P. Tanskanen

THE ELECTRIC FIELD AND WAVE EXPERIMENT FOR THE CLUSTER MISSION

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-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-1 with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach 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.

RAPID: The imaging energetic particle spectrometer on Cluster

The RAPID spectrometer (Research with Adaptive Particle Imaging Detectors) for the Cluster mission is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20–400 keV for electrons, 40 keV–1500 keV (4000 keV) for hydrogen, and 10 keV nucl-1–1500 keV (4000 keV) for heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for either species. Identification of the ionic component (particle mass A) is based on a two-dimensional analysis of the particles velocity and energy. Electrons are identified by the well-known energy-range relationship. Details of the detection techniques and in-orbit operations are described. Scientific objectives of this investigation are highlighted by the discussion of selected critical issues in geospace.

Correlative study of solar activity and cosmic ray intensity

We perform a correlative study of solar activity (sunspot numbers) and cosmic ray intensity (neutron monitor count rates) for the last four solar cycles. Analysis of the running cross correlation between the two series shows that the behavior of cosmic ray modulation is similar, in general, for particles with different energy. However, a strong rigidity dependence as well as an unusual behavior of the cross correlation function is found for the descending phase of cycle 20. We study the evolution of cosmic ray and solar activity cycles in a three-dimensional phase space by means of the delayed component method. While all solar activity cycles and most cosmic ray cycles are planar, cosmic ray cycle 20 is significantly three-dimensional. A concept of the momentary phase of a cycle is introduced, and the phase evolution of cosmic ray and solar activity cycles is studied. We also discuss the heliospheric conditions responsible for the unusual behavior of cosmic ray modulation in the descending phase of cycle 20.

Dispersive Pc1 bursts observed by Freja

Electric field measurements by the Freja double probe sensor are used to study equatorially generated ion cyclotron waves, also called Pc1 pulsations. We have examined the global occurrence and spectral properties of these waves in the upper ionosphere during 12-hour period on Nov. 18, 1992, when a long chain of structured Pc1 waves (pearls) was observed on ground. In agreement with ground observations, Pc1 waves were found to occur as short bursts of 10–25 s in early morning to postnoon MLT sector. Most Pc1 activity was detected within a small latitudinal range, extending from 60° CGMlat at dawn to 63° CGMlat at noon. The latitudinal width of the source was only about 0.5° CGMlat. Observations give evidence for a plasmapause connected source region that was several hours wide in MLT and active during many hours. One burst displayed a fully developed classical dispersive Pc1 pearl, now detected for the first time above the ionosphere. In all studied Pc1 events, two spectral maxima (bands) were observed. The longer Pc1 wave bursts showed evidence for a small time delay between the lower and upper frequency bands, unveiling a new dispersive phenomenon.

The Double Probe Electric Field Experiment on Freja: Experiment Description and First Results

A description is given of theFreja double-probe electric field instrument. Its capability to perform high-resolution measurements of the aurora and its fine-structure as well as collect information on sub-auroral and low-latitude phenomena is illustrated by selected results from the first six months of operation. The instrument is highly flexible and possible to operate in a number of different modes. It is also equipped with a 4-Megabyte burst memory for high data sampling rate and temporary storage of data. It has been fully operational since October 1992, and delivers data from ≈22 hr day−1 including about 5–6 auroral crossings of the northern and southern auroral ionosphere. New and important information on the auroral fine structure and electrodynamics is obtained by means of burst resolution data (6144 samples s−1) and normal resolution data (768 samples s−1). Common burst data collection triggered by the electric field event detector has turned out to be very useful for the selection of scientifically interesting events. This is illustrated by high-resolution data of a pair of extremely intense and narrow electric field structures (1 V m−1) which are associated with a total absence of precipitating particles, depletions of the thermal plasma and with an intense wave activity. The low inclination of theFreja orbit provides a new perspective for studying largescale phenomena associated with east-west gradients as is exemplified by electric field data from a satellite crossing over north-south oriented auroral structures presumably resulting from rotational distortions of east-west aligned auroral arcs. The different plasma regimes encountered byFreja are continuously monitored by means of current sweeps applied to the probes and by the satellite potential. In addition, overview data (8 samples s−1) are collected from full orbits and stored in the on-board memory and have proved to be extremely valuable, providing new information on global electric field phenomena at subauroral and lower latitudes, such as the intense poleward electric fields and Pc-1 observations that have been made near the plasmapause during substorm activity.

Cassini Plasma Spectrometer Investigation

Cassini/Huygens is a joint project of NASA and the European Space Agency designed to explore the Saturnian system in depth during its four-year mission. Cassini, the orbiter spacecraft, will carry twelve hardware investigations while Huygens, the Titan atmospheric probe, will carry an additional six. The Cassini Plasma Spectrometer (CAPS), one of 12 orbiter investigations, includes 3 plasma sensors designed to cover the broadest possible range of plasma energy, composition, and temporal variation. It is conservatively estimated that CAPS will provide a factor of ten or more improvement in measurement capabilities over those of the Voyager spacecraft at Saturn.

