2001:
André, M.; Behlke, R.; Wahlund, J. E.; Vaivads, A.; Eriksson, A.; Tjulin, A.; Carozzi, T. D.; Cully, C.; Gustafsson, G.; Sundkvist, D.; Khotyaintsev, Y.; Cornilleau-Wehrlin, N.; Rezeau, L.; Maksimovic, M.; Lucek, E.; Balogh, A.; Dunlop, M.; Lindqvist, P-A.; Mozer, F.; Pedersen, A.; Fazakerley, A.
Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp.1471-1481
Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e., they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e., over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large scale depression wave, i.e., an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large scale wave is associated with a rather static small scale electric field, while a more turbulent backside of the large scale wave is associated with small scale time varying electric field wave packets.
Bates, I., Balikhin, M., Alleyne, H. and Andre, M.
Minimum-variance free determination of magnetosheath wave propagation vectors
Les Woolliscroft Memorial Conference Sheffield Space Plasma Meeting: Multipoint Measurements versus Theory. Proceedings (ESA SP-492) (2001) pp. 133
The direction of propagation of waves in the plasma is important when studying space plasma turbulence and the linear and nonlinear processes that occur. The usual procedure of determining this propagation direction involves minimum variance analysis. This technique has many limitations, such as not being suitable for plane-polarised waves or for multiple waves at the same frequency, but is the only way of finding the propagation direction from dual satellite measurements. With Cluster II affording simultaneous four-point measurements the reliance on minimum variance analysis is removed and for the first time the determination of wave propagation directions directly from the instrument measurements is possible. The propagation directions of magnetosheath waves measured by EFW on Cluster II are determined using this minimum variance analysis-free method. Further information, such as FGM measurements and the basic plasma parameters, are needed to make a full physical interpretation of these results
Décréau, P.M.E.; Fergeau, P.; Krasnosels'kikh, V.; Le Guirriec, E.; Lévêque , M.; Martin, Ph.; Randriamboarison, O.; Rauch, J. L.; Sené, F. X.; Séran, H. C.; Trotignon, J. G. ; Canu, P.; Cornilleau, N.; de Féraudy, H.; Alleyne, H. ; Yearby, K.; Mögensen; P. B.; Gustafsson, G.; André, M.; Gurnett, D.C.; Darrouzet, F.; Lemaire, J.
Early results from the Whisper instrument on CLUSTER: an overview
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1241-1258
The Whisper instrument yields two data sets : (i) the electron density determined via the relaxation sounder, and (ii) the spectrum of natural plasma emissions in the frequency band 2-80 kHz. Both data sets allow three dimensional exploration of the magnetosphere by the Cluster mission. The total electron density can be derived unambiguously by the sounder in most magnetospheric regions, provided it is in the range 0.25 to 80 cm-3. The natural emissions already observed by earlier spacecraft are fairly well measured by the Whisper instrument, thanks to the digital technology which largely overcomes the limited telemetry allocation. The natural emissions are usually related to the plasma frequency as identified by the sounder, and the combination of active sounding operation and passive survey operation allows a time resolution for the total density determination of respectively 2.2 s in normal telemetry mode and 0.3 s in burst mode telemetry. Recorded aboard the four spacecraft, the Whisper density data set forms a reference for other techniques measuring the electron population. We give examples of Whisper density data used to derive the vector gradient, and estimate the drift velocity of density structures. Wave observations are also of crucial interest for studying small scale structures, as demonstrated in an example in the fore-shock region. Early results from the Whisper instrument are very encouraging, and demonstrate that the four-point Cluster measurements indeed bring a unique and completely novel view of the regions explored.
Gustafsson, G.; André, M.; Carozzi, T.; Eriksson, A. I.; Fälthammar, C. G.; Grard, R.; Holmgren, G.; Holtet, J. A.; Ivchenko, N.; Karlsson, T.; Khotyaintsev, Y.; Klimov, S.; Laakso, H.; Lindqvist, P. A.; Lybekk, B.; Marklund, G.; Mozer, F.; Mursula, K.; Pedersen, P.; Popielawska, B.; Savin, S.; Stasiewicz, K.; Tanskanen, P.; Vaivads, A.; Wahlund, J. E.
First results of electric field and density observations by CLUSTER EFW based on initial months of operation
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1219-1240
Highlights are presented from studies of electric field data from various regions along the CLUSTER orbit. They all point towards very high coherence for phenomena recorded on four spacecraft separated by a few hundred kilometers for structures over the whole range of apparent frequencies from 1 mHz to 9 kHz. This presents completely new opportunities to study spatial-temporal plasma phenomena in the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.
