuttle Spacelab 2 mission in 1985. The heritage of the HRTS program lies in the NRL ultraviolet spectrograph on the Skylab Apollo Telescope Mount (ATM experiment S082B, Bartoe et al. 1977) and its rocket-born predecessors. This spectrograph obtained spectra in the 970-1970 and 1940-3940 wavelength regions of a 2" x 60" area of the sun. Since ground based observation in visible light have shown the existence of considerable structure at the arc-second level, the central goal was to design a spectrograph that could simultaneously record an extended ultraviolet spectrum with arc- second resolution. In addition, it would need to be a double-dispersion spectrograph in order to eliminate stray light. This was accomplished by the invention of the tandem-Wadsworth spectrograph by G. E. Brueckner and J.-D. F. Bartoe [1*]. In this design, the astigmatism is reduced to a very low level by requiring all light rays to be imaged onto the normal of each grating. In this manner, stigmatic spectra along the whole slit and at all wavelengths included in the spectrograph are obtained. Also, because the dispersions of the two gratings are additive, a smaller focal length can be used. Four requirements were laid out for a flight spectrograph: (a) wavelength coverage, 1200-1700 , (b) f number, 15, (c) resolvable element size at the film plane over the entire wavelength range, 10 x 10 m, (spectral and spatial), (d) dispersion, 5 /mm. All four requirements were shown to be fulfilled by the symmetric tandem Wadsworth spectrograph [1].

A conventional Cassegrain telescope was then coupled with the tandem-Wadsworth spectrograph to form a sounding rocket payload. Because of the requirements on resolution and large format, photographic film was the only suitable detector and Kodak 101 was selected. A rocket payload must fit inside a cylinder roughly 270 cm long and 44 cm in diameter. This accommodates a 30 cm f/15 Cassegrain telescope with a focal length of 450 cm and the tandem-Wadsworth spectrograph with a folding mirror for compactness. An H visible light system was incorporated to provide real time video images of the UV spectrograph slit jaws for pointing and to record these images photographically for post flight analysis. The first flight demonstration of the HRTS was on a sounding rocket.

The 1170-1700 region of the spectrum contains numerous spectral lines and continua that are valuable diagnostics of the temperature minimum region, chromosphere and transition region, which are all important regions for understanding the strong nonradiative heating that maintains the outer solar atmosphere. The temperature minimum region can be studied with the 1600 continuum. The intensity and velocity patterns in the chromosphere are mapped, for example, by the resonance and intercombination lines of C I near 1560 and 1613 , respectively. In the transition region, there are density sensitive line ratios of Si III and O IV in addition to the strong lines of Si IV and C IV. Several weak coronal lines and a strong flare line of Fe XXI complement the primary investigations at lower temperatures.

The First Rocket Flight: July 21, 1975

The first spectra were extremely impressive. They dramatically demonstrated the highly inhomogeneous nature of the chromospheric and transition zone intensity and velocity fields. The pointing of the slit was chosen so that it intersected a sun spot and the quiet solar limb. The first observations of a variety of supersonic phenomena were made: coronal jets, explosive events and sunspot downflows. The high velocity outflows led to the suggestion that they were a potential source for the solar wind [5,30]. Molecular lines of CO and H2 were discovered in sunspots where they were excited by H Ly fluorescence.

The Second Rocket Flight: February 13, 1978

For the second HRTS rocket flight, a broadband ultraviolet spectroheliograph was added to view the UV spectrograph slit jaws. The design is similar to the tandem-Wadsworth spectrograph except that the normals to the two gratings are parallel so that the dispersions of the two gratings cancel rather than add [4]. The bandpass is about 50 and when centered at 1600 , the temperature minimum continuum emission dominates the chromospheric emissions on the disk.

The second flight again placed the slit radially through a sunspot and the quiet limb. While waiting in the tower at White Sands, the temperature in the telescope rose beyond its nominal operating range and so it was somewhat out of focus during flight. The spectroheliograph images showed that the majority of emission in the quiet temperature minimum is produced in small grains near the resolution of the instrument [12,29]. A spectacular example of a repeating 500 km s-1 coronal jet was found in an undistinguished part of the quiet sun [30]. Transition region flows of up to 300 km s-1 were found to extend over a good portion of a major sunspot which contained a large light bridge [14].

