Solar Wind Region Research Articles

Excitation of a magnetospheric MHD cavity by Kelvin-Helmholtz instability

The Kelvin-Helmholtz instability is investigated analytically by using a one-dimensional nonuniform model of the Earth’s magnetosphere and the adjacent solar wind region. Its properties are shown to be essentially governed by the presence of an MHD cavity that arises in the magnetosphere because of the non-uniformity of the latter and also because of the jump in the parameters of the medium at the magnetopause (the outer boundary of the magnetosphere). System oscillations constitute a discrete spectrum of eigenmodes, which are determined by the wave vector kt along the tangential discontinuity and also by the mode number n = 0, 1, 2, …, playing the role of the wavenumber along a coordinate normal to the magnetopause. Analytic expressions are obtained for the frequency and instability growth rate of each eigenmode and for the functions describing its spatial structure. All these quantities depend parametrically on the solar wind velocity VW, or more precisely, on the Doppler frequency shift ωW = kt · VW. For each eigenmode, there is a lower instability threshold depending on the parameter ωW and a sharp maximum in the growth rate at the eigenfrequency of the magnetospheric cavity. For ωW values below the threshold, the properties of an eigenmode are highly sensitive to the type of solar wind nonuniformity. Three cases are considered: a uniform solar wind and solar winds in which the speed of sound increases or decreases away from the magnetopause.

Understanding the Behavior of the Heliospheric Magnetic Field and the Solar Wind During the Unusual Solar Minimum Between Cycles 23 and 24

The properties of the heliospheric magnetic field and the solar wind were substantially different in the unusual solar minimum between Cycles 23 and 24: the magnetic-field strength was substantially reduced, as were the flow properties of the solar wind, such as the mass flux. Explanations for these changes are offered that do not require any substantial reconsideration of the general understandings of the behavior of the heliospheric magnetic field and the solar wind that were developed in the minimum of Cycle 22 – 23. Solar-wind composition data are used to demonstrate that there are two distinct regions of solar wind: solar wind likely to originate from the stalk of the streamer belt (the highly elongated loops that underlie the heliospheric current sheet), and solar wind from outside this region. The region outside the streamer-stalk region is noticeably larger in the minimum of Cycle 23 – 24; however, the increased area can account for the reduction in the heliospheric magnetic-field strength in this minimum. Thus, the total magnetic flux contained in this region is the same in the two minima. Various correlations among the solar-wind mass flux and coronal electron temperature inferred from solar-wind charge states were developed for the Cycle 22 – 23 solar minimum. The data for the minimum of Cycle 23 – 24 suggest that the correlations still hold, and thus the basic acceleration mechanism is unchanged in this minimum.

Structure of the earth’s magnetosheath at small fluctuations in the interplanetary magnetic field direction

Spatial structure of the magnetosheath of the Earth was studied under the conditions when no sharp (more than 40° during 5 min) changes in the interplanetary magnetic field direction were observed. On the basis of 24 flights of the Interball-1 satellite through the magnetosheath, it is found that three regions differing from each other by parameters of the field and plasma can be observed in the magnetosheath under the above-indicated conditions. These regions also differ from the solar wind region before front of the Earth’s magnetospheric bow shock. Empirical distributions of parameters were studied in each region. Taking into account the influence of the interplanetary magnetic field direction on the processes in the magnetosheath, the cases of quasi-perpendicular and quasi-parallel shock waves were considered separately. The study showed that the distribution of parameters in the selected regions (in the solar wind before front of the bow shock, in the magnetosheath behind the bow shock (post-shock), in the region of the magnetosheath with minimal fluctuations in the field, and in the inner magnetosheath) differ from each other at any interplanetary magnetic field direction.

