skip to main content

Title: Kinetic-scale Current Sheets in the Solar Wind at 1 au: Scale-dependent Properties and Critical Current Density
Abstract

We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s−1magnetic field resolution. The CSs have thickness,λ,from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λpin terms of local proton inertial length,λp. We found that the current density is larger for smaller-scale CSs,J0≈ 6 nAm−2· (λ/100 km)−0.56, but does not statistically exceed a critical value,JA,corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units:J0/JA0.17·(λ/λp)0.51. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale:Δθ19°·(λ/λp)0.5. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.

Authors:
; ; ; ; ;
Award ID(s):
2026680
Publication Date:
NSF-PAR ID:
10362905
Journal Name:
The Astrophysical Journal Letters
Volume:
926
Issue:
2
Page Range or eLocation-ID:
Article No. L19
ISSN:
2041-8205
Publisher:
DOI PREFIX: 10.3847
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s−1and a velocity dispersion ofσrv=2.50.8+1.3km s−1, which results in a dynamical mass ofM1/2(rh)=84+12×105Mand a mass-to-light ratio ofM/LV=440250+650M/L. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M/L). However, we do not resolve a metallicity dispersion (σ[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in linemore »with its orbital parameters. Intriguingly, Grus I has among the lowest central densities (ρ1/23.52.1+5.7×107Mkpc−3) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

    « less
  2. Abstract

    We use ALMA observations of CO(2–1) in 13 massive (M*≳ 1011M) poststarburst galaxies atz∼ 0.6 to constrain the molecular gas content in galaxies shortly after they quench their major star-forming episode. The poststarburst galaxies in this study are selected from the Sloan Digital Sky Survey spectroscopic samples (Data Release 14) based on their spectral shapes, as part of the Studying QUenching at Intermediate-z Galaxies: Gas, anguLarmomentum, and Evolution (SQuIGGLE) program. