Magnetic Field Signatures Research Articles

Magnetized CMB observables: A dedicated numerical approach

Large-scale magnetic fields affect the scalar modes of the geometry whose ultimate effect is to determine the anisotropies of the Cosmic Microwave Background (CMB in what follows). For the first time, a consistent numerical approach to the magnetized CMB anisotropies is pursued with the aim of assessing the angular power spectra of temperature and polarization when the scalar modes of the geometry and a stochastic background of inhomogeneous magnetic fields are simultaneously present in the plasma. The effects related to the magnetized nature of the plasma are taken into account both at the level of the dynamical equations and at the level of the initial conditions of the Einstein-Boltzmann hierarchy. The temperature and polarization observables are exploited to infer the peculiar signatures of a pre-equality magnetic field. Using the extrapolated best fit to the three year WMAP data the increase and distortions of the first seven peaks in the TT autocorrelations are monitored for different values of the regularized magnetic field intensity and for the physical range of spectral indices. Similar analyses are also conducted for the first few anticorrelation (and corrrelation) peaks of the TE power spectra. Possible interesting degeneracies and stimulating perspectives are pointed out and explored.

Titan's magnetic field signature during the Cassini T34 flyby: Comparison between hybrid simulations and MAG data

During the T34 flyby on 19 July 2007, the Cassini spacecraft passed through the magnetic pile‐up region at Titan's ramside. The magnetic environment of Titan during this flyby is studied using a three‐dimensional hybrid simulation model. This approach treats the electrons of the plasma as a massless, charge‐neutralizing fluid, whereas the effects of finite ion gyroradii are taken into account by modeling the ions as individual particles. The simulation results are compared to data collected by the Cassini Magnetometer Instrument. The key features of the measured magnetic field signature have shown to be fully reproducible in the framework of the simulation model. Several signatures in the observed magnetic field can be ascribed to the passage of the Cassini spacecraft through the magnetic barrier upstream of Titan.

Reduction of a Ship's Magnetic Field Signatures

Decreasing the magnetic field signature of a naval vessel will reduce its susceptibility to detonating naval influence mines and the probability of a submarine being detected by underwater barriers and maritime patrol aircraft. Both passive and active techniques for reducing the magnetic signatures produced by a vessel's ferromagnetism, roll-induced eddy currents, corrosion-related sources, and stray fields are presented. Mathematical models of simple hull shapes are used to predict the levels of signature reduction that might be achieved through the use of alternate construction materials. Also, the process of demagnetizing a steel-hulled ship is presented, along with the operation of shaft-grounding systems, paints, and alternate configurations for power distribution cables. In addition, active signature reduction technologies are described, such as degaussing and deamping, which attempt to cancel the fields surrounding a surface ship or submarine rather than eliminate its source. Table of Contents: Introduction / Passive Magnetic Silencing Techniques / Active Signature Compensation / Summary

Evidence for collisionless magnetic reconnection at Mars

Using data from Mars Global Surveyor (MGS) in combination with Particle‐In‐Cell (PIC) simulations of reconnection, we present the first direct evidence of collisionless magnetic reconnection at Mars. The evidence indicates that the spacecraft passed through the diffusion region where reconnection is initiated and observed the magnetic field signatures of differential electron and ion motion – the Hall magnetic field – that uniquely indicate the reconnection process. These are the first such in‐situ reconnection observations at an astronomical body other than the Earth. Reconnection may be the source of Mars' recently discovered auroral activity and the changing boundaries of the closed regions of crustal magnetic field.

