near-Earth Plasma Research Articles

Perturbations of ionosphere-magnetosphere coupling by powerful VLF emissions from ground-based transmitters

The characteristics of the plasma-wave disturbances stimulated in the near-Earth plasma by powerful VLF radiation from ground-based transmitters are investigated. Radio communication VLF transmitters of about 1 MW in power are shown to produce artificial plasma-wave channels (density ducts) in the near-Earth space that originate in the lower ionosphere above the disturbing emission source and extend through the entire ionosphere and magnetosphere of the Earth along the magnetic field lines. Measurements with the onboard equipment of the DEMETER satellite have revealed that under the action of emission from the NWC transmitter, which is one of the most powerful VLF radio transmitters, the generation of quasi-electrostatic (plasma) waves is observed on most of the satellite trajectory along the disturbed magnetic flux tube. This may probably be indicative of stimulated emission of a magnetospheric maser.

Dispersion relation for ballooning modes and condition of their stability in the near-earth plasma

The ballooning disturbances in a finite-pressure plasma in a curvilinear magnetic field are described by a system of coupled equations for Alfven and slow magnetosonic modes. The local dispersion relation obtained in a WKB approximation is the simplest and most evident method that can be used to characterize the properties of these disturbances. This dispersion relation is widely used to predict the possible instabilities and spectral properties of LF oscillations in the nightside magnetosphere. The formal derivation of the dispersion relation from the initial system of coupled MHD modes and the transition to different limiting cases have been traced. The behavior of dispersion curves in different oscillation branches and the possible development of instabilities and formation of regions where waves cannot propagate have been studied in detail. This made it possible to specify the results of previous works and even indicate the incorrectness in some works. In particular, it has been indicated that a fast Alfven branch of oscillations is always stable and an aperiodic instability can originate on a slow magnetosonic oscillation branch.

The Near-Earth Plasma Environment

An overview of the plasma environment near the earth is provided. We describe how the near-earth plasma is formed, including photo-ionization from solar photons and impact ionization at high latitudes from energetic particles. We review the fundamental characteristics of the earth’s plasma environment, with emphasis on the ionosphere and its interactions with the extended neutral atmosphere. Important processes that control ionospheric physics at low, middle, and high latitudes are discussed. The general dynamics and morphology of the ionized gas at mid- and low-latitudes are described including electrodynamic contributions from wind-driven dynamos, tides, and planetary-scale waves. The unique properties of the near-earth plasma and its associated currents at high latitudes are shown to depend on precipitating auroral charged particles and strong electric fields which map earthward from the magnetosphere. The upper atmosphere is shown to have profound effects on the transfer of energy and momentum between the high-latitude plasma and the neutral constituents. The article concludes with a discussion of how the near-earth plasma responds to magnetic storms associated with solar disturbances.

Dust-acoustic soliton in a plasma with multi-species ions and kappa-distributed electrons

In this work, we consider the formation of electrostatic, dust-acoustic solitary structure in a unmagnetized plasma with Lorentzian electrons (kappa-distributed) and more than one species of thermal ions (Maxwellian). The work is inspired by results of different space-based observations of electrostatic solitary waves (ESW) in the near-earth and magnetospheric plasmas and recent experimental realization of existence of superthermal electron component in various space plasmas. We have, in this work, shown that existence of compressive potential structure is possible only with more than one species of thermal ions. Besides, formation of compressive double layers is also possible which depends on the amount of deviation of the electron thermal velocities from a Maxwellian distribution. We show that both dust-temperature and super-thermal electrons lead to a decrease in the soliton amplitude.

Observations of kinetic Alfvén waves by THEMIS near a substorm onset

Low frequency electromagnetic fluctuations in the vicinity of a magnetospheric substorm onset were investigated using simultaneous observations by THEMIS multiple probes in the near-Earth plasma sheet in the magnetotail. The observations indicate that in the vicinity of a substorm onset, kinetic Alfvén waves can be excited in the high-β plasma sheet (β=2µ0 nT/B 2, the ratio of plasma thermal pressure to magnetic pressure) within the near-Earth magnetotail. The kinetic Alfvén wave has a small spatial scale in the high-β plasma. The parallel electric field accompanying kinetic Alfvén waves accelerates the charged particles along the magnetic field. The kinetic Alfvén waves play an important role in the substorm trigger process, and possibly in the formation of a substorm current wedge.

