Ionospheric Boundary Research Articles

Dynamics of the ionospheric electric currents and auroral luminosity boundaries during strong magnetic storms

The regularities in the southward drift of the ionospheric current centers and luminosity boundaries during strong magnetic storms of November 2003 and 2004 (with Dst ≈ −400 and −470 nT, respectively) are studied based on the global geomagnetic observations and TV measurements of auroras. It has been indicated that the eastward and westward electrojets in the dayside and nightside sectors simultaneously shift equatorward to minimal latitudes of Φmin° ∼53°–55°. It has been obtained that the Φmin° latitude decreases with increasing negative values of Dst, IMF Bz component, and westward electric field strength in the solar wind. The dependence of the electrojet equatorward shift velocity (Vav) on the rate of IMF Bz variations (ΔBz/Δt) has been determined. It is assumed that the electrojet dynamics along the meridian is caused by a change in the structure of the magnetosphere and electric fields in the solar wind and the Earth’s magnetosphere.

Discharge processes, electric field, and electron energy in ISUAL‐recorded gigantic jets

This article reports the first high time resolution measurements of gigantic jets from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment. The velocity of the upward propagating fully developed jet stage of the gigantic jets was ∼107 m s−1, which is similar to that observed for downward sprite streamers. Analysis of spectral ratios for the fully developed jet emissions gives a reduced E field of 400–655 Td and average electron energy of 8.5–12.3 eV. These values are higher than those in the sprites but are similar to those predicted by streamer models, which implies the existence of streamer tips in fully developed jets. The gigantic jets studied here all contained two distinct photometric peaks. The first peak is from the fully developed jet, which steadily propagates from the cloud top (∼20 km) to the lower ionosphere at ∼90 km. We suggest that the second photometric peak, which occurs ∼1 ms after the first peak, is from a current wave or potential wave–enhanced emissions that originate at an altitude of ∼50 km and extend toward the cloud top. We propose that the fully developed jet serves as an extension of the local ionosphere and produces a lowered ionosphere boundary. As the attachment processes remove the charges, the boundary of the local ionosphere moves up. The current in the channel persists and its contact point with the ionosphere moves upward, which produces the upward surging trailing jets. Imager and photometer data indicate that the lightning activity associated with the gigantic jets likely is in‐cloud, and thus the initiation of the gigantic jets is not directly associated with cloud‐to‐ground discharges.

Magnetospheric amplification and emission triggering by ELF/VLF waves injected by the 3.6 MW HAARP ionospheric heater

The HF dipole array of the High Frequency Active Auroral Research Program (HAARP) in Gakona, Alaska, was recently upgraded to 180 elements, facilitating operations at a total radiated power level of 3.6 MW and an effective radiated power of ∼575 MW. In the first experiments at the new power level, the HAARP array is used for magnetospheric wave injection. Modulated heating of auroral electrojet currents in the ionosphere yields radiation in the ELF/VLF frequency range. The HAARP‐generated signals are injected into the magnetosphere, where they propagate in the whistler mode in field‐aligned “ducts,” allowing them to be observed at the conjugate point on a ship‐borne receiver and on autonomous buoy platforms. The observation of the 1‐hop signals is accompanied by the observation of associated 2‐hop components in the northern hemisphere, which have reflected from the ionospheric boundary in the southern hemisphere. The observed signals are accompanied by triggered emissions and exhibit temporal amplification of 15–25 dB/s and bandwidth broadening to ∼50 Hz. Amplification occurs at injected signal frequencies selected in near real time on the basis of observations of natural emission activity, and only certain components of the frequency‐time formats transmitted are amplified. Observations at multiple sites and dispersion analysis show that the signals are injected into the magnetosphere directly above the HF heater. The duration of echo observation and the prevalence of 1‐hop observations are consistent with statistics from 1986 Siple Station experiments. The particle‐trapping wave amplitude near the magnetic equator is estimated in the range 0.1–0.4 pT and gyroresonance with 10 keV–100 keV electrons.

