Ionospheric Field Research Articles

A statistical study of the low‐altitude ionospheric magnetic fields over the north pole of Venus

AbstractExamination of Venus Express (VEX) low‐altitude ionospheric magnetic field measurements during solar minimum has revealed the presence of strong magnetic fields at low altitudes over the north pole of Venus. A total of 77 events with strong magnetic fields as VEX crossed the northern polar region were identified between July 2008 and October 2009. These events all have strong horizontal fields, slowly varying with position. Using the superposed epoch method, we find that the averaged peak field is about 45 nT, which is well above the average ambient ionospheric field of 20 nT, with a full width at half maximum duration of 32 s, equivalent to a width of about 300 km. Considering the field orientation preference and spacecraft trajectory geometry, we conclude that these strong fields are found over the northern hemisphere with an occurrence frequency of more than 33% during solar minimum. They do not show a preference for any particular interplanetary magnetic field (IMF) orientation. However, they are found over the geographic pole more often when the interplanetary field is in the Venus orbital plane than when it is perpendicular to the orbital plane of Venus. The structures were found most frequently in the −E hemisphere, determined from the IMF orientation. The enhanced magnetic field is mainly quasi perpendicular to solar wind flow direction, and it is suggested that these structures form in the low‐altitude collisional ionosphere where the diffusion and convection times are long.

NOAA/NGDC candidate models for the 12th generation International Geomagnetic Reference Field

The International Geomagnetic Reference Field (IGRF) is a model of the geomagnetic main field and its secular variation, produced every 5 years from candidate models proposed by a number of international research institutions. For this 12th generation IGRF, three candidate models were solicited: a main field model for the 2010.0 epoch, a main field model for the 2015.0 epoch, and the predicted secular variation for the five-year period 2015 to 2020. The National Geophysical Data Center (NGDC), part of the National Oceanic and Atmospheric Administration (NOAA), has produced three candidate models for consideration in IGRF-12. The 2010 main field candidate was produced from Challenging Minisatellite Payload (CHAMP) satellite data, while the 2015 main field and secular variation candidates were produced from Swarm and Ørsted satellite data. Careful data selection was performed to minimize the influence of magnetospheric and ionospheric fields. The secular variation predictions of our parent models, from which the candidate models were derived, have been validated against independent ground observatory data.

Electrical conductivity of the Earth's mantle from the first Swarm magnetic field measurements

AbstractWe present a 1‐D electrical conductivity profile of the Earth's mantle down to 2000 km derived from L1b Swarm satellite magnetic field measurements from November 2013 to September 2014. We first derive a model for the main magnetic field, correct the data for a lithospheric field model, and additionally select the data to reduce the contributions of the ionospheric field. We then model the primary and induced magnetospheric fields for periods between 2 and 256 days and perform a Bayesian inversion to obtain the probability density function for the electrical conductivity as function of depth. The conductivity increases by 3 orders of magnitude in the 400–900 km depth range. Assuming a pyrolitic mantle composition, this profile is interpreted in terms of temperature variations leading to a temperature gradient in the lower mantle that is close to adiabatic.

Propagation of Pi2 pulsations in a dipole model of the magnetosphere

AbstractLocalized fast flows that impinge on the inner magnetosphere from the plasma sheet are observed to oscillate on time scales of minutes. The compression ahead of these flows will launch fast mode waves, while the velocity shears at the edges of these flows directly excite shear Alfvén waves. These waves, which are coupled by gradients in the Alfvén speed, have been suggested as a source for the Pi1 and Pi2 waves that are observed at both high and low latitudes in the ionosphere. A new three‐dimensional simulation of the propagation of ULF waves in the dipolar region of the magnetosphere has been developed to study these coupled wave modes. This model includes a height‐resolved ionospheric conductivity so that ionospheric fields can be more realistically determined, as well as a direct calculation of ground magnetic fields to compare with ground magnetometers using an inductive ionosphere model. Results from this model show that a plasmaspheric resonance can be set up by waves with periods about 1 min and that field line resonances can be excited both inside and outside the plasmasphere. The use of the inductive ionosphere model leads to the conclusion that even a uniform Hall conductivity can break the dawn‐dusk symmetry of the convection pattern. Waves from a source at 10 Earth radii reach the ionosphere with time delays between high and low latitudes of tens of seconds, with implications for the timing of substorm phenomena observed by spacecraft and by ground magnetometers and radars.

