Dayside Auroral Zone Research Articles

Time-fractional KdV equation for plasma of two different temperature electrons and stationary ion

Using the time-fractional KdV equation, the nonlinear properties of small but finite amplitude electron-acoustic solitary waves are studied in a homogeneous system of unmagnetized collisionless plasma. This plasma consists of cold electrons fluid, non-thermal hot electrons, and stationary ions. Employing the reductive perturbation technique and the Euler-Lagrange equation, the time-fractional KdV equation is derived and it is solved using variational method. It is found that the time-fractional parameter significantly changes the soliton amplitude of the electron-acoustic solitary waves. The results are compared with the structures of the broadband electrostatic noise observed in the dayside auroral zone.

Effect of the presence of excess superthermal hot electrons on electron-acoustic solitary waves in auroral zone plasma

A theoretical investigation is carried out for the nonlinear properties of small amplitude electron acoustic solitary waves (EAWs) in an unmagnetized collisionless plasma consisting of a cold electron fluid and hot electrons obeying κ velocity distribution, and stationary ions. The Korteweg de Vries (KdV) equation that contains the lowest-order nonlinearity and dispersion is derived from the lowest order of perturbation and a linear inhomogeneous (KdV-type) equation that accounts for the higher-order nonlinearity and dispersion is obtained. A stationary solution for equations resulting from higher-order perturbation theory has been found using the renormalization method. The effects of the spectral index κ and the higher-order corrections are found to significantly change the properties (viz. the amplitude, width, electric field ) of the EASWs. A comparison with the Viking Satellite observations in the dayside auroral zone are also discussed.

Propagation of three-dimensional electron-acoustic solitary waves

Theoretical investigation is carried out for understanding the properties of three-dimensional electron-acoustic waves propagating in magnetized plasma whose constituents are cold magnetized electron fluid, hot electrons obeying nonthermal distribution, and stationary ions. For this purpose, the hydrodynamic equations for the cold magnetized electron fluid, nonthermal electron density distribution, and the Poisson equation are used to derive the corresponding nonlinear evolution equation, Zkharov–Kuznetsov (ZK) equation, in the small- but finite- amplitude regime. The ZK equation is solved analytically and it is found that it supports both solitary and blow-up solutions. It is found that rarefactive electron-acoustic solitary waves strongly depend on the density and temperature ratios of the hot-to-cold electron species as well as the nonthermal electron parameter. Furthermore, there is a critical value for the nonthermal electron parameter, which decides whether the electron-acoustic solitary wave’s amplitude is decreased or increased by changing various plasma parameters. Importantly, the change of the propagation angles leads to miss the balance between the nonlinearity and dispersion; hence, the localized pulses convert to explosive/blow-up pulses. The relevance of this study to the nonlinear electron-acoustic structures in the dayside auroral zone in the light of Viking satellite observations is discussed.

Superposed epoch analysis of the ionospheric convection evolution during substorms: IMF BY dependence

We present superposed epoch analyses of the average ionospheric convection response in the northern and southern hemispheres to magnetospheric substorms occurring under different orientations of the interplanetary magnetic field (IMF). Observations of the ionospheric convection were provided by the Super Dual Auroral Radar Network (SuperDARN) and substorms were identified using the Far Ultraviolet (FUV) instrument on board the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. We find that during the substorm growth phase the expected IMF BY‐dependent dawn‐dusk asymmetry is observed over the entire convection pattern, but that during the expansion phase this asymmetry is retained only in the polar cap and dayside auroral zone. In the nightside auroral zone the convection is reordered according to the local substorm electrodynamics with any remaining dusk‐dawn asymmetry being more closely related to the magnetic local time of substorm onset, itself only weakly governed by IMF BY. Owing to the preponderance of substorms occurring just prior to magnetic midnight, the substorm‐asymmetry tends to be an azimuthal extension of the dusk convection cell across the midnight sector, a manifestation of the so‐called “Harang discontinuity.” This results in the northern (southern) hemisphere nightside auroral convection during substorms generally resembling the expected pattern for negative (positive) IMF BY. When the preexisting convection pattern in the northern (southern) hemisphere is driven by positive (negative) IMF BY, the nightside auroral convection changes markedly over the course of the substorm to establish this same “Harang” configuration.