On the development of a magnetospheric substorm influenced by a storm sudden commencement: Ground, balloon, and satellite observations

Previous statistical investigations have revealed a relationship between storm sudden commencements (ssc) and magnetospheric substorm onsets. Little is known about the physical processes constituting this relationship. We used a comprehensive data set for a detailed case study. The ssc occurred on July 6, 1979, at 1930 UT. The substorm expansion phase started 5 min later. The event was preceded by a loading phase of more than 1.5 hours. The loading phase developed in three steps. During each step the cross-tail current sheet suddenly expanded earthward and intensified. The third step, at 1930 UT, coincided with the ssc. It was very likely caused by the interaction of ions with magnetohydrodynamic waves generated by the ssc. This step was followed at 1935 UT by the onset of the expansion phase accompanied by PiB magnetic pulsations, impulsive electron precipitation, and energetic ion injection at GEOS 2 orbit. The dipolarization of the geomagnetic field started 9 min after the expansion phase onset together with energetic electron injection and a decrease in the energetic ion flux. Signatures of Birkeland currents at dipolarization, and a pressure anisotropy P∥ions > P⊥ions during the 4 min before dipolarization, indicate field-aligned processes consistent with partial diversion of the cross-tail current into the ionosphere. The observations are discussed in the framework of a model in which the ballooning mode instability (BMI) developing in the near-Earth plasma sheet is regarded as the trigger process for the expansion phase. Quantitative estimates of the instability criteria show that the necessary conditions for the BMI are fulfilled during the whole loading phase. An energetic ion pressure gradient, which is needed to drive the BMI, was observed during the interval 1931 UT (ssc) to 1944 UT (dipolarization). Signatures of the instability itself, in terms of regular ion pressure gradient variations, were recorded during the same time interval. We conclude that the magnetosphere was potentially unstable for the BMI when the ssc occurred and that the interaction of the ssc with the magnetospheric particles expanded the cross-tail current sheet further earthward, thereby creating plasma conditions in which the BMI could grow. The instability started at 1931 UT, and the BMI in turn triggered the expansion phase at 1935 UT.

A modulated multiband Pc1 event observed by POLAR/EFI around the plasmapause

Abstract We study an electromagnetic ion cyclotron (EMIC) wave event observed simultaneously by the Electric Field Instrument (EFI) on board the POLAR spacecraft and by the Finnish pulsation magnetometer chain on April 25, 1997, when the two were in a good conjunction. EFI recorded waves at two frequency bands from L = 4.3 (in the outer plasmasphere) to L=6.2 (just outside the plasmapause). Both bands were observed in several conjugate stations on ground. The waves showed repetitive variations in amplitude, corresponding to classical Pc1 pearls. The repetition period was the same on ground and in space. Moreover, the repetition period of Pc1 pearls coincided with the period of simultaneous Pc4 waves observed by POLAR and on ground. The observations suggest that Pc1 pearls (EMIC waves in general) are modulated by Pc4 waves rather than result from the bouncing of a wave packet from one hemisphere to another.

Simulations of the response function of a plasma ion beam spectrometer for the Cassini mission to Saturn

To obtain very high (∼1%) energy resolution with spherical‐section electrostatic analyzers requires high precision in both fabrication and in the alignment process. In order to aid in the calibration of the instrument and to help minimize fabrication costs, we have applied simulation models to the ion beam spectrometer for the NASA/ESA Cassini mission to Saturn. In our previous article we studied the effects of misalignment and simple irregularities of the hemispherical surfaces on the performance of an electrostatic analyzer. We have considered a hemispherical electrostatic analyzer equipped with an aperture plate to collimate the stray electric field at the entrance apertures. The influence of a curved entrance aperture has also been added to the simulation model, and its effects have been studied in detail. A cylindrical three‐dimensional simultaneous overrelaxation algorithm has been introduced to solve for the stray electric field. The maximum loss of transmitted particles with respect to the transmi...

Comparison between simulations and calibrations of a high resolution electrostatic analyzer

The ion beam spectrometer (IBS) is one of the three spectrometers in the Cassini plasma spectrometer instrument aboard the Cassini/Huygens spacecraft. The IBS is a very high energy resolution hemispherical electrostatic analyzer. The design values of the IBS are analyzer gap 2.5 mm, and middle radius 100 mm. Because of the high energy resolution required, special care had to be used in the design and manufacturing of the instrument. A simulation was developed in order to aid the designing process. Here we show that the best fit to the laboratory calibration of the IBS flight model is obtained using the simulation model, where the inner hemisphere is misaligned by about 20 μm, where a maximum asymmetry of 75 μm is included in the inner hemisphere, and where the analyzer gap is increased from the design value by about 0.1 mm. We show here that geometric factors postulated in the theoretical model resulted in a better agreement between calibrations and simulations than the geometric factors calculated from c...