Solar wind ion charge state distributions and compound cross sections for solar wind charge exchange X-ray emission

Solar Wind Charge eXchange X-ray(SWCX) emission in the heliosphere and Ea rth’s exosphere is a hard to avoid signal in soft Xray obse rvations of astrophysical targets.On the other hand,the X-ray imaging possibilities offered by the SWCX process has led to an increasing number of future dedicated space missions for investigating the solar wind-terrestrial inte ractions and magnetospheric interfaces.In both cases,accurate modelling of the SWCX emission is key to correctly interpret its signal,and remove it from obse rvations,when needed.In this paper,we compile solar wind abundance measurements from ACE for different solar wind types,and atomic data from literature,including charge exchange cross-sections and emission probabilities,used fo r calculating the compound cross-section a for the SWCX X-ray emission.We calculate a values for charge-exchange with H and He,relevant to soft X-ray energy bands(0.1-2.0 keV)for various solar wind types and solar cycle conditions.

Two methods for separating the magnetospheric solar wind charge exchange soft X-ray emission from the diffuse X-ray background

Solar wind charge exchange(SWCX)is the process of solar wind high-valence ions exchanging charges with neutral components and generating soft X-rays.Recently,detecting the SWCX emission from the magnetosphere is proposed as a new technique to study the magnetosphere using panoramic soft X-ray imaging.To better prepare for the data analysis of upcoming magnetospheric soft X-ray imaging missions,this paper compares the magnetospheric SWCX emission obtained by two methods in an XMM-Newton observation,during which the solar wind changed dramatically.The two methods differ in the data used to fit the diffuse X-ray background(DXB)parameters in spectral analysis.The method adding data from the ROSAT All-Sky Survey(RASS)is called the RASS method.The method using the quiet observation data is called the Quiet method,where quiet observations usually refer to observations made by the same satellite with the same target but under weaker solar wind conditions.Results show that the spectral compositions of magnetospheric SWCX emission obtained by the two methods are very similar,and the changes in intensity over time are highly consistent,although the intensity obtained by the RASS method is about 2.68±0.56 keV cm^(-2)s^(-1)sr^(-1)higher than that obtained by the Quiet method.Since the DXB intensity obtained by the RASS method is about 2.84±0.74 keV cm^(-2)s^(-1)sr^(-1)lower than that obtained by the Quiet method,and the linear correlation coefficient between the difference of SWCX and DXB obtained by the two methods in diffe rent energy band is close to-1,the diffe rences in magnetospheric SWCX can be fully attributed to the diffe rences in the fitted DXB.The difference between the two methods is most significant when the energy is less than 0.7 keV,which is also the main energy band of SWCX emission.In addition,the difference between the two methods is not related to the SWCX intensity and,to some extent,to solar wind conditions,because SWCX intensity typically va ries with the solar wind.In summary,both methods are robust and reliable,and should be considered based on the best available options.

Evaluation of Coronal and Interplanetary Magnetic Field Extrapolation Using PSP Solar Wind Observation

Using solar wind observation near PSP perihelions as constraints,we have investigated the parameters in various PFSS model methods.It is found that the interplanetary magnetic field extrapolation with source surface height R_(SS)=2 Rs is better than that with R_(SS)=2.5 Rs.HMI and GONG magnetograms show similar performances in the simulation of magnetic field variation,but the former appears to have a slight advantage in reconstruction of intensity while the latter is more adaptable to sparser grids.The finite-difference method of constructing eigenvalue problems for potential fields can achieve similar accuracy as the analytic method and greatly improve the computational efficiency.MHD modeling performs relatively less well in magnetic field prediction,but it is able to provide rich information about solar-terrestrial space.

Effect of High-Speed Solar Winds Turbulence Upstream of the Earth’s Magnetosphere: Case of the Outer Minima of Solar Cycles 20, 21, 22, 23 and 24

Highly turbulent environment, the solar wind is a stream of very energetic particles mainly made of protons and electrons. During its trip in the interplanetary space, this solar flow becomes more accelerated during the outer minima (descending phases) of the solar cycles and can therefore influence all of humanity and its technology. These disturbances lead to socio-economic consequences requiring a precise knowledge of the climate variability. Using a statistical approach, we evaluate the response of the Earth’s magnetosphere to the High-Speed Solar Winds (HSSW) forcing during the peaks of the last five outer minima. To do so, 1UA data of solar wind and magnetic field parameters were extracted from OMNI browser. Analysis of the energetic solar plasma particles shows that strong geomagnetic field variations can occur even in the absence of large solar disturbances. While the normalized reconnection rate was estimated to be ~21% of the total variance of the magnetospheric variables, the upstream of the magnetic cavity was perturbed 80% of the time with large energies recorded. As a result, Earth’s magnetosphere becomes denser (i.e., more drag), which is a problem for spacecraft. Thus, the coupled solar wind-magnetosphere system follows scale-invariant dynamics and is in a state far from equilibrium. Our analysis provides insight into the main cause of geomagnetic storms with more than 97% of HSSW imposed in the range 300 - 850 km/s. These high-speeds lead to auroras that can disrupt electrical and communication systems.

Distributions and Structure of the Solar Wind during Solar Cycles 23 and 24

To observe the level of interaction between the solar wind and the geomagnetic activity, we analyzed the distribution of the solar wind speeds according to the different classes of geomagnetic activity and the different phases of solar activity. We found that, the magnetic quiet activity reccord 80% of the solar wind speeds V s observed 88% of solar wind speeds V > 450 km/s. The shock activity observes 82% of the solar wind speeds V > 450 km/s. About 70% of the solar wind speeds V > 450 km/s, are observed in the corotating activity class. The cloud shock activity and fluctuating activity classes observed respectively 37% and 55% of the wind speeds V > 450 km/s. Furthermore, slow solar winds are mainly observed at the minimum phase of each solar cycle;but exceptionally the solar maximum phase of solar cycle 24, records a significant rate of slow solar wind. Shock winds are mainly observed around the solar maximum and recurrent winds are mainly observed at the descending phase of the solar cycle. Corotating stable winds and moderate shock winds dominate respectively at the descending phase and at the maximum phase.

Solar Wind Interaction with Jupiter’s Magnetosphere

This paper studies the effects of the solar wind on Jupiter’s magnetosphere. The solar wind parameters are characterized using the Michigan Solar Wind Model (mSWiM) solar wind data propagated to Jupiter from 1997 to 2016. This analysis covers almost solar cycles 23 and 24. Interplanetary fast shocks: Forward shocks (FS), Reverse shocks (RS), and solar wind dynamic pressure were obtained and analyzed during the apparent opposition periods. The fast forward (FS) shocks were predominant during this period. Generally, the solar wind dynamic pressure from FS and RS shocks follows the solar cycles 23 and 24.

Alfvn Waves in the Solar Wind

Alfvn waves are found to be ubiquitous in the solar wind.Recent progress in observational studies of the waves is reviewed to formulate a microscopic picture for the Alfvenic fluctuations. The main aspects of the observational properties of these waves,including the wave intervals, propagation,evolution,origin and generation,are presented.Then Alfven wave heating and acceleration of the solar wind plasma are briefly introduced.The relation of the waves to rotational and tangential discontinuities,magnetic decreases,and other relatively large-scale structures such as flux tubes/ropes,magnetic clouds and interplanetary coronal mass ejections in the solar wind is particularly investigated.Finally,some remaining open questions are also indicated due to their fundamental importance of understanding of the physical nature of Alfven waves and the role of the waves in heating and accelerating the solar wind.

Tianwen-1 MINPA observations in the solar wind

The Mars Ion and Neutral Particle Analyzer(MINPA)is one of the three scientific instruments onboard the Tianwen-1 orbiter to investigate the Martian space environment.During Tianwen-1’s transfer orbit to Mars,the MINPA was switched on to measure the solar wind ions.Here,we present the first results of the MINPA observations in the solar wind.During cruise,nearly half of the MINPA ion field-of-view(FOV)was blocked by the lander capsule;thus only the solar-wind ions with azimuthal speeds pointing towards the unblocked FOV sectors could be detected.We perform a detailed comparison of the MINPA’s solar wind observations with data from Earth-based missions when MINPA reached its count-rate peak,finding a general consistency of the ion moments between them.The blocking effect due to the lander is evaluated quantitatively under varying solar-wind velocity conditions.Despite the blocking effect,the MINPA’s solar wind measurements during the transfer orbit suggest a good performance.

Ray Structure of the Coronal Streamer Belt and Its Manifestation as Sharp Large Peaks of Solar Wind Plasma Density at the Earth's Orbit

The white-light corona calibrated data with processing level L1 from the LASCO-C2/SOHO instrument, and data from the Wind spacecraft with one-hour and one-minute time resolution on quasi-stationary slow (v between 300-450 km/s at the Earth's orbit) the Solar Wind (SW) parameters in the absence of sporadic SW streams are examined. Within distances from the Sun's center less than R in the range of 20-30 Rs,(Rs, the solar radius), slow wind is known as the streamer belt, and at larger distances it is called the Heliospheric Plasma Sheet (HPS). It is shown that the streamer belt comprises a sequence of pairs of rays. In general, ray brightnesses in each pair can differ, and the magnetic field is oppositely directed in them. The neutral line of the radial magnetic field of the Sun runs along the belt between the rays of each of the pairs.The area in which the streamer belt intersects the ecliptic plane and which lies at the central meridian, will be recorded at the earth's orbit with a time delay of 5-6 days, in the form of one or several peaks with Nmax ＞ 10 cm-3. Furthermore, the simplest density profile of the portion of the HCS has the form of two peaks of a different or identical amplitude . The such a profile is observed in cases where the angle of intersection of the streamer belt with the ecliptic plane near the Sun is sufficiently large, i.e. close to 90°. The two-ray structure of the cross-section of the streamer-belt moves from the Sun to the Earth, it retains not only the angular size of the peaks but also the relative density variations, and the position of the neutral line(sector boundary) in between. At the Earth's orbit the ray structure of the streamer belt provides the source for sharp (i.e. with steep fronts of a duration of a few minutes or shorter) solar wind plasma density peaks (of a duration of several hours) with maximum values Nmax ＞ 10 cm-3.

Non-storm erosion of MeV electron outer radiation belt down to L^(*)<4.0 associated with successive enhancements of solar wind density

We report an unusual non-storm erosion event of outer zone MeV electron distribution during three successive solar wind number density enhancements(SWDEs)on November 27-30,2015.Loss of MeV electrons and energy-dependent narrowing of electron pitch angle distributions(PAD)first developed at L^(*)=5.5 and then moved down to L^(*)<4.According to the evolution of the electron phase space density(PSD)profile,losses of electrons with small pitch angles at L^(*)>4 during SWDE1 are mainly due to outward radial diffusion.However during SWDE2&3,scattering loss due to EMIC waves is dominant at 4<L^(*)<5.As for electrons with large pitch angles,outward radial diffusion is the primary loss mechanism throughout all SWDEs which is consistent with the incursion of the Last Closed Drift Shell(LCDS).The inner edge of EMIC wave activity moved from L^(*)~5 to L^(*)~4 and from L~6.4 to L~4.2 from SWDE1 to SWDE2&3,respectively,observed by Van Allen Probes and by ground stations.This is consistent with the inward penetration of anisotropic energetic protons from L^(*)=4.5 to L^(*)=3.5,suggesting that the inward extension of EMIC waves may be driven by the inward injection of anisotropic energetic protons from the dense plasma sheet.

Propagation of large-scale solar wind events in the outer heliosphere from a numerical MHD simulation

Voyager 1 occasionally detected sudden jumps of the local interstellar magnetic field strength since its heliopause crossing in August 2012.These events were believed to be associated with outward propagating solar wind shocks originating in the inner heliosphere.Here we investigate the correlation between interstellar shocks and large-scale solar wind events by means of numerical MHD simulation.The solar wind is simplified as a symmetric flow near the equatorial plane,and the interstellar neutrals are treated as a constant flow with a fixed density distribution along the upwind direction of the local interstellar medium.The charge exchanges between the solar wind plasma and the interstellar neutrals are taken into account.At a heliocentric distance of 1 AU,the solar wind data from OMNI,STEREO A and B during the period between 2010 and 2017 are used as the inner boundary conditions to drive the simulation.The simulation results showed that the solar wind gradually merges into large-scale structures as the radial distance increases,consistent with observations by New Horizons.After propagating into the inner heliosheath,the shocks are fully developed and the corresponding pressure pulses roughly agree with the observations by Voyager 2 in the inner heliosheath.The arrival of the shocks beyond the heliopause is estimated and found to be consistent with the observed signatures of interstellar shocks by Voyager 1.The possible origins of interstellar shocks in the inner heliosheath are discussed based on the simulation.

