INTERNATIONAL SPACE SCIENCE INSTITUTE No 9, June 2002 Published by the Association Pro ISSI The Fourfold Way Through the Magnetosphere: The Cluster Mission Editorial Imagine a lonely spacecraft trav- currently Director of the Inter- Impressum elling through space. Its mission national Space Science Institute in is to probe the value of, say, the Bern and Senior Scientists at the strength of the local magnetic Max Planck Institute for extra- field. Its instruments continuously terrestrial Physics in Garching SPATIUM monitor the field strength and de- (Germany) is one of the fathers of Published by the liver the data to the on-board Cluster. He is also the Principle Association Pro ISSI telecommunication system, which Investigator of the Electron Drift twice a year forwards them to the ground con- Instrument designed to determine trol station on Earth. There, scien- the strength of the local electric tists try to interpret the data. field around the spacecraft. INTERNATIONAL SPACE SCIENCE But now a basic dilemma arises: In November 2001, very shortly INSTITUTE due to the continuous motion and after the commissioning of the Association Pro ISSI temporal evolution of the ambi- Cluster spacecraft G. Paschmann Hallerstrasse 6, CH-3012 Bern ent medium and its boundaries, presented the first results to our Phone ++41 31 631 48 96 the recorded data are a complicat- members. In the mean time, this Fax ++41 31 631 48 97 ed mix. The observer cannot dis- mission has delivered a wealth of tinguish whether the magnetic additional data, which partially President field experienced a change in time could be considered in the present Prof. Hermann Debrunner, or it varied spatially or both hap- revised version of his lecture. We University of Bern pened simultaneously. are very indebted to G. Pasch- Publisher mann for his kind permission to Dr. Hansjörg Schlaepfer, That was the problem faced by publish the present text on the legenda schläpfer wort & bild, the fathers of the European Space fascinating world of magnetic Winkel Agency’s Cluster mission in 1982 fields. Layout intended to exploring the Earth’s Marcel Künzi, marketing · kom- magnetosphere. Their idea was to munikation, CH-8483 Kollbrunn solve the problem by flying a Printing quartet of closely spaced identical Hansjörg Schlaepfer Druckerei Peter + Co dpc spacecraft, since time and space Bern, June 2002 CH-8037 Zurich need a set of three co-ordinates to be defined completely. The in- struments aboard the four space- craft may now take measure- ments at identical times but at different places in space allowing the scientists to work out the rele- vant temporal and spatial charac- teristics of the magnetic field and Front Cover: The aurora is the the other quantities that charac- only visible proof of the Earth’s terize the ambient medium, such magnetosphere. This wonderful as the electric field, and plasma picture shows the glow from out- density, velocity and temperature. bursts of ionic particles from the Sun in the ionosphere. (Credit & The author of the present issue Copyright: Dennis Mammana, of Spatium, Dr. Götz Paschmann, Skyscapes) SPATIUM 9 2 The Fourfold Way Through the Magnetosphere: The Cluster Mission *) Götz Paschmann,International Space Science Institute,Bern and Max Planck Institut für extraterrestrische Physik,Garching Introduction ral evolution. Cluster is the first forward. One should not forget attempt to do this in space, admit- though that because of the com- tedly with only the minimum plex structure of the magnetos- number that can be arranged in phere and the strong interrela- three dimensions, namely four. tionship between its parts, there is The Earth’s magnetosphere has There have previous attempts the danger that spacecraft flying in been investigated by direct in-situ with two spacecraft (ISEE and close formation will miss the measurements since the begin- AMPTE), but with two stations large-scale context. Fortunately, ning of the space age and has one can only infer variations along observations from the ground and turned out to be the site of many the line between the spacecraft, from other spacecraft are available fascinating phenomena and pro- and three stations always form a that serve to fill in this global cesses that must be studied in-situ plane. Thus Cluster is a giant step context. because they are not accessible by remote sensing techniques. Exam- ples are shock formation, magnet- ic reconnection, particle accelera- Interplanetary Magnetic Field tion, wave-particle interaction and Tail Current turbulence. While interesting in their own right, as part of the ex- ploration of the Earth’s environ- ment, they can also guide our Plasma Mantle understanding of similar pro- cesses on the Sun or the distant Magnetotail Northern Lobe universe, places where one can never make in-situ measure- Polar Cusp ments. Figure 1 shows a view of the Plasma Sheet Plasma- Neutral Sheet magnetosphere that emphasizes sphere Current its main