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Earth magnetic field mapping from planetary orbiting satellites requires a spacecraft magnetic field environment control program combined with the deployment of the magnetic sensors on a boom in order to reduce the measurement error caused by the local spacecraft field. Magnetic mapping missions (Magsat, Oersted, CHAMP, SAC-C MMP and the planned ESA Swarm project) carry a vector magnetometer and an absolute scalar magnetometer for in-flight calibration of the vector magnetometer scale values and for monitoring of the inter-axes angles and offsets over time intervals from months to years. This is done by comparing the two magnetometer outputs for several days and for as many different external field directions and amplitudes in the satellite frame as available. The vector and the scalar sensor may be placed of the order of 2 m apart and at the end of an about 10 m long boom counted from the spacecraft centre-of-gravity. In line with the classical dual vector sensors technique for monitoring the spacecraft magnetic field, this paper proposes and demonstrates that a similar combined scalar/vector gradiometry technique is feasible by using the measurements from the boom-mounted scalar and vector sensors onboard the Oersted satellite. For Oersted, a large difference between the pre-flight determined spacecraft magnetic field and the in-flight estimate exists causing some concern about the general applicability of the dual sensors technique.

A novel highly stable magnetic fluxgate vector gradiometer is presented in this paper. It is based on two triaxial fluxgate sensors with the Compact Spherical Coil (CSC) feedback to demonstrate the feasibility of such instrument (the full gradient instrument will consist of at least four triaxial sensors for measuring all the components of the gradient tensor). The sensors have been designed and constructed for the geopotential German satellite CHAMP, and are based on the instrument flying on the Danish satellite Orsted dedicated to measure the Earth's magnetic field with very high precision. The transducers are of the ringcore type with very low noise and high thermal stability. They use amorphous metal magnetic core (Vitrovac 6025). The cores have been annealed in two different processes with different temperatures and stress. With this instrument, three components of the gradient tensor can be measured with a precision better than 100 pT rms /m. The noise of the gradiometer is 93 pT rms /m in the band 0.01–10 Hz (30.1 pT rms /(Hz 1/2 m) at 1 Hz).

The shift of the NMR spectral line frequency in a proton free precession absolute scalar magnetometer using the omni-directional toroid container for a proton-rich liquid depends on the magnetic susceptibility of the liquid and on the direction of the external field relative to the axis of the toroid. The theoretical shift is estimated for water by computing the additional magnetic field from the magnetization of the liquid and comparing it to the theoretical field in a spherical container. Along the axis the estimated average shift is −0.08 nT and perpendicular to the axis the shift is +0.08 nT relative to that of a spherical sensor. The field inhomogeneity introduced by the toroid shape amounts to 0.32 nT over the volume of the sensor and is not expected to significantly affect the signal decay time, when considering the typical water line width of about 2.5 nT.

International audience; DYNAMO is a small multi-instrument payload aimed at characterizing current atmospheric escape, which is still poorly constrained, and improving gravity and magnetic field representations, in order to better understand the magnetic, geologic and thermal history of Mars. The internal structure and evolution of Mars is thought to have influenced climate evolution. The collapse of the primitive magnetosphere early in Mars history could have enhanced atmospheric escape and favored transition to the present arid climate. These objectives are achieved by using a low periapsis orbit. DYNAMO has been proposed in response to the AO released in February 2002 for instruments to be flown as a complementary payload onboard the CNES Orbiter to Mars (MO-07), foreseen to be launched in 2007 in the framework of the French PREMIER Mars exploration program. MO-07 orbital phase 2b (with an elliptical orbit of periapsis 170 km), and in a lesser extent 2a, offers an unprecedented opportunity to investigate by in situ probing the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, and therefore the present atmospheric escape rate. Ultraviolet remote sensing is an essential complement to characterize high, tenuous, layers of the atmosphere. One Martian year of operation, with about 5,000 low passes, should allow DYNAMO to map in great detail the residual magnetic field, together with the gravity field. Additional data on the internal structure will be obtained by mapping the electric conductivity, sinergistically with the NETLANDER magnetic data. Three options have been recommended by the International Science and Technical Review Board (ISTRB), who met on July 1st and 2nd, 2002. One of them is centered on DYNAMO. The final choice, which should be made before the end of 2002, will depend on available funding resources at CNES.

