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Register a new userTime-dependent occurrence rate of electromagnetic cyclotron waves in the solar wind: evidence for effect of alpha particles?
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Previous studies revealed that electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency exist widely in the solar wind, and the majority of ECWs are left-handed (LH) polarized waves. Using the magnetic field data from the STEREO mission, this Letter carries out a survey of ECWs over a long period of 7 years, and calculates the occurrence rates of ECWs with different polarization senses. Results show that the occurrence rate is nearly a constant for the ECWs with right-handed polarization, but it varies significantly for the ECWs with LH polarization. Further investigation of plasma conditions reveals that the LH ECWs take place preferentially in a plasma characterized by higher temperature, lower density, and larger velocity. Some considerable correlations between the occurrence rate of LH ECWs and the properties of ambient plasmas are discussed. The present research may provide evidence for effect of alpha particles on generation of ECWs.
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Electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency are frequently observed in the solar wind, yet their generation mechanism is still an open question. Based on the Wind data during the years 2005$-$2015, this paper carries out a statistical study on plasma characteristics associated with the occurrence of ECWs. The probability density distributions (PDDs) of proton temperature anisotropy ($T_perp/T_parallel$) and proton parallel beta ($beta_parallel$) are investigated, where $perp$ and $parallel$ refer to perpendicular and parallel to the background magnetic field, respectively. The PDDs depend on solar wind types as well as wave polarizations, and those for ECWs with left-handed (LH) polarization exhibit considerable differences from the PDDs for ambient solar winds. The distributions of occurrence rates of LH ECWs in ($beta_parallel$, $T_perp/T_parallel$) space show a tendency that the occurrence rates increase with proton temperature anisotropies. The $beta_parallel$ with maximum of occurrence rates is near 0.1 when $T_perp/T_parallel > 1$ while it is around 1 when $T_perp/T_parallel < 1$. The presence of alpha$-$proton differential flow with large kinetic energy corresponds to a much high occurrence rate as well as the domination of LH polarization of ECWs. Based on these observations and existing theories, we propose that the proton cyclotron and parallel firehose instabilities with effects of alpha$-$proton differential flow are likely responsible for the local generation of LH ECWs in the solar wind. The generation mechanism of right-handed ECWs seems to be complicated and more discussions are needed in future researches.
Observations of plasma waves by the Fields Suite and of electrons by the Solar Wind Electrons Alphas and Protons Investigation (SWEAP) on Parker Solar Probe provide strong evidence for pitch angle scattering of strahl-energy electrons by narrowband whistler-mode waves at radial distances less than ~0.3 AU. We present two example intervals of a few hours that include 8 waveform captures with whistler-mode waves and 26 representative electron distributions that are examined in detail. Two were narrow; 17 were clearly broadened, and 8 were very broad. The two with narrow strahl occurred when there were either no whistlers or very intermittent low amplitude waves. Six of the eight broadest distributions were associated with intense, long duration waves. Approximately half of the observed electron distributions have features consistent with an energy dependent scattering mechanism, as would be expected from interactions with narrowband waves. A comparison of the wave power in the whistler-mode frequency band to pitch angle width and a measure of anisotropy provides additional evidence for the electron scattering by whistler-mode waves. The pitch angle broadening occurs in over an energy range comparable to that obtained for the n=1 (co-streaming) resonance for the observed wave and plasma parameters. The additional observation that the heat flux is lower in the interval with multiple switchbacks may provide clues to the nature of switchbacks. These results provide strong evidence that the heat flux is reduced by narroweband whistler-mode waves scattering of strahl-energy electrons.
Direct evidence of an inertial-range turbulent energy cascade has been provided by spacecraft observations in heliospheric plasmas. In the solar wind, the average value of the derived heating rate near 1 au is $sim 10^{3}, mathrm{J,kg^{-1},s^{-1}}$, an amount sufficient to account for observed departures from adiabatic expansion. Parker Solar Probe (PSP), even during its first solar encounter, offers the first opportunity to compute, in a similar fashion, a fluid-scale energy decay rate, much closer to the solar corona than any prior in-situ observations. Using the Politano-Pouquet third-order law and the von Karman decay law, we estimate the fluid-range energy transfer rate in the inner heliosphere, at heliocentric distance $R$ ranging from $54,R_{odot}$ (0.25 au) to $36,R_{odot}$ (0.17 au). The energy transfer rate obtained near the first perihelion is about 100 times higher than the average value at 1 au. This dramatic increase in the heating rate is unprecedented in previous solar wind observations, including those from Helios, and the values are close to those obtained in the shocked plasma inside the terrestrial magnetosheath.
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.
We investigate the spatial correlation properties of the solar wind using simultaneous observations by the ACE and WIND spacecraft. We use mutual information as a nonlinear measure of correlation and compare this to linear correlation. We find that the correlation lengthscales of fluctuations in density and magnetic field magnitude vary strongly with the solar cycle, whereas correlation lengths of fluctuations in B field components do not. We find the correlation length of |B| ~ 120 Re at solar minimum and ~ 270 Re at maximum and the correlation length of density ~ 75 Re at minimum and ~ 170 Re at minimum. The components of the B field have correlation lengths ~ correlation length |B| at minimum.
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