Magnetic Fields of the Outer Planets

The rapidly rotating giant planets of the outer solar system all possess strong dynamo-driven magnetic fields that carve a large cavity in the flowing magnetized solar wind. Each planet brings a unique facet to the study of planetary magnetism. Jupiter possesses the largest planetary magnetic moment, 1.55×10²⁰ Tm³, 2×10⁴ times larger than the terrestrial magnetic moment whose axis of symmetry is offset about 10° from the rotation axis, a tilt angle very similar to that of the Earth. Saturn has a dipole magnetic moment of 4.6×10¹⁸ Tm³ or 600 times that of the Earth, but unlike the Earth and Jupiter, the tilt of this magnetic moment is less than 1° to the rotation axis. The other two gas giants, Uranus and Neptune, have unusual magnetic fields as well, not only because of their tilts but also because of the harmonic content of their internal fields. Uranus has two anomalous tilts, of its rotation axis and of its dipole axis. Unlike the other planets, the rotation axis of Uranus is tilted 97.5° to the normal to its orbital plane. Its magnetic dipole moment of 3.9×10¹⁷ Tm³ is about 50 times the terrestrial moment with a tilt angle of close to 60° to the rotation axis of the planet. In contrast, Neptune with a more normal obliquity has a magnetic moment of 2.2×10¹⁷ Tm³ or slightly over 25 times the terrestrial moment. The tilt angle of this moment is 47°, smaller than that of Uranus but much larger than those of the Earth, Jupiter and Saturn. These two planets have such high harmonic content in their fields that the single flyby of Voyager was unable to resolve the higher degree coefficients accurately. The four gas giants have no apparent surface features that reflect the motion of the deep interior, so the magnetic field has been used to attempt to provide this information. This approach works very well at Jupiter where there is a significant tilt of the dipole and a long baseline of magnetic field measurements (Pioneer 10 to Galileo). The rotation rate is 870.536° per day corresponding to a (System III) period of 9 h 55 min 26.704 s. At Saturn, it has been much more difficult to determine the equivalent rotation period. The most probable rotation period of the interior is close to 10 h 33 min, but at this writing, the number is still uncertain. For Uranus and Neptune, the magnetic field is better suited for the determination of the planetary rotation period but the baseline is too short. While it is possible that the smaller planetary bodies of the outer solar system, too, have magnetic fields or once had, but the current missions to Vesta, Ceres and Pluto do not include magnetic measurements.     

Energetics of the magnetosphere

Energy flow in various large-scale processes of the Earth's magnetosphere is examined. This energy comes from the solar wind, via the dawn-to-dusk convection electric field, a field established primarily by magnetic merging but with viscous-like boundary interaction as a possible contributor. The convection field passes about 5 × 10¹¹ W to the near-Earth part of the plasma sheet, and also moves the plasma earthward. In addition, 1–3 × 10¹¹ W are given to the complex system of the Birkeland currents: about 4 × 10¹⁰ of this, on the average, goes to parallel acceleration, chiefly of auroral electrons, about 2–3 times that amount to joule heating of the ionosphere, and the rest heats the ring current. The ring current stores energy (mainly as kinetic energy of particles) of the order of 2 × 10¹⁵ J, and this value rises and decays during magnetic storms, on time scales ranging from a fraction of a day to several days. The tail can store comparable amounts as magnetic energy, and appreciable fractions of its energy may be released in substorms, on time scales of tens of minutes. The sporadic power level of such events reaches the order of 3 × 10¹² W. The role of magnetic merging in such releases of magnetic energy is briefly discussed, as is the correlation between properties of the solar wind and magnetospheric power levels.     

The Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) Experiment

The Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment is one of five experiments which together comprise the Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, and a spectrum analyser to calculate in near-real time aboard the spacecraft, the complete auto- and cross-spectral matrices using the three magnetic and two electric components of the electromagnetic field. The magnetic waveform at frequencies below either 10 Hz or 180 Hz is also transmitted. The sensitivity of the search coil is adapted to the phenomena theo be studied: the values 3 × 10^-3 nT Hz^-1/2 and 3 × 10^-5 nT Hz^-1/2 are achieved respectively at 1 Hz and 100 Hz. The dynamic range of the STAFF instruments is about 96 dB in both waveform and spectral power, so as to allow the study of waves near plasma boundaries. Scientific objectives of the STAFF investigations, particularly those requiring four point measurements, are discussed. Methods by which the wave data will be characterised are described with emphasis on those specific to four-point measurements, including the use of the Field Energy Distribution function.     

Galactic effects of the cosmic-ray gas

On an astronomical scale cosmic rays must be considered a tenuous and extremely hot (relativistic) gas. The pressure of the cosmic-ray gas is comparable to the other gas and field pressures in interstellar space, so that the cosmic-ray pressure must be taken into account in treating the dynamical properties of the gaseous disk of the galaxy. This review begins with a survey of present knowledge of the cosmic-ray gas. Then the kinetic properties of the gas are developed, followed by an exposition of the dynamical effects of the cosmic-ray gas on a large-scale magnetic field embedded in a thermal gas. The propagation of low-frequency hydromagnetic waves is worked out in the fluid approximation.The dynamical properties of the gaseous disk of the galaxy are next considered. The equations for the equilibrium distribution in the direction perpendicular to the disk are worked out. It is shown that a self-consistent equilibrium can be constructed within the range of the observational estimates of the gas density, scale height, turbulent velocity, field strength, cosmic-ray pressure, and galactic gravitational acceleration. Perturbation calculations then show that the equilibrium is unstable, on scales of a few hundred pc and in times of the order 2 × 10⁷ years. The instability is driven about equally by the magnetic field and the cosmic-ray gas and dominates self-gravitation. Hence the instability dominates the dynamics of the interstellar gas and is the major effect in forming interstellar gas clouds. Star formation is the end result of condensation of the interstellar gas into clouds, indicating, then, that cosmic rays play a major role in initiating star formation in the galaxy.The cosmic rays are trapped in the unstable gaseous disk and escape from the disk only in so far as their pressure is able to inflate the magnetic field of the disk. The observed scale height of the galactic disk, the short life (10⁶ years) of cosmic-ray particles in the disk of the galaxy, and their observed quiescent state in the disk, indicate that the galactic magnetic field acts as a safety valve on the cosmic ray pressure P so that PB ²/8. We infer from the observed life and quiescence of the cosmic rays that the mean field strength in the disk of the galaxy is 3–5 × 10^–6 gauss.     

The Rosat mission

The primary scientific objective of the ROSAT mission is to perform the first all sky survey with an imaging X-ray telescope leading to an improvement in sensitivity by several orders of magnitude compared with previous surveys. Consequently a large number of new sources (> 10⁵) will be discovered and located with an accuracy of 1 arcmin. After completion of the survey which will take about half a year the instrument will be used for detailed observations of selected targets.The X-ray telescope consists of a fourfold nested Wolter type I mirror system with 80 cm aperture and 240 cm focal length, and three focal plane detectors. In the baseline version these will be imaging proportional counters (0.1 – 2 keV) providing a field of view of 2⁰ × 2⁰.     

The FAST Satellite Fields Instrument

We describe the electric field sensors and electric and magnetic field signal processing on the FAST (Fast Auroral SnapshoT) satellite. The FAST satellite was designed to make high time resolution observations of particles and electromagnetic fields in the auroral zone to study small-scale plasma interactions in the auroral acceleration region. The DC and AC electric fields are measured with three-axis dipole antennas with 56 m, 8 m, and 5 m baselines. A three-axis flux-gate magnetometer measures the DC magnetic field and a three-axis search coil measures the AC magnetic field. A central signal processing system receives all signals from the electric and magnetic field sensors. Spectral coverage is from DC to 4 MHz. There are several types of processed data. Survey data are continuous over the auroral zone and have full-orbit coverage for fluxgate magnetometer data. Burst data include a few minutes of a selected region of the auroral zone at the highest time resolution. A subset of the burst data, high speed burst memory data, are waveform data at 2×10⁶ sample s^–1. Electric field and magnetic field data are primarily waveforms and power spectral density as a function of frequency and time. There are also various types of focused data processing, including cross-spectral analysis, fine-frequency plasma wave tracking, high-frequency polarity measurement, and wave-particle correlations.     

