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Register a new userEffects of alpha-proton differential flow on proton temperature anisotropy instabilities in the solar wind: Wind observations
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Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of evolution of the solar wind. Based on the Wind data during 2005-2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha-proton differential flow Vd. As Vd rotates from anti-Sunward direction to Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of Vd further illustrates that large kinetic energies of alpha-proton differential flow correspond to high occurrence rates of ECWs. These results are well consistent with theoretical predictions for effects of alpha-proton differential flow on proton temperature anisotropy instabilities.
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According to emph{Wind} observations between June 2004 and May 2019, this Letter investigates the proton and alpha particle temperatures in the space of ($theta_d$, $V_d/V_A$) for the first time, where $theta_d$ and $V_d$ are the radial angle and magnitude of alpha$-$proton differential flow vector respectively, $V_A$ is the local Alfven speed. Results show that the temperatures significantly depend on $theta_d$ as well as $V_d/V_A$. In case of low proton parallel beta ($beta_{p{parallel}} < 1$), it is found that the proton perpendicular temperature is clearly enhanced when $theta_d$ is small ($lesssim 45^circ$) and $V_d/V_A gtrsim 0.5$. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when $theta_d$ is large ($gtrsim 90^circ$) or $V_d/V_A$ is sufficiently small. The maximum of proton parallel temperature takes place at $theta_d sim 90^circ$ accompanied by higher $beta_{p{parallel}}$ and by larger turbulence amplitude of magnetic fluctuations in inertial range. This study should present strong evidence for cyclotron resonance heating of protons and alpha particles in the solar wind. Other mechanisms including Landau resonance and stochastic heating are also proposed, which tend to have different ($theta_d$, $V_d/V_A$) spaces than cyclotron resonance heating.
Kinetic plasma processes have been investigated in the framework of solar wind turbulence, employing Hybrid Vlasov-Maxwell (HVM) simulations. The dependency of proton temperature anisotropy T_{perp}/T_{parallel} on the parallel plasma beta beta_{parallel}, commonly observed in spacecraft data, has been recovered using an ensemble of HVM simulations. By varying plasma parameters, such as plasma beta and fluctuation level, the simulations explore distinct regions of the parameter space given by T_{perp}/T_{parallel} and beta_{parallel}, similar to solar wind sub-datasets. Moreover, both simulation and solar wind data suggest that temperature anisotropy is not only associated with magnetic intermittent events, but also with gradient-type structures in the flow and in the density. This connection between non-Maxwellian kinetic effects and various types of intermittency may be a key point for understanding the complex nature of plasma turbulence.
We use fluctuating magnetic helicity to investigate the polarisation properties of Alfvenic fluctuations at ion-kinetic scales in the solar wind as a function of $beta_p$, the ratio of proton thermal pressure to magnetic pressure, and $theta_{vB}$, the angle between the proton flow and local mean magnetic field, $mathbf{B}_0$. Using almost 15 years of textit{Wind} observations, we separate the contributions to helicity from fluctuations with wave-vectors, $textbf{k}$, quasi-parallel and oblique to $mathbf{B}_0$, finding that the helicity of Alfvenic fluctuations is consistent with predictions from linear Vlasov theory. This result suggests that the non-linear turbulent fluctuations at these scales share at least some polarisation properties with Alfven waves. We also investigate the dependence of proton temperature in the $beta_p$-$theta_{vB}$ plane to probe for possible signatures of turbulent dissipation, finding that it correlates with $theta_{vB}$. The proton temperature parallel to $mathbf{B}_0$ is higher in the parameter space where we measure the helicity of right-handed Alfvenic fluctuations, and the temperature perpendicular to $mathbf{B}_0$ is higher where we measure left-handed fluctuations. This finding is inconsistent with the general assumption that by sampling different $theta_{vB}$ in the solar wind we can analyse the dependence of the turbulence distribution on $theta_{kB}$, the angle between $textbf{k}$ and $mathbf{B}_0$. After ruling out both instrumental and expansion effects, we conclude that our results provide new evidence for the importance of local kinetic processes that depend on $theta_{vB}$ in determining proton temperature in the solar wind.
We report proton temperature anisotropy variations in the inner heliosphere with Parker Solar Probe (PSP) observations. Using a linear fitting method, we derive proton temperature anisotropy with temperatures measured by the Solar Probe Cup (SPC) from the SWEAP instrument suite and magnetic field observations from the FIELDS instrument suite. The observed radial dependence of temperature variations in the fast solar wind implies stronger perpendicular heating and parallel cooling than previous results from Helios measurements made at larger radial distances. The anti-correlation between proton temperature anisotropy and parallel plasma beta is retained in fast solar wind. However, the temperature anisotropies of the slow solar wind seem to be well constrained by the mirror and parallel firehose instabilities. The perpendicular heating of the slow solar wind inside 0.24 AU may contribute to its same trend up against mirror instability thresholds as fast solar wind. These results suggest that we may see stronger anisotropy heating than expected in inner heliosphere.
Based on in-situ measurements by Wind spacecraft from 2005 to 2015, this letter reports for the first time a clearly scale-dependent connection between proton temperatures and the turbulence in the solar wind. A statistical analysis of proton-scale turbulence shows that increasing helicity magnitudes correspond to steeper magnetic energy spectra. In particular, there exists a positive power-law correlation (with a slope $sim 0.4$) between the proton perpendicular temperature and the turbulent magnetic energy at scales $0.3 lesssim krho_p lesssim 1$, with $k$ being the wavenumber and $rho_p$ being the proton gyroradius. These findings present evidence of solar wind heating by the proton-scale turbulence. They also provide insight and observational constraint on the physics of turbulent dissipation in the solar wind.
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