IL NUOVO CIMENTO VOL. 15 C, N. 5 Settembre-Ottobre 1992

EUV-FUV Spectroscopy of TSS Optical Phenomena (*).

C. BATALLI COSMOVICI(') and R. STALIO (2)

(') lstituto di F~sica dello Spazio Interplanetario del CNR, 00044 Frascati, Italia ('~) Dipartimento di Astronomia, Universit~ di Trieste, Italia

(ricevuto il 14 Luglio 1992)

S u m m a r y . - We propose to use the IEH (International Ultraviolet Hitch- hiker), a multidisciplinary facility (Astronomy, Solar System, Earth's atmo- sphere) to be mounted on the Shuttle pallet as a Hitchhiker flight opportunity, in order to obtain 2D images in the EUV-FUV ((400 -- 1300) .~) of the optical phenomena occurring in the neighborhood of the TSS satellite. These peculiar phenomena, not detectable during the first TSS mission, are primarily due to the interaction of a high-potential conductive body with the surrounding ionospheric plasma.

PACS 94.20 - Physics of the ionosphere. PACS 95.85.Mt -Astronomical observations: ultraviolet ( 1 0 0 - 3000) /~

1. - Spacecraft glow.

Orbiting vehicles were found to emit optical radiat ion while travelling through the a tmosphere [1]. During the third Shut t le flight in 1982, images taken by the as t ronauts have shown an orange glow over the Shut t le tail and thrus te rs concen- t ra ted a round the areas exposed to the ,orbital wind,. The glow spreads for about 20 cm over the Shut t le surface.

This p h e n o m e n o n is due main ly to chemica l processes which are e n h a n c e d because of the catalytic effect of the orbi ter ram surfaces [2]. Atomic oxygen at 250 km alt i tude cont r ibutes with about 80% to the a tmospher ic gas composit ion. It in terac ts with the Shut t le surface at 8 k m / s (orbital velocity) cor responding to an energy of 1.3 eV. This collisional energy and the high ior iospher ic t em p e ra tu r e (about 1000 K) make oxygen ex t remely reactive and lead to the format ion of NO and NO2 molecules on the materials surface. These molecules are then re leased in an exci ted state emit t ing photons in the orange spectral region.

(*) Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27-30, 1990.

703

704

2. - Electron b e a m s emit ted from the orbiter.

C. BATALLI COSMOVIC1 and R. STALIO

Beside these phenomena observable in all Shuttle flights, during the TSS-1 mission two electron guns will be mounted on the Shuttle pallet: the Italian EGA (Electron Gun Assembly), operating mainly in a continuous mode with a max- imum current of 750 mA and the American FPEG (Fast-Pulse Electron Gun) operating in a pulse mode at about 100 mA.

The physics of the interaction between spacecraft-emitted electron beams and the ionospheric plasma is not well understood. Previous measurements on Shuttle flights STS-3 and STS-9 on the optical luminosities observed showed clearly that there exists an anomalous broadening of the beam near the Electron Gun and it appeared that the beam plasma discharge takes place already at a level of 100 mA beam. In some cases the beam was seen to strike the orbiter's surface causing a visible fluorescing region.

At low current levels ( < 100 mA) the orbiter was lit up, while the short-duration electron pulses were emitted.

It is not clear whether the source of the light may be caused by some beam plasma discharge phenomena surrounding the orbiter or by fluorescence from returning electrons which were collected by the orbiter to neutralize itself.

In sia, electron detectors have seen strong return fluxes of electrons with energies in excess of the energy of the emitted beam energy. It seems that nonlinear plasma effects would accelerate the electrons above the energy of the injected beam.

We could demonstrate in our Plasma Chamber at Frascati, when the EGA was tested and its interaction with the surrounding neutrals and ions was observed at different pressures and magnetic-field intensities and direction, that the optical phenomena are relevant and observable also with t he naked eye. The TV-cameras mounted on the Shuttle pallet and the TOP-camera [3], a special spectrophotometric camera which will be used by the astronauts, will cover the monitoring of the optical phenomena (in the visible) due to the interaction of the electron beams with the ionospheric plasma in order to correlate electrodynamic with optical measurements.

3. - TSS optical p h e n o m e n a .

The Tethered Satellite System (TSS), according to the last NASA flight schedule, should be flown in 1992.

