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At least 343 records · Page 19

All sources NASA NTRS DOE OSTI

DOE OSTI2014

On the area expansion of magnetic flux tubes in solar active regions

We calculated the three-dimensional (3D) distribution of the area expansion factors in a potential magnetic field, extrapolated from the high-resolution Hinode/SOT magnetogram of the quiescent active region NOAA 11482. Retaining only closed loops within the computational box, we show that the distribution of area expansion factors show significant structure. Loop-like structures characterized by locally lower values of the expansion factor are embedded in a smooth background. These loop-like flux tubes have squashed cross-sections and expand with height. The distribution of the expansion factors show an overall increase with height, allowing an active region core characterized by low values of the expansion factor to be distinguished. The area expansion factors obtained from extrapolation of the Solar Optical Telescope magnetogram are compared to those obtained from an approximation of the observed magnetogram by a series of 134 submerged charges. This approximation retains the general flux distribution in the observed magnetogram, but removes the small-scale structure in both the approximated magnetogram and the 3D distribution of the area expansion factors. We argue that the structuring of the expansion factor can be a significant ingredient in producing the observed structuring of the solar corona. However, due to the potential approximation used, these results may not be applicable to loops exhibiting twist or to active regions producing significant flares.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Magnetic flux density from the relative circular motion of stars and partially ionized gas in the galaxy mid-plane vicinity

Observations suggest a slower stellar rotation relative to gas rotation in the outer part of the Milky Way Galaxy. This difference could be attributed to an interaction with the interstellar magnetic field. In a simple model, fields of order 10 μG are then required, consistently with the observed values. This coincidence suggests a tool for estimating magnetic fields in spiral galaxies. A north–south asymmetry in the rotation of gas in the Galaxy could be of magnetic origin too.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2017

Is There a CME Rate Floor? CME and Magnetic Flux Values for the Last Four Solar Cycle Minima

The recent prolonged activity minimum has led to the question of whether there is a base level of the solar magnetic field evolution that yields a ''floor'' in activity levels and also in the solar wind magnetic field strength. Recently, a flux transport model coupled with magneto-frictional simulations has been used to simulate the continuous magnetic field evolution in the global solar corona for over 15 years, from 1996 to 2012. Flux rope eruptions in the simulations are estimated (Yeates), and the results are in remarkable agreement with the shape of the SOlar Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment coronal mass ejection (CME) rate distribution. The eruption rates at the two recent minima approximate the observed-corrected CME rates, supporting the idea of a base level of solar magnetic activity. In this paper, we address this issue by comparing annual averages of the CME occurrence rates during the last four solar cycle minima with several tracers of the global solar magnetic field. We conclude that CME activity never ceases during a cycle, but maintains a base level of 1 CME every 1.5 to approx. 3 days during minima. We discuss the sources of these CMEs.

Webb, D. F.↗

DOE OSTI2017

Magnetic Flux Emergence and Decay Rates for Preceder and Follower Sunspots Observed with HMI

We quantify the emergence and decay rates of preceder (p) and follower (f) sunspots within 10 active regions from 2010 to 2014 using Space-weather Helioseismic Magnetic Imager Active Region Patch data. The sunspots are small to mid-sized regions and contain a signed flux within a single polarity sunspot of (1.1--6.5)×10{sup 21} Mx. The net unsigned flux within the regions, including plage, ranges from (5.1--20)×10{sup 21} Mx. Rates are calculated with and without intensity contours to differentiate between sunspot formation and flux emergence. Signed flux emergence rates, calculated with intensity contours, for the p (f) spots average 6.8(4.9)×10{sup 19} Mx hr{sup −1}, while decay rates are −1.9(−3.4)×10{sup 19} Mx hr{sup −1}. The mean, signed flux emergence rate of the regions, including plage, is 7.1×10{sup 19} Mx hr{sup −1}, for a mean peak flux of 5.9×10{sup 21} Mx. Using a synthesis of these results and others reported previously, there is a clear trend for larger flux regions to emerge faster than smaller ones. Observed emergence rates (dϕ/dt, Mx hr{sup −1}) scale with total signed peak flux, ϕ-tilde {sub max}, as a power law with an exponent of 0.36, i.e., dϕ/dt=A ϕ-tilde {sub max}{sup 0.36}. The observed rates may assist in constraining the boundary and initial conditions in simulations which already demonstrate increased rates for flux tubes with higher buoyancy and twist, or in the presence of a strong upflow. Overall, the observed emergence rates are smaller than those in simulations, which may indicate a slower rise of the flux in the interior than what is captured in simulations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Investigation of magnetic flux transport and shock formation in a staged Z-pinch

