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THE FREJA MISSION THE FREJA MISSION edited by RICKARD LUNDIN Swedish Institute ofS pace Physics. Kiruna. Sweden GERHARD HAERENDEL Max-Planck-Institut jUr Extraterrestrische Physik. Garching bei Munchen. Germany and SVENGRAHN Swedish Space Corporation. Stockholm. Sweden Reprinted from Space Science Reviews, VoI. 70, Nos. 3-4, 1994 SPRINGER SCIENCE+BUSINESS MEDIA, B.V. A C.I.P. Catalogue record for this book is available from the Library of Congress. ISBN 978-94-010-4132-4 ISBN 978-94-011-0299-5 (eBook) DOI 10.1007/978-94-011-0299-5 Printed an acid1ree paper AII Rights Reserved © 1995 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1995 Softcover reprint ofthe hardcover Ist edition 1995 No pact of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. TABLE OF CONTENTS R. LUNDIN, G. HAERENDEL, and S. GRAHN / The Freja Science Mission [1] J. S. MURPHREE, R. A. KING, T. PAYNE, K. SMITH, D. REID, J. ADEMA, B. GORDON, and R. WLOCHOWICZ / The Freja Ultraviolet Imager [17] G. PASCHMANN, M. BOEHM, H. HOFNER, R. FRENZEL, P. PARIGGER, F. MELZNER, G. HAERENDEL, C. A. KLETZING, R. B. TORBERT, and G. SARTORI/The Electron Beam Instrument (F6) on Freja [43] FREJA MAGNETIC FIELD EXPERIMENT TEAM / Magnetic Field Experiment on the Freja Satellite [61] G.T. MARKLUND, L.G. BLOMBERG, P.-A. LINDQVIST, c.-G. FALTHAMMAR, G. HAERENDEL, F .. MOZER, A. PEDERSEN, and P. TANSKANEN / The Double Probe Electric Field Experiment on Freja: Experiment Description and First Results [79] M. BOEHM, G. PASCHMANN, J. CLEMMONS, H. HOFNER, R. FRENZEL, M. ERTL, G. HAERENDEL, P. HILL, H. LAUCHE, L. ELIAS SON, and R. LUNDIN / The TESP Electron Spectrometer and Correlator on Freja [105] B. A. WHALEN, D. J. KNUDSEN, A. W. YAU, A. M. PILON, T. A. CAMERON, J. F. SEBESTA, D. J. MCEWEN, J. A. KOEHLER, N. D. LLOYD, G. POCOBELLI, J. G. LAFRAMBOISE, W. LI, R. LUNDIN, L. ELIAS SON, S. WATANABE, and G. S. CAMPBELL / The Freja F3C Cold Plasma Analyzer [137] L. ELIASSON, O. NORBERG, R. LUNDIN, K. LUNDIN, S. OLSEN, H. BORG, M. ANDRE, H. KOSKINEN, P. RIIHELA, M. BOEHM, and B. WHALEN / The Freja Hot Plasma Experiment - Instrument and First Results [159] B. HOLBACK, S.-E. JANSSON, L. AHLEN, G. LUNDGREN, L. L YNGDAL, S. POWELL, and A. MEYER / The Freja Wave and Plasma Density Experiment [173] P.-A. LINDQVIST, G. T. MARKLUND, and L. G. BLOMBERG / Plasma Characteristics Determined by the Freja Electric Field Instrument [189] THE FREJA SCIENCE MISSION R. LUNDIN Swedish Institute of Space Physics, Kiruna, Sweden G.HAERENDEL Max-Planck lnstitut fur Extraterrestrische Physik, Garching, Germany and S.GRAHN Swedish Space Corporation, Stockholm, Sweden (Received 11 April, 1994) Abstract. Freja*, a joint Swedish and German scientific satellite launched on october 6 1992, is designed to give high temporal/spatial resolution measurements of auroral plasma characteristics. A high telemetry rate (520 kbits S-I) and ~15 Mbyte distributed on board memories that give on the average 2 Mbits S-I for one minute enables Freja to resolve meso and micro scale phenomena in the 100 m range for particles and I -10m range for electric and magnetic fields. The on-board UV imager resolve auroral structures of kilometer size with a time resolution of one image per 6 s. Novel plasma instruments give Freja the capability to increase the spatial/temporal resolution orders of magnitudes above that achieved on satellites before. The scientific objective of Freja is to study the interaction between the hot magnetospheric plasma with the topside atmosphere/ionosphere. This interaction leads to a strong energization of magnetospheric and ionospheric plasma and an associated erosion, and loss, of matter from the Terrestrial exosphere. Freja orbits with an altitude of ~600- 1750 km, thus covering the lowcr part of the auroral accelcration region. This altitudc range hosts processes that heat and energize the ionospheric plasma above the auroral zone, leading to the escape of ionospheric plasma and the formation of large density cavities. 