The Third Rocket Flight: March 1, 1979

The observations of transient high velocity outflows in both previous rocket flights gave considerable support to the notion that they were responsible for the generation of the solar wind. However, because of the small areas of the solar surface sampled, the extrapolation to a global phenomena was a large and uncertain jump. It was decided that a program to perform sequential spatial rasters of a more extended region of the sun would be very worthwhile. On the third rocket flight the slit was rastered in 2" increments to obtain 6 exposures of a 10" wide region. This sequence was repeated to build up 10 complete rasters and one incomplete raster. By limiting the spectrograph range to 50 roughly centered on the C IV lines near 1550 , it was possible to use a smaller but faster ruled grating for an increase in throughput of about a factor of 2. With the slit also opened up by a factor of 2, exposure times of 3 s could be used. While a large number of explosive events (100 km s-1) were observed, none of the high velocity (500 km s-1) coronal jets were found. The conclusion was that, although the events were not energetically important, there was possibly enough mass ejected in the explosive events to account for the mass requirements for the solar wind [64]. From the two dimensional profile data obtained in the rasters, it was possible to reconstruct images of the intensity and velocity fields in the chromospheric lines of C I and Fe II and the transition zone lines of C IV as well as intensity images of the temperature minimum continuum. These showed that the quiet transition zone consisted of structures that were essentially extensions of the chromospheric spicules [39]. Simultaneous H observations from Sac Peak showed that the Fe II emissions were produced in the same plasmas responsible for the formation of the spicules seen in the blue wing of H . The Doppler shifts in the ultraviolet lines were less than 3 km s-1, an order of magnitude below the velocities inferred from H limb observations. Instead, extremely small regions in the supergranular cell centers were found with blue shifts in the C I lines which indicated flow velocities of about 20 km s-1 [28]. The time sequences of the C IV images showed that the rare blue shifted structures were generally associated with expulsions and ejections rather than some sort of laminar flow along magnetically confined stream lines [42]. The spectroheliograph was tuned to 1550 in order to see transition region structure above the disk. These also suggested the existence of transient ejecta but the stray light from the disk emissions masked many of these weak phenomena [13].

The Fourth Rocket Flight: March 7, 1983

The goal of the fourth rocket flight was to observe the fine-scale structure of the solar transition zone and chromosphere at the limb. A curved slit jaw, with a radius of curvature twice that of the Sun, was installed into the spectrograph. In order to eliminate straylight in the spectroheliograph above the limb, a low-reflectance coating for visible light was used on one of the slit jaw surfaces. Further, a mechanism for translating the seconding mirror in flight to achieve the best focus of the telescope was also employed. Pointing and focusing of the experiment were to be controlled from the ground, based on the H alpha video downlink. Unfortunately the telemetry link to the rocket malfunctioned and it was not possible to properly focus the telescope. Nevertheless, profiles of coronal lines of Si VIII, Fe X,XI, and XII were obtained for quiet coronal structures as well as inside a coronal hole for the Si VIII line formed at 9 x 105 K. This is a unique data set because it places the only available limits on the Alfven wave energy flux that is often invoked to explain the acceleration of high speed solar wind streams. This problem is under current study.

Spacelab-2: July 29-August 6, 1985

The Spacelab 2 flight brought about the first opportunity to make extended observations of the sun with the HRTS. The HRTS was mounted on a cruciform in the Shuttle bay together with two other high resolution solar experiments (SOUP and CHASE) and a full sun ultraviolet monitor (SUSIM). Pointing of the four experiments was provided by the Instrument Pointing System (IPS). Payload specialists inside the Shuttle initiated complex instrument observing sequences planned on the ground and performed target selection with the help of video H images of the HRTS slit jaw and white-light images from the SOUP. The mission itself provided an extreme test in the operations planning, both for the complete shuttle/Spacelab combination and for the solar experiments in particular. Solar planning was supported by NOAA personnel who coordinated the gathering of various ground-based observations and forecasting. The mission was greatly complicated by unexpectedly high temperatures in the Shuttle bay and a problematic IPS. Nevertheless, a number of high quality observing programs were executed.

