Engineering topics

Srnka, L. J.

Publications and source records attributed to Srnka, L. J..

17 records

NASA NTRS1984

Lunar magnetism

Aspects of lunar paleomagnetic and electromagnetic sounding results which appear inconsistent with the hypothesis that an ancient core dynamo was the dominant source of the observed crustal magnetism are discussed. Evidence is summarized involving a correlation between observed magnetic anomalies and ejecta blankets from impact events which indicates the possible importance of local mechanisms involving meteoroid impact processes in generating strong magnetic fields at the lunar surface. A reply is given to the latter argument which also presents recent evidence of a lunar iron core.

Hood, L. L.↗

NASA NTRS1980

Cometary collisions on the moon and Mercury

Unusual swirl patterns of bright and dark material on the moon and Mercury are proposed to be remnants of collisions with gas/dust-rich regions within a cometary coma. This interpretation provides important new clues for understanding cometary fine structure, impact effects of low-density material, and the origin of certain pronounced magnetic anomalies.

Schultz, P. H.↗

NASA NTRS1979

A study of the Rima Sirsalis lunar magnetic anomaly

The source of the lunar magnetic anomaly associated with the Rima Sirsalis linear rille has been modelled using the vector field intensities due to arbitrary uniform magnetization in a rectangular prism. It is shown that in order to match the Apollo 16 subsatellite data, the lunar surface near the rille must have a vertical magnetization of 6,000-9,000 G if the anomaly is due to flux leakage from a gap in the crust with the dimensions of the rille. An alternative explanation is that Rima Sirsalis and its surroundings are the site of a vertical magnetization contrast of 100,000-10,000 G which is at least as wide as the rille and extends to a depth of tens of kilometers in the crust. A wider magnetic source reduces the required magnetization (or depth) proportionately, since to first order the field at high altitude is proportional to the magnetic dipole moment per unit length.

Srnka, L. J.↗

NASA NTRS1979

Theory of global thermoremanent magnetization of planetary lithospheres in dipole fields of internal origin

A model is presented for the global thermoremanent magnetization of spherical lithospheres which cool in the presence of central dipole fields. Reversals and intensity variations of the field are incorporated in this model, which is applicable to bodies whose interiors have remained above the Curie point throughout their evolution. The model demonstrates that even considering Runcorn's (1975) magnetostatics theorems for spherical shells, a nonzero magnetic permeability and a finite cooling rate in the lithosphere permit the acquisition of a sizable global remanent dipole moment, which would be detectable by external measurements after the magnetizing field has disappeared. Preliminary application of this model to Mercury, Venus, and Mars suggests that only the combination of a nonreversing ancient source field with a surface value near 1 Oe plus a sizable concentration (about 1% by volume) of ferromagnetic material in their crusts could produce remanent planetary dipole fields as large as those measured by spacecraft. On the other hand, if ancient reversing dynamos existed in these planets, it is unlikely that large planetary-scale fields like those observed at Mercury could be due to remanence in their crusts, irrespective of their composition.

Srnka, L. J.↗

NASA NTRS1979

On the detection of lunar volatile emissions

This letter shows that the Apollo lunar-surface Suprathermal Ion Detection Experiment (SIDE) instruments lack the sensitivity to detect even large emissions of radiogenic gases from the moon if the venting of these gases occurs primarily at a few-well-defined sites of lunar transient phenomena (LTPs). It is suggested that specific flight instruments for the proposed ESA Polar Orbiting Lunar Observatory (POLO) mission, which could detect active venting, would help determine the energy source for LTPs and would increase knowledge of lunar geophysics. A critical-velocity model for the LTP energy source is briefly discussed.

Srnka, L. J.↗

NASA NTRS1979

Magnetic field and shock effects and remanent magnetization in a hypervelocity impact experiment

The impact of aluminum projectiles onto high-alumina terrestrial basalt blocks at 13-15 km/s in the presence of a variable magnetic field is studied. Plasma production but not field production was detected, and characteristics of the remanence and the shocked basalt are reported. Mineralogical data suggest that the magnetization acquired in the material near the craters is shock remanence. The experimental results might indicate that shock effects or possibly thermoremanence in ejecta fragments, may be responsible for part of the magnetization of the lunar surface.

Srnka, L. J.↗

NASA NTRS1979

Models of an early lunar dynamo

The hypothesis that a dynamo once existed in the moon, and hence was the source of lunar paleomagnetism, is tested using a model for the global thermoremanent magnetization of spherical lithospheres. Various models of an ancient lunar dipole field are used which incorporate intensity variations and reversals, and which are consistent with lunar sample paleointensities. It is found that non-reversing lunar dynamos which have simple exponential decay histories beginning at 4.6 b.y. ago are inconsistent with this limit, unless the moon has been thoroughly demagnetized to a depth of tens of kilometers by impacts or other processes. An early lunar dynamo cannot be excluded by global scalar measurements unless the permanent lunar dipole moment is shown to be significantly less than 10 to the 13th power G/cu cm by future spacecraft measurements, which may be technologically impossible; vector measurements of lunar magnetic anomalies over the whole moon offer a possibility of determining the source of the field which magnetized the lunar crust.

Srnka, L. J.↗

NASA NTRS1978

Spin-related magnetism of interstellar grains

The magnetic dipole moments and internal magnetic fields due to the spin of electrically charged elongated nonmagnetic interstellar grains in kinetic equilibrium with their surroundings are computed for the grain-size range from 0.01 to 1.0 micron. It is shown that the induced magnetic moments and internal magnetic fields of charged spinning nonmagnetic grains of arbitrary composition and prolate spheroidal shape can be appreciable, possibly even exceeding 0.01 emu/cu cm for 0.01-micron grains. The results indicate that virtually all grains smaller than 0.1 micron in mean diameter, and all elongated grains smaller than about 1 micron in length, are immersed in local magnetic fields due to spin that are much larger than the ambient galactic field. Some implications of this effect are discussed in relation to the polarization of starlight by aligned dust grains and the primordial remanent magnetization found in primitive carbonaceous chondrites.

Srnka, L. J.↗

NASA NTRS1978

An effect of nuclear electric quadrupole moments in thermonuclear fusion plasmas

Consideration of the nuclear electric quadrupole terms in the expression for the fusion Coulomb barrier suggests that this electrostatic barrier may be substantially modified from that calculated under the usual plasma assumption that the nuclei are electric monopoles. This effect is a result of the nonspherical potential shape and the spatial quantization of the nuclear spins of the fully stripped ions in the presence of a magnetic field. For monopole-quadrupole fuel cycles like p-B-11, the fusion cross-section may be substantially increased at low energies if the protons are injected at a small angle relative to the confining magnetic field.

De, B. R.↗

NASA NTRS1977

Recent activity in the moon; Proceedings of the Special Symposium, Houston, Tex., March 16, 1976

The papers review evidence for recent activity within the moon as manifested by lunar grid system, transient phenomena, moonquakes, and episodic emissions of radiogenic gases. Topics include a survey of lunar transient phenomena, possible causes of such phenomena, evidence that high-frequency seismic events may be shallow moonquakes, lunar seismicity and tectonics, a hypothesis on the nature of sites of lunar gas venting, and a search for sporadic gas emissions from the moon. Other contributions discuss the release of radiogenic argon-40 from the moon, radon-222 emission as an indicator of current activity on the moon, upper limits to gas emission from sites of lunar transient phenomena, physical processes that could produce transient changes on the lunar surface, critical-velocity gas-plasma interaction as a mechanism for lunar transient phenomena, and tidal triggering of moonquakes, transient phenomena, and radiogenic-gas emissions.