The Broad Band X-Ray Telescope

The Broad-Band X-Ray Telescope (BBXRT) has been designed to perform high sensitivity, moderate resolution spectrophototnetry of X-ray sources in the 0.3–12 keV band from the Shuttle. It consists of a coaligned pair of high throughput, conical X-ray imaging mirrors, with a cryogenically-cooled, multiple element, Si(Li) spectrometer at the focus of each. On axis, BBXRT will have an effective area of 580 cm² at 2 keV and 250 cm² at 7 keV, and a spectral resolution of 110 eV at 2 keV and 150 eV at 7 keV. A 10⁴ s observation with BBXRT will allow a determination of the continuum spectral shape for sources near the Einstein deep survey limit.     

LISM structure — Fragmented superbubble shell?

Small scale structure in local interstellar matter (LISM) is considered. Overall morphology of the local cloud complex is inferred from Ca II absorption lines and observations of H I in white dwarf stars. Clouds with column densities ranging from 2–100 × 10¹⁷ cm^–2 are found within 20 pc of the Sun. Cold (50 K) dense (10⁵ cm^–3) small (5–10 au) clouds could be embedded and currently undetected in the upwind gas. The Sun appears to be embedded in a filament of gas with thickness 0.7 pc, and cross-wise column density 2 × 10¹⁷ cm^–2. The local magnetic field direction is parallel to the filament, suggesting that the physical process causing the filamentation is MHD related. Enhanced abundances of refractory elements and LISM kinematics indicate outflowing gas from the Scorpius-Centaurus Association. The local flow vector and Sco data are consistent with a 4,000,000 year old superbubble shell at –22 km s^–1, which is a shock front passing through preshock gas at –12 km s^–1, and yielding cooled postshock gas at –26 km s^–1in the upwind direction. A preshock magnetic field strength of 1.6 G, and postshock field strength of 5.2 G embedded in the superbubble shell, are consistent with the data.Abbreviations LISM Local ISM - SIC Surrounding Interstellar Cloud - LIC Local Interstellar Cloud     

An EXOSAT observation of the morphology of the coronal X-ray emission from algol

The X-ray emission from Algol is thought to originate in a corona associated with the K star in this system. We report the results of a 35 hr continuous EXOSAT observation through secondary optical eclipse that was designed to measure the structure of the corona. No obvious X-ray eclipse was seen. The spectrum measured by the ME gives a temperature of 2.5 × 10⁷ K, consistent with the hard component previously seen by the Einstein SSS. The soft component previously reported by the SSS would only contribute at most 25% to the count rate seen in the LE (used with Al/P). The lack of a hard X-ray eclipse indicates the dimensions of the higher temperature emission region to be comparable to or greater than the size of the K star. An X-ray flare was detected with a peak luminosity of 1.4 × 10³¹ erg s^-1 and a total duration of 8 hours. The peak temperature was 5.0 keV with an emission measure of 9.4 × 10⁵³ cm^-3. The thermal nature of the flare is confirmed by the detection of an iron line with an EW of 2 keV. By equating the observed decay time of the flare to a known cooling law gives a dimension for the flaring loop of 0.3 stellar radii. This is much smaller than the dimensions of the hard component inferred from the lack of an eclipse. It seems probable that the flare occurred in one of the loops responsible for the lower temperature component seen by the SSS.     

2A0526-328: The white dwarf rotation period revealed

We report results from EXOSAT observations of the intermediate polar system 2A0526-328 (TV Col). The hard X-ray emission (2–8 keV) is modulated with a period of 1943 s, interpreted as the white-dwarf rotation period. Soft and hard X-ray emission show intensity minima, in phase with the orbital period of 0.2286 days; analysis of the hard X-ray spectra shows that these minima are caused by an extra low-energy absorption corresponding to a H column density of 4 × 10²² cm^-2.     

Observability of magnetically strongly shifted iron line emission from X-ray pulsars

As an application of our extensive calculations of energies and intensities of atomic lines in very intense magnetic fields of the order of 10¹¹-10¹³ G we discuss the possibility of observing magnetically strongly shifted iron lines in the spectra of pulsating X-ray sources. Careful estimates of the relevant parameters lead us to the conclusion that it would be profitable to look for magnetically shifted iron line enission in magnetic neutron stars of low luminosity using spectrometers working in the energy range 10 – 100 keV with sensitivities of 10^–4 cm^–2 s^–1 and energy resolutions E/E 10 – 100.This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG).     

