Geomagnetic Core Field Secular Variation Models

We analyse models describing time changes of the Earth’s core magnetic field (secular variation) covering the historical period (several centuries) and the more recent satellite era (previous decade), and we illustrate how both the information contained in the data and the a priori information (regularisation) affect the result of the ill-posed geomagnetic inverse problem. We show how data quality, frequency and selection procedures govern part of the temporal changes in the secular variation norms and spectra, which are sometimes difficult to dissociate from true changes of the core state. We highlight the difficulty of resolving the time variability of the high degree secular variation coefficients (i.e. the secular acceleration), arising for instance from the challenge to properly separate sources of internal and of external origin. In addition, the regularisation process may also result in artificial changes in the model norms and spectra. Model users should keep in mind that such features can be mis-interpreted as the signature of physical mechanisms (e.g. diffusion). Finally, we present perspectives concerning core field modelling: imposing dynamical constraints (e.g. by means of data assimilation) reduces the non-uniqueness of the geomagnetic inverse problem.     

Short Timescale Core Dynamics: Theory and Observations

Fluid motions in the Earth’s core produce changes in the geomagnetic field (secular variation) and are also an important ingredient in the planet’s rotational dynamics. In this article we review current understanding of core dynamics focusing on short timescales of years to centuries. We describe both theoretical models and what may be inferred from geomagnetic and geodetic observations. The kinematic concepts of frozen flux and magnetic diffusion are discussed along with relevant dynamical regimes of magnetostrophic balance, tangential geostrophy, and quasi-geostrophy. An introduction is given to free modes and waves that are expected to be present in Earth’s core including axisymmetric torsional oscillations and non-axisymmetric Magnetic-Coriolis waves. We focus on important recent developments and promising directions for future investigations.     

Observations and Models of the Long-Term Evolution of Earth’s Magnetic Field

The geomagnetic signal contains an enormous temporal range—from geomagnetic jerks on time scales of less than a year to the evolution of Earth’s dipole moment over billions of years. This review compares observations and numerical models of the long-term range of that signal, for periods much larger than the typical overturn time of Earth’s core. On time scales of 10⁵–10⁹ years, the geomagnetic field reveals the control of mantle thermodynamic conditions on core dynamics. We first briefly describe the general formalism of numerical dynamo simulations and available paleomagnetic data sets that provide insight into paleofield behavior. Models for the morphology of the time-averaged geomagnetic field over the last 5 million years are presented, with emphasis on the possible departures from the geocentric axial dipole hypothesis and interpretations in terms of core dynamics. We discuss the power spectrum of the dipole moment, as it is a well-constrained aspect of the geomagnetic field on the million year time scale. We then summarize paleosecular variation and intensity over the past 200 million years, with emphasis on the possible dynamical causes for the occurrence of superchrons. Finally, we highlight the geological evolution of the geodynamo in light of the oldest paleomagnetic records available. A summary is given in the form of a tentative classification of well-constrained observations and robust numerical modeling results.     

Geomagnetic Jerks: Rapid Core Field Variations and Core Dynamics

The secular variation of the core field is generally characterized by smooth variations, sometimes interrupted by abrupt changes, named geomagnetic jerks. The origin of these events, observed and investigated for over three decades, is still not fully understood. Many fundamental features of geomagnetic jerks have been the subject of debate, including their origin internal or external to the Earth, their occurrence dates, their duration and their global or regional character. Specific tools have been developed to detect them in geomagnetic field or secular variation time series. Recently, their investigation has been advanced by the availability of a decade of high-quality satellite measurements. Moreover, advances in the modelling of the core field and its variations have brought new perspectives on the fluid motion at the top of the core, and opened new avenues in our search for the origin of geomagnetic jerks. Correlations have been proposed between geomagnetic jerks and some other geophysical observables, indicating the substantial interest in this topic in our scientific community. This paper summarizes the recent advances in our understanding and interpretation of geomagnetic jerks.     

