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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Identification of frequency components in electron content data by nonlinear filtering

A nonlinear filtering technique, called multiplicative homomorphic signal processing, is used to identify three groups of frequency components in electron content data. The first group of components results from the modulation of solar activity produced by the 27 day solar rotation. The second group is of lunar origin. The lunar frequencies are caused by nonlinear processes which probably occur in the dynamo region. The precise nature of these processes is not fully understood. The third group of frequency components, with a fundamental period of 13.9 days, has an unknown origin.

Bernhardt, P. A.↗
NASA NTRS1979

Lunar and Planetary Science Conference, 10th, Houston, Tex., March 19-23, 1979, Proceedings. Volume 3 - Planetary interiors and surfaces

The areas covered on the present symposium include geophysical investigations based on laboratory and planetary measurements; impact and volcanic processes; and planetary surface features and processes. The topics discussed include models of an early lunar dynamo, parameterized convection within the moon and the terrestrial planets, the bounds of the heat production rate with the moon, the shock metamorphism of granulated lunar basalt, asymmetric terracing of lunar highland craters, large impact basins on Mercury, impact melting in early lunar history, and evidence for ancient mare volcanism.

Merrill, R. B.↗
NASA NTRS1979

Primordial heating of asteroidal parent bodies

Most meteorites show evidence of thermal processing either because of metamorphic changes or as a result of melting and differentiation. Proposed mechanisms for supplying this energy generally rely upon short-lived radioisotopes or electrical induction, though accretion is sometimes mentioned, and more exotic models have been discussed. Interest in isotopic heating has been heightened by the discovery of Al-26 in Allende inclusions and also by the proposal that a lunar core and dynamo resulted from the radioactive decay of superheavy elements during the early solar system. Electrical induction as a heat source can be scaled to a broad range of solar system conditions, but corroborative evidence for these conditions is inconclusive. The accretion mechanism is probably not viable for the asteroidal and meteorite parent bodies, because the high kinetic energy requirement is inconsistent with the formation of the objects and their regoliths in the presence of a weak gravitational field.

Sonett, C. P.↗
NASA NTRS1980

Interplanetary shock waves and magnetospheric substorms

It is shown that substorm activity after a storm sudden commencement (SSC) depends on whether or not an interplanetary shock wave is accompanied by a large increase of the solar wind-magnetosphere energy coupling function. It has long been thought that substorm activity associated with an SSC results from sudden conversion of magnetic energy stored in the magnetotail, and that this conversion is triggered by the shock wave. However, the present result implies that the magnetospheric substorm is not a sudden conversion of stored magnetic energy, but is a direct consequence of increased efficiency of the solar wind-magnetosphere dynamo.

Akasofu, S.-I.↗
NASA NTRS1980

Midlatitude measurements of the ionospheric electric field during the ALADDIN programme

Three measurements of ionospheric electric field were made during the 24-h ALADDIN rocket program at Wallops Island on June 29-30, 1974. The first of these used a double probe instrument, flown at 1500 Local Solar Time, and the second and third measurements were made by barium cloud releases at evening and morning twilight. These three electric field vectors have been compared with the predictions of a number of models of electric field due to the dynamo effects of various atmospheric tides, and also of a possible magnetospheric origin. On the assumption that the measurements were made at a location equatorward of the afternoon convergence and poleward of the morning divergence in the electric field patterns related to the Sq current system, Stening's (1973) model of the diurnal variation of the electric field induced by the (1, -2) tidal model at the time of the summer solstice correctly predicts the directions of the observed electric field. Forbes and Lindzen's (1976, 1977) model, incorporating the three major propagating tidal modes as well as the evanescent (1, -2) mode, also bears an acceptable relationship to the ALADDIN electric field directions. The ALADDIN E-field magnitudes are comparable with those obtained by ground-based observations from Millstone Hill and from Saint Santin but are about half of Stening's model values, and three times those of Forbes and Lindzen. Saint Santin E-field directions, at the same latitude but 75 deg difference in longitude, are distinctly different from ALADDIN, implying that longitudinal differences are significant.

Rees, D.↗
NASA NTRS1980

A modeling of magnetic field variations during magnetospheric substorms

Magnetic field variations in the noon-midnight plane during the magnetospheric substorm are studied in terms of changes of three current systems: the dynamo-driven current on the magnetopause, the cross-tail current and the field-aligned current-auroral electrojet system. The field-aligned current is assumed to be generated as a result of interruption and subsequent diversion of the cross-tail current to the ionosphere. It is concluded that the available observations are consistent with a large increase of the three currents.

Akasofu, S.-I.↗
NASA NTRS1980

Lunar asymmetry and palaeomagnetism

A model is proposed for the early lunar evolution which accounts for the compositional asymmetry between the nearside and farside of the moon and the natural remanent magnetism of lunar rocks. According to the model, the preferred gravitational energy state consisted of an asymmetric accumulation of a liquid iron alloy (Fe-Ni and a small amount of sulfur) which displaces upwards the cold primordial undifferentiated core. The resulting depth asymmetry of the outer partially molten zone leads eventually to the subcrustal accumulation of light magnesium-rich pyroxenes and olivine, preferentially in one hemisphere, sufficient to explain the offset and also indirectly providing a possible explanation for the nearside concentration of KREEP and mass basalt. Slow downward migration of iron releases gravitational energy sufficient for convection and dynamo generation in an iron layer for about a billion years.

Stevenson, D. J.↗
NASA NTRS1981

Solar magnetic fields - The generation of emerging flux

X-ray observations have provided information about magnetic fields on the sun, and the implications of these observations are discussed. The pattern of small-scale flux emergence is quite different from that of active regions. It is inferred that the small-scale fields originate fairly high in the convective zone, while the fields in active regions have a deeper origin. The small-scale turbulent fields are only loosely related to the fields that define the normal solar cycle. The way in which dynamo models must be modified in the light of these results is indicated.

Golub, L.↗
NASA NTRS1980

Limits on the possible intrinsic magnetic field of Venus

Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined for possible surface-correlated features and any possible intrinsic magnetic moment. The field variations observed in the wake do not resemble those expected for a solar wind interaction with even a weak intrinsic magnetic field. Little orbit-to-orbit persistence of features is found in the magnetic records. The magnetic field measurements in the wake are averaged in 10 deg x 10 deg bins to minimize the effects of external field sources. In these 37 bins, the average fields appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, a maximum intrinsic magnetic dipole moment is obtained of 4.3 + or - 2.0 x 10 to the 21st G cu cm, approximately an order of magnitude less than previous estimates. It is noted that a more conservative estimate of the probable error of the mean is 5.5 x 10 to the 21st G cu cm. The Pioneer Venus measurements are thus consistent with zero planetary moment. The present measurements are found to be far below estimates made on the basis of angular momentum, the so-called magnetic Bode's law, and far below the dynamo scaling law of Busse.

Russell, C. T.↗
NASA NTRS1981

Equatorward shift of the cusp during magnetospheric substorms

The equatorward shift of the midday part of the auroral oval (the cusp region) during the expansion phase of magnetospheric substorms is reexamined. A magnetospheric model developed by Akasofu and Corrick (1980) is used, which is constructed by using the earth's dipole field, an image dipole field of the earth and 40 bisected circular loops. A new dayside current system represented by six current loops separated by 15 deg in longitude, each carrying 100 KA, is added to this model. For a total current of .6 MA, the latitude of the cusp shifts equatorward by 1.8 deg; for a current of 1.8 MA, the cusp location shifts by more than 5 deg. Thus, the magnetic field of an enhanced dynamo current is the dayside boundary layer and of the connected circuit can account for the observed equatorward shift of the cusp region.

Akasofu, S.-I.↗
NASA NTRS1981

On the coronae of rapidly rotating stars. I - The relation between rotation and coronal activity in RS CVn systems

Soft X-ray observations are presented of a nearly complete sample of RS Canum Venaticorum systems taken with the Einstein X-ray Observatory. It is shown that the quiescent coronal activity, as measured by the ratio of the X-ray to bolometric flux, is directly proportional to the angular velocity of the star with the active chromosphere in these systems. This relation is found to hold over two decades in angular velocity. It is also found that the stellar surface gravity has no obvious influence on the ratio of the X-ray luminosity to the bolometric luminosity over two decades in surface gravity. It is pointed out that the linear relation between the ratio of the X-ray luminosity to the bolometric luminosity on the one hand, and the angular velocity, on the other, holds important implications for dynamo theories of the generation of stellar magnetic fields.

Walter, F. M.↗
NASA NTRS1981

On the coronae of rapidly rotating stars. II - A period-activity relation in G stars

Observational evidence supporting the contention that coronal activity in G stars obeys the same relation with angular velocity as that observed in RS CVn systems is presented. It is found, however, that the level of the quiescent coronal activity, as measured by the ratio of the X-ray luminosity to the bolometric luminosity, at a given angular velocity is approximately one order of magnitude lower in G stars than in the RS CVn systems. Data on additional K stars indicate that these stars are indistinguishable from the K components of RS CVn systems. No evidence is found that the surface gravity has any appreciable effect on the level of coronal activity. The observation that the ratio of the X-ray luminosity to the bolometric luminosity is directly proportional to the product of the angular velocity and some function of the depth of the convective zone is seen as supporting a dynamo mechanism for the generation of the stellar magnetic flux.