Grad-Shafranov Reconstruction of Magnetic Flux Ropes in the Near-Earth Space

Electric currents permeate space plasmas and often have a significant component along the magnetic field to form magnetic flux ropes. A larger spatial perspective of these structures than from the direct observation along the satellite path is crucial in visualizing their role in plasma dynamics. For magnetic flux ropes that are approximately two-dimensional equilibrium structures on a certain plane, Grad-Shafranov reconstruction technique, developed by Bengt Sonnerup and his colleagues (see Sonnerup et al. in J. Geophys. Res. 111:A09204, 2006), can be used to reveal two-dimensional maps of associated plasma and field parameters. This review gives a brief account of the technique and its application to magnetic flux ropes near the Earth’s magnetopause, in the solar wind, and in the magnetotail. From this brief survey, the ranges of the total field-aligned current and the total magnetic flux content for these magnetic flux ropes are assessed. The total field-aligned current is found to range from ∼0.14 to ∼9.7×10⁴ MA, a range of nearly six orders of magnitude. The total magnetic flux content is found to range from ∼0.25 to ∼2.3×10⁶ MWb, a range of nearly seven orders of magnitude. To the best of our knowledge, this review reports the largest range of both the total field-aligned current and the total magnetic flux content for magnetic flux ropes in space plasmas.     

Simultaneous EXOSAT-IUE observations of NGC 4151

NGC 4151 was observed four times in Nov. 83. The results indicate that: a) there exists a correlation between the X-ray and UV fluxes on the long term; b) the soft X-ray excess between 0.1 and 1 keV is probably steeper than expected from the leaky absorber model by Holt et al (1980); c)the spectral fit to the ME data, after correction for a soft component, yields =1,73±0.27, N_H=(15.2±2.2)×10²² cm^–2, E.W.(Fe line)=0.208±0.084 keV, and does not require a strong overabundance of Fe in the absorber. The relationship between N_H and the strength of the broad emission lines is commented.     

Time variation of the pulse period of Vela X-1

X-ray pulsar Vela X-l was observed with the X-ray astronomy satellite HAKUCHO in five occasions between March 1979 and March 1981. An increase of the pulsation period at an average rate of P/P 3.0 × 10^–4 yr^–1 was observed over the time span of two years. Besides, variations of the pulse period in the time scale of 10 days were resolved in superposition on the secular spin-down trend. The observed rate of change P - 3 × 10^–8, for both spin-up and spin-down, is an order of magnitude greater than the secular spin-down rate.     

ROMAP: Rosetta Magnetometer and Plasma Monitor

The scientific objectives, design and capabilities of the Rosetta Lander’s ROMAP instrument are presented. ROMAP’s main scientific goals are longterm magnetic field and plasma measurements of the surface of Comet 67P/Churyumov-Gerasimenko in order to study cometary activity as a function of heliocentric distance, and measurements during the Lander’s descent to investigate the structure of the comet’s remanent magnetisation. The ROMAP fluxgate magnetometer, electrostatic analyser and Faraday cup measure the magnetic field from 0 to 32 Hz, ions of up to 8000 keV and electrons of up to 4200 keV. Additional two types of pressure sensors – Penning and Minipirani – cover a pressure range from 10⁻⁸ to 10¹ mbar. ROMAP’s sensors and electronics are highly integrated, as required by a combined field/plasma instrument with less than 1 W power consumption and 1 kg mass.     

The Effects of Transverse Magnetic Field and Density Variations on the Particle Energy Spectra in a Reconnecting 3D Current Sheet

Electron and proton acceleration by a super-Dreicer electric field is further investigated in a non-neutral reconnecting current sheet (RCS) with a variable plasma density. The tangential B _z and transverse magnetic field components B _x are assumed to vary with the distances x and z from the X nullpoint linearly and exponentially, respectively; the longitudinal component (a ‘guiding field’) is accepted constant. Particles are found to gain a bulk of their energy in a thin region close to the X nullpoint where the RCS density increases with z exponentially with the index λ and the tangential magnetic field B _x also increases with z exponentially with the index α. For the RCS with a constant density (λ = 0), the variations of the tangential magnetic field lead to particle power-law energy spectra with the spectral indices γ₁ being dependent on the exponent α as: for protons and for electrons in a strong guiding field (β > 10⁻²) and for electrons in a moderate or weak guiding field (β > 10⁻⁴). For the RCS with an exponential density increase in the vicinity of the X nullpoint (λ≥ 0) there is a further increase of the resulting spectral indices γ that depends on the density exponent index λ as for protons and for electrons in weaker guiding fields and as for electrons in stronger guiding fields. These dependencies can explain a wide variety (1.5–10) of particle spectral indices observed in solar flares by the variations of a magnetic field topology and physical conditions in a reconnecting region. This can be used as a diagnostic tool for the investigation of the RCS dynamics from the accelerated particle spectra found from hard X-ray and microwave emission.     

