Solar Wind Acceleration Research Articles

Energization of Plasma Species by Intermittent Kinetic Alfvén Waves

We propose a new phase-mixing sweep model of coronal heating and solar wind acceleration based on dissipative properties of kinetic Alfven waves (KAWs). The energy reservoir is provided by the intermittent ∼1 Hz MHD Alfven waves excited at the coronal base by magnetic restructuring. These waves propagate upward along open magnetic field lines, phase-mix, and gradually develop short wavelengths across the magnetic field. Eventually, at 1.5–4 solar radii they are transformed into KAWs. We analyze several basic mechanisms for anisotropic energization of plasma species by KAWs and find them compatible with observations. In particular, UVCS (onboard SOHO) observations of intense cross-field ion energization at 1.5–4 solar radii can be naturally explained by non-adiabatic ion acceleration in the vicinity of demagnetizing KAW phases. The ion cyclotron motion is destroyed there by electric and magnetic fields of KAWs.

Kinetic Physics of the Solar Corona and Solar Wind

Kinetic plasma physics of the solar corona and solar wind are reviewed with emphasis on the theoretical understanding of the in situ measurements of solar wind particles and waves, as well as on the remote-sensing observations of the solar corona made by means of ultraviolet spectroscopy and imaging. In order to explain coronal and interplanetary heating, the microphysics of the dissipation of various forms of mechanical, electric and magnetic energy at small scales (e.g., contained in plasma waves, turbulences or non-uniform flows) must be addressed. We therefore scrutinise the basic assumptions underlying the classical transport theory and the related collisional heating rates, and also describe alternatives associated with wave-particle interactions. We elucidate the kinetic aspects of heating the solar corona and interplanetary plasma through Landau- and cyclotron-resonant damping of plasma waves, and analyse in detail wave absorption and micro instabilities. Important aspects (virtues and limitations) of fluid models, either single- and multi-species or magnetohydrodynamic and multi-moment models, for coronal heating and solar wind acceleration are critically discussed. Also, kinetic model results which were recently obtained by numerically solving the Vlasov‐Boltzmann equation in a coronal funnel and hole are presented. Promising areas and perspectives for future research are outlined finally.

Wave acceleration of the fast solar wind

Observations show that the solar wind exhibits two modes of outflow: the slow (∼400 km s −1), high density, highly variable wind that emerges from equatorial solar regions, and the high latitude, fast (∼800 km s −1), low density, steady solar wind. The bi-modal solar wind outflow is most evident near minima of solar activity. Theoretical studies of the fast solar wind in open magnetic structures motivated by SOHO, Ulysses, and Helios observations indicate that both, high frequency kinetic waves, and low frequency MHD waves play a role in its acceleration and heating. Ion-cyclotron waves have been suggested as the main energy source of the solar wind. However, there are theoretical difficulties with the ion-cyclotron wave heating of the protons, and these waves do not heat electrons. Low frequency MHD waves are still the best candidates to transport momentum and energy far from the Sun, to accelerate the solar wind on large spatial scales. I will present recently developed two-dimensional three-fluid model that includes explicit wave acceleration, and visco-resistive dissipation. The model describes electrons, protons, and minor ions as three coupled fluids that are heated by different heating processes with the parameters constrained by observations. I will present the results of 2.5D three-fluid simulations of the fast solar wind plasma that combine the effects of MHD waves self-consistently, and ion-cyclotron waves parametrically on the acceleration and heating processes. I will present the results of hybrid kinetic models of ion-cyclotron wave heating of the heavy ions in the solar wind plasma.

Doppler redistribution of anisotropic radiation and resonance polarization in moving scattering media

The present paper puts the Doppler redistribution theory of anisotropic radiation and resonance polarization in moving scattering media into a predictive application following Sahal-Bréchot et al. (1998, A&A, 340, 579; P), using the example of the 1032 Å line of the corona. Numerical results are presented for different heights over the limb compatible with observations of different instruments on SOHO (SUMER, CDS, UVCS) but far from coronal holes in order to avoid inhomogeneities of the transition zone incident radiation. The orders of magnitude of our numerical results confirm the theoretical predictions of P and show that interpretation of polarimetric data of the 1032 Å line of the corona, associated to the shift and the Doppler-dimming effect, offers a powerful diagnostic method of the complete velocity field vector, provided that the partial anisotropy of the incident radiation field be taken into account. These results also explain the precision of the diagnostic of interpretation polarimetric observations achieved by the ultraviolet spectrometer SUMER in the south polar coronal hole at above the solar limb; e.g. the solar wind acceleration region.

