Vector Magnetic Data Research Articles

A SELF-CONSISTENT NONLINEAR FORCE-FREE SOLUTION FOR A SOLAR ACTIVE REGION MAGNETIC FIELD

Nonlinear force-free solutions for the magnetic field in the solar corona constructed using photospheric vector magnetic field boundary data suffer from a basic problem: the observed boundary data are inconsistent with the nonlinear force-free model. Specifically, there are two possible choices of boundary conditions on vertical current provided by the data, and the two choices lead to different force-free solutions. A novel solution to this problem is described. Bayesian probability is used to modify the boundary values on current density, using field-line connectivity information from the two force-free solutions and taking into account uncertainties, so that the boundary data are more consistent with the two nonlinear force-free solutions. This procedure may be iterated until a set of self-consistent boundary data (the solutions for the two choices of boundary conditions are the same) is achieved. The approach is demonstrated to work in application to Hinode/SOT observations of NOAA active region 10953.

A CRITICAL ASSESSMENT OF NONLINEAR FORCE-FREE FIELD MODELING OF THE SOLAR CORONA FOR ACTIVE REGION 10953

Nonlinear force-free field (NLFFF) models are thought to be viable tools for investigating the structure, dynamics and evolution of the coronae of solar active regions. In a series of NLFFF modeling studies, we have found that NLFFF models are successful in application to analytic test cases, and relatively successful when applied to numerically constructed Sun-like test cases, but they are less successful in application to real solar data. Different NLFFF models have been found to have markedly different field line configurations and to provide widely varying estimates of the magnetic free energy in the coronal volume, when applied to solar data. NLFFF models require consistent, force-free vector magnetic boundary data. However, vector magnetogram observations sampling the photosphere, which is dynamic and contains significant Lorentz and buoyancy forces, do not satisfy this requirement, thus creating several major problems for force-free coronal modeling efforts. In this article, we discuss NLFFF modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT, STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process illustrate the three such issues we judge to be critical to the success of NLFFF modeling: (1) vector magnetic field data covering larger areas are needed so that more electric currents associated with the full active regions of interest are measured, (2) the modeling algorithms need a way to accommodate the various uncertainties in the boundary data, and (3) a more realistic physical model is needed to approximate the photosphere-to-corona interface in order to better transform the forced photospheric magnetograms into adequate approximations of nearly force-free fields at the base of the corona. We make recommendations for future modeling efforts to overcome these as yet unsolved problems.

Nonlinear Force-Free Modeling of Coronal Magnetic Fields. II. Modeling a Filament Arcade and Simulated Chromospheric and Photospheric Vector Fields

We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional, and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model. When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though, like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various codes will not necessarily converge to the correct, or even the same, solution.

Magnetic perturbations seen by CHAMP and evaluated using the TIE-GCM

Abstract. The Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) is a self-consistent, global, atmospheric model that can be used to estimate magnetic perturbations at satellite altitude. These computed perturbations can then be compared with the magnetic vector data provided by low-earth orbiting satellites. In this initial study, the quietest day of each month from 2001–2005 was selected for comparison. CHAMP magnetic vector residuals were computed for these intervals using the CHAOS model to remove core and crustal geomagnetic contributions. Under various input parameters, the TIE-GCM predictions were compared with the CHAMP residuals on an orbit by orbit basis. Initial results demonstrate a reasonable agreement between the TIE-GCM estimates and the CHAMP residuals in non-polar, dayside regions (±50° magnetic latitude) where both are able to resolve the Equatorial Electro-Jet (EEJ) and solar quiet (Sq) current systems. Although no clear component or temporal correlation was discerned, evidence showing the decrease in residual comparisons presents the possibility of using the TIE-GCM to pre-process geomagnetic data for main field modeling purposes.

Characteristics of crustal magnetic structures in the Tsushima (Ulleung) and Japan Basins from vector magnetic anomalies

Abstract Magnetic surveys by a Deep-tow Three-Component Magnetometer (DTCM) were conducted in the northeastern part of the Japan Basin and the central part of the Tsushima (Ulleung) Basin. Magnetic lineations are recognized clearly in the former area, whereas they were not recognized by previous studies in the latter area. The high-quality vector magnetic anomaly data obtained by DTCM enables the precise determination of the strikes of magnetic lineations and the positions of magnetic boundaries. Magnetic anomalies measured by DTCM show the characteristics of linear magnetic anomalies in both basins. The strikes of magnetic lineations are N47°E in the Japan Basin and N82°E in the Tsushima Basin. The estimated magnetization intensities of magnetic source models constructed from the amplitudes of analytic signal calculated from vector anomalies and the crustal structures determined by seismic studies are similar to those of typical extrusive basalt in both basins. The observed anomalies in the Japan Basin contain a short wavelength anomaly which cannot be explained by the model. Their ages may be chrons C5Cr (16.726–17.277 Ma), C5Dn (17.277–17.615 Ma), C5Dr (17.615–18.281 Ma), and sub-chron C5Dr.1n which was identified by a paleomagnetic study. The estimated half-spreading rate is 2.0 cm/yr, which is slower than that estimated by previous study. The observed anomalies in the Tsushima Basin show that there is a partial magnetization high. This may indicate that not all of the sources of magnetic lineations in the Tsushima Basin changed to low magnetization by the effect of thick sediment cover and the intrusions of a large amount of dikes after the formation.