Transport and diffusion of particles due to transverse drift waves

Transport and diffusion of plasma particles perpendicular and parallel to the magnetic field is discussed in the framework of the transverse drift wave theory. The starting model includes the density and magnetic field gradients perpendicular to the magnetic field vector. In such an inhomogeneous environment the transverse drift wave naturally develops. The transverse drift wave is a low frequency mode, with the frequency far below the ion gyro-frequency, it is driven by these gradients and it propagates perpendicular to them. The mode is also purely perpendicular to the magnetic field and it is electromagnetically transverse, which implies that when its wave vector is perpendicular to the magnetic field vector, the perturbed electric field is along the equilibrium magnetic field, while in the same time the perturbed magnetic field is in the direction of the background gradients. In application to the solar wind, it is shown that very small wave electric field amplitude, of the order of 10 −7 V/m, within one wave period can produce the drift of protons in both directions, perpendicular to the ecliptic plane and also along the background magnetic field, to distances measured in millions of kilometers. The electric field along the magnetic field vector implies particle acceleration in the same direction. When a critical threshold velocity of the particle is achieved, the particle motion becomes stochastic. This is a completely new nonlinear stochastic mechanism which follows from the very specific geometry of the transverse drift mode. Particle drift perpendicular to the magnetic field vector means a diffusion of particles, with the effective diffusion coefficient for ions that is at least 11 orders of magnitude larger than the classic diffusion coefficient. The features of this diffusion are: within certain time interval, initially faster particles will diffuse to larger distances, and the same holds for protons in comparison to heavier ions. For electrons the effective diffusion coefficient can easily match the one obtained from observations, i.e., to become of the order of 10 17 m 2 /s. It is also expected that the wave-induced stochastic motion will considerably increase the effective collision frequency in such an environment which is, with respect to its mean parameters, practically collision-lees. Hence, the solar wind regions affected by such a stochastic acceleration may show various unexpected features that are typical for collisional plasmas.

Spectropolarimetric forward modelling of the lines of the Lyman-series using a self-consistent, global, solar coronal model

Context. The presence and importance of the coronal magnetic field is illustrated by a wide range of phenomena, such as the abnormally high temperatures of the coronal plasma, the existence of a slow and fast solar wind, the triggering of explosive events such as flares and CMEs. Aims. We investigate the possibility of using the Hanle effect to diagnose the coronal magnetic field by analysing its influence on the linear polarisation, i.e. the rotation of the plane of polarisation and depolarisation. Methods. We analyse the polarisation characteristics of the first three lines of the hydrogen Lyman-series using an axisymmetric, self-consistent, minimum-corona MHD model with relatively low values of the magnetic field (a few Gauss). Results. We find that the Hanle effect in the above-mentioned lines indeed seems to be a valuable tool for analysing the coronal magnetic field. However, great care must be taken when analysing the spectropolarimetry of the Lα line, given that a non-radial solar wind and active regions on the solar disk can mimic the effects of the magnetic field, and, in some cases, even mask them. Similar drawbacks are not found for the Lβ and Lγ lines because they are more sensitive to the magnetic field. We also briefly consider the instrumental requirements needed to perform polarimetric observations for diagnosing the coronal magnetic fields. Conclusions. The combined analysis of the three aforementioned lines could provide an important step towards better constrainting the value of solar coronal magnetic fields.

COSMIC-RAY MODULATION BY THE GLOBAL MERGED INTERACTION REGION IN THE HELIOSHEATH

It was found by Voyager 1 and Voyager 2 that the Global Merged Interaction Region (GMIR) still has effects on cosmic-ray transport in the heliosheath. The GMIR produced by the intense solar activities of 2005 September propagated into the heliosheath in 2006, causing several decreases in the cosmic-ray flux at Voyager 1. Motivated by these observations, we investigate cosmic-ray modulation by the GMIR in the heliosheath using a three-dimensional Galactic cosmic-ray modulation code that includes the drift effect, termination shock, and a propagating GMIR. The simulation reproduces all major modulation features found by previous cosmic-ray observations in the supersonic solar wind region. However, when the simulation is made for the heliosheath region during a passage of the GMIR, the cosmic-ray modulation shows a new feature. The cosmic-ray fluxes at many locations in the heliosheath region experience two distinguished decreases. One coincides with the GMIR's arrival at the termination shock, the other with its arrival at the observing points. Based on this feature, we use the Voyager observations of the 2006 GMIR modulation events to derive the radial distance of the termination shock of ~91 AU and the GMIR shock propagation speed inside the heliosheath of ~172 km s-1 for this event.