Commission 49: Interplanetary Plasma and Heliosphere

AbstractCommission 49 covers research on the solar wind, shocks and particle acceleration, both transient and steady-state, e.g., corotating, structures within the heliosphere, and the termination shock and boundary of the heliosphere. During the last three years there was considerable progress made in studies of solar energetic particles, compositional and other signatures in the heliosphere, solar wind pickup ions, the termination shock, which was finally crossed by a spacecraft, and the boundary between the heliosphere and interstellar medium, and in solar wind modeling and space weather. These topics have been summarized here in five articles, each with extensive references that will guide the reader who wants further details. Observations from the following spacecraft have extensively used during this period: Ulysses, Cassini, Voyager 1 and 2, MESSENGER, ACE, Genesis, SOHO, Wind, and RHESSI.

Making the Corona and the Fast Solar Wind: A Self-consistent Simulation for the Low-Frequency Alfvén Waves from the Photosphere to 0.3 AU

By performing a one-dimensional magnetohydrodynamic simulation with radiative cooling and thermal conduction, we show that the coronal heating and the fast solar wind acceleration in the coronal holes are natural consequences of the footpoint fluctuations of the magnetic fields at the photosphere. We initially set up a static open flux tube with a temperature of 104 K rooted at the photosphere. We impose transverse photospheric motions corresponding to the granulations with a velocity dv⊥ = 0.7 km s-1 and a period between 20 s and 30 minutes, which generate outgoing Alfven waves. We self-consistently treat these waves and the plasma heating. After attenuation in the chromosphere by 85% of the initial energy flux, the outgoing Alfven waves enter the corona and contribute to the heating and acceleration of the plasma mainly by the nonlinear generation of the compressive waves and shocks. Our result clearly shows that the initially cool and static atmosphere is naturally heated up to 106 K and accelerated to 800 km s-1.

MHD Waves and Heating in Coronal Holes

Coronal holes have been identified as source regions of the fast solar wind, and MHD wave activity has been detected in coronal holes by remote sensing, and in situ in fast solar wind streams. I review some of the most suggestive wave observations, and discuss the theoretical aspects of MHD wave heating and solar wind acceleration in coronal holes. I review the results of single fluid 2.5D MHD, as well as multi-fluid 2.5D MHD models of waves in coronal holes, the heating, and the acceleration of the solar wind be these waves.

Study of Hα Macrospicules in Coronal Holes: Magnetic Structure and Evolution in Relation to Photospheric Magnetic Setting

Small-scale solar dynamic events such as spicules, macrospicules, and microflares may play an important role in the coronal heating and solar wind acceleration in coronal holes. In these regions, the network fields concentrated along edges of supergranules are probably the source of the fine-scale activity that may drive the heating and acceleration. Recent Hα limb observations from Big Bear Solar Observatory (BBSO) have shown that most macrospicules have two different forms of magnetic structure—a spiked jet or an erupting loop, suggesting two different formation mechanisms. In this paper, we analyze BBSO Hα images and magnetograms of a coronal hole region near the disk center to study the evolution of the two types of macrospicule in relation to the magnetic arrangement at their base. We identified 78 macrospicules from the best day of 3 days of observations. Of these, 65 events were in the form of a spiked jet and were rooted in compact bipolar fields at the edges of the magnetic network. This supports the idea that spiky macrospicules are driven by reconnection between the network bipole and open magnetic fields. We also found five macrospicules that were in the form of an erupting loop oriented along a neutral line between the positive and negative network flux. They appear to be minifilament eruptions. Our results verify the magnetic structure inferred from our previous limb observations and support scenarios of coronal heating and solar wind generation through fine-scale explosive reconnection events seated in the magnetic network.

Origin and Acceleration of the Slow Solar Wind

This Letter uses Doppler dimming measurements by SOHO UVCS to elucidate the origin and acceleration of the slow solar wind. By investigating plasma flow in the corona over an active region during 2000 May 14-16, we confirm what has been suggested by the presence of the imprint of active regions in the solar wind near Earth orbit, that active regions are a source of slow wind. The observed active region does not have an associated streamer in the outer corona. We explain how this implies that any related heliospheric current sheet must be transverse to the line of sight. It is this favorable geometry of a transverse heliospheric current sheet that allows the plasma flow over the active region to be isolated in path-integrated Doppler dimming measurements. The results also show that acceleration of the slow wind associated with active regions toward its terminal speed is faster than that along the heliospheric current sheet. These differences in acceleration explain why the signatures of the heliospheric current sheet are dissimilar in velocity, but not in density, between the corona and solar wind measured near Earth orbit.

