Fast Solar Wind Research Articles

Noble gas elemental abundances in three solar wind regimes as recorded by the Genesis mission

We discuss elemental abundances of noble gases in targets exposed to the solar wind (SW) onboard the “Genesis” mission during the three different SW “regimes”: “Slow” (interstream, IS) wind, “Fast” (coronal hole, CH) wind and solar wind related to coronal mass ejections (CME). To this end we first present new Ar, Kr, and Xe elemental abundance data in Si targets sampling the different regimes. We also discuss He, Ne, and Ar elemental and isotopic abundances obtained on Genesis regime targets partly published previously. Average Kr/Ar ratios for all three regimes are identical to each other within their uncertainties of about 1% with one exception: the Fast SW has a 12% lower Xe/Ar ratio than do the other two regimes. In contrast, the He/Ar and Ne/Ar ratios in the CME targets are higher by more than 20% and 10%, respectively, than the corresponding Fast and Slow SW values, which among themselves vary by no more than 2–4%. Earlier observations on lunar samples and Genesis targets sampling bulk SW wind had shown that Xe, with a first ionisation potential (FIP) of ∼12 eV, is enriched by about a factor of two in the bulk solar wind over Ar and Kr compared to photospheric abundances, similar to many “low FIP” elements with a FIP less than ∼10 eV. This behaviour of the “high FIP” element Xe was not easily explained, also because it has a Coulomb drag factor suggesting a relatively inefficient feeding into the SW acceleration region and hence a depletion relative to other high FIP elements such as Kr and Ar. The about 12% lower enrichment of Xe in Genesis’ Fast SW regime observed here is, however, in line with the hypothesis that the depletion of Xe in the SW due to the Coulomb drag effect is overcompensated as a result of the relatively short ionisation time of Xe in the ion-neutral separation region in the solar chromosphere. We will also discuss the rather surprising fact that He and Ne in CME targets are quite substantially enriched (by 20% and 10%, respectively) relative to the other solar wind regimes, but that this enrichment is not accompanied by an isotopic fractionation. The Ne isotopic data in CMEs are consistent with a previous hypothesis that isotopic fractionation in the solar wind is mass-dependent.

Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind

We present three-dimensional direct numerical simulations and an analytic model of reflection-driven magnetohydrodynamic (MHD) turbulence in the solar wind. Our simulations describe transverse, non-compressive MHD fluctuations within a narrow magnetic flux tube that extends from the photosphere, through the chromosphere and corona and out to a heliocentric distance $r$of 21 solar radii $(R_{\odot })$. We launch outward-propagating ‘$\boldsymbol{z}^{+}$fluctuations’ into the simulation domain by imposing a randomly evolving photospheric velocity field. As these fluctuations propagate away from the Sun, they undergo partial reflection, producing inward-propagating ‘$\boldsymbol{z}^{-}$fluctuations’. Counter-propagating fluctuations subsequently interact, causing fluctuation energy to cascade to small scales and dissipate. Our analytic model incorporates dynamic alignment, allows for strongly or weakly turbulent nonlinear interactions and divides the$\boldsymbol{z}^{+}$fluctuations into two populations with different characteristic radial correlation lengths. The inertial-range power spectra of$\boldsymbol{z}^{+}$and$\boldsymbol{z}^{-}$fluctuations in our simulations evolve toward a$k_{\bot }^{-3/2}$scaling at$r>10R_{\odot }$, where$k_{\bot }$is the wave-vector component perpendicular to the background magnetic field. In two of our simulations, the$\boldsymbol{z}^{+}$power spectra are much flatter between the coronal base and$r\simeq 4R_{\odot }$. We argue that these spectral scalings are caused by: (i) high-pass filtering in the upper chromosphere; (ii) the anomalous coherence of inertial-range$\boldsymbol{z}^{-}$fluctuations in a reference frame propagating outwards with the$\boldsymbol{z}^{+}$fluctuations; and (iii) the change in the sign of the radial derivative of the Alfvén speed at$r=r_{\text{m}}\simeq 1.7R_{\odot }$, which disrupts this anomalous coherence between$r=r_{\text{m}}$and$r\simeq 2r_{\text{m}}$. At$r>1.3R_{\odot }$, the turbulent heating rate in our simulations is comparable to the turbulent heating rate in a previously developed solar-wind model that agreed with a number of observational constraints, consistent with the hypothesis that MHD turbulence accounts for much of the heating of the fast solar wind.

