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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Mapping the self-generated magnetic fields due to thermal Weibel instability

The origin of the seed magnetic field that is amplified by the galactic dynamo is an open question in plasma astrophysics. Aside from primordial sources and the Biermann battery mechanism, plasma instabilities have also been proposed as a possible source of seed magnetic fields. Among them, thermal Weibel instability driven by temperature anisotropy has attracted broad interests due to its ubiquity in both laboratory and astrophysical plasmas. However, this instability has been challenging to measure in a stationary terrestrial plasma because of the difficulty in preparing such a velocity distribution. Here, we use picosecond laser ionization of hydrogen gas to initialize such an electron distribution function. We record the 2D evolution of the magnetic field associated with the Weibel instability by imaging the deflections of a relativistic electron beam with a picosecond temporal duration and show that the measured k -resolved growth rates of the instability validate kinetic theory. Concurrently, self-organization of microscopic plasma currents is observed to amplify the current modulation magnitude that converts up to ~1% of the plasma thermal energy into magnetic energy, thus supporting the notion that the magnetic field induced by the Weibel instability may be able to provide a seed for the galactic dynamo.

42 ENGINEERING↗
DOE OSTI2022

Measuring the melting curve of iron at super-Earth core conditions

The discovery of more than 4500 extrasolar planets has created a need for modeling their interior structure and dynamics. Given the prominence of iron in planetary interiors, we require accurate and precise physical properties at extreme pressure and temperature. A first-order property of iron is its melting point, which is still debated for the conditions of Earth’s interior. Here, we used high-energy lasers at the National Ignition Facility and in situ x-ray diffraction to determine the melting point of iron up to 1000 gigapascals, three times the pressure of Earth’s inner core. We used this melting curve to determine the length of dynamo action during core solidification to the hexagonal close-packed (hcp) structure. We find that terrestrial exoplanets with four to six times Earth’s mass have the longest dynamos, which provide important shielding against cosmic radiation.

58 GEOSCIENCES↗
DOE OSTI2022

On the Origin of Pulsar and Magnetar Magnetic Fields

Abstract In order to address the generation of neutron star magnetic fields, with particular focus on the dichotomy between magnetars and radio pulsars, we consider the properties of dynamos as inferred from other astrophysical systems. With sufficiently low (modified) Rossby number, convective dynamos are known to produce dipole-dominated fields whose strength scales with convective flux, and we argue that these expectations should apply to the convective protoneutron stars (PNSs) at the centers of core-collapse supernovae. We analyze a suite of three-dimensional simulations of core collapse, featuring a realistic equation of state and full neutrino transport, in this context. All our progenitor models, ranging from 9 M ⊙ to 25 M ⊙ , including one with initial rotation, have sufficiently vigorous PNS convection to generate dipole fields of order ∼10 15 Gauss, if the modified Rossby number resides in the critical range. Thus, the magnetar/radio pulsar dichotomy may arise naturally in part from the distribution of core rotation rates in massive stars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1961

Theory of Electrostatic Fields in the Ionosphere at Equatorial Latitudes

The properties of the elongated electrostatic fields that are required to provide the coupling mechanism in the dynamo-motor concept of the E and F regions of the ionosphere are examined theoretically for the conditions that prevail in equatorial latitudes. The analysis is developed for an electrostatic field of arbitrary horizontal scale in a horizontally stratified partly ionized gas subject to an imposed magnetic field having the form of a parabolic arch over the equator. The anisotropic character and continuous variation with height of the conductivity are retained throughout , and numerical solutions are determined for the attenuation of the electric field with distance along the field line. The results are similar qualitatively to those found previously upon analysis of the corresponding problem for middle latitudes, but the attenuation of the electrostatic field with height is considerably greater. It develops, in particular, that the coupling between E and F regions is very small for fields having horizontal wavelengths of a few kilometers. It follows that the dynamo-motor concept could not be used to account for the presence of irregularities of this scale. On the other hand, it is found that almost all the attenuation occurs at heights near that of the E region. If an electrostatic field having a horizontal wavelength of a few kilometers could be produced at heights of, say 200 km or greater, it follows that it would be very elongated and extend with little change in amplitude from hemisphere to hemisphere. The results also indicate that effective coupling could be achieved for fields having horizontal wavelengths of the order of tens of kilometers, but the assumptions introduced to simplify the analysis may impair the quantitative reliability of the results for fields of this scale.