Kauristie, K., Pulkkinen, T.I., Amm, O., Viljanen, A., Syrjäsuo, M., Janhunen, P., Janhunen, P., Massetti, S., Orsini, S., Candidi, M., Watermann, J., Donovan, E., Prikryl, P., Mann, I., Eglitis, P., Smith, C., Denig, W.F., Opgenoorth, H. and Lockwood, M.
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1683-1696
On Dec 07 2000, during 1330-1530 UT the MIRACLE allsky camera at Ny Ålesund observed auroras at high latitudes (MLAT ~76) simultaneously when the Cluster spacecraft were skimming the magnetopause in the same MLT sector (at ~ 16-18 MLT). The location of the auroras (near the ionospheric convection reversal boundary) and the clear correlation between their dynamics and IMF variations suggests their close relationship with R1 currents. Consequently we can assume that the Cluster spacecraft were making observations in the magnetospheric region associated with the auroras, although exact magnetic conjugacy between the ground-based and satellite observations did not exist. The solar wind variations appeared to control both the behaviour of auroras and magnetopause dynamics. Auroral structures were observed at Ny Ålesund especially during periods of IMF Bz negative. Then also the Cluster spacecraft experienced periodic (T ~ 4-6 min) encounters between magnetospheric and magnetosheath plasmas. These undulations of the boundary can be interpreted to be a consequence of tail-ward propagating magnetopause surface waves. Simultaneous dusk sector ground-based observations show weak, but discernible magnetic pulsations (Pc5) and occasionally periodic variations (T ~ 2-3 min) in the high-latitude auroras. In the dusk sector Pc5 activity was stronger and had characteristics consistent with field line resonance type activity. When IMF Bz stayed positive for a longer period the auroras were dimmer and the spacecraft stayed at the outer edge of the magnetopause where they observed electromagnetic pulsations with T ~ 1 min. We find these observations interesting especially from the viewpoint of previously presented studies relating poleward moving high-latitude auroras with pulsation activity and MHD waves propagating at the magnetospheric boundary layers.
Laakso, H., Grard, R., Masson, A., Moullard, O., Bale, S., Mozer, F., Pedersen, A., André, M., Ericsson, A., Gustafsson, G. and Lindqvist, P.-A.
Multi-point electric field observations in the high-latitude magnetosphere
Proceedings of the Sheffield Space Physics Conference: Multipoint Measurements Versus Theory, ESA SP-XXX, Estec, Noordwijk, 2001 (Submitted)
We use multi-point electric field observations from the four Cluster satellites to study the dynamical behavior of the high-latitude magnetosphere on February 13-14, 2001, 20-02 UT. At 20:00 UT the vehicles enter the cusp where three satellites observe a 500-volt potential drop. It implies that at lower altitudes there likely exist some parallel electric fields that accelerate electrons downward and ions upward. In the following 2-3 hours the satellites move over the southern polar cap where all four satellites pass through a number of stationary, large-scale density enhancements that are associated with 200-volt potential drops. The observed events are possibly ionospheric ion outflows, occurring during enhancements of geomagnetic activity. At 23:20 UT, the satellites move in the distant plasma sheet, and an hour later they have a brief encounter with the auroral region where a density cavity of a few degrees wide is observed. At the equatorward edge of the cavity, large electric fields of 100 mV/m are observed, which are likely related to an auroral arc. Similar observations are collected from all four satellites within a few minutes, but a detailed comparison reveals plenty of differences, apparently due to small spatial and temporal scale sizes. Near the perigee pass, the vehicles traverse the plasma trough near local midnight, where they all detect a ULF wave event. A preliminary analysis of the event shows that it is a resonant mode of a 120-sec period. Surprisingly the observations from four satellites are not well correlated, which suggests a short spatial and temporal scale for the event. A possible source mechanism for ULF waves at this local time sector is drifting ring current protons.
