Figure 4 - uploaded by Lan K Jian
Content may be subject to copyright.
Ion measurements from the Wind spacecraft during Event #1 on 19 March 2005. (first column) The stepwise black lines show the relative flux measurements from the SWE instrument as functions of energy/charge for five different values of θ Bn , the angle between B o and the projection axis on which the distribution was measured. The smooth curves display the ion distribution fits for proton core (red), proton beam, (blue), and alpha particle (violet) components. (second column) The proton and alpha particle velocity distributions in v ||-v ⊥ space as represented by the proton core, proton beam, and alpha particle fits.
Source publication
Intervals of enhanced magnetic fluctuations have been frequently observed in the solar wind. But it remains an open question as to whether these waves are generated at the Sun and then transported outward by the solar wind or generated locally in the interplanetary medium. Magnetic field and plasma measurements from the Wind spacecraft under slow s...
Context in source publication
Context 1
... analysis shows that the fluctuations of Events #1, #6, and #7 are predominantly left-hand polarized, whereas Event #4 corresponds primarily to right-hand polarization fluctuations as measured in the spacecraft frame. Figure 4 shows selected ion measurements from the SWE instrument during Event #1. Here θ Bn indicates the angle between the magnetic field vector and the projection axis on which the distribution was measured (a selection of 5 out of the 40 spin angles that make up the fit). ...
Similar publications
Anisotropic velocity distributions of protons have long been considered as free energy sources for exciting electromagnetic ion cyclotron (EMIC) waves in the Earth's magnetosphere. Here we rigorously calculated the proton anisotropy parameter using proton data obtained from Van Allen Probe-A observations. The calculations are performed for times du...
Citations
... The observations of right-handed fluctuations in the spacecraft frame were suggested to be caused by the Doppler shift of Alfvén ion-cyclotron waves propagating sunward in the plasma frame. The stability analysis of ion velocity distribution functions at 1 AU showed however that the low-frequency fluctuations can be also fast magnetosonic waves, right-handed in the plasma frame [6,7]. The occurrence of the low-frequency fluctuations is probably much higher than a few percent reported at 0.3-1 AU [3,4,5], since turbulence can mask their presence [8,9]. ...
... In both cases the amplitude of plasma density fluctuations is about 0.01N 0 . The amplitude of the compressible velocity component V || /v A ≈ 0.01 is smaller than the amplitude of the incompressible component, V ⊥ /v A ≈ 0.03, but still larger than in cold plasma, where according to Eqs. (6) and (7) we would have V || /V ⊥ ≈ tan θ ≈ 0.1. Both panels demonstrate that even by the time of 500 ω −1 ci none of the quantities exhibit nonlinear evolution. ...
Parker Solar Probe measurements have recently shown that coherent fast magnetosonic and Alfvén
ion-cyclotron waves are abundant in the solar wind and can be accompanied by higher-frequency
electrostatic fluctuations. In this letter we reveal the nonlinear process capable of channelling the
energy of low-frequency electromagnetic to higher-frequency electrostatic fluctuations observed
aboard Parker Solar Probe. We present Hall-MHD simulations demonstrating that low-frequency
electromagnetic fluctuations can resonate with the ion-sound mode, which results in steepening of
plasma density fluctuations, electrostatic spikes and harmonics in the electric field spectrum. The
resonance can occur around the wavenumber determined by the ratio between local sound and Alfvén
speeds, but only in the case of oblique propagation to the background magnetic field. The resonance
wavenumber, its width and steepening time scale are estimated, and all indicate that the revealed
two-wave resonance can frequently occur in the solar wind. This process can be a potential channel
of energy transfer from cyclotron resonant ions producing the electromagnetic fluctuations to Landau
resonant ions and electrons absorbing the energy of the higher-frequency electrostatic fluctuations.
... A series of studies has reported widespread observations of enhanced magnetic fluctuations near proton/electron cyclotron frequencies in the inner heliosphere (Jian et al. 2009;Breneman et al. 2010;Lacombe et al. 2014;Boardsen et al. 2015;Gary et al. 2016;Stansby et al. 2016;Zhao et al. 2019;Tong et al. 2019;Agapitov et al. 2020;Bowen et al. 2020;Cattell et al. 2020;Verniero et al. 2020;Liu et al. 2023). These electromagnetic waves at the ion/electron scale include Alfven ion cyclotron, fast magnetosonic, and whistler waves, and they are believed to play a crucial role in heating, accelerating, scattering coronal and solar wind plasma particles and regulating heat flux in solar wind through wave-particle resonant interactions (Pagel et al. 2007;Cattell & Vo 2021;Bowen et al. 2022;Squire et al. 2022;Raouafi et al. 2023). ...
... These electromagnetic waves at the ion/electron scale include Alfven ion cyclotron, fast magnetosonic, and whistler waves, and they are believed to play a crucial role in heating, accelerating, scattering coronal and solar wind plasma particles and regulating heat flux in solar wind through wave-particle resonant interactions (Pagel et al. 2007;Cattell & Vo 2021;Bowen et al. 2022;Squire et al. 2022;Raouafi et al. 2023). While many issues concerning the origin of such kinetic-scale waves are still debated, recent observations suggest that these waves could be locally generated in an interplanetary medium through plasma kinetic instabilities (Gary et al. 2016;Liu et al. 2023). Despite the well-known fact that velocity space micro-instabilities arise when the plasma deviates far from thermal equilibrium, the underlying physical mechanisms giving rise to the non-thermal features of particle velocity distributions observed in the inner heliosphere remain unclear. ...
The Korean heliospheric community, led by the Korea Astronomy and Space Science Institute (KASI), is currently assessing the viability of deploying a spacecraft at the Sun-Earth Lagrange Point L4 in collaboration with National Aeronautics and Space Administration (NASA). The aim of this mission is to utilize a combination of remote sensing and in situ instruments for comprehensive observations, complementing the capabilities of the L1 and L5 observatories. The paper outlines longterm scientific objectives, underscoring the significance of multi-point in-situ observations to better understand critical heliospheric phenomena. These include coronal mass ejections, magnetic flux ropes, heliospheric current sheets, kinetic waves and instabilities, suprathermal electrons and solar energetic particle events, as well as remote detection of solar radiation phenomena. Furthermore, the mission’s significance in advancing space weather prediction and space radiation exposure assessment models through the integration of L4 observations is discussed. This article is concluded with an emphasis on the potential of L4 observations to propel advancements in heliospheric science.
... Plasma waves driven by these departures from LTE are directly measurable in magnetic and electric field data and often probed via examination of magnetic field power spectra, magnetic field ellipticity (polarization), Poynting vectors, etc. (He et al. 2011;Podesta & Gary 2011a;Bowen et al. 2020a). Empirically (Jian et al. 2009(Jian et al. , 2010(Jian et al. , 2014Gary et al. 2016;Wicks et al. 2016;Klein et al. 2018;Bowen et al. 2020b;Verniero et al. 2020;Martinović et al. 2021), it suffices to restrict attention to quasi-parallel propagation with k × B = 0, in which case there are three main types of ion instability driven modes physically relevant in the solar wind. Alfvén/ion cyclotron (A/IC) instabilities are triggered by temperature anisotropies with T ⊥ /T ∥ > 1 and produce lefthanded (LH) circularly polarized waves in the particles' rest frame. ...
... However, the excellent agreement between the distribution of proton measurements in the (R p , β p,∥ ) plane and instability contours of constant growth rate (Kasper et al. 2002;Gary et al. 2003;Bale et al. 2009;Matteini et al. 2012), as well as statistical enhancements in the amplitude of magnetic field fluctuations at the boundaries of these measurements (Bale et al. 2009), is strong evidence that various kinetic instabilities are indeed active in the solar wind and are limiting the range of proton temperature anisotropies. However, the fact that it is the oblique rather than parallel instabilities that appear to constrain the data in (R p , β p,∥ ) space better, while evidence from both statistical surveys and case studies (Jian et al. 2010;Gary et al. 2016;Klein et al. 2018Klein et al. , 2019Martinović et al. 2021) suggest that parallel instabilities should be dominant, is perhaps evidence of the shortcomings of this approach and that other effects such as additional species or drift speeds need to be taken into account. Similar effects on the distribution of alpha particle measurements have also been observed (Gary et al. 2003;Maruca et al. 2012), suggesting analogous mechanisms at work. ...