특이치 분해 방법에 의한 함정 자기원 다이폴 모델링 방안 연구

본 논문에서는 모델 함정에 의해 발생하는 정 자기장 신호 특성을 수학적으로 모델링하는 방법에 대해 기술하고 있다. 특정 위치에 설치된 자기센서들에 의해 계측된 자기장 신호 값들로 부터 특이치 분해(singular value decomposition) 방법을 이용한 등가 자기원 다이폴 모델링 기법을 제안하였으며, 제안된 기법의 타당성은 비자성 자기실험실에서 측정된 자기장 값들과의 비교를 통해 성공적으로 검증되었다. 본 논문에서 제안된 기법은 모델 함정 뿐 만 아니라 실 함정에서의 다양한 심도 변화에 따른 정 자기장 신호 분포 특성 예측 시 직접 활용이 가능하다. This paper describes the mathematical modeling method for the static magnetic field signature generated by a magnetic scale model. we proposed the equivalent dipole modeling method utilizing a singular value decomposition technique from magnetic field signatures by magnetic sensors are located special depths below the scale model. The proposed dipole modeling method was successfully verified through comparisons with the real measured values in our non-magnetic laboratory. Using the proposed method, it is possible to predict and analyze static magnetic field distributions at any difference depths generated from the real ships as well as a scale model ship.

Hybrid simulation of Titan's magnetic field signature during the Cassini T9 flyby

Titan's induced magnetotail is analyzed in terms of a three‐dimensional hybrid model, treating the electrons as a massless, charge‐neutralizing fluid, whereas a completely kinetic approach is used to cover ion dynamics. Since during the T9 flyby of Cassini, the spacecraft passed through Titan's induced magnetotail in a distance of about 4 Titan radii downstream of the obstacle, this flyby offers a unique chance to study the magnetic lobe structure in Titan's wake region. The key features of the measured magnetic field signature have shown to be completely reproducible in the framework of the hybrid approximation. Besides, the influence of the ionospheric production rates as well as the upstream plasma direction is investigated. While changes of the ion production rate yield only a slight modification of the magnetotail structure, the magnetic field enhancement in the lobes is strongly modified by changes of the upstream flow direction.

Solar wind interaction with lunar crustal magnetic anomalies

Using Lunar Prospector data, we review the magnetic field and electron signatures of solar wind interaction with lunar crustal magnetic sources. Magnetic field amplifications, too large to represent direct measurements of crustal fields, appear in the solar wind over strong crustal sources, with the chance of observing these amplifications depending on upstream solar wind parameters. We often observe increases in low-energy (≲100 eV) electron energy fluxes simultaneously with large magnetic field amplifications, consistent with an increase in plasma density across a shock surface. We also often observe low frequency wave activity in the magnetic field data (both broadband turbulence and monochromatic waves), often associated with electron energization, sometimes up to keV energies. Electron energization appears to be correlated more closely with wave activity than with magnetic amplifications. Detailed studies of the interaction region will be necessary in order to understand the physics of the Moon–solar wind interaction. At present, the Moon represents the only natural laboratory available to us to study solar wind interaction with small-scale crustal magnetic fields, though simulation results and theoretical work can also help us understand the physical processes at work.

Internal structure of a magnetic flux rope from Cluster observations

We investigate a magnetic flux rope (MFR) observed by Cluster in the magnetotail during a substorm on 2001 August 22. The MFR was aligned with its principal axis closely along the dawn‐dusk direction and had a small size of ∼2 RE with a total current of ∼0.8 MA. The four spacecraft traversed the MFR at different distances from its center based on the magnetic field signature. This fortuitous situation reveals the irregular magnetic field structure in its inner core, which is a feature reported here for the first time. At the leading edge, the y‐component of the electric field was dawnward against the current density direction (dynamo action) and the x‐component of the Lorentz force was Earthward. These parameters reversed in direction at its trailing edge (load).

Simultaneous radio and satellite optical measurements of high‐altitude sprite current and lightning continuing current

We report coordinated measurements of lightning and resulting sprites using ground‐level magnetic field sensors (<0.1 Hz to 30 kHz bandwidth) and the ISUAL instrument on the FORMOSAT‐2 satellite. These measurements demonstrate two distinct elements of the connection between the radio and optical emissions. First, the quasi‐static magnetic field signature is tightly correlated with the low‐altitude optical emissions from the lightning flash, indicating that this radio signature is produced by continuing lightning current. Second, in two events with strong postreturn stroke extremely low frequency (ELF) magnetic pulses, the optical emissions demonstrate that there are no observable intensifications of low‐altitude optical emissions associated with those pulses. If they were produced by a lightning process, such as an M‐component, the connection between optical emissions and current seen in the return stroke and the continuing current suggests they should be visible. However, as has been observed previously, the bright, high‐altitude optical emissions associated with the sprite are simultaneous with the ELF pulse. This is strong evidence that these ELF pulses originate in high‐altitude electric current in the sprite itself and are not produced by a low‐altitude lightning process.