Turbulence in the inner solar wind determined from frequency fluctuations of the downlink signals from the ULYSSES and GALILEO spacecraft

The results of several sets of measurements of the frequency of radio signals during coronal-sounding experiments carried out from 1991 to 2000 using the ULYSSES and GALILEO spacecraft are presented and analyzed. The S-band signals (carrier frequency f = 2295 MHz) were received at the three 70-m widely spaced ground stations of the NASA Deep Space Network. As a rule, the frequency-fluctuation spectra at frequencies above 1 mHz are power-laws. At small heliocentric distances, R < 10R ⊙ (R ⊙ is the solar radius), the spectral index is close to zero; this corresponds to a spectral index for the one-dimensional turbulence spectrum p 1 = 1. The index of the frequency-fluctuation spectra in the region of the supersonic solar wind at distances R > 30 R ⊙ is between 0.5 and 0.7 (p 1 = 1.5–1.7). The results demonstrate a substantial difference between the turbulence regimes in these regions: in the region of the established solar wind, the power-law spectra are determined by nonlinear cascade processes that pump energy from the outer turbulence scale to the small-scale part of the spectrum, whereas such cascade processes are absent in the solar wind acceleration region. Near the solar minimum, the change in the turbulence regime of the fast, high-latitude solar wind occurs at greater distances than for the slow, low-latitude solar wind. Spectra with a sharp cutoff at high frequencies have been detected for the first time. Such spectra are observed only at R < 10 R ⊙ and at sufficiently low levels of the electron density fluctuations. The measured cutoff frequencies are between 10 and 30 mHz; the cutoff frequency tends to increase with heliocentric distance. The variance of the plasma-density fluctuations has been estimated for the slow, low-latitude solar wind. These estimates suggest that the relative fluctuation level at distances 7 R ⊙ < R < 30 R ⊙ does not depend on heliocentric distance. The cross correlation of the frequency fluctuations recorded at widely spaced ground stations increases with the index of the frequency-fluctuation spectrum. At distances R ≈ 10 R ⊙, the rate of temporal changes in irregularities on the scale of several thousand kilometers is less than or comparable to the solar wind velocity.

Effect of a Converging Flow at the Streamer Cusp on the Genesis of the Slow Solar Wind

The observation by the Large Angle and Spectrometric Coronagraph (LASCO) instrument on the Solar and Heliospheric Observatory of blobs of plasma generated near the cusp region of the streamer belt has sparked the hope of gaining new insight into the genesis of the slow solar wind. The observations suggest that open and closed field lines reconnect near the cusp of the helmet streamer to form blobs of higher density plasmas that are ejected into the slow solar wind. Models of this process have been suggested, and simulation studies have been conducted. We take the recent simulation work of Einaudi et al. as our starting point to investigate the formation of the slow solar wind and of the blobs observed by LASCO. We report two main conclusions. First, our results confirm the overall scenario suggested in the above paper (Einaudi et al.), which is that reconnection indeed causes the formation of blobs and the acceleration of the slow solar wind. Second, by employing more realistic two-dimensional configurations including the cusp and bent field lines with fast solar wind flow, we find that reconnection giving rise to plasma blobs is driven by converging flows with reconnection rates largely insensitive to changes in resistivity.