The satellite will be deployed upward from the Shuttle and connected by a 20 km long conducting tether in order to carry out electrodynamic investigations in the Earth's ionosphere at about 300 km altitude. (fig. 1).

All these investigations are based on the exploitation of the Faraday-induced electric field and the Lorentz force associated with the interaction between the orbiting system velocity, the geomagnetic field and the tether current [4].

The current passing through the tether will strongly depend on the plasma sheaths near the tether ends because of the high impedance introduced in the equivalent electric circuit. The TSS-1 configuration utilizes the subsatellite as passive electron collector, while the orbiter is equipped with an electron gun (EGA) to enhance the quite low ion-collected current with a high amount of electrons expulsion.

E U V - F U V SPECTROSCOPY OF T S S OPTICAL PHENOMENA 705

Fig. l. - Artistic ~qew of the TSS 1 mission with the satellite deployer above the Shuttle. (Courtesy by Alenia Spazio)

706 c. BATALLI COSMOVICI and R. STALIO

A more promising method to enhance the emitted or collected electron current is to generate two high-density plasma clouds (plasma contactors) in the vicinity of the tether ends with the main purpose to reduce the space charge limitations in the electric-current flow process and increase the surface collection equivalent area. The most appropriate plasma sources for this purpose seem to be the ,,hollow cathodes,, and in our laboratory at Frascati many experiments have been carried out in order to optimize them, allowing their utilization in the second TSS flight [5, 6]. Since the potential of the satellite with respect to the ionospheric plasma will be of the order of 1 kV, the electrons incident on the satellite may reach sufficiently high-energies causing optical emissions. There is also the possibility that the high electric field expected around the satellite will concentrate the current and create a discharge, as already seen around a tethered rocket payload [3]. During neutral- gas releases (N2) from the satellite nozzles, the discharge phenomena will be particularly enhanced.

Therefore, on the TSS-1 mission the observation of the tethered satellite and its neutral-gas release jets provide a unique opportunity to observe the ionized plasma which is produced during a neutral gas release by the satellite's active control system.

Moreover, if a hollow cathode is used in the second TSS mission as a Plasma Contactor for the satellite, xenon or argon will be pumped in the surrounding ionosphere. A wide spectrum of ion - and neutral - lines can thus be used as tracer in order to visualize the optical phenomena occuring around the satellite independently from the neutral N2 release, which will occur only for few seconds at fixed times during the mission. In order to check the possibility of observing strong EUV-FUV spectral lines, detailed laboratory spectra of hollow cathodes have been obtained using the Arcetri's EUV-spectrograph [7]. The noble gases inves- tigated were Xe, Ar, Kr and Ne, covering the wavelength region between 400 and 2500 A with a resolution of 1 .~.

Figure 2 shows a typical Argon spectrum between 500 and 1000 A. The obtained line intensities are quite promising for TSS Space Spectroscopy. We

4 Ar II(932A) 3

-.h-----.

11

136 19 29

14 /28 33

10 17 )'1 21

34 ~ 30 3" 36 Ar II(519.~)

o

Z (A)

Fig. 2. - Typical EUV spectrum of Argon II excited in a hollow-cathode plasma source. Spectrum taken with the UV vacuum spectrograph of the Arcetri Observatory in the spectral region ( 5 0 0 - 1000) A.

EUV-FUV SPECTROSCOPY OF T S S OPTICAL PHENOMENA 707

propose therefore to use a small telescope mounted on the Shuttle pallet in order to achieve high-spatial resolution 2D images in the EUV-FUV spectral region of the optical phenomena occurring close to the satellite.

4. - The IEH (International Extreme Ultraviolet Hitchhiker).

The Hitchhiker Ultraviolet Telescope called IEH is developed in cooperation between the University of Trieste and the University of Arizona [8-10]. Before the Challenger disaster, three flights on the Shuttle and one on the Spartan were approved by NASA. Now, according to the last NASA flight schedule, the first flight shoud take place in 1993, about one year after the TSS-1.