In-Situ Solar Wind and Magnetic Field Signatures of Interplanetary Coronal Mass Ejections

The heliospheric counterparts of coronal mass ejections (CMEs) at the Sun, interplanetary coronal mass ejections (ICMEs), can be identified in situ based on a number of magnetic field, plasma, compositional and energetic particle signatures as well as combinations thereof. We summarize these signatures and their implications for understanding the nature of these structures and the physical properties of coronal mass ejections. We conclude that our understanding of ICMEs is far from complete and formulate several challenges that, if addressed, would substantially improve our knowledge of the relationship between CMEs at the Sun and in the heliosphere.

Primordial magnetic fields and CMB anisotropies

AbstractPossible signatures of primordial magnetic fields on the Cosmic Microwave Background (CMB) temperature and polarization anisotropies are reviewed. The signals that could be searched for include excess temperature anisotropies particularly at small angular scales below the Silk damping scale, B‐mode polarization, and non‐Gaussian statistics. A field at a few nG level produces temperature anisotropies at the 5 µK level, and B‐mode polarization anisotropies 10 times smaller, and is therefore potentially detectable via the CMB anisotropies. An even smaller field, with B 0 < 0.1 nG, could lead to structure formation at high redshift z > 15, and hence naturally explain an early re‐ionization of the Universe. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cluster observations of flux rope structures in the near-tail

Abstract. An investigation of the 2003 Cluster tail season has revealed small flux ropes in the near-tail plasma sheet of Earth. These flux ropes manifest themselves as a bipolar magnetic field signature (usually predominantly in the Z-component) associated with a strong transient peak in one or more of the other components (usually the Y-component). These signatures are interpreted as the passage of a cylindrical magnetic structure with a strong axial magnetic field over the spacecraft position. On the 2 October 2003 all four Cluster spacecraft observed a flux rope in the plasma sheet at X (GSM) ~-17 RE. The flux rope was travelling Earthward and duskward at ~160 kms-1, as determined from multi-spacecraft timing. This is consistent with the observed south-then-north bipolar BZ signature and corresponds to a size of ~0.3 RE (a lower estimate, measuring between the inflection points of the bipolar signature). The axis direction, determined from multi-spacecraft timing and the direction of the strong core field, was close to the intermediate variance direction of the magnetic field. The current inside the flux rope, determined from the curlometer technique, was predominantly parallel to the magnetic field. However, throughout the flux rope, but more significant in the outer sections, a non-zero component of current perpendicular to the magnetic field existed. This shows that the flux rope was not in a "constant α" force-free configuration, i.e. the magnetic force, J×B was also non-zero. In the variance frame of the magnetic field, the components of J×B suggest that the magnetic pressure force was acting to expand the flux rope, i.e. directed away from the centre of the flux rope, whereas the smaller magnetic tension force was acting to compress the flux rope. The plasma pressure is reduced inside the flux rope. A simple estimate of the total force acting on the flux rope from the magnetic forces and surrounding plasma suggests that the flux rope was experiencing an expansive total force. On 13 August 2003 all four Cluster spacecraft observed a flux rope at X (GSM) ~-18 RE. This flux rope was travelling tailward at 200 kms-1, consistent with the observed north-then-south bipolar BZ signature. The bipolar signature corresponds to a size of ~0.3 RE (lower estimate). In this case, the axis, determined from multi-spacecraft timing and the direction of the strong core field, was directed close to the maximum variance direction of the magnetic field. The current had components both parallel and perpendicular to the magnetic field, and J×B was again larger in the outer sections of the flux rope than in the centre. This flux rope was also under expansive magnetic pressure forces from J×B, i.e. directed away from the centre of the flux rope, and had a reduced plasma pressure inside the flux rope. A simple total force calculation suggests that this flux rope was experiencing a large expansive total force. The observations of a larger J×B signature in the outer sections of the flux ropes when compared to the centre may be explained if the flux ropes are observed at an intermediate stage of their evolution after creation by reconnection at multiple X lines near the Cluster apogee. It is suggested that these flux ropes are in the process of relaxing towards the force-free like configuration often observed further down the tail. The centre of the flux ropes may contain older reconnected flux at a later evolutionary stage and may therefore be more force-free.