Solar Activity Cycle in Solar-wind Sources and Flows

Experiments based on multi-source radio occultation measurements of the circumsolar plasma at R∼4.0−70RS were carried out during 1997 – 2008 to locate the inner boundary of the solar-wind transonic transition region, Rin. The data obtained were used to correlate the solar-wind stream structure and magnetic fields on the source surface (R=2.5RS) in the solar corona. The method of the investigation is based on the analysis of the dependence Rin=F(|BR|) in the correlation diagrams, where Rin is the inner boundary of the solar-wind transition region and |BR| is the intensity of the magnetic field at the source surface. On such diagrams, the solar wind is resolved into discrete branches, streams of different types. The analysis of the stream types using a continuous series of data from 1997 to 2008 allowed us to propose a physical criterion for delimiting the epochs in the current activity cycle.

COMPARING SHOCKS IN PLANETARY NEBULAE WITH THE SOLAR WIND TERMINATION SHOCK

We show that suprathermal particles, termed pick-up ions (PUIs), might reduce the postshock temperature of the fast wind and jets in some planetary nebulae (PNs) and in symbiotic systems. The goal is to explain the finding that the temperature of the hot bubble formed by the post-shock gas in some PNs and symbiotic nebulae is lower, sometimes by more than an order of magnitude, than the value expected from simple hydrodynamical calculations. Although various explanations have been proposed, there is as yet no prefered solution for this low tempeature problem. PUIs have been invoked to explain the low temperature behind the termination shock of the solar wind. While in the case of the solar wind the neutral atoms that turn into PUIs penetrate the pre-shock solar wind region from the interstellar medium (ISM), in PNs the PUI source is more likely slowly moving clumps embedded in the fast wind or jets. These clumps are formed by instabilities or from backflowing cold gas. Our estimates indicate that in young PNs these PUIs will thermalize before leaving the system. Only in older PNs whose sizes exceed ~5000 AU and for which the fast-wind mass loss rate is below ~10^{-7} Mo/yr do we expect the PUIs to be an efficient carrier of energy out of the postshock region (the hot bubble).

Frequency fluctuation of spacecraft radio signals in the circumsolar plasma

Frequency fluctuation data of monochromatic radio waves propagating through the circumsolar plasma in 1975–2002 are systematized and analyzed. The radial dependences of the intensity of the frequency fluctuations are obtained for the decimeter radio waves in the circumsolar plasma from the results of radio sounding using the Ulysses and Galileo spacecrafts. It is demonstrated that the radial profile of the rms frequency fluctuations can be approximated by a power-law function whose exponent is determined by the intensity of the plasma inhomogeneities, the velocity of solar wind, the spectral index of the spatial spectrum of the plasma fluctuations, and the outer scale of turbulence. It is also shown that three different frequency fluctuation regimes are found in the solar wind acceleration region and in the inner and outer regions of the stable solar wind.

Observation of the multifractal spectrum in solar wind turbulence by Ulysses at high latitudes

The aim of our study is to examine the question of multifractal scaling properties of turbulence in the solar wind at high latitudes. We analyze time series of the velocities of the solar wind during solar minimum (1994–1997, 2006–2007) at various heliographic latitudes measured in situ by Ulysses, which is the only mission that has investigated parameters of the solar wind out of the ecliptic plane also in the polar regions of the Sun. We consider the non‐homogeneous energy transfer rate in the turbulent cascade leading to the phenomenon of intermittency. To quantify the degree of multifractality and the degree of asymmetric scaling of solar wind turbulence, we consider a generalized two‐scale weighted Cantor set with two different scales describing nonuniform distribution of the kinetic energy flux between cascading eddies of various sizes. It is worth noting that both characteristics exhibit latitudinal dependence with some symmetry with respect to the ecliptic plane. Generally, at high latitudes during solar minimum in the fast solar wind streams we observe a somewhat smaller degree of multifractality and intermittency as compared with those at the ecliptic and a roughly symmetric multifractal singularity spectrum. The minimum of intermittency is observed at midlatitudes, possibly related to the transition from the region where the interaction of the fast and slow streams takes place to a more homogeneous region of the pure fast solar wind.

A COMPARISON OF ELEMENTAL ABUNDANCE RATIOS IN SEP EVENTS IN FAST AND SLOW SOLAR WIND REGIONS

Abstract : The solar energetic (E > 1 MeV per nucleon) particles (SEPs) observed in gradual events at 1 AU are assumed to be accelerated by coronal/interplanetary shocks from ambient thermal or suprathermal seed particles. If so, then the elemental abundances of SEPs produced in different solar wind (SW) stream types (transient, fast, and slow) might be systematically distinguished from each other. We look for these differences in SEP energy spectra and in elemental abundance ratios (including Mg/Ne and Fe/C, which compare low/high first ionization potential elements), in a large number of SEP time intervals over the past solar cycle. The SW regions are characterized by the three-component stream classification of Richardson et al. Our survey shows no significant compositional or energy spectral differences in the 5-10 MeV per nucleon range for SEP events of different SW stream types. This result extends the earlier finding that SEP events are observed frequently in fast SW streams, although their higher Alfven and SW flow speeds should constrain SEP production by coronal mass ejection-driven shocks in those regions. We discuss the implications of our results for shock seed populations and cross-field propagation.

THREE-DIMENSIONAL FEATURES OF THE OUTER HELIOSPHERE DUE TO COUPLING BETWEEN THE INTERSTELLAR AND INTERPLANETARY MAGNETIC FIELDS. III. THE EFFECTS OF SOLAR ROTATION AND ACTIVITY CYCLE

We investigate the effects of the 11 year solar cycle and 25 day rotation period of the Sun on the interaction of the solar wind (SW) with the local interstellar medium (LISM). Our models take into account the partially ionized character of the LISM and include momentum and energy transfer between the ionized and neutral components. We assume that the interstellar magnetic field vector belongs to the hydrogen deflection plane as discovered in the SOHO SWAN experiment. This plane is inclined at an angle of about 60° toward the ecliptic plane of the Sun, as suggested in recent publications relating the local interstellar cloud properties to the radio emission observed by Voyager 1. It is assumed that the latitudinal extent of the boundary between the slow and fast SW regions, as well as the angle between the Sun's rotation and magnetic-dipole axes, are periodic functions of time, while the polarity of the interstellar magnetic field changes sign every 11 years at the solar maximum. The global variation of the SW-LISM interaction pattern, the excursions of the termination shock and the heliopause, and parameter distributions in certain directions are investigated. The analysis of the behavior of the wavy heliospheric current sheet in the supersonic SW region shows the importance of neutral atoms on its dynamics.

Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock

The solar wind blows outward from the Sun and forms a bubble of solar material in the interstellar medium. The termination shock occurs where the solar wind changes from being supersonic (with respect to the surrounding interstellar medium) to being subsonic. The shock was crossed by Voyager 1 at a heliocentric radius of 94 au (1 au is the Earth-Sun distance) in December 2004 (refs 1-3). The Voyager 2 plasma experiment observed a decrease in solar wind speed commencing on about 9 June 2007, which culminated in several crossings of the termination shock between 30 August and 1 September 2007 (refs 4-7). Since then, Voyager 2 has remained in the heliosheath, the region of shocked solar wind. Here we report observations of plasma at and near the termination shock and in the heliosheath. The heliosphere is asymmetric, pushed inward in the Voyager 2 direction relative to the Voyager 1 direction. The termination shock is a weak, quasi-perpendicular shock that heats the thermal plasma very little. An unexpected finding is that the flow is still supersonic with respect to the thermal ions downstream of the termination shock. Most of the solar wind energy is transferred to the pickup ions or other energetic particles both upstream of and at the termination shock.