On Mechanisms of Proton Perpendicular Heating in the Solar Wind: Test Results Based on Wind Observations

The solar wind protons undergo significant perpendicular heating when they propagate in the interplanetary space.Stochastic heating and cyclotron resonance heating due to kinetic Alfvén waves(KAWs) are two proposed mechanisms. Which mechanism accounts for the perpendicular heating is still an open question. This paper performs tests for the two mechanisms based on Wind observations during 2004 June and 2019 May. Results show that heating rates in terms of stochastic heating theory considerably depend on the parameter of plasma β. For the solar wind with moderately high β, the theoretical heating rates are comparable to or larger than empirical heating rates, suggesting that the stochastic heating could be a powerful mechanism. For the solar wind with low β, on the contrary, the majority of data have theoretical heating rates much lower than empirical heating rates, showing that the stochastic heating seems to be weak in this case. On the other hand, it is found that, when the propagation angles of KAWs are around 70°, theoretically predicted damping wavenumbers of KAWs are equal to the observed wavenumbers at which magnetic energy spectra become significantly steep. This may imply that resonance heating due to cyclotron damping of KAWs could be another mechanism if KAWs have propagation angles around 70°.

Analysis of the Distribution,Rotation and Scale Characteristics of Solar Wind Switchbacks:Comparison between the First and Second Encounters of Parker Solar Probe

The S-shaped magnetic structure in the solar wind formed by the twisting of magnetic field lines is called a switchback,whose main characteristics are the reversal of the magnetic field and the significant increase in the solar wind radial velocity.We identify 242 switchbacks during the first two encounters of Parker Solar Probe.Statistics methods are applied to analyze the distribution and the rotation angle and direction of the magnetic field rotation of the switchbacks.The diameter of switchbacks is estimated with a minimum variance analysis(MVA)method based on the assumption of a cylindrical magnetic tube.We also make a comparison between switchbacks from inside and the boundary of coronal holes.The main conclusions are as follows:(1)the rotation angles of switchbacks observed during the first encounter seem larger than those of the switchbacks observed during the second encounter in general;(2)the tangential component of the velocity inside the switchbacks tends to be more positive(westward)than in the ambient solar wind;(3)switchbacks are more likely to rotate clockwise than counterclockwise,and the number of switchbacks with clockwise rotation is 1.48 and 2.65 times those with counterclockwise rotation during the first and second encounters,respectively;(4)the diameter of switchbacks is about 10;km on average and across five orders of magnitude(10^(3)–10^(7)km).

Testing the effect of solar wind parameters and geomagnetic storm indices on Galactic cosmic ray flux variation with automatically-selected Forbush decreases

Forbush decrease(FD),discovered by Scott E.Forbush about 80 years ago,is referred to as the non-repetitive short-term depression in Galactic cosmic ray(GCR)flux,presumed to be associated with large-scale perturbations in solar wind and interplanetary magnetic field(IMF).It is the most spectacular variability in the GCR intensity which appears to be the compass for investigators seeking solar-terrestrial relationships.The method of selection and validation of FD events is very important to cosmic ray(CR)scientists.We have deployed new computer software to determine the amplitude and timing of FDs from daily-averaged CR data at Oulu Neutron Monitor station.The code selected 230 FDs between 1998 and 2002.In an attempt to validate the new FD automated catalog,the relationship between the amplitude of FDs,and IMF,solar wind speed(SWS)and geomagnetic storm indices(Dst,kp,ap)is tested here.A two-dimensional regression analysis indicates significant linear relationship between large FDs(CR(%)≤-3)and solar wind data and geomagnetic storm indices in the present sample.The implications of the relationship among these parameters are discussed.