Определение критической частоты F-слоя по данным наблюдений ионосферного альфвеновского резонанса

 The parameters of the ionospheric Alfven (IAR) and Schumann (SR) resonances are determined by the intensity of their source (global thunderstorm activity) and properties of the propagation medium (the ionosphere and magnetosphere of the Earth). Our observations and results of other authors show that the SR depends primarily on the global parameters of the source and propagation medium. Otherwise the IAR features are mainly determined by local characteristics of the near-earth plasma. The latter circumstance was the basis for the authors to analyze the possibility of diagnostics of the local ionosphere using the IAR data. ELF recordings, obtained at the Ukrainian Antarctic Station (UAS) and low-frequency observatory IRA NAS Ukraine (LFO) were used for the analysis. The presence of long-term data sets obtained at the points with a spatial spacing of about 15 thousand kilometers, allowed us to study the basic seasonal and diurnal behavior of the eigenfrequencies and “observability”of IAR, as well as to investigate the dependence of these parameters from the site position. IAR characteristics were compared with synchronous data of the ionospheric sounding and the total electron content (TEC) over the observation points. A joint analysis of data showed that plasma parameters at the height of the ionosphere F-region have a major impact on the variation of eigenfrequencies of IAR. The article describes a phenomenological model and proposes the technique of estimation of the magnitude of the critical frequency of F layer - F2f0 based on the calculation of frequency interval between the IAR peaks -Δf. The reliability of this technique is confirmed comparing the reconstructed values F2f0 with the vertical sounding data obtained at UAS.

Formation of strong stationary vortex turbulence in the terrestrial magnetosheath

The microscopic consequences of the presence of nonlinear vortex structures in the near-Earth plasma dispersive medium are studied in this work. In dispersive media, strongly localized vortex structures contain trapped particles, cause pronounced density fluctuations, and intensify transfer processes, mixing in a medium; i.e., they can form strong vortex turbulence. Turbulence is represented as a gas in the ensemble of strongly localized (therefore, weakly interacting) identical vortices composing the ground state. Vortices with different amplitudes are randomly located in space (since they interact with one another) and are described statistically. It is assumed that the steady turbulent state is formed through a balance of mutually competing effects: spontaneous generation of vortices due to nonlinear steepening of the disturbance front, ^noise transfer to small scales, and collisional or collisionless damping of disturbances in the HF region. Noise scaling in the inertial interval takes place since structures merge during their collision. A magnetized plasma medium in the magnetosheath is considered. A new type of turbulent fluctuation spectra with respect to wavenumbers k −8/3, which is in satisfactory agreement with satellite observations in space plasma, has been determined. The medium particle diffusion on an ensemble of vortices has also been studied. It has been established that the interaction between structures themselves and between structures and medium particles causes anomalous diffusion in the medium. The effective diffusion coefficient square roothly depends on the noise stationary level.

A statistical study of plasma sheet electrons carrying auroral upward field-aligned currents measured by Time History of Events and Macroscale Interactions during Substorms (THEMIS)

[1] We have statistically investigated the electron density ne,M and temperature Te,M in the near-Earth plasma sheet in terms of the magnetosphere-ionosphere coupling process, as measured by the electrostatic analyzer (ESA) on board the Time History of Events and Macroscale Interactions during Substorms (THEMIS-D) satellite from November 2007 to January 2010. To find out when and where an aurora can occur, either with or without electron acceleration, the thermal current j∥th and the conductivity K along the magnetic field line were also estimated from observations of the magnetospheric electrons with pitch angle information inside 12 RE. The thermal current, j∥th(∝ ne,M Te,M1/2), represents the upper limit of the field-aligned current that can be carried by magnetospheric electrons without a field-aligned potential difference. The conductivity, K(∝ ne,M Te,M−1/2), relates the upward field-aligned current, j∥, to the field-aligned potential difference, V∥, assuming adiabatic electron transport. The thermal current is estimated by two methods: (1) from the relation by using ne,M and Te,M and (2) from the total downward electron number flux. We find that in the dawnside inner magnetosphere, the thermal currents estimated by both methods are sufficient to carry typical region 2 upward field-aligned current. On the other hand, in the duskside outer magnetosphere, a field-aligned potential difference is necessary on the region 1 current since the estimated thermal current is smaller than the typical region 1 current. By using the relationship, j∥ = KV∥, where K is the conductivity estimated from Knight's relation and j∥ is the typical auroral current, we conclude that a field-aligned potential difference of V∥ = 2–5 kV is necessary on the duskside region 1 upward field-aligned current.