features, except for the t bow shock that has been left out n e r for clarity. u r Ring C While much has been learned Magnetopause about the magnetosphere through Solar Magneto- measurements from single satel- pause Field-Aligned Current Current lites, a limitation has been the Wind fundamental inability of a single observer to unambiguously dis- tinguish spatial from temporal changes. The magnetosphere is constantly changing its shape and Figure 1 size, and many of the processes Cutaway view of the Earth’s magnetosphere that identifies the key regions en- countered by Cluster: the solar wind approaching from the left; the magnetopause governing it are known to vary on (yellow surface) that is the outer boundary of the magnetosphere; the polar cusp, a short spatial and temporal scales. funnel-shaped indentation in the magnetopause; the magnetotail, with the hot A solution to this dilemma is a plasma sheet at the centre and the lobes on either side. Thin black lines with arrows fleet of mobile stations, very are the magnetic field lines, dashed lines show plasma flow. The black arrows mark electric current flow. The bow shock that stands in the solar wind ahead of the much like what one does to study magnetopause, and the region in between, the magnetosheath, have been omitted weather patterns and their tempo- for clarity. th *) Pro ISSI lecture, Bern, 8 November 2001 SPATIUM 9 3 The Cluster Spatium Nr. 2. Cluster was not so and successfully launched on two fortunate. Ready for launch in Soyuz-Fregate launchers from Mission 1995, the four spacecraft were de- Baikonur on 16 July and 12 August stroyed when the Ariane 5 rocket 2000, respectively. After extensive exploded during its maiden flight commissioning of the payload, the on 4 June 1996. At first it seemed science phase officially began on that the only remaining option 1 February 2001. was to put together a single space- History craft from the leftover parts, ap- This being a Pro ISSI lecture, I propriately named Phoenix after should point out that several peo- Cluster’s history goes back to the mythical bird that rose from ple associated with ISSI were November 1982, when a group of the ashes. But recognizing the deeply involved in Cluster. Two European scientists proposed the unique importance of the mis- of its directors, Bengt Hultqvist mission. In February 1986 ESA sion, the ESA Science Programme and myself, and a long-time mem- chose Cluster and the Solar and Committee (SPC) on 3 April ber of its Science Committee, Heliospheric Observatory (SOHO) 1997 agreed that three new Gerhard Haerendel, were among as the first “Cornerstone” in its Cluster spacecraft should be built the seven who proposed the mis- Horizon 2000 Science Pro- alongside Phoenix, and thus sion in 1982. Roger Bonnet, then gramme. SOHO was launched in Cluster was reborn. Through a Director of Scientific Programmes December 1995 and has sent back tremendous effort by all involved, at ESA, was the key force behind a stream of exciting data about the fully instrumented satellites the recovery initiative, and Hans the Sun, as vividly described in were rebuilt in only three years Balsiger was chairman of ESA’s ck magnetosheath sho w bo magnetopause Bow shock Polar cusp cusp Auroral zone neutral sheet tra region Plasmas- phere lobes Magnetopause Solar wind Figure 2 Cluster orbit in February, as it cuts through the key regions of the magnetosphere. SPATIUM 9 4 SPC, when the decision to repeat superimposed on a cut of the mag- 10 5 Nominal phase Extension phase Cluster was finally made in April netosphere, and illustrates that 1997. Last but not least, Hans Cluster moves outbound over the 10 4 Peter Schneiter and Contraves northern polar cap, crosses the mag- 10 3 Space had a leading role in the re- netopause and bow shock into the max vival efforts by the industry that solar wind, before recrossing those min 10 2 had built Cluster. It is therefore boundaries in reverse order and cusp only natural that the interpretation moving over the southern polar tail 10 1 of the Cluster measurements will cap back towards perigee. Figure 2 distance [km]Inter-spacecraft Aug-00 01 02 03 04 05 date be one of ISSI’s future workshop applies to February of each year. Figure 4 activities. As the orbit is inertially fixed, it Spacecraft separation distances as rotates around the Earth once a planned for the entire mission. year, as the latter revolves around Orbits and Separations the Sun. As a result, the apogee regular tetrahedron when cross- of the orbit will be located in the ing the major boundaries. The After their successful launch from geomagnetic tail half a year later, separation distances between the Baikonur, the Cluster spacecraft as shown in Figure 3. In the course spacecraft are being changed be- were placed in nearly identical, of the mission Cluster will thus tween 100 km and tens of thou- highly eccentric polar orbits, with encounter the regions of interest sands of km during the mission.