Offset Dipole Research Articles

Energetic particle dynamics in Mercury's magnetosphere

We investigate the drift paths of energetic particles in Mercury's magnetosphere by tracing their motion through a model magnetic field. Test particle simulations solving the full Lorentz force show a quasi‐trapped energetic particle population that gradient and curvature drift around the planet via “Shabansky” orbits, passing though high latitudes in the compressed dayside by equatorial latitudes on the nightside. Due to their large gyroradii, energetic H+ and Na+ ions will typically collide with the planet or the magnetopause and will not be able to complete a full drift orbit. These simulations provide direct comparison for recent spacecraft measurements from MESSENGER. Mercury's offset dipole results in an asymmetric loss cone and therefore an asymmetry in particle precipitation with more particles precipitating in the southern hemisphere. Since the planet lacks an atmosphere, precipitating particles will collide directly with the surface of the planet. The incident charged particles can kick up neutrals from the surface and have implications for the formation of the exosphere and weathering of the surface.

Low‐degree structure in Mercury's planetary magnetic field

The structure of Mercury's internal magnetic field has been determined from analysis of orbital Magnetometer measurements by the MESSENGER spacecraft. We identified the magnetic equator on 531 low‐altitude and 120 high‐altitude equator crossings from the zero in the radial cylindrical magnetic field component,Bρ. The low‐altitude crossings are offset 479 ± 6 km northward, indicating an offset of the planetary dipole. The tilt of the magnetic pole relative to the planetary spin axis is less than 0.8°. The high‐altitude crossings yield a northward offset of the magnetic equator of 486 ± 74 km. A field with only nonzero dipole and octupole coefficients also matches the low‐altitude observations but cannot yield off‐equatorialBρ= 0 at radial distances greater than 3520 km. We compared offset dipole and other descriptions of the field with vector field observations below 600 km for 13 longitudinally distributed, magnetically quiet orbits. An offset dipole with southward directed moment of 190 nT‐RM3yields root‐mean‐square (RMS) residuals below 14 nT, whereas a field with only dipole and octupole terms tuned to match the polar field and the low‐altitude magnetic equator crossings yields RMS residuals up to 68 nT. Attributing the residuals from the offset‐dipole field to axial degree 3 and 4 contributions we estimate that the Gauss coefficient magnitudes for the additional terms are less than 4% and 7%, respectively, relative to the dipole. The axial alignment and prominent quadrupole are consistent with a non‐convecting layer above a deep dynamo in Mercury's fluid outer core.

A global hybrid model for Mercury's interaction with the solar wind: Case study of the dipole representation

The interaction of the solar wind (SW) with the magnetic field of Mercury is investigated by means of a three dimensional parallelized multispecies hybrid model. A comparison between two mathematical representations of Mercury's intrinsic magnetic field is studied. The first model is an Offset Dipole (OD) having the offset and dipolar moment reported by Anderson et al. (2011). The second model is a combination of a Dipole and a Quadrupole (DQ), the total field is fitted to the offset dipolar field, for northern latitudes greater than 50°. Simulations reproduce the features which characterize Mercury's interaction with the SW, encompassing the Bow Shock (BS), the magnetosheath, the magnetotail, the “cusps” region and the neutral current sheet. Global hybrid simulations of the Hermean magnetosphere run for the OD and DQ models demonstrate that the southern parts of the magnetospheres produced by the OD and DQ models differ greatly in topology and volume meanwhile their northern parts‐are quite similar. In particular the DQ model exhibits a dome of closed field lines around the south pole in contrast to the OD. Without further information on the intrinsic magnetic field of the planet in the southern region which should be provided by BepiColombo after year 2020, we can only speculate on the influence of the different magnetic topologies on the magnetospheric dynamics.

Wireless Medical Telemetry Characterization for Ingestible Capsule Antenna Designs

This letter presents a comprehensive and systematic antenna performance evaluation and wireless medical telemetry characterization between two different ingestible antenna designs, an inverted conical helical antenna and a reference conformal meandered offset dipole antenna. The antenna performance comparison matrix includes matching, radiation patterns, and polarization. Both these antennas are independently characterized inside a human body environment (box-model human). Specific absorption rate (SAR) evaluations are described, and finally a link budget is developed for both the antenna systems based on certain parameters (frequency, power limitations) with the goal of establishing a reliable communication link for indoor communication for biotelemetry applications.