Walter, F. M.↗
NASA NTRS1981

On a theory of temporal fluctuations in the electrostatic potential structures associated with auroral arcs

A possible mechanism is presented for the generation of large-amplitude temporal fluctuations in the structure of the electron energization region associated with auroral arcs. The mechanism is based on the observation that the auroral arc system resembles a laboratory circuit consisting of the series connection of battery, resistance and a forward biased diode containing collisionless plasma in which large-amplitude relaxation oscillations are sometimes observed to be superimposed on the steady-state current. It is shown that in both the laboratory and auroral systems, in which a localized auroral arc dynamo, the ionosphere and the electron energization region are involved, the oscillations are controlled by the times for ions and electrons to traverse the acceleration region, which also characterize the low- and high-frequency structure of the fluctuating waveform. It is demonstrated that a plausible one-dimensional double-layer model of the auroral arc acceleration region exhibits the dynamic negative resistance necessary for the generation of oscillations by the present mechanism. Finally, consideration is given to two kinds of auroral phenomena which might be associated with the mechanism: the 10-Hz quasi-periodic flickering aurora and 10-Hz modulations in the intensity of electrostatic hydrogen cyclotron waves.

Silevitch, M. B.↗
NASA NTRS1981

The magnetic fields of Jupiter and Saturn

The magnetic fields of Jupiter and Saturn and the characteristics of their magnetospheres, formed by interaction with the solar wind, are discussed. The origins of both magnetic fields are associated with a dynamo process deep in the planetary interior. The Jovian magnetosphere is analogous to that of a pulsar magnetosphere: a massive central body with a rapid rotation and an associated intense magnetic field. Its most distinctive feature is its magnetodisk of concentrated plasma and particle flux, and reduced magnetic field intensity. The magnetopause near the subsolar point has been observed at radial distances ranging over 50 to 100 Jovian radii, implying a relatively compressible obstacle to solar wind flow. The composition of an embedded current sheet within the magnetic tail is believed to be influenced by volcanic eruptions and emissions from Io. Spectral troughs of the Jovian radiation belts have been interpreted as possible ring particles. The Saturnian magnetosphere appears to be more like the earth in its topology. It is mainly characterized by a dipole axis parallel to the rotational axis of the planet and a magnetic field intensity much less than expected.

Ness, N. F.↗
NASA NTRS1981

On the seat of the solar cycle

A discussion of some of the issues raised in connection with the seat of the solar cycle are presented. Is the cycle controlled by a strictly periodic oscillator that operates in the core, or is it a turbulent dynamo confined to the convection zone and possibly a thin boundary layer beneath it? Sunspot statistics are discussed, with a view to ascertaining the length of the memory of the cycle, without drawing a definitive conclusion. Also discussed are some of the processes that might bring about variations delta L and delta R in the luminosity and the radius of the photosphere. It appears that the ratio W = delta lnR/delta lnL increases with the depth of the disturbance that produces the variations, so that imminent observations might determine whether or not the principal dynamical processes are confined to only the outer layers of the Sun.

Gough, D.↗
NASA NTRS1982

Torsional waves on the sun and the activity cycle

The detailed relation of the sun's torsional oscillation velocity feature to magnetic activity indicates (1) that these motions represent a fundamental oscillation within the sun which is responsible for the solar activity cycle, and (2) that they are not a natural consequence of an alpha-omega dynamo. It is demonstrated that a solar torsional oscillation with wave number 1/hemisphere exists.

Labonte, B. J.↗
NASA NTRS1981

Pulsar disk systems

It is proposed that radio pulsars have the same basic physical features as X-ray pulsars. Specifically, it is suggested that active radio pulsars are rotating neutron stars surrounded by fossil disks left over from the collapse event and that energy is extracted from the rotation of the neutron star by interaction with the disk to produce pulsar luminosity. Attention is given to a model in which the neutron star acts as a unipolar generator (or Faraday disk dynamo) and the disk acts as a load. The self-excitation of the disk/pulsar system is considered along with aspects of disk persistence, pulsed emission, and magnetically field aligned currents to the disk. An investigation is conducted regarding the possibility of a deposition of material in the form of a disk about a pulsar, taking into account questions concerning disk survival. It is found that there are some promising features regarding a disk system.

Michel, F. C.↗
NASA NTRS1980

The core and the magnetic field of Uranus

The existence of a magnetic field on Uranus is suggested by the tentative observation of 0.5 MHz pulses from the planet which are analogous to the Jovian decametric radiation. Analysis of a recent three-layer model leads to the conclusion that the electrical conductivity, physical state, and thermal gradients are such that a thermally driven hydromagnetic dynamo is probably operating in the core. An investigation is conducted to see whether the considered structure is suitable for the generation of a field. It is found that the existence of a magnetic field on Uranus can be understood as being a natural result of gravitational differentiation in a liquid, metallic core. The presence of a heat source in the core implies that the central temperature is approximately 8400 K rather than 7000 K in the original model

Torbett, M.↗