1. Introduction Plasma is the main state of matter in the Universe, the condensed state being limited to rare islands of 'space debris' (planets, asteroids, etc.) or to thinly dis persed interstellar/interplanetary gas/dust. Our largest plasma object - the Sun - represents an almost 1000 times higher mass compared to the accumulated mass of condensed matter in our solar system. In addition the Sun provides through its solar wind a plasma environment for all other bodies within the heliosphere. The result of an intrinsic magnetic field of these bodies is that a 'magnetosphere' is formed immersed in the plasma flow from the Sun. Our magnetosphere, though controlled by electromagnetic forces from the Sun, is populated to a large degree by plasma of terrestrial origin. In general, plasma systems organized by magnetic fields may result in a strong chemical separation of matter into cellular structures - * Freja, the goddess of fertility in Nordic mythology, was not a gentle' Afrodite of the north'. She was the empress of Folkvang, the estate of the Nordic Gods, and she stood close to Odin, the almighty. She is a female warrior like Pallas Athena in the Greek mythology. Her power encompassed life and death, love and battle, fertility and black magic. Half of the heros killed in battle were her toll, sent to her for her amusement. [1] Space Science Reviews 70: 405-419, 1994. © 1994 Kluwer Academic Publishers. 406 R. LUNDIN ET AL. with very modest flow of matter across the 'cell walls', as noted by Alfven (1981) in discussions concerning The Plasma Universe. The Sun represents not only the main source of energy but also a strong source of matter interacting with the planets in our solar system. The solar wind, the loss of matter from the Sun, may correspond to about five Earth masses per billion years. This wind of solar plasma interacts with, e.g., planets and comets in a complicated way that is highly dependent on their intrinsic magnetic properties. An important common consequence of the interaction is, however, the erosion of matter from planets and comets as a result of the energy and momentum transfer from the solar wind. The Earth is more effectively protected from this interaction by a strong intrinsic magnetic field which provides a 'magnetic umbrella' shielding its topside atmosphere. Despite this the Earth is losing matter at a rate of 2-4 kg s -1 (Chappell et al., 1987) as a result of the heating and acceleration of plasma along the auroral oval topside ionosphere. The loss rate may appear high, but it is nevertheless insignificant on cosmogonic time scales. It will for instance take at least 10 billion years to evacuate the present terrestrial atmosphere at that rate. Other planets, such as Venus and Mars may have been less fortunate in retaining a hydrosphere and a 'habitable' atmosphere due to their lack of a strong intrinsic magnetic field. There, the solar wind interacts direclty with the entire fronts ide ionosphere and the momentum transfer and relative atmospheric loss rate becomes correspondingly larger. The Sun-Earth relationship is complex involving direct electromagnetic forc ing from radiation as well as indirect electromagnetic forcing from the solar wind plasma interacting with the terrestrial magnetic field. Because the solar radiation input is 4 to 5 orders of magnitude higher than the solar wind energy input, one would expect solar radiation to dominate the dynamics of the topside ionosphere. However, contemporary space research has demonstrated that the solar wind forc ing may become more important than solar radiation for the redistribution and loss of ionospheric plasma. There is a strong coupling between the ionization of the upper atmosphere by solar UV and EUV and the electromagnetic forcing induced by solar wind plasma interacting with the Earth's magnetic field. An increased conductivity in the upper atmosphere due to ionization means that electromagnetic energy more easily dissipates there and a complex chain of transport, dissipation, and loss can be set up as a consequence of the magnetosphere-ionosphere coupling. In fact, dissipation of solar wind electromagnetic energy to the topside atmosphere and ionosphere of the Earth requires a finite electrical conductivity, the solar wind electromagnetic energy being dumped to the ionosphere as waves/electric fields (providing Joule heating) or via charged particle precipitation. The former (Joule heating) can be described by a current circuit analogy where the field-alignedlBirkeland currents connects the ionospheric load to the solar wind dynamo. Although Birkeland currents and their connection to the solar wind dynamo were extensively studied during the seventies and eighties (e.g., Ijima and Potemra, 1976) our knowledge of the properties of the [2] THE FREJA SCIENCE MISSION 407 source (dynamo) region remains relatively poor. The implications of the Birkeland currents are better known, though. The most important one being their relation to the auroral energization process. Aurora, in particular its most spectacular and intense form, the discrete aurora, manifests itself over the polar regions as a result of plasma energization processes primarily taking place in the altitude range 1000 to 10000 km. This has been a well accepted fact for about 15 years now - gradually evolving through an era of rising maturity of space