M. Lockwood, H. Opgenoorth, A.P. van Eyken, A. Fazakerley, J.-M. Bosqued, W. Denig, J.A. Wild, C. Cully, R. Greenwald, G. Lu, O. Amm, H. Frey, A. Strømme, P. Prikryl, M.A. Hapgood, M.N. Wild, R. Stamper, M. Taylor, I. McCrea, K. Kauristie, T. Pulkkinen, F. Pitout, A. Balogh, M. Dunlop, H. Rème, R. Behlke, T. Hansen, G. Provan, P. Eglitis, S.K. Morley, D. Alcayde, P.-L. Blelly, J. Moen, E. Donovan, M. Engebretson, M. Lester, J. Watermann, M.F. Marcucci
Coordinated Cluster, ground-based instrumentation and low-altitude satellite observations of transient poleward-moving events in the ionosphere and in the tail lobe
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1589-1612
During the interval 8:00-9:30 on January 14, 2001, the four Cluster spacecraft were moving from the central magnetospheric lobe, through the dusk sector mantle, on their way towards intersecting the magnetopause near 15:00 MLT and 15:00 UT. Throughout this interval, the EISCAT Svalbard Radar (ESR) at Longyearbyen observed a series of poleward-moving transient events of enhanced F-region plasma concentration ("polar cap patches"), with a repetition period of order 10 min. Allowing for the estimated solar wind propagation delay of 75 ± 5 min, the interplanetary magnetic field (IMF) had a southward component during most of the interval. The magnetic footprint of the Cluster craft, mapped to the ionosphere using the Tsyganenko T96 model (with input conditions prevailing during this event), was to the east of the ESR beams. Around 09:05 UT, the DMSP-F12 satellite flew over the ESR and showed a sawtooth cusp ion dispersion signature that also extended into the electrons on the equatorward edge of the cusp, revealing pulsed magnetopause reconnection. The consequent enhanced ionospheric flow events were imaged by the SuperDARN HF backscatter radars. The average convection patterns (derived using the AMIE technique on data from magnetometers, the EISCAT and SuperDARN radars, and the DMSP satellites) show that the associated poleward-moving events also convected over the predicted footprint of the Cluster craft. Cluster observed enhancements in the fluxes of both electrons and ions. These events were found to be essentially identical at all four craft, indicating that they had a much larger spatial scale than the satellite separation of order 600 km. Some of the events show a correspondence between the lowest energy magnetosheath electrons detected by the PEACE instrument on Cluster (10-20 eV) and the topside ionospheric enhancements seen by the ESR (at 400-700 km). We suggest that a potential barrier at the magnetopause, which prevents the lowest energy electrons from entering the magnetosphere, is reduced when and where the boundary-normal magnetic field is enhanced and that the observed polar cap patches are produced by the consequent enhanced precipitation of the lowest energy electrons, making them and the low energy electron precipitation fossil remnants of magnetopause reconnection rate pulses.
M. Lockwood, A. Fazakerley , H. Opgenoorth, J. Moen, A.P. van Eyken, M. Dunlop, J.-M. Bosqued, G. Lu, C.Cully, P. Eglitis, I.W. McCrea, M.A. Hapgood, M.N. Wild, R. Stamper, W. Denig, M. Taylor, J.A. Wild, G. Provan, O. Amm, K. Kauristie, T. Pulkkinen, A. Strømme, P. Prikryl, F. Pitout, A. Balogh, H. Rème, R. Behlke, T. Hansen, R. Greenwald, H. Frey, S.K. Morley, D. Alcayde, P.-L. Blelly, E. Donovan, M. Engebretson, M. Lester, J. Waterman, M.F. Marcucci
Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to reconnection pulses and FTE signatures
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1613-1640
We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster craft during an outbound pass through the mid-afternoon magnetopause ([XGSM, YGSM, ZGSM] " [2, 7, 9] RE). The events take place during an interval of northward IMF, as seen in data from the ACE satellite and lagged by a propagation delay of 75 min that is well defined by two separate studies: (1) of the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) of the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, this issue). With an additional lag of 16.5 min, the transient LLBL events correlate well with swings of the IMF clock angle (in GSM) to near 90°. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10-15 min to propagate eastward to15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by cross-correlation of the signatures seen on the four Cluster craft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found nearer dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Events (FTE), with allowance for magnetospheric ion reflection off Alfvénic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1RE in their direction of motion and show a rise in the magnetic field strength but, unlike FTEs, in general they show no pressure excess in their core and hence no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed where the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge into the exterior boundary (where the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of flux transfer events at middle altitudes.
Marklund, G.T., Ivchenko, N., Karlsson, T., Fazakerley, A., Dunlop, M., Lindqvist, P.A., Buchert, S., Owen, C., Taylor, M., Vaivalds, A., Carter, P., Andre, M. and Balogh, A.
Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere
Nature (2001) Vol. 414, No. 6865, pp. 724-727
The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field(1-3). On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth(4-11). Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted(12), but how they could be maintained is still a matter for debate(13). The spatial and temporal behaviour of the electric fields-a knowledge of which is crucial to an understanding of their nature-cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.
J. Moen, J. A. Holtet , A. Pedersen, B. Lybekk, K. Svenes, K. Oksavik, F.Søraas, M. André
Cluster boundary-layer measurements and optical observations at magnetically conjugate sites
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1655-1668
The Cluster spacecraft experienced several boundary-layer encounters when flying outbound from the magnetosphere to the magnetosheath in the dusk sector on January 14, 2001. The boundary layer was populated by magnetosheath electrons, but in not quite as high densities as in the magnetosheath itself. The Cluster ground track was calculated using the Tsyganenko 96 model for the magnetic field, which was found to be quite sensitive to the IMF orientation. Two of Cluster&rsquos boundary-layer encounters are associated with auroral intensifications in the 15-17 MLT sector west of Svalbard. NOAA-12 probed the auroral precipitation associated with the second encounter, and associated the discrete aurora in the 1630 MLT sector to an 10 keV electron beam poleward of the 30 keV electron-trapping boundary. A sequence of three moving auroral forms emanating from this activity region are likely candidates for flux transfer events. The auroral signatures are discussed in relation to earlier observations, and appear to be an example of the boundary plasma sheet on open field lines.
H. J. Opgenoorth, M. Lockwood, D. Alcayde, E. Donovan, M. J. Engebretson, A. P. van Eyken, K. Kauristie, M.Lester, J. Moen, J. Waterman, H. Alleyne, M. André, M. W. Dunlop, N. Cornilleau-Wehrlin, A. Masson, A. Fazerkerley, H. Reme, R. Andre, O. Amm, A. Balogh, R. Behlke, P.L. Blelly, H. Boholm, E. Borälv, J.M. Bosqued, S. Buchert, M. Candidi, J.C. Cerisier, Ch. Cully, W.F. Denig,R. Doe, P.Eglitis, R. A. Greenwald, B. Jackal, J. D. Kelly, Ian Krauklis, G. Lu, I. R. Mann, M.F. Marcucci, I. W. McCrea, M. Maksimovic, S. Massetti, P. M. E. Decreau, D. K. Milling, S. Orsini, F. Pitout, G. Provan, J. M. Ruohoniemi, J. C Samson, J. J. Schott, F. Sedgemore-Schulthess, R. Stamper, P. Stauning, A. Strömme, M. Taylor, A. Vaivads, J. P. Villain, I. Voronkov, J. Wild, and M. Wild
Coordinated Ground-Based, Low Altitude Satellite and Cluster Observations on Global and Local Scales During a Transient Postnoon Sector Excursion of the Magnetospheric Cusp
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp.1367-1398
On January 14, 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the postnoon dayside magnetopause over Greenland between 1300 and 1400 UT. During that interval, most probably caused by a direction change of the Solar wind magnetic field, a sudden reorganisation of the high latitude dayside convection pattern resulted after 1320 UT in an eastward and poleward directed flow-channel as monitored by the SuperDARN radar network and also by arrays of ground-based magnetometers in Canada, Greenland and Scandinavia. After an initial eastward and later poleward expansion of the flow channel between 1320 and 1340 UT UT the four Cluster spacecraft, and the field-line footprints covered by the eastward looking scan cycle of the Söndre Strömfjord incoherent scatter radar were engulfed by cusp-like precipitation and transient magnetic and electric field signatures. In addition the EISCAT Svalbard Radar detected strong transient effects of the convection reorganisation, poleward moving precipitation and a fast ion flow channel in association with auroral structures suddenly forming to the west and north of the radar. From a detailed analysis of the coordinated Cluster and ground-based data it was found that this extraordinary transient convection pattern, indeed, moved the cusp from its former pre-noon position into the late post-noon sector, allowing for the first and quite unexpected encounter of the Cusp by the Cluster spacecraft. Our findings illustrate the large amplitude of cusp dynamics even in response to moderate solar wind forcing. The global ground-based data proves as an invaluable tool to monitor the dynamics and width of the affected magnetospheric regions.
Pedersen, A., Decreau, P., Escoubet, C.P., Gustafsson, G., Laakso, H., Lindqvist, P.-A. , Lybekk, B., Masson, A., Mozer, F. and Vaivads, A.
