Slow Solar Wind Streams Research Articles

Two-parametric representation of the solar and heliospheric protons

Abstract The dynamics of the interplanetary proton spectra in the energy range 1–40 MeV have been investigated using the data obtained on board the IMP-8 (1973–1992) and Meteor (1970–1996) satellites. Phase diagrams of the proton events are constructed in the two-parametric space { Y,g }, where Y=g −1 lg( I p ), g is the differential energy spectral index, I p represents the >1 MeV proton intensity. The results show that the phase trajectories allow separation of the background proton fluxes, proton populations accelerated mainly in the solar flares, at the propagating interplanetary shocks associated with coronal mass ejections and also in the corotating interaction regions between fast and slow solar wind streams. The estimations of the maximal and minimal >1 MeV proton fluxes in the near Earth heliosphere are obtained.

Highlight Results from Ulysses

Launched in October 1990, the ESA-NASA Ulysses mission has conducted the very first survey of the heliosphere within 5 AU of the Sun over the full range of heliolatitudes. The first polar passes took place in 1994 and 1995, enabling Ulysses to characterise the global structure of the heliosphere at solar minimum, when the corona adopts its simplest configuration. The most important findings to date include a confirmation of the uniform nature of the high-speed (~ 750 km s−1) solar wind flow from the polar coronal holes, filling two-thirds of the volume of the inner heliosphere; the sharp boundary, existing from the chromosphere through the corona, between fast and slow solar wind streams; the latitude independence of the radial component of the heliospheric magnetic field; the lower-than-expected latitude gradient of galactic and anomalous cosmic rays; the continued existence of recurrent increases in the flux of low-energy ions and electrons up to the highest latitudes.

Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer

Using improved, self‐consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi‐stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing‐in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well‐known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low‐FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low‐FIP bias in the slow wind and a weaker (and perhaps conceptually different) low‐FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi‐stationary solar wind types.

Global three‐dimensional MHD simulation of a space weather event: CME formation, interplanetary propagation, and interaction with the magnetosphere

A parallel adaptive mesh refinement (AMR) finite‐volume scheme for predicting ideal MHD flows is used to simulate the initiation, structure, and evolution of a coronal mass ejection (CME) and its interaction with the magnetosphere‐ionosphere system. The simulated CME is driven by a local plasma density enhancement on the solar surface with the background initial state of the corona and solar wind represented by a newly devised “steady state” solution. The initial solution has been constructed to provide a reasonable description of the time‐averaged solar wind for conditions near solar minimum: (1) the computed magnetic field near the Sun possesses high‐latitude polar coronal holes, closed magnetic field flux tubes at low latitudes, and a helmet streamer structure with a neutral line and current sheet; (2) the Archimedean spiral topology of the interplanetary magnetic field is reproduced; (3) the observed two‐state nature of the solar wind is also reproduced with the simulation yielding fast and slow solar wind streams at high and low latitudes, respectively; and (4) the predicted solar wind plasma properties at 1 AU are consistent with observations. Starting with the generation of a CME at the Sun, the simulation follows the evolution of the solar wind disturbance as it evolves into a magnetic cloud and travels through interplanetary space and subsequently interacts with the terrestrial magnetosphere‐ionosphere system. The density‐driven CME exhibits a two‐step release process, with the front of the CME rapidly accelerating following the disruption of the near‐Sun closed magnetic field line structure and then moving at a nearly constant speed of ∼560 km/s through interplanetary space. The CME also produces a large magnetic cloud (> 100 RS across) characterized by a magnetic field that smoothly rotates northward and then back again over a period of ∼2 days at 1 AU. The cloud does not contain a sustained period with a strong southward component of the magnetic field, and, as a consequence, the simulated CME is somewhat ineffective in generating strong geo‐magnetic activity at Earth. Nevertheless, the simulation results illustrate the potential, as well as current limitations, of the MHD‐based space weather model for enhancing the understanding of coronal physics, solar wind plasma processes, magnetospheric physics, and space weather phenomena. Such models will provide the foundation for future, more comprehensive space weather prediction tools.

Flow shear across solar wind discontinuities: WIND observations

We examine the tangential magnetic field and velocity shears across directional discontinuities (DDs) with significant change in magnetic field intensity observed by WIND in slow and fast solar wind streams. The magnetic field rotation sense in fast wind DDs is that predicted by theory for outward propagating rotational discontinuities (RDs), but flow shear magnitude and orientation do not always satisfy RD theory. Alternatively, DDs with small normal magnetic field can be regarded as tangential discontinuities (TDs); the observed shears imply that the length scale over which the proton velocity distribution changes at the discontinuity can be both smaller or larger than that of the electron distribution. The slow wind includes a larger fraction of DDs that disagree with RD theory. It is shown that the flow shear orientations allowed in a TD provide a continuous transition between the opposite orientations for RDs propagating along or against the magnetic field direction.

Particle propagation channel detected at 4.7 AU inside a corotating interaction region

This study presents an interplanetary particle (electrons and ions) event which was detected at 4.7 AU by the instruments on the Ulysses spacecraft inside a corotating interaction region between a fast and slow solar wind stream. A particle propagation channel is identified where the particles can propagate from the Sun nearly scatter free within a region bounded by tangential discontinuities. This channel is found to be rooted in the vicinity of a solar active region where the flare associated with the interplanetary event was produced. The ions (energy over 600 keV) detected inside this channel are shown to be of solar origin: dispersion occurs in the proton arrival time versus proton energy, there is a strong anisotropy in the proton pitch angle distribution, and a hard energy‐spectrum for the protons. The electrons were already present when the spacecraft crossed the structure. The chemical composition is also indicative of a solar origin, with a decrease in the 4He/H (4He in the energy range 389–1278 keV and H in the range 480–1204 keV) abundance ratio by a factor of 2 inside the channel. Mapped back to the solar surface, the size of the propagation channel is estimated to be of the order of 30,000 km. The channel is found to be magnetically quiet, without a preferred direction for the minimum of variance.

Measurement of the Abundance of Helium-3 in the Sun and in the Local Interstellar Cloud with SWICS on Ulysses

The abundance of 3He in the present day local interstellar cloud (LIC) and in the sun has important implications for the study of galactic evolution and for estimating the production of light nuclei in the early universe. Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses is used to measure the isotopic ratio of helium (3He/4He = Г) both in the solar wind and the local interstellar cloud. For the solar wind, the unique high-latitude orbit of Ulysses allows us to study this ratio in the slow and highly dynamic wind in the ecliptic plane as well as the steady high-latitude wind of the polar coronal holes. The 3He+ /4He+ ratio in the local cloud is derived from the isotopic ratio of pickup helium measured in the high-speed solar wind. In the LIC the ratio is found to be (2.48 −0.62 +0.68 ) × 10−4 with the 1-σ uncertainty resulting almost entirely from statistical error. In the solar wind, Г is determined with great statistical accuracy but shows systematic differences between fast and slow solar wind streams. The slow wind ratio is variable. Its weighted average value (4.08 ± 0.25) × 10−4 is, within uncertainties, in agreement with the Apollo SWC results. The high wind ratio is less variable but smaller. The average Г in the fast wind is (3.3 ± 0.3) × 10−4.

Alfvén waves in the solar corona and solar wind

Alfvén waves have long been known to be a major component of the turbulence measured in situ in the interplanetary medium. Until recently, however, observations had been limited to the ecliptic plane, where the solar wind structure is complicated by the interaction of fast and slow solar wind streams, the Alfvénic turbulence being essentially limited to high-speed streams in well defined magnetic sectors. The Ulysses spacecraft has shown how this structure disappears with increasing latitude, leading to a relatively constant high-speed stream originating from polar coronal holes. Within this region the radial magnetic field appears to be relatively constant with latitude, and the fluctuations are everywhere dominated by large-amplitude Alfvén waves propagating away from the sun, covering a broad band of wavelengths. Here we discuss the origin and evolution of solar wind Alfvén waves; the possible role played by such fluctuations in the heating of the corona and acceleration of the high-speed wind is explored in the light of both analytical models and numerical simulations.

Magnetohydrodynamic Turbulence in the Solar Wind

The fluctuations in magnetic field and plasma velocity in the solar wind possess many features expected of fully developed magnetohydrodynamic turbulence. Understanding this nonlinear system is complicated by the dynamical effects of velocity shear between fast and slow solar wind streams, by the spherical expansion of the solar wind, and by its compressibility. Direct spacecraft observations from 0.3 to over 20 AU, radio scintillation observations of plasmas near the Sun, numerical simulations, and various models provide complementary methods that have shown convincingly that the fluctuations in the wind parameters undergo significant dynamical evolution independent of whatever turbulence might exist in the solar photosphere and corona. This rich area of study allows one to test theories of turbulence against direct observation, and to use observations to guide and motivate development of turbulence theories. The solar wind thus provides an excellent laboratory for studying many fundamental questions about turbulent plasmas.

Interactions of solar wind streams and related small structures

Coronal holes produce high‐speed, low‐density, and high‐temperature streams that propagate into the interplanetary space. These streams interact with the slow‐speed, high‐density, and low‐temperature stream of the ambient solar wind. We investigate this problem using the time‐dependent, two‐dimensional hydrodynamic model in the spherical‐equatorial coordinate system. More accurate numerical methods and finer difference meshes used enable us to track the evolution of detailed features of the fast and slow stream interaction. An analysis of formation of shock pairs (forward and reverse shocks) is presented for both erupting and corotating parts of fast streams. Further, it is shown that the process of interaction of fast and slow solar wind streams may contain richer structures. Such structures may originate during the reconfinement process (internal shocks), spatial substructures (flux tubes), and small temporal modulations (shock wings). They may influence the global shape of stream interfaces and heating of the plasma. Finally, conclusion can be made that boundaries between the fast and slow coronal streams seem to be stable against small random fluctuations and against small introduced disturbances and the Kelvin‐Helmholtz instability is not initiated.

Parametric study of interaction effects of fast and slow co-rotating solar wind streams

Through numerical calculations, a study is made of the influence of main plasma and magnetic field characteristics, and of the solar wind (SW) inhomogeneity near the Sun upon the effectiveness of interaction of fast and slow SW streams in interplanetary space. A two-dimensional, two-fluid magnetohydrodynamical SW model with the inclusion of Alfvén waves was used. The study has revealed a group of parameters that have a substantial effect on solar wind characteristics in the region of interaction of its fast and slow streams. Alfvén waves have been found to influence the effectiveness of interaction of such streams. In terms of the model used in the parametric investigation of the interaction of co-rotating streams, it was also possible to obtain new results concerning the SW acceleration characteristics in a 2-D approximation. On this basis we suggest the idea of a likely physical mechanism that governs the velocity of SW outflow from coronal holes, depending on their size.

Ulysses observations of a pure electron event and its associated type III radio emission on December 15, 1990

We describe an electron event associated with a type III burst observed by Ulysses. This event was observed when the spacecraft crossed the region of strong compression between a fast and a slow solar wind stream, characterized by a relatively high density and a complex behaviour of the plasma density, magnetic field and energetic electron anisotropy. There is a clear influence of the local plasma frequency on the time profile of the type III burst, even at frequencies significantly higher than the average local plasma frequency, which can be interpreted as due to the reflection of the electromagnetic radiation by randomly distributed regions of high density.

Interplanetary magnetic field structure and solar wind parameters as inferred from solar magnetic field observations and by using a numerical 2-D MHD model

An attempt is made to infer parameters of the solar corona and the solar wind by means of a numerical, self-consistent MHD simulation. Boundary conditions for the magnetic field are given from the observations of the large-scale magnetic field at the Sun. A two-region, planar (the ecliptic plane is assumed) model for the solar wind flow is considered. Region I of transonic flow is assumed to cover the distances from the solar surface up to 10R S (R S is the radius of the Sun). Region II of supersonic, super-Alfvenic flow extends between 10R S and the Earth's orbit. Treatment for region I is that for a mixed initial-boundary value problem. The solution procedure is similar to that discussed by Endler (1971) and Steinolfson, Suess, and Wu (1982): a steady-state solution is sought as a relaxation to the dynamic equilibrium of an initial state. To obtain a solution to the initial value problem in region II with the initial distribution of dependent variables at 10R S (deduced from the solution for region I), a numerical scheme similar to that used by Pizzo (1978, 1982) is applied. Solar rotation is taken into account for region II; hence, the interaction between fast and slow solar wind streams is self-consistently treated. As a test example for the proposed formulation and numerical technique, a solution for the problem similar to that discussed by Steinolfson, Suess, and Wu (1982) is obtained. To demonstrate the applicability of our scheme to experimental data, solar magnetic field observations at Stanford University for Carrington rotation 1682 are used to prescribe boundary conditions for the magnetic field at the solar surface. The steady-state solution appropriate for the given boundary conditions was obtained for region I and then traced to the Earth's orbit through region II. We compare the calculated and spacecraft-observed solar wind velocity, radial magnetic field, and number density and find that general trends during the solar rotation are reproduced fairly well although the magnitudes of the density in comparison are vastly different.

The interaction effect of fast and slow solar wind streams in interplanetary space on wind characteristics at the Earth's orbit

By analyzing observational data, it has been possible to determine quantitative relationships that represent the role of the interaction of fast and slow solar wind (SW) streams in the formation of characteristic SW properties at the Earth's orbit.

Energetic protons associated with a forward-reverse interplanetary shock pair at 1 A.U.

A forward-reverse interplanetary shock was observed on 25 March 1969 by the magnetometer and plasma detector on the HEOS-1 satellite. This relatively rare event was described by Chao et al (1972) who concluded that the shock pair was formed at a distance 0.10–0.13 A.U. upstream of the Earth as a result of the interaction between a fast and a slow solar wind streams. Simultaneous observations of 1 MeV solar proton fluxes were also performed on HEOS-1. A characteristic intensity peak was observed as the forward shock passed by the spacecraft. The evolution of the proton intensity, together with a detailed analysis of anisotropies and pitch angle distributions show a complex dynamic picture of the effect of the forward shock on the ambient proton population. Significant changes in particle fluxes are seen to be correlated with fluctuations in the magnetic field. It is suggested that simple geometrical models of shock-associated acceleration should be expanded to include the effect of magnetic fluctuations on particle fluxes. The interaction region limited by the forward and reverse shocks contained a large variety of magnetic fluctuations. Following the tangential discontinuity separating the fast solar wind stream from the preceding slow stream, a sunward flow was observed in the proton data, followed by a small but significant drop in intensity prior to the reverse shock.

Correlation of interplanetary scintillation and spacecraft plasma density measurements

Interplanetary scintillation and spacecraft measurements of proton density are observed to be strongly correlated. This result confirms the association between corotating sectors of enhanced scintillation and compression regions located at the interface between fast and slow solar wind streams; it also suggests that scintillation observations provide a means of predicting, several days in advance, the arrival at the Earth, of solar plasma of enhanced density.