Large-scale Solar Wind Research Articles

An asynchronous and parallel time-marching method: Application to three-dimensional MHD simulation of solar wind

An asynchronous and parallel time-marching method for three-dimensional (3D) time-dependent magnetohydrodynamic (MHD) simulation is used for large-scale solar wind simulation. It uses different local time steps in the corona and the heliosphere according to the local Courant-Friedrichs-Levy (CFL) conditions. The solar wind background with observed solar photospheric magnetic field as input is first presented. The simulation time for the background solar wind by using the asynchronous method is <1/6 of that by using the normal synchronous time-marching method with the same computation precision. Then, we choose the coronal mass ejection (CME) event of 13 November, 2003 as a test case. The time-dependent variations of the pressure and the velocity configured from a CME model at the inner boundary are applied to generate transient structures in order to study the dynamical interaction of a CME with the background solar wind flow between 1 and 230 Rs. This time-marching method is very effective in terms of computation time for large-scale 3D time-dependent numerical MHD problem. In this validation study, we find that this 3D MHD model, with the asynchronous and parallel time-marching method, provides a relatively satisfactory comparison with the ACE spacecraft observations at L1 point.

Solar cycle changes of large-scale solar wind structure

AbstractIn this paper, I present the results on large-scale evolution of density turbulence of solar wind in the inner heliosphere during 1985–2009. At a given distance from the Sun, the density turbulence is maximum around the maximum phase of the solar cycle and it reduces to ~70%, near the minimum phase. However, in the current minimum of solar activity, the level of turbulence has gradually decreased, starting from the year 2005, to the present level of ~30%. These results suggest that the source of solar wind changes globally, with the important implication that the supply of mass and energy from the Sun to the interplanetary space has significantly reduced in the present low level of activity.

Catalog of large-scale solar wind phenomena during 1976–2000

The main goal of this paper is to compile a catalog of large-scale phenomena in the solar wind over the observation period of 1976-2000 using the measurement data presented in the OMNI database. This work included several stages. At first the original OMNI database was supplemented by certain key parameters of the solar wind that determine the type of the solar wind stream. The following parameters belong to this group: the plasma ratio β , thermal ( NkT ) and kinetic ( mNV 2 ) pressures of the solar wind, the ratio T / T exp of measured and expected temperatures, gradients of the plasma velocity and density, and the magnetic field gradient. The results of visualization of basic plasma parameters that determine the character of the solar wind stream are presented on the website of the Space Research Institute, Moscow. Preliminary identification of basic types of the solar wind stream (FAST and SLOW streams, Heliospheric Current Sheet (HCS), Corotating Interaction Region (CIR), EJECTA (or Interplanetary Coronal Mass Ejections), Magnetic Cloud (MC), SHEATH (compression region before EJECTA/MC), rarified region RARE, and interplanetary shock wave IS) had been made with the help of a preliminary identification program using the preset threshold criteria for plasma and interplanetary magnetic field parameters. Final identification was done by comparison with the results of visual analysis of the solar wind data. In conclusion, histograms of distributions and statistical characteristics are presented for some parameters of various large-scale types of the solar wind.

Models of Solar Wind Structures and Their Interaction with the Earth’s Space Environment

The discipline of “Space Weather” is built on the scientific foundation of solar-terrestrial physics but with a strong orientation toward applied research. Models describing the solar-terrestrial environment are therefore at the heart of this discipline, for both physical understanding of the processes involved and establishing predictive capabilities of the consequences of these processes. Depending on the requirements, purely physical models, semi-empirical or empirical models are considered to be the most appropriate. This review focuses on the interaction of solar wind disturbances with geospace. We cover interplanetary space, the Earth’s magnetosphere (with the exception of radiation belt physics), the ionosphere (with the exception of radio science), the neutral atmosphere and the ground (via electromagnetic induction fields). Space weather relevant state-of-the-art physical and semi-empirical models of the various regions are reviewed. They include models for interplanetary space, its quiet state and the evolution of recurrent and transient solar perturbations (corotating interaction regions, coronal mass ejections, their interplanetary remnants, and solar energetic particle fluxes). Models of coupled large-scale solar wind–magnetosphere–ionosphere processes (global magnetohydrodynamic descriptions) and of inner magnetosphere processes (ring current dynamics) are discussed. Achievements in modeling the coupling between magnetospheric processes and the neutral and ionized upper and middle atmospheres are described. Finally we mention efforts to compile comprehensive and flexible models from selections of existing modules applicable to particular regions and conditions in interplanetary space and geospace.

