Fast Solar Wind Research Articles

Ion Heating Resulting from the Deceleration of Alpha Particles by a Proton-alpha Drift Instability in a Nonuniform Solar-wind Plasma

Abstract The deceleration of alpha particle observed in the fast solar wind can contribute to the plasma heating between 0.3 and 1 au. The observational data suggest that the energy released from the deceleration has to be channeled to perpendicular heating of the protons. A possible mechanism of the energy conversion is a proton-alpha drift instability. We present hybrid numerical simulations of this instability in a warm plasma with particle-in-cell ions and a neutralizing electron fluid. The parallel temperature of the alpha particles is assumed to be larger than the perpendicular temperature. This sense of the anisotropy makes parallel-propagating fast magnetosonic waves the most easily excited modes. For typical ion beta values at 0.3 to 1 au, we find that the instability does not produce evident perpendicular heating of the protons if the initial background plasma is uniform. The lack of the heating is related to inefficient cyclotron interaction of the protons with the parallel-propagating fast modes. However, the background plasma in the solar wind is unlikely to be uniform. We consider the background variations across the mean magnetic field in the form of single or multiple equilibrium structures. The inhomogeneity modifies the unstable waves by making them oblique. Furthermore, their wavenumber spectrum extends to perpendicular wavenumbers of the order of the inverse proton gyroradius. Such waves can interact with the protons more efficiently. We show that significant and preferentially perpendicular heating of the protons is present in the nonuniform plasma.

Quasilinear Consequences of Turbulent Ion Heating by Magnetic Moment Breaking

Abstract The fast solar wind emerging from coronal holes is likely heated and accelerated by the dissipation of magnetohydrodynamic turbulence, but the specific kinetic mechanism resulting in the perpendicular ion heating required by observations is not understood. A promising mechanism has been proposed by Chandran et al., which in this paper we call “magnetic moment breaking” (MMB). As currently formulated, MMB dissipation operates only on the ion perpendicular motion, and does not influence their parallel temperature. Thus, the MMB mechanism acting by itself produces coronal hole proton distributions that are unstable to the ion-cyclotron (IC) anisotropy instability. This quasilinear instability is expected to operate faster than the nonlinear turbulent cascade, scattering ions into the parallel direction and generating quasi-parallel-propagating IC waves. To investigate the consequences of this instability on the MMB-heated protons, we construct a homogeneous model for protons with coronal hole properties. Using a simplified version of the resonant cyclotron interaction, we heat the protons by the MMB process and instantaneously scatter them to lower anisotropy while self-consistently generating parallel-propagating IC waves. We present several illustrative cases, finding that the extreme anisotropies implied by the MMB mechanism are limited to reasonable values, but the distinctive shape of the proton distribution derived by Klein & Chandran is not maintained. We also find that these combined processes can result in somewhat higher particle energization than the MMB heating alone. These quasilinear consequences should follow from any kinetic mechanism that primarily increases the perpendicular ion temperature in a collisionless plasma.

Correlation of the Radial and Meridional Components of IMF and the Solar Wind Velocity: Dependence on the Time and the Fluctuation Frequency

The behavior of the correlation between the radial BR and normal BN components of the interplanetary magnetic field (IMF) in the dependence on the time, the flow type of the solar wind, and the frequency of fluctuations has been studied with consideration of the peculiarities in IMF sectors of different signs. Based on the data of the Wind spacecraft obtained in 1995−2011, it has been found that the correlation between BR and BN is strongest in the periods of the solar activity minima and that the annual averages of the correlation coefficient obey the rules CRN > 0 or CRN < 0 northward or southward of the heliospheric current sheet, respectively. During the solar cycle, the correlation coefficient changes almost similarly in slow and fast solar wind streams. The dependence of the correlation between BR and BN on the fluctuation frequency was analyzed in a range of 1.2 × 10−5 to 8.3 × 10−3 Hz; it has been found that the correlation is maximal at low frequencies and that it slowly decreases, starting from 10−4 Hz, with increases in the frequency. It has been also found that, fluctuations in the components of the solar wind velocity VR and VN correlate analogously to the IMP components BR and BN at frequencies around 10−4 Hz or higher.

Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations

Spacecraft observations have shown that the proton temperature in the solar wind falls off with radial distance more slowly than expected for an adiabatic prediction. Usually, previous studies have been focused on the evolution of the solar-wind plasma by using the bulk speed as an order parameter to discriminate different regimes. In contrast, here, we study the radial evolution of pure and homogeneous fast streams (i.e. well-defined streams of coronal-hole plasma that maintain their identity during several solar rotations) by means of re-processed particle data, from the HELIOS satellites between 0.3 and 1 AU. We have identified 16 intervals of unperturbed high-speed coronal hole plasma, from three different sources and measured at different radial distances. The observations show that, for all three streams, (i) the proton density decreases as expected for a radially expanding plasma, unlike previous analysis that found a slower decrease; (ii) the magnetic field deviates from the Parker prediction, with the radial and tangential components decreasing more slowly and quickly than expected, respectively; (iii) the double-adiabatic invariants are violated and an increase of entropy is observed; (iv) the proton-core temperature anisotropy is constrained by mirror mode instability; (v) the collisional frequency is not constant, but decreases as the plasma travels away from the Sun. The present work provides an insight into the heating problem in pure fast solar wind, fitting in the context of the next solar missions, and, especially for Parker Solar Probe, it enables us to predict the high-speed solar-wind environment much closer to the Sun.

On the Dependence of the Solar Wind Velocity on the Fractional Area of Coronal Holes in Longitude

The relationship between the fractional area of coronal holes and the maximum velocity of the fast solar wind at 1 AU based on AIA/SDO and ACE/SWEPAM observations is considered for the period from June 2015 to March 2017. Assuming the ballistic model of the solar wind propagation it has been shown that the coronal holes within the meridional slice ±10° make a basic contribution to the ecliptic solar wind. The maximum correlation coefficient between the area of coronal holes and the peak solar wind velocity at latitudes within ±40° was found to be equal to 0.762 ± 0.145. The probable causes of the discrepancy between the predicted and observed values of the solar wind velocity are discussed.

Structure of the Front of a Collisionless Oblique Interplanetary Shock Wave from High Time Resolution Measurements of Solar-Wind Plasma Parameters

Data from the Fast Solar Wind Monitor (BMSW) of the science payload onboard the SPEKTR-R satellite and data from instruments on board the WIND spacecraft are used to study statistically the structure of the front of oblique interplanetary shock waves with respect to the θBn angle and the fulfillment of the Rankine–Hugoniot conditions at the fronts of collisionless shock waves. The experimental wavelength of oscillations upstream of the ramp is compared with the estimated theoretical wavelength to determine that the dispersion of oblique magnetosonic waves plays the decisive role in the formation of the fronts of quasiperpendicular (45° ≤ θBn < 90°) collisionless interplanetary shock waves with small Mach numbers МA < 3 and a parameter of β1 < 1. Comparison of the Rankine–Hugoniot relations MA(ρ2/ρ1), which were measured at the fronts of 47 interplanetary shock waves with β1 < 5 and Alfven Mach numbers of 1 < МА < 10 by calculations performed within the ideal magnetic hydrodynamics (MHD), has revealed that the effective adiabatic index γ, which characterizes the processes inside the shock front, lies mainly within the range from 2 to 5/3.

Geoeffectiveness of Stream Interaction Regions From 1995 to 2016

AbstractStream interaction regions (SIRs) are important sources of geomagnetic storms. In this work, we first extend the end time of the widely used SIR catalog developed by Jian et al. (2006, https://doi.org/10.1007/s11207-006-0132-3), which covered the period from 1995 to 2009, to the end of 2016. Based on this extended SIR catalog, the geoeffectiveness of SIRs is discussed in detail. It was found that 52% of the SIRs caused geomagnetic storms with Dstmin ≤−30 nT, but only 3% of them caused intense geomagnetic storms with Dstmin ≤−100 nT. Furthermore, we found that 10 of the intense geomagnetic storms caused by SIRs were associated with complex structures due to interactions between SIRs and interplanetary coronal mass ejections (ICMEs). In such a structure, an ICME is embedded in the SIR and located between the slow and fast solar wind streams. In addition, we found that the geoeffectiveness of SIRs interacting with ICMEs is enhanced. The possibility of SIR‐ICME interaction structures causing geomagnetic storms is markedly higher than that of isolated SIRs or isolated ICMEs. In particular, the geoeffectiveness of SIR‐ICME interaction structures is similar to that of the Shock‐ICME interaction structures, which have been demonstrated to be the main causes of geomagnetic storms.

Solar wind and kinetic heliophysics

Abstract. This paper reviews recent aspects of solar wind physics and elucidates the role Alfvén waves play in solar wind acceleration and turbulence, which prevail in the low corona and inner heliosphere. Our understanding of the solar wind has made considerable progress based on remote sensing, in situ measurements, kinetic simulation and fluid modeling. Further insights are expected from such missions as the Parker Solar Probe and Solar Orbiter. The sources of the solar wind have been identified in the chromospheric network, transition region and corona of the Sun. Alfvén waves excited by reconnection in the network contribute to the driving of turbulence and plasma flows in funnels and coronal holes. The dynamic solar magnetic field causes solar wind variations over the solar cycle. Fast and slow solar wind streams, as well as transient coronal mass ejections, are generated by the Sun's magnetic activity. Magnetohydrodynamic turbulence originates at the Sun and evolves into interplanetary space. The major Alfvén waves and minor magnetosonic waves, with an admixture of pressure-balanced structures at various scales, constitute heliophysical turbulence. Its spectra evolve radially and develop anisotropies. Numerical simulations of turbulence spectra have reproduced key observational features. Collisionless dissipation of fluctuations remains a subject of intense research. Detailed measurements of particle velocity distributions have revealed non-Maxwellian electrons, strongly anisotropic protons and heavy ion beams. Besides macroscopic forces in the heliosphere, local wave–particle interactions shape the distribution functions. They can be described by the Boltzmann–Vlasov equation including collisions and waves. Kinetic simulations permit us to better understand the combined evolution of particles and waves in the heliosphere.

Flow-line Wandering in the Turbulent Solar Wind and Space Environment Forecasts

Abstract Space environment forecasts are based on ab initio modeling of the solar wind (SW) wherein solar magnetic fields and plasmas are propagated from an initial/boundary model source surface in the lower solar corona out to 1 au. Testing of these space environment forecasts relative to in situ measurements at 1 au typically shows uncertainties in the arrival times of fast SW streams of the order of a day, with broad distributions, means/medians of the order of half a day, and large variations between models but no definite winner. Here the effect of flow-line (FL) wandering due to the higher frequency velocity fluctuations within the turbulent SW on the arrival-time statistics of parcels of SW plasma transiting through the inner heliosphere out to 1 au is evaluated for a range of cutoff timescales in the velocity fluctuations. Used for this evaluation are in situ SW velocity measurements onboard Wind at 1 au, detailed spectral analysis of these measurements, WKB extrapolations to the inner heliosphere and simple application of a newly extended theoretical calculation of the mean SWFL cross-flow and “flow-aligned” displacements from the measured spectra. It is found that the velocity fluctuations near 1 au have little effect on the arrival times. The effect of the velocity fluctuations increases sunward, however, to a level sufficient to explain the large and broadly distributed “uncertainties” found in the testing of the forecasts.

Long-Term Independence of Solar Wind Polytropic Index on Plasma Flow Speed.

The paper derives the polytropic indices over the last two solar cycles (years 1995–2017) for the solar wind proton plasma near Earth (~1 AU). We use ~92-s datasets of proton plasma moments (speed, density, and temperature), measured from the Solar Wind Experiment instrument onboard Wind spacecraft, to estimate the moving averages of the polytropic index, as well as their weighted means and standard errors as a function of the solar wind speed and the year of measurements. The derived long-term behavior of the polytropic index agrees with the results of other previous methods. In particular, we find that the polytropic index remains quasi-constant with respect to the plasma flow speed, in agreement with earlier analyses of solar wind plasma. It is shown that most of the fluctuations of the polytropic index appear in the fast solar wind. The polytropic index remains quasi-constant, despite the frequent entropic variations. Therefore, on an annual basis, the polytropic index of the solar wind proton plasma near ~1 AU can be considered independent of the plasma flow speed. The estimated all-year weighted mean and its standard error is γ = 1.86 ± 0.09.

Parametric Decay and the Origin of the Low-frequency Alfvénic Spectrum of the Solar Wind

Abstract The fast solar wind shows a wide spectrum of transverse magnetic and velocity field perturbations. These perturbations are strongly correlated in the sense of Alfvén waves propagating mostly outward, from the Sun to the interplanetary medium. They are likely to be fundamental to the acceleration and the heating of the solar wind. However, the precise origin of the broadband spectrum is unknown to date. Typical periods of chromospheric Alfvén waves are limited to a few minutes, and any longer period perturbations should be strongly reflected at the transition region. In this work, we show that minute long Alfvénic fluctuations are unstable to the parametric instability. Parametric instability enables an inverse energy cascade by exciting several-hour-long periods of Alfvénic fluctuations together with strong density fluctuations (typically between 1 and 20 R ⊙). These results may improve our understanding of the origin of the solar wind turbulent spectrum and will be tested by the Parker Solar Probe.

First observations of magnetic holes deep within the coma of a comet

The Rosetta spacecraft of the European Space Agency made ground-breaking observations of comet 67P/Churyumov-Gerasimenko and of its cometary environment. We search for magnetic holes in that environment, i.e., significant depressions in the magnetic field strength, measured by the Rosetta fluxgate Magnetometer (MAG) in April and May 2015. In that time frame of two months, we identified 23 magnetic holes. The cometary activity was intermediate and increasing because Rosetta was on the inbound leg toward the Sun. While in April solar wind protons were still observed by Rosetta near the comet, in May these protons were already mostly replaced by heavy cometary ions. Magnetic holes have frequently been observed in the solar wind. We find, for the first time, that magnetic holes exist in the cometary environment even when solar wind protons are almost absent. Some of the properties of the magnetic holes are comparable to those of solar wind holes; they are associated with density enhancements, sometimes associated with co-located current sheets and fast solar wind streams, and are of similar scales. However, particularly in May, the magnetic holes near the comet appear to be more processed, featuring shifted density enhancements and, sometimes, bipolar signatures in magnetic field strength rather than simple depressions. The magnetic holes are of global size with respect to the coma. However, at the comet, they are compressed owing to magnetic field pile-up and draping so that they change in shape. There, the magnetic holes become of comparable size to heavy cometary ion gyroradii, potentially enabling kinetic interactions.

A Model for Solar Wind Structure and Dynamics Based on Alfvén Wave Turbulence

We present a 3D reduced magnetohydrodynamic (RMHD) model of reflection-driven Alfvén wave turbulence in an open magnetic field at the center of a coronal hole. The non-linear interactions between outward (dominant) and inward (minority) propagating waves generate turbulence. The RMHD equations describing the turbulence include the effects of solar wind outflow velocity on the dissipation of waves. The Alfvén wave turbulence results in the transfer of energy to small perpendicular scales (“direct cascade”) or to larger scales (“inverse cascade”). We first show the result of our calculation for a model where there is smooth variation of plasma parameters such as Alfvén speed and density with height along the flux tube. However, this model does not produce the required energy to heat the plasma and accelerate the fast solar wind. Therefore, we introduce our second model, where we include additional density fluctuation along the open field. These density variations simulate the effects of compressive MHD waves in the solar wind. We find that these variations in density enhance the turbulence dissipation rates, and thereby increase the heating rate and the acceleration of the solar wind.

Angular Independence of Break Position for Magnetic Power Spectral Density in Solar Wind Turbulence

Abstract The break in power spectral density (PSD) around the ion scales indicates the onset of dissipation and/or dispersion of kinetic turbulence. For Alfvén waves in the kinetic regime, the dissipation and dispersion are individually dependent on the propagation angle, θ kB, which has θ RB (the angle between radial direction and local mean magnetic field direction) as a proxy in solar wind measurements. The relation between θ RB and the break position helps us find the role of dissipation and/or dispersion for deforming the PSD profile. In order to locate the spectral break position automatically and quantitatively, we develop a dual-power-law fitting method to fit the PSD profiles in both MHD and kinetic ranges simultaneously. The break position f b is found to change little with θ RB, suggesting an angular independence of the spectral break. Furthermore, f b in our statistical study of fast solar wind near 1 au is consistent with a wavenumber k satisfying k(ρ p + d p) ∼ 1 (ρ p is the thermal proton gyroradius and d p is the proton inertial length), independently of θ RB. To interpret this independence, we incorporate the effects of both dissipation and dispersion in a unified description, which is the breakdown of the magnetic frozen-in condition in wavenumber space (k ∥, k ⊥). The breakdown of the frozen-in condition is relatively isotropic compared to the strong anisotropy of dispersion and dissipation. Furthermore, the spatial scale for the onset of the breakdown frozen-in condition is estimated to be the sum of ρ p and d p.