Brightening of onset arc precedes the dipolarization onset: THEMIS observations of two events on 1 March 2008

Abstract. We present a new M-I coupling model of substorm during southward IMF based on the THEMIS observations of two events on 1 March 2008. The first event (E-1) was classified as a pseudo-breakup: brightening of the onset arc preceded the first dipolarization onset by ∼71 ± 3 s, but the breakup arcs faded within ∼5 min without substantial poleward expansion and the dipolarization stopped and reversed to thinning. The second event (E-2) was identified as a substorm: brightening of the second onset arc preceded the second dipolarization onset by ∼80 ± 3 s, leading to a full-scale expanding auroral bulge during the substorm expansion phase for ∼20 min. The Alfvén travel time from the ionosphere to the dipolarization onset region is estimated at ∼69.3 s in E-1; at ∼80.3 s in E-2, which matched well with the observed time delay of the dipolarization onset after the brightening of the onset arc, respectively in E-1 and E-2. Brightening of the onset arc precedes the dipolarization onset suggest that the onset arc brightening is caused by the intense upward field-aligned currents originating from the divergence of the Cowling electrojet in the ionosphere. The Cowling electrojet current loop (CECL) is formed to close the field-aligned currents at all times. The closure current in the Alfvén wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfvén wavefront incident on the near-Earth plasma sheet to disrupt the cross-tail current in the dipolarization region. Slow MHD waves dominate the disruption of the cross-tail current in the dipolarization region.

Geospheric effects of the solar flare of December 13, 2006

Disturbances in the magnetic field and magnetospheric and ionospheric plasma registered on December 14–16, 2006, during a strong magnetic storm caused by a solar flare of 4B/X3.4 class are studied. It is shown that in the north of Yakutia, interactions between the Earth’s magnetosphere and the region of high dynamic pressure of the solar wind led to the formation of sporadic layers in the ionospheric E and F regions, depletion of the critical frequency of the F2 layer, and total absorption. At the end of the magnetic storm’s main phase, anomalously high values of foF2 exceeding the quiet level by a factor of 1.5–1.7 were detected. It was found that the disturbances detected by ground-based observatories had developed on the background of changes in the temperature, density, and the pitch-angle distribution of particles at the geostationary orbit manifesting radial shifts of magnetospheric structures (magnetopause, cusp/cleft, and plasma sheet) relative to the observation points. A conclusion is drawn that in this case, changes in the near-Earth plasma and magnetic field manifest the dynamics of the physical conditions at the magnetospheric boundary and diurnal rotation of the Earth.

Model calculations of the total electron content for the system of GPS satellites

The numerical global self-consistent model of the Earth’s thermosphere, ionosphere and protonosphere (GSM TIP), which makes it possible to calculate all the main parameters of the near-Earth plasma, is used to calculate the total electron content (TEC). Calculations have been performed along the radiosignal propagation trajectory between a surface receiving point and a GPS satellite. The TEC value calculated from the satellite data have been compared with such a “true model” TEC value for magnetically quiet conditions of the spring equinox and moderate solar activity. The relative errors in determining the satellite data-based TEC for two European (Troms have been calculated. It has been indicated that an increase in the number of satellites not always results in an increase in accuracy of the TEC value measured on satellites.

Relations between multiple auroral streamers, pre-onset thin arc formation, and substorm auroral onset

[1] Recent ground-based imager observations have provided evidence of precursor auroral activity leading to substorm auroral onset, where the precursor is initiated by a poleward boundary intensification (PBI) followed by an auroral streamer moving equatorward toward the onset latitude leading to substorm auroral onset. However, since many streamers do not lead to substorms, the question arises as to what conditions are required for streamers to lead to onset. Using 382 events detected by the THEMIS all-sky imagers during 2007–2009, we examined the properties of latitudinally thin, quiet arcs that eventually break up during the substorm auroral onset and the relationship of such quiet arcs to streamers. We found that a pre-existing latitudinally thin quiet arc that leads to auroral onset is much brighter than prior thin arcs that do not lead to onset, and that streamers that do not lead to onset form or intensify such quiet arcs. The newly formed or intensified quiet arc remains bright for a few to tens of minutes (∼20 min on average) until a subsequent streamer leads to substorm auroral onset along the pre-existing arc. The pre-onset sequence proposed here suggests that both types of streamers, which do and do not lead to substorms, enhance auroral luminosity near the equatorward boundary of the oval, and that a sufficiently intense, quiet time thin arc near the poleward edge of proton precipitation, likely corresponding to a large plasma pressure gradient in the near-Earth plasma sheet, reflects important pre-conditions for a precursor flow burst to trigger substorm auroral onset.

