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At least 433 records · Page 24

Magnetic reversals of Jupiter and Saturn

The possibility that the gas-giant planets Jupiter and Saturn undergo solar-type magnetic reversals is examined using dynamo theory and radiotelescope data on decametriic emissions from Jupiter. Possible values are found for the effects of the fluctuating velocity field, the magnetic diffusivity, and change in the rotation rate of a dynamo over a characteristic length. The radio emissions from Jupiter decreased in intensity from 1961-72 and rose steadily to the end of 1978, which could have been caused by a change in the Jovian magnetic field. Since Jupiter may have a small rocky core embedded in metallic hydrogen which comprises 75 percent of the radius of the planet, the planetary magnetic field may extend into the cores of its satellites. The dynamo characteristics, like those of Saturn, would be chaotic, although quasi-periodic reversals could occur over intervals on the order of centuries instead of decades such as with the sun and much longer periods such as with the earth.

Hathaway, D. H.↗

Orbital studies of lunar magnetism

Limitations of present lunar magnetic maps are considered. Optimal processing of satellite derived magnetic anomaly data is also considered. Studies of coastal and core geomagnetism are discussed. Lunar remanent and induced lunar magnetization are included.

Mcleod, M. G.↗

Numerical calculations of the collapse of nonrotating, magnetic gas clouds

Results of the first self-consistent numerical calculations of the dynamic collapse of a magnetized protostellar gas cloud are presented. Symmetry about an axis parallel to the initial magnetic field direction has been assumed, so that the calculations could be performed on a two-dimensional grid. Also, the cloud was taken to be nonrotating and isothermal, and the magnetic field was assumed to remain frozen in to the gas. As starting models for the calculations, gas spheres with uniform density and magnetic field were used. The time evolution of the clouds has been calculated for roughly two initial free-fall times, at which point the central density has increased by a factor of approximately 10,000 to 1,000,000. Several such calculations have been performed for different values of the cloud's initial thermal, magnetic, and gravitational energies. In virtually all cases it is found that, once a flattened core forms in the cloud, the central magnetic field strength, B, varies with gas density, rho, according to (d log B/d log rho) = 1/2. This behavior is independent of the initial energy ratios mentioned above. It is also found that the magnetic field is able to prevent completely the collapse of part of the outer envelope of the cloud.

Scott, E. H.↗

Filament Eruptions, Jets, and Space Weather

Previously, from chromospheric H alpha and coronal X‐ray movies of the Sun's polar coronal holes, it was found that nearly all coronal jets (greater than 90%) are one or the other of two roughly equally common different kinds, different in how they erupt: standard jets and blowout jets (Yamauchi et al 2004, Apl, 605, 5ll: Moore et all 2010, Apj, 720, 757). Here, from inspection of SDO/AIA He II 304 A movies of 54 polar x-ray jets observed in Hinode/XRT movies, we report, as Moore et al (2010) anticipated, that (1) most standard x-ray jets (greater than 80%) show no ejected plasma that is cool enough (T is less than or approximately 10(exp 5K) to be seen in the He II 304 A movies; (2) nearly all blownout X-ray jets (greater than 90%) show obvious ejection of such cool plasma; (3) whereas when cool plasma is ejected in standard X-ray jets, it shows no lateral expansion, the cool plasma ejected in blowout X-ray jets shows strong lateral expansion; and (4) in many blowout X-ray jets, the cool plasma ejection displays the erupting-magnetic-rope form of clasic filament eruptions and is thereby seen to be a miniature filament eruption. The XRT movies also showed most blowout X-ray jets to be larger and brighter, and hence to apparently have more energy, than most standard X-ray jets. These observations (1) confirm the dichotomy of coronal jets, (2) agree with the Shibata model for standard jets, and (3) support the conclusion of Moore et al (2010) that in blowout jets the magnetic-arch base of the jet erupts in the manner of the much larger magnetic arcades in which the core field, the field rooted along the arcade's polarity inversion line, is sheared and twisted (sigmoid), often carries a cool‐plasma filament, and erupts to blowout the arcade, producing a CME. From Hinode/SOT Ca II movies of the polar limb, Sterling et al (2010, ApJ, 714, L1) found that chromospheric Type‐II spicules show a dichotomy of eruption dynamics similar to that found here for the cool‐plasma component of coronal X‐ray jets. This favors the idea that Type‐II spicules are miniature counterparts of coronal X‐ray jets. In Moore et al (2011, ApJ, 731, L18), we pointed out that if Type‐II spicules are magnetic eruptions that work like coronal X‐ray jets, they carry an area‐averaged mechanical energy flux of approximately 7x10)(exp 5) erg cm(exp -2) s(exp-1) into the corona in the form of MHD waves and jet outflow, enough to power the heating of the global corona and solar wind. On this basis, from our observations of mini‐filament eruptions in blowout X‐ray jets, we infer that magnetic explosions of the type that have erupting filaments in them are the main engines of both (1) the steady solar wind and (2) the CMEs that produce the most severe space weather by blasting out through the corona and solar wind, making solar energetic particle storms, and bashing the Earth's magnetosphere. We conclude that in focusing on prominences and filament eruptions, Einar had his eye on the main bet for understanding what powers all space weather, both the extreme and the normal.