Four-point high time resolution information on electron densities by the electric field experiments (EFW) on Cluster
Annales Geophysicae (the Special issue: First Cluster results) (2001) Vol. 19, No. 6, pp. 1483-1489
For accurate measurements of electric fields spherical double probes are electronically controlled to be at a positive potential of approximately 1 volt relative to the ambient magnetospheric plasma. The spacecraft will acquire a potential which balances photoelectrons escaping to the plasma and the electron flux collected from the plasma. The probe to plasma potential difference can be measured with a time resolution of a fraction of a second, and provides information on electron density over a wide range of electron densities from the lobes (~0.01cm-3) to the magnetosheath (>10 cm-3) and the plasmasphere (> 100 cm-3) . This technique has been perfected and calibrated against other density measurements on GEOS, ISEE-1, CRRES, GEOTAIL and POLAR. The CLUSTER spacecraft potential measurements open for new approaches, particularly near boundaries and gradients where four-point measurements will provide information never obtained before. Another interesting point is that onboard data storage of this simple parameter can be done for complete orbits and thereby will provide background information for the shorter full data collection periods on CLUSTER. Preliminary calibrations against other density measurements on CLUSTER will be reported.
Szita, S., Fazakerley, A.N., Carter, P.J., James, A.M., Travnicek, P., Watson, G., André, M., Ericsson, A. and Torkar, K.
Cluster PEACE observations of electrons of spacecraft origin
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1711-1720
The two PEACE (Plasma Electron And Current Experiment) sensors onboard each Cluster spacecraft sample the electron velocity distribution across the full 4 ¡ solid angle and the energy range 0.7eV to 26keV with a time resolution of 4 seconds. We present high energy and angular resolution 3D observations of electrons of spacecraft origin in the various environments encountered by the Cluster constellation, including a lunar eclipse interval where the spacecraft potential was reduced but remained positive, and periods of ASPOC (Active Spacecraft POtential Control) operation which reduced the spacecraft potential. We demonstrate how the spacecraft potential may be found from a gradient change in the PEACE low energy spectrum, and show how the observed spacecraft electrons are confined by the spacecraft potential. We identify an intense component of the spacecraft electrons with energies equivalent to the spacecraft potential, the arrival direction of which is seen to change when ASPOC is switched on. Another spacecraft electron component, seen in the sunward direction, is reduced in eclipse but unaffected by ASPOC, and we believe this is produced in the analyser by solar UV. We find that PEACE anodes with a look direction along the spacecraft surfaces are more susceptible to spacecraft electron contamination than those which look perpendicular to the surface, which justifies the decision to mount PEACE with its field of view radially outward rather than tangentially.
K. Torkar, W. Riedler, C.P. Escoubet, M. Fehringer, R. Schmidt, R.J.L. Grard, H. Arends, F. Rüdenauer, W. Steiger, B.T. Narheim, K. Svenes, R. Torbert, M. André, A. Fazakerley, R. Goldstein, R.C. Olsen, A. Pedersen, E. Whipple, and Hua Zhao
Active spacecraft potential control for cluster implementation and first results
Annales Geophysicae (the Special issue: First Cluster results ) (2001) Vol. 19, No. 6, pp. 1289-1302
Electrostatic charging of a spacecraft modifies the distribution of electrons and ions before the particles enter the sensors mounted at the spacecraft body. The floating potential of magnetospheric satellites in sunlight very often reaches several tens of volts, making measurements of the cold (several eV) component of the ambient ions impossible. The plasma electron data get contaminated by large fluxes of photo-electrons attracted back into the sensors. The Cluster spacecraft are equipped with emitters of the liquid metal ion source type, producing indium ions at 5 to 9 keV energy at currents of some tens of microampere. This current shifts the equilibrium potential of the spacecraft to moderately positive values. The design and principles of the operation of the instrument for active spacecraft potential control (ASPOC) are presented in detail. Experience with spacecraft potential control from the commissioning phase and the first two months of the operational phase are now available. The instrument is operated with constant ion current for most of the time, but tests with varying currents and a "feedback" mode with the instrument EFW measuring the spacecraft potential have been carried out. The spacecraft potential is reduced to values according to expectations. Also the low energy electron measurements show substantially reduced fluxes of photo-electrons as expected. The flux decrease of photo-electrons returning to the spacecraft, however, goes at the expense of an enlarged sheath around the spacecraft which causes problems for boom-mounted probes.
