Norman F. Ness

A model of interplanetary and coronal magnetic fields

A model of the large-scale magnetic field structure above the photosphere uses a Greens function solution to Maxwells equations. Sources for the magnetic field are related to the observed photospheric field and to the field computed at a ‘source’ surface about 0.6 R⊙ above the photosphere. The large-scale interplanetary magnetic field sector pattern is related to the field pattern at this ‘source’ surface. The model generates magnetic field patterns on the ‘source’ surface that compare well with interplanetary observations. Comparisons are shown with observations of the interplanetary magnetic field obtained by the IMP-3 satellite.

Observational constraints on the dynamics of the interplanetary magnetic field dissipation range

The dissipation range for interplanetary magnetic field fluctuations is formed by those fluctuations with spatial scales comparable to the gyroradius or ion inertial length of a thermal ion. It is reasonable to assume that the dissipation range represents the final fate of magnetic energy that is transferred from the largest spatial scales via nonlinear processes until kinetic coupling with the background plasma removes the energy from the spectrum and heats the background distribution. Typically, the dissipation range at 1 AU sets in at spacecraft frame frequencies of a few tenths of a hertz. It is characterized by a steepening of the power spectrum and often demonstrates a bias of the polarization or magnetic helicity spectrum. We examine Wind observations of inertial and dissipation range spectra in an attempt to better understand the processes that form the dissipation range and how these processes depend on the ambient solar wind parameters (interplanetary magnetic field intensity, ambient proton density and temperature, etc.). We focus on stationary intervals with well-defined inertial and dissipation range spectra. Our analysis shows that parallel-propagating waves, such as Alfven waves, are inconsistent with the data. MHD turbulence consisting of a partly slab and partly two-dimensional (2-D) composite geometry is consistent with the observations, while thermal paxticle interactions with the 2-D component may be responsible for the formation of the dissipation range. Kinetic Alfven waves propagating at large angles to the background magnetic field are also consistent with the observations and may form some portion of the 2-D turbulence component.

Magnetic Field Observations near Venus: Preliminary Results from Mariner 10

The NASA-GSFC magnetic field experiment on Mariner 10 is the first flight of a dual magnetometer system conceived to permit accurate measurements of weak magnetic fields in space in the presence of a significant and variable spacecraft magnetic field. Results from a preliminary analysis of a limted data set are summarized in this report, which is restricted primarily to Venus encounter. A detached bow shock wave that develops as the super Alfv�nic solar wind interacts with the Venusian atmosphere has been observed. However, the unique coincidence of trajectory position and interplanetary field orientation at the time of bow shock crossing led to a very disturbed shock profile with considerably enhanced upstream magnetic fluctuations. At present it is not possible to ascertain the nature and characteristics of the obstacle responsible for deflecting the solar wind flow. Far downstream disturbances associated with the solar wind wake have been observed.

Dissipation range dynamics : Kinetic Alfvén waves and the importance of βe

In a previous paper we argued that the damping of obliquely propagating kinetic Alfven waves, chiefly by resonant mechanisms, was a likely explanation for the formation of the dissipation range for interplanetary magnetic field fluctuations. This suggestion was based largely on observations of the dissipation range at 1 AU as recorded by the Wind spacecraft. We pursue this suggestion here with both a general examination of the damping of obliquely propagating kinetic Alfven waves and an additional examination of the observations. We explore the damping rates of kinetic Alfven waves under a wide range of interplanetary conditions using numerical solutions of the linearized Maxwell-Vlasov equations and demonstrate that these waves display the nearly isotropic dissipation properties inferred from the previous paper. Using these solutions, we present a simple model to predict the onset of the dissipation range and compare these predictions to the observations. In the process we demonstrate that electron Landau damping plays a significant role in the damping of interplanetary magnetic field fluctuations which leads to significant heating of the thermal electrons.

Magnetic field studies at jupiter by voyager 1: preliminary results.

Results obtained by the Goddard Space Flight Center magnetometers on Voyager 1 are described. These results concern the large-scale configuration of the Jovian bow shock and magnetopause, and the magnetic field in both the inner and outer magnetosphere. There is evidence that a magnetic tail extending away from the planet on the nightside is formed by the solar wind-Jovian field interaction. This is much like Earths magnetosphere but is a new configuration for Jupiters magnetosphere not previously considered from earlier Pioneer data. We report on the analysis and interpretation of magnetic field perturbations associated with intense electrical currents (approximately 5 x 106 amperes) flowing near or in the magnetic flux tube linking Jupiter with the satellite Jo and induced by the relative motion between Io and the corotating Jovian magnetosphere. These currents may be an important source of heating the ionosphere and interior of Io through Joule dissipation.

Heating of the low-latitude solar wind by dissipation of turbulent magnetic fluctuations

We test a theory presented previously to account for the turbulent transport of magnetic fluctuation energy in the solar wind and the related dissipation and heating of the ambient ion population. This theory accounts for the injection of magnetic energy through the damping of large-scale flow gradients, such as wind shear and compression, and incorporates the injection of magnetic energy due to wave excitation by interstellar pickup ions. The theory assumes quasi-two-dimensional spectral transport of the fluctuation energy and subsequent dissipation that heats the thermal protons. We compare the predictions of this theory with Voyager 2 and Pioneer 11 observations of magnetic fluctuation energy, magnetic correlation lengths, and ambient proton temperatures. Near-Earth Omnitape observations are used to adjust for solar variability, and the possibility that high-latitude effects could mask possible radial dependences is considered. We find abundant evidence for in situ heating of the protons, which we quantify, and show that the observed magnetic energy is consistent with the ion temperatures.

Magnetic field experiment for Voyagers 1 and 2

The magnetic field experiment to be carried on the Voyager 1 and 2 missions consists of dual low field (LFM) and high field magnetometer (HFM) systems. The dual systems provide greater reliability and, in the case of the LFMs, permit the separation of spacecraft magnetic fields from the ambient fields. Additional reliability is achieved through electronics redundancy. The wide dynamic ranges of ± 0.5 G for the LFMs and ± 20 G for the HFMs, low quantization uncertainty of ± 0.002 γ (γ = 10−5 G) in the most sensitive (± 8 γ) LFM range, low sensor RMS noise level of 0.006 γ, and use of data compaction schemes to optimize the experiment information rate all combine to permit the study of a broad spectrum of phenomena during the mission. Objectives include the study of planetary fields at Jupiter, Saturn, and possibly Uranus; satellites of these planets; solar wind and satellite interactions with the planetary fields; and the large-scale structure and microscale characteristics of the interplanetary magnetic, field. The interstellar field may also be measured.

Power spectra of the interplanetary magnetic field

Power spectra based on Pioneer 6 interplanetary magnetic field data in early 1966 exhibit a frequency dependence of f−2 in the range 2.8 × 10−4 to 1.6 × 10−2 cps for periods of both quiet and disturbed field conditions. Both the shape and power levels of these spectra are found to be due to the presence of directional discontinuities in the microstructure (< 0.01 AU) of the interplanetary magnetic field. Power spectra at lower frequencies, in the range of 2.3 × 10−6 to 1.4 × 10−4 cps, reflect the field macrostructure (> 0.1 AU) and exhibit a frequency dependence roughly between f−1 and f−3/2. The results are related to theories of galactic cosmic-ray modulation and are found to be consistent with recent observations of the modulation.

Observations of Mercury's magnetic field

This paper presents a study of magnetic field data obtained by Mariner 10 during the third and final encounter with the planet Mercury on 16 March 1975. A well developed bow shock and modest magnetosphere, previously observed at first encounter on 29 March 1974, were again observed. In addition, a much stronger magnetic field near closest approach, 400γ versus 98γ, was observed at an altitude of 327 km and approximately 68° north Mercurian latitude. Spherical harmonic analysis of the data provides an estimate of the centered planetary magnetic dipole of 5.0 × 1022 gauss-cm3 with the axis tilted 12° to the rotation axis and in the same sense as Earths. The interplanetary field was sufficiently different between first and third encounters that in addition to the very large field magnitude observed it argues strongly against a complex induction process generating the observed planetary field. While a possibility exists that Mercury possesses a remanent field due to magnetization early in its formation, a present day active dynamo seems to be a more likely candidate for its origin. The existence of such a dynamo argues for a mature planetary interior with a well-developed core.

MERCURY: MAGNETIC FIELD AND INTERIOR

Between 1965 and 1975, our knowledge of Mercury and its physical characteristics improved dramatically. Radar studies of the planetary orbit and rotation rate and Mariner 10 spacecraft studies of its surface, atmosphere, magnetic field and plasma environment provided startling new results on what had been the least understood member of the terrestrial planets. With a highly cratered surface and a modest magnetic field, Mercury is a differentiated planet with fractionally the largest iron core of all.