Revisiting the Rigidly Rotating Magnetosphere model for Ori E - I. Observations and data analysis★

We have obtained 18 new high-resolution spectropolarimetric observations of the B2Vp star sigma Ori E with both the Narval and ESPaDOnS spectropolarimeters. The aim of these observations is to test, with modern data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki (2005), applied to the specific case of sigma Ori E by Townsend et al. (2005). This model includes a substantially offset dipole magnetic field configuration, and approximately reproduces previous observational variations in longitudinal field strength, photometric brightness, and Halpha emission. We analyze new spectroscopy, including H I, He I, C II, Si III and Fe III lines, confirming the diversity of variability in photospheric lines, as well as the double S-wave variation of circumstellar hydrogen. Using the multiline analysis method of Least-Squares Deconvolution (LSD), new, more precise longitudinal magnetic field measurements reveal a substantial variance between the shapes of the observed and RRM model time-varying field. The phase resolved Stokes V profiles of He I 5876 A and 6678 A lines are fit poorly by synthetic profiles computed from the magnetic topology assumed by Townsend et al. (2005). These results challenge the offset dipole field configuration assumed in the application of the RRM model to sigma Ori E, and indicate that future models of its magnetic field should also include complex, higher-order components.

Global configuration of Saturn's magnetic field derived from observations

Magnetic field observations from the Cassini spacecraft are averaged into 1 × 1 RS (1 RS = 60238 km) meridional bins at Saturn. A Runge‐Kutta procedure uses the bin‐averaged field components to estimate the global configuration of Saturn's magnetic field and map field lines from the equator to Saturn's ionosphere. Within ∼18 RS of Saturn, the mapping gives a good representation of the meridional shape of Saturn's magnetic field and confirms that beyond L ∼ 6, the field departs from a dipole and becomes a smoothly‐warped magnetodisk. The disk warps upwards by as much as ∼1 RS at radial distances of ∼15 RS, with the warping increasing with radial distance. When traced back to Saturn, the bin average field lines intersect the ionosphere at latitudes similar to those expected on the basis of recent magnetic field models; the ionospheric pierce latitudes can differ by as much as 6° from those expected from a simple offset dipole.

Zeeman tomography of magnetic white dwarfs

The magnetic fields of the accreting white dwarfs (WDs) in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects. We determine the structure of the surface magnetic fields of the WDs primaries in magnetic CVs using Zeeman tomography. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree l(max)=3 or 5 and include all tesseral and sectoral components with 0<=m<=l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, CP Tuc and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the WD. The results suggest that the field geometries of the WDs in short-period mCVs are quite complex with strong contributions from multipoles higher than the dipole in spite of a typical age of the WDs in CVs in excess of 1 Gyr. It is feasible to derive the surface field structure of an accreting WD from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure.

A model of the ionosphere of Saturn's rings and its implications

The detection of cold O + and O + ions in the vicinity of Saturn’s rings during the Cassini Orbiter orbit insertion confirmed expectations that the rings would have a water product atmosphere and ionosphere. These observations prompted a new look at their origin and nature by Johnson et al. [Johnson, R.E., Luhmann, J.G., Tokar, R.L., Bouhram, M., Berthelier, J.J., Sittler, E.C., Cooper, J.F., Hill, T.W., Crary, F.J., Young, D.T., 2006. Icarus 180, 393–402], but also raised questions about the ionosphere’s spatial distribution and fate that inspired the ionospheric model described in this report. Here a test particle model with some Monte Carlo aspects is used to consider the behavior of the O + and O + ions produced in the atmosphere of Saturn’s rings. Key features of these calculations include the Johnson et al. description of the production of the ring atmosphere, and the effects of the offset dipole magnetic field of Saturn. The results suggest that the latter should produce some possibly observable asymmetries in both the inner ring ionosphere and the precipitation of ring ions into the atmosphere of Saturn. Further in situ observations of the rings are not currently planned, but remote sensing instruments on Cassini may provide future observational tests of the model.  2005 Elsevier Inc. All rights reserved.