Interplanetary origin of multiple‐dip geomagnetic storms

In this paper, we have systematically investigated the interplanetary drivers of major dips during intense (Dst ≤ −100 nT) geomagnetic storms in 1996–2006. A major dip is defined as a temporary decrease in Dst index with amplitude larger than 14.5 nT. Multiple dips result in a storm if regions of geoeffective solar wind with strong southward magnetic fields are separated by less geoeffective solar wind. Among these 90 intense storms, we found that only 34% (31 events) showed a classical “one‐dip” profile, while 49% (44 events) had two dips. Another 17% (15 events) had triple or more dips. We found that of a total of 165 major dips associated with the 90 storms, about 45% (74 dips) were caused by interplanetary coronal mass ejections (ICMEs), or ejecta, and 30% (49 dips) were caused by sheaths (SHs) that lie between shocks driven by ICMEs and leading edges of the ICMEs. About 7% (11 dips) were caused by a shock driven by an ICME running into a preceding ICME and intensifying its magnetic field (PICME‐SH). About 11% (18 dips) were due to corotating interaction regions (CIRs) formed by the interaction of high‐speed solar wind from coronal holes with the preceding slower solar wind. Another 7% (12 dips) were caused by various solar wind structures prior the onset of the storm. Among these different types of drivers, the largest storms dips on average were produced by shocks propagating through preceding ICMEs (PICME‐SH). One frequent cause of a two‐dip storm is that the first dip is produced by the upstream sheath and the second dip is produced by the driving ICME. Another common cause of a two‐dip or multiple‐dip storm is the presence of multiple subregions of southward magnetic field within a complex solar wind flow, resulting from two successive, closely spaced ICMEs.

Termination Shock Asymmetries as Seen by theVoyagerSpacecraft: The Role of the Interstellar Magnetic Field and Neutral Hydrogen

We show that asymmetries of the termination shock due to the influence of the interstellar magnetic field (ISMF) are considerably smaller in the presence of neutral hydrogen atoms, which tend to symmetrize the heliopause, the termination shock, and the bow shock due to charge exchange with charged particles. This leads to a much stronger restriction on the ISMF direction and its strength. We demonstrate that in the presence of the interplanetary magnetic field the plane defined by the local interstellar medium (LISM) velocity and magnetic field vectors does not exactly coincide with the plane defined by the interstellar neutral helium and hydrogen velocity vectors in the supersonic solar wind region, which limits the accuracy of the inferred direction of the ISMF. We take into account the tilt of the LISM velocity vector with respect to the ecliptic plane and show that magnetic fields as strong as 3 μG or greater may be necessary to account for the observed asymmetry. Estimates are made of the longitudinal streaming anisotropy of energetic charged particles at the termination shock caused by the nonalignment of the interplanetary magnetic field with its surface. By investigating the behavior of interplanetary magnetic field lines that cross the Voyager 1 trajectory in the inner heliosheath, we estimate the length of the trajectory segment that is directly connected by these lines to the termination shock. A possible effect of the ISMF draping over the heliopause is discussed in connection with radio emission generated in the outer heliosheath.

Statistics of low-frequency variations in solar wind, foreshock and magnetosheath: INTERBALL-1 and CLUSTER data

The properties of small-scale ion flux and magnetic field fluctuations in the regions of undisturbed solar wind, foreshock and magnetosheath have been investigated. The systematic measurements of these parameters with very high time resolution, which were obtained onboard INTERBALL-1 spacecraft during 1996–2000, make it possible to examine their variations in a broad range of timescales (1–1800 s). The results from similar analysis of CLUSTER data are then compared with those from INTERBALL-1 data. The magnetic field variations from these two different satellites data are consistent not only in quality but also in quantity. Statistical results show that the level of the relative variations of parameters in magnetosheath and foreshock is about 3 times larger than those in undisturbed solar wind. Very intensive fluctuations in the magnetosheath are observed even under the “quiet” conditions of upstream solar wind. Properties of the small-scale fluctuations in the magnetosheath are strongly controlled by the angle Θ Bn between the interplanetary magnetic field vector and the bow shock normal, i.e. the level of fluctuations grows while the angle Θ Bn decreases. Wave properties of the plasma and magnetic field fluctuations are also very different in undisturbed solar wind, foreshock, quasi-parallel and quasi-perpendicular magnetosheath.

