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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Hematite Versus Magnetite as the Signature for Planetary Magnetic Anomalies?

Crustal magnetic anomalies are the result of adjacent geologic units having contrasting magnetization. This magnetization arises from induction and/or remanence. In a planetary context we now know that Mars has significant crustal magnetic anomalies due to remanent magnetization, while the Earth has some anomalies where remanence can be shown to be important. This picture, however, is less clear because of the nature and the magnitude of the geomagnetic field which is responsible for superimposed induced magnetization. Induced magnetization assumes a magnetite source, because of its much greater magnetic susceptibility when compared with other magnetic minerals. We investigated the TRM (thermoremanent magnetization) acquisition of hematite, in weak magnetic fields up to 1 mT, to determine if the remanent and induced magnetization of hematite could compete with magnetite. TRM acquisition curves of magnetite and hematite show that multi-domain hematite reaches TRM saturation (0.3 - 0.4 A sq m/kg) in fields as low as 100 microT. However, multi-domain magnetite reaches only a few percent of its TRM saturation in a field of 100 microT (0.02 - 0.06 A sq m/kg). These results suggest that a mineral such as hematite and, perhaps, other minerals with significant remanence and minor induced magnetization may play an important role in providing requisite magnetization contrast. Perhaps, and especially for the Mars case, we should reevaluate where hematite and other minerals, with efficient remanence acquisition, exist in significant concentration, allowing a more comprehensive explanation of Martian anomalies and better insight into the role of remanent magnetization in terrestrial crustal magnetic anomalies.

Kletetshka, Gunther↗
NASA NTRS2002

Particle Events as a Possible Source of Large Ozone Loss during Magnetic Polarity Transitions

The energy deposition in the mesosphere and stratosphere during large extraterrestrial charged particle precipitation events has been known for some time to contribute to ozone losses due to the formation of potential ozone destroying species like NO(sub x), and HO(sub x). These impacts have been measured and can be reproduced with chemistry models fairly well. In the recent past, however, even the impact of the largest solar proton events on the total amount of ozone has been small compared to the dynamical variability of ozone, and to the anthropogenic induced impacts like the Antarctic 'ozone hole'. This is due to the shielding effect of the magnetic field. However, there is evidence that the earth's magnetic field may approach a reversal. This could lead to a decrease of magnetic field strength to less than 25% of its usual value over a period of several centuries . We show that with realistic estimates of very large solar proton events, scenarios similar to the Antarctic ozone hole of the 1990s may occur during a magnetic polarity transition.

vonKoenig, M.↗
NASA NTRS1995

The Sources of Bz Fluctuations within CIRs: Magnetic Storms During the Descending Phase of the Solar Cycle

This presentation examines the magnetic field fluctuations within Corotating Interaction Regions (CIRs) detected by Ulysses at mid- and low-latitudes. CIRs are formed by the interaction of high-speed streams flowing from the polar coronal hole with slow-speed streams. Several wave modes are identified, and the effectiveness of these waves causing magnetic storms at Earth will be discussed.

Space Physics Magnetic Storms Solar Cycles Sun↗
NASA NTRS2007

The Cosmic Battery Revisited

We reinvestigate the generation and accumulation of magnetic flux in optically thin accretion flows around active gravitating objects. The source of the magnetic field is the azimuthal electric current associated with the Poynting-Robertson drag on the electrons of the accreting plasma. This current generates magnetic field loops which open up because of the differential rotation of the flow. We show through simple numerical simulations that what regulates the generation and accumulation of magnetic flux near the center is the value of the plasma conductivity. Although the conductivity is usually considered to be effectively infinite for the fully ionized plasmas expected near the inner edge of accretion disks, the turbulence of those plasmas may actually render them much less conducting due to the presence of anomalous resistivity. We have discovered that if the resistivity is sufficiently high throughout the turbulent disk while it is suppressed interior to its inner edge, an interesting steady-state process is established: accretion carries and accumulates magnetic flux of one polarity inside the inner edge of the disk, whereas magnetic diffusion releases magnetic flux of the opposite polarity to large distances. In this scenario, magnetic flux of one polarity grows and accumulates at a steady rate in the region inside the inner edge and up to the point of equipartition when it becomes dynamically important. We argue that this inward growth and outward expulsion of oppositely-directed magnetic fields that we propose may account for the approx. 30 min cyclic variability observed in the galactic microquasar GRS1915+105.

Contopoulos, Ioannis↗
NASA NTRS2012

What Properties of CMEs are Most Important for Space Weather?