A brief review of “solar flare effects” on the ionosphere

The study of solar flare effects (SFEs) on the ionosphere is having a renaissance. The development of GPS ground and satellite data for scientific use has opened up new means for high time resolution research on SFEs. At present, without continuous flare photon spectra (X rays, EUV, UV, and visible) monitoring instrumentation, the best way to model flare spectral changes within a flare is through ionospheric GPS studies. Flare EUV photons can increase the total electron content of the subsolar ionosphere by up to 30% in ∼5 min. Energetic particles (ions) of 10 keV to GeV energies are accelerated at the flare site. Electrons with energies up to several MeV are also created. A coronal mass ejection (CME) is launched from the Sun at the time of the flare. Fast interplanetary CMEs (ICMEs) have upstream shocks which accelerate ions to ∼10 keV to ∼10 MeV. Both sources of particles, when magnetically connected to the Earth's magnetosphere, enter the magnetosphere and the high‐latitude and midlatitude ionosphere. Those particles that precipitate into the ionosphere cause rapid increases in the polar atmospheric ionization, disruption of transpolar communication, and cause ozone destruction. Complicating the picture, when the ICME reaches the magnetosphere ∼1 to 4 days later, shock compression of the magnetosphere energizes preexisting 10–100 keV magnetospheric electrons and ions, causing precipitation into the dayside auroral zone (∼60°–65° MLAT) ionospheres. Shock compression can also trigger supersubstorms in the magnetotail with concomitant energetic particle precipitation into the nightside auroral zones. If the interplanetary sheath or ICME magnetic fields are southwardly directed and last for several hours, a geomagnetic storm will result. A magnetic storm is characterized by the formation of an unstable ring current with energetic particles in the range ∼10 keV to ∼500 keV. The ring current decays away by precipitation into the middle latitude ionosphere over timescales of ∼10 h. A schematic of a time line for the above SFE ionospheric effects is provided. Descriptions of where in the ionosphere and in what time sequence is provided in the body of the text. Much of the terminology presently in use describing solar, interplanetary, magnetospheric, and ionospheric SFE‐related phenomena are dated. We suggest physics‐based terms be used in the future.

Modulational instability of electron-acoustic waves: an application to auroral zone plasma

By using the standard reductive perturbation technique, a nonlinear Schrodinger equation is derived to study the modulational instability of finite amplitude electron-acoustic waves in an unmagnetized plasma consisting of cold electron fluid and nonthermal electrons. It is found that the presence of nonthermally distributed electrons modifies the domain of the modulational instability and solitary structures. Possibility of stationary states of the wave packets that can appear as envelope solitons under different conditions is explored. The present investigation is relevant to observation from the Viking satellite in the dayside auroral zone.

Nonlinear electron-acoustic waves with vortex-like electron distribution and electron beam in a strongly magnetized plasma

Abstract The nonlinear properties of solitary waves structure in a strongly magnetized plasma consisting of cold electron fluid, hot electrons obeying trapped/vortex-like distribution, warm electron beam, and stationary ions, are investigated. Applying a reductive perturbation theory (RPT), a nonlinear modified Korteweg–de Vries (mKdV) equation and a linear inhomogeneous mKdV-type equation are derived. The stationary solutions of these coupled equations are obtained using a renormalization method. However, both of the mKdV- and mKdV-type solutions present a positive potential, that corresponds to a hole (hump) in the cold (hot) electron number density. The dependence of the wave amplitude, width and velocity on the system parameters is investigated. The application of our results in interpreting the salient features of the broadband electrostatic noise (BEN) in the dayside auroral zone of the Earth’s magnetosphere is discussed.

Higher-order contribution to obliquely nonlinear electron-acoustic waves with electron beam in a magnetized plasma

Propagation of electron-acoustic waves in a strongly magnetized four-component plasma consisting of cold and hot electrons, a warm electron beam, and stationary ions is investigated. The present model considered weakly dispersive and strongly magnetized plasma in the limit of long wavelengths. The introduction of an electron beam allows the existence of electron-acoustic solitons with velocity related to the beam velocity. With increasing the beam velocity and the beam temperature, both the soliton amplitude and the width increase. Applying a reductive perturbation theory, a nonlinear Zakharov-Kuznetsov (ZK) equation for the first-order perturbed potential and a linear inhomogeneous Zakharov-Kuznetsov (ZK-type) equation for the second-order perturbed potential are derived. Stationary solutions of these coupled equations are obtained using a renormalization method. These solutions admit either compressive or rarefactive soliton type depending on the electron-beam parameters. Moreover, the dependence of the solution on the beam parameters, obliqueness on the magnetic field, and the magnetic field itself is also investigated. The application of the present investigation to the broadband electrostatic noise in the dayside auroral zone of the Earth’s magnetosphere is considered.