The compact spherical coil (CSC) vector-feedback magnetometer on the Danish Orsted geomagnetic mapping satellite underwent extensive calibrations and verifications prior to integration and launch. The theory of the 'thin shell' calibration procedure is introduced. Spherical harmonic modelling was developed and tested over several years and used for Orsted and other missions at test facilities in Europe, the United States and the Republic of South Africa. The verification of the test coil system using an Overhauser absolute scalar proton magnetometer is explained and the overall calibration results are given. The temperature calibrations are explained and reported on. The overall calibration model standard deviation is about 100 pT rms. Comparisons with the later in-flight calibrations show that, except for the unknown satellite offsets, an agreement within 4 nT was obtained. Finally an rf interference between the CSC and the Overhauser magnetometer is discussed, which may account for some of this discrepancy.

The vector fluxgate magnetometer of the Orsted satellite is routinely calibrated by comparing its output with measurements of the absolute magnetic intensity from the Overhauser instrument, which is the second magnetometer of the satellite. We describe the method used for and the result obtained in that calibration. Using three years of data the agreement between the two magnetometers after calibration is 0.33 nT rms (corresponding to better than ± 1 nT for 98% of the data, and better than ± 2 nT for 99.94% of the data). We also report on the determination of the transformation between the magnetometer coordinate system and the reference system of the star imager. This is done by comparing the magnetic and attitude measurements with a model of Earth’s magnetic field. The Euler angles describing this rotation are determined in this way with an accuracy of better than 4 arcsec.

The Swedish micro-satellite Astrid-2 was successfully launched into a near polar orbit in December 1998. Despite the fact that the primary science mission was auroral research, the magnetic instrument was designed to accomplish high-resolution and high-precision vector field magnetic measurements, and therefore mapping of the Earth's magnetic field was possible. The spacecraft spins about a highly stable axis in space. This fact and the globally distributed data make the magnetic measurements well suited for the estimate of a magnetic field model at the spacecraft altitude (about 1000 km). This paper describes the initial analysis of the Astrid-2 magnetic data. As a result of the study of a single day (February 7, 1999), magnetically fairly quiet, it was possible to in-flight adjust the calibration of the magnetometer and find a magnetic field model fitting the scalar component of the measurements to better than 5 nT(rms) for latitudes Equatorward of 50degrees. Several methods for field modeling are discussed in this paper under the assumption that the direction of the spin axis in inertial space is nearly constant, and this assumption is corroborated by the observations. The approximate inertial orientation of the magnetometer could then be determined simultaneously with the instrument intrinsic calibration and the estimate of main field model coefficients. Hence, apart from the scientific use of the magnetic data, the attitude of the spacecraft may be estimated with high precision.

The single axis magnetic gradiometer based on two compact detector compensation (CDC) fluxgate ringcore sensors separated 20 cm is described. Despite its high stability and precision better than 1 nT, the calibration procedures are not straightforward. Firstly, the mono-axial measurement does not provide vector information about the magnetic field. Secondly, one of the sensors measures the ambient magnetic field and is used to compensate for the main field at both sensors. Several methods have been developed for characterization of the gradiometer, and the calibration of the gradient measurements is achieved by using a magnetic dipole of strength 2 mAm 2 . In a coil facility, the gradient can be determined with an accuracy of 0.3 nT/m RMS .