Moore, Ronald↗

Simultaneous stochastic inversion for geomagnetic main field and secular variation. II - 1820-1980

With the aim of producing readable time-dependent maps of the geomagnetic field at the core-mantle boundary, the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation, described by Bloxham (1987), was applied to survey data from the period 1820-1980 to yield two time-dependent geomagnetic-field models, one for the period 1900-1980 and the other for 1820-1900. Particular consideration was given to the effect of crustal fields on observations. It was found that the existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances: (1) when sequences of measurements are made at one particular site, and (2) for measurements made at satellite altitude. The present model shows many of the features in the earth's magnetic field at the core-mantle boundary described by Bloxham and Gubbins (1985) and supports many of their earlier conclusions.

Bloxham, Jeremy↗

Models of the earth's core

Combined inferences from seismology, high-pressure experiment and theory, geomagnetism, fluid dynamics, and current views of terrestrial planetary evolution lead to models of the earth's core with five basic properties. These are that core formation was contemporaneous with earth accretion; the core is not in chemical equilibrium with the mantle; the outer core is a fluid iron alloy containing significant quantities of lighter elements and is probably almost adiabatic and compositionally uniform; the more iron-rich inner solid core is a consequence of partial freezing of the outer core, and the energy release from this process sustains the earth's magnetic field; and the thermodynamic properties of the core are well constrained by the application of liquid-state theory to seismic and labroatory data.

Stevenson, D. J.↗

Magnetorotational iron core collapse

During its final evolutionary stages, a massive star, as considered in current astrophysical theory, undergoes rapid collapse, thereby triggering a sequence of a catastrophic event which results in a Type II supernova explosion. A remnant neutron star or a black hole is left after the explosion. Stellar collapse occurs, when thermonuclear fusion has consumed the lighter elements present. At this stage, the core consists of iron. Difficulties arise regarding an appropriate model with respect to the core collapse. The present investigation is concerned with the evolution of a Type II supernova core including the effects of rotation and magnetic fields. A simple neutrino model is developed which reproduced the spherically symmetric results of Bowers and Wilson (1982). Several two-dimensional computational models of stellar collapse are studied, taking into account a case in which a 15 solar masses iron core was artificially given rotational and magnetic energy.

