Sharp boundaries of solar wind plasma structures and their relationship to solar wind turbulence

Sharp (<10 min) and large (>20%) solar wind ion flux changes are common phenomena in turbulent solar wind plasma. These changes are the boundaries of small- and middle-scale solar wind plasma structures which can have a significant influence on Earth’s magnetosphere. These solar wind ion flux changes are typically accompanied by only a small change in the bulk solar wind velocity, hence, the flux changes are driven mainly by plasma density variations. We show that these events occur more frequently in high-density solar wind. A characteristic of solar wind turbulence, intermittency, is determined for time periods with and without these flux changes. The probability distribution functions (PDF) of solar wind ion flux variations for different time scales are calculated for each of these periods and compared. For large time scales, the PDFs are Gaussian for both data sets. For small time scales, the PDFs from both data set are more flat than Gaussian, but the degree of flatness is much larger for the data near the sharp flux change boundaries.     

Two-velocity structure observed in the inner solar wind

Measurements of the motion of plasma density inhomogeneities in the inner solar wind are presented. The speeds were estimated using a cross-correlation analysis of radio frequency fluctuations of the Galileo spacecraft measured simultaneously at widely spaced ground stations. The radial projections of the correlation baselines on the pattern plane were of the order of several thousand kilometers. For cross-correlation functions calculated with comparatively short averaging times, we find that a pronounced two-velocity configuration is occasionally observed over the range of heliocentric distances 20 R < R < 40 R. The typical mean speed for such observations is about 300–400 km/s and the difference between the two predominant speeds is about 150–200 km/s. These results may indicate that the density fluctuations are associated with slow magnetosonic waves propagating in opposite directions at the local speed of sound in the reference frame moving with the mean solar wind speed. Quite reasonable estimates of the solar wind speed and speed of sound are obtained from this model. Another possible explanation of the two-velocity structures is that two independent solar wind streams are present simultaneously along different segments of the radio ray path.     

Study of the solar Slow Sonic,Alfvén and Fast Magnetosonic transition surfaces

In this paper we study the shape, extend and time variations of the solar wind transition surfaces using the Lima and Priest (1993) hydrodynamic model adequately adapted for the case of the solar wind flow. The transition surfaces, namely the Slow (Sonic), the Alfvén, and the Fast Magnetosonic surface, are important boundaries around the Sun and play a crucial role in the development of the solar wind and the structure of the inner heliosphere. We determine the shape and dimension of these surfaces as a function of heliographic latitude using measurements from Ulysses spacecraft, and we also study their temporal variation using data from spacecrafts at 1 AU (OMNI database). Furthermore, we establish their dependence with the solar activity, demonstrating their shape and location for the last two solar cycles. From this we noticed that the temporal variation of all transition surfaces follows the 11-year solar cycle. Finally, from the OMNI database, we have studied the temporal variation over the past 40 years of the plasma β parameter, the kinetic to magnetic and the kinetic to thermal energy ratios, at a distance of 1 AU from the Sun.     

Effect of current sheets on the power spectrum of the solar wind magnetic field using a cell model

A puzzling observation of solar wind MHD turbulence is the often seen Kolmogorov scaling of k^-5/3$k^{-5/3}$, even though the solar wind MHD turbulence is dominated by Alfvénic fluctuations. Recently Li et al. (2011) proposed that the presence of current sheets may be the cause of the Kolmogorov scaling. Here, using a cell model of the solar wind we examine the effect of current sheets on the power spectrum of the solar wind magnetic field. We model the solar wind as multiple cells separated by current sheets. We prescribe the spectra of turbulent magnetic field in individual cells as IK-like and examine the spectra along trajectories that cross multiple boundaries. We find that these spectra become softer and are consistent with the Kolmogorov-scaling. Our finding supports our recent proposal of Li et al. (2011).     