Target preheating is an integral component of magnetized inertial fusion in reducing convergence ratio. In the staged Z-pinch concept, it is achieved via one or more shocks. Previous work [Narkis et al., Phys. Plasmas 23, 122706 (2016)] found that shock formation in the target occurred earlier in higher-Z liners due to faster flux transport to the target/liner interface. However, a corresponding increase in magnitude of magnetic pressure was not observed, and target implosion velocity (and therefore shock strength) remained unchanged. To investigate other means of increasing the magnitude of transported flux, a Korteweg-de Vries-Burgers equation from the 1-D single-fluid, resistive magnetohydrodynamic equations is obtained. Solutions to the nondispersive (i.e., Burgers) equation depend on nondimensional coefficients, whose dependence on liner density, temperature, etc., suggests an increase in target implosion velocity, and therefore shock strength, can be obtained by tailoring the mass of a single-liner gas puff to a double-liner configuration. In the selected test cases of 1-D simulated implosions of krypton on deuterium, the peak Mach number increased from ∼5 to ∼8. While a notable increase was seen, Mach numbers exceeding 10 (implosion velocities exceeding ∼25 cm/μs) are necessary for adequate shock preheating.

Narkis, J. (ORCID:0000000154368933)↗

NASA NTRS1973

Magnetic flux density environment of several electron-bombardment ion thrusters

Measurements of the external magnetic field of several electron-bombardment mercury ion thrusters were made using a single axis fluxgate magnetometer. The axial dipole moment accounted for most of the external field. In addition, residual fields from an electromagnet thruster were approximately 20 percent of the initial field. Simple degaussing of the axial magnets reduced the residual fields to 1 to 2 percent of the initial field.

Danilowicz, R. L.↗

NASA NTRS2010

Two Types of Magnetic Flux Cancellation in the Solar Eruption of 2007 May 20

We study a solar eruption of 2007 May 20, in an effort to understand the cause of the eruption's onset. The event produced a GOES class B6.7 flare peaking at 05:56 UT, while ejecting a surge/filament and producing a coronal mass ejection (CME). We examine several data sets, including H-alpha images from the Solar Optical Telescope (SOT) on Hinode, EUV images from TRACE, and line-of-sight magnetograms from SOHO/MDI. Flux cancelation occurs among two different sets of flux elements inside of the erupting active region: First, for several days prior to eruption, opposite-polarity sunspot groups inside the region move toward each other, leading to the cancelation of approximately 10^{21} Mx of flux over three days. Second, within hours prior to the eruption, positive-polarity moving magnetic features (MMFs) flowing out of the positive-flux spots at approximately 1 kilometer per second repeatedly cancel with field inside a patch of negative-polarity flux located north of the sunspots. The filament erupts as a surge whose base is rooted in the location where the MMF cancelation occurs, while during the eruption that filament flows out along the polarity inversion line between the converging spot groups. We conclude that a plausible scenario is that the converging spot fields brought the magnetic region to the brink of instability, and the MMF cancelation pushed the system "over the edge." triggering the eruption.

Sterlin, Alphonse C.↗

NASA NTRS2016

MAVEN Observations of Magnetic Flux Ropes with a Strong Field Amplitude in the Martian Magnetosheath During the ICME Passage on 8 March 2015

We present initial results of strong field amplitude flux ropes observed by Mars Atmosphere and Volatile EvolutioN (MAVEN) mission around Mars during the interplanetary coronal mass ejection (ICME) passage on 8 March 2015. The observed durations were shorter than 5 s and the magnetic field magnitudes peaked above 80 nT, which is a few times stronger than those usually seen in the magnetosheath barrier. These are the first unique observations that MAVEN detected such flux ropes with a strong field at high altitudes (greater than 5000 km). Across these structures, MAVEN coincidentally measured planetary heavy ions with energies higher than a few keV. The spatial properties inferred from the Grad-Shafranov equation suggest that the speed of the structure can be estimated at least an order of magnitude faster than those previously reported quiet-time counterparts. Hence, the space weather event like the ICME passage can be responsible for generating the observed strong field, fast-traveling flux ropes.