X-ray and optical observations of Nova Muscae 1983 during its nebular stage

The detection of X-rays from Nova Muscae 1983 (discovered on January 18, 1983) constitutes the first detection of X-rays from a classical nova during outburst. X-ray observations were carried out on 1984 April 20 and July 14 when Nova Mus had entered the nebular stage. During both observations no significant flux was observed with the medium energy detectors (2–50 keV). The source was detected with the low energy detector (.04–2 keV) using 3000 Å Lexan and Parlene- N-Aluminium filters; counting rates of (3.4 ± 1.2) × 10³ and (3.7 ± 1.2 × 10^-3 counts/sec were measured with the respective filters. The source was detected again on July 14 with about the same intensity. Either a shocked shell of circumstellar gas emitting 10⁷ thermal bremsstrahlung at 10³⁵ erg/sec intensity or a white dwarf remnant emitting 3.5 × 10⁵ blackbody radiation at 10³⁷ erg/sec luminosity are compatible with the measurements.Spectra taken in the visual spectral range show strong forbidden coronal emission lines of [FeVII] 6085, [FeX] 6374, and as never observed before in such a strength, [FeXIV] 5303 requiring excitation temperatures of 2 × 10⁶ °K.     

The discovery of a 25 min regular modulation in the X-ray flux from 2S0142+61

A 13 hr observation of 2S0142+61 on 1984 August 27 by EXOSAT shows the X-ray flux of 2S0142+61 to be modulated with a period of 1456+/-6 s. The 1–10 keV spectrum is two component with a 0.7 keV thermal and 0.0 energy index power law, with 30% of the total luminosity in the thermal component. The spectrum is absorbed by 1 × 10²² H cm^-2. Only the hard component is pulsed with a 3 to 10 keV peak to mean amplitude of 35%. Below 2 keV the modulation is less than a few percent. The total 1–10 keV luminosity is 3.5 × 1032 erg s^-1 for a distance of 100 pc. Possible optical counterparts are discussed.     

EXOSAT observations of the BL Lac object Mkn 421

The BL Lac object Mkn 421 was observed by EXOSAT four times over a period of six days in February 1984. Significant X-ray variability was apparent on a timescale of less than a day, but with no accompanying spectral change. The source exhibited a very soft power law X-ray spectrum with an extremely low intrinsic column density (N_H10²⁰ cm^–2). There was no evidence for an additional hard component attributable to synchrotron self-Compton emission. The observations when combined with other published data imply that significant changes occur in the form of the broad-band UV/X-ray continuum of this source.     

Coronal heating by dissipation of magnetic structure

Coronal loops are heated by the release of stored magnetic energy and by the dissipation of MHD waves. Both of these processes rely on the presence of internal structure in the loop. Tangled or sheared fields dissipate wave energy more efficiently than smooth fields. Also, a highly structured field contains a large reservoir of free magnetic energy which can be released in small reconnection events (microflares and nanoflares). The typical amount of internal structure in a loop depends on the balance between input at the photosphere and dissipation. This paper describes measures of magnetic structure, how these measures relate to the magnetic energy, and how photospheric motions affect the structure of a loop.The magnetic energy released during a reconnection event. can be estimated if one knows the equilibrium energy before and after the event. For a loop with highly tangled field lines, a direct solution of the equilibrium equations may be difficult. However, lower bounds can be placed on the energy of the equilibrium field, given a measure of the tangling known as the crossing number. These bounds lead to an estimate of the buildup of energy in a coronal loop caused by random photospheric motions. Parker's topological dissipation model can plausibly supply the 10⁷ erg cm^–2 s^–1 needed to heat the active region corona. The heating rate can be greatly enhanced by fragmentation of flux tubes, for example by the breakup of photospheric footpoints and the formation of new footpoints.