Severe space weather is characterized by intense particle radiation from the Sun and major geomagnetic storm caused by magnetized solar plasmas arriving at Earth. Coronal mass ejections (CMEs) are key players in both these aspects. CMEs traveling at super-Alfv nic speeds drive fast-mode MHD shocks that create the high levels of particle radiation. When a CME arrives at Earth, the CME-associated magnetic fields reconnect with Earth s magnetopause fields resulting in solar plasma entry into the magnetosphere and a geomagnetic storm depending on the magnetic structure of the CME. Particle radiation starts affecting geospace as soon as the CMEs leave the Sun and the geospace may be immersed in the radiation for several days. On the other hand, the geomagnetic storm happens only upon CME arrival at Earth. The requirements for the production of particles and magnetic storms by CMEs are different in a number of respects: solar source location, CME magnetic structure, conditions in the ambient solar wind, and shock-driving ability of CMEs. Intense shocks arriving at Earth have additional space weather effects such as sudden impulse that shrinks the magnetosphere often exposing satellites in geosynchronous orbit to the solar wind and energetic storm particle events. This paper highlights these space weather effects using CME observations space and ground based instruments during of solar cycles 23 and 24.

Gopalswamy, Nat↗
NASA NTRS1990

Preparation of dilute magnetic semiconductor films by metalorganic chemical vapor deposition

A method for preparation of a dilute magnetic semiconductor (DMS) film is provided, wherein a Group II metal source, a Group VI metal source and a transition metal magnetic ion source are pyrolyzed in the reactor of a metalorganic chemical vapor deposition (MOCVD) system by contact with a heated substrate. As an example, the preparation of films of Cd.sub.1-x Mn.sub.x Te, wherein 0.ltoreq..times..ltoreq.0.7, on suitable substrates (e.g., GaAs) is described. As a source of manganese, tricarbonyl (methylcyclopentadienyl) maganese (TCPMn) is employed. To prevent TCPMn condensation during the introduction thereof int the reactor, the gas lines, valves and reactor tubes are heated. A thin-film solar cell of n-i-p structure, wherein the i-type layer comprises a DMS, is also described; the i-type layer is suitably prepared by MOCVD.

Nouhi, Akbar↗
NASA NTRS1987

Simulations of the sun's polar magnetic fields during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, the evolution of the radial component of the solar photospheric magnetic field is simulated during 1976-1984, and the corresponding evolution of the line-of-sight polar fields as seen from earth is derived. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 sq km/sec. For an assumed 300 sq km/sec rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m/s reached at about 5 deg latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m/s, the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.

Devore, C. Richard↗
NASA NTRS1988

Preparation of dilute magnetic semiconductor films by metalorganic chemical vapor deposition

A method for preparation of a dilute magnetic semiconductor (DMS) film is provided, in which a Group II metal source, a Group VI metal source and a transition metal magnetic ion source are pyrolyzed in the reactor of a metalorganic chemical vapor deposition (MOCVD) system by contact with a heated substrate. As an example, the preparation of films of Cd(sub 1-x)Mn(sub x)Te, in which 0 is less than or equal to x less than or equal to 0.7, on suitable substrates (e.g., GaAs) is described. As a source of manganese, tricarbonyl (methylcyclopentadienyl) manganese (TCPMn) is employed. To prevent TCPMn condensation during its introduction into the reactor, the gas lines, valves and reactor tubes are heated. A thin-film solar cell of n-i-p structure, in which the i-type layer comprises a DMS, is also described; the i-type layer is suitably prepared by MOCVD.

Nouhi, Akbar↗
NASA NTRS1994

Hybrid simulations of whistler waves generation and current closure by a pulsed tether in the ionosphere

The dynamic response of a magnetized collisionless plasma to an externally driven, finite size, sudden switch-on current source across the magnetic field has been studied using a two dimensional hybrid code. It was found that the predominant plasma response was the excitation of whistler waves and the formation of current closure by induced currents in the plasma. The results show that the current closure path consists of: (a) two antiparallel field-aligned current channels at the end of the imposed current sheet; and (b) a cross-field current region connecting these channels. The formation of the current closure path occured in the whistler timescale much shorter than that of MHD and the closure region expanded continuously in time. The current closure process was accompanied by significant energy loss due to whistler radiation.

Chang, C. L.↗
NASA NTRS2021

The Correct Sense of Faraday Rotation

The phenomenon of Faraday rotation of linearly polarized synchrotron emission in a magneto-ionized medium has been understood and studied for decades. But since the sense of the rotation itself is irrelevant in most contexts, some uncertainty and inconsistencies have arisen in the literature about this detail. Here, we start from basic plasma theory to describe the propagation of polarized emission from a background radio source through a magnetized, ionized medium in order to rederive the correct sense of Faraday rotation. We present simple graphics to illustrate the decomposition of a linearly polarized wave into right and left circularly polarized modes, the temporal and spatial propagation of the phases of those modes, and the resulting physical rotation of the polarization orientation. We then re-examine the case of a medium that both Faraday-rotates and emits polarized radiation and show how a helical magnetic field can construct or destruct the Faraday rotation. This paper aims to resolve a source of confusion that has arisen between the plasma physics and radio astronomy communities and to help avoid common pitfalls when working with this unintuitive phenomenon.