Structure of the high-altitude cusp formed by the horizontal IMF

The importance of a sign of the IMF B Z component for solar wind plasma entry into the magnetosphere was suggested about 40 years ago and later it was documented by experimental investigations. When IMF B Z is negative, subsolar reconnection fills the cusp with the magnetosheath plasma which partly precipitate to the ionosphere and partly is reflected and create a plasma mantle. During periods of a positive IMF B Z , the reconnection site moves tailward of the cusp. However, such situations are rather rare because B Z is often a minor IMF component. In spite of this, the structure of the cusp during periods of a small IMF B Z component is generally unknown; partly due to fully 3-D nature of interaction and partly due to a lack of experimental data. In the present paper, we are analyzing two-point observations of the cusp–magnetosheath boundary during a prolonged interval of dawnward pointing IMF. These observations are supported by an analysis of the precipitation patterns observed by the several spacecraft in different altitudes of the dayside auroral zone. An analysis shows that when the IMF B Y component is large enough the reconnection sites in both hemispheres move away from the local noon but in an opposite sense. Each of these sites is able to supply the precipitation in both hemispheres but the amount and parameters of precipitating particles are controlled by a magnetosheath speed above reconnection sites.

Substorm related changes in precipitation in the dayside auroral zone – a multi instrument case study

Abstract. A period (08:10–14:40 MLT, 11 February 1997) of enhanced electron density in the D- and E-regions is investigated using EISCAT, IRIS and other complementary instruments. The precipitation is determined to be due to substorm processes occurring close to magnetic midnight. Energetic electrons drift eastward after substorm injection and precipitate in the morning sector. The precipitation is triggered by small pulses in the solar wind pressure, which drive wave particle interactions. The characteristic energy of precipitation is inferred from drift timing on different L-shells and apparently verified by EISCAT measurements. The IMF influence on the precipitation in the auroral zone is also briefly discussed. A large change in the precipitation spectrum is attributed to increased numbers of ions and much reduced electron fluxes. These are detected by a close passing DMSP satellite. The possibility that these ions are from the low latitude boundary layer (LLBL) is discussed with reference to structured narrow band Pc1 waves observed by a search coil magnetometer, co-located with IRIS. The intensity of the waves grows with increased distance equatorward of the cusp position (determined by both satellite and HF radar), contrary to expectations if the precipitation is linked to the LLBL. It is suggested that the ion precipitation is, instead, due to the recovery phase of a small geomagnetic storm, following on from very active conditions. The movement of absorption in the later stages of the event is compared with observations of the ionospheric convection velocities. A good agreement is found to exist in this time interval suggesting that E × B drift has become the dominant drift mechanism over the gradient-curvature drift separation of the moving absorption patches observed at the beginning of the morning precipitation event.Key words. Ionosphere (auroral ionosphere; particle precipitation) Magnetospheric physics (storms and substorms)

Electron‐acoustic solitary waves via vortex electron distribution

We consider the nonlinear propagation of electron‐acoustic waves in a plasma composed of a cold electron fluid, hot electrons obeying a trapped/vortex‐like distribution, and stationary ions. The properties of small but finite amplitude electron‐acoustic solitary waves (EASWs) are studied by employing the reductive perturbation technique, whereas those of arbitrary amplitude EASWs are investigated by means of the pseudopotential approach. It is found that the present plasma model supports EASWs having a positive potential, which corresponds to a hole (hump) in the cold (hot) electron number density. It is also shown that as the trapped electron temperature increases, the amplitude of the EASWs increases but their width decreases. The application of our results in interpreting the salient features of the broadband electrostatic noise in the dayside auroral zone is discussed.