Hitchhiker is the name given to the class of payloads that utilize the ,,Shuttle Payload of Opportunity Carrier~ system. It is intended to make the Shuttle available to a whole new class of users, by fitting between those who want a Get Away Special (GAS) canister with no services and those who want and can afford a lot of special services on a Spacelab pallet. The Hitchhiker concept should meet the following obiectives: 1) provide reduced lead time to flight opportunity, 2) provide increased reflight opportunities, 3) reduce integration costs and 4) maximize Shuttle load factors. The instrument described here has the clear potential to produce significant scientific results as a Hitchhiker program. Demonstration of its successful operation in this program would naturally lead to the consideration of a more ambitious instrument designed to conduct long term UV monitoring of astronomical- and solar-system objects. This instrument could also be accomodated by standard satellite buses or platforms without requirements for a high degree of three-axis stability or precise pointing [9].

The main scientific relevance of the instrument is its capability to achieve 2-dimension spectra in a spectral range ((400 -- 1300) A) essentially unexplored so far. The 2D images in the different spectral lines will permit to study in detail extended sources like comets, interstellar clouds and galaxies. In our case the ion cloud generated around the TSS satellite would be an ideal object.

The satellite will be on station for electrodynamic studies at two different altitudes: 20 km and 2.4 km.

In the first case our FOV of 3' x 15' will correspond to 18 m x 87 m and the spatial resolution (4.5" x 4.5") will correspond to 44 cm • 44 cm. In the second case the values will be reduced to 2.2 m • 10.6 m and 5.3 cm x 5.3 cm, thus giving the possibility of a detailed analysis of the optical phenomena occuring around the satellite. Moreover the instrument will be equipped with the most up-to-date technologies for UV detection: microchannel plates coupled to CCD detectors via fiber optics.

The CCD (Charge Coupled Devices) importance for space research has been demonstrated by the success of the GIOTTO-HMC Camera which provided pictures of the cometary nucleus of exceptional quality with an exposure time of only 1 ps. The IEH instrumental characteristics are given in table I and II.

The instrument is essentially an objective grating spectrograph employing a mechanical collimator to define the FOV (fig. 3). Its most important advances are: 1) the use of an areal detector having good spatial-resolution capabilities, 2) high spectral resolution (1/~) and high sensitivity. The concave grating images, the FOV on the detector, dispersing in wavelength and forming monochromatic

708 c. BATALLI COSMOVICI and R. STALIO

TABLE I. - - Characteristics of the IEH instrument.

Spectrometer 1 Spectrometer 2

Mirror size (15 • 15) cm 2 (15 x 1 5 ) c m ~ Central obscurat ion 25 cm 2 25 cm 2 Effective area 200 cm 2 200 cm 2 Mechanical collimator

Field of view 0.05 ~ x 0.25 ~ 0.05 ~ x 0.25 ~ Point source t ransmiss ion (Tr 0.59 0.59 Solid angle of acceptance 3.8 • 10 -6 sr 3.8 x 10 -6 sr

Spatial resolution 4.5" • 4.5" 4.5" x 4.5"

Spectral range (400 -- 860) /~ (840 -- 1300) A Spectral dispersion 0.4 ~ / p i x e l 0.4 ,~/pixel

Concave gratings (parabolic, holografic) Radius of curvature 2 m 2 m Ruling density 600 1 /mm 600 1 /mm Blaze angle 1.03 ~ 1.85 ~ Blaze wavelength 600 /~ 1075 A Angle of inc idence 2.06 ~ 3.70 ~ Angle of diffraction (cent re) 0 ~ 0 ~ Plate dispersion 16.7 .s 16.7 .h./mm Reflective coating Os (?) Os (?) Estimated efficiency (~/) on blaze 7% 7%

Intensif ier Cathode LiF CsI Quan tum efficency (centre wavelength) 0.40 0.30 Active area 25 m m D. 25 m m D. Wavelength t ransducer (Ph) P20 P20 Area of fiber coupler (approx) (4.25 x 25) mm 2 (4.5 x 25) m m 2

CCD (EEV, P8602) Pixel size 0.022 m m 2 0.022 m m 2 Array of Pixels 385 • 576 px. 385 • 576 px. Area (8.47 x 12.67) m m 2 (8.47 x 12.67) m m 2

TABLE II. - IEH: Instrumentation as foreseen for the f irst f l ight including the solar experiment wi th a rare-gas absorption cell.

. , Two objective grating spectrometers.