Polarity Reversals from Paleomagnetic Observations and Numerical Dynamo Simulations

Recent advances in the study of geomagnetic field reversals are reviewed. These include studies of the transitional field during the last geomagnetic reversal and the last geomagnetic excursion based on paleomagnetic observations, and analysis of reversals in self-consistent 3D numerical dynamo simulations. Field models inferred from observations estimate reversal duration in the range of 1–10 kyr (depending on site location). The transitional fields during both the Matuyama/Brunhes reversal and the Laschamp excursion are characterized by low-latitude reversed flux formation and subsequent poleward migration. During both events the dipole as well as the non-dipole field energies decrease. However, while the non-dipole energy dominates the dipole energy for a period of 2 kyr in the reversal, the non-dipole energy merely exceeds the dipole energy for a very brief period during the excursion. Numerical dynamo simulations show that stronger convection, slower rotation, and lower electrical conductivity provide more favorable conditions for reversals. A non-dimensional number that depends on the typical length scale of the flow and represents the relative importance of inertial effects, termed the local Rossby number, seems to determine whether a dynamo will reverse or not. Stable polarity periods in numerical dynamos may last about 1 Myr, whereas reversals may last about 10 kyr. Numerical dynamo reversals often involve prolonged dipole collapse followed by shorter directional instability of the dipole axis, with advective processes governing the field variation. Magnetic upwellings from the equatorial inner-core boundary that produce reversed flux patches at low-latitudes of the core-mantle boundary could be significant in triggering reversals. Inferences from the observational and modeling sides are compared. We summarize with an outlook on some open questions and future prospects.     

An Overview of Earth’s Global Electric Circuit and Atmospheric Conductivity

The Earth’s global atmospheric electric circuit depends on the upper and lower atmospheric boundaries formed by the ionosphere and the planetary surface. Thunderstorms and electrified rain clouds drive a DC current (～1 kA) around the circuit, with the current carried by molecular cluster ions; lightning phenomena drive the AC global circuit. The Earth’s near-surface conductivity ranges from 10^?7 S?m^?1 (for poorly conducting rocks) to 10^?2 S?m^?1 (for clay or wet limestone), with a mean value of 3.2 S?m^?1 for the ocean. Air conductivity inside a thundercloud, and in fair weather regions, depends on location (especially geomagnetic latitude), aerosol pollution and height, and varies from ～10^?14 S?m^?1 just above the surface to 10^?7 S?m^?1 in the ionosphere at ～80 km altitude. Ionospheric conductivity is a tensor quantity due to the geomagnetic field, and is determined by parameters such as electron density and electron–neutral particle collision frequency. In the current source regions, point discharge (coronal) currents play an important role below electrified clouds; the solar wind-magnetosphere dynamo and the unipolar dynamo due to the terrestrial rotating dipole moment also apply atmospheric potential differences. Detailed measurements made near the Earth’s surface show that Ohm’s law relates the vertical electric field and current density to air conductivity. Stratospheric balloon measurements launched from Antarctica confirm that the downward current density is ～1 pA m^?2 under fair weather conditions. Fortuitously, a Solar Energetic Particle (SEP) event arrived at Earth during one such balloon flight, changing the observed atmospheric conductivity and electric fields markedly. Recent modelling considers lightning discharge effects on the ionosphere’s electric potential (～+250 kV with respect to the Earth’s surface) and hence on the fair weather potential gradient (typically ～130 V?m^?1 close to the Earth’s surface. We conclude that cloud-to-ground (CG) lightning discharges make only a small contribution to the ionospheric potential, and that sprites (namely, upward lightning above energetic thunderstorms) only affect the global circuit in a miniscule way. We also investigate the effects of mesoscale convective systems on the global circuit.     