First observation of the anomalous electric field in the topside ionosphere by ionospheric modification over EISCAT

AbstractWe have developed an active ground‐based technique to estimate the steady state field‐aligned anomalous electric field (E*) in the topside ionosphere, up to ~600 km, using the European Incoherent Scatter (EISCAT) ionospheric modification facility and UHF incoherent scatter radar. When pumping the ionosphere with high‐power high‐frequency radio waves, the F region electron temperature is significantly raised, increasing the plasma pressure gradient in the topside ionosphere, resulting in ion upflow along the magnetic field line. We estimate E* using a modified ion momentum equation and the Mass Spectrometer Incoherent Scatter model. From an experiment on 23 October 2013, E* points downward with an average amplitude of ~1.6 μV/m, becoming weaker at higher altitudes. The mechanism for anomalous resistivity is thought to be low‐frequency ion acoustic waves generated by the pump‐induced flux of suprathermal electrons. These high‐energy electrons are produced near the pump wave reflection altitude by plasma resonance and also result in observed artificially induced optical emissions.

A comparison of the effects of initializing different thermosphere‐ionosphere model fields on storm time plasma density forecasts

AbstractData assimilation has been used successfully for real‐time ionospheric specification, but it has not yet proved advantageous for forecasting. The most challenging and important ionospheric events to forecast are storms. The work presented here examines the effectiveness of data assimilation in a storm situation, where the initial conditions are known and the model is considered to be correct but the external solar and geomagnetic drivers are poorly specified. The aim is to determine whether data assimilation could be used to improve storm time forecast accuracy. The results show that, in the case of the storm of Halloween 2003, changes made to the model's initial thermospheric conditions improve electron density forecasts by at least 10% for 18 h, while changes to ionospheric fields alone result in >10% forecast accuracy improvement for less than 4 h. Further examination shows that the neutral composition is especially important to the accuracy of ionospheric electron density forecasts. Updating the neutral composition gives almost all the benefits of updating the complete thermospheric state. A comparison with real, globally distributed observations of vertical total electron content confirms that updating the thermospheric composition can improve forecast accuracy.

Swarm SCARF Dedicated Lithospheric Field Inversion chain

The geomagnetic daily variation at mid-to-low latitudes, referred to as the geomagnetic Sq field, is generated by electrical currents within the conducting layers of the ionosphere on the dayside of the Earth. It is enhanced in a narrow equatorial band, due to the equatorial electrojet. The upcoming ESA Swarm satellite mission, to be launched end of 2013, will consist of three satellites in low-Earth orbit, providing a dense spatial and temporal coverage of the ionospheric Sq field. A Satellite Constellation Application and Research Facility (SCARF) has been set up by a consortium of research institutions, aiming at producing various level-2 data products during the Swarm mission. The Dedicated Ionospheric Field Inversion (DIFI) chain is a SCARF algorithm calculating global, spherical harmonic models of the Sq field at quiet times. It describes seasonal and solar cycle variations, separates primary and induced magnetic fields based upon advanced 3D-models of the mantle electrical conductivity, and relies on core, lithospheric and magnetospheric field models derived from other SCARF algorithms for removing non-ionospheric fields from the data. The DIFI chain was thoroughly tested on synthetic data during the SCARF preparation phase; it is now ready to be used for deriving models from real Swarm data.

Swarm SCARF Dedicated Ionospheric Field Inversion chain

The geomagnetic daily variation at mid-to-low latitudes, referred to as the geomagnetic Sq field, is generated by electrical currents within the conducting layers of the ionosphere on the dayside of the Earth. It is enhanced in a narrow equatorial band, due to the equatorial electrojet. The upcoming ESA Swarm satellite mission, to be launched end of 2013, will consist of three satellites in low-Earth orbit, providing a dense spatial and temporal coverage of the ionospheric Sq field. A Satellite Constellation Application and Research Facility (SCARF) has been set up by a consortium of research institutions, aiming at producing various level-2 data products during the Swarm mission. The Dedicated Ionospheric Field Inversion (DIFI) chain is a SCARF algorithm calculating global, spherical harmonic models of the Sq field at quiet times. It describes seasonal and solar cycle variations, separates primary and induced magnetic fields based upon advanced 3D-models of the mantle electrical conductivity, and relies on core, lithospheric and magnetospheric field models derived from other SCARF algorithms for removing non-ionospheric fields from the data. The DIFI chain was thoroughly tested on synthetic data during the SCARF preparation phase; it is now ready to be used for deriving models from real Swarm data.

The characteristics of quasistatic electric field perturbations observed by DEMETER satellite before large earthquakes

This paper presents new results after processing the ULF electric field (DC-15Hz) observed by DEMETER satellite (h=660–710km). Typical perturbations were picked up in quasistatic electric field around some large earthquakes in 2010 at first. And then, 27 earthquakes were selected to be analyzed on quasistatic electric field in two seismic regions of Indonesia and Chile at equatorial and middle latitude area respectively. Three-component electric field data related to earthquakes were collected along all the up-orbits (in local nighttime) in a limited distance of 2000km to the epicenters during 9days with 7days before and 1day after those cases, and totally 57 perturbations were found around them. All the results show that the amplitude of quasistatic electric field perturbations varies from 1.5 to 16mV/m in the upper ionosphere, mostly smaller than 10mV/m. And the perturbations were mainly located just over the epicentral area or at the end of seismic faults constructed by a series of earthquakes where electromagnetic emissions may be easily formed during preparation and development processes of seismic sequences. Among all 27 cases, there are 10 earthquakes with perturbations occurring just one day before, which demonstrates the close correlation in time domain between quasistatic electric field in ionosphere and large earthquakes. Finally, combined with in situ observation of plasma parameters, the coupling mechanism of quasistatic electric field in different earth spheres was discussed.

Linear mechanism of generation and intencification of internal gravity waves in the ionosphere at their interaction with a nonuniform zonal wind: 1. Model of the medium and initial dynamic equations

The specific features of the generation and intensification of internal gravity wave structures in different atmospheric-ionospheric regions, caused by zonal local nonuniform winds (shear flows), are studied. The model of the medium has been explained and an initial closed system of equations has been obtained in order to study the linear and nonlinear dynamics of internal gravity waves (IGWs) when they interact with the geomagnetic field in a dissipative ionosphere (for the D, E, and F regions).

Modeling of the ionospheric Alfvén resonator in dipolar geometry

A new model for the propagation of ultralow‐frequency (ULF) waves in dipolar geometry has been developed. This model features a full height‐resolved ionosphere including finite Pedersen, Hall, and parallel conductivities. By using a nonorthogonal coordinate system, this model is capable of calculating the ground magnetic field produced by ULF waves and comparing these fields to those measured in the ionosphere and magnetosphere. This model has been used to investigate the properties of the ionospheric Alfvén resonator (IAR) in a dipolar magnetosphere. Although the IAR mode frequencies are not strongly affected by the finite magnetic zenith angle, the damping of these waves is enhanced by the presence of the height‐resolved ionosphere. Pedersen conductivity shields higher frequency ULF waves such as Pc1 from penetrating through the ionosphere, limiting the magnitude of the ground magnetic field, whereas Hall conductivity and finite azimuthal wave number enhance the coupling to the ground. Results for runs in which a wave packet is introduced into the model show that the ground magnetic field is enhanced when the central frequency of the wave packet matches a resonant frequency of the IAR. Including a more realistic height‐resolved ionosphere yields a more direct calculation of ionospheric fields, allowing a comparison between ground and ionospheric fields.

Correcting ionospheric effects and monitoring two-dimensional displacement fields with multiple-aperture InSAR technology with application to the Yushu earthquake

Differential synthetic aperture radar interferometry (D-InSAR) can only measure one-dimensional surface displacements along the line-of-sight (LOS) direction which greatly inhibits its development and application. In this paper, we introduce a novel approach to measuring two-dimensional (2-D) surface displacements by exploiting a single InSAR pair, which is called multi-aperture InSAR (MAI) technology. We study the effects of baseline errors and the ionosphere on MAI technology and develop a directional filter and interpolator to minimize the ionospheric effects. A PALSAR image pair covering the 2010 Yushu earthquake is used to estimate the 2-D displacement fields of the earthquake using the MAI approach. The experimental results show that MAI is superior to conventional Offset-Tracking and therefore has great potential in co-seismic displacement measurement and source parameter inversion.

Modeling the dynamics of a high specific energy plasma bulge in the upper atmosphere: 1. Physics of the processes and numerical simulation of the early stage of plasma bulge expansion and interaction with the ionosphere and geomagnetic field: A review

The physicomathematical features of the plasma explosion region dynamics at altitudes higher than 100 km at the early stage of its development, which culminates in the transfer of a considerable portion of the initial plasma energy into the environment, are discussed in the first part of the work. The radiation gas dynamic, inertial, and shock stages of the disturbed region development are calculated.

Progress in polar upper atmospheric physics research in China

The Chinese Antarctic Great Wall, Zhongshan, Kunlun and Arctic Yellow River stations have unique geographical locations, well suited to carry out polar upper atmospheric observations. This paper reviews the tremendous history of nearly 30 years of Chinese polar expeditions and major progress in polar upper atmospheric physics research. This includes the polar upper atmospheric physics conjugate observation system at Zhongshan Station in the Antarctic and Yellow River Station in the Arctic, and original research achievements in polar ionospheric fields, aurora and particle precipitation, the polar current system, polar plasma convection, geomagnetic pulsations and space plasma waves, inter-hemispheric comparisons of the space environment, space weather in polar regions, power spectrum of the incoherent scatter radar, ionospheric heating experiments and polar mesospheric summer echoes, polar ionosphere-magnetosphere numerical simulation and others. Finally, prospects for Chinese polar upper atmospheric physics research are outlined.

Extremely low frequency plasma turbulence recorded by the DEMETER satellite in the ionosphere over the Abruzzi region prior to the April 6, 2009, L’Aquila earthquake

The present study analyzes the low frequency fluctuations of the magnetic and electric fields prior to the earthquake with magnitude M.6.3 on April 6, 2009, in the Abruzzi region in Italy. This includes the disturbances of the ionospheric electromagnetic field prior to the earthquake, starting two weeks before the event, with detailed analysis for the days of March 26, April 1, April 2, and April 4, as 11, 5, 4 and 2 days prior to the earthquake, when the effects observed were strongest. Special attention is given to the characteristics of the spectra of these variations and the search for nonlinear effects. This analysis is possible in the time intervals when the extremely low frequency waveform was transmitted. Although the mechanism of the energy transmission from the earthquake preparation zone to the ionosphere is not determined, it can be said that the ionospheric plasma is an unstable medium and that even a small perturbation can lead to variations in the electromagnetic field and plasma parameters. Some aspects of this are discussed in the present report. The observations by the DEMETER satellite that are discussed suggest the presence of plasma turbulence over an area above the Abruzzi earthquake that occurred on April 6, 2009. The characteristics of the observed disturbances are studied through wavelet and bispectral analysis, the tools that are most relevant to the analysis of plasma turbulence. Furthermore, some statistical features are also presented, including the probability distribution function, and the kurtosis and skewness recently introduced into studies of such turbulence.

Investigating magnetospheric interaction effects on Titan’s ionosphere with the Cassini orbiter Ion Neutral Mass Spectrometer, Langmuir Probe and magnetometer observations during targeted flybys

In the ∼6years since the Cassini spacecraft went into orbit around Saturn in 2004, roughly a dozen Titan flybys have occurred for which the Ion Neutral Mass Spectrometer (INMS) measured that moon’s ionospheric density and composition. For these, and for the majority of the ∼60 close flybys probing to altitudes down to ∼950km, Langmuir Probe electron densities were also obtained. These were all complemented by Cassini magnetometer observations of the magnetic fields affected by the Titan plasma interaction. Titan’s ionosphere was expected to differ from those of other unmagnetized planetary bodies because of significant contributions from particle impact due to its magnetospheric environment. However, previous analyses of these data clearly showed the dominance of the solar photon source, with the possible exception of the nightside. This paper describes the collected ionospheric data obtained in the period between Cassini’s Saturn Orbit Insertion in 2004 and 2009, and examines some of their basic characteristics with the goal of searching for magnetospheric influences. These influences might include effects on the altitude profiles of impact ionization by magnetospheric particles at the Titan orbit location, or by locally produced pickup ions freshly created in Titan’s upper atmosphere. The effects of forces on the ionosphere associated with both the draped and penetrating external magnetic fields might also be discernable. A number of challenges arise in such investigations given both the observed order of magnitude variations in the magnetospheric particle sources and the unsteadiness of the magnetospheric magnetic field and plasma flows at Titan’s (∼20Rs (Saturn Radius)) orbit. Transterminator flow of ionospheric plasma from the dayside may also supply some of the nightside ionosphere, complicating determination of the magnetospheric contribution. Moreover, we are limited by the sparse sampling of the ionosphere during the mission as the Titan interaction also depends on Saturn Local Time as well as possible intrinsic asymmetries and variations of Titan’s neutral atmosphere. We use organizations of the data by key coordinate systems of the plasma interaction with Titan’s ionosphere to help interpret the observations. The present analysis does not find clear characteristics of the magnetosphere’s role in defining Titan’s ionosphere. The observations confirm the presence of an ionosphere produced mainly by sunlight, and an absence of expected ionospheric field signatures in the data. Further investigation of the latter, in particular, may benefit from numerical experiments on the inner boundary conditions of 3D models including the plasma interaction and features such as neutral winds.

Statistical analysis of dayside equatorial ionospheric electric fields and electrojet currents produced by magnetospheric substorms during sawtooth events

Substorms cause significant disturbances in the ionosphere. However, it has not been well understood how the electric field and electrojet in the dayside equatorial ionosphere respond to substorm onset. Previous studies found that the equatorial electric field, after substorm onset, could be eastward or westward. Because the onset of isolated substorms is often related to a northward turning of the interplanetary magnetic field (IMF), the measured total electric field is determined by contributions from both IMF northward turning and substorm onset and is not necessarily the signature of the onset. In order to exclude the effect of IMF northward turning, we analyze the variations of ionospheric electric field and electrojet during storm time substorms when the IMF remains stable. Thus, the ionospheric variations can be identified to be caused solely by substorms. The electric field data are measured by the Jicamarca radar, and the electrojet is derived from magnetometers at Jicamarca and Piura. It is found that substorm onset induces an eastward electric field and electrojet in the dayside equatorial ionosphere when the IMF remains continuously southward across the onset. The equatorial electrojet starts to increase at the onset, reaches a maximum value ∼30 min after the onset, and then decreases to the pre‐onset value ∼60 min after the onset. Westward electric field and counter‐electrojet occur only if the substorm onset is associated with a northward turning of the IMF. It is concluded that the effect of substorm onset on the dayside equatorial ionosphere, without involvement of IMF reorientations, is an enhanced eastward electric field.

On a Correlation between the Ionospheric Electric Field and the Time Derivative of the Magnetic Field

A correlation of the ionospheric electric field and the time derivative of the magnetic field was noticed over thirty years ago and has yet to be explained. Here we report on another set of examples during the superstorm of November 2004. The electric field in the equatorial ionosphere, measured with the Jicamarca incoherent scatter radar, exhibited a 3 mV/m electric field pulse that was not seen in the interplanetary medium. It was, however, accompanied by a correlation with the time derivative of the magnetic field measured at two points in Peru. Our inclination was to assume that the field was inductive. However, the time scale of the pulse was too short for the magnetic field to penetrate the crust of the Earth. This means that the area threaded by∂B/∂twas too small to create the observed electric field by induction. We suggest that the effect was caused by a modulation of the ring current location relative to the Earth due to the electric field. This electric field is required, as the magnetic field lines are considered frozen into the plasma in the magnetosphere. The closer location of the ring current to the Earth in turn increased the magnetic field at the surface.

Magneto-ionospheric effects of active experiments conducted on February 18, 2004

Results of radiophysical and magnetometric observations of dynamical processes in the ionosphere and geomagnetic field that followed launches of four rockets of different power and made from different launching sites are analyzed. It is found that the rocket launches were accompanied by an intensification of wave activity in the ionosphere and geomagnetic field. The medium reactions from various sources (various rockets) were overlapping, which made their identification more difficult. Both ionospheric and geomagnetic disturbances have, most probably, two groups of propagation velocities: about 1.2–1.7 km s−1 and 440–760 m s−1. Slow MHD waves and internal gravity waves corresponded to these velocities.

Relationship between ULF wave mode mix, equatorial electric fields, and ground magnetometer data

[1] The magnetized plasma of near-Earth space supports the shear and fast Alfven, ultralow-frequency (ULF; 1–100 mHz), magnetohydrodynamic wave modes. The fast mode may propagate across the magnetic field, spreading ULF wave energy throughout the magnetosphere, and couple with the shear Alfven mode to form field line resonances (FLRs). The FLR electric field in the magnetosphere may have sufficiently large amplitudes to energize electrons and enhance radiation belt particle diffusion rates. Ozeke et al. (2009) recently described a technique that related the observed north-south ULF magnetic component at the ground, bνg, with the radial electric field component in the equatorial plane of the magnetosphere, eνeq, via the fields in the ionosphere. In this paper we use a fully coupled ULF wave model to determine the ratio eνeq/bνg for a 5 mHz FLR formed at high latitudes. We find that Ozeke et al. (2009) underestimated the ULF wave magnetic field on the ground which varies with ionosphere Hall conductance. This difference is found to be caused by assuming a decoupled wave mode model for the ionosphere fields. Any relationship that involves ULF wavefields in the ionosphere must include the effects of ULF wave mode mixing.