8 March 2015↗

DOE OSTI2022

MAGNETIC FLUX EXPULSION IN SUPERCONDUCTING RADIO-FREQUENCY NIOBIUM CAVITIES MADE FROM COLD WORKED NIOBIUM

Trapped residual magnetic field during the cool down of superconducting radio frequency (SRF) cavities is one of the primary sources of RF residual losses leading to lower quality factor. Historically, SRF cavities have been fabricated from high purity fine grain niobium with grain size ~50 to 100 µm as well as large grain with grain size of the order of few centimeters. Non-uniform recrystallization of fine-grain Nb cavities after the post fabrication heat treatment leads to higher flux trapping during the cool down, and hence the lower quality factor. We fabricated two 1.3 GHz single cell cavities from cold-worked niobium from different vendors and processed along with cavities made from SRF grade Nb. The flux expulsion and flux trapping sensitivity were measured after successive heat treatments in the range 800 to 1000°C. The flux expulsion from cold-worked fine-grain Nb cavities improves after 800°C/3h heat treatments and it becomes similar to that of standard fine-grain Nb cavities when the heat treatment temperature is higher than 900°C.

Khanal, B. D.↗

DOE OSTI2022

MAGNETIC FLUX EXPULSION IN SUPERCONDUCTING RADIO-FREQUENCY NIOBIUM CAVITIES MADE FROM COLD WORKED NIOBIUM

Khanal, B. D.↗

NASA NTRS1979

Sunspots and the physics of magnetic flux tubes. IX - Umbral dots and longitudinal overstability

The dynamical properties of the sunspot field and of a column of hot gas confined by such a vertical magnetic field are examined in order to understand the umbral dot within the context of the magnetic sunspot structure. Attention is given to the conditions necessary for gas intrusion, longitudinal as well as convective overstability, the growing modes, and the even mode. With the hypothesis that the subsurface magnetic field of a sunspot splits into many separate flux tubes with field-free gas between, it is suggested that the field-free columns occasionally punch their way up through the overlying magnetic field to the surface, appearing there as the bright, field-free umbral dots. Effects fostering the phenomenon are also discussed, that is, the enhanced temperature of a column of rising gas, the strongly reduced overhead magnetic pressure, and the initiated upward intrusion; these effects are illustrated with examples.

Parker, E. N.↗

NASA NTRS2012

Using Statistical Multivariable Models to Understand the Relationship Between Interplanetary Coronal Mass Ejecta and Magnetic Flux Ropes

In-situ measurements of interplanetary coronal mass ejections (ICMEs) display a wide range of properties. A distinct subset, "magnetic clouds" (MCs), are readily identifiable by a smooth rotation in an enhanced magnetic field, together with an unusually low solar wind proton temperature. In this study, we analyze Ulysses spacecraft measurements to systematically investigate five possible explanations for why some ICMEs are observed to be MCs and others are not: i) An observational selection effect; that is, all ICMEs do in fact contain MCs, but the trajectory of the spacecraft through the ICME determines whether the MC is actually encountered; ii) interactions of an erupting flux rope (PR) with itself or between neighboring FRs, which produce complex structures in which the coherent magnetic structure has been destroyed; iii) an evolutionary process, such as relaxation to a low plasma-beta state that leads to the formation of an MC; iv) the existence of two (or more) intrinsic initiation mechanisms, some of which produce MCs and some that do not; or v) MCs are just an easily identifiable limit in an otherwise corntinuous spectrum of structures. We apply quantitative statistical models to assess these ideas. In particular, we use the Akaike information criterion (AIC) to rank the candidate models and a Gaussian mixture model (GMM) to uncover any intrinsic clustering of the data. Using a logistic regression, we find that plasma-beta, CME width, and the ratio O(sup 7) / O(sup 6) are the most significant predictor variables for the presence of an MC. Moreover, the propensity for an event to be identified as an MC decreases with heliocentric distance. These results tend to refute ideas ii) and iii). GMM clustering analysis further identifies three distinct groups of ICMEs; two of which match (at the 86% level) with events independently identified as MCs, and a third that matches with non-MCs (68 % overlap), Thus, idea v) is not supported. Choosing between ideas i) and iv) is more challenging, since they may effectively be indistinguishable from one another by a single in-situ spacecraft. We offer some suggestions on how future studies may address this.