Five Years of Stereo Magnetospheric Imaging by TWINS

Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) is the first stereoscopic magnetospheric imager. TWINS is a NASA Explorer Mission of Opportunity performing simultaneous energetic neutral atom (ENA) imaging from two widely-separated Molniya orbits on two different spacecraft, and providing nearly continuous coverage of magnetospheric ENA emissions. The ENA imagers observe energetic neutrals produced from global ion populations, over a broad energy range (1–100 keV/u) with high angular (4^°×4^°) and time (about 1-minute) resolution. TWINS distinguishes hydrogen ENAs from oxygen ENAs. Each TWINS spacecraft also carries a Lyman-α geocoronal imager to monitor the cold exospheric hydrogen atoms that produce ENAs from ions via charge exchange. Complementing the imagers are detectors that measure the local charged particle environment around the spacecraft. During its first five years of science operations, TWINS has discovered new global properties of geospace plasmas and neutrals, fostered understanding of causal relationships, confirmed theories and predictions based on in situ data, and yielded key insights needed to improve geospace models. Analysis and modeling of TWINS data have: (1) obtained continuous (main phase through recovery) global ion spectra, (2) revealed a previously unknown local-time dependence of global pitch angle, (3) developed quantitative determination of ion fluxes from low altitude ENAs (4) determined dynamic connections between local pitch angle and global ion precipitation, (5) confirmed local-time dependence of precipitating ion temperature, (6) imaged global dynamic heating of the magnetosphere, (7) explained why the oxygen ring current survives longer into recovery than hydrogen, and (8) revealed new global exospheric density features and their influence upon ring current decay rates. Over the next several years of the solar cycle, TWINS observations of three-dimensional (3D) global ion dynamics, composition, origins and destinies are crucial to capture the system-level view of geospace over the full range of geomagnetic and solar activity conditions.     

Magnetospheric Science Objectives of the <Emphasis Type="Italic">Juno</Emphasis> Mission

In July 2016, NASA’s Juno mission becomes the first spacecraft to enter polar orbit of Jupiter and venture deep into unexplored polar territories of the magnetosphere. Focusing on these polar regions, we review current understanding of the structure and dynamics of the magnetosphere and summarize the outstanding issues. The Juno mission profile involves (a) a several-week approach from the dawn side of Jupiter’s magnetosphere, with an orbit-insertion maneuver on July 6, 2016; (b) a 107-day capture orbit, also on the dawn flank; and (c) a series of thirty 11-day science orbits with the spacecraft flying over Jupiter’s poles and ducking under the radiation belts. We show how Juno’s view of the magnetosphere evolves over the year of science orbits. The Juno spacecraft carries a range of instruments that take particles and fields measurements, remote sensing observations of auroral emissions at UV, visible, IR and radio wavelengths, and detect microwave emission from Jupiter’s radiation belts. We summarize how these Juno measurements address issues of auroral processes, microphysical plasma physics, ionosphere-magnetosphere and satellite-magnetosphere coupling, sources and sinks of plasma, the radiation belts, and the dynamics of the outer magnetosphere. To reach Jupiter, the Juno spacecraft passed close to the Earth on October 9, 2013, gaining the necessary energy to get to Jupiter. The Earth flyby provided an opportunity to test Juno’s instrumentation as well as take scientific data in the terrestrial magnetosphere, in conjunction with ground-based and Earth-orbiting assets.     

连续地月转移系统动力学研究与能量分析

为了研究新型连续地月转移系统的动力学及能量需求,采用Lagrange方法,在系绳为刚性杆假设的前提下,同时忽略第三体引力、地球扁率和系绳轴向变形等扰动因素的影响,建立了驱动型动量交换绳系卫星(MMET)系统的三维刚性动力学模型。对所建立的动力学模型进行了数值仿真及对比分析,仿真结果验证了所建模型的正确性。研究表明,外力矩对系统轨道运动参数影响甚小,对姿态运动参数影响明显。采用MMET方式进行载荷转移,推导出了实现载荷地月轨道转移所需的入口速度条件以及时间周期条件,并求解出了载荷在2次任务之间的时间间隔。给定初始条件下,当MMET系统以0.231 6 rad/s的旋转角速度绕其质心旋转1 448.5圈,其绕地心刚好运行5圈时,载荷可顺利进入地月转移轨道。最后,对连续地月转移系统实现载荷的地月转移进行了能量对比分析,结果表明,相同条件下,MMET载荷转移方式相比于传统脉冲变轨方式在载荷转移过程中消耗更少的能量。     

The Lunar Radar Sounder (LRS) Onboard the KAGUYA (SELENE) Spacecraft

The Lunar Radar Sounder (LRS) onboard the KAGUYA (SELENE) spacecraft has successfully performed radar sounder observations of the lunar subsurface structures and passive observations of natural radio and plasma waves from the lunar orbit. After the transfer of the spacecraft into the final lunar orbit and antenna deployment, the operation of LRS started on October 29, 2007. Through the operation until June 10, 2009, 2363 hours worth of radar sounder data and 8961 hours worth of natural radio and plasma wave data have been obtained. It was revealed through radar sounder observations that there are distinct reflectors at a depth of several hundred meters in the nearside maria, which are inferred to be buried regolith layers covered by a basalt layer with a thickness of several hundred meters. Radar sounder data were obtained not only in the nearside maria but also in other regions such as the farside highland region and polar region. LRS also performed passive observations of natural plasma waves associated with interaction processes between the solar wind plasma and the moon, and the natural waves from the Earth, the sun, and Jupiter. Natural radio waves such as auroral kilometric radiation (AKR) with interference patterns caused by the lunar surface reflections, and Jovian hectometric (HOM) emissions were detected. Intense electrostatic plasma waves around 20 kHz were almost always observed at local electron plasma frequency in the solar wind, and the electron density profile, including the lunar wake boundary, was derived along the spacecraft trajectory. Broadband noises below several kHz were frequently observed in the dayside and wake boundary of the moon and it was found that a portion of them consist of bipolar pulses. The datasets obtained by LRS will make contributions for studies on the lunar geology and physical processes of natural radio and plasma wave generation and propagation.     

Polar auroras

Conclusion We have reviewed the somewhat conflicting data which have accumulated on such a vast scale in recent years. It is now becoming clearer which studies are likely to produce significant results, and this in itself may be a very important consequence of the assimilation of accumulated data. We must however ask in conclusion: does the outer radiation belt exist during the polar aurora? If the interplanetary media or the solar wind, carry magnetic fields, then these fields can be of two kinds. Firstly, they may be magnetic lines of force dragged by the plasma from the Sun. Secondly, the interplanetary medium or the solar wind are capable of carrying closed magnetic lines of force which are not related to the Sun. When such fields approach the Earth, the high-latitude geomagnetic lines of force which previously passed through the equatorial plane on the boundary of the magnetosphere, may deform in such a way as to pass out of one geomagnetic poles, miss the equatorial plane, enter the interplanetary plasma, and after passing through a very considerable volume of this plasma reach the other geomagnetic pole. This will in effect amount to an attachment through the medium of magnetic lines of force of enormous regions of ionised interplanetary matter or of solar wind to the Earth's magnetosphere. As these extraneous magnetic fields depart from the Earth's neighbourhood, the original dipole field will be reestablished. Rapid variations in the configuration of the geomagnetic field will occur during the interaction. It is possible that energetic particles appear with a very high degree of probability on the boundary of the geomagnetic field during such deformations. If this is so, then the outer radiation belt is merely a temporary formation appearing during the quiet intervals between geomagnetic disturbances, and containing a small residue of energetic charged particles, which exist during the polar auroras but do not succeed in entering the lower atmosphere during this time. In this process the particles giving rise to the polar auroras originate in the plasma of the solar corpuscular streams flowing past the Earth.Under the action of a solar wind the geomagnetic field is compressed at the front and elongated at the rear. This resembles the original Chapman theory of geomagnetic storms more closely than any other theory. Since the elongated geomagnetic field on the night side of the Earth is of a lower intensity, it may be associated with the magnetic fields brought in by the incident medium right down to very great depths. This may be responsible for the observed displacement at the zone of the polar auroras towards lower geomagnetic latitudes at night.Translated by the Express Translation Servies, Wimbledon, London.     