Space-based, high-precision magnetometry is essential for understanding a variety of phenomena ranging from secular variation of the Earth's main field, through the signatures of crustal magnetism and the effects of plasma currents flowing externally to the Earth. Orsted, Denmark's first satellite, was launched on February 23, 1999 into a polar, low-Earth orbit to provide the first near-global set of high-precision geomagnetic observations since the Magsat mission of 1979–1980 (see Magsat Special Issue of Geophysical Research Letters., vol. 9, no. 4, pp. 239–379, 1982). With the new mapping of the Earth's magnetic field, the International Geomagnetic Reference Field model (IGRF), a standard model used for navigation, prospecting, and other practical purposes, has been determined with improved precision for epoch 2000 [Olsen et al., 2000a; Mandea and Langlais, 2000]. The satellite has routinely provided high-precision vector data since August 1999, and the mission is continuing well beyond its nominal 14-month lifetime into 2001.

Orsted is the first satellite to conduct high-precision magnetometer observations from low-altitude noon-midnight orbits passing through the polar cusp regions. Field-aligned currents (FAC) derived from Orsted magnetic field measurements have been combined with ionospheric current patterns inferred from ground-based magnetic observations to define the structure and location of cusp currents and their dependencies on interplanetary magnetic field (IMF) conditions. Example cases illustrate the close relation between IMF By-related FAC and horizontal ionospheric currents in the cusp region. Our statistical analysis defines for the noon region the variations in FAC latitude with IMF Bz. Comparisons with the statistical cusp location indicate that the more equatorward region of IMF By-related FAC is located on field lines closing at the dayside, while the more poleward FAC are on “open” field lines. High-energy electron measurements from the satellite confirm this result.

The magnetic measurements by the Orsted satellite in noon-midnight orbits have enabled the derivation of the global dawn-dusk oriented ionospheric currents from an Ampere's law closed loop line integral of the geomagnetic vector field along the satellite track. The globally integrated dawn-to-dusk ionospheric current is found to be proportional to the geo-effective solar wind electric field and is around 1 million ampere for a typical solar wind electric field of 2 mV/m. Dividing the Ampere integral into semi-orbit parts has enabled us to show that the hemispherical total current intensities depend on the respective polar cap conductivities, which relate to the daily and seasonally varying solar illumination. The more illuminated hemisphere conveys up to three times more current from dawn to dusk than does the less illuminated.

The current-output fluxgate may be tuned by using a serial capacitor. Such tuning increases the sensor sensitivity in the situation when the pick-up coil has a low number of turns. We achieved a signal/feedthrough ratio improvement by a factor of 5. The measured parameters fit the simplified theoretical model within 20% deviation.

We report here three-point measurements of bursty, velocity-dispersed, field-aligned electron precipitation at the poleward edge of a northward-moving, post-breakup, nightside auroral arc. The three-point measurement allows detection of the proper motion of the inverted-V arc, which is shown to be 550 m/sec northward. The velocity dispersion patterns are fitted to find the source altitude of the precipitation bursts as a function of distance from the poleward edge of the arc. These source points are interpreted to trace out the low-altitude boundary of the inverted-V potential drop, which is seen to rise both in time, and in the northward direction. The precipitation bursts under the inverted-V are seen to have an arc-aligned velocity which varies with time, and which is consistent with the measured E × B local drift speed.

The Auroral Turbulence II (AT2) sounding rocket carried three payloads into the auroral ionosphere where they crossed several arc structures. At the border of an auroral arc a quasiperiodic structure was observed by the magnetic and electric field instruments as well as by the particle detectors. The variations were temporal oscillations, but existed only in a narrow (≈ 7 km) region transverse to the arc, with a correlation length along the arc of at least several km. The relation between the electric and magnetic field amplitude indicates the Alfvenic nature of the variations. Field aligned electron precipitation is correlated to the field variations. The narrow band nature of the oscillations and frequency around 0.6 Hz is consistent with waves confined in the ionospheric Alfven resonator.

