A Mechanism for Electrostatic Solitary Waves Observed in Ganymede’s Magnetopause

A mechanism for electrostatic solitary waves observed in the reconnection jet region of the Earth’s magnetotail

The formation of compressive (hump) and rarefactive (dip) ion acoustic solitons is studied in magnetized O+- H+- e and O+- H−- e plasmas. The hydrodynamics equations are described for cold heavy (oxygen) ions, warm light (hydrogen) ions and isothermal Boltzmann distributed electrons along with Poisson equations in the presence of a magnetic field. The reductive perturbation method is used to derive the nonlinear Zakharov–Kuznetsov (ZK) equation for an ion acoustic wave in magnetized two-ion component plasma. It is found that two modes of ion acoustic waves with fast and slow speeds can propagate in the linear limit in such a plasma. It is noticed that, in the case of positively charged light hydrogen ions O+- H+- e plasmas, the slow ion acoustic wave solitons formed both potential hump as well as dip structures, while fast ion acoustic wave solitons give only hump structures. However in the case of negatively charged light hydrogen ions O+- H−- e plasmas, the slow ion acoustic wave solitons formed potential hump structures while fast ion acoustic wave solitons produce dip structures. The variations in the amplitude and width of the nonlinear slow and fast ion acoustic wave structures with density, temperature of light ions and magnetic field intensity are obtained in magnetized two-ion component plasmas. The magnetic field has its effect only on the width of the nonlinear ion acoustic wave structures in two-ion component plasmas.

Ion- and electron-acoustic solitons and double layers in multi-component space plasmas

A study of large amplitude ion-acoustic solitons is conducted for a model composed of cool and hot ions and cool and hot electrons. Using the Sagdeev pseudo-potential formalism, the scope of earlier studies is extended to consider why upper Mach number limitations arise for slow and fast ion-acoustic solitons. Treating all plasma constituents as adiabatic fluids, slow ion-acoustic solitons are limited in the order of increasing cool ion concentrations by the number densities of the cool, and then the hot ions becoming complex valued, followed by positive and then negative potential double layer regions. Only positive potentials are found for fast ion-acoustic solitons which are limited only by the hot ion number density having to remain real valued. The effect of neglecting as opposed to including inertial effects of the hot electrons is found to induce only minor quantitative changes in the existence regions of slow and fast ion-acoustic solitons.

Weak double layers (WDLs) and coherent electrostatic waves in the range of frequencies above the proton plasma frequency, $\mathrm{f}_{{\mathrm {pi}}}$, and smaller than or of the order of the electron plasma frequency, $\mathrm{f}_{{\mathrm {pe}}}$, have been observed in the solar wind at 1 AU. A soliton model, which treats the solar wind plasma as a fluid of hot protons and hot $\alpha$ particles streaming with respect to protons, and suprathermal electrons having $\kappa$-distribution, is found to sustain slow and fast ion-acoustic solitons and double layers. The slow ion-acoustic mode is a new mode that occurs due to the presence of alpha particles. This mode can support both positive and negative solitons and double layers. The slow ion-acoustic mode can exist even when the relative streaming, U 0 , between alphas and protons is zero, provided alpha temperature, $\mathrm{T}_{{\mathrm {i}}}$, is not exactly equal to 4 times the proton temperature, $\mathrm{T}_{{\mathrm {p}}}$. An increase of the $\kappa$-index leads to an increase in the critical Mach number, maximum Mach number and the maximum amplitude of both slow and fast ion-acoustic solitons. The fast ion-acoustic mode can support only positive potential solitons. The predicted amplitudes and widths of slow ion-acoustic double layers are found to be in an excellent agreement with the observed amplitudes and widths of WDLs. The fast Fourier transform (FFT) of the ion-acoustic solitons/DLs would produce a broadband spectrum with a main peak between 0.35 kHz to 1.6 kHz, and $\mathrm { E } = ( 0.01 - 0.7 ) \mathrm { mV } \mathrm { m } ^ { - 1 }$ which are in excellent agreement with the observed electric fields $\sim ( 0.0054 - 0.54 ) \mathrm { mV } \mathrm { m } ^ { - 1 }$ associated with the low-frequency waves observed in the solar wind at 1 AU. It is proposed that WDLs and low-frequency coherent electrostatic waves, observed by Wind spacecraft in the solar wind at 1 AU [1], might be generated by the slow and fast ion-acoustic solitons and double layers.