UV spectroheliograph images of the limb inside a polar coronal hole provided a sequence of images at a rapid cadence to study the evolution of macrospicules, some of which arose and disappeared during a time faster than could be measured in previous observations on Skylab [59]. The emergence of magnetic flux near a sunspot resulted in plasma turbulent and flow velocities of 300 km s-1 [52]. A comparison of the locations of explosive events with nearly simultaneous Kitt Peak magnetograms and He I 10830 images indicated that they generally occurred on the edges of the strong magnetic network [80]. The spatial distribution of supersonic sunspot downflows were mapped [58]. It was demonstrated that transition region emissions are formed in discrete structures with length scales of 2000 km but which themselves must be an ensemble of extremely fine scale structures with size scales less than 30 km [48]. High speed outflows of 60 km s-1 were found in an active region filament [60]. By combining the observational information on the transition region velocities at resolved and unresolved scales, it was shown that there is enough power in the transition zone velocity field at scales small enough that there is sufficient dissipation to account for the radiative losses [61].

The Fifth HRTS Rocket Flight: December 11, 1987

The fifth flight was conceived as a dual rocket campaign involving both the HRTS and the AS&E X-ray rocket payloads. The primary goal was to determine the coronal manifestation, if any, of the coronal jets and explosive events observed in the HRTS transition region spectra. After a frustrating series of problems with the rocket guidance system, the two rockets were launched within a half hour of each other. The AS&E payload returned a series of exposures of the full disk through several filter combinations. The HRTS performed a large area raster in the spectral region near 1550 , which includes lines of C I, Fe II and C IV. Coordinated observations were also obtained by the Big Bear Solar Observatory (video magnetograms) and the Kitt Peak Observatory (He 10830 spectroheliograms and magnetograms). The analysis of this data set is in progress but preliminary results indicate that the explosive events are not related to X-ray bright points but tend to occur at the borders of the supergranular network [103,117]. It is possible that they are related to the coalescence and cancellation of magnetic flux which is known to occur in those regions (Martin, 1988). There also seems to be some tendency for the explosive events to occur where the X-ray emission is relatively weak.

The Sixth Rocket Flight: November 20, 1988

The coronal jets and explosive events observed with the HRTS have been suggested as potential sources of the solar wind. However, the HRTS has never obtained spectra in a coronal hole on the solar disk which is generally considered to be the source region for high speed coronal wind streams. EUV spectra obtained by Rottman, Orrall and Klimchuk (1981,1982) suggest a relative blueshift in coronal holes on the disk but these spectra lack an absolute wavelength calibration. The HRTS spectra can be referenced to narrow chromospheric line to determine a near absolute velocity scale. The sixth rocket flight was successful in observing a well defined coronal hole on the disk. The C IV spectra show that there is a significantly greater probability of outflows inside the coronal hole. Approximately 26% of the coronal hole shows blueshifts and the average blueshift velocity is 5 km s-1. Nevertheless, the average velocity in the coronal hole is a 2 km s-1 downflow [71].

In addition to the coronal hole, an emerging active region and an emerging flux region were within the field of view of the slit rasters. A map of the positions of the explosive events in these regions showed a greatly enhanced population. In the emerging active region, the explosive events outlined the borders of high magnetic flux regions and, in particular, the magnetic neutral line. In the emerging flux region, the explosive events occurred along three well defined lines, apparently again demarcating the neutral line of the emerging flux and the boundary between the new flux and the preexisting flux. Combined with the strong evidence in the Spacelab 2 data that explosive events were associated with emerging flux, it was proposed that the mechanism driving the explosive events was magnetic reconnection whether in the emerging flux regions or in the majority of the explosive events which have , as yet, no defined relationship with magnetic signatures. We have further proposed that the majority of explosive events are the result of the process of magnetic cancellation of photospheric flux elements observed in the Big Bear video magnetograph and identify this process as magnetic reconnection. By equating the velocity of the explosive events with the local Alfven speed and measuring the density with density sensitive line ratios of O IV, a magnetic field strength of 20 gauss is derived for the reconnection volume [80].