Average profiles of the solar wind and outer radiation belt during the extreme flux enhancement of relativistic electrons at geosynchronous orbit

Abstract. We report average profiles of the solar wind and outer radiation belt during the extreme flux enhancement of relativistic electrons at geosynchronous orbit (GEO). It is found that seven of top ten extreme events at GEO during solar cycle 23 are associated with the magnetosphere inflation during the storm recovery phase as caused by the large-scale solar wind structure of very low dynamic pressure (&lt;1.0 nPa) during rapid speed decrease from very high (&gt;650 km/s) to typical (400–500 km/s) in a few days. For the seven events, the solar wind parameters, geomagnetic activity indices, and relativistic electron flux and geomagnetic field at GEO are superposed at the local noon period of GOES satellites to investigate the physical cause. The average profiles support the "double inflation" mechanism that the rarefaction of the solar wind and subsequent magnetosphere inflation are one of the best conditions to produce the extreme flux enhancement at GEO because of the excellent magnetic confinement of relativistic electrons by reducing the drift loss of trapped electrons at dayside magnetopause.

Large-scale solar wind density enhancement and its boundaries: Helios 1, 2 and IMP 8 observations

In the present paper, we investigate a large pulse in the solar wind density observed on April 1–2, 1977 by the Helios 1, 2 probes and 2 days later by the IMP 8 spacecraft in the “line-up” condition (all three spacecraft had the positions along the Sun–Earth line). In this pulse the strong enhancement in density (∼50% relative to the undisturbed level) was not accompanied by significant changes in other main solar wind parameters. The outcomes of detailed analysis of physical properties as well as of geometrical features of the observed plasma pulse structure are discussed. One of the characteristic peculiarities of this pulse was its sharp boundaries (with durations of several minutes only). The main scientific result of our study is that the trailing edge of the pulse was found to be very stable (conserved its shape and duration) propagating over as large distance as 0.6 AU (90×10 6 km) or during 2.3 days of the solar wind motion.

Energetic particle observations by Ulysses during the declining phase of solar cycle 23

We present recent energetic particle measurements from 1–20 MeV/n recorded by the Ulysses/COSPIN/LET instrument from mid‐October to the end of November 2003 and from May 2004 to the end of February 2005. Long‐lasting periods with enhanced solar activity occurred during the declining phase of the current solar cycle contributing to the high‐intensity particle events and heavy ion signatures observed by Ulysses near the ecliptic plane, at ∼5 AU from the Sun. The energetic particles injected from the Sun during the January 2005 intense solar activity were not observed at Ulysses until a stream interface that acted as a barrier for the particles reached the spacecraft. Our observations during this period thus show that the propagation of energetic particles is largely governed by the existence of large‐scale solar wind structures that impede the transport of the particles. We use the elemental composition of the particle fluxes recorded by COSPIN/LET to gain insight into the possible origin of the particle events. We study and provide a possible interpretation of the composition signatures observed at ∼5 AU during periods of CME/CIR combinations. Although predominantly SEP‐like, the observed composition showed evidence for enhancements in He, which we suggest is of interstellar origin, consistent with CIR shocks accelerating ions from multiple sources.