A model of a fluxgate magnetometer based on the field interactions in the fluxgate core has been derived. The non-linearity of the ringcore sensors due to large uncompensated fields transverse to the measuring axis are calculated and compared with measurements. Measurements of the non-linearity are made with a spectrum analyzer measuring the higher harmonics of an applied sinusoidal field. For a sensor with a permalloy ringcore of 1 in. in diameter the deviation from linearity is measured to about 15 nT p-p in the earth's field and the measurements are shown to fit well the calculations. Further, the measurements and the calculations are also compared with a calibration model of the fluxgate sensor onboard the ‘MAGSAT’ satellite. The later has a deviation from linearity of about 50 nT p-p but shows basically the same form of non-linearity as the measurements.

The construction of a triaxial fluxgate sensor with very high axis stability and low temperature coefficients is described. The axis orthogonalities change less than 2.1 s of are in the whole testing temperature range +20 to −10°C. The temperature coefficients for the sensitivities of the three axes are 6.7, 10.1 and 13.3 ppm K −1 , respectively. This high stability is achieved by using a newly developed ceramic, CSiC, as the supporting construction material.

The full electric and magnetic field data set from the subpayload of the CRIT II sounding rocket experiment is presented for the first time. CRIT II was an ionospheric injection experiment aimed at studying the critical ionization velocity (CIV) effect. It consisted of two payloads located on nearly the same magnetic field line. By using the data from both payloads, we are able to reach a good understanding of the momentum transfer between the injected ions and the ambient ionosphere. The data also make it possible to resolve the conflict between the two competing models for the energy transfer from the newly created ions to hot electrons in the CIV process. The results give a natural coupling between the energy and momentum transfer processes in CIV experiments.

Two sounding rockets launched from Greenland on March 31, 1987, present one of the most detailed pictures of polar cap auroral arc dynamics ever obtained. The electrodynamics show a clear picture of energy flux carried by energetic, precipitating electrons accompanied by a comparable magnitude Poynting flux immediately adjacent to the electron precipitation. The result is a W-shaped region of energy flux at the locations of auroral arcs. In addition, the current carried by the energetic, precipitating electrons is somewhat greater than that indicated by the magnetometer measurements, suggesting that the background ionosphere in the polar cap may have closed the upward electron current with a downward current along the same field lines. The measurements are compared to a model of quasi-static auroral acceleration based on that by Lyons [1981]. It is found that this model reproduces well most of the electrodynamic features; energy flux, electric fields, and parallel currents determined by the model are in generally good agreement with the in situ measurements.

We present new data from the subpayload of the GRIT II ionospheric active injection experiment. The analysis made possible by these data provides a good understanding of the momentum transfer between the injected ions and the ambient ionosphere. It resolves the conflict between the two competing models for the energy transfer from the newly created ions to hot electrons, while also giving a natural coupling between the energy and momentum transfer processes.

Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6–13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to δB ∼ 3 nT and δE ∼ 4 mV/m and have downward field-aligned Poynting fluxes of up to ∼10−5 W/m². Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous ground-based optical data show that the arc was flickering at frequencies of 8–15 Hz. The observed frequencies would match the oxygen cyclotron frequency at ∼4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500–5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region.

The experiments and theoretical considerations leading to the construction of a high-performance three-axis fluxgate magnetometer are described. The magnetometer will be used (1996) in the Earth's field mapping satellite named 'OErsted'. The fluxgate sensors are based on stress-annealed metallic glass ribbons as core materials. It is shown that very simple physical models can be used to explain the fluxgate mode of operation, thereby making it easy to calculate the overall sensor performance from first principles. Special attention is drawn to the core excitation current which is analysed on the basis of nonlinear electrical circuitry. It is furthermore shown that the ring-core demagnetizing field obeys a simple cosine law which permits the calculation of the sensor sensitivity with high accuracy. The sensitivity, that is the signal-to-noise ratio, is ultimately determined by the sensor noise which is about 15 pT RMS (0.06-10 Hz), corresponding to a noise power density (1/f noise) of 6.2 pT Hz-1/2 at 1 Hz. The actual magnetometer operating range and sensitivity is determined by the 1 bit resolution of the Earth's field represented by the output from the 18 bit AD converted used in the instrument (+or-65536 nT with 0.5 nT resolution). The maximum attainable bandwidth is half the sensor excitation frequency (1/2*15 kHz) but the OErsted magnetometer bandwidth is limited to 250 Hz. The thermal stability of the sensor has been measured to be better than 1 nT in the temperature range -20 to +60 degrees C.

