Ionospheric Boundary Research Articles

Solar wind‐ and EUV‐dependent models for the shapes of the Martian plasma boundaries based on Mars Express measurements

AbstractThe long operational life (2003‐present) of Mars Express (MEX) has allowed the spacecraft to make plasma measurements in the Martian environment over a wide range of upstream conditions. We have analyzed ∼7000 MEX orbits, covering three orders of magnitude in solar wind dynamic pressure, with data from the on board Analyzer of Space Plasmas and Energetic Particles (ASPERA‐3) package, mapping the locations where MEX crosses the main plasma boundaries, induced magnetosphere boundary (IMB), ionosphere boundary (IB), and bow shock (BS). A coincidence scheme was employed, where data from the Ion Mass Analyzer (IMA) and the Electron Spectrometer (ELS) had to agree for a positive boundary identification, which resulted in crossings from 1083 orbit segments that were used to create dynamic two‐parameter (solar wind density, nsw, and velocity vsw) dependent global dynamic models for the IMB, IB, and BS. The modeled response is found to be individual to each boundary. The IMB scales mainly dependent on solar wind dynamic pressure and EUV intensity. The BS location closely follows the location of the IMB at the subsolar point, though under extremely low nsw and vsw the BS assumes a more oblique shape. The IB closely follows the IMB on the dayside and changes its nightside morphology with different trends for nsw and vsw. We also investigate the influence of extreme ultraviolet (EUV) radiation on the IMB and BS, finding that increased EUV intensity expands both boundaries.

Effects of the major sudden stratospheric warming event of 2009 on the subionospheric very low frequency/low frequency radio signals

AbstractThis paper presents effects of the major sudden stratospheric warming (SSW) event of 2009 on the subionospheric very low frequency/low frequency (VLF/LF) radio signals propagating in the Earth‐ionosphere waveguide. Signal amplitudes from four transmitters received by VLF/LF radio networks of Germany and Japan corresponding to the major SSW event are investigated for possible anomalies and atmospheric influence on the high‐ to middle‐latitude ionosphere. Significant anomalous increase or decrease of nighttime and daytime amplitudes of VLF/LF signals by ∼3–5 dB during the SSW event have been found for all propagation paths associated with stratospheric temperature rise at 10 hPa level. Increase or decrease in VLF/LF amplitudes during daytime and nighttime is actually due to the modification of the lower ionospheric boundary conditions in terms of electron density and electron‐neutral collision frequency profiles and associated modal interference effects between the different propagating waveguide modes during the SSW period. TIMED/SABER mission data are also used to investigate the upper mesospheric conditions over the VLF/LF propagation path during the same time period. We observe a decrease in neutral temperature and an increase in pressure at the height of 75–80 km around the peak time of the event. VLF/LF anomalies are correlated and in phase with the stratospheric temperature and mesospheric pressure variation, while minimum of mesospheric cooling shows a 2–3 day delay with maximum VLF/LF anomalies. Simulations of VLF/LF diurnal variation are performed using the well‐known Long Wave Propagating Capability (LWPC) code within the Earth‐ionosphere waveguide to explain the VLF/LF anomalies qualitatively.

Propagation of a whistler wave incident from above on the lower nighttime ionosphere

Abstract. The problems of reflection and transmission of a whistler wave incident in the nighttime ionosphere from above are considered. Numerical solution of the wave equations for a typical condition of the lower ionosphere is found. The solution area comprises both the region of strong wave refraction and a sharp boundary of the nighttime ionosphere (∼ 100 km). The energy reflection coefficient and horizontal wave magnetic field on the ground surface are calculated. The results obtained are important for analysis of the extremely low-frequency and very low-frequency (ELF–VLF) emission phenomena observed from both the satellites and the ground-based observatories.

