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تسجيل مستخدم جديدValidity of the Taylor Hypothesis for Linear Kinetic Waves in the Weakly Collisional Solar Wind
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The interpretation of single-point spacecraft measurements of solar wind turbulence is complicated by the fact that the measurements are made in a frame of reference in relative motion with respect to the turbulent plasma. The Taylor hypothesis---that temporal fluctuations measured by a stationary probe in a rapidly flowing fluid are dominated by the advection of spatial structures in the fluid rest frame---is often assumed to simplify the analysis. But measurements of turbulence in upcoming missions, such as Solar Probe Plus, threaten to violate the Taylor hypothesis, either due to slow flow of the plasma with respect to the spacecraft or to the dispersive nature of the plasma fluctuations at small scales. Assuming that the frequency of the turbulent fluctuations is characterized by the frequency of the linear waves supported by the plasma, we evaluate the validity of the Taylor hypothesis for the linear kinetic wave modes in the weakly collisional solar wind. The analysis predicts that a dissipation range of solar wind turbulence supported by whistler waves is likely to violate the Taylor hypothesis, while one supported by kinetic Alfven waves is not.
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A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfven-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflecti
on-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ($P_{rm AWb}$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$, which we denote $chi_{rm H}(r)$, and the fraction that is transferred via work, which we denote $chi_{rm W}(r)$. We find that $chi_{rm W}(r_{rm A})$ ranges from 0.15 to 0.3, where $r_{rm A}$ is the Alfven critical point. This value is small compared to~one because the Alfven speed $v_{rm A} $ exceeds the outflow velocity $U$ at $r<r_{rm A}$, so the AWs race through the plasma without doing much work. At $r>r_{rm A}$, where $v_{rm A} < U$, the AWs are in an approximate sense stuck to the plasma, which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $r=r_{rm A}$, so the total rate at which AWs do work on the plasma at $r>r_{rm A}$ is a modest fraction of $P_{rm AWb}$. We find that heating is more effective than work at $r<r_{rm A}$, with $chi_{rm H}(r_{rm A})$ ranging from 0.5 to 0.7. The reason that $chi_{rm H} geq 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfven-speed scale height in RDAWT, and there are a few Alfven-speed scale heights between the coronal base and $r_{rm A}$.
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The joint distribution of the normalized reduced magnetic helicity ${sigma}_m ({theta}_{RB}, {tau})$ is obtained, where ${theta}_{RB}$ is the angle between the local mean magnetic field and the radial direction and ${tau}$ is the temporal scale. Two populations around ion scales are identified: the first population has ${sigma}_m ({theta}_{RB}, {tau}) < 0$ for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for $60^{circ} < {theta}_{RB} < 130^{circ}$, corresponding to kinetic Alfven waves (KAWs), and the second population has ${sigma}_m ({theta}_{RB}, {tau}) > 0$ in the frequency range [1.4, 4.9] Hz for ${theta}_{RB} > 150^{circ}$, corresponding to Alfven ion Cyclotron Waves (ACWs). This demonstrates for the first time the co-existence of KAWs and ACWs in the slow solar wind in the inner heliosphere, which contrasts with previous observations in the slow solar wind at 1 AU. This discrepancy between 0.18 and 1 AU could be explained, either by i) a dissipation of ACWs via cyclotron resonance during their outward journey, or by ii) the high Alfvenicity of the slow solar wind at 0.18 AU that may be favorable for the excitation of ACWs.
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