Transport of Ionospheric up‐Flowing O+ ION along the Field Line at Various Longitude to the Magnetosphere

AbstractAccording to ion's distribution function found from the dynamic equation, the static flux distribution of the up‐flowing O+ ion originating from various ionospheric regions along the field line at various longitude and the ion transport features are studied. The results show that: (1) The ions originating from the equatorial side of the dayside auroral zone and lower latitude region mainly transport into dayside magnetospheric region. The ions originating from the polar gap and polar side of the dayside auroral zone mainly transport into the magnetomantle and lob. The ions originating from equatorial side of the nightside auroral zone and lower latitude region mainly transport into the plasmasheet and the closed field line region. (2) The ionospheric up‐flowing ions mainly distribute in near geo‐space and ion's flux has a negative gradient versus geo‐center distance. (3) If Kp is higher, the probability of the up‐flowing ions to entry the magnetosphere will be bigger and the region where the up‐flowing ions are dominant will be bigger. Our theoretical results can interpret some features of the geopause, and the theoretical flux of the ion in the magnetotail is consistent with the observation.

Auroral zone dayside precipitation during magnetic storm initial phases

Significant charged-particle precipitation occurs in the dayside auroral zone during and after interplanetary shock impingements on the Earth's magnetosphere. The precipitation intensities and spatial and temporal evolution are discussed. Although the post-shock energy flux (10– 20 erg cm −2 s −1 ) is lower than that of substorms, the total energy deposition rate may be considerably greater (∼ an order of magnitude) than nightside energy rates due to the greater area of the dayside portion of the auroral oval (defined as extending from 03 MLT through noon to 21 MLT). This dayside precipitation represents direct solar wind energy input into the magnetosphere/ionosphere system. The exact mechanisms for particle energization and precipitation into the ionosphere are not known at this time. Different mechanisms are probably occurring during different portions of the storm initial phase. Immediately after shock compression of the magnetosphere, possible precipitation-related mechanisms are: (1) betatron compression of preexisting outer zone magnetospheric particles. The anisotropic plasma is unstable to loss-cone instabilities, leading to plasma wave growth, resonant particle pitch-angle scattering and electron and proton losses into the upper ionosphere. (2) The compression of the magnetosphere can also lead to enhanced field-aligned currents and the formation of dayside double-layers. Finally (3) in the latter stages of the storm initial phase, there is evidence for a long-lasting viscous-like interaction occurring on the flanks of the magnetopause. Ground-based observations identifying the types of dayside auroral forms would be extremely useful in identifying the specific solar wind energy transfer mechanisms.

Generation of electron-acoustic waves in the magnetosphere

Generation of electron-acoustic waves (EAWs) is examined in an unmagnetized, four-component plasma consisting of stationary cold and hot Maxwellian electrons, drifting electron beam and ions. The addition of hot background electrons to the previously existing three-component model reduces the excited EAW frequencies and growth rates. Electrostatic linear dispersion equation is solved analytically for the growth rate and real frequency. The linear theory of EAWs is applied to the dayside auroral zone and other regions of the magnetosphere. The mechanism can generate frequencies over a wide range of 5 Hz – 6 kHz , 3 Hz – 4 kHz and 5 Hz – 6 kHz in the dayside auroral zone, plasma sheet boundary layer and polar cusp, respectively. The e-folding time of the instability is of the order of a few milliseconds, and the waves are expected to grow nonlinearly to large amplitudes. Our four-component model can explain the broadband electrostatic noise (BEN) observations below the total electron plasma frequencies in these regions fairly well.

<i>Letter to the Editor:</i> First complementary observations by ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar

Abstract. Experimental results are presented from ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar. Tomographic measurements on 10 October 1996, showing a narrow, field-aligned enhancement in electron density in the post-noon sector of the dayside auroral zone, are related to a temporal increase in the plasma concentration observed by the incoherent scatter radar in the region where the HF radar indicated a low velocity sunwards convection. The results demonstrate the complementary nature of these three instruments for polar-cap ionospheric studies.Key words. Ionosphere · Auroral ionosphere · Polar ionosphere · Radio science (ionospheric physics)

The dayside auroral zone as a hard target for coherent HF radars

Observations from the CUTLASS Finland coherent HF radar on 23 February 1996 are employed to demonstrate that changes in propagation mode from ½F to 1½F and back again, determined from elevation angle measurements, do not significantly alter the ranges over which ionospheric backscatter is observed. This indicates that the latitudinal extent of backscatter in the dayside auroral oval and cusp region correspond to the boundaries of geophysical processes, as opposed to limits in the illumination of the F region ionosphere by the radar. Hence, the HF radar technique is confirmed as an excellent diagnostic of the cusp and other dayside regions.