Plasma Diagnostics of the Large‐Scale Corona with SUMER. I. Measurements at the West Limb

In the present work we analyze the physical properties of the quiet-Sun plasma measured in a 0.5 × 1.8 R☉ region above the west solar limb (R☉ = solar radius). We make use of large scans obtained with the SUMER spectrometer on board SOHO to construct two-dimensional spatial maps of line intensities, electron temperature, emission measure, element abundances, line widths and nonthermal velocities, and photoexcitation effects covering the entire field of view. Electron densities were measured in a more limited portion of the field of view. The aim of the paper is to identify tracers of coronal hole and quiet-Sun plasma at high altitudes that allow us to measure the position of the coronal hole/quiet-Sun boundaries, and to provide a comprehensive, empirical picture of the off-limb solar corona that can provide theoreticians with experimental constraints to their models of the large-scale coronal structure, coronal heating, and solar wind acceleration.

A comparative study on 3-D solar wind structure observed by Ulysses and MHD simulation

During Ulysses’ first rapid pole-to-pole transit from September 1994 to June 1995, its observations showed that middle-or high-speed solar winds covered all latitudes except those between -20° and +20° near the ecliptic plane, where the velocity was 300–450 km/s. At poleward 40°, however, only fast solar winds at the speed of 700–870 km/s were observed. In addition, the transitions from low-speed wind to high-speed wind or vice versa were abrupt. In this paper, the large-scale structure of solar wind observed by Ulysses near solar minimum is simulated by using the three-dimensional numerical MHD model. The model combines TVD Lax-Friedrich scheme and MacCormack II scheme and decomposes the calculation region into two regions: one from 1 to 22 Rs and the other from 18 Rs to 1 AU. Based on the observations of the solar photospheric magnetic field and an addition of the volumetric heating to MHD equations, the large-scale solar wind structure mentioned above is reproduced by using the three-dimensional MHD model and the numerical results are roughly consistent with Ulysses’ observations. Our simulation shows that the initial magnetic field topology and the addition of volume heating may govern the bimodal structure of solar wind observed by Ulysses and also demonstrates that the three-dimensional MHD numerical model used here is efficient in modeling the large-scale solar wind structure.

Numeric simulations of in situ observations of interacting ejecta near 1 AU

Abstract We use a simple numerical model (Gonzalez-Esparza, J.A., Santillan, A., Ferrer, J. A numerical study of the interaction between two ejecta in the interplanetary medium: one and two dimensional hydrodynamic simulations, Ann. Geophys. 22, 3741–3749, 2004) to study the evolution of three events in the solar wind reported by Wang et al. (Wang, Y.M., Ye, P.Z., Wang, S. Multiple magnetic clouds: several examples during March–April 2001. J. Geophys. Res. 108, 1370, 2003, doi:10.1029/2003JA009850 ), where two interacting ejecta detected in situ by ACE near 1 AU were related to CMEs observed previously by SOHO-LASCO. The study is based on a 1-D hydrodynamic model using the ZEUS code (Stone, J.M., Norman, M. ZEUS 2-D: A radiation magnetohydrodynamics code for astrophysical flows in two dimensions, I, the hydrodynamics algorithms and tests, Astrophys. J. 80, 753, 1992). Although this model cannot address either the magnetic field dynamics or the complex geometrical effects intrinsic in the three-dimensional nature of the phenomena, it illuminates the transferring of momentum and evolution of interacting large-scale solar wind disturbances in those cases where there is no merging (magnetic reconnection) between the two ejecta. This model can reproduce, in some cases, characteristics of the events such as transit times and flow signatures as inferred from the two-point measurements from spacecraft.

Three-dimensional MHD simulation for the solar wind structure observed by Ulysses

Ulysses has been the first spacecraft to explore the high latitudinal regions of the heliosphere till now. During its first rapid pole-to-pole transit from September 1994 to June 1995, Ulysses observed a fast speed flow with magnitude reaching 700–800 km/s at high latitudinal region except ±20° area near the ecliptic plane where the velocity is 300–400 km/s. The observations also showed a sudden jump of the velocity across the two regions. In this note, based on the characteristic and representative observations of the solar magnetic field and K-coronal polarized brightness, the large-scale solar wind structure mentioned above is reproduced by using a three-dimensional MHD model. The numerical results are basically consistent with those of Ulysses observations. Our results also show that the distributions of magnetic field and plasma number density on the solar source surface play an important role in governing this structure. Furthermore, the three-dimensional MHD model used here has a robust ability to simulate this kind of large-scale wind structure.

