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Probability density distribution of solar wind data in the βp-θvB plane, calculated using Equation 18. The dashed black lines indicate the isocontours of σ ̃ m (θvB ) = 0 mirrored about the line θvB = 90◦ (see main text).
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We use fluctuating magnetic helicity to investigate the polarization properties of Alfvénic fluctuations at ion-kinetic scales in the solar wind as a function of βp, the ratio of proton thermal pressure to magnetic pressure, and θvB, the angle between the proton flow and local mean magnetic field, B0. Using almost 15 yr of Wind observations, we sep...
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... After removing the T-V relationship from different PSP encounters (see Appendix), Fig. 7 shows that there was a consistent relationship between the change in temperature ( ) and magnetic field angle from radial ( ). This is more suggestive of an in situ heating mechanisms that is dependent on the angle of the magnetic field (Woodham et al. 2021b;Opie et al. 2022), rather than the presence of the magnetic deflection itself. ...
... The consistent rate of temperature increase across the PSP encounters does suggest that the temperature enhancements are created by some in situ mechanism that is dependent on the magnetic field angle (Opie et al. 2022). For example, using Wind spacecraft data at 1 AU, Woodham et al. (2021b) interpreted changes in , ∥ and ,⊥ with as turbulent dissipation through kinetic Alfvén waves and Alfvén ion cyclotron waves, respectively. Similarly, D' Amicis et al. (2019) also demonstrated a strong positive correlation with ,⊥ and using Wind, which they linked to ion-cyclotron resonance. ...
... After removing the larger scale T-V relationship, we demonstrated that the proton core temperature increases associated with the deflections were remarkably consistent with magnetic field angle across the PSP encounters. The strong link between temperature and magnetic field angle does imply that an underlying mechanism is responsible for the characteristic temperature increases, such as: heating through the phase steepening of Alfvén waves (Vasquez & Hollweg 1998;González et al. 2021); turbulent dissipation (Woodham et al. 2021b); shear induced effects (Del Sarto & Pegoraro 2018); or wave-particle interactions (D'Amicis et al. 2019;Opie et al. 2022). So while we believe our results do support the growing evidence from remote sensing and simulation domains, we cannot yet rule out in situ mechanisms from our results alone. ...
Measurements of transverse magnetic field and velocity components from Parker Solar Probe have revealed a coherent quasi-periodic pattern in the near-Sun solar wind. As well as being Alfvénic and arc-polarised, these deflections were characterised by a consistent orientation and an increased proton core temperature, which was greater parallel to the magnetic field. We show that switchbacks represent the largest deflections within this underlying structure, which is itself consistent with the expected outflow from interchange reconnection simulations. Additionally, the spatial scale of the deflections was estimated to be around 1 Mm on the Sun, comparable to the jetting activity observed at coronal bright points within the base of coronal plumes. Therefore, our results could represent the in situ signature of interchange reconnection from coronal bright points within plumes, complementing recent numerical and observational studies. We also found a consistent relationship between the proton core temperature and magnetic field angle across the Parker Solar Probe encounters and discussed how such a persistent signature could be more indicative of an in situ mechanism creating a local increase in temperature. In future, observations of minor ions, radio bursts and remote sensing images could help further establish the connection between reconnection events on the Sun and signatures in the solar wind.
... Using the above frequency and wavenumber information, we find that about 40% (44%) of the LH wave events are limited to f pl /f cp < 0.5 (λ p k < 0.8) and about 74% (85%) of the RH wave events are limited to f pl /f cp < 1 (λ p k < 0.8) in the data set in Figure 4. We note that this estimation is sensitive to the frequency and wavenumber thresholds, which are closely related to the plasma parameters (Liu et al. 2021). Woodham et al. 2021). Consequently, this sampling effect is one key factor determining the occurrence rate of the observed ion-scale waves (Bowen et al. 2020b). ...
Determining the mechanism responsible for plasma heating and particle acceleration is a fundamental problem in the study of the heliosphere. Due to efficient wave–particle interactions of ion-scale waves with charged particles, these waves are widely believed to be a major contributor to ion energization, and their contribution considerably depends on the wave occurrence rate. By analyzing the radial distribution of quasi-monochromatic ion-scale waves observed by the Parker Solar Probe, this work shows that the wave occurrence rate is significantly enhanced in the near-Sun solar wind, specifically 21%–29% below 0.3 au, in comparison to 6%–14% beyond 0.3 au. The radial decrease of the wave occurrence rate is not only induced by the sampling effect of a single spacecraft detection, but also by the physics relating to the wave excitation, such as the enhanced ion beam instability in the near-Sun solar wind. This work also shows that the wave normal angle θ , the absolute value of ellipticity ϵ , the wave frequency f normalized by the proton cyclotron frequency f cp , and the wave amplitude δ B normalized by the local background magnetic field B 0 slightly vary with the radial distance. The median values of θ , ∣ ϵ ∣, f , and δ B are about 9°, 0.73, 3 f cp , and 0.01 B 0 , respectively. Furthermore, this study proposes that the wave mode natures of the observed left-handed and right-handed polarized waves correspond to the Alfvén ion cyclotron mode wave and the fast magnetosonic whistler mode wave, respectively.