A new approach to solar wind monitoring

The monitoring of solar wind parameters is a key problem of the space weather program. We are presenting a new solution of plasma parameter determination suitable for small and fast solar wind monitors. The first version will be launched during the SPECTR-R project into a highly elongated orbit with apogee ∼350,000 km. The method is based on simultaneous measurements of the total ion flux and ion integral energy spectrum by six identical Faraday cups. Three of them are dedicated to determination of the ion flow direction, the other three (equipped with control grids supplied by a retarding potential) are used for determination of the density, temperature, and speed of the plasma flow. The version under development is primarily designed for the measurements in the solar wind and tail magnetosheath, thus for velocities range from 270 to 750 km/s, temperatures from 1 to 30 eV, and densities up to 200 cm⁻³. However, the instrument design can be simply modified for measurements in other regions with a substantial portion of low-energy plasma as a subsolar magnetosheath, cusp or low-latitude boundary layer. Testing of the engineering model shows that the proposed method can provide reliable plasma parameters with a high time resolution (up to 8 Hz). The paper presents not only the method and its technical realization but it documents all advantages and peculiarities of the suggested approach.     

The characteristics of sharp (small-scale) boundaries of solar wind plasma and magnetic field structures

We investigate properties of large (>20%) and sharp (<10 min) solar wind ion flux changes using INTERBALL-1 and WIND plasma and magnetic field measurements from 1996 to 1999. These ion flux changes are the boundaries of small-scale and middle-scale solar wind structures. We describe the behavior of the solar wind velocity, temperature and interplanetary magnetic field (IMF) during these sudden flux changes. Many of the largest ion flux changes occur during periods when the solar wind velocity is nearly constant, so these are mainly plasma density changes. The IMF magnitude and direction changes at these events can be either large or small. For about 55% of the ion flux changes, the sum of the thermal and magnetic pressure are in balance across the boundary. In many of the other cases, the thermal pressure change is significantly more than the magnetic pressure change. We also attempted to classify the types of discontinuities observed.     

Flows and obstacles in the solar wind

Understanding the physics of the various disturbances in the solar wind is critical to successful forecasts of space weather. The STEREO mission promises to bring us new and deeper understanding of these disturbances. As we stand on the threshold of the first results from this mission, it is appropriate to review what we know about solar wind disturbances. Because of their complementary nature we discuss both the disturbances that arise within the solar wind due to the stream structure and coronal mass ejecta and the disturbances that arise when the solar wind collides with planetary obstacles, such as magnetospheres.     

Self-similar stochastic processes in solar wind turbulence

Solar wind data is used to estimate the autocorrelation function for the stochastic process x(τ) = y(t + τ) − y(t), considered as a function of τ, where y(t) is any one of the quantities B²(t), n_p(t)V²(t), or n_p(t). This process has stationary increments and a variance that increases like a power law τ^2γ where γ is the scaling exponent. For the kinetic energy density and the proton density the scaling exponent is close to the Kolmogorov value γ = 1/3, for the magnetic energy density it is slightly larger. In all three cases, it is shown that the autocorrelation function estimated from the data agrees with the theoretical autocorrelation function for a self-similar stochastic process with stationary increments and finite variance. This is far from proof, but it suggests that these stochastic processes may be self-similar for time scales in the small scale inertial range of the turbulence, that is, from approximately 10 to 10³ s.     

二维磁流体动力学平衡日冕

应用解析和数值相结合的方法,本文处理了球对称太阳引力场中具有磁场的非粘、可压缩、完全导电等离子体的二维磁流体动力学平衡问题.得到了两种不同的解:(1)一种相应于磁场具有闭合区和开场区的稳定、自洽的等离子体流动,开场区几个太阳半径之外等离子体径向速度达到大值,超过了局地声速和局地Alfén速度;(2)等离子体速度总是小于局地声速和局地Alfvén速度,并当径向距离很大时速度趋于零.     