exploration. The first in-situ measurements, performed in the sixties by sounding rockets, of monoenergetic beams of electrons over auroral arcs (e.g., McIlwain, 1960; Albert, 1967; Evans, 1968) and strongly field-aligned fluxes of electrons (Hoffman and Evans, 1968) became qualitative evidence for the hypothesis by Alfven (1958) of electric potentials being established along the magnetic field lines. During the seventies various models for the acceleration region above the auro ral oval were moulded from numerous measurements by sounding rockets and low-altitude orbiting satellites. For instance, field-aligned pitch angle distributions and monoenergetic beams of electrons with rising and subsequently falling energies versus latitude, denoted inverted V : s, were interpreted as regions with parallel electric fields (e.g., Frank and Ackerson, 1971; Gumett and Frank, 1973). The prime obstacle for establishing parallel electrostatic fields as proposed by Alfven, the apparent infinite conductivity along magnetic field lines, was removed by intro ducing, e.g., 'anomalous' resistivity due to wave-particle interaction (Papadopoulos and Coffey, 1974). The first modelling attempt by Knight (1973) and Lennartson (1976) introduced a scheme whereby a nonresistive acceleration may occur simply because of the magnetic mirroring geometry. A quite successful model of the electron distribution resulting from such a nonresistive acceleration, including the backscattering of electrons, was introduced by Evans (1974). Many other models also followed based on the concept by Evans (see, e.g., FaIthammar, 1983, for a review). Further indirect means of establishing a current-voltage relationship in the nonresistive acceleration region were made from sounding rocket measurements (e.g., Lundin and Sandahl, 1978; Lyons et al., 1979). Conclusive evidence for the mid-altitude energization of ions and electrons came from the S3-3 satellite, traversing for the first time with appropriate instrumentation the core of the acceleration region. The S3-3 particle and electric field data (e.g;, Shelley et at., 1976; Mizera and Fennell, 1977) frequently displayed many of the characteristic features expected from an electrostatic acceleration process with, e.g., electrons accelerated in one direction (downward) and ions accelerated in the opposite direction (upward). A comparison between measured and simulated particle distribution functions within a field-aligned electrostatic potential (e.g., Chiu and Schultz, 1978) also showed good quantitative agreement with theory. The discovery of perpendicular electric 'shocks' (Mozer et al., 1977) and weak double layers (Temerin et at., 1982) introduced further complexity to the perhaps [3] 408 R. LUNDIN ET AL. too primitive conjecture of electrostatic field-aligned potentials along magnetic field lines. Instead of constituting a smooth potential well structure the electric field measurements implied considerably more temporaVspatial variations acting within the acceleration region. DE-l and in particular Viking confirmed this considerably more complex picture of the auroral energization region. The launch of DE-l in 1981 into a somewht higher orbit (apogee:::::: 23 000 km) than S3-3 (apogee:::::: 8000 km) added further discoveries from the auroral energiza tion region. For instance, so-called 'electron conic' signatures (Menietti and Burch, 1985) are features of the electron distribution function that cannot be attributed to a simple electrostatic acceleration. On the other hand, the high-low aspect intro duced by the co-orbiting DE-l and DE-2 indicated that at least the time-averaged properties of the acceleration region were in good agreement with mid-altitude acceleration constituting a quasi-electrostatic potential well structure (Reiff et al., 1988). Similarly, the launch of Viking in 1986 and the very detailed measurements of particles and fields carried out during the about I-year lifetime of the space craft marks another landmark for the understanding of the mid-altitude auroral energization region. The apogee of Viking (::::::13 500 km) was ideal because that appears to mark an upper bound of at least the dayside midaltitude energization. a Most of the traversals of the AKR generation are below:::::: 1 000 km, indicating that the main field-aligned potential is below that altitude (e.g., Bahnsen et al., 1989). This upper altitude limit is consistent with Viking ion data of field-aligned beams (Thelin et al., 1990). Viking data also showed that transverse energization of ions continued up to at least the Viking apogee. Transversely energized ions, first studied in detail from low-altitude satellites such as ISIS 2 (e.g., Klumpar et al., 1979), have