This paper describes an experiment where the flux-gate sensor broad band output signal is digitized with 8-bit resolution at a high rate, and subsequently numerically analysed in order to extract information on the external magnetic field. The results show that it is feasible to obtain a noise level of 1 nT for a data output rate of 100 Hz. The method will allow construction of a magnetometer based entirely on digital techniques, with only a minimum of analogue circuits.

The fluxgate null feed-through signal is discussed theoretically giving an expression for the excitation current and for the core flux coupling coefficients to the secondary coil. Experimental results show that the two signals cannot be nulled simultaneously by rotating the core inside the secondary coil, and give the two coupling coefficients. The excitation current coupling depends critically on the symmetry of the winding, and it is shown that the two signals are associated with excess noise adding to the basic core noise.

Measurements inside a high velocity neutral barium beam show a factor of six increase in plasma density in a moving ionizing front. This region was co-located with intense electric fields (δE ≈ 300 mV/m2) perpendicular to the local geomagnetic field and field aligned currents all fluctuating at frequencies well under the lower hybrid frequency for barium but above the oxygen cycloton frequency. It was determined that these structures were moving with the barium stream near the neutral barium velocity. Large quasi-dc electric and magnetic field fluctuations were also detected. The heart of the ionizing front, a cross beam current of nearly 10 mA/m2, was estimated from the magnetic field variation. This is three orders of magnitude higher than typical auroral zone currents associated with auroral arcs. This current sheet was co-located with fluxes of soft electrons which saturated the particle detector. An Alfven wave with a finite electric field component parallel to the geomagnetic field was observed to propagate along Bo where it was detected by an instrumented sub-payload.

The output current impulses of the short-circuited fluxgate depend on the input magnetic field, and on a number of core parameters such as the demagnetization factor, the cross sectional area, the time spent in saturation, etc. Following a theoretical treatment, experimental tests of the parameter dependences are reported, and additional experimental evidence of the noise performance is presented.

The sensor offset as a function of the ring rotation angle with respect to the detector coil has been studied for three different fluxgate sensors of the ringcore type. The core material consists of metallic glass ribbons which have been heat treated in different ways so that different magnetization curves and noise levels are obtained. For the best core the offsets are rather low (a few nT) and only slightly dependent on the rotation angle.

The Swarm mission under the ESA’s Living Planet Programme is planned for launch in 2010 and consists of a constellation of three satellites at LEO. The prime objective of Swarm is to measure the geomagnetic field with unprecedented accuracy in space and time. The magnetometry package consists of an extremely accurate and stable vector magnetometer, which is co-mounted in an optical bench together with a start tracker system to ensure mechanical stability of the measurements.

The paper presents design and implementation of the new self-compensating excitation circuitry to the new generation of high-precise space vector magnetometers. The application starts with complex study including design of new robust model of the non-linear inductor leading to investigation of the most crucial points, continuous by design of the self-compensating excitation unit and concludes with unit complex testing and application to the magnetometer. The application of the self-compensation of the excitation decreases temperature drift of the magnetometer offset caused by the temperature drift of the sensor (dominant source of the offset drift) by factor of 7.

Measurements from rocket-borne sensors inside a high-velocity neutral barium beam show a-factor-of-six increase in plasma density in a moving ionizing front. This region was colocated with intense fluctuating electric fields at frequencies well under the lower hybrid frequency for a barium plasma. Large quasi-dc electric and magnetic field fluctuations were also detected with a large component of the current and the electric field parallel to B(0). An Alfven wave with a finite electric field component parallel to the geomagnetic field was observed to propagate along B(0), where it was detected by an instrumented subpayload.