Earth's lithospheric magnetic field determined to spherical harmonic degree 90 from CHAMP satellite measurements

SUMMARY The CHAMP magnetic field mission is providing highly reliable measurements from which the global lithospheric magnetic field can be determined in unprecedented resolution and accuracy. Using almost 5 yr of data, we derive our fourth generation lithospheric field model termed MF4, which is expanded to spherical harmonic degree and order 90. After subtracting from the full magnetic field observations predicted fields from an internal field model up to degree 15, an external field model up to degree two, and the predicted magnetic field signatures for the eight dominant ocean tidal constituents, we fit and remove remaining external fields and polar electrojet signatures in a track-by-track scheme. From a subset of least disturbed tracks, we estimate the MF4 model by least squares, damping ill-determined coefficients by regularization. The resulting MF4 model provides a good representation of the lithospheric field down to an altitude of about 50 km at lower latitudes, with reduced accuracy in the polar regions. Crustal features come out significantly sharper than in previous models. In particular, bands of magnetic anomalies along subduction zones become visible by satellite for the first time.

Exploitation of A Ship's Magnetic Field Signatures

Abstract Surface ship and submarine magnetic field signatures have been exploited for over 80 years by naval influence mines, and both underwater and airborne surveillance systems. The generating mechanism of the four major shipboard sources of magnetic fields is explained, along with a detailed description of the induced and permanent ferromagnetic signature characteristics. A brief historical summary of magnetic naval mine development during World War II is followed by a discussion of important improvements found in modern weapons, including an explanation of the damage mechanism for non-contact explosions. A strategy for selecting an optimum mine actuation threshold is given. A multi-layered defensive strategy against naval mines is outlined, with graphical explanations of the relationships between ship signature reduction and minefield clearing effectiveness. In addition to a brief historical discussion of underwater and airborne submarine surveillance systems and magnetic field sensing principles, ma...

Effects of kinetic processes in shaping Io's global plasma environment: A 3D hybrid model

The global dynamics of the ionized and neutral gases in the environment of Io plays an important role in the interaction of Jupiter's corotating magnetospheric plasma with Io. Stationary simulations of this problem have already been done using the magnetohydrodynamics (MHD) and the electrodynamics approaches. One of the major results of recent simplified two-fluid model simulations [Saur, J., Neubauer, F.M., Strobel, D.F., Summers, M.E., 2002. J. Geophys. Res. 107 (SMP5), 1–18] was the production of the structure of the double-peak in the magnetic field signature of the I0 flyby. These could not be explained before by standard MHD models. In this paper, we present a hybrid simulation for Io with kinetic ions and fluid electrons. This method employs a fluid description for electrons and neutrals, whereas for ions a particle approach is used. We also take into account charge-exchange and photoionization processes and solve self-consistently for electric and magnetic fields. Our model may provide a much more accurate description for the ion dynamics than previous approaches and allows us to account for the realistic anisotropic ion velocity distribution that cannot be done in fluid simulations with isotropic temperatures. The first results of such a simulation of the dynamics of ions in Io's environment are discussed in this paper. Comparison with the Galileo I0 flyby results shows that this approach provides an accurate physical basis for the interaction and can therefore naturally reproduce all the observed salient features.

Dynamics of the Saturnian inner magnetosphere: First inferences from the Cassini magnetometers about small‐scale plasma transport in the magnetosphere

The Cassini magnetometers reveal a very dynamic plasmadisc within the inner Saturnian magnetosphere during the first three orbits of the Cassini orbital tour. This corotation‐dominated region is known to contain various neutral and plasma populations and Voyager spacecraft observations suggest important radial transport processes redistribute the locally created plasma out to the remote magnetospheric regions, by a mechanism yet to be identified. In this work we report on anomalous magnetic field signatures in the inner regions of the magnetosphere (inside of 8 Saturn radii) that may participate in the radial transport and that seem to be consistent with signatures of interchanging flux tubes. These unusual events are characterized by sharp boundaries and abrupt changes of the magnetic pressure, consisting of depressions or enhancements of the field magnitude. They present some similarities with previously identified signatures of interchanging flux tubes in the Io torus, allowing the interesting possibility of carrying out a comparative study of these two environments.

