skip to main content


Title: Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind
Abstract

Parker Solar Probe (PSP) observed sub-Alfvénic solar wind intervals during encounters 8–14, and low-frequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in super-Alfvénic wind. We apply a new mode decomposition analysis to the sub-Alfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/nonpropagating Alfvén vortices, forward and backward fast and slow magnetosonic (MS) modes. Density fluctuations are primarily and almost equally entropy- and backward-propagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumberk) for each mode. Entropy density fluctuations have a wavenumber anisotropy ofkk, whereas slow-mode density fluctuations havek>k. Magnetic field fluctuations are primarily magnetic island modes (δBi) with anO(1) smaller contribution from unidirectionally propagating Alfvén waves (δBA+) giving a variance anisotropy ofδBi2/δBA2=4.1. Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorov-likek1.6in perpendicular wavenumber, and the unidirectional Alfvén wave spectra arek1.6andk1.5. Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due toβp≪ 1. Transverse velocity fluctuations are the dominant velocity component in fast MS modes, and longitudinal fluctuations dominate in slow modes. Mode decomposition is an effective tool in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes.

 
more » « less
NSF-PAR ID:
10502660
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ;
Publisher / Repository:
DOI PREFIX: 10.3847
Date Published:
Journal Name:
The Astrophysical Journal
Volume:
966
Issue:
1
ISSN:
0004-637X
Format(s):
Medium: X Size: Article No. 75
Size(s):
Article No. 75
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    The Parker Solar Probe (PSP) entered a region of sub-Alfvénic solar wind during encounter 8, and we present the first detailed analysis of low-frequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, cross-helicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor’s hypothesis to sub- and super-Alfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (z±, respectively) are presented, showing thatz+modes dominatezby an order of magnitude or more, and thez+spectrum is a power law in frequency (parallel wavenumber)f−3/2(k3/2) compared to the convexzspectrum withf−3/2(k3/2) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) tof−1.25at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropykk3/4, that thez+fluctuations experience primarily nonlinear interactions, and that the minorityzfluctuations experience both nonlinear and Alfvénic interactions withz+fluctuations. The density spectrum is a power law that resembles neither thez±spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly super-Alfvénic intervals are similar.

     
    more » « less
  2. Abstract

    We present thez≈ 6 type-1 quasar luminosity function (QLF), based on the Pan-STARRS1 (PS1) quasar survey. The PS1 sample includes 125 quasars atz≈ 5.7–6.2, with −28 ≲M1450≲ −25. With the addition of 48 fainter quasars from the SHELLQs survey, we evaluate thez≈ 6 QLF over −28 ≲M1450≲ −22. Adopting a double power law with an exponential evolution of the quasar density (Φ(z) ∝ 10k(z−6);k= −0.7), we use a maximum likelihood method to model our data. We find a break magnitude ofM*=26.380.60+0.79mag, a faint-end slope ofα=1.700.19+0.29, and a steep bright-end slope ofβ=3.841.21+0.63. Based on our new QLF model, we determine the quasar comoving spatial density atz≈ 6 to ben(M1450<26)=1.160.12+0.13cGpc3. In comparison with the literature, we find the quasar density to evolve with a constant value ofk≈ −0.7, fromz≈ 7 toz≈ 4. Additionally, we derive an ionizing emissivity ofϵ912(z=6)=7.231.02+1.65×1022ergs1Hz1cMpc3, based on the QLF measurement. Given standard assumptions, and the recent measurement of the mean free path by Becker et al. atz≈ 6, we calculate an Hiphotoionizing rate of ΓH I(z= 6) ≈ 6 × 10−16s−1, strongly disfavoring a dominant role of quasars in hydrogen reionization.

     
    more » « less
  3. Abstract

    A test of lepton flavor universality inB±K±μ+μandB±K±e+edecays, as well as a measurement of differential and integrated branching fractions of a nonresonantB±K±μ+μdecay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions ats=13TeVrecorded in 2018, by the CMS experiment at the LHC, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractionsB(B±K±μ+μ)toB(B±K±e+e)is determined from the measured double ratioR(K)of these decays to the respective branching fractions of theB±J/ψK±withJ/ψμ+μande+edecays, which allow for significant cancellation of systematic uncertainties. The ratioR(K)is measured in the range1.1<q2<6.0GeV2, whereqis the invariant mass of the lepton pair, and is found to beR(K)=0.780.23+0.47, in agreement with the standard model expectationR(K)1. This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the sameq2range,B(B±K±μ+μ)=(12.42±0.68)×108, is consistent with the present world-average value and has a comparable precision.

     
    more » « less
  4. Abstract

    We measure the CO-to-H2conversion factor (αCO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements ofαCOfor CO (2–1) and (1–0), respectively. The mean values forαCO (2–1)andαCO (1–0)are9.35.4+4.6and4.22.0+1.9Mpc2(Kkms1)1, respectively. The CO-intensity-weighted mean is 5.69 forαCO (2–1)and 3.33 forαCO (1–0). We examine howαCOscales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength (U¯). Among them,U¯, ΣSFR, and the integrated CO intensity (WCO) have the strongest anticorrelation with spatially resolvedαCO. We provide linear regression results toαCOfor all quantities tested. At galaxy-integrated scales, we observe significant correlations betweenαCOandWCO, metallicity,U¯, and ΣSFR. We also find thatαCOin each galaxy decreases with the stellar mass surface density (Σ) in high-surface-density regions (Σ≥ 100Mpc−2), following the power-law relationsαCO(21)Σ0.5andαCO(10)Σ0.2. The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease inαCOwith increasing Σas a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction inαCO. The decrease inαCOat high Σis important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburst-like” conversion factors.

     
    more » « less
  5. Abstract

    We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲z≲ 2.6 (zmean= 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass oflog(M*/M)med=8.290.43+0.51and a median star formation rate ofSFRHαmed=2.251.26+2.15Myr1. We measure the faint electron-temperature-sensitive [Oiii]λ4363 emission line at 2.5σ(4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of12+log(O/H)direct=7.880.22+0.25(0.150.06+0.12Z). We investigate the applicability at highzof locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixedM*, our composite is well represented by thez∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories(log(M*/M)med=8.920.22+0.31), we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixedM*and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii]λ3729/[Oii]λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density ofne=10+215cm3(ne=10+74cm3) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies atz∼ 2.

     
    more » « less