Slow shock interactions in the heliosphere using an adaptive grid MHD model

Abstract. A one-dimensional (1-D), time-dependent, adaptive-grid MHD model with solar wind structure has been used in the past to study the interaction of shocks. In the present study, we wish to study some fundamental processes that may be associated with slow shock genesis and their possible interactions with other discontinuities. This adaptive-grid model, suitable for appropriate spatial and temporal numerical simulations, is used for this purpose because its finer grid sizes in the vicinity of the steep gradients at shocks make it possible to delineate the physical parameters on both sides of the shocks. We found that a perturbation with deceleration of solar wind will generate an ensemble consisting of a forward slow shock, a fast forward wave and a reverse slow shock. On the other hand, a perturbation with an increase in acceleration of solar wind will generate both a slow shock and a fast shock. These two perturbations, although not unique, may be representative of momentum and pressure changes at the solar surface. During the transition of a fast shock overtaking a slow shock from behind, the slow shock might disappear temporarily. Also, during the process of the merging of two slow shocks, a slow shock-like structure is formed first; later, the slow shock-like structure evolves into an intermediate shock-like structure. This intermediate shock-like structure then evolves into an intermediate wave and a slow shock-like structure. Finally, the slow shock-like structure evolves into a slow shock, but the intermediate wave disappears by interacting with the non-uniform solar wind. This complex behavior demonstrates the non-unique nature of the formation of slow shocks, intermediate shocks and their derivative structures. We emphasize the main aim of this work to be both: (a) non-unique input physical parameters to explain the paucity of observed slow shocks, as well as (b) the impossibility of backward tracing to the history of input boundary conditions in view of the present inability to describe unambiguous inputs at the Sun.

Solar wind electron temperature and density measurements on the Solar Orbiter with thermal noise spectroscopy

The measurement of the solar wind electron temperature in the unexplored region between 1 and 45 R s is of prime importance for understanding the solar wind acceleration. Solar Orbiter’s location, combined with the fact that the spacecraft will nearly co-rotate with the sun on some portions of its orbit, will furnish observations placing constraints on solar wind models. We discuss the implementation of the plasma thermal noise analysis for the Solar Orbiter, in order to get accurate measurements of the total electron density and electron temperature and to correct the spacecraft charging effects which affect the electron analyzers.

Properties of solar wind turbulence near the Sun as deduced from coronal radio sounding experiments

The characteristics of plasma turbulence in the inner solar wind, as deduced from radio frequency fluctuation measurements recorded during solar conjunction, are reviewed. Special emphasis is placed on results from the radio occultation experiments performed with the Galileo and Ulysses spacecraft in the interval 1991–2000. Estimates of the power spectral index are obtained over the range of heliocentric distances 5 R ⊙ < R < 80 R ⊙ and the radial evolution of this parameter is discussed. Low-heliolatitude data from Galileo are compared with high-heliolatitude data from Ulysses during the period of solar activity minimum. Unusual power spectra with a sharp break at high fluctuation frequencies are detected for the first time in the solar wind acceleration region. The observations are interpreted within the framework of a theoretical model based on local generation of density fluctuations via nonlinear interactions of Alfvén waves propagating away from the Sun.

Relation between solar wind velocity and properties of its source region

Abstract Different kinds of coronal holes are sources of different kind of solar winds. A successful solar wind acceleration model should be able to explain all those solar winds. For the modeling it is important to find a universal relation between the solar wind physical parameters, such as velocity, and coronal physical parameters such as magnetic field energy. To clarify the physical parameters which control the solar wind velocity, we have studied the relation between solar wind velocity and properties of its source region such as photospheric/coronal magnetic field and the size of each coronal hole during the solar minimum. The solar wind velocity structures were derived by using interplanetary scintillation tomography obtained at Solar-Terrestrial Environment Laboratory, Japan. Potential magnetic fields were calculated to identify the source region of the solar wind. HeI 1083 nm absorption line maps obtained at Kitt Peak National Solar Observatory were used to identify coronal holes. As a result, we found a relation during solar minimum between the solar wind velocity and the coronal magnetic condition which is applicable to different kind of solar winds from different kind of coronal holes.

Intermittent Heating of the Solar Corona by Heat Flux–generated Ion Cyclotron Waves

Recently, we suggested that the source of ion heating in solar coronal holes is small-scale reconnection events (microflares) at the coronal base. The microflares launch intermittent heat flux up into the corona exciting ion cyclotron waves through a plasma microinstability. The ions are heated by these waves during the microflare bursts and then evolve with no energy input between the bursts. In this paper, we show that the structure of the proton distribution in the relatively long time periods between the microflares is determined by collisions at small heliocentric distances. At greater distances, the collisional processes can be replaced by similar processes due to secondary instabilities. These are excited by the distortion of the distribution under the action of the mirror force. At the same time, the heating during the microflare bursts is not affected by either the collisions or the secondary instabilities because of the short duration of the bursts. We demonstrate that in each intermittent heating event the protons diffuse approximately along one-dimensional curves in the phase space and can develop a quasi-plateau. The corresponding temperature increase can then be calculated without solving the diffusion equations. The overall coronal heating by this mechanism is a summed effect of all microflare bursts during the expansion time of the solar wind and adiabatic cooling between the microflares. The calculations for the collision-dominated region suggest that the overall heating is efficient enough to account for the acceleration of the fast solar wind in this region.

