Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
10
result(s) for
"Davis, Nooshin"
Sort by:
The Evolution of the 1/f Range within a Single Fast-solar-wind Stream between 17.4 and 45.7 Solar Radii
The power spectrum of magnetic field fluctuations in the fast solar wind (V SW > 500 km s−1) at magnetohydrodynamic scales is characterized by two different power laws on either side of a break frequency f b. The low-frequency range at frequencies f smaller than f b is often viewed as the energy reservoir that feeds the turbulent cascade at f > f b. At heliocentric distances r exceeding 60 solar radii (R s), the power spectrum often has a 1/f scaling at f < f b, i.e., the spectral index is close to −1. In this study, measurements from the Parker Solar Probe's Encounter 10 with the Sun are used to investigate the evolution of the magnetic field power spectrum at f < f b at r < 60 R s during a fast radial scan of a single fast-solar-wind stream. We find that the spectral index in the low-frequency part of the spectrum decreases from approximately −0.61 to −0.94 as r increases from 17.4 to 45.7 R s. Our results suggest that the 1/f spectrum that is often seen at large r in the fast solar wind is not produced at the Sun, but instead develops dynamically as the wind expands outward from the corona into the interplanetary medium.
Journal Article
Higher-order Analysis of Three-dimensional Anisotropy in Imbalanced Alfvénic Turbulence
We examine the applicability of homogeneous, negligible cross-helicity magnetohydrodynamic (MHD) turbulence models incorporating “critical balance” (CB) and “scale-dependent dynamic alignment” (SDDA) to Parker Solar Probe observations of a highly Alfvénic, high cross-helicity solar wind stream. At energy-injection scales, both Alfvénic modes satisfy χλ±≡τA±/τnl±≪1 , where τA± and τnl± denote the linear and nonlinear timescales. The outward cascade remains weak, χλ+≈0.2 , across all scales, while the inward cascade reaches CB at the onset of the inertial range ( χλ−∼1 ), yet with spectral scalings that depart from the canonical weak-to-strong transition. Within the domain conventionally designated as the inertial range, we identify two statistically distinct subranges. At larger scales (R2; 200–6000, di), the average eddy displays a field-aligned, tube-like morphology. An inverse correlation is observed between alignment angle and gradient intensity, and the conditional structure-function exponents ζn—measured perpendicular to both the local mean field and fluctuation direction—agree with the predictions of B. D. G. Chandran et al. and A. Mallet & A. A. Schekochihin, although the parallel and displacement components show more concave scaling than anticipated. At smaller scales (R1; 10–100, di), spectra steepen, eddies become ribbon-like, and intermittency weakens. Within a narrow interval near the ion characteristic scales, eddies approach isotropy before the trend of increasing anisotropy resumes at smaller scales. Analysis using five-point increments further demonstrates a stronger multifractal character at kinetic scales than is resolved with conventional two-point methods. Finally, we discuss the influence of solar wind expansion, finite cross helicity, “anomalous coherence,” and the emergence of a “helicity barrier” in modifying CB/SDDA phenomenology and shaping the statistical properties of solar wind turbulence.
Journal Article
New Observations of Solar Wind 1/f Turbulence Spectrum from Parker Solar Probe
The trace magnetic power spectrum in the solar wind is known to be characterized by a double power law at scales much larger than the proton gyro-radius, with flatter spectral exponents close to −1 found at the lower frequencies below an inertial range with indices closer to [−1.5, −1.67]. The origin of the 1/f range is still under debate. In this study, we selected 109 magnetically incompressible solar wind intervals (δ∣ B ∣/∣ B ∣ ≪ 1) from Parker Solar Probe encounters 1–13 that display such double power laws, with the aim of understanding the statistics and radial evolution of the low-frequency power spectral exponents from Alfvén point up to 0.3 au. New observations from closer to the Sun show that in the low-frequency range solar wind, turbulence can display spectra much shallower than 1/f, evolving asymptotically to 1/f as advection time increases, indicating a dynamic origin for the 1/f range formation. We discuss the implications of this result on the Matteini et al. conjecture for the 1/f origin as well as example spectra displaying a triple power law consistent with the model proposed by Chandran et al., supporting the dynamic role of parametric decay in the young solar wind. Our results provide new constraints on the origin of the 1/f spectrum and further show the possibility of the coexistence of multiple formation mechanisms.