Working Group 2 Report: Energy Input, Heating, and Solar Wind Acceleration in Coronal Holes

Theories and observations of energy input, heating and acceleration mechanisms in the low corona were presented and discussed. The main topics of discussion were large-scale solar wind simulations, theoretical heating mechanisms, observational constraints, confronting theory with observations and observational issues.

Effects of decreasing ionospheric pressure on the solar wind interaction with non-magnetized planets

The large-scale solar wind interaction with the ionosphere of non-magnetized planets is numerically simulated in the framework of three-dimensional (3-D) magnetohydrodynamics (MHD) with a two-component plasma. The finite-volume total variation diminishing (TVD) scheme is used to solve this problem. Numerical results are given for two cases of different solar extreme ultraviolet (EUV) flux values. In case 1, solar EUV ionization is set so the peak ionospheric plasma pressure is below the incident solar wind dynamic pressure. In case 2, on the other hand, it is set so the peak ionospheric pressure exceeds the solar wind dynamic pressure. While the formation of the bow shock and the magnetic barrier in the upstream region is seen in both cases, a clear formation of the ionopause is seen only in case 2. In case 1, the interplanetary magnetic field (IMF) penetrates from the magnetosheath to the dayside ionosphere so as to adjust the ionospheric total pressure. Penetrating IMF affects the vertical motion of the ionospheric plasma to cause anomalous stratifications of the terminator ionosphere. However, formation process of the ionotail is little affected by the penetrating IMF. Another important process predicted from the present study is partial penetration of the IMF from the magnetic barrier to the terminator ionosphere. This nonideal MHD process characterized by the penetration of flowing magnetized plasma into non-magnetized plasma plays a principal role in the mixing interaction between the solar wind and the planetary ionosphere.

A large magnetic depression observed in the solar wind close to the Earth's bow shock

Two large magnetic depressions have been observed in the solar wind just upstream of the Earth's bow shock. These depressions possess similar properties to the magnetic hole phenomena that have been observed in the solar wind, but the structures observed in this case were much larger, the depressions lasting for ∼10 mins. The larger depression was interspersed with foreshock phenomena and so the observations describe a large-scale solar wind structure and its interaction with the foreshock region. Particle and magnetic field data from AMPTE UKS and ISEE1 are presented. The isotropic nature of the electrons within the solar wind structure appears to suggest the depression is a magnetically separate structure propagating with the solar wind e.g. a flux rope.

Ion acceleration processes in the Hermean and terrestrial magnetospheres

The magnetosphere of Mercury differs in several aspects from that of the other Earth-like planets in our solar system. For instance, the lack of a dense atmosphere and ionosphere implies a different solar-wind magnetosphere interaction compared to that of for example the Earth. However, the solar wind interaction with Mercury might be strongly dependent on a tenuous, but yet significant, exoionosphere. Indeed, the scale sizes and the time constants are different from that of the Earth's magnetosphere, but the large-scale solar wind energy and mass transfer and the intrinsic plasma acceleration processes near Mercury may have very much in common with plasma processes near the Earth. In this report we discuss potential ion acceleration and plasma outflow processes at Mercury in context with results obtained from observations of ion acceleration and outflow in the Earth's magnetosphere and topside ionosphere. We conclude that the acceleration and outflow processes at Mercury, although present in a more extreme environment compared to the other Earth-like planets, are expected to have many similarities with the acceleration and outflow processes near the Earth. Furthermore, we demonstrate that charge-exchange processes in the Hermean magnetosphere are expected to be rather efficient, resulting in “bright flashes” of energetic neutral atom (ENA) generation regions in the nightside atmosphere of Mercury and subsequent loss of magnetospheric plasma. A future mission to Mercury should therefore, besides appropriate plasma experiments, also incorporate ENA imaging experiments.