... Our identification of the ion-scale wave modes active during this event as all being anti-sunward propagating is in agreement with previous case studies (Gary et al. 2016;Verniero et al. 2020), and statistical surveys Bowen et al. 2020a;Martinović et al. 2021), which concluded that the majority of observed ion-scale waves are propagating outward, thus maintaining their intrinsic plasma frame polarizations. Similarly, previous results (Jian et al. 2009;Bowen et al. 2020a) have suggested that LH events are more frequent than RH ones. ...
Ion-scale wave events or wave storms in the solar wind are characterized by enhancements in magnetic field fluctuations as well as coherent magnetic field polarization signatures at or around the local ion cyclotron frequencies. In this paper, we study in detail one such wave event from Parker Solar Probe's (PSP) fourth encounter, consisting of an initial period of left-handed (LH) polarization abruptly transitioning to a strong period of right-handed (RH) polarization, accompanied by a clear core beam structure in both the alpha and proton velocity distribution functions. A linear stability analysis shows that the LH-polarized waves are anti-sunward propagating Alfvén/ion cyclotron waves primarily driven by a proton cyclotron instability in the proton core population, and the RH polarized waves are anti-sunward propagating fast magnetosonic/whistler waves driven by a firehose-like instability in the secondary alpha beam population. The abrupt transition from LH to RH is caused by a drop in the proton core temperature anisotropy. We find very good agreement between the frequencies and polarizations of the unstable wave modes as predicted by linear theory and those observed in the magnetic field spectra. Given the ubiquity of ion-scale wave signatures observed by PSP, this work gives insight into which exact instabilities may be active and mediating energy transfer in wave–particle interactions in the inner heliosphere, as well as highlighting the role a secondary alpha population may play as a rarely considered source of free energy available for producing wave activity.
... 5281/zenodo.8313659. This treatment of the Wind SWE data has been previously employed in multiple studies [41][42][43][44] . ...
Both solar wind and ionospheric sources contribute to the magnetotail plasma sheet, but how their contribution changes during a geomagnetic storm is an open question. The source is critical because the plasma sheet properties control the enhancement and decay rate of the ring current, the main cause of the geomagnetic field perturbations that define a geomagnetic storm. Here we use the solar wind composition to track the source and show that the plasma sheet source changes from predominantly solar wind to predominantly ionospheric as a storm develops. Additionally, we find that the ionospheric plasma during the storm main phase is initially dominated by singly ionized hydrogen (H⁺), likely from the polar wind, a low energy outflow from the polar cap, and then transitions to the accelerated outflow from the dayside and nightside auroral regions, identified by singly ionized oxygen (O⁺). These results reveal how the access to the magnetotail of the different sources can change quickly, impacting the storm development.
... Once U pα crosses a local instability threshold, small-scale fluctuations in the electromagnetic field grow at the expense of the U pα , and, in turn, the instabilities regulate the value of U pα by limiting it to the local threshold (Marsch et al. 1982;Marsch & Livi 1987;Gary et al. 2000a;Verscharen et al. 2013a). The proton-α differential flow primarily excites two kinds of kinetic instabilities: a group of Alfvén/ion-cyclotron (A/IC) instabilities and the fast-magnetosonic/whistler (FM/W) instability (Gary 1993;Verscharen et al. 2013b;Gary et al. 2016). The parallel FM/W instability has lower thresholds than other α-particle-driven instabilities when β p is large, while the oblique A/IC instabilities have lower thresholds at large beam density or small β p (Gary et al. 2000a), where ...
Large-scale compressive slow-mode-like fluctuations can cause variations in the density, temperature, and magnetic-field magnitude in the solar wind. In addition, they also lead to fluctuations in the differential flow U p α between α -particles and protons (p), which is a common source of free energy for the driving of ion-scale instabilities. If the amplitude of the compressive fluctuations is sufficiently large, the fluctuating U p α intermittently drives the plasma across the instability threshold, leading to the excitation of ion-scale instabilities and thus the growth of corresponding ion-scale waves. The unstable waves scatter particles and reduce the average value of U p α . We propose that this “fluctuating-drift effect” maintains the average value of U p α well below the marginal instability threshold. We model the large-scale compressive fluctuations in the solar wind as long-wavelength slow-mode waves using a multi-fluid model. We numerically quantify the fluctuating-drift effect for the Alfvén/ion-cyclotron and fast-magnetosonic/whistler instabilities. We show that measurements of the proton– α differential flow and compressive fluctuations from the Wind spacecraft are consistent with our predictions for the fluctuating-drift effect. This effect creates a new channel for a direct cross-scale energy transfer from large-scale compressions to ion-scale fluctuations.