Symbalisty, E. M. D.↗

Initiation of Coronal Mass Ejections

This paper is a synopsis of the initiation of the strong-field magnetic explosions that produce large, fast coronal mass ejections. Cartoons based on observations are used to describe the inferred basic physical processes and sequences that trigger and drive the explosion. The magnetic field that explodes is a sheared-core bipole that may or may not be embedded in surrounding strong magnetic field, and may or may not contain a flux rope before it starts to explode. We describe three different mechanisms that singly or in combination trigger the explosion: (1) runaway internal tether-cutting reconnection, (2) runaway external tether-cutting reconnection, and (3) ideal MHD instability or loss or equilibrium. For most eruptions, high-resolution, high-cadence magnetograms and chromospheric and coronal movies (such as from TRACE and/or Solar-B) of the pre-eruption region and of the onset of the eruption and flare are needed to tell which one or which combination of these mechanisms is the trigger. Whatever the trigger, it leads to the production of an erupting flux rope. Using a simple model flux rope, we demonstrate that the explosion can be driven by the magnetic pressure of the expanding flux rope, provided the shape of the expansion is "fat" enough.

Moore, Ronald L.↗

Fe-Mg-Mn relations of ureilite olivines and pyroxenes and the genesis of ureilites

Microprobe analyses of ureilite and pigeonite cores are studied. The Fe/Mn-Fe/Mg relationship in the olivine core is examined. It is observed that magnetic processs such as fractional crystallization and partial melting, and FeO reduction contribute to the olivine core composition. The study of the Mg/Mn and Fe/Mn distributions reveals that these two distributions are not in equilibrium in the olivine and pigeonite cores. The effect of a reducing agent, carbon, on the ureilite genesis is investigated. It is concluded that fractional crystallization and FeO reduction are the major processes of ureilite genesis.

Mittlefehldt, D. W.↗

Non-steady Reconnection in Global Simulations of Magnetosphere Dynamics

To analyze the non-steady magnetic reconnection during quasi-steady solar wind driving we employed high resolution global MHD model BATSRUS with non-MHD corrections in diffusion regions around the reconnection sites. To clarify the role of small-scale non-MHD effects on the global magnetospheric dynamic we performed simulations with different models of dissipation. We found that magnetopause surface is not in steady state even during extended periods of steady solar wind conditions. The so-called tilted reconnection lines become unstable due to formation of pressure bubbles, strong core field flux tubes, vortices, and traveling magnetic field cavities. Non-steady dayside reconnection results in formation of flux tubes with bended axis magnetically connecting magnetic field cavities generated at flanks and strong core segments formed near the subsolar region. We found that the rate of magnetic flux loading to the tail lobes is not very sensitive to the dissipation mechanism and details of the dayside reconnection. On the other hand the magnetotail reconnection rate, the speed of the reconnection site retreat and the global magnetotail dynamics strongly depend on the model of dissipation. THEMIS and Cluster observations are consistent with signatures predicted by simulations.

Kuznetsova, M. M.↗

Improved Readout Scheme for SQUID-Based Thermometry

An improved readout scheme has been proposed for high-resolution thermometers, (HRTs) based on the use of superconducting quantum interference devices (SQUIDs) to measure temperature- dependent magnetic susceptibilities. The proposed scheme would eliminate counting ambiguities that arise in the conventional scheme, while maintaining the superior magnetic-flux sensitivity of the conventional scheme. The proposed scheme is expected to be especially beneficial for HRT-based temperature control of multiplexed SQUIDbased bolometer sensor arrays. SQUID-based HRTs have become standard for measuring and controlling temperatures in the sub-nano-Kelvin temperature range in a broad range of low-temperature scientific and engineering applications. A typical SQUIDbased HRT that utilizes the conventional scheme includes a coil wound on a core made of a material that has temperature- dependent magnetic susceptibility in the temperature range of interest. The core and the coil are placed in a DC magnetic field provided either by a permanent magnet or as magnetic flux inside a superconducting outer wall. The aforementioned coil is connected to an input coil of a SQUID. Changes in temperature lead to changes in the susceptibility of the core and to changes in the magnetic flux detected by the SQUID. The SQUID readout instrumentation is capable of measuring magnetic-flux changes that correspond to temperature changes down to a noise limit .0.1 nK/Hz1/2. When the flux exceeds a few fundamental flux units, which typically corresponds to a temperature of .100 nK, the SQUID is reset. The temperature range can be greatly expanded if the reset events are carefully tracked and counted, either by a computer running appropriate software or by a dedicated piece of hardware.

Penanen, Konstantin↗

Expedition UT-GOM2-2 Methods

Methods used during the University of Texas (UT) Deepwater Hydrate Coring Expedition (UT-GOM2-2) include work done onboard the Helix Q4000 in the offshore Gulf of America (Gulf of Mexico), herein “the Gulf”, “dockside” in Salt Lake City, Utah, and some shore-based work in individual laboratories. The goal of this report is two-fold: to provide enough detail on the methods so they can be repeated by others; and to provide a reference document for the team to enhance cross-disciplinary understanding and knowledge. Methods include drilling operations, depth references and depth modification, downhole tool deployment, coring tool performance assessment, core processing, lithostratigraphy, biostratigraphy, as well as physical properties, including core logging and imaging, rock magnetism, dissolved methane concentration, hydrate saturation, microbiology, and geochemistry. An extensive amount of operational work and planning was required before mobilization of the expedition to permit, build mobile labs, and test downhole tools for deepwater drilling.

03 NATURAL GAS↗

Comparison of high frequency, high temperature core loss and B-H loop characteristics of an 80 Ni-Fe crystalline alloy and two iron-based amorphous alloys

Limited experimental data exists for the specific core loss and dynamic B-H loops for soft magnetic materials for the combined conditions of high frequency and high temperature. This experimental study investigates the specific core loss and dynamic B-H characteristics of a nickel-iron crystalline magnetic alloy (Supermalloy) and two iron-based amorphous magnetic materials (Metglas 2605S-3A and Metglas 2605SC) over the frequency range of 1-50 kHz and temperature range of 23-300 C under sinusoidal voltage excitation. The effects of maximum magnetic flux density, frequency, and temperature on the specific core loss and on the size and shape of the B-H loops are examined. The Supermalloy and Metglass 2605S-3A and 2605SC data are used to compare the core loss of transformers with identical kVA and voltage ratings.

Wieserman, William R.↗

The putative center in NGC 1052

Context. Many active galaxies harbor powerful relativistic jets, however, the detailed mechanisms of their formation and acceleration remain poorly understood.Aims. To investigate the area of jet acceleration and collimation with the highest available angular resolution, we study the innermost region of the bipolar jet in the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC 1052.Methods. We combined observations of NGC 1052 taken with VLBA, GMVA, and EHT over one week in the spring of 2017. Our study is focused on the size and continuum spectrum of the innermost region containing the central engine and the footpoints of both jets. We employed a synchrotron-self absorption model to fit the continuum radio spectrum and we combined the size measurements from close to the central engine out to ∼1 pc to study the jet collimation.Results. For the first time, NGC 1052 was detected with the EHT, providing a size of the central region in-between both jet bases of 43 μas perpendicular to the jet axes, corresponding to just around 250 RS (Schwarzschild radii). This size estimate supports previous studies of the jets expansion profile which suggest two breaks of the profile at around 3 × 103 RS and 1 × 104 RS distances to the core. Furthermore, we estimated the magnetic field to be 1.25 Gauss at a distance of 22 μas from the central engine by fitting a synchrotron-self absorption spectrum to the innermost emission feature, which shows a spectral turn-over at ∼130 GHz. Assuming a purely poloidal magnetic field, this implies an upper limit on the magnetic field strength at the event horizon of 2.6 × 104 Gauss, which is consistent with previous measurements.Conclusions. The complex, low-brightness, double-sided jet structure in NGC 1052 makes it a challenge to detect the source at millimeter (mm) wavelengths. However, our first EHT observations have demonstrated that detection is possible up to at least 230 GHz. This study offers a glimpse through the dense surrounding torus and into the innermost central region, where the jets are formed. This has enabled us to finally resolve this region and provide improved constraints on its expansion and magnetic field strength.Key words: methods: observational / techniques: high angular resolution / techniques: interferometric / galaxies: active / galaxies: jets / galaxies: Seyfert⋆⋆ NASA Hubble Fellowship Program, Einstein Fellow.