Three-dimensional Simulation of the Fast Solar Wind Driven by Compressible Magnetohydrodynamic Turbulence

Abstract Using a three-dimensional compressible magnetohydrodynamic (MHD) simulation, we have reproduced the fast solar wind in a direct and self-consistent manner, based on the wave/turbulence-driven scenario. As a natural consequence of Alfvénic perturbations at the coronal base, highly compressional and turbulent fluctuations are generated, leading to heating and acceleration of the solar wind. The analysis of power spectra and structure functions reveals that the turbulence is characterized by its imbalanced (in the sense of outward Alfvénic fluctuations) and anisotropic nature. The density fluctuation originates from the parametric decay instability (PDI) of outwardly propagating Alfvén waves and plays a significant role in the Alfvén-wave reflection that triggers turbulence. Our conclusion is that the fast solar wind is heated and accelerated by compressible MHD turbulence driven by PDI and resultant Alfvén-wave reflection.

Thermodynamics of pure fast solar wind: radial evolution of the temperature–speed relationship in the inner heliosphere

ABSTRACT A strong correlation between speed and proton temperature has been observed, across many years, on hourly averaged measurements in the solar wind. Here, we show that this relationship is also observed at a smaller scale on intervals of a few days, within a single stream. Following the radial evolution of a well-defined stream of coronal-hole plasma, we show that the temperature–speed (T–V) relationship evolves with distance, implying that the T–V relationship at 1 au cannot be used as a proxy for that near the Sun. We suggest that this behaviour could be a combination of the anticorrelation between speed and flux-tube expansion factor near the Sun and the effect of a continuous heating experienced by the plasma during the expansion. We also show that the cooling index for the radial evolution of the temperature is a function of the speed. In particular, T⊥ in faster wind, although higher close to the Sun, decreases more quickly with respect to slower wind, suggesting that it has less time to interact with the mechanism(s) able to heat the plasma. Finally, we predict the expected T–V relationship in fast streams closer to the Sun with respect to the Helios observations, which Parker Solar Probe will explore in the near future.

Quasilinear approach of the cumulative whistler instability in fast solar wind: Constraints of electron temperature anisotropy

Context. Solar outflows are a considerable source of free energy that accumulates in multiple forms such as beaming (or drifting) components, or temperature anisotropies, or both. However, kinetic anisotropies of plasma particles do not grow indefinitely and particle-particle collisions are not efficient enough to explain the observed limits of these anisotropies. Instead, self-generated wave instabilities can efficiently act to constrain kinetic anisotropies, but the existing approaches are simplified and do not provide satisfactory explanations. Thus, small deviations from isotropy shown by the electron temperature (T) in fast solar winds are not explained yet. Aims. This paper provides an advanced quasilinear description of the whistler instability driven by the anisotropic electrons in conditions typical for the fast solar winds. The enhanced whistler-like fluctuations may constrain the upper limits of temperature anisotropy A ≡ T⊥/T∥ > 1, where ⊥, ∥ are defined with respect to the magnetic field direction. Methods. We studied self-generated whistler instabilities, cumulatively driven by the temperature anisotropy and the relative (counter)drift of electron populations, for example, core and halo electrons. Recent studies have shown that quasi-stable states are not bounded by linear instability thresholds but an extended quasilinear approach is necessary to describe these quasi-stable states in this case. Results. Marginal conditions of stability are obtained from a quasilinear theory of cumulative whistler instability and approach the quasi-stable states of electron populations reported by the observations. The instability saturation is determined by the relaxation of both the temperature anisotropy and relative drift of electron populations.