Runcorn, S. K.↗

NASA NTRS1977

Critical velocity phenomena and the LTP

When the relative velocity between magnetized plasma and neutral gas exceeds a critical value, the gas-plasma interaction is dominated by collective phenomena which rapidly excite and ionize the neutrals. The interaction of the solar wind with a large cloud (between 10 to the 24th and 10 to the 28th power neutrals) vented from the moon should be of this type. Line radiation from such an interaction can yield an apparent lunar surface brightness rivaling reflected sunlight levels over small areas, if the kinetic-energy flow density of the gas is sufficiently high. The aberrated solar-wind flow past the moon would enhance the visibility of such interactions near the lunar sunrise terminator, supporting the statistical studies which indicate that the 'Lunar Transient Phenomena' (anomalous optical phenomena on the moon) are significantly correlated with the position of the terminator on the lunar surface.

Srnka, L. J.↗

NASA NTRS1977

Spontaneous magnetic field generation in hypervelocity impacts

Hypervelocity impacts of meteoroids onto early planetary surfaces may have generated short-lived magnetic fields. The high specific power densities of the impacts, plasma production in the ejecta clouds, and the chemically layered targets of the meteoroids are analyzed in describing the evolution of the magnetic fields. Durations from about one millionth of a minute to one minute, as well as strengths up to 100 tesla, are posited for the impact-generated magnetic fields. The analogy of magnetic-field generation in laser-target experiments is also mentioned. The acquisition of shock remanence and thermoremanence by the ejecta and nearby rock following impact is discussed.

Srnka, L. J.↗

NASA NTRS1976

Sources of lunar limb compressions - Topographic correlations

Apollo 15 and 16 subsatellite observations of lunar limb compressions in the solar wind have been studied statistically by correlating occurrence rates for the compressions with seven topographic parameters derived from Apollo laser altimetry, with the use of a 5-deg x 5-deg selenographic grid. Maximum, rms, and average sunset-facing slopes are significantly correlated with occurrence rates in the source regions, but no correlation with elevation differences is found. Variance percentages suggest that a slope-related source mechanism generates about 6-8% of the limb compressions originating in source areas covered by the laser altimetry. These results confirm the preliminary conclusions of an earlier statistical study, based on Explorer 35 limb-compression observations, which used a coarser 15-deg x 15-deg grid

Srnka, L. J.↗

NASA NTRS1976

Magnetic dipole moment of a spherical shell with TRM acquired in a field of internal origin

The acquisition of thermoremanent magnetization (TRM) by a cooling spherical shell is studied for internal magnetizing dipole fields, using Runcorn's (1975) theorems on magnetostatics. If the shell cools progressively inward, inner regions acquire TRM in a net field composed of the dipole source term plus a uniform field due to the outer magnetized layers. In this case, the global dipole moment and external remanent field are nonzero when the whole shell has cooled below the Curie point and the source dipole has disappeared. The remanent field outside the shell is found to depend on the thickness, radii, and cooling rate of the shell, as well as the coefficient of TRM and the intensity of the magnetizing field. Some implications for the moon's remanent dipole moment are discussed.

Srnka, L. J.↗

NASA NTRS1976

On the global TRM of the lunar lithosphere

A global thermoremanent magnetization (TRM) theory is developed which takes into account the direction and magnitude of a hypothetical source dipole field, the distribution of magnetic permeability and the coefficient of TRM and the cooling rate in the lithosphere, and models the low-field TRM characteristics of the material. When applied to the moon in conjunction with lunar magnetic parameter ranges now available, the theory gives a permanent lunar dipole moment which is uncertain to a factor of about 10 to the 5th power.

Srnka, L. J.↗

NASA NTRS1975

Lunar topography and the limb compression source regions

Data from the Apollo 15, 16, and 17 laser altimeters have been used to study slopes, elevations, and roughness in the identifiable regions on the moon which sporadically produce plasma compressions and magnetic-field enhancements in the solar wind/lunar void boundary when those regions are at a flow limb. It is found that occurrence rates for such 'limb compressions' derived from Explorer 35 satellite measurements are significantly correlated with peak, average, and RMS slopes in the source regions, whereas rates derived from Apollo 15 and 16 subsatellite data are not correlated with topography. This suggests that two or more mechanisms operate in the source regions to produce limb compressions. Together with the known correlation between limb compressions and local surface remanent magnetic fields, the results indicate that lunar magnetization is not strongly related to surface features.

Srnka, L. J.↗

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.↗