Three‐fluid model of the heating and acceleration of the fast solar wind

A new three‐fluid, two‐dimensional, wave‐driven model that includes, for the first time, heat conduction, viscous, and resistive dissipation for protons and electrons in two‐dimensional coronal hole is presented. The fast solar wind model includes electron, proton, and He++ or O5+ ion fluids. The heating of the solar wind plasma due to MHD waves is modeled as follows: A broadband spectrum of low‐frequency Alfvén waves is launched from the base of the corona. The waves deposit momentum and heat into the plasma to produce the fast solar wind. The values of the resistivity and shear viscosity coefficients required to produce the hot and fast solar wind consistent with observations are orders of magnitude larger than classical values. An empirical heating term that represents the contribution of additional heating processes, such as resonant heating by ion cyclotron waves, is included for the heavy ions and for protons in three out of four cases in the present study.

A Transonic Collisionless Model of the Solar Wind

Because of the semicollisional nature of the solar wind, the collisionless or exospheric approach and the hydrodynamic one are both inaccurate. However, the advantage of simplicity makes them useful for enlightening us on some basic mechanisms of solar wind acceleration. Previous exospheric models have been able to reproduce winds that were already nearly supersonic at the exobase, the altitude above which there are no collisions. In order to allow transonic solutions, a lower exobase has to be considered, in which case the protons are experiencing a nonmonotonic potential energy profile. This is done in the present work. In this model, the electron velocity distribution in the corona is assumed to be nonthermal. Parametric results are presented and show that the high acceleration obtained does not depend on the details of the nonthermal distributions. This acceleration seems, therefore, to be a robust result produced by the presence of a sufficient number of suprathermal electrons. A method for improving the exospheric description is also given, which consists of mapping particle orbits in terms of their invariants of motion.

The Magnetic Structure of Hα Macrospicules in Solar Coronal Holes

Measurements by Ulysses in the high-speed polar solar wind have shown the wind to carry some fine-scale structures in which the magnetic field reverses direction by having a switchback fold in it. The lateral span of these magnetic switchbacks, translated to the Sun, is of the scale of the lanes and cells of the magnetic network in which the open magnetic flux of the polar coronal hole and polar solar wind are rooted. This suggests that the magnetic switchbacks might be formed from network-scale magnetic loops that erupt into the corona and then undergo reconnection with the open field. This possibility motivated us to undertake the study reported here of the structure of H-alpha macrospicules observed at the limb in polar coronal holes, to determine whether a significant fraction of these eruptions appear to be erupting loops. From a search of the polar-coronal holes in 6 days of image-processed full-disk H-alpha movies from Big Bear Solar Observatory, we found a total of 35 macrospicules. Nearly all of these (32) were of one or the other of two different forms: 15 were in the form of an erupting loop, and 17 were in the form of a single-column spiked jet. The erupting-loop macrospicules are appropriate for producing the magnetic switchbacks in the polar wind. The spiked-jet macrospicules show the appropriate structure and evolution to be driven by reconnection between network-scale closed field (a network bipole) and the open field rooted against the closed field. This evidence for reconnection in a large fraction of our macrospicules (1) suggests that many spicules may be generated by similar but smaller reconnection events, and (2) supports the view that coronal heating and solar wind acceleration in coronal holes and in quiet regions and corona are driven by explosive reconnection events in the magnetic network.

Origin and Acceleration of Fast and Slow Solar Wind

Before the advent of the Solar and Heliospheric Observatory (SOHO, launched in 1995), we had little information on how coronal plasma gets accelerated to the high speed measured in situ. The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO, acquiring UV data over the first few solar radii, a region unexplored in this spectral range, allowed us to build a profile of the outflow plasma speed vs. heliocentric distance, based on empirical constraints. Still, much has to be learnt about the behavior of solar wind in the extended corona. In the following, after briefly reviewing the general properties of the solar wind, I'll focus on controversial issues — like the identification of the sources of fast and slow wind, and the acceleration of fast vs. slow wind streams — and I'll illustrate recent contributions to the solution of these problems.