79 ASTRONOMY AND ASTROPHYSICS↗

Pulse and Polarization Structures in Axion-Converted X-Rays from Pulsars

Neutron stars (NSs) with their strong magnetic fields and hot dense cores could be powerful probes of axions, a classic benchmark of feebly coupled new particles, through abundant production of axions with the axion-nucleon coupling and subsequent conversion into x-rays due to the axion-photon coupling. In this Letter, we point out that the pulsation structures in both the intensity and polarization of x-rays from NSs could provide us additional information about axions and their couplings. We develop new analytical formalisms of pulsation-polarization structure applicable to a wide range of NSs in the axion scenario and argue that they hold in complicated astrophysical environments. As a case study, we apply our formalism to a representative X-ray Dim Isolated Neutron Star, RX J1856.6-3754, with an unexpected hard x-ray excess which might be axion induced. We show with an updated fit that the axion explanation is compatible with both the intensity and pulsation data available, and combining the pulsation data does not shift the posterior by more than 1⁢𝜎. Yet, the preferred parameter space is close to being excluded by other astrophysical constraints. With a 75% reduction of the uncertainties in the pulsation data, we could potentially draw a definite conclusion on the axion-induced x-rays at more than 3⁢𝜎 level.

Axions↗

Solid State Transformer Architecture and Control Compensation for Common Mode Currents

A high-altitude electromagnetic pulse (HEMP) or similar geomagnetic disturbance (GMD) has the potential to impact the operation of large-scale electric power grids. By introducing low-frequency common-mode (CM) currents, these events can degrade the performance of critical system components, such as large power transformers by introducing CM currents which can lead to magnetic saturation of the transformer core. In this work, a solid-state transformer (SST) is developed to replace susceptible equipment and improve grid resiliency by safely absorbing these CM disturbances. This device will be referred to as a common-mode solid-state transformer (CM-SST). An SST architecture based on a four-legged AC/DC converter is developed. This architecture enables active control of CM signals without disturbing the AC voltages or the real and reactive power delivery capabilities. A system-level model of this architecture is created, and time-domain simulations are performed to evaluate the SST’s performance in response to simulated CM disturbances. A control strategy for mitigating CM current is also investigated. Hamiltonian surface shaping and power flow control (HSSPFC) is used to design a nonlinear controller for the SST’s output inverter. The objectives of the controller are to suppress CM-induced AC current offsets and regulate AC currents to desired setpoints. Nonlinear system analysis is applied to design and validate the controller. Two cases are tested: (a) the proposed four-leg inverter and (b) a standard three-leg inverter. The results show that the proposed controller rapidly mitigates CM disturbances while maintaining high-quality AC current waveforms in the four-leg configuration. Finally, the hardware performance of an SST prototype is evaluated. In particular, the ability of the SST to safely redirect and absorb CM currents is demonstrated, showing how it can protect neighboring conventional transformers in the system. The study confirms that appropriate control laws allow the SST to protect both itself and adjacent transformers during a HEMP or GMD event.

24 POWER TRANSMISSION AND DISTRIBUTION↗

Early-type magnetic stars.

Surface magnetic field variations in early stars, suggesting mechanism for magnetic energy transfer from convective core to surface

Finzi, A.↗

Application of the minimum correlation technique to the correction of the magnetic field measured by magnetometers on spacecraft

Some aspects of the problem of obtaining precise, absolute determination of the vector of low magnetic fields existing in the interplanetary medium are addressed. In the case of a real S/C, there is always the possibility of a spurious field which includes the spacecraft residual field and/or possible field from the sensors, due to both electronic drifts or changes of the magnetic properties of the sensor core. These latter effects may occur during storage of the sensors prior to launching and/or in-flight. The reliability is demonstrated for a method which postulates that there should be no correlation between changes in measured field magnitude and changes in the measured inclination of the field with respect to any one of three fixed Cartesian component directions. Application of this minimum correlation technique to data from IMP-8 and Helios 1-2 shows it is appropriate for determination of the zero offset corrections of triaxial magnetometers. In general, a number of the order of 1000 consecutive data points is sufficient for a good determination.

Mariani, F.↗