Spontaneous Magnetic Fluctuations and Collisionless Regulation of Turbulence in the Earth’s Magnetotail

Among the fundamental and most challenging problems of laboratory, space, and astrophysical plasma physics is to understand the relaxation processes of nearly collisionless plasmas toward quasi-stationary states and the resultant states of electromagnetic plasma turbulence. Recently, it has been argued that solar wind plasma β and temperature anisotropy observations may be regulated by kinetic instabilities such as the ion cyclotron, mirror, electron cyclotron, and firehose instabilities; and it has been argued that magnetic fluctuation observations are consistent with the predictions of the fluctuation–dissipation theorem, even far below the kinetic instability thresholds. Here, using in situ magnetic field and plasma measurements by the THEMIS satellite mission, we show that such regulation seems to occur also in the Earth’s magnetotail plasma sheet at the ion and electron scales. Regardless of the clear differences between the solar wind and the magnetotail environments, our results indicate that spontaneous fluctuations and their collisionless regulation are fundamental features of space and astrophysical plasmas, thereby suggesting the processes is universal.

Relationship between geomagnetic storm development and the solar wind parameter ß

We have analyzed the dynamics of solar wind and interplanetary magnetic field (IMF) parameters during the development of 933 isolated geomagnetic storms, observed over the period from 1964 to 2010. The analysis was carried out using the epoch superposition method at intervals of 48 hrs before and 168 hrs after the moment of Dst minimum. The geomagnetic storms were selected by the type of storm commencement (sudden or gradual) and by intensity (weak, moderate, and strong). The dynamics of the solar wind and IMF parameters was compared with that of the Dst index, which is an indicator of the development of geomagnetic storms. The largest number of storms in the solar activity cycle is shown to occur in the years of minimum average values (close in magnitude to 1) of the solar wind parameter β (β is the ratio of plasma pressure to magnetic pressure). We have revealed that the dynamics of the Dst index is similar to that of the β parameter. The duration of the storm recovery phase follows the characteristic recovery time of the β parameter. We have found out that during the storm main phase the β parameter is close to 1, which reflects the maximum turbulence of solar wind plasma fluctuations. In the recovery phase, β returns to background values β~2‒3.5. We assume that the solar wind plasma turbulence, characterized by the β parameter, can play a significant role in the development of geomagnetic storms.

Relationship between geomagnetic storm development and the solar wind parameter β

We have analyzed the dynamics of solar wind and interplanetary magnetic field (IMF) parameters during the development of 933 isolated geomagnetic storms, observed over the period from 1964 to 2010. The analysis was carried out using the epoch superposition method at intervals of 48 hrs before and 168 hrs after the moment of Dst minimum. The geomagnetic storms were selected by the type of storm commencement (sudden or gradual) and by intensity (weak, moderate, and strong). The dynamics of the solar wind and IMF parameters was compared with that of the Dst index, which is an indicator of the development of geomagnetic storms. The largest number of storms in the solar activity cycle is shown to occur in the years of minimum average values (close in magnitude to 1) of the solar wind parameter β (β is the ratio of plasma pressure to magnetic pressure). We have revealed that the dynamics of the Dst index is similar to that of the β parameter. The duration of the storm recovery phase follows the characteristic recovery time of the β parameter. We have found out that during the storm main phase the β parameter is close to 1, which reflects the maximum turbulence of solar wind plasma fluctuations. In the recovery phase, β returns to background values β~2‒3.5. We assume that the solar wind plasma turbulence, characterized by the β parameter, can play a significant role in the development of geomagnetic storms.