Three‐dimensional MHD simulations of the magnetosphere of Uranus

We have successfully simulated the magnetosphere of Uranus for the time period of the Voyager 2 flyby in January 1986. On the basis of the Voyager measurements, a self‐consistent numerical solution is obtained with the parallel block adaptive three‐dimensional (3‐D) MHD code BATS‐R‐US. The time‐dependent simulation has been carried out with a new explicit‐implicit time integration scheme. By comparing corotating steady state solutions and a fully time‐dependent 3‐D simulation with the Voyager data, we show that the magnetosphere of Uranus at the time of the flyby can be regarded as stationary relative to the frame corotating with the planet. We obtained excellent agreement with the observed magnetic field vector along the whole path of the flyby, which includes the near‐Uranus offset dipole field as well as several current sheet crossings in the tail. The location of the bow shock and the magnetopause also agree to high accuracy. We are confident that our numerical solution is a good representation of the three‐dimensional magnetosphere of Uranus during the flyby. The numerical solution shows a twisted magnetotail with field lines that are also stretched due to the flow of plasma in the magnetotail.

Determination of the properties of Mercury's magnetic field by the MESSENGER mission

The MESSENGER mission to Mercury, to be launched in 2004, will provide an opportunity to characterize Mercury's internal magnetic field during an orbital phase lasting one Earth year. To test the ability to determine the planetary dipole and higher-order moments from measurements by the spacecraft's fluxgate magnetometer, we simulate the observations along the spacecraft trajectory and recover the internal field characteristics from the simulated observations. The magnetic field inside Mercury's magnetosphere is assumed to consist of an intrinsic multipole component and an external contribution due to magnetospheric current systems described by a modified Tsyganenko 96 model. Under the axis-centered-dipole approximation without correction for the external field the moment strength is overestimated by ∼4% for a simulated dipole moment of 300 nTR M 3 , and the error depends strongly on the magnitude of the simulated moment, rising as the moment decreases. Correcting for the external field contributions can reduce the error in the dipole term to a lower limit of ∼1–2% without a solar wind monitor. Dipole and quadrupole terms, although highly correlated, are then distinguishable at the level equivalent to an error in the position of an offset dipole of a few tens of kilometers. Knowledge of the external magnetic field is therefore the primary limiting factor in extracting reliable knowledge of the structure of Mercury's magnetic field from the MESSENGER observations.

Extensively Interlocked 2-D Supramolecular Architecture of Self-Adhesive Double-Tailed 1,3-Planar-2,4-upright-Calix[4]arene

Abstract A novel interlocked 2-D supramolecular architecture is constructed through the synergistic intermolecular interactions of calix[4]arene 1,3-bis(4-nitrobenzoate) in the crystal, which are highly stabilized by the regular π–π and offset stacking, edge-to-face, and/or dipole–dipole interactions.

WD 1953-011: a magnetic white dwarf with peculiar field structure

We present Hα spectra of the magnetic white dwarf star WD 1953−011 which confirm the presence of the broad Zeeman components corresponding to a field strength of ∼500 kG found by Maxted & Marsh. We also find that the line profile is variable over a time-scale of a day or less. The core of the Hα line also shows a narrow Zeeman triplet corresponding to a field strength of ∼100 kG which appears to be almost constant in shape. These observations suggest that the magnetic field on WD 1953−011 has a complex structure, and that the star has a rotational period of hours or days which causes the observed variability of the spectra. We argue that neither an offset dipole model nor a double-dipole model is sufficient to explain our observations. Instead, we propose a two-component model consisting of a high-field region of magnetic field strength ∼500 kG covering about 10 per cent of the surface area of the star, superimposed on an underlying dipolar field of mean field strength ∼70 kG. Radial velocity measurements of the narrow Zeeman triplet show that the radial velocity is constant to within a few km s−1, so this star is unlikely to be a close binary.

Model-based characterization of CPS-fed printed dipole for innovative design of uniplanar integrated antenna

A printed dipole radiator fed by coplanar stripline (CPS) is modeled by using a full-wave method of moments (MoM), and its input impedance is accurately extracted (de-embedded) by applying a so-called "short-open calibration" (SOC) scheme. An offset CPS-fed dipole radiator is studied, and its pertinent results show a significant reduction of its input impedance with enlarging the offset. Two new uniplanar integrated antennas in the form of single and twin dipole radiators are designed jointly with an innovative hybrid FGCPW and CPS feeder, and predicted characteristics are well confirmed by our measurements.