Vaivads, A.; André, M.; Buchert, S.; Eriksson, A.; Olsson, A.; Wahlund, J-E.; Janhunnen, P.; Marklund, G.; Kistler, L.M.; Mouikis, C.; Winningham, D.; Fazakerley, A.; Newell, P.
What high altitude observations tell us about the auroral acceleration: a Cluster/DMSP conjunction
Accepted for publication by Geophysical Research Letters
2002:
Balikhin, M. A.; Nozdrachev, M.; Dunlop, M.; Krasnoselskikh, V.; Walker, S. N.; Alleyne, H. St.C. K.; Formisano, V.; André, M.; Balogh, A.; Eriksson, A. and Yearby, K.
Observations of the terrestrial bowshock in quasi-electrostatic sub-shock regime
J. Geophys. Res., 107(8), 10, 1029/2001JA000327, (2002)
Balikhin, M. A.; Pokhotelov, O. A.; Walker, S. N.; Amata, E.; Alleyne, H. St.C. K.; André, M.; Dunlop, M.; Bates, I.
Minimum variance free wave identification: Application to Cluster electtric field measurements in the magnetosheath
Geophys. Res. Lett. (submitted) (2002)
Balikhin, M. A.; Pokhotelov, O. A.; Walker, S. N. and André, M.
Identification of low frequency waves in the vicinity of the terrestrial bow shock
J. Geophys. Res. (submitted) (2002)
Behlke, R.; André, M.; Buchert, S. C.; Vaivads, A.; Eriksson, A. I.; Lucek, E. A. and Balogh, A.
Multi-point electric field measurements of Short Large-Amplitude Magnetic Structures (SLAMS) at the Earth´s quasi-parallel bowshock
Geophys. Res. Lett. (accepted) (2002)
Cattel, C.; Dombeck, J.; Wygant, J.; Mozer, F. S. and André, M.
The role of waves in magnetotail dynamics
International Conference on Substorms, R. Winglee, ed., pg. 443 (2002)
Cattel, C.; Neiman, C.; Dombeck, J.; Crumley, J.; Wygant, J.
Large amplitude solitary waves in and near the earth´s magnetosphere, magnetopause and bow shock: Polar and Cluster observations
Nonlinear Processes in Geophysics (accepted) (2002)
Cully, C. M.; Donovan, E. F.; Buchert, S. C.; Opgenoorth, H. J.; Fazakerley, A.; Bosqueg, J. M.; André, M. et al
Magnetic reconnection near substorm onset: Multipoint in-situ observations
Geophys. Res. Lett. (submitted) (2002)
Décréau, P. M. E.; Le Guirriec, E.; Rauch, J. L.; Trotignon, J. G.; CAnu, P.; DArrouzet, F.; Lemaire, J.; Masson, A,; Sedgemore, F.; André, M.
Plasmapause formation and density irregularities: Cluster observations in the dusk sector
Advances in Space Research (submitted) (2002)
Kim, K.-H. , Cattel, C.A., Lee, D.-H. , Takahashi, K., Shiokawa, K., Singer, H., Mozer, F. S., André, M., Elphic, R.C. and Balogh, A.
Magnetospheric responses to sudden and quasi-periodic solar wind variations.
Accepted for publication by J. Geophys. Res. (2002)
Maksimovic, M.; Bale, S. D.; Horbury, T. S.; André, M.
Bow shock motions observed with Cluster
Geophys. Res. Lett. (submitted) (2002)
Maynard, N. C.; Ober, D. M.; Burke, W. J.; Scudder, J. D,; Mozer, F. S.; Lester, M.; Cowley, S.; Wild, J.; Grocott, A.; Dunlop, M.; Siscoe, G. L.; Russell, C. T.; Farrugia, J.; Lund, E.; Weimer, D. R.; Siebert, K. D.; Balogh, A.; André, M.; Reme, H
Temporal / spatial evolution of merging at the dayside magnetopause with a southward IMF component: Observation from Polar, Cluster and SuperDARN
J. Geophys. Res. (submitted) (2002)
O. Moullard, A. Masson, H. Laakso, M. Parrot, P. Décréau, O. Santolik, M. André
Density modulated whistler mode emissions observed near the plasmapause
Geophys. Res. Lett., 29(20), 1975, 2002
Electron density fluctuations are regularly observed near the plasmapause together with electromagnetic waves below the electron cyclotron frequency (usually called hiss or chorus). Instruments on board CLUSTER spacecraft often observe two such emission bands with fluctuating wave intensities that suggest wave ducting in density enhancements as well as troughs. Near perigee the CLUSTER density measurements are usually limited to the electron density from 0.2 to 80 cm-3. To establish a correlation between density and wave intensity deeper inside the outer plasmasphere, we extrapolate the electron density from the spacecraft potential after fitting a relationship between observed plasma frequency and spacecraft potential. During a plasmapause crossing on June 5, 2001 (near the geomagnetic equator, L = 4 - 6, afternoon sector), density fluctuations up to hundreds cm-3 are found while whistler mode waves are observed in two separate frequency bands, at 100-500 Hz (correlated to the density fluctuations) and 3-6 kHz (anti-correlated).Behlke, R, M.