Riley, P.↗

DOE OSTI2023

Residual loss due to magnetic flux trapping in niobium superconducting microwave cavities

One of the significant source of radio frequency loss in superconducting microwave cavities is the trapped residual magnetic field during the cooldown due to the incomplete Messier effect. The sensitivity of flux trapping depends on the distribution of impurities, network of dislocations and defects which pin vortices. Recent advancement in cavity processing techniques resulted in very high quality factors with impurity doping, however the rf loss related to the flux trapping increases. High quality factor in SRF cavities is not only limited to particle accelerators but also emerging as an application to quantum computing and quantum information science. In this presentation, we present the results of measurements of flux trapping sensitivity on several 1.3 GHz elliptical accelerating cavities with different grain size, impurity content and surface preparations.

Dhakal, P.↗

NASA NTRS1987

Energy buildup in coronal magnetic flux tubes

A time-dependent two-dimensional MHD simulation is used to study the response of the magnetic field in coronal loops to photospheric motion. From an initially uniform field, circular sections of the ends of the loop are slowly rotated to represent the photospheric motion. The evolution of the field and flow is characterized by three phases: (1) a phase of negligible kinetic energy where the current and field are predominantly parallel; (2) a phase where the field twist increases, the axial field at and near the axis increases, and the axial field decreases in two cylindrical regions away from the axis; and (3) a phase in which a significant portion of the field makes several rotations at large radii, with a corresponding reducton in the axial field to a few percent of the initial value.

Steinolfson, R. S.↗

DOE OSTI2016

Triode for Magnetic Flux Quanta

In an electronic triode, the electron current emanating from the cathode is regulated by the electric potential on a grid between the cathode and the anode. Here we demonstrate a triode for single quantum magnetic field carriers, where the flow of individual magnetic vortices in a superconducting film is regulated by the magnetic potential of striae of soft magnetic strips deposited on the film surface. By rotating an applied in-plane field, the magnetic strip potential can be varied due to changes in the magnetic charges at the strip edges, allowing accelerated or retarded motion of magnetic vortices inside the superconductor. Scaling down our design and reducing the gap width between the magnetic stripes will enable controlled manipulation of individual vortices and creation of single flux quantum circuitry for novel high-speed low-power superconducting electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1993

Evidence for siphon flows with shocks in solar magnetic flux tubes

We synthesize profiles of the infrared line Fe I 15648.5 A (g = 3) for a recently developed theoretical model of siphon flows along photospheric magnetic loops. The synthesized line profiles are compared with the observations from which Rueedi et al. (1992) deduced the presence of such flows across the neutral line of an active region plage. This comparison supports the interpretation of Rueedi et al. (1992). It also suggests that the average footpoint separation of the observed loops carrying the siphon flow is 8-15 sec and that the siphon flow experiences a standing tube shock in the downstream leg near the top of the arch.

Degenhardt, D.↗

NASA NTRS1990

Magnetic flux tubes and their relation to continuum and photospheric features

An investigation is made of the relationship between photospheric 'filigree' light points, line-center brightness, and magnetic field, as inferred from sets of individual images and films showing a distinct difference between two classes of magnetic regions. While in the first such region the vertical velocity field is average and the magnetic field is mostly confined in narrow lanes, the granulation pattern of the second scale is much smaller, the vertical velocity is lower, and the magnetic field is less compact. Where granulation is normal, excellent correlation is obtained between bright continuum, line-center, and magnetic field line structure.

Title, A.↗

DOE OSTI2017

Gas Gun-Configured Magnetic Flux Compression Generator, NLV-013-17, Year 1 of 3

Presentation on project to explore the concept of achieving current amplification by principles of high-explosive magnetic compression generation but without high explosives and demonstrate that a gas gun or launcher could be configured to provide the input energy and action necessary for magnetic compression generation performance in a more controlled, cost-effective manner with improved safety.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