The magnetic field configurations of AM Herculis binaries

We have considered equilibrium configurations of a white dwarf and a red dwarf companion in AM Herculis type binary system where the red dwarf is assumed to have an intrinsic centered dipole field, and the white dwarf an offset dipole or a centered dipole plus a quadrupole field, and the interaction is purely magnetic. The restoring torque, due to the system being perturbed from its stable equilibrium configuration, is compared with the accretion torque. The results are used to show that the observed alignment of the main cyclotron emission region relative to the red star can be explained quite simply by these models without having to invoke gravitational torques. We also provide a possible explanation of the observation that the more strongly accreting pole has the weaker field strength in the few systems where fields have been determined at both poles.

Discovery of two cool magnetic white dwarfs

We report the discovery of magnetism in two cool white dwarfs, LHS 1044 and LHS 1734. Optical and infrared photometry, as well as high signal-to-noise spectroscopy, are presented for both objects. The observed energy distributions along with the measured trigonometric parallaxes are used to derive the effective temperatures and surface gravities. LHS 1734, with T(eff) = 5230 K, becomes the coolest observed magnetic white dwarf. The strength of each magnetic field is estimated from a detailed line profile analysis. Offset dipole models with dipole strengths of 16.7 and 7.3 MG, for LHS 1044 and LHS 1734, respectively, are shown to reproduce successfully the observed line profiles. In both objects, the dipole needs to be offset away from the observer by an amount equal to nearly one-fifth of the stellar radius.

New polarimetric observations and a two pole model for the cyclotron emission from AM Herculis

We present high signal-to-noise ratio phase dependent linear polarization and intensity data on AM Herculis which show the presence of structured linear pulses in the optical and near IR regions. The data have been analysed to construct a detailed model which reproduces closely the observed pulse structure and polarization angle variations and clearly demonstrates the presence of two interfering cyclotron emission regions located close to the foot point of a closed field line in an offset dipole field distribution

Two successive reversal transitions from Crete described by a two-dipole model with standing and time varying components.

A two-dipole model is used to simulate the behaviour of the geomagnetic field during two successive polarity transitions (R→N followed by N→R) reported by VALET and LAJ (1981) from a 6Ma old marine clay sequence in western Crete. The model consists of an axial geocentric dipole and an offset minor dipole located at the northern core-mantle boundary. During the transitions the geocentric dipole decays exponentially through zero and then recovers its opposite polarity state. The axial offset dipole has two components: a time-varying and a standing one. In the middle part of the reversal process the standing component controls the transitional field and is responsible for the steep negative inclinations observed in both transitions.The advantage of the two-dipole model compared to some other transition models is that it is capable of describing not only the inclination and intensity transitions but also the properties of the field during intervals of stable polarity, including the departure of inclinations from the axial geocentric dipole value, the symmetry of the older (R→N) reversal and the asymmetry of the younger (N→R) reversal. In a global perspective the model predicts that the shapes of intensity and inclination curves during the transitions depend on the observer's site latitude and that the onset and termination parts of these curves may differ. Because the model is axial it cannot produce the observed declination transitions. In spite of this the model produces VGP-latitude vs. stratigraphic height curves and field intensity vs. VGP-latitude curves consistent with results derived from Hoffman's flooding model.