Briggs, Benjamin R.↗
NASA NTRS1972

Convection electric fields and polar thermospheric winds.

Use of the qualitative ideas of convection electric fields over the earth's polar regions to demonstrate the importance of ion drag in establishing a thermospheric wind system. Recent measurements indicate that uniform electric fields of 10 to 40 mV/m are a regular feature of the polar-cap ionosphere. Calculations of the neutral thermospheric wind, using these measured fields in a simple ionospheric model, have been made. The time scale for motion of the neutral gas ranges from less than 1 hour at F-region heights to about 2 hours in the dynamo region of the ionosphere. It has been found that the viscosity of the atmosphere is important in determining the winds in the dynamo region. Results are given that show ion-temperature enhancements of hundreds of degrees that are due to ion-neutral frictional effects. In addition, the total deposition rate of convection energy in the polar thermosphere is shown to be of the same order of magnitude as that due to absorption of solar EUV radiation. The implications of these results for the dynamics and energetics of the thermosphere are discussed.

Fedder, J. A.↗
NASA NTRS1973

Jupiter's radiation belts.

A model for the production and loss of energetic electrons in Jupiter's radiation belt is presented. It is postulated that the electrons originate in the solar wind and are diffused in toward the planet by perturbations which violate the particles' third adiabatic invariant. At large distances, magnetic perturbations, electric fields associated with magnetospheric convection, or interchange instabilities driven by thermal plasma gradients may drive the diffusion. Inside about 10 Jupiter radii, the diffusion is probably driven by electric fields associated with the upper atmosphere dynamo which is driven by neutral winds in the ionosphere. The diurnal component of the dynamo wind fields produces a dawn-dusk asymmetry in the decimetric radiation from the electrons in the belts, and the lack of obvious measured asymmetries in the decimetric radiation measurements provides estimates of upper limits for these Jovian ionospheric neutral winds.

Brice, N.↗
NASA NTRS1973

Planetary magnetism in the outer solar system.

A brief review of the salient considerations which apply to the existence of magnetic fields in connection with planetary and subplanetary objects in the outer solar system is given. Consideration is given to internal dynamo fields, fields which might originate from interaction with the solar wind or magnetospheres (externally driven dynamos) and lastly fossil magnetic fields such as have been discovered on the moon. Where possible, connection is made between magnetism, means of detection, and internal body properties.

Sonett, C. P.↗
NASA NTRS1973

Electric fields and plasma convection in the plasmasphere.

A review is given of present theoretical and experimental information on quasistatic electric fields in and near the plasmasphere with the aim of discussing plasma flow within the plasmasphere, dynamo electric fields, and the penetration of electric fields of magnetospheric origin to low latitudes. Data are presented that suggest that the mean temporal variation of electric fields at all latitudes is diurnal, that dynamo electric fields are more important near L = 3 and are less important near and beyond L = 4 than are fields of magnetospheric origin, and that magnetospheric fields with periods that are a reasonable fraction of one day efficiently penetrate the plasmapause.

Mozer, F. S.↗
NASA NTRS1973

A numerical study of polar ionospheric currents.