Evolution of Relative Drifts in the Expanding Solar Wind: Helios Observations

Ion velocity distribution functions (VDFs) measured in-situ in the solar wind show often large deviations from a simple Maxwellian distribution. Even a main part of the proton VDF (proton core) situated around the global maximum maintains a bi-Maxwellian character. Moreover, skewness, related to a non-thermal tail or proton beam, is often observed. In addition to these two proton populations, various heavy ion species are present and their VDFs also exhibit the mentioned non-thermal features. In the proton core frame, minor components including ion beams drift along the interplanetary magnetic field and their average differential velocities decrease with an increasing distance from the Sun. We present reprocessing of the VDFs measured by the Helios spacecraft with a motivation to discuss evolutions of relative drifts between three dominant components – the proton core, proton beam, and $\upalpha $ -particle core at different distances from the Sun. Our processing is based on assumptions that partial VDFs of all these components can be approximated by a bi-Maxwellian distribution. We compare results of this novel procedure with those from previous computation results and introduce basic characteristics of the VDF components in both slow and fast solar wind streams. Finally, we investigate the correlated variations of the velocity ratios on the ion components and magnetic field orientations occurring independently on the radial distance from the Sun.

Radial Evolution of Stochastic Heating in Low-β Solar Wind

Abstract We consider the radial evolution of perpendicular ion heating due to the violation of magnetic moment conservation caused by Alfvén and kinetic Alfvén wave turbulence. This process, referred to as stochastic heating (SH), can be quantified by the ratio between the average velocity fluctuations at the ion gyroradius and the perpendicular ion thermal speed . Using 17 yr of Helios observations, we constrain how much energy could be dissipated by this mechanism between 0.29 and 0.98 au. We find that SH likely operates throughout the entire inner heliosphere, but that its radial dependence is steeper than that of empirically derived dissipation rates, with r −2.5 being compared with r −2.08. This difference is significantly increased in fast solar wind streams to r −3.1 compared with r −1.8.

Multifractal Analysis of Heliospheric Magnetic Field Fluctuations Observed by Ulysses

Abstract We present the multifractal study of the intermittency of the magnetic field turbulence in the fast and slow solar wind beyond the ecliptic plane during two solar minima (1997–1998, 2007–2008) and solar maximum (1999–2001). More precisely, we consider 126 time intervals of Ulysses magnetic field measurements, obtain the multifractal spectra, and examine the degree of multifractality as the measure of intermittency in the MHD range of scales, for a wide range of heliocentric distances and heliolatitudes. The results show a slow decrease of intermittency with the radial distance, which is more significant for the fast than for the slow solar wind. Analysis of Alfvénic and compressive fluctuations confirms the decrease of intermittency with distance and latitude. This radial dependence of multifractality/intermittency may be explained by a slower evolution of turbulence beyond the ecliptic plane and by the reduced efficiency of intermittency drivers with the distance from the Sun. Additionally, our analysis shows that the greatest differences between magnetic field components are revealed close to the Sun, where intermittency is the strongest. Moreover, we observe that the slow solar wind from the maximum of the solar cycle 23 exhibits in general, a lower level of multifractality (intermittency) than fast solar wind, which can be related to the idea of a new type of Alfvénic slow solar wind.

The Intense Substorm Incidence in Response to Interplanetary Shock Impacts and Influence on Energetic Electron Fluxes at Geosynchronous Orbit

AbstractThe interaction between interplanetary (IP) shocks and the Earth's magnetosphere would generate/excite various types of geomagnetic phenomena. In order to analyze what kind of IP shock is more likely to trigger intense substorms (SME/AE > 1,000 nT) and how the energetic electrons response to intense substorms at geosynchronous orbit, we perform a systematic survey of 246 IP shock events using SuperMag and LANL observations between 2001 and 2013. The statistical analysis shows that intense substorms (SME > 1,000 nT) triggered by IP shocks are most likely to occur under the southward interplanetary magnetic field (IMF) and fast solar wind preconditions. Besides, intense substorms after the IP shock arrival are much more likely to occur when IMF points toward (away from) the Sun around spring (autumn) equinox, which can be ascribed to the Russell‐McPherron effect. Thus, the IMF Bs precondition of an IP shock and the Russell‐McPherron effect can be considered as precursors of an intense substorm. Furthermore, after the shock arrival associated with intense substorms, low‐energy (<200 keV) electron fluxes increase significantly at geosynchronous orbit, and high‐energy (>200 keV) electron fluxes decrease. The spectral index becomes softer with intense substorms and remains almost unchanged with moderate substorms no matter whether the IMF precondition is southward or northward.