... The interpretation of the thermal-ion instabilities in the solar wind is usually based on the assumption of a homogeneous and stationary background (Gary et al. 2015;Jian et al. 2016;Bowen et al. 2020). However, the unstable waves are not the only observed fluctuations. ...
We investigate a secondary proton beam instability coexisting with the ambient solar wind turbulence at 50 R ☉ . Three-dimensional hybrid numerical simulations (particle ions and a quasi-neutralizing electron fluid) are carried out with the plasma parameters in the observed range. In the turbulent background, the particle distribution function, in particular the slope of the “bump-on-tail” responsible for the instability, is time-dependent and inhomogeneous. The presence of the turbulence substantially reduces the growth rate and saturation level of the instability. We derive magnetic power spectra from the observational data and perform a statistical analysis to evaluate the average turbulence intensity at 50 R ☉ . This information is used to link the observed frequency spectrum to the wavenumber spectrum in the simulations. We verify that Taylor’s frozen-in hypothesis is valid for this purpose to a sufficient extent. To reproduce the typical magnetic power spectrum of the instability observed concurrently with the background turbulence, an artificial spacecraft probe is run through the simulation box. The thermal-ion instabilities are often seen as power elevations in the kinetic range of scales above an extrapolation of the turbulence spectrum from larger scales. We show that the elevated power in the simulations is much higher than the background level. Therefore, the turbulence at the average intensity does not obscure the secondary proton beam instability, as opposed to the solar wind at 1 au, in which the ambient turbulence typically obscures thermal-ion instabilities.
... The departure from local thermodynamic equilibrium provides sufficient free energy to drive kinetic instabilities generating unstable waves (Chen et al. 2016;Gary et al. 2016;Klein et al. 2021). The instabilities have been comprehensively discussed in the framework of linear theory, considering both temperature anisotropies for the two proton populations and the differential flow along the background magnetic field (Gary 1993;Yoon 2017). ...
... It enables the global optimum by optimizing a fitness function, in contrast to the traditional Levenberg-Marquardt (LM) method, which is more likely to attain the local optimum. Previous works (Gary et al. 2016;Wicks et al. 2016;Bowen et al. 2020b) on fitting observed VDFs usually assume that the beam only drifts along the local magnetic field. They employ the fitting procedure in 1D or 2D velocity space, which requires fewer fitting parameters. ...
... We find only one unstable mode branch (Figure 2(c)). Typically, there are four modes associated with the bi-Maxwellian fitting result (Gary et al. 2016;Klein et al. 2021): parallel-and antiparallel-propagating A/IC modes and FM/W modes. In this study, the only unstable mode is the antiparallelpropagating (ω r /ω c,p < 0, where ω r is the wave frequency and (h, i) Two snapshots of the isosurfaces of the VDFs (red for core, blue for beam, and green for total) at ω c,p t/2π = 0, which are viewed from two orthogonal perspectives, both perpendicular to the background magnetic field. ...
The proton beam is an important population of the non-Maxwellian proton velocity distribution in the solar wind, but its role in wave activity remains unclear. In particular, the velocity vector of the proton beam and its influence on wave growth/damping have not been addressed before. Here we explore the origin and the associated particle dynamics of a kinetic wave event in the solar wind by analyzing measurements from Solar Orbiter and comparing them with theoretical predictions from linear Vlasov theory. We identify the waves as outward-propagating circularly polarized fast magnetosonic/whistler (FM/W) waves. The proton’s velocity distribution functions can destabilize FM/W waves. According to linear Vlasov theory, the velocity fluctuations of the core and the beam associated with FM/W waves render the original field-aligned background drift velocity non-field-aligned. This non-field-aligned drift velocity carrying the information of the velocity fluctuations of the core and the beam is responsible for the wave growth/damping. Specifically, for the FM/W waves we analyze, the non-field-aligned fluctuating velocity of the beam population is responsible for the growth of these unstable waves in the presence of a proton beam. In contrast, the core population plays the opposite role, partially suppressing the wave growth. Remarkably, the observed drift velocity vector between the core and the beam is not field aligned during an entire wave period. This result contrasts the traditional expectation that the proton beam is field aligned.