On the time lag between solar wind dynamic parameters and solar activity UV proxies

The solar activity displays variability and periodic behaviours over a wide range of timescales, with the presence of a most prominent cycle with a mean length of 11 years. Such variability is transported within the heliosphere by solar wind, radiation and other processes, affecting the properties of the interplanetary medium. The presence of solar activity–related periodicities is well visible in different solar wind and geomagnetic indices, although their time lags with respect to the solar cycle lead to hysteresis cycles. Here, we investigate the time lag behaviour between a physical proxy of the solar activity, the Ca II K index, and two solar wind parameters (speed and dynamic pressure), studying how their pairwise relative lags vary over almost five solar cycles. We find that the lag between Ca II K index and solar wind speed is not constant over the whole time interval investigated, with values ranging from 6 years to $～$1 year (average 3.2 years). A similar behaviour is found also for the solar wind dynamic pressure. Then, by using a Lomb-Scargle periodogram analysis we obtain a 10.21-year mean periodicity for the speed and 10.30-year for the dynamic pressure. We speculate that the different periodicities of the solar wind parameters with respect to the solar 11-year cycle may be related to the overall observed temporal evolution of the time lags. Finally, by accounting for them, we obtain empirical relations that link the amplitude of the Ca II K index to the two solar wind parameters.     

Chaos and multifractals in the solar wind

By using the false-nearest-neighbours method, we have argued that the deterministic component of solar wind plasma dynamics should be low-dimensional. In fact, the results we have obtained using the method of topological embedding indicate that the behaviour of the solar wind can be approximately described by a low-dimensional chaotic attractor in the inertial manifold, which is a subspace of system phase space. We have also shown that the multifractal spectrum of the solar wind attractor is consistent with that for the multifractal measure of the self-similar generalized weighted Cantor set with two different scaling parameters and one probability measure parameter responsible for nonuniform compression in phase space and multifractality. The values of the parameters fitted also demonstrate that the complex solar wind system could only be weakly non-conservative (small dissipation) and quantify nonlinear dynamics; some parts of the attractor in phase space are visited much more frequently than other parts. In addition, to quantify the multifractality of space plasma intermittent turbulence, we consider that generalized Cantor set also in the context of scaling properties of solar wind turbulence. We investigate the resulting multifractal spectrum of a one-dimensional phenomenological model of turbulence cascade depending on its parameters, especially for asymmetric scaling. In particular, we have shown that intermittent pulses are stronger for the cascade model with two different scaling parameters. Even thought solar wind turbulence appears to be rather space filling, a better agreement with the data is obtained, especially for the negative index of generalized dimensions. Therefore we argue that there is a need to use a two-scale asymmetric cascade model. We hope that this generalized multifractal model will be a useful tool for analysis of intermittent turbulence in space plasmas. We thus believe that fractal analysis of chaotic systems could lead us to a deeper understanding of their nature, and maybe even to predict their seemingly unpredictable behaviour.     

Kinetic theory of sheath formation in solar wind plasma

We present a general self-consistent kinetic theory for plasma sheath formation in solar wind plasma. The theory could be applied to anisotropic, as well as to isotropic collisionless plasma without resorting to any simplifications, limitations, or assumptions, such as the necessary existence of a ‘pre-sheath’ region of ions acceleration to ensure the Bohm criterion. The kinetic framework is first applied to sheath formation around an arbitrary oriented planar absorbing surface, charged by solar wind anisotropic plasma, under the condition of negligible photoelectric effect. We then make use of our kinetic approach for the plane geometry in isotropic collisionless plasma, as a particular case of a planar electrode orientation parallel to plasma streaming velocity, also analyzing the sheath structure around spherical and cylindrical absorbing electrodes submerged in isotropic collisionless plasma. Obtained results demonstrate principal differences in spatial charge distributions in sheath regions between spherical or cylindrical electrodes of large size and an unbound planar surface submerged in isotropic plasma. In the case of a planar electrode, we directly compare results obtained in our kinetic and hydrodynamic theories and conventional hydrodynamic theory of plasma sheath formation. The outcome from the present study have direct implications to the analysis of plasma sheath structure and associated distribution in space of charged dust grains, which is relevant to the moon exploration near the optical terminator region or in shadowed craters in the moon.     