The Seventh Rocket Flight: November 21, 1990

The seventh rocket flight of the HRTS occurred jointly with a launch of the AS&E X-ray telescope to obtain nearly simultaneous X-ray images and ultraviolet spectra of a flare- producing active region. The payloads were ready for flight from White Sands in early June but the Sun decided that it would remain inactive for a period of about 2 months at the peak of the solar cycle. Finally, a flaring active region appeared with a tongue of one magnetic polarity intruding into plage of the opposite magnetic polarity. Flares occurred along this tongue throughout the day. The two rockets were fired within a half hour of each other and both observed a solar flare. The HRTS spectra showed that the core of the hot flare, seen in Fe XXI 1354 (107 K), was only a few arc-seconds in extent and bridged the neutral line on one side of the intruding polarity. Explosive events were also observed on both sides of this tongue. The first ultraviolet spectra of an Ellerman bomb were obtained. These showed extremely wide profiles in transition zone lines with velocities of up to 200 km s-1. The first concrete association of an explosive event with changes in the photospheric magnetic configuration was found, in support of the idea that the explosive events are the direct result of magnetic reconnection.

The Eighth Rocket Flight: August 24, 1992

On August 24, 1992 the NRL High Resolution Telescope and Spectrograph (HRTS) was launched on a Black Brant rocket to observe an active region at the solar limb in the ultraviolet with 1 arc-second resolution. The instrument configuration consisted of a 30 cm gregorian telescope, a tandem-Wadsworth spectrograph, an ultraviolet spectroheliograph and an H spectroheliograph with video and photographic cameras. In order to reduce stray light at the spectrograph slit, a disk occulter was placed at the prime focus of the primary mirror and a Lyot stop was employed as well. The spectrograph covered the 1850-2670 wavelength range which includes strong lines of Fe II, Si II, Si III, C III and Fe XII spanning the 104 - 106 K temperature range. The spectrograph slit was curved to conform to the solar limb and was successively stepped above the limb in 0.10 R increments. The spectroheliograph included a new narrow band filter to image the C IV emissions near 1550 .

Above the limb, the C IV image shows that the active region transition zone consists of a variety of threads, twisted threads, disconnected threads and point-like objects that extend high above the limb and bear more resemblance to a prominence than to typical coronal loop structures. On the disk, the active region seen in C IV consists of diffuse structures, small loops and bright points. The quiet sun C IV structures on the disk are generally elongated, suggesting the extensions of the spicules but the fine, high-contrast sorts of structures seem above the limb are not apparent.

The characteristics of the coronal line profiles observed in the ultraviolet spectra show a marked contrast to those of the chromospheric and transition region lines. The coronal lines reveal only slowly varying changes in intensity as a function of position while the chromospheric and transition region lines show considerable structure at the 1 arc-second resolution of the instrument. Velocities in the coronal lines are uniformly low whereas the chromospheric and transition region lines show strong variations. Supersonic velocities as high as 70 km s-1 are seen in loop structures and may be related to the high speed downflows seen in sunspot spectra on the disk in previous HRTS flights. Density sensitive line ratios of Si III/C III and Fe XII are available to determine pressures and fill factors once the instrumental calibration is completed.

The Next Rocket Flight

The HRTS is being reconfigured to observe the near-UV lines of Mg I and Mg II near 2700A. The intent of this flight is to study the variations in the Mg I and II profiles to understand their contribution to observed changes in the solar irradiance. This will be accomplished by rastering the slit across a sunspot, a plage and regions of the quiet sun. The tentative launch date is Fall 1996.

References

Also see A Bibliography of HRTS Scientific Publications



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