Field line resonant frequencies and ionospheric conductance: Results from a 2‐D MHD model

The magnetised plasma of the near‐Earth space environment supports ultra‐low frequency (ULF; 1–100 mHz), magnetohydrodynamic (MHD) oscillations. For sufficiently large ionospheric conductances, field line resonances (FLRs) form between the northern and southern ionospheres. These conditions are usually met for daytime ionosphere conductance values. The FLRs are normal modes of the system and may be used to remote sense plasma mass densities in the magnetosphere. The oscillations lose energy in the ionosphere whose properties determine the boundary conditions, particularly resonance damping effects. Using a two dimensional (2D) MHD model of the magnetosphere and realistic ionosphere boundary conditions, the variation in resonant frequency with ionosphere conductivity is reported. For typical mid to low latitude summer and winter ionosphere parameters, the FLRs change by less than 5%. This translates to an uncertainty of 7% in plasma mass density.

On the generation mechanism of terminator times in subionospheric VLF/LF propagation and its possible application to seismogenic effects

Abstract. The signals from VLF/LF transmitters are known to propagate in the Earth-ionosphere waveguide, so that the subionospheric propagation characteristics are very sensitive to the condition of the lower ionosphere. We know that there appear the terminator times just around the sunrise and sunset in the diurnal variation of subionospheric VLF/LF signal (amplitude and phase). These terminator times are found to shift significantly just around an earthquake, which enables us to infer the change in the ionosphere during the earthquake. In this paper we try to understand the physical mechanism on the generation of those terminator times for relatively short propagation path (less than 2000 km) by means of wave-hop method. It is found that the lowering of the ionosphere boundary during an earthquake decreases the path length of the sky wave and this alters the interference condition of this wave with the ground wave, which lead to an appearance of terminator times as the destructive interference between the ground and sky waves. Finally, we suggest a possible use of terminator time shifts to investigate the lower ionospheric plasma changes during the earthquakes.

A magnetospheric energy principle for hydromagnetic stability problems

A magnetospheric energy principle is formulated to study hydromagnetic stability of a magnetospheric plasma. The magnetospheric plasma is either in a two‐dimensional or three‐dimensional static equilibrium. It is surrounded by lateral perfectly conducting walls and ideal ionospheres in both cases and also by dawn‐dusk periodic boundaries in the two‐dimensional case. The two‐dimensional case has a translational symmetry and has no unperturbed magnetic field component in the dawn‐dusk direction. Unlike the conventional energy principle for a plasma surrounded by a perfectly conducting wall, field lines are assumed to vertically thread the ionospheric boundary, which is not a perfectly conducting rigid wall. Ideal ionospheric boundary conditions are obtained, so that the force operator becomes self‐adjoint and the magnetospheric energy principle is valid. There are four ideal ionospheric boundary conditions to satisfy these requirements: horizontally free, conducting, free, and rigid. A change in the potential energy becomes equal to the sum of the change in the fluid energy and an ionospheric surface contribution, which is negative and thus destabilizing for horizontally free and free ionospheric boundary conditions. A minimization condition for the change in the potential energy is obtained. When an unperturbed field‐aligned current vanishes, the horizontally free, conducting, free, and rigid boundary conditions allow interchange, incompressible ballooning, incompressible ballooning, and compressible ballooning modes, respectively. Different characteristics of those three pressure‐driven modes are clarified. Existing interchange stability criteria are compared and results of several different numerical stability analyses of ballooning instabilities for different magnetospheric equilibria are discussed systematically in light of the present magnetospheric energy principle.

Energy dissipation via electron energization in standing shear Alfvén waves

A two-dimensional hybrid magnetohydrodynamic-kinetic electron model in dipolar coordinates is used to study the case of a fundamental mode toroidal field line resonance (FLR) centered on an L=10 closed dipolar magnetic field line. The model is initialized via a perturbation of the azimuthal shear Alfvén velocity so that only upward field aligned currents (corresponding to downwelling electrons) are present at the ionospheric boundaries during the first half wave period. It is found that the acceleration of the electrons to carry the field aligned currents can be a significant sink of Alfvén wave energy depending on the width of the flux tube. For a FLR with an equatorial perpendicular wavelength of 0.25 RE about 20% of the wave energy is dissipated over a half cycle. This varies inversely with the width of the flux tube increasing to 40% by a width of 0.15 RE, which, unless the system is driven, can completely damp the resonance in about 2–3cycles.

Ballooning perturbations in the inner magnetosphere of the Earth: Spectrum, stability and eigenmode analysis

We investigate the generation of ballooning perturbations in the inner magnetosphere of the Earth in the dipole model of the geomagnetic field taking into account ionospheric boundary conditions. The ionosphere is considered as a thin layer with finite conductivity. The eigenmode spectrum is discrete and consists of Alfvén, slow magnetosonic, flute and incompressible modes. Their interaction depends on ionospheric conductivity. The decay rate is small in noon and night sectors and large in dawn and dusk sectors. The lowest stability threshold α/ γ ≈ 4.25 is determined by flute modes.

Global aspects of solar wind–ionosphere interactions

Recent observations have quantified the auroral wind O + outflow in response to magnetospheric inputs to the ionosphere, notably Poynting energy flux and precipitating electron density. For moderate to high activity periods, ionospheric O + is observed to become a significant or dominant component of plasma pressure in the inner plasma sheet and ring current regions. Using a global circulation model of magnetospheric fields and its imposed ionospheric boundary conditions, we evaluate the global ionospheric plasma response to local magnetospheric conditions imposed by the simulation and evaluate magnetospheric circulation of solar wind H +, polar wind H +, and auroral wind O +. We launch and track the motions of millions of test particles in the global fields, launched at randomly distributed positions and times. Each particle is launched with a flux weighting and perpendicular and parallel energies randomly selected from defined thermal ranges appropriate to the launch point. One sequence is driven by a two-hour period of southward interplanetary magnetic field for average solar wind intensity. A second is driven by a 2-h period of enhanced solar wind dynamic pressure for average interplanetary field. We find that the simulated ionospheric O + becomes a significant plasma pressure component in the inner plasma sheet and outer ring current region, particularly when the solar wind is intense or its magnetic field is southward directed. We infer that the reported empirical scalings of auroral wind O + outflows are consistent with a substantial pressure contribution to the inner plasma sheet and plasma source surrounding the ring current. This result violates the common assumption that the ionospheric load is entirely confined to the F layer, and shows that the ionosphere is often an important dynamic element throughout the magnetosphere during moderate to large solar wind disturbances.

Recent studies of Schumann resonance and ELF transients

Schumann resonance (SR) is the global electromagnetic phenomenon occurring in the Earth – ionosphere cavity at frequencies of 8, 14, 20 Hz, etc (the ELF band). Phenomenon was named after the author of the pioneer publication on the planetary resonance. Since natural electromagnetic signal originates from lightning strokes distributed worldwide, parameters of oscillations reflect the average condition of ionospheric plasma, spatial distribution and current intensity of thunderstorms. ‘Regular’ lightning strokes occur at a rate of 50 events per second, and their electromagnetic radiation forms the SR background. An irregular or transient component exists caused by relatively rare powerful strokes (the rate is about one event per minute or even less) that provide pulses ten times higher than an ordinary discharge. Three orthogonal field components (vertical electric and two crossed horizontal magnetic fields) are recorded at a Schumann resonance field site, and the power spectra are obtained in the frequency band up to 40 Hz averaged over a few minutes with the typical frequency resolution of 0.1 Hz. The main objective of report is to review the recent results in the Schumann resonance studies. The report consists of two parts; one of them summarizes the experimental data obtained during last 3 – 5 years, while the second describes model studies of ELF radio propagation. We try to mention the most important results in the Schumann resonance. The report surveys the data obtained by using ‘classical’ power spectra acquired at different observatories. Power spectra give information on the properties of the lower ionosphere boundary and on the dynamics of the global thunderstorms. For instance, the sudden changes can be detected in the space weather such as Solar Proton Events (SPE); the regular seasonal motion of thunderstorms is monitored, which is associated with the ground surface temperature; unusual changes might be found in the SR spectra prior to disastrous earthquakes, etc. The cross-spectra of both magnetic fields and magnetic and electric fields (Poynting vector) might be obtained when three field components are recorded. Cross-spectra enhance the coherent fields and reduce the noise thus presenting a more exact information on both the source location and the SR parameters. Elliptical polarization of horizontal magnetic field is measured via the cross-spectra technique, and recent comparison with the model computations proved the detection of Schumann resonance line splitting. Accumulated experimental data present a wide area for resolving the inverse electromagnetic problems directed to establishing an unknown instant distribution of the global lightning activity. Resonance parameters measured worldwide allowed for constructing the effective globally averaged models of the lower ionosphere and the dispersion relation for ELF radio waves. Recent model developments are connected with applications of finite element methods to the resonance problem. Simultaneously, the ‘classical’ spectral field representation was transformed into the time domain providing an effective computational algorithm. Analytical time domain solution allowed generating the quasi–random pulse successions modeling the ELF radio noise. The model time domain ELF signals might be synthesized with predetermined properties. In the conclusion of review, we note a renewal of interest towards the extra-terrestrial Schumann resonance studies. The Huygens probe mission on Titan stimulated interest to distant detection of feasible lightning strokes at the planets and their global location by using the planetary resonance concurrently with studies of ionospheres.