Analytical Model Nonequilibrium Inhomogeneous Plasma of the Heliosphere

We prove that a nonequilibrium inhomogeneous giant gas discharge is realized in the heliosphere with huge values of the parameter <i>E</i>/<i>N</i>, which determines the temperature of electrons. This quasi-stationary discharge determines the main parameters of the weak solar wind (SW) in the heliosphere. In connection with the development of space technologies and the human spacewalk, the problem of the nature of the SW is acute. The study of the interference of gravitational and electrical potentials at the Earth's surface began with the work of Hilbert 1600. Such polarization effects – the interference of Coulomb and gravitational forces – have not been studied well enough even in the heliosphere. Our article is devoted to this problem. Pannekoek-Rosseland-Eddington model do not take into account the important role of highly energetic running (away from the Sun) electrons and, accordingly, the duality of electron fluxes. According to an alternative model formulated by we, highly energetic (escaping from the Sun) electrons leave the Sun and the heliosphere, and weakly energetic ones, unable to leave the Coulomb potential well (hole) – the positively charged Sun and the heliosphere, return to the Sun. The weak difference between the opposite currents of highly energetic (escaping from the Sun) electrons and weakly energetic (returning to the Sun) electrons is compensated by the current of positive ions and protons from the Sun – SW. These dynamic processes maintain a quasi-constant effective dynamic charge of the Sun and the entire heliosphere. At the same time, quasi-neutrality in the Sun and heliosphere is well performed up to 10^-36. According to experiments and analytical calculations based on our model: 1) the plasma in the corona is nonequilibrium; 2) the maximum electron temperature is T_e ~ 1-2 million degrees; 3) T_e grows from 1000 km away from the Sun and 4) the role of highly energetic electrons escaping from the plasma leads to a significant increase in the effective: solar charge and electric fields in the heliosphere in relation to the Pannekoek-Rosseland-Eddington model. This is due to the absence of a compensation layer that screens the effective charge of the Sun. It is not formed at all due to the escape of highly energetic electrons (as in a conventional gas discharge) in the entire heliosphere with high temperatures exceeding the temperature of the Sun's surface. Thus, the process of escape of highly energetic electrons forms the internal EMF of the entire heliosphere. Interference of gravitational and Coulomb potentials in the entire heliosphere is considered, it is being manifested in generation of two opposite flows of particles: 1) that are neutral or with a small charge (to the Sun), and 2) in the form of high-energy electrons (escaping from the positively charged Sun) and a solar wind (from the Sun). Calculated values of the registered ion parameters in the solar wind were compared with experimental observations. Reasons for generating the ring current in inhomogeneous heliosphere and inapplicability of the Debye theory in describing processes in the solar wind (plasma with current) are considered.

Analytical Model Nonequilibrium Inhomogeneous Plasma of the Heliosphere

We prove that a nonequilibrium inhomogeneous giant gas discharge is realized in the heliosphere with huge values of the parameter <i>E</i>/<i>N</i>, which determines the temperature of electrons. This quasi-stationary discharge determines the main parameters of the weak solar wind (SW) in the heliosphere. In connection with the development of space technologies and the human spacewalk, the problem of the nature of the SW is acute. The study of the interference of gravitational and electrical potentials at the Earth's surface began with the work of Hilbert 1600. Such polarization effects – the interference of Coulomb and gravitational forces – have not been studied well enough even in the heliosphere. Our article is devoted to this problem. Pannekoek-Rosseland-Eddington model do not take into account the important role of highly energetic running (away from the Sun) electrons and, accordingly, the duality of electron fluxes. According to an alternative model formulated by we, highly energetic (escaping from the Sun) electrons leave the Sun and the heliosphere, and weakly energetic ones, unable to leave the Coulomb potential well (hole) – the positively charged Sun and the heliosphere, return to the Sun. The weak difference between the opposite currents of highly energetic (escaping from the Sun) electrons and weakly energetic (returning to the Sun) electrons is compensated by the current of positive ions and protons from the Sun – SW. These dynamic processes maintain a quasi-constant effective dynamic charge of the Sun and the entire heliosphere. At the same time, quasi-neutrality in the Sun and heliosphere is well performed up to 10^-36. According to experiments and analytical calculations based on our model: 1) the plasma in the corona is nonequilibrium; 2) the maximum electron temperature is T_e ~ 1-2 million degrees; 3) T_e grows from 1000 km away from the Sun and 4) the role of highly energetic electrons escaping from the plasma leads to a significant increase in the effective: solar charge and electric fields in the heliosphere in relation to the Pannekoek-Rosseland-Eddington model. This is due to the absence of a compensation layer that screens the effective charge of the Sun. It is not formed at all due to the escape of highly energetic electrons (as in a conventional gas discharge) in the entire heliosphere with high temperatures exceeding the temperature of the Sun's surface. Thus, the process of escape of highly energetic electrons forms the internal EMF of the entire heliosphere. Interference of gravitational and Coulomb potentials in the entire heliosphere is considered, it is being manifested in generation of two opposite flows of particles: 1) that are neutral or with a small charge (to the Sun), and 2) in the form of high-energy electrons (escaping from the positively charged Sun) and a solar wind (from the Sun). Calculated values of the registered ion parameters in the solar wind were compared with experimental observations. Reasons for generating the ring current in inhomogeneous heliosphere and inapplicability of the Debye theory in describing processes in the solar wind (plasma with current) are considered.

Ion-driven Instabilities in the Inner Heliosphere. I. Statistical Trends

Instabilities described by linear theory characterize an important form of wave–particle interaction in the solar wind. We diagnose unstable behavior of solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it to fits of ∼1.5M proton and α particle Velocity Distribution Functions (VDFs) observed by Helios I and II. The variation of the fraction of unstable intervals with radial distance from the Sun is linear, signaling a gradual decline in the activity of unstable modes. When calculated as functions of the solar wind velocity and Coulomb number, we obtain more extreme, exponential trends in the regions where collisions appear to have a notable influence on the VDF. Instability growth rates demonstrate similar behavior, and significantly decrease with Coulomb number. We find that for a nonnegligible fraction of observations, the proton beam or secondary component might not be detected, due to instrument resolution limitations, and demonstrate that the impact of this issue does not affect the main conclusions of this work.