Paleomagnetic results from Luzon and the central Philippines

Samples were collected from 86 paleomagnetic sites from the islands of Luzon, Marinduque, Mindoro, Panay, Negros, Cebu, and Mindanao in the Philippine Arc. The sampling sites range in age from Pleistocene to Jurassic. Characteristic directions of magnetization of the samples were determined by the use of vector plots. Curie temperature determinations, thin section studies, and hysteresis studies showed that remanence of these samples is carried by fine‐grained (pseudo‐single domain) magnetite. Positive fold tests from Miocene data from Panay, Jurassic data from Mindoro, and Cretaceous data from Cebu suggest that the magnetization of these regions was acquired prior to folding. Rotations reported below are measured with respect to the axial goecentric dipole field. The Plio‐Pleistocene data set shows no resolvable rotation for the 22 sites. This data set suggests that the various terranes that make up the Philippine Arc have behaved as a single unit during the past 5 m.y. or that deformation has been below the limits of resolution. The inclination data from the Plio‐Pleistocene sites have anomalously shallow inclination and are consistent with other Plio‐Pleistocene data from Vietnam, Taiwan, and the Marianas. These data support earlier suggestions for a late Neogene offset dipole effect. The late Miocene sites fall into two separate groups. Ten sites from western Luzon show evidence for around 20° of clockwise rotation. In contrast to this, late Miocene samples from the Bicol region, Negros, Marinduque, and Mindanao are not rotated. The cause of the postlate Miocene clockwise rotation of Luzon is unknown, but a Pliocene collision of the North Luzon Arc with Taiwan is suggested. Early Neogene results also separate into two different populations. The population from Marinduque shows evidence for a large counterclockwise rotation. The second early Neogene population comes from Panay, Cebu, and Mindanao and clearly shows evidence for a clockwise rotation. The validity of this rotation is further supported by a fold test and a reversal test. These early Neogene data sets are consistent with a middle to late Miocene collision of the Palawan Continental Terrane and the Central Philippine Arc. Data from six dikes of possible Oligocene age from the Zambales Ophiolite are highly discordant from the present field, being rotated approximately 60° clockwise. The directions from these dikes are similar to a direction reported earlier from late Oligocene sediments also from the Zambales region. These two data sets support the interpretation that the Eocene direction from Zambales is recording a large clockwise rotation of the region. Data from the Mesozoic sites are from two regions. Data from the Cretaceous Pandan formation of Cebu are discordant with data from the Upper Jurassic from Mindoro. The presence of a fold test from each region and a reversal test from Mindoro supports the interpretation that each of these data sets is reliable. The VGP of Mindoro is displaced southward from the Late Jurassic VGP of South China, suggesting a post‐Jurassic southward migration of Mindoro.

Palaeointensity Determinations on the Miocene of New Zealand

Measurements have been made on a collection of New Zealand lavas, from 18 to 6 Ma in age, to determine the intensity of the ancient geomagnetic field. The predicted geocentric virtual dipole moment (VDM) derived from the New Zealand rocks (2.24×1022Am2) is much lower than that obtained from Icelandic rocks of similar age (7.11×1022Am2). The offset dipole model is discussed as a possible explanation for this discrepancy.

Paleomagnetism and neogene tectonics of the Northern Channel Islands, California

Geologic observations and previous paleomagnetic studies have suggested that the western Transverse Ranges arrived at their anomalous east‐west orientation by clockwise tectonic rotation. A paleomagnetic study of the northern Channel Islands was undertaken, in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern California Borderland. The islands of Anacapa, Santa Cruz, Santa Rosa, and San Miguel form an east‐west trending chain and are considered part of the Transverse Range physiographic province. Oligocene through Miocene volcanic and intrusive rocks occur on these islands, and were sampled for this paleomagnetic study. Eocene sandstones were sampled on San Miguel Island, and Eocene sandstones and Miocene siltstones were sampled on Santa Cruz Island. Paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69° to 81° from expected directions and inclinations which are too shallow by 10° to 25°. Normal and reversely magnetized units yield antipodal mean directions. The mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests. These results suggest that the northern Channel Islands have been tectonically rotated into place since early(?) Miocene time as the outer borderland area translated northwestward within a large shear zone between the Pacific and North American Plates. The data from the northern Channel Islands when averaged with the data from the Santa Monica Mountains yield a result of I = 36.1° ± 5.1°, D = 72.6° ± 6.3° for the southern part of the western Transverse Ranges (52 units, 405 samples). The shallow paleomagnetic inclinations suggest a northward latitude translation of 14.0°+3.7° or −3.9° for the western Transverse Ranges. However, most plate tectonic reconstructions suggest only 4°±3° of northward translation. The discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal. Other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components, or motions of small tectonic plates between the North American and Pacific plates as the Farallan plate broke up and was subducted.