Journal Article
On the Evolution of the Anisotropic Scaling of Magnetohydrodynamic Turbulence in the Inner Heliosphere
We analyze a merged Parker Solar Probe (PSP) and Solar Orbiter (SO) data set covering heliocentric distances 13 R ⊙ ≲ R ≲ 220 R ⊙ to investigate the radial evolution of power and spectral index anisotropy in the wavevector space of solar wind turbulence. Our results show that anisotropic signatures of turbulence display a distinct radial evolution when fast, V sw ≥ 400 km s−1, and slow, V sw ≤ 400 km s−1, wind streams are considered. The anisotropic properties of slow wind in Earth orbit are consistent with a “critically balanced” cascade, but both spectral index anisotropy and power anisotropy diminish with decreasing heliographic distance. Fast streams are observed to roughly retain their near-Sun anisotropic properties, with the observed spectral index and power anisotropies being more consistent with a “dynamically aligned” type of cascade, though the lack of extended fast wind intervals makes it difficult to accurately measure the anisotropic scaling. A high-resolution analysis during the first perihelion of PSP confirms the presence of two subranges within the inertial range, which may be associated with the transition from weak to strong turbulence. The transition occurs at κ d i ≈ 6 × 10−2 and signifies a shift from −5/3 to −2 and from −3/2 to −1.57 scaling in parallel and perpendicular spectra, respectively. Our results provide strong observational constraints for anisotropic theories of MHD turbulence in the solar wind.
Journal Article
Formation of Magnetic Switchbacks via expanding Alfvén Waves
Context. Large-amplitude inversions of the solar wind's interplanetary magnetic field have long been documented; however, observations from the Parker Solar Probe (PSP) mission have renewed interest in this phenomenon as such features, often termed switchbacks, may constrain both the sources of the solar wind as well as in-situ nonlinear dynamics and turbulent heating. Aims. We aim to show that magnetic field fluctuations in the solar wind are consistent with Alfvénic fluctuations that naturally form switchback inversions in the magnetic field through expansion effects. Methods. We examine PSP observations of the evolution of a single stream of solar wind in a radial scan from PSP's tenth perihelion encounter from approximately 15-50 solar radii. We study the growth and radial scaling of normalized fluctuation amplitudes in the magnetic field, \\( B/B\\), within the framework of spherical polarization. We compare heating rates computed via outer-scale decay from consideration of wave-action to proton heating rates empirically observed through considering adiabatic expansion. Results. We find that the magnetic field fluctuations are largely spherically polarized and that the normalized amplitudes of the magnetic field, \\( B/B\\), increases with amplitude. The growth of the magnetic field amplitude leads to switchback inversions in the magnetic field. While the amplitudes do not grow as fast as predicted by the conservation of wave action, the deviation from the expected scaling yields an effective heating rate, which is close to the empirically observed proton heating rate. Conclusions. The observed scaling of fluctuation amplitudes is largely consistent with a picture of expanding Alfvén waves that seed turbulence leading to dissipation. The expansion of the waves leads to the growth of wave-amplitudes, resulting in the formation of switchbacks.
Paper
The Evolution of the 1/f Range Within a Single Fast-Solar-Wind Stream Between 17.4 and 45.7 Solar Radii
The power spectrum of magnetic-field fluctuations in the fast solar wind (\\(V_ SW> 500 km s^-1\\)) at magnetohydrodynamic (MHD) scales is characterized by two different power laws on either side of a break frequency \\(f_ b\\). The low-frequency range at frequencies \\(f\\) smaller than \\(f_ b\\) is often viewed as the energy reservoir that feeds the turbulent cascade at \\(f>f_ b\\). At heliocentric distances \\(r\\) exceeding \\(60\\) solar radii (\\(R_ s\\)), the power spectrum often has a \\(1/f\\) scaling at \\(f
Paper
On the evolution of the Anisotropic Scaling of Magnetohydrodynamic Turbulence in the Inner Heliosphere
We analyze a merged Parker Solar Probe (\\(PSP\\)) and Solar Orbiter (\\(SO\\)) dataset covering heliocentric distances \\(13 \\ R_ R 220\\) \\(R_\\) to investigate the radial evolution of power and spectral-index anisotropy in the wavevector space of solar wind turbulence. Our results show that anisotropic signatures of turbulence display a distinct radial evolution when fast, \\(V_sw ~ 400 ~km ~s^-1\\), and slow, \\(V_sw ~ 400 ~km ~s^-1\\), wind streams are considered. The anisotropic properties of slow wind in Earth orbit are consistent with a ``critically balanced'' cascade, but both spectral-index anisotropy and power anisotropy diminish with decreasing heliographic distance. Fast streams are observed to roughly retain their near-Sun anisotropic properties, with the observed spectral index and power anisotropies being more consistent with a ``dynamically aligned'' type of cascade, though the lack of extended fast-wind intervals makes it difficult to accurately measure the anisotropic scaling. A high-resolution analysis during the first perihelion of PSP confirms the presence of two sub-ranges within the inertial range, which may be associated with the transition from weak to strong turbulence. The transition occurs at \\( d_i 6 10^-2\\), and signifies a shift from -5/3 to -2 and -3/2 to -1.57 scaling in parallel and perpendicular spectra, respectively. Our results provide strong observational constraints for anisotropic theories of MHD turbulence in the solar wind.
Paper
New Observations of Solar Wind 1/f Turbulence Spectrum from Parker Solar Probe
The trace magnetic power spectrum in the solar wind is known to be characterized by a double power law at scales much larger than the proton gyro-radius, with flatter spectral exponents close to -1 found at the lower frequencies below an inertial range with indices closer to \\([-1.5,-1.6]\\). The origin of the \\(1/f\\) range is still under debate. In this study, we selected 109 magnetically incompressible solar wind intervals (\\( | B|/| B| 1\\)) from Parker Solar Probe encounters 1 to 13 which display such double power laws, with the aim of understanding the statistics and radial evolution of the low frequency power spectral exponents from Alfvén point up to 0.3 AU. New observations from closer to the sun show that in the low frequency range solar wind turbulence can display spectra much shallower than \\(1/f\\), evolving asymptotically to \\(1/f\\) as advection time increases, indicating a dynamic origin for the \\(1/f\\) range formation. We discuss the implications of this result on the Matteini et al. (2018) conjecture for the \\(1/f\\) origin as well as example spectra displaying a triple power law consistent with the model proposed by Chandran et al. (2018), supporting the dynamic role of parametric decay in the young solar wind. Our results provide new constraints on the origin of the \\(1/f\\) spectrum and further show the possibility of the coexistence of multiple formation mechanisms.
Paper
Higher-Order Analysis of Three-Dimensional Anisotropy in Imbalanced Alfvénic Turbulence
We analyze in-situ observations of imbalanced solar wind turbulence to evaluate MHD turbulence models grounded in \"Critical Balance\" (CB) and \"Scale-Dependent Dynamic Alignment\" (SDDA). At energy injection scales, both outgoing and ingoing modes exhibit a weak cascade; a simultaneous tightening of SDDA is noted. Outgoing modes persist in a weak cascade across the inertial range, while ingoing modes shift to a strong cascade at \\( 3 10^4 d_i\\), with associated spectral scalings deviating from expected behavior due to \"anomalous coherence\" effects. The inertial range comprises two distinct sub-inertial segments. Beyond \\( 100 d_i\\), eddies adopt a field-aligned tube topology, with SDDA signatures mainly evident in high amplitude fluctuations. The scaling exponents \\(_n\\) of the \\(n\\)-th order conditional structure functions, orthogonal to both the local mean field and fluctuation direction, align with the analytical models of Chandran et al. 2015 and Mallet et al. 2017, indicating \"multifractal\" statistics and strong intermittency; however, scaling in parallel and displacement components is more concave than predicted, possibly influenced by expansion effects. Below \\( 100 d_i\\), eddies become increasingly anisotropic, evolving into thin current sheet-like structures. Concurrently, \\(_n\\) scales linearly with order, marking a shift towards \"monofractal\" statistics. At \\( 8 d_i\\), the increase in aspect ratio halts, and the eddies become quasi-isotropic. This change may signal tearing instability, leading to reconnection, or result from energy redirection into the ion-cyclotron wave spectrum, aligning with the \"helicity barrier\". Our analysis utilizes 5-point structure functions, proving more effective than the traditional 2-point method in capturing steep scaling behaviors at smaller scales.
Paper
Genetic and molecular reappraisal of spindle cell adamantinoma of bone reveals a small subset of misclassified intraosseous synovial sarcoma
Adamantinoma represents a distinct group of bone tumors showing both mesenchymal and epithelial differentiation most commonly involving the tibial diaphysis. Most adamantinomas contain a fibro-osseous component and an epithelial component consisting of squamous or basaloid cells. Adamantinomas are considered malignant neoplasms requiring en bloc excision that frequently recur locally and can rarely metastasize. Rare adamantinomas show an epithelial component consisting predominantly of monomorphic spindle cells, which, combined with an epithelial immunophenotype, can mimic monophasic synovial sarcoma. Synovial sarcoma is very rare in bone. It is considered a high-grade sarcoma that typically necessitates chemotherapy. However, the relationship between spindle cell adamantinoma and intraosseous synovial sarcoma has not been investigated. The current study was prompted by identification of a presumed spindle cell adamantinoma of the tibia with diffuse keratin expression that harbored a
SS18
gene region rearrangement. FISH of eight additional bone tumors initially classified as spindle cell adamantinoma based on clinicoradiopathologic findings revealed one additional case with
SS18
rearrangement. Histologically, both intraosseous synovial sarcoma and spindle cell adamantinoma demonstrated uniform fusiform nuclei with scant cytoplasm, short fascicles and low mitotic activity. The adamantinomas, but not the synovial sarcomas, were more likely to show overt epithelial differentiation in the form of pseudoglands or squamous nests. Immunohistochemistry of all cases, irrespective of
SS18
status, showed diffuse keratin positivity in the spindle cell component, and less consistent EMA positivity. Clinical follow-up was available in both intraosseous synovial sarcomas, one of which recurred and the other metastasized. Two of the six spindle cell adamantinomas with follow-up metastasized. The above findings highlight the morphologic and immunophenotypic overlap between spindle cell adamantinoma and intraosseous synovial sarcoma of the tibia. Investigation of
SS18
status to exclude synovial sarcoma is suggested prior to rendering a diagnosis of spindle cell adamantinoma.
Journal Article