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Register a new userThe Radial Dependence of Proton-scale Magnetic Spectral Break in Slow Solar Wind during PSP Encounter 2
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Magnetic field fluctuations in the solar wind are commonly observed to follow a power law spectrum. Near proton-kinetic scales, a spectral break occurs which is commonly interpreted as a transition to kinetic turbulence. However, this transition is not yet entirely understood. By studying the scaling of the break with various plasma properties, it may be possible to constrain the processes leading to the onset of kinetic turbulence. Using data from Parker Solar Probe (textit{PSP}), we measure the proton scale break over a range of heliocentric distances, enabling a measurement of the transition from inertial to kinetic scale turbulence under various plasma conditions. We find that the break frequency $f_b$ increases as the heliocentric distance $r$ decreases in the slow solar wind following a power law $f_bsim r^{-1.11}$. We also compare this to the characteristic plasma ion scales to relate the break to the possible physical mechanisms occurring at this scale. The ratio between $f_b$ and $f_c$, the Doppler shifted ion cyclotron resonance scale, is approximately unity for all plasma $beta_p$. At high $beta_p$ the ratio between $f_b$ and $f_rho$, the Doppler shifted gyroscale, is approximately unity; while at low $beta_p$ the ratio between $f_b$ and $f_d$, the Doppler shifted proton-inertial length is unity. Due to the large comparable Alfven and solar wind speeds, we analyze these results using both the standard and modified Taylor hypothesis, demonstrating robust statistical results.
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The solar wind shows periods of highly Alfvenic activity, where velocity fluctuations and magnetic fluctuations are aligned or anti-aligned with each other. It is generally agreed that solar wind plasma velocity and magnetic field fluctuations observed by Parker Solar Probe (PSP) during the first encounter are mostly highly Alfvenic. However, quantitative measures of Alfvenicity are needed to understand how the characterization of these fluctuations compares with standard measures from prior missions in the inner and outer heliosphere, in fast wind and slow wind, and at high and low latitudes. To investigate this issue, we employ several measures to quantify the extent of Alfvenicity -- the Alfven ratio $r_A$, {normalized} cross helicity $sigma_c$, {normalized} residual energy $sigma_r$, and the cosine of angle between velocity and magnetic fluctuations $costheta_{vb}$. We show that despite the overall impression that the Alfvenicity is large in the solar wind sampled by PSP during the first encounter, during some intervals the cross helicity starts decreasing at very large scales. These length-scales (often $> 1000 d_i$) are well inside inertial range, and therefore, the suppression of cross helicity at these scales cannot be attributed to kinetic physics. This drop at large scales could potentially be explained by large-scale shears present in the inner heliosphere sampled by PSP. In some cases, despite the cross helicity being constant down to the noise floor, the residual energy decreases with scale in the inertial range. These results suggest that it is important to consider all these measures to quantify Alfvenicity.
The power spectral density of magnetic fluctuations in the solar wind exhibits several power-law-like frequency ranges with a well defined break between approximately 0.1 and 1 Hz in the spacecraft frame. The exact dependence of this break scale on solar wind parameters has been extensively studied but is not yet fully understood. Recent studies have suggested that reconnection may induce a break in the spectrum at a disruption scale $lambda_D$, which may be larger than the fundamental ion kinetic scales, producing an unusually steep spectrum just below the break. We present a statistical investigation of the dependence of the break scale on the proton gyroradius $rho_i$, ion inertial length $d_i$, ion sound radius $rho_s$, proton-cyclotron resonance scale $rho_c$ and disruption scale $lambda_D$ as a function of $beta_{perp i}$. We find that the steepest spectral indices of the dissipation range occur when $beta_e$ is in the range of 0.1-1 and the break scale is only slightly larger than the ion sound scale (a situation occurring 41% of the time at 1 AU), in qualitative agreement with the reconnection model. In this range the break scale shows remarkably good correlation with $lambda_D$. Our findings suggest that, at least at low $beta_e$, reconnection may play an important role in the development of the dissipation range turbulent cascade and causes unusually steep (steeper than -3) spectral indices.
Fluctuations of solar wind magnetic field and plasma parameters exhibit a typical turbulence power spectrum with a spectral index ranging between $sim -5/3$ and $sim -3/2$. In particular, at $1$ AU, the magnetic field spectrum, observed within fast corotating streams, also shows a clear steepening for frequencies higher than the typical proton scales, of the order of $sim 3times10^{-1}$ Hz, and a flattening towards $1/f$ at frequencies lower than $sim 10^{-3}$ Hz. However, the current literature reports observations of the low-frequency break only for fast streams. Slow streams, as observed to date, have not shown a clear break, and this has commonly been attributed to slow wind intervals not being long enough. Actually, because of the longer transit time from the Sun, slow wind turbulence would be older and the frequency break would be shifted to lower frequencies with respect to fast wind. Based on this hypothesis, we performed a careful search for long-lasting slow wind intervals throughout $12$ years of Wind satellite measurements. Our search, based on stringent requirements not only on wind speed but also on the level of magnetic compressibility and Alfvenicity of the turbulent fluctuations, yielded $48$ slow wind streams lasting longer than $7$ days. This result allowed us to extend our study to frequencies sufficiently low and, for the first time in the literature, we are able to show that the $1/f$ magnetic spectral scaling is also present in the slow solar wind, provided the interval is long enough. However, this is not the case for the slow wind velocity spectrum, which keeps the typical Kolmogorov scaling throughout the analysed frequency range. After ruling out the possible role of compressibility and Alfvenicity for the 1/f scaling, a possible explanation in terms of magnetic amplitude saturation, as recently proposed in the literature, is suggested.
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.
The nature of the plasma wave modes around the ion kinetic scales in highly Alfvenic slow solar wind turbulence is investigated using data from the NASAs Parker Solar Probe taken in the inner heliosphere, at 0.18 Astronomical Unit (AU) from the sun. 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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