Numerical calculations for the electric current in the polar ionosphere have been made by assuming some realistic distributions of the electric field and conductivity. Two dynamo actions are taken into account; one of which is induced by ionospheric winds and the other by the solar wind. For the solar wind dynamo action, it is found that the secondary polarization field caused by nonuniform distribution of ionospheric conductivity is much larger than the primary field induced by the solar wind, suggesting its important effect on charged particles in the magnetosphere, and that the irrotational current having a source and sink is of the same order of magnitude as the solenoidal current closing its circuit in the ionosphere. It is also found that the solar wind is, in general, more effective than the ionospheric winds in producing polar current systems such as DP 1 and 2, but in some cases the ionospheric winds have a significant effect on the current distribution.

Maeda, H.↗
NASA NTRS1973

On the improper neglect of certain terms in random function theory

This paper presents some exact solutions of problems in random function theory for the purpose of testing the validity of an approximate method known variously in the many different fields of its application as first order smoothing theory, first order cumulant discard, quasilinear theory, or the adiabatic approximation. The hydromagnetic dynamo equations are used here, as particularly appropriate for such an investigation. The calculations show that in one case the exact and approximate solutions agree. In the other case the approximate solution is wrong. Hence, in the absence of a general criterion for validity, a result based on first order smoothing theory is a conjecture rather than a fact. This impacts strongly on much of the recent work on hydromagnetic dynamos.

Lerche, I.↗
NASA NTRS1974

The aurora and the magnetosphere - The Chapman Memorial Lecture

Review of recent progress in magnetospheric physics, in particular, in understanding the magnetospheric substorm. It is shown that a number of magnetospheric phenomena can now be understood by viewing the solar wind-magnetosphere interaction as an MHD dynamo; auroral phenomena are powered by the dynamo. Also, magnetospheric responses to variations of the north-south and east-west components of the interplanetary magnetic field have been identified. The magnetospheric substorm is entirely different from the responses of the magnetosphere to the southward component of the interplanetary magnetic field. It may be associated with the formation of a neutral line within the plasma sheet and with an enhanced reconnection along the line. A number of substorm-associated phenomena can be understood by noting that the new neutral line formation is caused by a short-circuiting of a part of the magnetotail current.

Akasofu, S.-I.↗
NASA NTRS1975

Sheath-limited unipolar induction in the solar wind

A model of the steady-state interaction between the solar wind and an electrically conducting body having neither an atmosphere nor an intrinsic magnetic field sufficient enough to deflect the plasma flow is presented which considers some effects of a plasma surface sheath on unipolar induction. The Sonett-Colburn (1967, 1968) unipolar dynamo model is reviewed, and it is noted that the unipolar dynamo response of an electrically conducting body in the solar wind's motional field can be controlled by sheath effects in certain cases where the body radius is less than a certain critical value. It is shown that sheath effects do not limit the unipolar response of the moon or Mercury since their body radii are much larger than their critical radii. Sheath effects are also considered for asteroids, the Martian satellites, the irregular Jovian satellites, the outer satellites of Saturn, and meteorite parent bodies in a primordial enhanced solar wind.

Srnka, L. J.↗
NASA NTRS1975

Electric fields in the ionosphere

F-region drift velocities, measured by incoherent-scatter radar were analyzed in terms of diurnal, seasonal, magnetic activity, and solar cycle effects. A comprehensive electric field model was developed that includes the effects of the E and F-region dynamos, magnetospheric sources, and ionospheric conductivities, for both the local and conjugate regions. The E-region dynamo dominates during the day but at night the F-region and convection are more important. This model provides much better agreement with observations of the F-region drifts than previous models. Results indicate that larger magnitudes occur at night, and that daily variation is dominated by the diurnal mode. Seasonal variations in conductivities and thermospheric winds indicate a reversal in direction in the early morning during winter from south to northward. On magnetic perturbed days and the drifts deviate rather strongly from the quiet days average, especially around 13 L.T. for the northward and 18 L.T. for the westward component.

Kirchhoff, V. W. J. H.↗
NASA NTRS1976

Constraints on the nature of the ancient lunar magnetic field

Assuming that the physical properties of solar-wind ions have remained unchanged over the past 4 billion years, the observation of solar-wind ions in lunar breccias with compaction ages greater than 3.2 billion years places constraints on the nature and origin of the ancient lunar magnetic field. Solar-wind ions would not be expected to occur in old lunar breccias if a surface magnetic field of more than 0.03 gauss was present. Several explanations of this phenomenon are consistent with the global lunar dynamo theory of the origin of the lunar dipole field, including a wandering of the lunar dipole axis, late onset of dynamo action, and reversals of the lunar dipole field, producing a long-term field close to zero. Models invoking external field magnetization as the cause of the ancient lunar magnetic field constrain the dipole axis, precluding field reversals, and do not provide an alternative explanation for the observed occurrence of solar-wind ions in lunar breccias.

Goswami, J. N.↗
NASA NTRS1977

The magnetic field of Mercury

The Mariner 10 spacecraft encountered Mercury three times in 1974-1975. The first and third encounters provided detailed observations of a well-developed detached bow shock wave which results from the interaction of the solar wind. The planet possesses a global magnetic field and a modest magnetosphere, which deflects the solar wind. The field is approximately dipolar, with orientation in the same sense as earth, tilted 12 deg from the rotation axis. The magnetic moment corresponds to an undistorted equatorial field intensity of 350 gammas, approximately 1% of earth's. The field, while unequivocally intrinsic to the planet, may be due to remanent magnetization acquired from an extinct dynamo or a primordial magnetic field or due to a presently active dynamo. The latter possibility appears more plausible at present. In any case, the existence of the magnetic field provides very strong evidence of a mature differentiated planetary interior with a large core (core radius about 0.7 Mercury radius) and a record of the history of planetary formation in the magnetization of the crustal rocks.

Ness, N. F.↗
NASA NTRS1978

Magnetic field in the primitive solar nebula

Carbonaceous chondrites have apparently been magnetized in their early history in magnetic fields with intensities of 0.1 to 10 G, but the origin of the magnetizing field has remained obscured. It is suggested that the magnetic field recorded in the remanence of carbonaceous chondrites may have been produced by a self-excited hydromagnetic dynamo in the gaseous preplanetary nebula from which the solar system is thought to have formed. Recently computed models for the evolution of the preplanetary nebula, consisting of turbulent and differentially rotating gaseous disks with characteristic radial scales of several AU, are used to demonstrate the feasibility of this hypothesis. The maximum field intensity that might be realized by the dynamo production process is estimated to be as high as 1 to 10 G, taking into account two dynamical mechanisms that limit the strength of the field (the Coriolis force and ambipolar diffusion).

Levy, E. H.↗
NASA NTRS1979

The magnetic fields of Mercury, Mars, and moon

Mariner observations have shown a significant global magnetic field at Mercury with a dipole moment at a tilt of 14 + or - 5 deg relative to the normal of the orbit plane. A presently active dynamo is the most likely origin for the planet's magnetic field. Limited evidence for an intrinsic magnetic field on Mars was obtained by USSR spacecraft in 1971 and 1974. The Martian magnetic field, if it exists, may result from either remanent magnetism or an active dynamo. On the moon, local magnetic fields have been detected by the Apollo and Lunokhod missions, but no global correlation of the steady state values has been noted.

Ness, N. F.↗
NASA NTRS1979

The structure and the magnetic field of Uranus

The two-layer model of Uranus is recalculated using a rotation period of 24 rather than 10.8 hours and also the melting temperatures of H2O and of the constituents of the core are evaluated. It appears that the core is solid and the H2O-containing mantle is liquid. The conductivities of H2O, 'metallic' H2 and metallic H are roughly in the ratio of 1:100:10000 so that the convective velocities required for the operation of a dynamo would have to be about 10, 0.1 and 0.001 cm/sec, respectively. The very low thermal flux from Uranus suggests that, unless a precessional dynamo is invoked, metallically conducting H2 or metallic hydrogen has to be present in the mantle.

Torbett, M.↗