Alpha/proton Instability in the Presence of Proton and Alpha Temperature Anisotropy and its Application to the Deceleration of Alpha Particles in the Solar Wind

Observations of solar wind plasma find that the drift velocity between alpha beams and protons,vα,is reduced with the heliocentric distance,but the ratio of vαto vA(the local Alfven velocity)does not vary obviously and has a typical velocity ratio of vα/vA≤1.The alpha beam instability is believed to be responsible for the deceleration of alpha beams in the solar wind.However,the role of the temperature anisotropy of ions(protons and alpha particles)on the alpha beam instability has not been examined.Based on the kinetic theory of the PDRK solver,this study systemically investigates the alpha beam instability in the presence of proton and alpha temperature anisotropies and considers the effects of these temperature anisotropies on the instability at an arbitrary propagation angle relative to the ambient magnetic field.It is found that the real frequencies,growth rates and threshold conditions of the excited waves sensitively depend on the proton temperature anisotropy T_(i⊥)/T_(i‖)and the alpha temperature anisotropy T_(α⊥)/T_(α‖),as well as the parallel electron betaβe‖.In particular,for both alpha beam and proton temperature anisotropy driven cases,the parallel magnetosonic/whistler(PM/W),backward magnetosonic/whistler(BM/W)and oblique Alfvén/ion cyclotron(OA/IC)waves arise when T_(i⊥)/T_(i‖)<1,and the PM/W,OA/IC,parallel Alfven/ion cyclotron(PA/IC)and mirror waves occur when T_(i⊥)/T_(i‖)>1.The presence of anisotropic protons leads to a lower velocity threshold for the OA/IC and BM/W waves at T_(i⊥)/T_(i‖)<1 and for the OA/IC,PA/IC and mirror waves at T_(i⊥)/T_(i‖)>1.For both alpha beam and alpha temperature anisotropy driven cases,the PM/W and OA/IC waves are unstable when T_(α⊥)/T_(α‖)<1,and the PM/W,OA/IC,PA/IC,oblique magnetosonic/whistler(OM/W)and mirror waves are likely to grow as T_(α⊥)/T_(α‖)>1.The presence of anisotropic alpha particles results in a lower threshold velocity for the PM/W wave at T_(α⊥)/T_(α‖)<1 and for the PA/IC,OM/W and mirror waves at T_(α⊥)/T_(α‖)>1.On account of the influences of the anisotropic proton and alpha,these alpha beam instabilities can effectively constrain the alpha-beam drift velocity to less than or equal to the local Alfven velocity.These results may shed light on the evolution and deceleration mechanism of alpha particles in the solar wind.

Validation of the use of THEMIS-B and THEMIS-C as a near-Earth solar wind monitor

Two THEMIS(Time History of Events and Macroscale Interactions during Substorms)spacecraft,B and C,began orbiting the Moon in 2011 and have since provided routine measurements of the plasma conditions in the lunar orbit.In this study,we systematically compare these measurements in near-Earth space with solar wind measurements obtained from the Lagrangian 1(L1)point and propagated to the Earth,including measurements in the OMNI database and from the Wind spacecraft.A statistical comparison between THEMIS data and data from the OMNI database from September 2011 to December 2017 showed that the Y and Z components of the magnetic field and the flow speed are generally consistent between the two data sets.The ion number density and the dynamic pressure measured by THEMIS in near-Earth space are lower than those in the OMNI database,suggesting possible variation in the solar wind environment while propagating from the L1 point to near-Earth space.We further show two examples in which near-Earth solar wind measurements are applied in calculating the magnetopause location and in quantifying the magnetic field response to interplanetary shocks.Both examples suggest that using solar wind data from near-Earth space achieves better results than using solar wind data from the L1 point.These results provide validation of THEMIS-B and THEMIS-C as an alternative monitor of the near-Earth solar wind environment.

Is Solar Wind electron precipitation a source of neutral heating in the nightside Martian upper atmosphere?

Solar Wind(SW)electron precipitation is able to deposit a substantial amount of energy in the nightside Martian upper atmosphere,potentially exerting an influence on its thermal structure.This study serves as the first investigation of such an issue,with the aid of the simultaneous measurements of both neutral density and energetic electron intensity made on board the recent Mars Atmosphere and Volatile Evolution(MAVEN)spacecraft.We report that,from a statistical point of view,the existing measurements do not support a scenario of noticeable neutral heating via SW electron precipitation.However,during 3%−4%of the MAVEN orbits for which data are available,strong correlation between nightside temperature and electron intensity is observed,manifested as collocated enhancements in both parameters,as compared to the surrounding regions.In addition,our analysis also indicates that neutral heating via SW electron precipitation tends to be more effective at altitudes below 160 km for integrated electron intensities above 0.01 ergs·cm^−2·s^−1 over the energy range of 3−450 eV.The results reported here highlight the necessity of incorporating SW electron precipitation as a heat source in the nightside Martian upper atmosphere under extreme circumstances such as during interplanetary coronal mass ejections.