Titan's Magnetic Field Signature During the First Cassini Encounter

The magnetic field signature obtained by Cassini during its first close encounter with Titan on 26 October 2004 is presented and explained in terms of an advanced model. Titan was inside the saturnian magnetosphere. A magnetic field minimum before closest approach marked Cassini's entry into the magnetic ionopause layer. Cassini then left the northern and entered the southern magnetic tail lobe. The magnetic field before and after the encounter was approximately constant for approximately 20 Titan radii, but the field orientation changed exactly at the location of Titan's orbit. No evidence of an internal magnetic field at Titan was detected.

First of Many Returns

### Contents #### Viewpoint Intensive Titan Exploration Begins P. R. Mahaffy #### Reports Cassini Radar Views the Surface of Titan C. Elachi et al. Titan's Atmospheric Temperatures, Winds, and Composition F. M. Flasar et al. The Cassini UVIS Stellar Probe of the Titan Atmosphere D. E. Shemansky et al. [Ion Neutral Mass Spectrometer Results from the First Flyby of Titan][1] J. H. Waite Jr. et al. Cassini Measurements of Cold Plasma in the Ionosphere of Titan J.-E. Wahlund et al. Energetic Neutral Atom Emissions from Titan Interaction with Saturn's Magnetosphere D. G. Mitchell et al. [Titan's Magnetic Field Signature During the First Cassini Encounter][2] H. Backes et al. T itan, Saturn's largest moon, and Earth are the only two bodies in our solar system with nitrogen-rich atmospheres. Although Earth's atmosphere has evolved partly under the influence of life (and includes carbon dioxide as the main carbon species), Titan's remains more primitive, reducing, and rich in methane, and may resemble that hypothesized for the early Earth. Our first close view of Titan came from Pioneer and the two Voyager spacecraft, which flew by in the late 1970s and early 1980s. These spacecraft provided a taste of Titan's atmosphere and its structure, determined Titan's size and density (a mixture of rock and organic ices), and showed that the moon lacked a significant magnetic field, although it is affected greatly by its orbit in and out of Saturn's magnetic field. Detailed study of Titan, however, has been limited because its atmosphere is so thick that, like those of Venus, Jupiter, and Saturn, it is difficult to see the surface beneath the haze. Cassini is equipped with a variety of instruments to both sample the haze and reveal the surface beneath it, and observations of Titan were a primary goal of the mission. Cassini also carried and released the Huygens probe, which successfully sampled Titan's atmosphere and landed on its solid surface late last year. The papers in this issue, including a summary by Mahaffy (p. 969), describe results from the first two passes of Cassini by Titan, including through its uppermost atmosphere. To see through the haze, Cassini carries a radar mapper that with each pass will build up a map of Titan's surface. The initial flyby has already revealed much (see the cover); as discussed by Elachi et al. (p. 970), Titan apparently has a relatively young flat surface with only a few large and degraded impact craters, but with a striking variation in the deposits. Together these features seem to indicate that there has been active resurfacing, likely through some form of ice volcanism. ![Figure][3] CREDIT: NASA/JET PROPULSION LABORATORY/U.S. GEOLOGICAL SURVEY A variety of other spectrometers on Cassini sampled and viewed Titan's atmosphere, providing new information on its composition and structure and showing some parallels with Earth's atmosphere [see Waite et al. (p. 982), Shemansky et al. (p. 978), and Flasar et al. (p. [975][4])]. Methane is abundant close to the surface, and Cassini has revealed several complex hydrocarbon species higher in the atmosphere. A tantalizing result is that although radiogenic argon is present (probably produced by outgassing of the moon's interior), the other argon isotopes that should have been present in an initial atmosphere are absent. Cassini also measured the effects of Titan's passage through Saturn's magnetic field [see Wahlund et al. (p. [986][5]), Mitchell et al. (p. 989), and Backes et al. (p. 992)]. It found and passed through a magnetic ionopause in Titan's atmosphere, below which the field strength dropped to zero, confirming Voyager's observation that Titan lacks a magnetic field. Cassini is planning to revisit Titan more than 40 times during its mission, repeating many of these analyses and mapping different parts of the moon's surface. Together with the Huygens data, these data should help us understand the origin and evolution of Saturn's largest moon and, by analogy, Earth's early history. [1]: /lookup/doi/10.1126/science.1110652 [2]: /lookup/doi/10.1126/science.1109763 [3]: pending:yes [4]: /lookup/doi/10.1126/science.1111150 [5]: /lookup/doi/10.1126/science.1109807