Major pathways to electron distribution function formation in regions of diffuse aurora

AbstractThis paper discusses the major pathways of electron distribution function formation in the region of diffuse aurora. The diffuse aurora accounts for about of 75% of the auroral energy precipitating into the upper atmosphere, and its origin has been the subject of much discussion. We show that an earthward stream of precipitating electrons initially injected from the Earth's plasma sheet via wave‐particle interactions degrades in the atmosphere toward lower energies and produces secondary electrons via impact ionization of the neutral atmosphere. These electrons of magnetospheric origin are then reflected back into the magnetosphere along closed dipolar magnetic field lines, leading to a series of reflections and consequent magnetospheric interactions that greatly augment the initially precipitating flux at the upper ionospheric boundary (700–800 km). To date this, systematic magnetosphere‐ionosphere coupling element has not been included in auroral research models, and, as we demonstrate in this article, has a dramatic effect (200–300%) on the formation of the precipitating fluxes that result in the diffuse aurora. It is shown that wave‐particle interaction processes that drive precipitating fluxes in the region of diffuse aurora from the magnetospheric altitudes are only the first step in the formation of electron precipitation at ionospheric altitudes, and they cannot be separated from the atmospheric “collisional machine” that redistributes and transfers their energy inside the magnetosphere‐ionosphere‐atmosphere coupling system.

Investigation of the radiation properties of magnetospheric ELF waves induced by modulated ionospheric heating

Electromagnetic extremely low frequency (ELF) waves play an important role in modulating the Earth’s radiation belt electron dynamics. High-frequency (HF) modulated heating of the ionosphere acts as a viable means to generate artificial ELF waves. The artificial ELF waves can reside in two different plasma regions in geo-space by propagating in the ionosphere and penetrating into the magnetosphere. As a consequence, the entire trajectory of ELF wave propagation should be considered to carefully analyze the wave radiation properties resulting from modulated ionospheric heating. We adopt a model of full wave solution to evaluate the Poynting vector of the ELF radiation field in the ionosphere, which can reflect the propagation characteristics of the radiated ELF waves along the background magnetic field and provide the initial condition of waves for ray tracing in the magnetosphere. The results indicate that the induced ELF wave energy forms a collimated beam and the center of the ELF radiation shifts obviously with respect to the ambient magnetic field with the radiation power inversely proportional to the wave frequency. The intensity of ELF wave radiation also shows a weak correlation with the size of the radiation source or its geographical location. Furthermore, the combination of ELF propagation in the ionosphere and magnetosphere is proposed on basis of the characteristics of the ELF radiation field from the upper ionospheric boundary and ray tracing simulations are implemented to reasonably calculate magnetospheric ray paths of ELF waves induced by modulated ionospheric heating.

Is diffuse aurora driven from above or below?

AbstractIn the diffuse aurora, magnetospheric electrons, initially precipitated from the inner plasma sheet via wave‐particle interaction processes, degrade in the atmosphere toward lower energies, and produce secondary electrons via impact ionization of the neutral atmosphere. These initially precipitating electrons of magnetospheric origin can also be additionally reflected back into the magnetosphere, leading to a series of multiple reflections by the two magnetically conjugate atmospheres that can greatly impact the initially precipitating flux at the upper ionospheric boundary (700–800 km). The resultant population of secondary and primary electrons cascades toward lower energies and escape back to the magnetosphere. Escaping upward electrons traveling from the ionosphere can be trapped in the magnetosphere, as they travel inside the loss cone, via Coulomb collisions with the cold plasma, or by interactions with various plasma waves. Even though this scenario is intuitively transparent, this magnetosphere‐ionosphere coupling element is not considered in any of the existing diffuse aurora research. Nevertheless, as we demonstrate in this letter, this process has the potential to dramatically affect the formation of electron precipitated fluxes in the regions of diffuse auroras.

Ion gyroradius effects on particle trapping in kinetic Alfvén waves along auroral field lines

AbstractIn this study, a 2‐D self‐consistent hybrid gyrofluid‐kinetic electron model is used to investigate Alfvén wave propagation along dipolar magnetic field lines for a range of ion to electron temperature ratios. The focus of the investigation is on understanding the role of these effects on electron trapping in kinetic Alfvén waves sourced in the plasma sheet and the role of this trapping in contributing to the overall electron energization at the ionosphere. This work also builds on our previous effort by considering a similar system in the limit of fixed initial parallel current, rather than fixed initial perpendicular electric field. It is found that the effects of particle trapping are strongest in the cold ion limit and the kinetic Alfvén wave is able to carry trapped electrons a large distance along the field line yielding a relatively large net energization of the trapped electron population as the phase speed of the wave is increased. However, as the ion temperature is increased, the ability of the kinetic Alfvén wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular to the ambient magnetic field which reduces the amplitude of the wave. This reduction of wave amplitude in turn reduces both the parallel current and the extent of the high‐energy tails evident in the energized electron populations at the ionospheric boundary (which may serve to explain the limited extent of the broadband electron energization seen in observations). Even in the cold ion limit, trapping effects in kinetic Alfvén waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrons to keV levels coincides with the arrival of the wave at the ionospheric boundary.

Energization of outer radiation belt electrons during storm recovery phase

AbstractWe use test particle simulations incorporating an MHD model of ULF wave propagation in the magnetosphere with realistic ionosphere boundary conditions to study electron energization in the dayside outer Van Allen radiation belt, referenced to in situ particle and wave observations. On 7 January 2011 the THEMIS spacecraft detected 3 and 4–5 mHz waves simultaneous with flux enhancement of >10 keV electrons during the early recovery phase of a moderate geomagnetic storm. We find that internal energization of equatorially mirroring electrons via nonresonant ULF wave‐particle interactions can explain these observations. The wave poloidal components cause radial drift of electrons, increasing (decreasing) their kinetic energy as they move inward (outward). Electrons with initial kinetic energies of a few keV can be energized to double these values within an hour by interaction with the 3 mHz waves. The energization rate is somewhat less for the 4–5 mHz waves. An increase in the ionospheric conductance decreases the power of the fast mode wave, reducing radial drift velocities and hence decreasing the rate of energization. The fast mode poloidal field varies with radial distance and longitude, and this also affects energization. Electrons which drift outward encounter a region where the toroidal field due to the field line resonance becomes dominant and produces strong azimuthal drift. These electrons become trapped in an L‐shell range just outward of the resonance region and are not energized.

Latitudinal amplitude-phase structure of MHD waves: STARE radar and image magnetometer observations and modeling

We have developed a numerical model that yields a steady-state distribution of field components of MHD wave in an inhomogeneous plasma box simulating the realistic magnetosphere. The problem of adequate boundary condition at the ionosphere–magnetosphere interface for coupled MHD mode is considered. To justify the model’s assumptions, we have derived the explicit inequality showing when the ionospheric inductive Hall effect can be neglected upon the consideration of Alfven wave reflection from the ionospheric boundaries. The model predicts a feature of the ULF spatial amplitude/phase distribution that has not been noticed by the field line resonance theory: the existence of a region with opposite phase delays on the source side of the resonance. This theoretical prediction is supported by the amplitude-phase latitudinal structures of Pc5 waves observed by STARE radar and IMAGE magnetometers. A gradual decrease in azimuthal wave number m at smaller L-shells was observed at longitudinally separated radar beams.

Широтная амплитудно-фазовая структура МГД-волн: моделирование наблюдения на радаре STARE и магнитометрах сети IMAGE

We have developed a numerical model that yields a steady-state distribution of field components of MHD wave in an inhomogeneous plasma box simulating the realistic magnetosphere. The problem of adequate boundary condition at the ionosphere–magnetosphere interface for coupled MHD mode is considered. To justify the model’s assumptions, we have derived the explicit inequality showing when the ionospheric inductive Hall effect can be neglected upon the consideration of Alfven wave reflection from the ionospheric boundaries. The model predicts a feature of the ULF spatial amplitude/phase distribution that has not been noticed by the field line resonance theory: the existence of a region with opposite phase delays on the source side of the resonance. This theoretical prediction is supported by the amplitude-phase latitudinal structures of Pc5 waves observed by STARE radar and IMAGE magnetometers. A gradual decrease in azimuthal wave number m at smaller L-shells was observed at longitudinally separated radar beams.