СТРУКТУРА СОЛНЕЧНОГО ВЕТРА В ГЕЛИОСФЕРЕ В ЗАВИСИМОСТИ ОТ ФАЗЫ СОЛНЕЧНОГО ЦИКЛА: КРУПНОМАСШТАБНАЯ ДИНАМИКА ГЕЛИОСФЕРНОГО ТОКОВОГО СЛОЯ

A stationary axisymmetric MHD model of the solar wind has been constructed, which allows us to study the spatial distribution of the magnetic field and plasma characteristics at radial distances from 20 to 400 radii of the Sun at almost all heliolatitudes. The model takes into account the changes in the magnetic field of the Sun during a quarter of the solar cycle, when the dominant dipole magnetic field is replaced by a quadrupole. Selfconsistent solutions for the magnetic and velocity fields, plasma concentration and current density of the solar wind depending on the phase of the solar cycle are obtained. It is shown that during the domination of the dipole magnetic component in the solar wind heliospheric current sheet (HCS) is located in the equatorial plane, which is a part of the system of radial and transverse currents, symmetrical in the northern and southern hemispheres. As the relative contribution of the quadrupole component to the total magnetic field increases, the shape of the HCS becomes conical; the angle of the cone gradually decreases, so that the current sheet moves entirely to one of the hemispheres. At the same time, at high latitudes of the opposite hemisphere, a second conical HCS arises, the angle of which increases. When the quadrupole field becomes dominant (at maximum solar activity), both HCS lie on conical surfaces inclined at an angle of 35 degrees to the equator. The model describes the transition from the fast solar wind at high latitudes to the slow solar wind at low latitudes: a relatively gentle transition in the period of low solar activity gives way to more drastic when high solar activity. The model also predicts an increase in the steepness of the profiles of the main characteristics of the solar wind with an increase in the radial distance from the Sun. Comparison of the obtained dependences with the available observational data is discussed.

On the ion-inertial-range density-power spectra in solar wind turbulence

Abstract. A model-independent first-principle first-order investigation of the shape of turbulent density-power spectra in the ion-inertial range of the solar wind at 1 AU is presented. Demagnetised ions in the ion-inertial range of quasi-neutral plasmas respond to Kolmogorov (K) or Iroshnikov–Kraichnan (IK) inertial-range velocity–turbulence power spectra via the spectrum of the velocity–turbulence-related random-mean-square induction–electric field. Maintenance of electrical quasi-neutrality by the ions causes deformations in the power spectral density of the turbulent density fluctuations. Assuming inertial-range K (IK) spectra in solar wind velocity turbulence and referring to observations of density-power spectra suggest that the occasionally observed scale-limited bumps in the density-power spectrum may be traced back to the electric ion response. Magnetic power spectra react passively to the density spectrum by warranting pressure balance. This approach still neglects contribution of Hall currents and is restricted to the ion-inertial-range scale. While both density and magnetic turbulence spectra in the affected range of ion-inertial scales deviate from K or IK power law shapes, the velocity turbulence preserves its inertial-range shape in the process to which spectral advection turns out to be secondary but may become observable under special external conditions. One such case observed by WIND is analysed. We discuss various aspects of this effect, including the affected wave-number scale range, dependence on the angle between mean flow velocity and wave numbers, and, for a radially expanding solar wind flow, assuming adiabatic expansion at fast solar wind speeds and a Parker dependence of the solar wind magnetic field on radius, also the presumable limitations on the radial location of the turbulent source region.