Small-scale ion flux and magnetic field fluctuations in solar wind, foreshock and magnetosheath

We have continued investigation of waves in the regions of undisturbed solar wind, foreshock and magnetosheath. The analysis of ion flux and magnetic field variations with the time interval 1–240s was performed in the regions above. Very large variation in such a time interval can be considered the common feature of the foreshock and magnetosheath. The results of case and statistical studies showed that the level of relative variations of ion flux and magnetic field magnitude in foreshock is about 3 times larger than in undisturbed solar wind. Variations of these parameters in the magnetosheath topologically connected with the quasi-parallel bow shock are about two times larger than those behind the quasi-perpendicular. We also compared the results from Interball-1 data analysis with those from statistical analysis of cluster magnetic field measurements. The magnetic field variations obtained from the different satellite data coincide with each other very well not only in quality but also in quantity.

Abundance Variation at the Vicinity of an Active Region and the Coronal Origin of the Slow Solar Wind

The solar wind ion composition is generally frozen-in within 5 R☉ of the Sun. Many characteristics in the elemental abundances measured in the solar wind are believed to be set at the chromospheric and low coronal levels. By connecting plasma parameters in the coronal and in situ solar wind data, valuable insights can be obtained as to how and where the solar wind is produced. Elemental abundance is found to vary across solar structures and heliocentric heights, as well as in the solar wind. Therefore, solar wind elemental composition data (such as those from SWICS on board ACE), combined with spectroscopic observations of the inner corona (such as those from UVCS on board SOHO), are ideal for investigating the coronal origin of the solar wind. We present such joint analysis using data from SOHO UVCS and ACE SWICS. In 1999 October, UVCS observed the west limb corona at 1.64 R☉ for 7 days with the passing of an equatorial coronal hole followed by an active region complex. This corresponds to a rarefaction transition from fast to slow wind as measured by ACE. This presents an unprecedented opportunity for comparing the variations in the coronal plasma properties with those in the solar wind. We present the analysis for the electron temperature, density, and elemental abundances during this period. We compare these properties between the two data sets and discuss the implications on the formation region of the slow solar wind. Treatment of the line-of-sight effect in the UVCS data, along with 3D MHD coronal field models, are employed to connect the coronal and solar wind data.

Apparent angular sizes of the sources of microwave subsecond pulses and electron-density fluctuations in the lower solar corona

Data on the visible angular sizes of sources of microwave subsecond pulses (MSPs) obtained using the Siberian Solar Radio Telescope are analyzed assuming a dominant role for scattering on small-scale electron-density inhomogeneities in the solar corona. The observed dependence of the angular sizes of MSPs on the distance from the solar-disk center confirms that the MSP sources are localized in low layers of the solar corona. Both absolute and fractional levels of small-scale electron-density fluctuations have been estimated. These estimates suggest that flicker-noise-type turbulence power spectra are formed in the lower corona, and are preserved in the solar-wind acceleration region. A composite dependence of the scattering angle of a sounding radio wave on distance from the Sun is presented.

Is There Any Evident Effect of Coronal Holes on Gradual Solar Energetic Particle Events?

Gradual solar energetic particle (SEP) events are thought to be produced by shocks, which are usually driven by fast coronal mass ejections (CMEs). The strength and magnetic field configuration of the shock are considered the two most important factors for shock acceleration. Theoretically, both of these factors should be unfavorable for producing SEPs in or near coronal holes (CHs). Meanwhile, CMEs and CHs could impact each other. Thus, to answer the question whether CHs have real effects on the intensities of SEP events produced by CMEs, a statistical study is performed. First, a brightness gradient method is developed to determine CH boundaries. Using this method, CHs can be well identified, eliminating any personal bias. Then 56 front-side fast halo CMEs originating from the western hemisphere during 1997-2003 are investigated as well as their associated large CHs. It is found that neither CH proximity nor CH relative location manifests any evident effect on the proton peak fluxes of SEP events. The analysis reveals that almost all of the statistical results are significant at no more than one standard deviation, σ. Our results are consistent with the previous conclusion suggested by Kahler that SEP events can be produced in fast solar wind regions and there is no requirement for those associated CMEs to be significantly faster.