also been observed by sounding rockets at altitudes as low as 500 km (Whalen et al., 1978). Figure 1 illustrates the meso-scale structure of the parallel acceleration region driven by a magnetospheric dynamo which converts (magnetospheric) plasma kinetic energy into electromagnetic energy that drives the auroral current circuit. Energy dissipation within the current circuit takes place in primarily two regions, the topside ionosphere as plasma heating/acceleration and the mid-altitude accel eration region as field-aligned accelerated beams of electrons and ions. The fig ure illustrate an important consequence of the auroral acceleration processes, the energization and evacuation of the topside ionospheric plasma that leads to the formation of plasma cavities. 2. The Freja Mission The Freja mission represents a continuation of a line of magnetospheric research that began with the first Swedish satellite Viking. As on Viking, participation in the instrumentation for Freja is spread over a number of countries, but the satellite is mainly funded by Sweden (::::::75%) and Germany (::::::25%). The low cost approach [4] THE FREJA SCIENCE MISSION 409 AURORAL ACCELERA nON - MESO SCALE DE-/ -2.2 (8000 000 km) \ \ ........... ···.;:·: . ..... ·l .....\ VIKING (2000 - .D~namo , J,l \ . EJ: : • S3-3 . AKEBONO \ ... : (1500· 8 000 kmJ..._ ..... , ...... y\ \ . "·7.. .-. .-...-..-.... .,. . . .. .. "' Mid-Altitude Plasma Low-Altitude Plasma Evacuation + plasma cavities Meso Scale: • Inverted V,s • Discrete arcs • Evacuation Regions Fig. I. A diagrammatic illustration of the auroral acceleration region and its associated current circuit driven by a magnetospheric dynamo. The acceleration/energization of ionospheric plasma lead to evacuation and the formation of plasma density cavities. on Freja follows a highly successful tradition such as Viking and AMPTE (Ger many). A piggyback launch on the Chinese Long March 2C and a very stramlined project organization is utilized to keep the costs of the satellite+launch to within 100 million Swedish kronor (~13 million US dollars in 1993). Despite the low cost the project is not compromising much on system safety or payload quality. For instance, critical housekeeping systems such as transponders, etc., are doubled. Eight PI-instruments for particles, fields and auroral imaging provide fine-structure plasma measurements with an hitherto unprecedented temporal/spatial resolution from satellites. The satellite has the shape of a spinning disk with a diamter of 2.2 m. The axis of the disk points in the direction of the Sun so that the solar panel on the top of the satellite will be constantly illuminated. A relatively high power (max. 137 W) [5] 410 R. LUNDIN ET AL. / STAR 13A rocket motor - Cylinder probe / - Command reception antenna I / Sun sensors / ~ Solar panels b0'-=--=~~~ -Interface ring '.Jith CZ -2C Wire boom L assembly Three aXIs '.Jave magnetometer Auroral L S-band transmit anrenna Imager STAR 68 rocket motor Fig. 2. View of the Freja spacecraft with some of its equipment and experiments. is obtained from the solar panels. The spacecraft is kept in a Sun-pointing position using magnetic torquing technique. Freja was successfully launched as a piggy-back satellite together with a Chi nese satellite on a Chinese Long-March II rocket on October 6, 1992. After launch into a 63° inclination orbit Freja was lifted to a higher perigee (~600 km) and apogee (1750 km) by means of two separate solid fuel boost motors. The relatively low inclination means that good auroral oval coverage is obtained mainly over the American continent. Thus the Canadian Prince Albert ground station, is an important gorund segment for the direct transmission of data. However, Esrange in Sweden is the main operations centre where all uplink communications are per formed. As a third station, the Japanese Antarctic Syowa station is used to take data near perigee over the southern hemisphere. A key feature of the measurements is a high temporal and spatial resolution, made possible by the high data rate which Freja provides. For instance, Freja provides a data rate of 256-524 kilobits per second (kbps) for the 15-25 min passes over the two ground stations Esrange in Kiruna (Sweden) and Prince Albert (Canada). On board memories (> 15 Mbytes) makes it possible to provide burst mode snapshots with > 2 Mbps for 30-60 s. The on -board memory can also be used for storing data along orbits out of reach of ground stations (compressed mode) alternatively for providing overview data. A diagrammatic view of Freja is shown in Figure 2 and some data of the spacecraft and orbit are summarized in Table I. [6]
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