Interplanetary spread of solar energetic protons near a high-speed solar wind stream

Aims. We study how a fast solar wind stream embedded in a slow solar wind influences the spread of solar energetic protons in interplanetary space. In particular, we aim at understanding how the particle intensity and anisotropy vary along interplanetary magnetic field (IMF) lines that encounter changing solar wind conditions such as the shock waves bounding a corotating interaction region (CIR). Moreover, we study how the intensities and anisotropies vary as a function of the longitudinal and latitudinal coordinate, and how the width of the particle intensities evolves with the heliographic radial distance. Furthermore, we study how cross-field diffusion may alter these spatial profiles. Methods. To model the energetic protons, we used a recently developed particle transport code that computes particle distributions in the heliosphere by solving the focused transport equation (FTE) in a stochastic manner. The particles are propagated in a solar wind containing a CIR, which was generated by the heliospheric model, EUHFORIA. We study four cases in which we assume a delta injection of 4 MeV protons spread uniformly over different regions at the inner boundary of the model. These source regions have the same size and shape, yet are shifted in longitude from each other, and are therefore magnetically connected to different solar wind conditions. Results. The intensity and anisotropy profiles along selected IMF lines vary strongly according to the different solar wind conditions encountered along the field line. The IMF lines crossing the shocks bounding the CIR show the formation of accelerated particle populations, with the reverse shock wave being a more efficient accelerator than the forward shock wave. The longitudinal intensity profiles near the CIR are highly asymmetric in contrast to the profiles obtained in a nominal solar wind. For the injection regions that do not cross the transition zone between the fast and slow solar wind, we observe a steep intensity drop of several orders of magnitude near the stream interface (SI) inside the CIR. Moreover, we demonstrate that the longitudinal width of the particle intensity distribution can increase, decrease, or remain constant with heliographic radial distance, reflecting the underlying IMF structure. Finally, we show how the deflection of the IMF at the shock waves and the compression of the IMF in the CIR deforms the three-dimensional shape of the particle distribution in such a way that the original shape of the injection profile is lost.

Solar Wind Magnetic Field Turbulence over the Solar Activity Cycle Inferred from Coronal Sounding Experiments with Helios Linear-Polarized Signals

Results of experiments on polarized radio sounding of the outer solar corona using the Helios spacecraft from 1975 to 1984 are presented. The characteristic parameters of the temporal spectra of fluctuations in the Faraday rotation of the plane of polarization for heliocentric distances from 3.5 to 5.5 solar radii are obtained. The absolute level of these fluctuations and, consequently, the level of fluctuations of the magnetic field, is almost independent of the solar activity. It is well known that the global structure of the solar wind varies with the solar cycle such that there is slow solar wind at low latitudes and fast solar wind at high latitudes during solar minima. In contrast, a slow solar wind dominates at all latitudes during solar maxima. One explanation for the invariance of the fluctuations observed by sounding the circumsolar plasma is that the mean magnetohydrodynamic turbulence of the low-latitude, slow solar wind depends weakly on the phase of the solar cycle.

Features of the Spectral Characteristics of Plasma Fluctuations in Different Large-Scale Streams of the Solar Wind

The turbulent characteristics of plasma fluctuations in the solar wind (SW) may substantially change depending on the SW conditions. Large-scale streams of different types, such as undisturbed slow solar wind, fast solar wind, interplanetary coronal mass ejection (EJECTA), magnetic cloud (MC), the compression region at the boundary between fast and slow streams (CIR), and the compression region in front of the EJECTA/MC (SHEATH), are usually characterized by specific plasma parameter values, which may influence turbulent cascade formation. In this paper, we analyze the properties of the spectra of ion flux fluctuations in the SW in the region of transition from the magnetohydrodynamic (MHD) scale to the kinetic one. The analysis is based on measurements carried out with the BMSW plasma spectrometer onboard the SPEKTR-R spacecraft with a high time resolution. The observational intervals inside the SW streams of different large-scale types are considered, and the main turbulence characteristics in these streams are compared. It was shown that the properties of the fluctuation spectra may strongly depend on the SW type; in particular, the spectrum of kinetic-scale fluctuations usually becomes much steeper inside the MC regions and the compression regions in front of them, as well as in the CIR. The characteristics of the fluctuation spectra on MHD scale are less dependent on the type of large-scale SW structures, and, on average, they correspond to the Kolmogorov spectra. However, it can be noted that the smallest spectral slopes are observed in the slow undisturbed solar wind, which is indicative of the differences from the traditional Kolmogorov model of the developed turbulence.