... Determining the nature of these mechanisms requires observing 3D distributions of the turbulent fluctuations. Plasma turbulence is inherently anisotropic due to preferred directions associated with the IMF [11,40], radial expansion [41] and large-scale gradients [42][43][44]. If turbulent fluctuations vary primarily parallel to the IMF (slab-like) [45], then noncompressive, Alfvénic fluctuations would, at small scales, ultimately dissipate energy via ion-cyclotron wave (ICW) heating [46]. ...
... These studies also frequently assume that the turbulence is insensitive to the angle between the SW velocity and the magnetic field, using variations in θ vB to study the functional dependence of the turbulence on the angle between the wavevector and magnetic field θ kB . Recent work [41] suggests that this assumption may not be valid; verifying this claim will require sampling the turbulent structures both along and transverse to the magnetic field direction simultaneously, a measurement that HS is designed to produce. With HS, the Taylor hypothesis can be directly evaluated. ...
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.
... For the wave and turbulence in the solar wind, their observations from a single spacecraft are significantly affected by the sampling effect (e.g., Fredricks & Coroniti 1976;Howes & Quataert 2010;Horbury et al. 2012;Bowen et al. 2020b;Woodham et al. 2021). Consequently, this sampling effect is one key factor determining the occurrence rate of the observed ion-scale waves (Bowen et al. 2020b). ...
Determining the mechanism responsible for the plasma heating and particle acceleration is a fundamental problem in the study of the heliosphere. Due to efficient wave-particle interactions of ion-scale waves with charged particles, these waves are widely believed to be a major contributor to ion energization, and their contribution considerably depends on the wave occurrence rate. By analyzing the radial distribution of quasi-monochromatic ion-scale waves observed by the Parker Solar Probe, this work shows that the wave occurrence rate is significantly enhanced in the near-Sun solar wind, specifically 21%$-$29% below 0.3 au, in comparison to 6%$-$14% beyond 0.3 au. The radial decrease of the wave occurrence rate is not only induced by the sampling effect of a single spacecraft detection, but also by the physics relating to the wave excitation, such as the enhanced ion beam instability in the near-Sun solar wind. This work also shows that the wave normal angle $\theta$, the absolute value of ellipticity $\epsilon$, the wave frequency $f$ normalized by the proton cyclotron frequency $f_{\mathrm{cp}}$, and the wave amplitude $\delta B$ normalized by the local background magnetic field $B_0$ slightly vary with the radial distance. The median values of $\theta$, $|\epsilon|$, $f$, and $\delta B$ are about $9^\circ$, $0.73$, $3f_{\mathrm{cp}}$, and $0.01B_0$, respectively. Furthermore, this study proposes that the wave mode nature of the observed left-handed and right-handed polarized waves corresponds to the Alfv\'en ion cyclotron mode wave and the fast-magnetosonic whistler mode wave, respectively.
... A complex mixture of effects-nonlinear Alfvén wave dynamics on fluid scales, various kinetic instabilities on ion scales, and the solar wind expansion-depend also on solar wind structure and internal correlations in the solar wind (e.g., Perrone et al. 2019aPerrone et al. , 2019b, and references therein), where plasma flow speed (e.g., Bruno et al. 2014a;Verscharen et al. 2021) and plasma beta (e.g., Duan et al. 2020;Woodham et al. 2021a) control many fluctuation properties. The wide range of temporal and spatial scales as well as parameter ranges involved complicate their understanding via numerical simulation and investigations with spacecraft observations. ...
... However, there is no strong damping of density fluctuations at the low-frequency range in the simulation results. Observations and simulations show that the fluctuation intensity evolves differently for different frequency ranges, i.e., there is a spectrum shape change around ion-kinetic scales likely supported by local kinetic processes (e.g., plasma instabilities and Landau damping; see Hellinger et al. 2015Hellinger et al. , 2019Woodham et al. 2021a). ...
Revealing the formation, dynamics, and contribution to plasma heating of magnetic field fluctuations in the solar wind is an important task for heliospheric physics and for a general plasma turbulence theory. Spacecraft observations in the solar wind are limited to spatially localized measurements, so that the evolution of fluctuation properties with solar wind propagation is mostly studied via statistical analyses of data sets collected by different spacecraft at various radial distances from the Sun. In this study we investigate the evolution of turbulence in the Earth’s magnetosheath, a plasma system sharing many properties with the solar wind. The near-Earth space environment is being explored by multiple spacecraft missions, which may allow us to trace the evolution of magnetosheath fluctuations with simultaneous measurements at different distances from their origin, the Earth’s bow shock. We compare ARTEMIS and Magnetospheric Multiscale (MMS) Mission measurements in the Earth magnetosheath and Parker Solar Probe measurements of the solar wind at different radial distances. The comparison is supported by three numerical simulations of the magnetosheath magnetic and plasma fluctuations: global hybrid simulation resolving ion kinetic and including effects of Earth’s dipole field and realistic bow shock, hybrid and Hall-MHD simulations in expanding boxes that mimic the magnetosheath volume expansion with the radial distance from the dayside bow shock. The comparison shows that the magnetosheath can be considered as a miniaturized version of the solar wind system with much stronger plasma thermal anisotropy and an almost equal amount of forward and backward propagating Alfvén waves. Thus, many processes, such as turbulence development and kinetic instability contributions to plasma heating, occurring on slow timescales and over large distances in the solar wind, occur more rapidly in the magnetosheath and can be investigated in detail by multiple near-Earth spacecraft.