Magnetohydrodynamic Modeling of the Solar Corona with an Effective Implicit Strategy

In this paper, we design an effective and robust model to solve the 3D single-fluid solar wind plasma magnetohydrodynamics (MHD) problem of low plasma β. This MHD model is formulated on a six-component composite grid system free of polar singularities. The computational domain ranges from the solar surface to the super-Alfvénic region. As common to all MHD codes, this code must handle the physical positivity-preserving property, time-step enlargement, and magnetic field divergence-free maintenance. To maintain physical positivity, we employ a positivity-preserving Harten–Lax–van Leer Riemann solver and take a self-adjusting and positivity-preserving method for variable reconstruction. To loosen the time-step limitation, we resort to the implicit lower–upper symmetric Gauss–Seidel method and keep the sparse Jacobian matrix diagonally dominant to improve the convergence rate. To deal with the constant theme of a magnetic field that is divergence-free, we adopt a globally solenoidality-preserving approach. After establishing the solar wind model, we use its explicit and implicit versions to numerically investigate the steady-state solar wind in Carrington rotations (CRs) 2172 and 2210. Both simulations achieve almost the same results for the two CRs and are basically consistent with solar coronal observations and mapped in situ interplanetary measurements. Furthermore, we use the implicit method to conduct an ad hoc simulation by multiplying the initial magnetic field of CR 2172 with a factor of 6. The simulation shows that the model can robustly and efficiently deal with the problem of a plasma β as low as about 5 × 10−7. Therefore, the established implicit solar wind MHD model is very promising for simulating complex and strong magnetic environments.

Solar Wind Plasma Properties During Ortho-Parker IMF Conditions and Associated Magnetosheath Mirror Instability Response

The properties of the solar wind fraction that exhibits an Interplanetary Magnetic Field (IMF) orientation orthogonal to the classical Parker spiral (so-called ortho-Parker) are investigated. We make use of a solar wind plasma categorization scheme, using 10 years of OMNI data, and show that the fractions of the different plasma origins (streamer-belt-origin plasma, coronal-hole-origin plasma, sector-reversal-region plasma and ejecta) identified by this scheme are rather constant when expressed as a function of the IMF orientation whereas the Alfvén Mach number significantly depends on this orientation. This has direct implication on the magnetosheath dynamics and, as an example, the stability of the mirror mode in this compressed plasma is studied thanks to Rankine-Hugoniot anisotropic relations. This study sheds light on previously reported, yet unexplained, observations of a larger occurrence of mirror mode in the magnetosheath downstream of ortho-Parker IMF.

Mesoscale Structure in the Solar Wind

Structures in the solar wind result from two basic mechanisms: structures injected or imposed directly by the Sun, and structures formed through processing en route as the solar wind advects outward and fills the heliosphere. On the largest scales, solar structures directly impose heliospheric structures, such as coronal holes imposing high speed streams of solar wind. Transient solar processes can inject large-scale structure directly into the heliosphere as well, such as coronal mass ejections. At the smallest, kinetic scales, the solar wind plasma continually evolves, converting energy into heat, and all structure at these scales is formed en route. “Mesoscale” structures, with scales at 1 AU in the approximate spatial range of 5–10,000 Mm and temporal range of 10 s–7 h, lie in the orders of magnitude gap between the two size-scale extremes. Structures of this size regime are created through both mechanisms. Competition between the imposed and injected structures with turbulent and other evolution leads to complex structuring and dynamics. The goal is to understand this interplay and to determine which type of mesoscale structures dominate the solar wind under which conditions. However, the mesoscale regime is also the region of observation space that is grossly under-sampled. The sparse in situ measurements that currently exist are only able to measure individual instances of discrete structures, and are not capable of following their evolution or spatial extent. Remote imaging has captured global and large scale features and their evolution, but does not yet have the sensitivity to measure most mesoscale structures and their evolution. Similarly, simulations cannot model the global system while simultaneously resolving kinetic effects. It is important to understand the source and evolution of solar wind mesoscale structures because they contain information on how the Sun forms the solar wind, and constrains the physics of turbulent processes. Mesoscale structures also comprise the ground state of space weather, continually buffeting planetary magnetospheres. In this paper we describe the current understanding of the formation and evolution mechanisms of mesoscale structures in the solar wind, their characteristics, implications, and future steps for research progress on this topic.