Comparison of DMSP and SECS region-1 and region-2 ionospheric current boundary

The region-1 and region-2 boundary has traditionally been identified using data from a single spacecraft crossing the auroral region and measuring the large scale changes in the cross track magnetic field. With data from the AUTUMN, CANMOS, CARISMA, GIMA, DTU MGS, MACCS, McMAC, STEP, THEMIS, and USGS ground magnetometer arrays we applied a state-of-art technique based on spherical elementary current system (SECS) method developed by Amm and Viljanen (1999) in order to calculate maps of region-1 and region-2 current system over the North American and Greenland auroral region. Spherical elementary current (SEC) amplitude (proxy for vertical currents) maps can be inferred at 10s temporal resolution, ~1.5° geographic latitude (Glat), and 3.5° geographic longitude (Glon) spatial resolution. We compare the location of the region-1 and region-2 boundary obtained by the DMSP spacecraft with the region-1 and region-2 boundary observed in the SEC current amplitudes. We find that the boundaries typically agree within 0.2°±1.3°. These results indicate that the location of the region-1 and region-2 boundary can reasonably be determined from ground magnetometer data. The SECS maps represent a value-added product from the magnetometer database and can be used for contextual interpretation in conjunction with other missions as well as help with our understanding of magnetosphere-ionosphere coupling mechanisms using the ground arrays and the magnetospheric spacecraft data.

Seasonal and solar cycle variations in the ionospheric convection reversal boundary location inferred from monthly SuperDARN data sets

Abstract. Multi-year (1995–2013) velocity data collected by the Super Dual Auroral Network (SuperDARN) HF radars are considered to investigate seasonal and solar cycle variations of the convection reversal boundary (CRB) location for interplanetary magnetic field (IMF) Bz < 0. By considering monthly data sets we show that the CRB is at higher latitudes in summer between 1995 and 2007. The poleward shifts are on the order of 2–5°. After 2007, the seasonal effect weakens, and the highest latitudes for the CRB start to occur during the winter time. We show that the CRB latitudes decrease with an increase of the IMF transverse component at a rate of (1–2°)/2 nT. Because of this effect, on average, the CRB latitudes are lower during high solar activity periods with stronger IMFs. We also confirm the effect of the CRB dawn-dusk shifts related to the IMF changes in the IMF By sign.

Electron distribution function formation in regions of diffuse aurora

AbstractThe precipitation of high‐energy magnetospheric electrons (E ∼ 600 eV–10 KeV) in the diffuse aurora contributes significant energy flux into the Earth's ionosphere. To fully understand the formation of this flux at the upper ionospheric boundary, ∼700–800 km, it is important to consider the coupled ionosphere‐magnetosphere system. In the diffuse aurora, precipitating electrons initially injected from the plasma sheet via wave‐particle interaction processes degrade in the atmosphere toward lower energies and produce secondary electrons via impact ionization of the neutral atmosphere. These precipitating electrons can be additionally reflected upward from the two conjugate ionospheres, leading to a series of multiple reflections through the magnetosphere. These reflections greatly influence the initially precipitating flux at the upper ionospheric boundary (700–800 km) and the resultant population of secondary electrons and electrons cascading toward lower energies. In this paper, we present the solution of the Boltzman‐Landau kinetic equation that uniformly describes the entire electron distribution function in the diffuse aurora, including the affiliated production of secondary electrons (E < 600 eV) and its energy interplay in the magnetosphere and two conjugated ionospheres. This solution takes into account, for the first time, the formation of the electron distribution function in the diffuse auroral region, beginning with the primary injection of plasma sheet electrons via both electrostatic electron cyclotron harmonic waves and whistler mode chorus waves to the loss cone, and including their subsequent multiple atmospheric reflections in the two magnetically conjugated ionospheres. It is demonstrated that magnetosphere‐ionosphere coupling is key in forming the electron distribution function in the diffuse auroral region.