Dynamical Simulations of Plasmapause Deformations

Dynamical simulations have been developed at IASB-BIRA to model the deformations of the plasmasphere during geomagnetic substorms and other variations in the level of geomagnetic activity. The simulations are based on the mechanism of plasma instability and use the empirical Kp-dependent electric field E5D. The results of the simulations are compared with IMAGE observations that provide the first global comprehensive images of the Earth’s plasmasphere. The predicted plasmapause positions correspond generally rather satisfactorily with the EUV observations. The plasmasphere is rather extended in all MLT sectors during quiet periods. During or just after geomagnetic substorms, the plasmaspause is sharper and becomes closer to the Earth in the night sector. Periods of enhanced geomagnetic activity are associated to the formation of plumes that rotate with the plasmasphere. The simulations reproduce the formation and the motion of these plumes, as well as the development of other structures like shoulders observed at the plasmapause by EUV on IMAGE.     

Latitudinal structure of the solar wind and interplanetary magnetic field

A review is given of both observational and theoretical results concerning the latitudinal structure of the solar wind and interplanetary magnetic field. Observations are reported on the solar wind plasma and magnetic fields, obtained both from direct satellite measurements and indirect methods, such as the observation of comet tails, radio scintillations, the study of the polar geomagnetic field and the semi-annual variation of geomagnetic activity. Results of theoretical work, both on three-dimensional modelling of the solar wind and on gas-magnetic field interactions in the solar corona are summarized. Finally, an attempt is made to compare available observations and theories. This points to the open questions which, to be settled, will need direct observations of plasma and magnetic field at high heliographic latitudes.     

On the Detectability and Use of Normal Modes for Determining Interior Structure of Mars

The InSight mission to Mars is well underway and will be the first mission to acquire seismic data from a planet other than Earth. In order to maximise the science return of the InSight data, a multifaceted approach will be needed that seeks to investigate the seismic data from a series of different frequency windows, including body waves, surface waves, and normal modes. Here, we present a methodology based on globally-averaged models that employs the long-period information encoded in the seismic data by looking for fundamental-mode spheroidal oscillations. From a preliminary analysis of the expected signal-to-noise ratio, we find that normal modes should be detectable during nighttime in the frequency range 5–15 mHz. For improved picking of (fundamental) normal modes, we show first that those are equally spaced between 5–15 mHz and then show how this spectral spacing, obtained through autocorrelation of the Fourier-transformed time series can be further employed to select normal mode peaks more consistently. Based on this set of normal-mode spectral frequencies, we proceed to show how this data set can be inverted for globally-averaged models of interior structure (to a depth of \(\sim 250~\mbox{km}\)), while simultaneously using the resultant synthetically-approximated normal mode peaks to verify the initial peak selection. This procedure can be applied iteratively to produce a “cleaned-up” set of spectral peaks that are ultimately inverted for a “final” interior-structure model. To investigate the effect of three-dimensional (3D) structure on normal mode spectra, we constructed a 3D model of Mars that includes variations in surface and Moho topography and lateral variations in mantle structure and employed this model to compute full 3D waveforms. The resultant time series are converted to spectra and the inter-station variation hereof is compared to the variation in spectra computed using different 1D models. The comparison shows that 3D effects are less significant than the variation incurred by the difference in radial models, which suggests that our 1D approach represents an adequate approximation of the global average structure of Mars.     

Locating the LCROSS Impact Craters

The Lunar CRater Observations and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket stage into a permanently shadowed region near the lunar south pole. The Sheperding Spacecraft (SSC) separated ～9 hours before impact and performed a small braking maneuver in order to observe the Centaur impact plume, looking for evidence of water and other volatiles, before impacting itself. This paper describes the registration of imagery of the LCROSS impact region from the mid- and near-infrared cameras onboard the SSC, as well as from the Goldstone radar. We compare the Centaur impact features, positively identified in the first two, and with a consistent feature in the third, which are interpreted as a 20 m diameter crater surrounded by a 160 m diameter ejecta region. The images are registered to Lunar Reconnaisance Orbiter (LRO) topographical data which allows determination of the impact location. This location is compared with the impact location derived from ground-based tracking and propagation of the spacecraft’s trajectory and with locations derived from two hybrid imagery/trajectory methods. The four methods give a weighted average Centaur impact location of ?84.6796°, ?48.7093°, with a 1σ uncertainty of 115 m along latitude, and 44 m along longitude, just 146 m from the target impact site. Meanwhile, the trajectory-derived SSC impact location is ?84.719°, ?49.61°, with a 1σ uncertainty of 3 m along the Earth vector and 75 m orthogonal to that, 766 m from the target location and 2.803 km south-west of the Centaur impact. We also detail the Centaur impact angle and SSC instrument pointing errors. Six high-level LCROSS mission requirements are shown to be met by wide margins. We hope that these results facilitate further analyses of the LCROSS experiment data and follow-up observations of the impact region.     