... Figure 6 shows that T ANI approaches 1 in the radial IMF event. Before the discussions about the T ANI , we should point out that the uncertainties of perpendicular and parallel temperature determination change with the angle between the sampling and magnetic field directions [43]. When this angle approaches 0 • , the perpendicular uncertainty increases, and the parallel one decreases. ...
Long-lasting radial interplanetary magnetic field (IMF) intervals in which IMF points along the solar wind velocity for several hours have many interesting properties. We investigate the average parameters and the behavior of magnetic field fluctuations within 419 such radial intervals. The power spectral density (PSD) calculated over 1-h intervals of a radial IMF is compared with PSDs in adjacent regions prior to and after the radial IMF. We concentrate on (1) the power of IMF fluctuations, (2) the median slopes of PSDs in both inertial and kinetic ranges, (3) the proton temperature and its anisotropy, and (4) the occurrence rate of wavy structures and their polarization. We have shown that the fluctuation amplitude is low in the radial IMF intervals in both magnetohydrodynamic (MHD) and kinetic ranges, and the spectral power increases with the cone angle in the MHD range. We discuss this effect in the light of present knowledge on plasma turbulence and peculiarities of observations of magnetic field variations under the radial background magnetic field. We found that in the radial IMF events, the proton temperature is more isotropic, the occurrence rate of waves is higher, and the waves have no preferred polarization in the frequency range from 0.1 to 1 Hz. It suggests that the radial IMF structure leads to a different development of turbulence than the typical Parker-spiral structure.
... The reasons can be twofold-first, the effects of the pseudo-temperature and, second, intensive proton and/or alpha particle beams that can exceed 30% of the proton core current. The analysis of effects of temperature anisotropy determined from measurements of the FC onboard Wind spacecraft [35] shows a slight enhancement of T for cone angles around 90 • . This trend is consistent with our observations but applicability of their results to our measurements is limited because the Wind FC uses different principle for a temperature determination and, as they show, the uncertainty of T determination is as large as 30-40% under this condition. ...
Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind, thus the turbulent behavior of these quantities is intensively studied. The temperature is also highly fluctuating quantity but its variations are studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of temperature variations requires knowledge of the full velocity distribution with an appropriate time resolution but such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board Spektr-R used the Maxwellian approximation and provided the thermal velocity with a 32 ms resolution, investigating factors influencing the temperature power spectral density shape. We discuss the question whether the temperature spectra determined from Faraday cups are real or apparent and analyze mutual relations of power spectral densities of parameters like the density, parallel and perpendicular components of the velocity and magnetic field fluctuations. Finally, we compare their spectral slopes with the slopes of the thermal velocity in both inertial and kinetic ranges and their evolution in course of solar wind expansion.
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.
Alfvén ion cyclotron waves (ACWs) and kinetic Alfvén waves (KAWs) are found to exist at < 0.3~au observed by Parker Solar Probe in Alfvénic slow solar winds. To examine the statistical properties of the background parameters for ACWs and KAWs and related wave disturbances, both wave events observed by Parker Solar Probe are selected and analyzed. The results show that there are obvious difference in the background and disturbance parameters between ACWs and KAWs. ACW events have a relatively higher occurrence rate but with a total duration slightly shorter than KAW events. The median background magnetic field magnitude and the related background solar wind speed of KAW events are larger than those of ACWs. The distributions of the relative disturbances of the proton velocity, proton temperature, he proton number density, and β cover wider ranges for ACW events than for KAW events. The results may be important for the understanding of the nature and characteristics of Alfvénic slow solar wind fluctuations at ion scales near the Sun, and provide the information of the background field and plasma parameters and the wave disturbances of ACWs and KAWs for further relevant theoretical modelling or numerical simulations.
Evidence for the presence of ion cyclotron waves (ICWs), driven by turbulence, at the boundaries of the current sheet is reported in this paper. By exploiting the full potential of the joint observations performed by Parker Solar Probe and the Metis coronagraph on board Solar Orbiter, local measurements of the solar wind can be linked with the large-scale structures of the solar corona. The results suggest that the dynamics of the current sheet layers generates turbulence, which in turn creates a sufficiently strong temperature anisotropy to make the solar-wind plasma unstable to anisotropy-driven instabilities such as the Alfvén ion cyclotron, mirror-mode, and firehose instabilities. The study of the polarization state of high-frequency magnetic fluctuations reveals that ICWs are indeed present along the current sheet, thus linking the magnetic topology of the remotely imaged coronal source regions with the wave bursts observed in situ. The present results may allow improvement of state-of-the-art models based on the ion cyclotron mechanism, providing new insights into the processes involved in coronal heating.