Spectral ~,-Range Resolution Aperture Sensitivity Configuration Channe l (A) (A) (cm) (*)

EUV1 499-860 1 15 x 15 10.8 Objective grating FUV 840-1300 1 15 x 15 11.3 Objective grating

(*) Point source continuum flux at full spectral resolution S/N=10, 1200 s, in units 10 -13 erg/s/cm2/A.

eo ICCD detectors. ,o Mechanical collimators. ** Solar experiment: rare-gas absorption cell.

E U V - F U V SPECTROSCOPY OF T S S OPTICAL PHENOMENA 709

/ ]

Fig. 3. - The scientific relevance of the IEH comes both from the original concept of an objective grating spectrometer which uses a single optical surface for reflecting, disperding and focalizing on an advanced detector system (ICCD) the collimated radiation from the target, and from the fact that the EUV and FUV spectral regions are still essentially unexplored. 1) Objective gratings, 2) ICCD detectors, 3) mechanical collimators.

710 c. BATALLI COSMOVICI and R. STALIO

images of an ex tended emiss ion source within the FOV, i .e. an image of the source, will be formed in the light of each of its emission lines (fig. 4).

The i n s t rumen t operates in the 400 to 1300 A region where refractive material t ransmits poorly or not at all and even reflect ions are very inefficient. The reflective coating could be Pt or SiC instead of Os, following new industr ial specifications.

The de tec tor system, as men t ioned above, is an Intensif ied Charge Coupled Device, ICCD. The intensif ier is an ,open , m ic ro ch an n e l plate with p h o s p o r / f i b e r optic ou tput coupled coheren t ly to a CCD read-out.

planetary interstellar H Ly a

wind H Ly a

auroral ~ ~ ~ J H2 bands . Itlll ,.~

. i Jupiter

i,,ter+tenar " 1 % "1600 wind He 584 /~ , ~ . ~ ~ , z z ' ~ ~ r ] ~ l / l l [ ~ ~

L++ - 1 0 ~ ~ " - + 1000

--lu _/..,.', '~, ~ ~ ' ~ wavelength (/~)

+ance 2 +00 10 6 0 0

Fig. 4. - The figure shows as example how Jupiter and the Torus of Io would be observed with the IEH, and how an image would appear giving spectral and spatial information at the same time.

5 . - C o n c l u s i o n .

We think that the the IEH could be the most suitable i n s t rumen t for observing optical TSS p h e n o m e n a in the EUV-FUV spectral region and we propose there fore to use it dur ing the TSS-1 refly, s ince it could be also a un ique complemen ta ry observational tool to the TOP-camera m o u n t e d inside the Shutt le cabin and operat ing in the visible region at a m u c h lower spatial resolution.

EUV-FUV SPECTROSCOPY OF TSS OPTICAL PHENOMENA 711

R E F E R E N C E S

[1] S. B. MENDE, O. K. GARRIOTT and P. M. BANKS: Geophys. Res. Lett., 10, 122 (1983). [2] G. R. SWENSON, S. B. MENDE and K. S. CLIFTON: Geophys. Res. Lett., 12, 97 (1985). [3] S. B. MENDE: Tether Optical Phenomena; Experiment Description (Lockheed Space

Science Laboratory Palo Alto, 1989). [4] M. DOBROWOLNY: Riv. Nuovo Cimento, 10, No. 3 (1987). [5] C. B. COsMOVlCI, G. VaNNARONI and P. STALIO: ESA Special Publicatin, 277, 429 (1988). [6] G. VaNNARONI, P. GlOVI and F. DE VENUTO: this issue. [7] C. B. COSMOWCI, G. VaNNaROm and M. Mazzom: Spectroscopy of Noble Gases in

a Hollow Cathode Plasma Source from 400 to 10000 A, submitted to J. Astron. Sci (1992).

[8] IEH-Team: The International EUV/FUV Hitchhiker (IEH), in Mass OuO'lows from Stars and Galactic Nuclei, 433 (Kluwer Academic Pubblisher, Dordrecht, 1989).

[9] R. STALIO and C. B. COSMOVICI: Conf. Proc. SIF, edited by P. L. Bernacca and R. Ruffini, Vol. 17 (Editrice Compositori, Bologna, 1989), p. 251.

[10] R. STALIO, L. A. BROADFOOT and P. S. POLIDAN: Adv. Space Res., 6, 194 (1986).