Numerical simulations of large‐scale solar wind fluctuations observed by Ulysses at high heliographic latitudes

An analysis of Ulysses plasma and magnetic field data from the high southern latitude pass revealed that at large spatial scales corresponding to wave frequencies below 10−5 Hz in the spacecraft frame, there is a transition from highly Alfvénic turbulence in all three spatial components of the fields to fluctuations which are Alfvénic only in field components transverse to the radial direction with uncorrelated radial magnetic and velocity fields. We find that the general properties of the turbulence can be simulated using a spectral method solution of the magnetohydrodynamic (MHD) equations only when certain types of initial conditions are employed. These simulations place constraints on conditions in the solar atmosphere that can generate the observed fluctuations.

Large-scale solar wind stream structure at high heliolatitudes

The tilted rotator model is considered using the collisionless supersonic and superalfvenic approximation. The results show that the maximal and minimal solar wind corotating stream velocities appear at the intermediate heliospheric latitudes depending on the tilt angle. The high-speed streams with the strongest velocity difference between maximal and minimal values are expected to be observed at the middle heliolatitudes near 45 degrees. Other consequences of the model are indicated and may be checked during the rapid pole-to-pole Ulysses passage in 1994–1995.

Shock interactions in the outer heliosphere

Observations of plasma and magnetic fields by Pioneer 10 and 11 and Voyager 1 and 2 reveal that MHD shocks are an important component of the large-scale solar wind structures in the outer heliosphere. This review discusses recent progress in simulation studies of the nonlinear evolution of the solar wind structures, and in particular concentrates on the theoretical development and applications of the shock interactions model. Various stream propagation models, which do not use the Rankine-Hugoniot relations to calculate the jump conditions at shock crossings, have been used to simulate the essential evolution process of isolated streams and the formation and propagation of corotating and transient shocks. They produce fairly good results in the region up to a few AU. In 1984, the shock interactions model was introduced to study the evolution of large-scale solar wind structures in the region outside 1 AU up to several tens of AU. The model uses the exact Rankine-Hugoniot relations to calculate the shock speed and shock strength at all shock crossings. So that the model can more accurately calculate the shock speeds and the accumulated irreversible shock heating of plasma at several tens of AU. The applications of the shock interactions model are presented in three groups. (a) The first group covers the basic interaction of a shock with the ambient solar wind, the formation and propagation of shock pairs, and the collision and merging of shocks. (b) The second group covers the use of the shock interactions model to simulate the nonlinear evolution of large-scale solar wind structures in the outer heliosphere. These simulation results can provide the detailed evolution process for large-scale solar wind structures in the vast region not directly observed. Two selected studies are reported. (c) Finally, the shock interactions model is applied to studying the heating of the solar wind in the outer heliosphere. The model calculations support shocks being chiefly responsible for the heating of the solar wind plasma in the outer heliosphere at least up to 30 AU.

Coronal holes as sources of large‐scale solar wind disturbances and geomagnetic perturbations

In this paper we present theoretical and observational evidence that low‐latitude unstable coronal holes may be, in some circumstances, the source of widespread interplanetary shocks and of the consequent geomagnetic perturbations. We also reanalyze here the shock events tracked by interplanetary scintillation in the study of Hewish and Bravo (1986), which produced the sudden commencement of a geomagnetic storm, in terms of an alternative flare, disappearing filament, or coronal hole eruption origin. The main conclusion of this study is that there are good reasons to consider coronal holes as sources of shock type interplanetary disturbances and that more effort should be made to monitor coronal holes continuously in search of evidence of large short‐time changes in their structure and physical conditions.

Solar wind latitude/longitude variations derived from interplanetary scintillations

Continuous accumulation of IPS (interplanetary scintillation) observations of the solar wind since 1972 enables us to follow solar-cycle variations of large-scale solar wind stream structures. The longitude/latitude structure of the solar wind speed is largely determined by the geometry of the heliospheric neutral sheet; the low-speed solar wind tends to be observed along the neutral sheet, whereas large, predominantly unipolar magnetic regions on the source surface (coronal holes) are the sources of high-speed streams. Large-scale configuration of a transient interplanetary disturbance has been shown to be distorted by the solar wind stream structures. In some cases, strong deceleration of the shock speed will take place along the heliospheric neutral sheet.