Alpha particle thermodynamics in the inner heliosphere fast solar wind

Context.Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of different ion species. However, most in situ observations of positive ions in the solar wind are taken at 1 AU, where information on their solar source properties may have been irreversibly erased.Aims.In this study we aim to use the properties of alpha particles at heliocentric distances between 0.3 AU and 1 AU to study plasma processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous remote sensing observations of the plasma closer to the Sun.Methods.We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial variation of alpha particle temperatures in fast solar wind observed between 0.3 AU and 1 AU.Results.Between 0.3 AU and 1 AU alpha particles are heated in the magnetic field perpendicular direction and cooled in the magnetic field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.

Evolution of the α-proton Differential Motion across Stream Interaction Regions

Abstract A corotating interaction region (CIR) develops between the solar wind streams with different bulk speeds emanating from distinct coronal sources. The arising pressure perturbations redistribute momentum between adjacent streams forming the regions of the compressed solar wind around the stream interface. We focus on properties of α-particles with respect to protons in CIRs using measurements of the Wind and Helios spacecraft. In the slow solar wind in front of CIRs, the relative helium abundance A He is usually low (about 1%) and the α-proton differential drift, V αp is close to zero. In the high-speed stream behind CIRs, both of these characteristics are significantly higher. Inside CIRs, a large enhancement of A He accompanied by a decrease in V αp is often observed in the compressed and slowed down fast solar wind close to the CIR leading edge. On the other hand, a depletion of A He is sometimes present in the compressed and accelerated slow solar winds. We explain these observations in terms of magnetic mirroring of the multicomponent solar wind in a converging magnetic field that develops within CIRs.

Jensen–Shannon Complexity Measurements in Solar Wind Magnetic Field Fluctuations

Abstract Turbulent magnetic field fluctuations in the solar wind have been extensively investigated, but few studies have analyzed their complexity. Jensen–Shannon complexity maps of time series data provide a mathematical tool that can characterize fluctuations in laboratory experiments as stochastic, chaotic, or periodic phenomena. We apply this recently developed tool to characterize stochastic behavior in solar wind structures, including interplanetary coronal mass ejections (ICMEs), co-rotating interactions regions (CIRs), and turbulent magnetic fluctuation intervals. We find that the turbulent intervals observed by Helios, Wind, and Ulysses lie within the stochastic region of the complexity maps and that their complexity decreases while their normalized entropy increases with distance from the Sun. The complexity values associated with the fast solar wind (>550 km s−1) turbulence identified in Ulysses data beyond 5 au are highest at low latitudes (<10°) and lowest at latitudes above 20°. The Jensen–Shannon complexity maps show that fluctuations in the magnetic field, plasma flow, and density of the solar wind at 1 au are stochastic in ICMEs and CIRs identified in Wind data. Our analysis of Ulysses data between 1.4 and 5.4 au shows that the complexity of the ICMEs decreases with distance from the Sun, and the normalized entropy increases. Furthermore, the complexity values associated with magnetic field fluctuations in ICMEs identified in Ulysses data behave like the slow solar wind turbulence fluctuations within 4 au and take on values closer to the complexity values of the fast solar wind beyond 4 au.

Distribution of intense, moderate and weak geomagnetic storms over the solar cycle

The geomagnetic storms observed during different phases of a solar cycle have distinct primary solar sources. Therefore, we have statistically examined, in the present study, how the geomagnetic storms of different intensities are distributed over a solar cycle. We have considered previous four cycles, viz. cycle 21, 22, 23 and 24. We selected all the storms which were observed during these cycles and classified them into three categories as: intense (Dst ≤ − 100 nT), moderate (− 100 ≤ Dst ≤ − 50) and weak (− 50 ≤ Dst ≤ − 30). We found that geomagnetic storms are distributed in such a way that maximum number of intense storms occur around peak phase of a solar cycle, while maximum number of weak storms occur during the starting and ending phases of a cycle. About 73.20% of intense storms, 58.07% and 47.93% of moderate and weak storms occurred around the peak of a cycle. Therefore, it was concluded that peak phase is dominated by intense and very intense storms due to increase in frequency of occurrence of coronal mass ejections during such periods. At the same time, the ending phase of solar cycle is dominated by the occurrence of weak storms due to the frequent occurrences of fast solar wind streams during such periods. Moreover, the total number of storms occurring during the descending phase is greater than those observed during the ascending phase. The storms with peak Dst ≤ − 200 nT were classified as very intense and designated as outstanding Sun–Earth connections. It was found that 90% of such events occur around the peak phase of solar cycle. Therefore, it was concluded that the high-risk events occur during the peak phase of a solar cycle. Therefore, extreme care must be taken while launching new space missions and operating the existing ones, during this period.

Modelling three-dimensional transport of solar energetic protons in a corotating interaction region generated with EUHFORIA

Aims. We introduce a new solar energetic particle (SEP) transport code that aims at studying the effects of different background solar wind configurations on SEP events. In this work, we focus on the influence of varying solar wind velocities on the adiabatic energy changes of SEPs and study how a non-Parker background solar wind can trap particles temporarily at small heliocentric radial distances (≲1.5 AU) thereby influencing the cross-field diffusion of SEPs in the interplanetary space. Methods. Our particle transport code computes particle distributions in the heliosphere by solving the focused transport equation (FTE) in a stochastic manner. Particles are propagated in a solar wind generated by the newly developed data-driven heliospheric model, EUHFORIA. In this work, we solve the FTE, including all solar wind effects, cross-field diffusion, and magnetic-field gradient and curvature drifts. As initial conditions, we assume a delta injection of 4 MeV protons, spread uniformly over a selected region at the inner boundary of the model. To verify the model, we first propagate particles in nominal undisturbed fast and slow solar winds. Thereafter, we simulate and analyse the propagation of particles in a solar wind containing a corotating interaction region (CIR). We study the particle intensities and anisotropies measured by a fleet of virtual observers located at different positions in the heliosphere, as well as the global distribution of particles in interplanetary space. Results. The differential intensity-time profiles obtained in the simulations using the nominal Parker solar wind solutions illustrate the considerable adiabatic deceleration undergone by SEPs, especially when propagating in a fast solar wind. In the case of the solar wind containing a CIR, we observe that particles adiabatically accelerate when propagating in the compression waves bounding the CIR at small radial distances. In addition, for r ≳ 1.5 AU, there are particles accelerated by the reverse shock as indicated by, for example, the anisotropies and pitch-angle distributions of the particles. Moreover, a decrease in high-energy particles at the stream interface (SI) inside the CIR is observed. The compression/shock waves and the magnetic configuration near the SI may also act as a magnetic mirror, producing long-lasting high intensities at small radial distances. We also illustrate how the efficiency of the cross-field diffusion in spreading particles in the heliosphere is enhanced due to compressed magnetic fields. Finally, the inclusion of cross-field diffusion enables some particles to cross both the forward compression wave at small radial distances and the forward shock at larger radial distances. This results in the formation of an accelerated particle population centred on the forward shock, despite the lack of magnetic connection between the particle injection region and this shock wave. Particles injected in the fast solar wind stream cannot reach the forward shock since the SI acts as a diffusion barrier.

Inherentness of Non-stationarity in Solar Wind

Abstract Most studies of turbulence in the solar wind invoke stationarity as a working hypothesis. Unfortunately, this concept is difficult to verify in practice. To investigate the validity of the weak stationarity assumption we consider magnetic field measurements made by the WIND satellite and study the properties of the autocorrelation function (ACF), which is a classical gauge for characteristic times or scales. We find that the ACF suffers from a high variance, which precludes the routine interpretation of correlation times and scales. In addition, the ACF fails to converge toward a constant function, even when considering the longest available intervals of either fast or slow solar wind. The reasons behind this lack of convergence are better understood by considering the power spectral density (PSD) of the magnetic field and analyzing synthetic data that exhibit the same PSD. Interestingly, we find evidence for an f −1 scaling at low frequencies in both fast and slow solar winds. These results, together with the theoretical properties of processes with f −γ scaling all point to the non-stationary behavior of the solar wind, in particular for scales that correspond to the inertial range. They also impose strong constraints on the applicability of ACF analysis as a tool for characterizing statistical properties of solar wind turbulence.