Mursula, K.; Kerttula, R.; Asikainen, T.; Friedel, R.; Daly, P. W.; Fritz, T. A.; Carter, M.; Söraas, F.; Fennell, J. F.; Vaivads, A.; Balogh, A.
Cluster / Rapid energetic electron observations at the dayside magnetospheric boundary
Advances in Space Research (submitted) (2002)
Sergeev, V.; Runov, Baumjohann, Nakamura, Zhang, Volwek, Eichelberger, Balogh, A.; Reme, H.; Sauvaud, J-A.; Klecker, B.; André, M.
Flapping motions and current sheet structure during substorm activation as observed by Cluster
Geophys. Res. Lett. (submitted) (2002)
Wahlund, J-E.; Yilmaz, A.; Backrud, M.; Vaivads, A.; Winningham, D.; André, M.; Balogh, A.; Bonell, S.; Buchert, S.; Carozzi, T.; Cornilleau, N.; Dunlop, M.; Eriksson, A.I.; Fazakerley, A.; Gustafsson, G.; Parot, M.; Robert, P.; Sundkvist, D.; Tjulin, A.
Observations of auroral broadband emissions by Cluster
Geophys. Res. Lett. (accepted) (2002)
Vaivads, A.
Cluster II and space physics
The Starry Sky magazine (popular science periodical in Latvia) (2002) pp.6-9
Walker, S.N.; Alleyne, H.StC.K.; André, M.; Dunlop, M.; Balikhin, M.A.
Electric field scales at quasiperpendicular shocks
Planetary and Space Science (submitted) (2002
Opgenoorth, H.J., Eglitis, P. and Lockwood, M.
Upgraded ground-based facilities for coordinated observations with Cluster: opportunities for magnetospheric research, revisited
ESA SP-449 (2000) pp. 79-85
ESA's first multi-satellite mission Cluster- is unique in its concept of 4 satellites orbiting in controlled formations. At the same time the network of ground-based instrumentation for space physics research has never been so versatile and globally distributed as it was during the later ISTP period and will be for the coming years of the ISTP Solar-Maximum Extension and the Cluster II mission. The SuperDARN network has now reached truely global dimensions with multiple radars in both hemispheres, the EISCAT Svalbard radar is completed with two antennas and doubled transmitter power as compared to the status of 1996, the EISCAT mainland radar will be completely renovated by the summer of 2000, and in Scandinavia and Canada new multi-instrument networks have been established. This optimised combination of space physics instrumentation both in space and on the ground will give an unprecedented opportunity to study structure and dynamics of the magnetosphere, and the coupling to the solar wind and the ionosphere. The status of ground-based instrumentation available for support of the Cluster II mission is briefly reviewed, and the most common data sets are introduced. We will discuss ways in which such ground based remote-sensing observations of the ionosphere can be used to support the multi-point in-situ satellite measurements. In addition a number of WWW URL 's will be given, for immediate access to some of the preliminary ground-based data in a distributed form. In the future these multiple coordinated data sets will be accessible from a central server at the Cluster Ground-Based data centre at the Rutherford Appleton Laboratory, RAL, UK.
A. I. Eriksson,
Spectral Analysis, in Analysis Methods for Multi-Spacecraft Data
ISSI Scientific Report SR-001, p. 5-42, ISSI (Bern) and ESA publications division, 1998.
H. Opgenoorth and M. Lockwood,
Opportunities for magnetospheric research with coordinated Cluster and ground-based observations
Space Science Reviews, 79, 599-637, 1997.