The requirements for precise global mapping of the Earth's vector magnetic field from a LEO satellite are discussed.

International audience; In the frame of the international cooperation for Mars exploration, a set of 4 NetLanders developed by an European consortium is expected to land on the planet during the forthcoming years. Among other instruments, the geophysical package of each lander will include a magnetometer. The different possible contributions of magnetic measurements onboard the NetLander stations are presented. Intrinsic planetary field and remanent magnetisation investigations by means of magnetometers onboard a network of landers are first considered, and the information that can be thus derived on the Martian core dynamo and surface rocks, soil, and dust is discussed. The contribution of permanent recording of the magnetic transient variations at a network of surface stations is then discussed. The transient variations of the magnetic field at the surface of a planet has a primary external source, the interaction between the environment of the planet and solar radiation, and a secondary source, the electric currents induced in the conductive planet. The continuous recording of the time variations of the magnetic field at the surface of Mars by means of three component magnetometers installed onboard NetLander stations will therefore allow study of both the internal structure of Mars and dynamics of its ionised environment. The expected characteristics of transient magnetic variations, and their relation with plasma flow and current in the Mars ionised environment are discussed. The use of the network magnetic data to probe the internal structure of Mars is also considered. The used techniques are presented, and the information that can be thus obtained on the Mars permafrost, lithosphere and mantle structure illustrated by numerical simulations. Finally, the specifications of the instrument allowing to achieve these objectives are discussed, and the instrument described.

International audience; Magnetic measurements taken by the Ørsted satellite during geomagnetic quiet conditions around Jan-uary 1, 2000 have been used to derive a spherical harmonic model of the Earth's magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Orsted mission, this model supercedes IGRF 2000.

Freja is a Swedish scientific satellite mission to study fine scale auroral processes. Launch was October 6, 1992, piggyback on a Chinese Long March 2C, to the present 600 × 1750 km, 63° inclination orbit. The JHU/APL provided the Magnetic Field Experiment (MFE), which includes a custom APL-designed Forth language microprocessor. This approach has led to a truly generic and flexible design with adaptability to differing mission requirements and has resulted in the transfer of significant ground analysis to on-board processing. Special attention has been paid to the analog electronic and digital processing design in an effort to lower system noise levels, verified by inflight data showing unprecedented system noise levels for near-Earth magnetic field measurements, approaching the fluxgate sensor levels. The full dynamic range measurements are of the 3-axis Earth’s magnetic field taken at 128 vector samples s-1 and digitized to 16 bit resolution, primarily used to evaluate currents and the main magnetic field of the Earth. Additional 3-axis ‘AC channels are bandpass filtered from 1.5 to 128 Hz to remove the main field spin signal, the range is ±650 nT. These vector measurements cover Pc waves to ion gyrofrequency magnetic wave signals up to the oxygen gyrofrequency (~40 Hz). A separate, seventh channel samples the spin axis sensor with a bandpass filter of 1.5 to 256 Hz, the signal of which is fed to a software FFT. This on-board FFT processing covers the local helium gyrofrequencies (~160 Hz) and is plotted in the Freja Summary Plots (FSPs) along with disturbance fields. First data were received in the U.S. October 16 from Kiruna, Sweden via the Internet and SPAN e-mail networks, and were from an orbit a few hours earlier over Greenland and Sweden. Data files and data products, e.g., FSPs generated at the Kiruna ground station, are communicated in a similar manner through an automatic mail distribution system in Stockholm to PIs and various users. Distributed management of spacecraft operations by the science team is also achieved by this advanced communications system.

This is a reply to a comment on our paper. The comment suggests an inconsistency in the relation between the magnetic field and the magnetometer measurements. This is resolved, once the existence of two physically different sensor systems is appreciated, as has also been discussed in some detail in our paper.