Meteor Phenomena and Bodies

Meteoroids can be observed at collision with the Earth's atmosphere as meteors. Different methods of observing meteors are presented: besides the traditional counts of individual events, exact methods yield also data on the geometry of the atmospheric trajectory; on the dynamics and ablation of the body in the atmosphere; on radiation; on the spectral distribution of radiation; on ionization; on accompanying sounds; and also data on orbits. Theoretical models of meteoroid interaction with the atmosphere are given and applied to observational data. Attention is paid to radar observations; to spectroscopic observations; to experiments with artificial meteors and to different types of meteor sounds. The proposed composition and structure of meteoroids as well as their orbits link them to meteorites, asteroids and comets. Meteor streams can be observed as meteor showers and storms. The rate of influx of meteoroids of different sizes onto Earth is presented and potential hazards discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Millennial Variations of the Geomagnetic Field: from Data Recovery to Field Reconstruction

Variations of the geomagnetic field over past millennia can be determined from archeomagnetic data and paleomagnetic sediment records. The resolution and validity of any field reconstruction depends on the reliability of such indirect measurements of past field values. Considerable effort is invested to ensure that the magnetic minerals carrying the ancient magnetization are good, if not ideal, recorders of the magnetic field. This is achieved by performing a wide array of rock magnetic and microscopy investigations, many of which are outlined here. In addition to data quality, the spatial and temporal distributions of archeomagnetic and sediment records play a significant role in the accuracy of past field reconstruction. Global field reconstructions enable studies of dynamic processes in Earth’s core. They rely on data compilations which ideally include information about the quality of a measurement and provide a useful archive for selecting data with the best characteristics. There is, however, a trade off between the total number of reliable data and the geographic or temporal coverage. In this review we describe the various types of paleomagnetic recorders, and the kind of measurements that are performed to gather reliable geomagnetic field information. We show which modeling strategies are most suitable, and the main features of the field that can be derived from the resulting models. Finally, we discuss prospects for progress in this kind of research.     

The Solar Wind Charge-eXchange Contribution to the Local Soft X-ray Background

The major sources of the Soft X-ray Background (SXRB), besides distinct structures as supernovae and superbubbles (e.g. Loop I), are: (i) an absorbed extragalactic emission following a power law, (ii) an absorbed thermal component (～2×10⁶ K) from the galactic disk and halo, (iii) an unabsorbed thermal component, supposedly at 10⁶ K, attributed to the Local Bubble and (iv) the very recently identified unabsorbed Solar Wind Charge-eXchange (SWCX) emission from the heliosphere and the geocorona. We study the SWCX heliospheric component and its contribution to observed data. In a first part, we apply a SWCX heliospheric simulation to model the oxygen lines (3/4 keV) local intensities during shadowing observations of the MBM 12 molecular cloud and a dense filament in the south galactic hemisphere with Chandra, XMM-Newton, and Suzaku telescopes. In a second part, we present a preliminary comparison of SWCX model results with ROSAT and Wisconsin surveys data in the 1/4 keV band. We conclude that, in the 3/4 keV band, the total local intensity is entirely heliospheric, while in the 1/4 keV band, the heliospheric component seems to contribute significantly to the local SXRB intensity and has potentially a strong influence on the interpretation of the ROSAT and Wisconsin surveys data in terms of Local Bubble hot gas temperature.