Self-consistent electrostatic simulations of reforming double layers in the downward current region of the aurora

Abstract. The plasma on a magnetic field line in the downward current region of the aurora is simulated using a Vlasov model. It is found that an electric field parallel to the magnetic fields is supported by a double layer moving toward higher altitude. The double layer accelerates electrons upward, and these electrons give rise to plasma waves and electron phase-space holes through beam–plasma interaction. The double layer is disrupted when reaching altitudes of 1–2 Earth radii where the Langmuir condition no longer can be satisfied due to the diminishing density of electrons coming up from the ionosphere. During the disruption the potential drop is in part carried by the electron holes. The disruption creates favourable conditions for double layer formation near the ionosphere and double layers form anew in that region. The process repeats itself with a period of approximately 1 min. This period is determined by how far the double layer can reach before being disrupted: a higher disruption altitude corresponds to a longer repetition period. The disruption altitude is, in turn, found to increase with ionospheric density and to decrease with total voltage. The current displays oscillations around a mean value. The period of the oscillations is the same as the recurrence period of the double layer formations. The oscillation amplitude increases with increasing voltage, whereas the mean value of the current is independent of voltage in the 100 to 800 V range covered by our simulations. Instead, the mean value of the current is determined by the electron density at the ionospheric boundary.

Effect of the ionosphere on the interaction between ULF waves and radiation belt electrons

AbstractThe diffusion and energization of electrons in the equatorial plane of Earth's magnetosphere by ULF waves under different ionosphere boundary conditions are examined. Using test‐particle simulations and considering intervals of weak geomagnetic activity, we find that the highest energization and minimum diffusion rates correspond to nightside ionosphere conditions. Conversely, the highest diffusion rates and minimum energization are seen for a perfectly reflecting ionosphere boundary. The maximum energies gained under dayside conditions, when Hall conductivity is included, are slightly greater than the maximum energy for similar conditions without Hall conductivity. The diffusion rates for dayside ionosphere conditions with only Pedersen conductivity are greater than the diffusion rates when Hall conductivity is included. These findings show that ULF wave‐particle interactions in Earth's magnetosphere depend on the ionosphere conductance.

Effect of the total solar eclipse of 20 March 2015 on the ELF propagation over high‐latitude paths

AbstractPropagation of the artificial electromagnetic waves with frequency of 82 Hz in the Earth‐ionosphere waveguide was observed during the solar eclipse on both partially and totally obscured high‐latitude paths with a length of 450–1200 km. Field excitation was monitored by the reference measurements in the near zone of the transmitter, which are free of the ionospheric influence. It is found that the amplitude of the field at the remote points varied depending on the solar illumination and solar elevation angle. We suppose that this effect was probably caused by the increase in the effective height of the ionospheric D layer, just as it was previously observed in VLF. The obtained results demonstrate the response of the propagating extremely low frequency (ELF, 3–300 Hz) wave to the change in the ionospheric boundary. This effect has been for the first time observed in this frequency range during a total solar eclipse.

Response of the ionospheric convection reversal boundary at high latitudes to changes in the interplanetary magnetic field

AbstractIn this paper we present a systematic study of the location of the convection reversal boundary for southward interplanetary magnetic field by using Defense Meteorological Satellite Program (DMSP) F13 and F15 spacecraft measurements during local summer seasons from 2000 to 2007 for both hemispheres. All the convection reversal boundaries are identified pass bypass by locating the highest‐latitude location where the plasma flow shows a large‐scale two‐cell pattern and reverses direction from sunward to antisunward. The location of the convection reversal boundaries are placed into 10 different categories based on By and the magnitude of southward Bz. Observations suggest that (1) the location of the boundary is well organized by the magnitude of Bz, being at lower latitudes for stronger negative Bz and also organized by the polarity of By, moving toward the dawnside/duskside when By changes from negative to positive in the northern/southern hemisphere; (2) the average latitudinal movement of the boundary associated with By changes is comparable to the average movement of the boundary with Bz changes; (3) an initial reconfiguration of the boundary near local noon is redistributed around the dawnside or duskside dependent on the direction of By; and (4) the boundary has a general spiral shape, which varies depending on Bz and By.