Reduced magnetic helicity behavior in different plasma regions of near-Earth space

We present a comparative study of the scaling laws of a signed measure calculated from the reduced magnetic helicity from Cluster observations in three different near-Earth plasma regions: solar wi ...

Near-Earth plasma sheet azimuthal pressure gradient and associated auroral development soon before substorm onset

[1] The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the premidnight region of the near-Earth plasma sheet during the substorm growth phase owing to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FAC variations associated with the substorm growth phase and may set up a plasma sheet precondition for the substorm onset being triggered near this region. We take advantage of two of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the R ∼10–12 RE region. Equatorial plasma pressure is estimated by removing the curvature force effect. Five events with the spacecraft footprints mapped very close to the aurora onset region were selected. These events show substantial duskward pressure gradient enhancement 1–5 min before onset. The onset arc, which results from enhanced energetic electron precipitation, was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs. Since the energy and energy flux of precipitating electrons reflect the upward FAC strength, these results indicate that the duskward azimuthal pressure gradient enhancement is associated with enhanced upward FAC during the late growth phase and leads to the intensification of the onset auroral arc soon before it breaks up. It is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc.

Multi-instrument observations of soft electron precipitation and its association with magnetospheric flows

[1] We present a multi-instrument study on the variations of optical auroras and ionospheric electron densities during an interval of a series of fast earthward flows in the magnetotail on 3 March 2009. The flow-related auroral signatures include intermittent higher-latitude (>68° magnetic latitude) intensifications manifested in green and blue line auroras and more latitudinally extended red line auroral intensifications and expansions. During the same interval the Poker Flat incoherent scatter radar (PFISR) detected F region ionospheric electron density enhancements which, together with the red line auroral intensifications, give evidence for soft electron (<1 keV) precipitation associated with fast magnetospheric flow activity. We demonstrated the southward motion of ionospheric electron density patches in correspondence to individual earthward flow bursts and auroral activations. By virtue of the multibeam technique of PFISR we construct the altitudinal profile of the density patches and estimate that the characteristic energies of the precipitating electrons were on order of a few hundred eV, comparable to the observed electron temperature in the near-Earth central plasma sheet (CPS). We propose that the fast flows give rise to enhanced ELF wave activity, which causes strong pitch angle diffusion of the soft electron population in the CPS via wave-particle interactions. The precipitation may be further aided with a moderate field-aligned potential drop comparable to or smaller than the CPS electron temperature. When the flows penetrate into the inner plasma sheet, the adiabatic drift motion of soft electrons may lead to a decreasing trend of electron energy with decreasing radial distance, which is manifested in PFISR observations as an ascending trend of the altitude range of the density patches toward the equatorward auroral border.

Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft

[1] Fast azimuthal auroral expansion and poleward expansion are characteristic features of the expansion phase of substorms. In the first study of its kind, we have investigated the azimuthal auroral expansion and its magnetospheric counterpart using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers and multiple spacecraft. During the tail season in 2008–2009, we found 16 events of azimuthally expanding aurora that passed near the magnetic footprints of the multiple spacecraft operating in the near-Earth plasma sheet. In the magnetosphere, these events commonly showed fast azimuthal and earthward flows associated with intense electric fields and magnetic dipolarization. The speed of the propagating structure, which was estimated from the time difference of the depolarization observed by the multiple spacecraft, was close to the measured azimuthal plasma flow velocity. We also found that this azimuthal plasma transport was dominated by the E × B drift speed associated with the enhanced electric field. In a statistical analysis, the averaged speeds of the leading edge of the westward and eastward auroral expansion were 8.8 and 5.3 km/s, respectively. When mapped onto the equatorial magnetosphere, these speeds (267 and 162 km/s) were comparable to the averaged azimuthal plasma (E × B) flow speeds observed by the spacecraft, which were 193 (239) km/s in the westward direction and 112 (139) km/s in the eastward direction. Our events showed that E × B flows and auroral expansion predominantly propagated westward, indicating an effect of westward background convection in the Harang flow shear. From these results, we concluded that the azimuthal auroral expansion was closely related to magnetic dipolarization which expanded azimuthally at the E × B drift speed. On the basis of the abrupt formation of the fast E × B flows and their propagation away from the onset location, we suggest that the effects of the intense large-scale electric fields, which are possibly generated through substorm onset turbulence, propagate to the ionosphere along the magnetic field lines and lead to azimuthal expansion of an auroral arc.