AC magnetic fields up to 3 kHz can be measured directly using a modified high frequency fluxgate magnetometer. The amorphous metal ring core sensor was excited at 15 kHz and in short circuited output mode has a -3 dB bandwidth of at least 2 kHz. The total RMS noise level is 93 pT (64 mHz to 3 kHz), and the noise power density decreases as 1/f over the entire frequency band.

Making use of the inverse Wiedemann effect in non-magnetostrictive hairpin shaped metallic glass ribbons we have made flux-gate sensors with small dimensions ( d × l = 3 × 8 mm 2 ) and RMS noise levels of 2 nT (0.02−1 Hz). The inverse Wiedemann effect arises from a helical anisotropy induced by torsional stress annealing.

This paper reports on results from a study of the poleward edge of the auroral oval in the morning sector using a comprehensive blend of in-situ and ground-based measurements. Three rockets, equipped to measure electric and magnetic fields, energetic particles, and plasma density flew into an auroral display whose dynamical features were reorded with a digital image into an auroral display intensified all-sky camera as well as with an incoherent scatter radar. In addition, a number of DMSP satellite measurements bracketed the launch time. Evidence is presented here that in a condition of declining magnetic activity Sun-aligned arcs are injected into the polar cap at velocities approximately 7 km/s from locations of periodic brightening along the morningside of the auroral oval. The multipoint in situ measurements allow some separation of temporal and spatial effects and strongly suggest a poleward contraction of the convention pattern of about 0.25 deg INVL in 70 s. The most equatorward of the two brightest arcs studied erupted into a region which already was characterized by strong sunward convection. The most poleward, however, pushed into a region that had been convecting in an antisunward direction at velocities exceeding 1 km/s less than 2 min earlier, and it is likely that sunward convection subsequently pertained poleward of that arc as well. We believe that these events mark the reconfiguration of the magnetosphere into a system characterized by a smaller polar cap.

In August 1976,two identically instrumented sounding rockets were launched from Danish Meteorological Institute Rocket Range at Sondre Stromiford, Greenland. One rocket (CUSP II) aimed to study the polar ionosphere cusp (or cleft) and the other (SEC II) to study the plasma irregularities in the ionosphere, especially those claimed to be related to the phenomena "Slant E Condition". The detailed cusp structure of electron density and temperature was obtained, showing distinct enhancements in density and temperature around the cusp boundaries. Both flights revealed the plasma irregularities in the ionospheric regions of about 95km to 120km in altitude. Dynamic frequency spectra of these irregularities together with ELF-VLF waves and DC-electric field observations preferably refer to the two stream instabilities.

Recent observations of partial penetration of the IMF B y into the magnetosphere (Fairfield, 1979; Cowley and Hughes, 1983) are shown to agree with the idea of a magnetopause current K y , induced by IMF B y (Primdahl and Spangslev, 1983). The slow decay of K y , caused by Joule heat losses in the cusp ionospheres is responsible for the appearance (on the average) of a fraction of IMF B y inside the magnetosphere, and this also explains the large statistical scatter of the data. A decay time constant of 4–5 days is derived from the average fraction of IMF B y observed inside the magnetosphere in good agreement with the 7-day time constant previously proposed.

On 27 January 1979, the S23H rocket was launched from Esrange, Kiruna, Sweden, shortly after the onset of an intense auroral breakup over Northern Scandinavia. The high altitude and short range trajectory of the rocket carried the instruments twice through the same magnetic flux tubes demonstrating some time variation imposed on a spatial structure. Several regions of intense Birkeland current sheets were penetrated, and examples are given where the northward electric field showed anticorrelation, correlation, as well as no correlation with the westward magnetic field perturbations. This is interpreted as examples of down-flowing and up-flowing electromagnetic energy as well as regions of little exchange of energy between the magnetosphere and the ionosphere.