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An overview

Abstract— The Bosumtwi impact crater in Ghana, arguably the best‐preserved complex young impact structure known on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1–2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near‐equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures.Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB‐07A and LB‐08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB‐07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB‐08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke‐dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim.Deep drilling results confirmed the gross structure of the crater as imaged by the pre‐drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post‐impact sediments (less than 1800 m/s) and the impactites (2600–3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence.The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre‐drilling geophysics, and presents an interesting problem for comparative studies and requires re‐evaluation of existing data from other terrestrial impact craters, as well as modeling parameters.

A model for estimating the relation between the Hall to Pedersen conductance ratio and ground magnetic data derived from CHAMP satellite statistics

Abstract. The goal of this study is to find a way to statistically estimate the Hall to Pedersen conductance ratio α from ground magnetic data. We use vector magnetic data from the CHAMP satellite to derive this relation. α is attained from magnetic satellite data using the 1-D Spherical Elementary Current Systems (SECS). The ionospheric equivalent current density can either be computed from ground or satellite magnetic data. Under the required 1-D assumption, these two approaches are shown to be equal, which leads to the advantage that the statistics are not restricted to areas covered by ground data. Unlike other methods, using magnetic satellite measurements to determine α ensures reliable data over long time sequences. The statistical study, comprising over 6000 passes between 55° and 76.5° northern geomagnetic latitude during 2001 and 2002, is carried out employing data from the CHAMP satellite. The data are binned according to activity and season. In agreement with earlier studies, values between 1 and 3 are typically found for α. Good compatibility is found, when α attained from CHAMP data is compared with EISCAT radar measurements. The results make it possible to estimate α from the east-west equivalent current density Jφ; [A/km]: α=2.07/(36.54/|Jφ|+1) for Jφ<0 (westward) and α=1.73/(14.79/|Jφ+1) for Jφ0 (eastward). Using the same data, statistics of ionospheric and field-aligned current densities as a function of geomagnetic latitude and MLT are included. These are binned with respect to activity, season and IMF BZ and BY. For the first time, all three current density components are simultaneously studied this way on a comparable spatial scale. With increasing activity, the enhancement and the equatorward expansion of the electrojets and the R1 and R2 currents is observed, and in the nightside, possible indications of a Cowling channel appear. During southward IMF BZ, the electrojets and the R1 and R2 currents are stronger and clearer than during northward BZ. IMF BY affects the orientation of the pattern.

The Role of the Kink Instability of a Long-Lived Active Region AR 9604

We have traced the long-term evolution of a non-Hale active region composed of NOAA 9604–9632–9672–9704–9738, which displayed strong transient activity with associated geomagnetic effects from September to December, 2001. By studying the development of spot-group and line-of-sight magnetic field together with the evolution of Hα filaments, the EUV and X-ray corona (TRACE 171 A, Yohkoh/SXT), we have found that the magnetic structure of the active region exhibited a continuous clockwise rotation throughout its entire life. Vector magnetic data obtained from Huairou Solar Observing Station (HSOS) and full-disk line-of-sight magnetograms from SOHO/MDI allowed the determination of the best-fit force-free parameter (proxy of twist), αbest, and the systematic tilt angle (proxy of writhe) which were both found to take positive values. Soft X-ray coronal loops from Yohkoh/SXT displayed a pronounced forward-sigmoid structure in period of NOAA 9704. These observations imply that the magnetic flux tube (loops) with the same handedness (right) of the writhe and the twist rotated clockwise in the solar atmosphere for a long time. We argue that the continuous clockwise rotation of the long-lived active region may be a manifestation that a highly right-hand twisted and kinked flux tube was emerging through the photosphere and chromosphere into the corona.

Geomagnetic paleointensity over 1.2 Ma from deep-tow vector magnetic data across the East Pacific Rise

Abstract Deep-tow vector magnetic data have been acquired across the fast-spreading southern East Pacific Rise 18°S and inverted to magnetization intensity variations. Vector magnetic data are used to determine continuous magnetic intensity within intervals of constant polarity over the Matuyama and Brunhes periods up to the Cobb Mountain event at 1.19 Ma. A comparison of our deep-tow vector data and a sediment core-derived geomagnetic paleointensity timescale suggests that the short-wavelength magnetic anomaly signal is indeed of geomagnetic origin and can be used to date the seafloor with a high resolution. The crustal age determined from our date reveals a highly asymmetric spreading rate for the recent period (since 0.3 Ma).

Electromagnetic induction modelling based on satellite magnetic vector data

SUMMARY The first results on the modelling of magnetic signals induced by transient equatorial ring currents in the magnetosphere with a timescale of the order of days and recorded at satellite altitudes are presented. The input of modelling consists of the X -component of the time-series recorded by the CHAMP vector magnetometer along individual night-time, mid-latitude satellite tracks. We have not considered the magnetic signals measured along day-time tracks and above polar regions because they are disturbed by signals with sources different from magnetospheric ring currents. The modelling procedure is divided into two parts. First, the X -components of satellite magnetic signals are processed by a two-step least-squares analysis. As a result, the X -data are represented in terms of series of Legendre polynomial derivatives. Four examples of the two-step analysis of the signals recorded by the CHAMP vector magnetometer are presented. Second, forward modelling of the electromagnetic induction response of a 2-D heterogeneous conducting sphere to a transient external current excitation is carried out in the time domain using a recently developed spectral finite-element technique (Martinec et al. 2003), which has been modified to include satellite magnetic data. The modified approach has been verified against a semi-analytical solution for a 2-D geometry: a spherical inclusion eccentrically nested in a homogeneous sphere. The output of the forward modelling of electromagnetic induction, that is, the predicted Z-component at satellite altitude, can then be compared with the satellite observations to obtain data to be minimized when adjusting electrical conductivity models in future inversion modelling. To outline such a procedure, we use a 5-layer spherically symmetric model of electrical conductivity and compare the predicted and observed Z-data for the magnetic storm recorded by the CHAMP satellite between 2001 September 25 and 2001 October 7. The agreement between observations and predictions is quantitatively satisfactory. The main advantage of the present method is its ability to use the satellite magnetic data directly, without the need to continue them from the satellite altitude to the ground or to decompose them into the inducing and induced parts by spherical harmonic analysis.

Airborne vector magnetics mapping of remanently magnetised banded iron-formations at Rocklea, Western Australia

Strongly remanent magnetic sources such as those found in the banded iron-formations (BIFs) of the Hamersley Basin impair the interpretation of standard total magnetic intensity survey data. The use of aircraft attitude information, provided by the FALCONTM airborne gravity gradiometer (AGG) system, makes it possible to reference geographically the vector magnetic information collected by the fluxgate triad on board the aircraft. A full aircraft compensation processing method, allowing for permanent, induced and eddy current effects, provides usable vector magnetic data. These data supply valuable extra information for the interpretation of strongly remanent BIFs. An example using data from a FALCONTM survey near the Rocklea Dome in the Hamersley Basin demonstrates improved mapping of the BIFs by the use of the vector-residual magnetic intensity data.

Airborne vector magnetics mapping of remanently magnetized banded iron formations at Rocklea, Western Australia

Strongly remanent magnetic sources such as those found in the banded iron formations (BIFs) of the Hamersley Basin impair the interpretation of standard total magnetic intensity survey data. The use of aircraft attitude information, provided by the FALCONTM airborne gravity gradiometer (AGG) system, makes it possible to reference geographically the vector magnetic information collected by the fluxgate triad on board the aircraft. A full aircraft compensation processing method, allowing for permanent, induced, and eddy current effects, provides usable vector magnetic data. These data supply valuable extra information for the interpretation of strongly remanent BIFs.An example using data from a FALCON survey near the Rocklea Dome in the Hamersley Basin demonstrates improved mapping of the BIFs by the use of the vector-residual magnetic intensity data.

Modelling of attitude error in vector magnetic data: application to Ørsted data

Analysis of data from the Orsted satellite indicates that the largest source of error arises from an anisotropic attitude uncertainty, related to poorly determined rotation about the axis of the star imager. For two data sets from May and December 1999, I quantify this error, and review and apply a formalism designed to allow for this problem. I argue that, when modelled correctly, this attitude uncertainty should not significantly damage the main field models obtained from Orsted data.

Observed currents on the Earth's high‐latitude magnetopause

This paper reports a survey of electrical currents on the Earth's magnetopause, principally at high latitudes, as inferred from magnetic vector data acquired by the University of Iowa/Langley Research Center/NASA satellite Hawkeye 1. The very eccentric orbit of this satellite had an inclination to the equator of approximately 90°, an initial apogee at a radial distance of about 21 RE (Earth radii) over the north pole, a relatively low altitude perigee, an essentially constant semi‐major axis of 11.0 RE, and a corresponding period of 51.3 hours. A total of 536 candidate crossings of the magnetopause were examined. Many of these were in bundles of multiple crossings and were considered essentially redundant. Others were poorly defined. A reduced data set of 139 selected cases was analyzed in detail though solar wind dynamic pressure data were available for only 117 of these cases. Some 17 crossings of the cusp/cleft region were included. Inferred values of the lineal current densities on the magnetopause are in the range 5.5 to 157.5 mA m−1 over a wide range of solar wind dynamic pressure P from 1.17 to 16.1 nPa. On the high‐latitude magnetopause near the noon‐midnight meridian plane, a suggested dependence of on P is among widely scattered data. The sense of the current system at high latitudes is counterclockwise as viewed from the Sun, in qualitative agreement with the Chapman‐Ferraro theory. The apparent normal thickness of the magnetopause current sheet ranges from 30 to 850 km with mean and median values of 185 and 158 km, respectively. It is argued provisionally that the radial rate of motion of the magnetopause is of the order of 2 km s−1 and hence that its true thickness is of similar magnitude, being of the order of one gyroradius for a 500‐eV proton moving perpendicular to the mean magnetic field in the current sheet or several gyroradii if moving more nearly parallel to the field. The relationship of our results to models of the geomagnetic field and to other related work is discussed briefly.

On the possibility of detecting large‐scale crustal remanent magnetization with MAGSAT vector magnetic anomaly data

Magnetic anomaly component data measured by Magsat is compared with synthetic anomaly component fields arising from an equivalent source dipole array at the Earth's surface generated from total field anomaly data alone. It is found that the synthetic components fit the component data regardless of the dipole orientation assigned to the equivalent sources and of the dipole spacing. Tentative conclusions are 1) over the U.S., vector anomaly fields can be determined to the accuracy of the measurements from the total field anomaly data alone, and 2) the equivalent source technique is not useful for determining the direction of large‐scale crustal magnetization.

Interpretation of vector magnetic data from boreholes

Interpretation of vector magnetic data from boreholes

Simultaneous measurements of auroral particles and electric currents by a rocket-borne instrument system: Introductory remarks

A rocket-borne experiment package has been designed to obtain simultaneous in situ measurements of the pitch angle distributions and energy spectra of primary auroral particles, the flux of neutral hydrogen at auroral energies, the electric currents flowing in the vicinity of the auroral arc as determined from vector magnetic data, and the modulation of precipitating electrons in the frequency range 0.5-10 MHz. The experiment package was launched by a Nike-Tomahawk rocket from Poker Flat, Alaska, at 0722 UT on Feb. 25, 1972, over a bright auroral band. This paper is intended to serve as an introduction to the detailed discussion of results given in the companion papers. As such it includes a brief review of the general problem, a discussion of the rocket instrumentation, a delineation of the auroral and geomagnetic conditions at the time of launch, and comments on the overall payload performance.

Vector magnetic data for detecting short polarity intervals in marine magnetic profiles

To identify short polarity intervals recorded during the early Cenozoic in the oceanic crust, a three-component magnetometer was flown across the magnetic stripes north of the Murray fracture zone. The vector magnetometer was flown at a low altitude (335 meters) in order to reduce attenuation of short-wavelength anomalies. The conventional means used to distinguish between magnetic stripes and anomalies due to three-dimensional sources is to correlate wiggles visually over a number of widely separated total-field profiles. The single profile of three orthogonal components was found to provide the distinction directly. The component parallel to the magnetic stripes hy recorded only anomalies due to three-dimensional sources, whereas the horizontal hx and vertical hz components perpendicular to hy were found to record both magnetic stripes and three-dimensional anomalies. By comparing profiles, it was possible to distinguish anomalies due to seamounts or other local sources from anomalies due to short polarity events. Analysis of this single profile from the north Pacific supports the existence of two short polarity events previously identified by stacking profiles of the total field recorded at the sea surface.

Spherical harmonic analysis: 2. A new model derived from magnetic observatory data for epoch 1960.0

A new method of spherical harmonic analysis was applied to the annual mean of vector magnetic data from 82 observatories for epoch 1960.0. The resulting model of the geomagnetic field, containing 77 Gaussian coefficients, fits the observatory data with an rms error of 358 γ, the project Magnet data to 394 γ and the Vanguard satellite data to 54 γ. For comparison, corresponding figures were obtained for 10 other published field models. Results for all models were obtained with and without an epoch correction using the Nagata and Oguti time derivative Gaussian coefficients.