Effects of a dipole-like crustal field on solar wind interaction with Mars

A three-dimensional four species multi-fluid magnetohydrodynamic (MHD) model was constructed to simulate the solar wind global interaction with Mars. The model was augmented to consider production and loss of the significant ion species in the Martian ionosphere, i.e., H^+, O2^+, O^+, CO^+2, associated with chemical reactions among all species. An ideal dipole-like local crustal field model was used to simplify the empirically measured Martian crustal field. Results of this simulation suggest that the magnetic pile-up region (MPR) and the velocity profile in the meridian plane are asymmetric, which is due to the nature of the multi-fluid model to decouple individual ion velocity resulting in occurrence of plume flow in the northern Martian magnetotail. In the presence of dipole magnetic field model, boundary layers, such as bow shock (BS) and magnetic pile-up boundary (MPB), become protuberant. Moreover, the crustal field has an inhibiting effect on the flux of ions escaping from Mars, an effect that occurs primarily in the region between the terminator (SZA 90°) and the Sun Mars line of the magnetotail (SZA 180°), partially around the terminator region. In contrast, near the tailward central line the crustal field has no significant impact on the escaping flux.

Seasonal Variations of Solar Wind Parameters during Solar Cycles 23 and 24

In this paper, we analyzed diurnal and annual seasonal variations of solar wind parameters such as interplanetary magnetic field (IMF), proton density (N), solar wind speed (V) and solar wind dynamic pressure (Pdym), during the solar cycles 23 and 24. Our study shows that strong geomagnetic disturbances are observed at the equinoxes during both solar cycles. The highest proton densities are observed at solstices during both solar cycles. The greatest solar wind speeds are observed at the equinoxes of solar cycle 23 and at the solstices of solar cycle 24. The highest solar wind dynamic pressures are observed at the solstices of both solar cycles. We also observed an asymmetrical evolution of the seasonal diurnal values of the solar wind parameters during the two cycles, except for the proton density. Our investigations also highlight the fact that the seasonal diurnal values of the solar wind parameters are significant at solar cycle 23 compared to solar cycle 24 characterized by a global weak in solar plasma conditions since the deep minimum that followed the solar cycle 23 leading to an absence of a persistent polar coronal hole. The drop observed in polar field and solar winds parameters during solar cycle 24 is reproduced on seasons (solstices and equinoxes). The solar cycle 23 and 24 appear to be two magnetically opposite solar cycles regardless the time scales.

Relativity and the Solar Wind: The Maxwell-Equation Origins of the Solar-Wind Motional Electric Field

The motional electric field of the solar wind as seen by the Earth is examined theoretically and with spacecraft measurements. As it flows outward from the sun, the solar-wind plasma carries a spatially structured magnetic field with it. To calculate the motional electric field of the solar wind the spatially structured magnetic field is Lorentz transformed;for a full physical understanding, it is also necessary to Lorentz transform the current densities and charge densities in the solar wind. Referring to Maxwell’s equations, two related questions are asked: 1) Is the source of the solar-wind motional electric field charge density in the solar wind, time derivatives of current densities in the solar wind, or both? 2) Is the solar-wind motional electric field at Earth an electrostatic field, an induction field, or a superposition of the two? A Helmholtz decomposition of the motional electric field of the solar wind is made into a divergence-origin (electrostatic) and a curl-origin (induction) electric field. The global electric field associated with the outward advection of the global Parker-spiral magnetic field is found to be electrostatic with its origin being a distributed charge density in the solar-wind plasma. The electrostatic versus induction nature of the time-varying electric field associated with the advection of mesoscale magnetic structure varies with time as differently shaped magnetic structures in the solar-wind plasma pass the Earth;the mesoscale structure of the solar-wind plasma contains sheets of space charge and sheets wherein the current density has nonzero time derivatives.