ISM↗
NASA NTRS2005

Modeling Solar Flare Hard X-ray Images and Spectra Observed with RHESSI

Observations obtained with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of a flare on February 20, 2002 indicate a hard X-ray (HXR) coronal source at or near the top of a flare loop (called a HXR looptop source). The existence of the HXR looptop source suggests that magnetic reconnection, which is believed to power flares, occurs above the loop. In order to explain this HXR looptop source, I created a steady-state particle transport model, in which high-energy electrons are continuously injected at the top of a semicircular flare loop. Based on the simulation results, I find that the model predictions are consistent with the RHESSI observations in many respects, but the spectrum of the looptop source obtained from the model is steeper than that from the RHESSI data. This suggests that, instead of being accelerated above the loop as generally believed, the particles might be accelerated in the looptop itself. RHESSI-observations of three other homologous flares that occurred between April 14 and 16, 2002, provide strong evidence for the presence of a large scale current sheet above a flare loop, which is the basis of standard flare models. The most convincing finding is the presence of the temperature distribution of a separate coronal source above the flare loops: the hotter part of the coronal source was located lower in altitude the cooler part. Together with the fact that the hotter flare loops are higher than the cooler loops, the observations support the existence of a large-scale current sheet between the top of the flare loops and the coronal source above. Blob-Like sources along a line above the loop in the decay phase of the April 15, 2002, flare, which are suggestive of magnetic islands initiated by the tearing-mode instability, and the observation of a cusp structure in microwaves, further support the presence of the current sheet. The observations of the three homologous flares reveal two other features which are beyond the predictions of the standard flare models: the downward motion of flare loops in the early impulsive phase of each flare, and an initially stationary coronal source above the loops. These features me believed to be related to the formation and development of a current sheet. In particular, the downward loop motion seem to be a common phenomenon in flares, suggesting the necessity for modifications to the existing standard flare. models. Finally, thanks to the broad energy coverage of the RHESSI spectra, a low-energy cutoff of 28 (plus or minus 2) keV in the nonthermal electron distribution was determined for the April 15, 2002, flare. As a result, the energy carried by the nonthermal electrons is found to be comparable to the thermal energy of the flare, but one order of magnitude larger than the kinetic energy of the associated coronal mass ejection. The method used to deduce the electron low-energy cutoff will be useful in the analyses of similar events.

Sui, Linhui↗
NASA NTRS1993

Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long-wavelength magnetic anomalies

Rock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudosingle-domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to the thermal demagnetization, with nearly 50 percent of the magnetization remaining after 400 C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long-wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanant magnetization along the present-day field is likely to dominate.

Kelso, Paul R.↗
NASA NTRS1995

Analysis of smooth high-frequency emission at Neptune

We present a detailed study of the highest-frequency component of smooth radio emission observed during the Voyager 2 encounter with Neptune in August 1989. This emission occurs during three distinct periods on August 24 and 25, 1989, in the frequency range of 550 to 900 kHz. By assuming straight-line propagation from sources of both fundamental and second harmonic gyroemission, we perform a detailed analysis of the observed polarization of the emission. The data are most consistent with an L-O mode source in the north magnetic polar region, around 50 deg W, 50 deg N. A second possible source is in the north magnetic polar region, around 270 deg W, 50 deg N. This source must emit in the R-X mode.

Morgan, D. D.↗
NASA NTRS1994

Pickup protons and pressure-balanced structures: Voyager 2 observations in merged interaction regions near 35 AU

Five pressure-balanced structures, each with a scale of the order of a few hundredths of an astonomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discountinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is transition region: the Sun is the source of the magnetic field, but the interstellar medium in source of pickups protons. Relative to the solar wind proton guyroadius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 +/- 0.14) x 10(exp -12) erg/cu cm, (5.8 +/- 0.4) x 10(exp 6) K and (7 +/- 2) x 10(exp -4)/cu cm.

Burlaga, L. F.↗
NASA NTRS1977

Magnetopause shapes - General solutions

The problem of determining the shape of a magnetopause, namely, the tangential discontinuity separating (typically) the magnetic field of a source from external plasma, is reduced to solving an integral equation. No symmetry assumptions whatsoever are used in the derivation, consequently, realistic problems such as model shapes for the earth's magnetosphere (tilted dipole, flowing plasma, etc.) can in principle be treated. Moreover, the magnetic field source itself need not be a simple dipole. A variational method is suggested whereby the optimum parameters for any finite term trial shape are obtained.

Michel, F. C.↗
NASA NTRS1982

Investigating tectonic and bathymetric features of the Indian Ocean using MAGSAT magnetic anomaly data

An equivalent source anomaly map and a map of the relative magnetization for the investigation region were produced. Gravimetry, bathymetry, and MAGSAT anomaly maps were contoured in pseudocolor displays. Finally, an autoregressive spectrum estimation technique was verified with synthetic data and shown to be capable of resolving exponential power spectra using small samples of data. Interpretations were made regarding the relationship between MAGSAT data spectra and crustal anomaly spectra.

Sailor, R. V.↗