Abstract. Large amplitude ion-acoustic and electron-acoustic waves in an unmagnetized multi-component plasma system consisting of cold background electrons and ions, a hot electron beam and a hot ion beam are studied using Sagdeev pseudo-potential technique. Three types of solitary waves, namely, slow ion-acoustic, ion-acoustic and electron-acoustic solitons are found provided the Mach numbers exceed the critical values. The slow ion-acoustic solitons have the smallest critical Mach numbers, whereas the electron-acoustic solitons have the largest critical Mach numbers. For the plasma parameters considered here, both type of ion-acoustic solitons have positive potential whereas the electron-acoustic solitons can have either positive or negative potential depending on the fractional number density of the cold electrons relative to that of the ions (or total electrons) number density. For a fixed Mach number, increases in the beam speeds of either hot electrons or hot ions can lead to reduction in the amplitudes of the ion-and electron-acoustic solitons. However, the presence of hot electron and hot ion beams have no effect on the amplitudes of slow ion-acoustic modes. Possible application of this model to the electrostatic solitary waves (ESWs) observed in the plasma sheet boundary layer is discussed.

We propose that the mechanism for the generation of weak double layers (WDLs) and low-frequency coherent electrostatic waves, observed by Wind in the solar wind at 1 AU, might be slow and fast ion-acoustic solitons and double layers. The solar wind plasma is modelled as a fluid of hot protons and hot $\alpha$ particles streaming with respect to protons, and suprathermal electrons having a $\kappa$ -distribution. The fast ion-acoustic mode is similar to the ion-acoustic mode of a proton–electron plasma and can support only positive-potential solitons. The slow ion-acoustic mode is a new mode that occurs due to the presence of $\alpha$ particles. This mode can support both positive and negative solitons and double layers. The slow ion-acoustic mode can exist even when the relative streaming, $U_{0}$ , between $\alpha$ particles and protons is zero, provided that the $\alpha$ temperature, $T_{i}$ , is not exactly equal to four times the proton temperature, $T_{p}$ . An increase of the $\kappa$ -index leads to an increase in the critical Mach number, maximum Mach number, and the maximum amplitude of both slow and fast ion-acoustic solitons. The slow ion-acoustic double layer can explain the amplitudes and widths, but not the shapes, of the observed WDLs in the solar wind at 1 AU by Wind spacecraft. The Fourier transform of the slow ion-acoustic solitons/double layers would produce broadband low-frequency electrostatic waves having main peaks between 0.35 kHz to 1.6 kHz, with an electric field in the range of $E = (0.01\,\mbox{--}\,0.7)~\mbox{mV}\,\mbox{m}^{-1}$ , in excellent agreement with the observed low-frequency electrostatic wave activity in the solar wind at 1 AU.

Electrostatic solitary waves and double layers are explored in a homogeneous, collisionless, and magnetized three-component plasma composed of hot protons, hot heavier ions (alpha particles, He++), and suprathermal electrons with kappa distribution. The Sagdeev pseudopotential technique is used to study the arbitrary amplitude ion-acoustic solitons and double layers. The effect of various parameters such as the number density of ions, ni0; the spectral index, κ; the Mach numbers, M; and the temperature ratio of ion to the electron σi on the evolution of ion-acoustic solitary waves as well as their existence domains is studied. The transition in the existence domain for slow-ion acoustic solitons from negative solitons/double layers to positive solitons/double layers is found to occur with a variation of the heavier ion temperature. It is observed that the width of the negative potential solitons increases as the amplitude increases, whereas for the positive potential solitons, the width decreases as the amplitude increases. Furthermore, it is found that the limitation on the attainable amplitudes of fast ion-acoustic solitons is attributed to that the number density of protons should remain real valued, while for the slow ion-acoustic solitons, the upper limit is provided by the requirement that the number density of heavier ions should remain real. In the presence of a double layer, the occurrence of the double layer limits the attainable amplitudes of the slow ion-acoustic solitons. The proposed plasma model is relevant to the coherent electrostatic structures observed in the solar wind at 1 AU.

The polarity of ion-acoustic solitons that arise in a plasma with two (same mass, different temperature) ion species and two (different temperature) electron species is investigated. Two different fluid models are compared. The first model treats all species as adiabatic fluids, while the second model treats the ion species as adiabatic, and the electron species as isothermal. Nonlinear structures are analysed via the reductive perturbation analysis and pseudo-potential analysis. Each model supports both slow and fast ion-acoustic solitons, associated with the two (slow and fast) ion-acoustic speeds. The models support both positive and negative polarity solitons associated with the slow ion-acoustic speed. Moreover, results are in good agreement, and both models support positive and negative polarity double layers. For the fast ion-acoustic speed, the first model supports only positive polarity solitons, while the second model supports solitons of both polarity, coexistence of positive and negative polarity solitons, double layers and supersolitons. A novel feature of our analysis is the evaluation of nonlinear structures at critical number densities where polarity changes occur. This analysis shows that solitons that occur at the acoustic speed are neither a necessary nor a sufficient condition for the phenomenon of coexistence. The relationship between the existence regions of supersolitons and soliton polarity is also discussed.