Fast plasma sheet flows and X line motion in the Earth's magnetotail: results from a global hybrid-Vlasov simulation

Abstract. Fast plasma flows produced as outflow jets from reconnection sites or X lines are a key feature of the dynamics in the Earth's magnetosphere. We have used a polar plane simulation of the hybrid-Vlasov model Vlasiator, driven by steady southward interplanetary magnetic field and fast solar wind, to study fast plasma sheet ion flows and related magnetic field structures in the Earth's magnetotail. In the simulation, lobe reconnection starts to produce fast flows after the increasing pressure in the lobes has caused the plasma sheet to thin sufficiently. The characteristics of the earthward and tailward fast flows and embedded magnetic field structures produced by multi-point tail reconnection are in general agreement with spacecraft measurements reported in the literature. The structuring of the flows is caused by internal processes: interactions between major X points determine the earthward or tailward direction of the flow, while interactions between minor X points, associated with leading edges of magnetic islands carried by the flow, induce local minima and maxima in the flow speed. Earthward moving flows are stopped and diverted duskward in an oscillatory (bouncing) manner at the transition region between tail-like and dipolar magnetic fields. Increasing and decreasing dynamic pressure of the flows causes the transition region to shift earthward and tailward, respectively. The leading edge of the train of earthward flow bursts is associated with an earthward propagating dipolarization front, while the leading edge of the train of tailward flow bursts is associated with a tailward propagating plasmoid. The impact of the dipolarization front with the dipole field causes magnetic field variations in the Pi2 range. Major X points can move either earthward or tailward, although tailward motion is more common. They are generally not advected by the ambient flow. Instead, their velocity is better described by local parameters, such that an X point moves in the direction of increasing reconnection electric field strength. Our results indicate that ion kinetics might be sufficient to describe the behavior of plasma sheet bulk ion flows produced by tail reconnection in global near-Earth simulations. Keywords. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet) – space plasma physics (numerical simulation studies)

Some Turbulent Predictions for Parker Solar Probe

From the the solar photosphere to the outer heliosphere, the Sun's plasma properties are fluctuating with a broad range of temporal and spatial scales. In fact, a turbulent cascade of energy from large to small scales is a frequently invoked explanation for heating the corona and accelerating the solar wind. NASA's Parker Solar Probe (PSP) is expected to revolutionize our understanding of coronal heating and magnetohydrodynamic (MHD) turbulence by performing in situ sampling closer to the Sun than any other prior space mission. This research note presents theoretical predictions for some properties of MHD turbulence (e.g., spacecraft-frame power spectra and variance anisotropies) in the regions to be explored by PSP. These results are derived from a previously published semi-empirical model of coupled Alfvenic and fast-mode turbulence in the fast solar wind. The primary reason for this research note is to show how straightforward it can be to extract useful predictions from existing theoretical models about measurable quantities that were not even considered when creating the models initially. The variance anisotropy, for example, may be an important quantity for distinguishing between different theoretical models for coronal heating and solar wind acceleration.

Realistic Worst Case for a Severe Space Weather Event Driven by a Fast Solar Wind Stream.

Satellite charging is one of the most important risks for satellites on orbit. Satellite charging can lead to an electrostatic discharge resulting in component damage, phantom commands, and loss of service and in exceptional cases total satellite loss. Here we construct a realistic worst case for a fast solar wind stream event lasting 5 days or more and use a physical model to calculate the maximum electron flux greater than 2 MeV for geostationary orbit. We find that the flux tends toward a value of 106 cm−2·s−1·sr−1 after 5 days and remains high for another 5 days. The resulting flux is comparable to a 1 in 150‐year event found from an independent statistical analysis of electron data. Approximately 2.5 mm of Al shielding would be required to reduce the internal charging current to below the National Aeronautics and Space Administration‐recommended guidelines, much more than is currently used. Thus, we would expect many satellites to report electrostatic discharge anomalies during such an event with a strong likelihood of service outage and total satellite loss. We conclude that satellites at geostationary orbit are more likely to be at risk from fast solar wind stream event than a Carrington‐type storm.

Response of Relativistic Electron Microbursts to the Arrival of High‐Speed Solar Wind Streams and its Relation to Flux Variation of Trapped Radiation Belt Electrons

AbstractRelativistic electron microbursts are frequently observed during high‐speed solar wind stream (HSS) events. However, solar wind parameters, which are important for enhanced microburst precipitation, have not been well understood. We perform a superposed epoch analysis of microburst occurrence frequency during HSS events, considering the north/south component of the interplanetary magnetic field (IMF Bz) and solar wind speed. The IMF Bz dependence is investigated considering the Russell‐McPherron effect following the method used by Miyoshi and Kataoka (2008, https://doi.org/10.1029/2007JA012506). We find that microbursts are most frequently observed during faster (&gt;500 km/s) solar wind speed streams with the IMF Bz shifted southward (fast SBZ‐HSS events), indicating that both fast solar wind speed and the southward IMF Bz are important for enhanced microburst precipitation. The occurrence frequency of chorus waves during the HSS events is investigated to gain an understanding of the different behavior of microburst activity in response to the HSS events. We show that relativistic electron microbursts are not always a good proxy for chorus waves, but a proxy for a subset of chorus that can resonate with MeV electrons. We also show that electron fluxes with energies from hundreds of keV to several MeV preferentially increase during the fast SBZ‐HSS events. These results indicate that observation of an enhanced occurrence of relativistic electron microbursts could be used as a proxy for the direct observation of the acceleration of MeV electrons by chorus waves in the heart of the outer radiation belt, not just a proxy for chorus wave activity in general.

Comparing Two Intervals of Exceptionally Strong Solar Rotation Recurrence of Galactic Cosmic Rays

AbstractTwo intervals of exceptionally strong recurrence of galactic cosmic ray (GCR) intensity at solar rotation period stand out in recent history, one in 2007–2008 and the other in 2014–2015. We use neutron monitor data from Oulu and Hermanus, solar wind (SW) data, and coronal images to study these intervals. We find that in both cases the source of solar rotation period recurrence was a coronal hole (CH), but CH structures were different. While a large, longitudinally asymmetric CH existed at high southern latitudes in 2014–2015, a low‐latitude CH caused the recurrence in 2007–2008. Spectral properties of GCR and SW parameters reflect these differences. In 2014–2015 the GCR power spectrum density was broad and peaked at a period of 28.9 days, longer than the simultaneous recurrence period of SW speed. In 2007–2008 the GCR power spectrum was narrow and peaked at 27.5 days, exactly the same as for SW speed. The effect of CHs to GCRs was somewhat different in the two cases because of opposite solar polarities in the two intervals. In 2014–2015, during positive polarity when GCRs drift inward from high latitudes, the convection of fast SW from CH reduces the inward GCR drift over a wide range of high heliolatitudes at certain heliolongitudes. In 2007–2008, during negative polarity when GCRs drift inward via the heliospheric current sheet, a low‐latitude CH depletes the GCR flux not only by convection but also by the deflecting heliospheric current sheet away from the ecliptic, whence GCR drift to higher latitudes in a limited longitude range.

Space weathering on the Moon: Farside-nearside solar wind precipitation asymmetry

The lunar surface is continuously under the impact of solar wind plasma, which breaks the chemical bonds of the surface material resulting in weathering of the surface and a modified chemical composition. Ion impact also sputters the surface material, affecting the composition of the lunar exosphere, and it controls the electrical properties of both the lunar surface and the near surface space.We have studied the lunar farside-nearside (FN) hemispherical asymmetry of the solar wind proton impact on the lunar surface along the orbit of the Moon during fast solar wind conditions. The analysis is based on a 3D hybrid model where ions are accelerated by the macroscopic j × B and pressure gradient forces.The derived proton impact surface map shows that the highest cumulative solar wind proton addition on the lunar surface is located on the farside while the most energetic protons precipitate on the nearside. The total ion impact rate was found to be smallest when the Moon is deep in the magnetotail. The total ion impact rate on the lunar surface varies while the Moon orbits the Earth and these longitudinal variations are caused by the magnetosphere and lunar tidal locking.