Correlations between neutral and ionized solar wind

We report results of a statistical study correlating ionized solar wind (ISW) fluxes observed by ACE during late 2000 and throughout 2001 with neutral solar wind (NSW) fluxes observed by IMAGE/LENA over the same period. The average correlation coefficient between the neutral and ionized solar wind is 0.66 with correlations greater than 0.80 occurring about 29% of the time. Correlations appear to be driven by high solar wind flux variability, similar to results obtained by in situ multi-spacecraft correlation studies. In this study, however, IMAGE remains inside the magnetosphere on over 95% of its orbits. As a function of day of year, or equivalently ecliptic longitude, the slope of the relationship between the neutral solar wind flux and the ionized solar wind flux shows an enhancement near the upstream direction, but the symmetry point appears shifted toward higher ecliptic longitudes than the interstellar neutral (ISN) flow direction by about 20°. The estimated peak interstellar neutral upstream density inside of 1 AU is about 7 × 10⁻³ cm⁻³.     

Coupling of earth’s magnetosphere,solar wind and lunar plasma environment

The Moon does not have significant atmosphere and magnetic field. So it was considered like a passive absorber of incoming plasma. The latest observation revealed that the significant deflected proton fluxes exist over magnetic anomalies at lunar surface. Such deflection implies that the magnetic anomalies may act as magnetosphere-like obstacles (mini-magnetospheres), modifying the upstream plasma.     

Charging of dust grains by anisotropic solar wind multi-component plasma

In this paper we study the charging process of small grain particles by anisotropic multi-component solar wind plasmas (electrons, protons and heavy ions), versus two-component (electron/proton) plasmas. We are focusing attention on the important characteristics of the charging process, namely the charging time, floating potential and current content as functions of plasma parameters such as He⁺⁺/H⁺ (α/p) number density and T_α/T_p temperature ratios of alpha particles to protons, as well as plasma streaming velocity v₀. Measured statistical properties of solar wind plasma parameters at 1 AU show considerable variations in α/p-temperature ratios from 1 to 10, in α/p-number density ratio from 0.01 to 0.35, as well as in values of streaming velocity v₀ from 200 km/s to 1000 km/s and more. Periods of these variations could last for several days each, leading to significant variability in the charging process, according to newly derived general analytical expressions. Numerical calculations performed for protons/alphas plasmas showed large disparity in the charging characteristics. For example, in anisotropic plasma, grain charging time varies up to 90% depending on α/p-particles temperature and number density ratios, whereas changes in floating potential are up to 40%. In contrast, in isotropic plasma, charging characteristic for grains do not change very much for the same plasma parameters variations, with charging time varying about 12% and floating potential only varying about 4%. It is also shown that in highly anisotropic plasma, with all ballistic electrons and ions, dust grains could not hold their charges, and characteristic discharged time is calculated. We note that the analysis is equally applicable to any sized body immersed in solar wind plasma.     

Statistical properties of the IMF clock angle in the solar wind with northward and southward interplanetary magnetic field based on ACE observation from 1998 to 2009: Dependence on the temporal scale of the solar wind

Utilizing ACE satellite observations from 1998 to 2009, we performed the elaborate study on the properties of the clock angle θ_CA (arctan(By/Bz) (?90° to 90°) of the interplanetary magnetic field (IMF) in the solar wind at 1?AU. The solar wind with northward IMF (NW-IMF) and southward IMF (SW-IMF) are analyzed, independently. Statistical analysis shows that the solar wind with SW-IMF and NW-IMF has similar properties in general, including their durations, the IMF Bz and By components, and the IMF θ_CA. Then, the solar wind with NW-IMF (SW-IMF) is classified into five different temporal scales according to the duration of the NW-IMF (SW-IMF), i.e., very-short wind of 10–30?min, short-scale wind of 0.5–1?h, moderate-scale wind of 1–3?h, long-scale wind of 3–5?h, and super-long wind >5?h. Our analysis reveals that the IMF θ_CA has a distinct decrease with increase of the temporal scale of the solar wind. Next, the solar wind is classified into two groups, i.e., the high-speed solar wind (>450?km/s) and the low-speed solar wind (<450?km/s). Our analysis indicates that the IMF θ_CA depends highly on the solar wind speed. Statistically, high-speed solar wind tends to have larger IMF θ_CA than low-speed solar wind. The evolutions of the solar wind and IMF with the solar activity are further studied, revealing no clear solar variation of the IMF θ_CA. Finally, we analyze the monthly variation of the IMF θ_CA. Superposed epoch result strongly suggests the seasonal variation of the IMF θ_CA.     

Radio sounding of the solar wind acceleration region with spacecraft signals

Data from coronal radio-sounding experiments carried out on various interplanetary spacecraft are used to derive the empirical radial dependence of solar wind velocity and density at heliocentric distances from 3 to 60 solar radii for heliolatitudes below 60° and for low solar activity. The radial dependencies of solar wind power and acceleration are derived from these results. Summaries of the radial behavior of characteristic parameters of the solar wind turbulence (e.g., the spectral index and the inner and outer turbulence scales), as well as the fractional density fluctuation, are also presented. These radio-sounding results provide a benchmark for models of the solar wind in its acceleration region.     

A MHD-turbulence model for solar corona

The disposition of energy in the solar corona has always been a problem of great interest. It remains an open question how the low temperature photosphere supports the occurence of solar extreme phenomena. In this work, a turbulent heating mechanism for the solar corona through the framework of reduced magnetohydrodynamics (RMHD) is proposed. Two-dimensional incompressible long time simulations of the average energy disposition have been carried out with the aim to reveal the characteristics of the long time statistical behavior of a two-dimensional cross-section of a coronal loop and the importance of the photospheric time scales in the understanding of the underlying mechanisms. It was found that for a slow, shear type photospheric driving the magnetic field in the loop self-organizes at large scales via an inverse MHD cascade. The system undergoes three distinct evolutionary phases. The initial forcing conditions are quickly “forgotten” giving way to an inverse cascade accompanied with and ending up to electric current dissipation. Scaling laws are being proposed in order to quantify the nonlinearity of the system response which seems to become more impulsive for decreasing resistivity. It is also shown that few, if any, qualitative changes in the above results occur by increasing spatial resolution.     

Gnevyshev gap, Forbush decreases, ICMEs and solar wind electric field: relationships

We have studied annual frequency distribution of the Forbush decreases for three solar cycles (20, 21, 22); most are associated with the fast ICMEs and SSCs. The frequency varies in step with the solar cycle but the distribution has a notable gap embedded in it, near the maximum of the cycle leading to two peaks in Forbush decreases per cycle. We show that the gap coincides with the epoch of solar polar field reversal. There is an indication of an odd/even cycle effect in the frequency distribution of Forbush decreases and the associated SSCs. We find that two peaks in Forbush decrease and SSC distributions are separated by the Gnevyshev gap; second peaks occur well before the onset of the high-speed streams in the descending phase of a cycle which do not cause Forbush decreases but do contribute to a peak in the geomagnetic activity index Ap. We compare Forbush decrease and SSC distributions with the corresponding distribution of the solar wind electric field and find that a large amplitude of the electric field of itself does not cause a Forbush decrease to occur unless it is also associated with a fast ICME/SSC.     

Changes in sea-level pressure over South Korea associated with high-speed solar wind events

We explore a possibility that the daily sea-level pressure (SLP) over South Korea responds to the high-speed solar wind event. This is of interest in two aspects: first, if there is a statistical association this can be another piece of evidence showing that various meteorological observables indeed respond to variations in the interplanetary environment. Second, this can be a very crucial observational constraint since most models proposed so far are expected to preferentially work in higher latitude regions than the low latitude region studied here. We have examined daily solar wind speed V, daily SLP difference ΔSLP, and daily log(BV²) using the superposed epoch analysis in which the key date is set such that the daily solar wind speed exceeds 800 km s⁻¹. We find that the daily ΔSLP averaged out of 12 events reaches its peak at day +1 and gradually decreases back to its normal level. The amount of positive deviation of ΔSLP is +2.5 hPa. The duration of deviation is a few days. We also find that ΔSLP is well correlated with both the speed of solar wind and log(BV²). The obtained linear correlation coefficients and chance probabilities with one-day lag for two cases are r ? 0.81 with P > 99.9%, and r ? 0.84 with P > 99.9%, respectively. We conclude by briefly discussing future direction to pursue.