A well established correlation exists between the IMF B y and the cusp field-aligned and horizontal currents (Wilhjelm et al. , 1978). The northern and southern cusp currents may be parts of one large scale current system (D'Angelo, 1980) flowing mainly at the magnetopause and driven by the z -component of the solar wind electric field. Primdahl and Spangslev (1981) suggested that the large scale current system seems to shield out the IMF B y from the interior of the magnetosphere. This paper proposes that the currents are induced by the change of sign of B y at the IMF sector boundary crossings, and argues that the time constant for decay of the currents may well be one week or larger. The percentage errors in inferring the IMF sector polarity from the Godhavn H magnetogram increases with increasing time since the last sector boundary crossing. This is in accordance with a steady decay of the induced currents. Finally experimental tests are proposed to demonstrate the feasability of and possibly distinguish between the mechanisms.

The release of dense chemical clouds in the ionosphere has strong influence on the electric field, plasma waves, and the energetic and thermal plasma. The observed effects are associated with the expanding neutral gas and with the localized conductivity enhancement caused by the ionized cloud. We report the first results of an experiment (Tor) performed at Esrange in October 1984. The observations are compared with observations made in a similar experiment (Trigger) in September 1977. In both experiments, strong electric field pulses were observed immediately after the releases. Also, in both experiments, enhanced energetic electron flux associated with the releases were observed. However, the details of the energetic electron response differ between the two experiments. This paper gives an overview of the experiment and the associated Eiscat measurements. Details of the observed electric field and particle observations are discussed in separate papers.

A method for measuring the demagnetisation of fluxgate sensors is introduced and used to evaluate the demagnetising factors for four ring-core sensors having 5, 10, 15 and 20 wraps of magnetic core ribbon. The demagnetising factor is proportional to the number of wraps, and the noise is also proportional to the number of wraps except for the more noisy 5-wrap core. The estimated internal core noise was 1 pT RMS taking into consideration the demagnetising factor and the relative permeability of the core material.

This paper presents initial results from the first comprehensively instrumented rocket flown through a Farley-unstable polar cap E-region. Ground-based ionosondes and magnetometers at two locations and HF radar backscatter at the launch site were used to determine the presence of a geographically widespread ionospheric plasma instability. The observed wave direction, electric field, and current density fit the predictions of the linear theory of the Farley instability, whereas the gradient-drift instability seems to be excluded by the geometry of the observations.

Previously published data by L. J. Cahill, Jr., from a rocket measurement of a horizontal polar E region sheet current have been further analyzed by using nearby ground-based magnetograms and ionograms. The combined data indicate that the requirements of the Farley-Buneman two-stream instability were fulfilled and give further support to the hypothesis that a plasma instability is revealed by the so-called ‘slant E condition’ on the ionograms previously described.

First results of comprehensive observations of magnetic and electric fields, and ambient and suprathermal plasmas above the dayside auroral oval with rocket-borne instrumentation which penetrated the cleft region are reported. Measurements were also obtained equatorward and poleward of the cleft. Convection velocities as inferred from electric field measurements were generally toward noon equatorward of the cleft and were antisunward over the polar cap. Our observations of electron temperatures, electric fields and low-frequency electrostatic noise provide strong evidence of a plasma instability (Farley-Buneman) in the E-layer, which is associated with the appearance of the ‘slant E condition’ identified in ground-acquired ionograms. The positions of these measurements relative to that of the cleft were firmly established via the determination of the plasma environment with an electrostatic analyzer.

The theoretical distributions over the polar cap of the growth rates of the gradient and the Farley--Buneman two-stream instabilities are compared to the measured distribution of the Slant E Condition (SEC) on polar cap ionograms. The Farley instability seems the most likely source of the SEC, and this fact opens the possibility by means of ground-based ionospheric Doppler radars of detecting whether the polar cap E-field is above or below a critical field and, if above, of determining the direction of the E-field and the magnitude of the critical field. (auth)