Charge balance and ionospheric potential dynamics in time‐dependent global electric circuit model

AbstractWe have developed a time‐dependent model of global electric circuit (GEC) in spherical coordinates. The model solves time‐dependent charge continuity equation coupled with Poisson's equation. An implicit time stepping is used to avoid a strict dielectric relaxation time step condition, and boundary conditions for Poisson's equation are implemented to allow accurate description of time evolution of the ionospheric potential. The concept of impulse response of GEC is introduced that allows effective representation of complex time dynamics of various physical quantities in the circuit using model results obtained for instantaneous deposition of a point charge. The more complex problems are then reconstructed using convolution and linearity principles. For a point charge instantaneously deposited at a typical thundercloud altitude the impulse response of the charge density shows induction of the same value and polarity charge at the ionospheric boundary, while charge of the same value but opposite sign is moving down logarithmically with time and neutralizes the source point charge on time scale corresponding to the dielectric relaxation time at altitude of the source point charge. The ionospheric potential is modified immediately with input of the source point charge based on free space solution of Poisson's equation. Then the ionospheric potential relaxes. It is shown that during formation of two main charge centers of the thundercloud, typically represented by a current dipole, the ionospheric potential can be determined from the difference of time integrals of two ionospheric potential impulse responses corresponding to charge locations at the opposite ends of the current dipole. For latitude‐ and longitude‐independent conductivity model, the total charge on the Earth is exactly zero at all times. During cloud‐to‐ground lightning discharge, the ionospheric potential changes instantaneously by a value proportional to the charge moment change produced by lightning and then relaxes to zero. For a typical charge moment change of 35Ckm and lightning frequency 10s−1, the ionospheric potential changes by 9.3kV; this value agrees well with the results presented by Rycroft et al. () and Rycroft and Odzimek ().

Toroidal quarter waves in the Earth’s magnetosphere: theoretical studies

An analytic model has been developed for toroidal quarter wave (QW) oscillations in the Earth’s magnetosphere using idealistic and highly asymmetric ionospheric boundary condition. The background magnetic field is dipolar and plasma density distribution is governed by a power law 1/r m where r is the geocentric distance of any point along the field line and m is the density index. The solution thus obtained has been compared with the numerical solutions. Earlier workers had developed the analytic model for trivial 1/r6 (m=6) type of plasma density distribution along the field line for which the period of the fundamental is twice that of corresponding half wave (i.e. toroidal oscillation in the symmetric ionospheric boundary). The present analytic model does reproduce this feature. In addition, it is seen that this ratio decreases for lower values of m. Moreover, for a particular value of m, this ratio shows a decreasing trend with increased harmonic number. The spatial characteristics of QW obtained from present analytic model are in excellent agreement with those computed numerically, thereby validating the model. The departure of frequency computed analytically from that obtained numerically is significant only for the fundamental and this departure reduces sharply with the increased harmonic number. It should be noted that there is no such departure for 1/r6 type of plasma density distribution and spatial structures as well as frequency computed from the present analytic model match perfectly with those computed numerically.

Dayside episodic ion outflow from Martian magnetic cusps and/or magnetosheath boundary motion associated with plasma oscillations

AbstractThe radar sounder on the Mars Express Spacecraft is able to make measurements of electron densities in the Martian ionosphere from both local electron plasma oscillations and remote soundings. A study of thousands of orbits shows that in some cases the electron plasma oscillations disappear and reappear abruptly near the upper boundary of the dayside ionosphere. In some cases, the Analyzer of Space Plasmas and Energetic Atoms (ASPERA‐3) data show clear evidence of upwardly accelerated ionospheric ions, on interconnected magnetic field lines. In other cases, ASPERA‐3 data show that when the plasma oscillations disappear, the spacecraft is in the magnetosheath and when they return, the ionospheric plasma reappears. These intermittent appearances of plasma suggest the multiple crossings of the magnetosheath boundary. The motion of the boundary or plasma clouds and ionospheric streamers (a relatively narrow strip of plasma attached to the ionosphere) can cause these multiple crossings.