The Kadomtsev–Petviashvili (KP) equation is derived for an electrostatic wave in unmagnetized multi-ion plasmas using the reductive perturbation method. It is found that two ion sound waves with fast and slow speeds can propagate in such a multi-ion plasma provided both ions are considered to be inertial and one of the ion component in two ion species plasma is taken to be warm as well. The potential hump (compressive) and dip (rarefactive) structures of the slow and fast ion acoustic waves are obtained for different combinations of multi-ion plasmas i.e., H+(warm)–O+(cold)–e, H+(warm)–H+(cold)–e, O+(cold)–H-(warm)–e, and H+(cold)–H-(warm)–e. It is found that, in the case of multi-ion H+(warm)–O+(cold)–e and H+(warm)–H+(cold)–e plasmas, the slow ion acoustic wave solitons form both potential hump (compressive) and dip (rarefactive) profiles depending on the temperature and concentration of the warm ion species, while the fast ion acoustic wave solitons only form the potential hump structures. However, in ...

Ion-acoustic solitons in a collisionless plasma with adiabatic positive and negative ions with equal ion temperature and isothermal electrons are studied by using the reductive perturbation method. The basic set of fluid equations is reduced for the fast ion-acoustic wave to the Korteweg–de Vries and modified Korteweg–de Vries equation and for the slow ion-acoustic wave to the Korteweg–de Vries equation. Stationary solutions of these equations are obtained and the effect of ion temperature on fast and slow ion-acoustic solitons is investigated.

Ion-acoustic Solitons in a collisionless two-electron-temperature plasma with adiabatic positive and negative ions with equal ion temperature and isothermal electrons are studied by using the reductive perturbation method. The basic set of fluid equations are reduced for fast ion-acoustic wave to the Korteweg-de Vries and modified Korteweg-de Vries equation and for slow ion-acoustic wave to the Korteweg-de Vries equation and solutions of these equations are obtained. The effect of ionic temperature, negative ions and small concentration of a cooler electron component on the amplitude and width of fast and slow ion-acoustic solitons is investigated.

The first close flyby of Ganymede by Juno on 2021 June 7 provided a unique opportunity to visit the previously unexplored wake and magnetopause regions. During the flyby, the Juno Waves instrument recorded traces of the existence of electrostatic solitary waves (ESWs) in the wake and magnetopause boundary. The flyby detected the presence of H2 + and H3 + ions in Ganymede’s wake for the first time, indicating that the wake is dominated by water products from the surface of Ganymede. The conditions for the occurrence of ESWs and double layers in the wake region and the associated existence domains (in parameter space) are explored in this paper. A magnetized fluid plasma is introduced to model Ganymede’s wake conditions, comprising of warm H+, O+, and H3 + ions and an electron beam, in addition to a non-Maxwellian background of suprathermal electrons. We have explored in particular the role of the third ion component H3 + on the properties of ESWs. Different acoustic modes are predicted and used as basis for our analysis, namely identified as one slow and two fast ion-acoustic modes and a higher-frequency electron-acoustic mode. Nonlinear analysis shows that the model may support the existence of positive and negative potential ion-acoustic solitons and double layers depending on the number density and on the temperature of H3 + ions. The theoretical prediction is correlated with observed ESWs in Ganymede’s wake.

In a series of papers by Maharaj et al., including “Existence domains of slow and fast ion-acoustic solitons in two-ion space plasmas” [Phys. Plasmas 22, 032313 (2015)], incorrect expressions for the Sagdeev potential are presented. In this paper, we provide the correct expression of the Sagdeev potential. The correct expression was used to generate the numerical results for the above-mentioned series of papers, so that all results and conclusions are correct, despite the wrong Sagdeev potential expressions printed in the papers. The correct expression of the Sagdeev potential presented here is a very useful generic expression in the sense that a single expression can be used to study nonlinear structures associated with any acoustic mode, despite the fact that the supersonic and subsonic species would vary if solitons associated with different linear modes are studied.

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