Behavior of the polar lower ionosphere during the solar flares in January 2005 according to the data of the partial reflections method

The results of the observations, using partial reflections, of the lower ionosphere over Tumannyi station in the Murmansk region (69.0° N, 35.7° E) during strong solar flares on January 15–20, 2005, are presented. The structure of the D region of the polar ionosphere and the effects of X-ray flares and fluxes of high energy protons on this region are considered. The anomalous values of electron density in the lower part of the D region, unusually low values of the height of the lower ionospheric boundary, complete or partial absorption of short and medium radiowaves, and bursts of the meter cosmic radio emission were detected during solar disturbances.

Io‐Jupiter interaction: Alfvén wave propagation and ionospheric Alfvén resonator

A linear, one‐dimensional gyrofluid code has been used to determine the characteristics of propagating Alfvén waves and the ionospheric Alfvén resonator on a Jupiter‐Io flux tube. This model includes electron inertia, electron pressure gradient, and finite ion gyroradius effects, as well as the displacement current correction to prevent the Alfvén velocity from exceeding the speed of light. A quasi‐steady Vlasov code provides realistic density profiles along the flux tube as input parameters for the gyrofluid model. In this paper, we demonstrate that the majority of the wave energy from an initial pulse with a long wavelength (∼0.1 RJ) is unable to reach Jupiter's ionosphere without wave breaking, phase mixing, and/or other nonlinear processes; however, a significant energy flux may be transferred via high‐frequency, small‐wavelength waves to the ionosphere. The waves that reach the ionosphere stimulate an ionospheric Alfvén resonator which is generated between the ionospheric boundary and the first velocity peak of the Alfvén phase speed. The ionospheric density and scale height play important roles to determine the resonant frequency. The eigenfrequency decreases with increasing scale height and with increasing ionospheric density. The fundamental frequency and higher harmonics of the Alfvén resonator are comparable to the observed reoccurring frequency of S bursts between a few and hundreds of Hz. On the basis of this information, we suggest the Alfvén resonator as the likely driver explaining multiple occurrences of S bursts.

The dynamics of the auroral oval and ionospheric trough boundaries according to data from the DMSP satellites and ground-based ionosonde network

This paper presents the results of a combined analysis of the experimental data of vertical- and oblique-incidence ionospheric sounding in the northeastern region of Russia and of the DMSP satellite data from 1999 to 2003. During geomagnetic disturbances period on ionograms we can see anomalous diffuse signals, which propagates outside great circle arc. The joint data analysis permits to suggest that the appearance of additional anomalous signals is connected with radio waves refraction in auroral oval and main ionospheric trough areas and with scattering on small-scale field-aligned irregularities close to auroral oval equatorial boundary in the region of electron diffuse precipitation. The basic attention is given to diagnostics of main ionospheric trough boundary and auroral oval zones position during geomagnetic disturbances periods.

Dissipation of Alfvén wave pulses propagating along dipole magnetic tubes with reflections at the ionosphere

A ratio of the maximal and minimal cross sections of the magnetic tube (contraction ratio) is a crucial parameter which affects very strongly on reflections of MHD wave pulses propagating along a narrowing magnetic flux tube. In cases of large contraction ratios of magnetospheric magnetic tubes, the wave energy flux at the ionospheric boundary can be rather small. Therefore the dissipation of the wave perturbations can be very weak for each reflection, in spite of a finite conductivity of the planet’s ionosphere. The dissipation is stronger for the pulses with shorter wave scales. Because of that, Alfvén wave pulses with sufficiently long wave scales have a very small energy loss for each reflection at the conducting ionosphere, and thus, they have many reflections without a noticeable decrease of their amplitude. This effect related to converging magnetic lines is dependent very strongly on the polarization of the Alfvén wave. In case of a dipole magnetic field, the effect is most pronounced for wave pulses characterized by velocity and magnetic perturbations in the meridional plane.

Parallel electric fields in a diverging flux tube with upward current: Nature of quasi‐static fields

Using one‐dimensional (1‐D) particle‐in‐cell (PIC) simulations of a diverging magnetic flux tube, the spatial distribution of parallel electric field in response to an applied potential drop is studied. Hot and cold plasma populations as appropriate for the upward current region are included. Simulations start with an initially empty flux tube representing an auroral density cavity. The cavity refills with plasmas flowing from the ionosphere at the bottom and the magnetosphere at the top. The ionospheric cold ions are significantly accelerated by means of plasma expansion processes in the cavity. When an appropriate warm secondary electron population at the ionospheric boundary is included in the simulations, a significant feature of the expanding ionospheric plasma is a rarefaction shock (RFS) near the bottom of the density cavity. Dynamics of the shock and its role in setting up a low‐altitude double layer (DL) are examined. Even after the shock's decay due to the heating of the ionospheric cold electrons by wave‐particle interactions, electron heating at low altitudes sustains the enhanced role of plasma expansion in upward acceleration of ions. The applied potential drop does localize like that in a strong DL, but only rarely a strong monotonic DL forms; often the DL is highly dynamic and evolves continually via plasma turbulence consisting of both electron and ion timescales. Averaging the fast fluctuations at the electron timescales, the potential distribution across the quasi‐static DL yields oscillatory and spiky parallel fields, which evolve with ion‐acoustic and ion hole turbulence. We compare our results with observations from satellites.

Influence of ionospheric conductivity on the ballooning modes in the inner magnetosphere of the Earth

The generation of ballooning perturbations in the inner magnetosphere of the Earth was investigated in the dipole model of the geomagnetic field taking into account the ionospheric boundary conditions. The ionosphere is considered as a thin layer with finite conductivity. A special attention was paid to the study of the influence of ionospheric conductivity on the stability of ballooning perturbations. It was demonstrated that in the case of insulating ionosphere, flute perturbations are generated and they determine the stability of magnetospheric magnetohydrodynamic modes. An analytical stability criterion was obtained for these perturbations. Conforming to the results of the paper (Hameiri, E., Ballooning modes on open magnetic field lines. Phys. Plasmas 6(3), 674–685, 1999), it also determines the stability boundary of magnetospheric magnetohydrodynamic perturbations in the resistive boundary case. In the case of resistive ionosphere, stable toroidal Alfvén waves are slowly decaying. The spectrum of coupled poloidal Alfvén and slow magnetosonic eigenmodes was also investigated for perfectly conductive ionosphere.

Structure of the magnetic pileup boundary at Mars and Venus

The magnetic pileup boundary (MPB) is a sharp, thin, and well‐defined plasma boundary located between the bow shock and the inner ionospheric boundary at comets, Mars, and Venus. This boundary separates the magnetosheath, a region of low magnetic fields with a conspicuous wave activity, from the magnetic pileup region dominated by strong, highly organized magnetic fields as a result of the pileup and draping of the interplanetary magnetic field. In the present paper we study the magnetic structure of the magnetic pileup boundary at Mars and Venus by means of the technique of minimum variance of the magnetic field. For each one of the crossings analyzed, we obtain a very well defined minimum variance vector. At Mars the direction of this vector agrees with the normal to the MPB fit obtained from Mars Global Surveyor crossings. The results confirm that the MPB is a well‐defined plasma boundary. According to empirical criteria based on minimum variance analysis results, the Martian and Venusian MPB crossings would resemble an MHD tangential discontinuity rather than a rotational discontinuity. However, spacecraft observations suggest that the nature of the MPB could be far more complex. We compare our results with similar studies at the MPB of comets and the magnetic tail boundary of Titan, and we discuss the nature of the boundary from a general perspective.

Balloon observations of temporal and spatial fluctuations in stratospheric conductivity

The first campaign of the Polar Patrol Balloon (PPB) experiment (1st-PPB) was carried out at Syowa Station in Antarctica during 1990–1991 and 1992–1993. Based on the results of the 1st-PPB experiment, the next campaign (2nd-PPB) was carried out in the austral summer of 2002–2003. This paper will present stratospheric conductivity results from the 2nd-PPB experiment. In that experiment, three balloons were launched for the purpose of upper atmosphere physics observation (three balloons). Payloads of these three flights were identical with each other, and were launched as close together in time as allowed by weather conditions to constitute a cluster of balloons during their flights. Such a “Balloon Cluster” is suitable to observe temporal evolution and spatial distribution of phenomena in the ionospheric regions and boundaries that the balloons traversed during their circumpolar trajectory. More than 20 days of simultaneous fair weather 3-axis electric field and stratospheric conductivity data were obtained at geomagnetic latitudes ranging from sub-auroral to the polar cap. Balloon separation varied from ∼60 to >1000 km. This paper will present stratospheric conductivity observations with emphasis on the temporal and spatial variations that were observed.

X-Ray Emission from Comets and Nonmagnetic Planets. Theory and Comparison with CHANDRA Observations

Mass loading of the solar wind by newly created cometary photoions as well as counterstreaming fluxes of protons in the mantle regions at the ionospheric boundaries of nonmagnetic planets Mars, Venus result in the modified two stream instability MTSI and excitation of the intense lower hybrid wave turbulence. Electrons are efficiently energized by these waves in the magnetic field aligned direction up to KeV energies confirmed by in situ observations and interacting with cometary or planetary atmospheres producing X-Ray emission by combination of Bremsstrahlung and line K-shell radiation. Analytical study of the nonlinear evolution of MTSI as well as numerical simulations of instability development in the mantle regions of nonmagnetic planets has been performed and it basically confirms the scenario of electron acceleration. Using the above described mechanism of X-ray emission produced in impacts of energetic electrons with ions and neutrals we employed ADAS – atomic data code to analyze X-ray spectrum emitted by comet Linear and obtained a good fit with recent CHANDRA observations. The theoretical model also predicts strong x-ray emission from nonmagnetic planets such as Mars and Venus.

Auroral particle acceleration by strong double layers: The upward current region

Satellite observations have established that parallel electric fields of both upward and downward current regions of the aurora are supported, at least in part, by strong double layers. The purpose of this article is to examine the role of double layers in auroral electron acceleration using direct measurements of parallel electric fields and the accompanying particle distributions, electrostatic waves, and nonlinear structures; the concentration is on the upward current region. Direct observations of the ionospheric boundary of the auroral cavity suggest that a stationary, oblique double layer carries a substantial, albeit a minority fraction (∼10% to ∼50%) of the auroral potential. An order of magnitude density gradient results in an asymmetric electric field signature. Oblique double layers with amplitudes greater than 100 mV/m have been verified in ∼3% and may occur in up to 11% of auroral cavity crossings, so it is feasible that strong double layers are a principal acceleration mechanism. In this article we also present a second type of double layer that has a symmetric electric field signature and is seen inside of the auroral cavity. These structures are a possible signature of a midcavity or high‐altitude acceleration mechanism. Numerical solutions of the Vlasov‐Poisson equations support the possibility of midcavity double layers and indicate that trapped electrons can play an important role in the double‐layer structure.