The European Space Agency's first multi-satellite mission Cluster is unique in its concept of four satellites orbiting in controlled formations. This will give an unprecedented opportunity to study structure and dynamics of the magnetosphere. The authors discuss ways in which ground-based remote-sensing observations of the ionosphere can be used to support the multipoint in situ satellite measurements. There are a very large number of potentially useful configurations between the satellites and any one ground-based observatory. These circumstances create a clear and pressing need for careful planning to ensure that the scientific return from Cluster is maximised by additional coordinated ground-based observations. For this reason, ESA established a working group to coordinate the observations on the ground with Cluster. The authors give a number of examples how the combined spacecraft and ground-based observations can address outstanding questions in magnetospheric physics. An online computer tool has been prepared to allow for the planning of conjunctions and advantageous constellations between the Cluster spacecraft and individual or combined ground-based systems. During the mission a ground-based database containing index and summary data will help to identify interesting datasets and allow to select intervals for coordinated studies. The authors illustrate the philosophy of their approach, using a few important examples of the many possible configurations between the satellite and the ground-based instruments.
Gustafsson, G., Boström, R., Holmgren, G., Lundgren, A., Stasiewicz, K., Åhlén, L., Mozer, F. S., Pankow, D., Harvey, P., Berg, P., Ulrich, R., Pedersen, A., Schmidt, R., Butler, A., Fransen, A. W. C., Klinge, D., Thomsen, M., Fälthammar, C.-G., Lindqvist, P.-A., Christensson, S., Holtet, J., Lybekk, B., Sten, T. A., Tanskanen, P., Lappalainen, K. and Wygant, J.
The electric Field and Wave Experiment for the Cluster Mission
Space Science Reviews, 79(1 - 2), 137 - 156, 1997.
The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s/sup -1/. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes up to 700 mV m/sup -1/ with high amplitude and time resolution, (2) measurements over short periods of time of up to five simultaneous waveforms (two electric signals and three magnetic signals from the search coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.
Stasiewicz, K. and Gustafsson, G.
Alfven waves and structures: What can we learn with multipoint measurements on CLUSTER II
ESA SP-449 (2000) pp. 177-182
We discuss the importance of EFW and EGM instruments on CLUSTER-II for studying Alfven waves and structures in the magnetosphere. The low frequency electromagnetic waves and structures are important for magnetosphere-ionosphere coupling transfer of energy from the solar wind into the magnetosphere and for wealth of auroral phenomena. The ambiguity in distinction between spatial and temporal variations are especially acute for low frequency electromagnetic phenomena related to Alfven waves. Using measurements from FREJA and POLAR satellites we discuss a number of key involved problems that can be addressed with multipoint capabilities of the CLUSTER II mission. We point out that the scale sizes of plasma and current structures in the magnetosphere cab be well below 100 km, which requires a small separation distances between the four CLUSTER spacecraft.
K. Stasiewicz
New Methods and Techniques in Visualization and Mapping of Magnetospheric Boundaries
in Physical Signatures of Magnetospheric Boundary Layer Processes, (J. A. Holtet and A. Egeland, eds.), Kluwer Academic Publishers, 433-447, 1994.
K. Stasiewicz
Magnetospheric Modelling in Support of Space Missions in Spatio-Temporal Analysis for Resolving Plasma Turbulence (START)
Proceedings ESA WPP-047, p. 345, 1993.
O. Moullard, A. Masson, H. Laakso, M. Parrot, P. Décréau, O. Santolik, M. André
Density modulated whistler mode emissions observed near the plasmapause
Geophys. Res. Lett., 29(20), 1975, 2002
Electron density fluctuations are regularly observed near the plasmapause together with electromagnetic waves below the electron cyclotron frequency (usually called hiss or chorus). Instruments on board CLUSTER spacecraft often observe two such emission bands with fluctuating wave intensities that suggest wave ducting in density enhancements as well as troughs. Near perigee the CLUSTER density measurements are usually limited to the electron density from 0.2 to 80 cm-3. To establish a correlation between density and wave intensity deeper inside the outer plasmasphere, we extrapolate the electron density from the spacecraft potential after fitting a relationship between observed plasma frequency and spacecraft potential. During a plasmapause crossing on June 5, 2001 (near the geomagnetic equator, L = 4 - 6, afternoon sector), density fluctuations up to hundreds cm-3 are found while whistler mode waves are observed in two separate frequency bands, at 100-500 Hz (correlated to the density fluctuations) and 3-6 kHz (anti-correlated).Behlke, R, M.