RCM-E simulation of ion acceleration during an idealized plasma sheet bubble injection

[1] In this paper, we investigate the role of plasma sheet bubbles in the ion flux variations at geosynchronous orbit during substorm injections by using the Rice Convection Model with an equilibrated magnetic field model (RCM-E). The bubble is initiated in the near-Earth plasma sheet with a localized reduction in entropy parameter PV5/3 following a substorm growth phase. In the expansion phase, characteristic features of substorm injections are reproduced; that is, there is a prominent dispersionless flux increase for energetic protons (>40 keV) and a flux decrease for lower-energy protons near midnight geosynchronous orbit while there is dispersive flux enhancement near the dusk sector. We find that the injection boundary is well coincident with the earthward boundary of the bubble, inside which the depletion of plasma content causes the magnetic field dipolarization, and in return, the magnetic field collapse energizes particles and alters the drift paths dramatically. Our results also show that a high-PV5/3 island is pushed ahead of the fast earthward propagating bubble, and a dipolarization front forms between them. Within the high-PV5/3 island, the diamagnetic effect makes the plasma pressure increase and the strength of the magnetic field decrease to a local minimum. We suggest that plasma sheet bubbles are elementary vehicles of substorm time particle injections from the main plasma sheet to the inner magnetosphere.

Effects of superthermal ring current ion tails on the electromagnetic ion cyclotron instability in multi-ion magnetospheric plasmas

[1] An important plasma source for the storm-enhanced ring current is the plasma sheet. Ion species in the near-Earth plasma sheet have been observed to have power law tails on their velocity distributions, which can be well fitted with kappa distributions under a variety of geomagnetic conditions. Motivated by these ideas, we investigate the electromagnetic ion cyclotron (EMIC) instability driven by hot ring current ions having velocity distributions that exhibit thermal anisotropy and power law tails of varying degrees of hardness (smallness of power index) for parameters consistent with the inner magnetosphere. With few exceptions, the presence of hard power law tails on the velocity distributions of the ring current ion species is observed to significantly enhance instability growth rates relative to a bi-Maxwellian ring current model. For a ring current composed of only hot protons, all EMIC branches are unstable, with the helium branch exhibiting the fastest growth rate for the thermal anisotropies considered. The addition of equal number densities of helium and oxygen ions to the ring current plasma has a dramatic stabilizing effect on the proton and helium branches. In this case it is frequently only the oxygen branch of the EMIC wave dispersion relation which is unstable. The detailed balance between cyclotron damping produced by one species and growth produced by another elevates the importance of the velocity distribution spectral index so that it can serve as a “switch” to turn on instability of certain branches.

Plasma sheet disruption by interchange-generated flow intrusions

[1] A three-dimensional particle-in-cell simulation is used to examine the interaction of interchange-generated flow intrusions with the near-Earth plasma sheet. The interchange heads narrow in cross-tail extent as they propagate earthward and dawnward, and the maximum Bz in the head increases to be comparable to the lobe field strength. The interaction of the flow heads with the near-Earth plasma sheet leads to a strong disruption involving localized reconnection on east-west scales of a few ion inertia lengths. This mechanism may explain several key signatures of substorm onset associated with pre-breakup auroral streamers and the disruption of the near-Earth plasma sheet.

Some new possibilities for diagnosing the magnetosphere based on whistler characteristics

Different types of natural electromagnetic emissions are generated in the Earth’s electronic radiation belts. The conditions for generation of these emissions depend on the plasma parameters, geometry of the system, wave transfer processes, and regularities of particle accumulation and precipitation from the magnetic trap. Effective interaction between waves and particles can often be described by the plasma magnetospheric maser theory. A plasma magnetospheric maser actually operates in several main regimes. These regimes are responsible for the generation of VLF emissions with different frequency spectrum dynamics. The regimes replace each other as a result of variations in the local and global characteristics of the magnetosphere. For example, the cyclotron generation dynamics largely depends on the the source power of energetic particles. Several new methods for diagnosing the near-Earth plasma can be implemented if the plasma magnetospheric maser theory is known.