... The proton-α differential flow primarily excites two kinds of kinetic instabilities: a group of A/IC instabilities and the FM/W instability (Gary 1993;Verscharen et al. 2013b;Gary et al. 2016). The parallel FM/W instability has lower thresholds than other αparticle-driven instabilities when β p is large, while the oblique A/IC instabilities have lower thresholds at large beam density or small β p (Gary et al. 2000a), where ...
... The proton-α differential flow primarily excites two kinds of kinetic instabilities: a group of A/IC instabilities and the FM/W instability (Gary 1993;Verscharen et al. 2013b;Gary et al. 2016). The parallel FM/W instability has lower thresholds than other αparticle-driven instabilities when β p is large, while the oblique A/IC instabilities have lower thresholds at large beam density or small β p (Gary et al. 2000a), where ...
... The compressive behavior of slow-mode-like fluctuations at large scales depends on the choice of the effective polytropic indexes γ j of the involved plasma species. For large-scale kinetic slow modes, the effective polytropic indexes are γ e = 1, γ p = 3, and γ α = 3 (Gary 1993). This choice represents one-dimensionally adiabatic protons and isothermal electrons. ...
Large-scale compressive slow-mode-like fluctuations can cause variations in the density, temperature, and magnetic-field magnitude in the solar wind. In addition, they also lead to fluctuations in the differential flow $U_{\rm p\alpha}$ between $\alpha$-particles and protons ($p$), which is a common source of free energy for the driving of ion-scale instabilities. If the amplitude of the compressive fluctuations is sufficiently large, the fluctuating $U_{\rm p\alpha}$ intermittently drives the plasma across the instability threshold, leading to the excitation of ion-scale instabilities and thus the growth of corresponding ion-scale waves. The unstable waves scatter particles and reduce the average value of $U_{\rm p\alpha}$. We propose that this "fluctuating-beam effect" maintains the average value of $U_{\rm p\alpha}$ well below the marginal instability threshold. We model the large-scale compressive fluctuations in the solar wind as long-wavelength slow-mode waves using a multi-fluid model. We numerically quantify the fluctuating-beam effect for the Alfv\'en/ion-cyclotron (A/IC) and fast-magnetosonic/whistler (FM/W) instabilities. We show that measurements of the proton-$\alpha$ differential flow and compressive fluctuations from the {\it Wind} spacecraft are consistent with our predictions for the fluctuating-beam effect. This effect creates a new channel for a direct cross-scale energy transfer from large-scale compressions to ion-scale fluctuations.
... However, most of these modes are not predicted to exist in the solar wind based on traditional models of Alfvénic turbulence, and most are strongly damped under typical solar wind conditions. Many instabilities expected to operate in the solar wind saturate by generating such wave modes, but a causal connection with regions of unstable plasma and the observed modes is difficult with single-spacecraft measurements, although attempts have been made [9]. Also, the sub-dominant modes likely generated by instabilities are difficult to resolve, because they are masked by the more energetic Alfvénic turbulence. ...
... Current observations do not provide clarity. For example, intense ICWs are commonly observed during times when plasma instabilities are present [114] in extended "storms" during quiet SW and radial IMF [115]. Because ICWs are capable of substantial heating of SW ions, it is important to understand exactly how often they occur. ...
HelioSwarm (HS) is a NASA Medium-Class Explorer mission of the Heliophysics Division designed to explore the dynamic three-dimensional mechanisms controlling the physics of plasma turbulence, a ubiquitous process occurring in the heliosphere and in plasmas throughout the universe. This will be accomplished by making simultaneous measurements at nine spacecraft with separations spanning magnetohydrodynamic and sub-ion spatial scales in a variety of near-Earth plasmas. In this paper, we describe the scientific background for the HS investigation, the mission goals and objectives, the observatory reference trajectory and instrumentation implementation before the start of Phase B. Through multipoint, multiscale measurements, HS promises to reveal how energy is transferred across scales and boundaries in plasmas throughout the universe.