Solar wind Alfvén waves: a source of pulsed ionospheric convection and atmospheric gravity waves

Abstract. A case study of medium-scale travelling ionospheric disturbances (TIDs) that are correlated with solar wind Alfvén waves is presented. The HF radar ground-scatter signatures of TIDs caused by atmospheric gravity waves with periods of 20-40min are traced to a source at high latitudes, namely pulsed ionospheric flows (PIFs) due to bursts in the convection electric field and/or the associated ionospheric current fluctuations inferred from ground magnetic field perturbations. The significance of PIFs and TIDs in the context of solar-terrestrial interaction is that Alfvénic fluctuations of the interplanetary magnetic field (IMF) observed in the solar wind plasma streaming from a coronal hole correlate with PIFs and TIDs. The link between the solar wind Alfvén waves and TIDs is corroborated by the ground magnetic field signatures of ionospheric current fluctuations that are associated with the IMF-By oscillations and TIDs. The observed PIFs and the associated negative-to-positive deflections of the ground magnetic field X component are interpreted as ionospheric signatures of magnetic reconnection pulsed by solar wind Alfvén waves at the dayside magnetopause. Although the clarity of the radar line-of-sight velocity data may have been affected by anomalous HF propagation due to intervening TIDs, the application of a pure state filtering technique to analyze the radar data time series reveals a one-to-one correspondence between PIFs, TIDs and solar wind Alfvén waves. The spectra of solar wind and ground magnetic field perturbations are similar to those of PIFs and TIDs. The ground-scatter signatures indicate TID wavelengths, phase velocities and travel times that are consistent with ray tracing, which shows a subset of possible gravity wave group paths that reach the F region from a source in the E region after the wave energy first travel downward to the upper mesosphere where the waves are reflected upward. The observed one-to-one correspondence between the convection electric field bursts and TIDs is consistent with the modeling results for large-scale TIDs by Millward et al. (1993a,b). The correlation with solar wind Alfvén waves points to very direct coupling of energy in the solar wind into the subauroral atmosphere.

Numerical simulation of the combined system of nighttime region 0, region 1, and region 2 field‐aligned currents

A numerical simulation is performed to identify the physical mechanisms for the generation of the nighttime region 0 and region 1 field‐aligned currents (FACs) in conditions of a steady solar wind with a southward or weak interplanetary magnetic field. The generation of region 0/region 1 FACs observed near the poleward edge of the auroral oval is closely associated with the dynamics of ion beams produced by nonadiabatic acceleration in the tail current sheet. After ejection from the current sheet, ion beams advance toward the Earth along the nearly straight field lines. During such a time interval, the ions have insignificant magnetic drift velocities. Subsequently, they are reflected at low‐altitude mirror points, and under the influence of inward convection they pass the equator on magnetic shells that are further in. At that moment they attain large magnetic (curvature) drift velocities. Accordingly, in a steady state, the magnetic drift speed averaged over all the ions in a flux tube increases with decreasing invariant latitude; it starts to increase significantly at a certain magnetic shell. In the present paper this magnetic shell is identified with the interface demarcating the nonadiabatic and adiabatic regions in the tail current sheet. An essential point for the region 1 FAC generation is distortion of this interface, namely inclination of the interface to the direction of the average magnetic drift velocity. The generation of region 0 FACs is due to anomalous cross‐field diffusion of energized ions near the poleward edge of the auroral oval. Latitudinal profiles of magnetic field perturbations arising from region 0/region 1 FACs in the simulation are remarkably similar to observed magnetic field signatures of the region 0/region 1 thin‐current‐sheet systems.