Modeling of microburst electron precipitation using pitch angle diffusion theory

Microburst electron precipitation is characterized by short bursty (duration ∼0.2–0.3 s) quasiperiodic precipitation of electrons in the dayside auroral zone. Pitch angle diffusion of electrons due to the interaction with whistler waves has been suggested previously as a possible mechanism for scattering electrons into the loss cone to cause microburst precipitation. In this paper we investigate the viability of the above mechanism through modeling. We assume the scattering to occur near the equatorial plane and solve the pitch angle diffusion equation numerically to find the time‐dependent form of the electron distribution function F, which results from a given time‐dependent diffusion coefficient D. Different aspects of our results (burst size, pitch angle dependence, risetime, burst width) are compared with observations from a recent rocket experiment on microburst launched from Poker Flat, Alaska. The comparison shows very good agreement, further supporting the idea that the pitch angle diffusion process is the driving mechanism for microbursts.

Akebono Observations of the Polar Wind and Suprathermal Auroral Ions: An Overview.

We present an overview of direct ion and electron observations of the polar wind and suprathermal auroral ions from the plasma instruments on Akebono. In the polar wind, the observed H+ ion reaches a velocity of 1 km/s at 2000 km altitude, as does He+ ion near 3000 km and O+ ion near 6000 km. At high altitudes, the O+ ion constitutes a significant component of the polar wind flow contrary to most polar wind model predictions. At 2000-4000 km, the H+ ion velocity in the polar wind is strongly correlated with the ambient electron temperature. The electron temperature (averaged thermal energy) in the upward magnetic field direction is a factor of 1.5-2 higher than that in the downward and field-perpendicular directions. This results in an upward heat flux which is comparable in magnitude to the heat flux carried by energetic (>10-eV) photoelectrons. The observed electron temperature anisotropy and correlation with ion velocity are consistent with the polar wind being driven by a large ambipolar electric field resulting from the presence of escaping atmospheric photoelectrons (required to maintain quasi-neutrality along the field line). At auroral latitudes, significant fluxes of “minor ions” are frequently observed, particularly atomic N+ and O++ ions and molecular NO+, N2+, and O2+ ions. The occurrence of molecular ions at high altitude (>3000 km) during extended periods of auroral activity points to their fast energization at the F-region and topside ionosphere. In the 3000-6000 km altitude region, transverse ion energization occurs frequently both on the dayside and the nightside. Different ion species are energized in the perpendicular direction to tens and at times hundreds of eV over a narrow spatial extent, and appear as “ion conics” at higher altitudes as they spiral up the field lines. In the 3000-9000 km altitude region, the average energy of the observed ion conics in the dayside auroral zone increases with altitude.

Turbulence generated by a gas of electron acoustic solitons

The characteristics of the electrostatic turbulence generated by a gas of electron acoustic solitons are investigated. Electron acoustic solitons are shown to propagate in magnetized plasmas up to about 30° off the parallel direction without significant changes in their properties relative to the nonmagnetized case. Using the conservation properties of the Korteweg‐deVries equation, the velocity distribution function of the soliton gas is calculated in the small‐amplitude limit. For low cold to hot electron density ratios and high‐mean‐square electric fields, a significant number of solitons with high velocities and amplitudes is produced, implying the generation of an intense broadband electrostatic turbulence. These theoretical results are compared with the properties of the broadband electrostatic noise (BEN) emissions observed by the Viking satellite in the dayside auroral zone. The electric spectra generated by the electron acoustic soliton gases which can be derived from Viking plasma and wave observations are calculated numerically. These spectra are shown to agree with experimental data and in particular to explain the high‐frequency part of BEN emissions, which lies in a range forbidden for linear electrostatic waves.

Ionospheric signatures of dayside magnetopause transients: A case study using satellite and ground measurements

A case study is presented of coordinated ground and space measurements featuring a set of transient, auroral fragments located on the poleward side of a stable cusp/cleft arc. Optical ground data from Ny Alesund (Svalbard) and Heiss Island (Franz Josef Land) were combined with DMSP F9 satellite measurements to examine the characteristics of these auroral features. A stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the IMF. Poleward of the red arc were several, westward moving auroral jets having characteristics similar to midday auroral breakup events. Such events may be ground signatures of transitory magnetic merging at the dayside magnetopause. If so, the driven convective motion of these structures should contribute to the polar cap potential. Within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential.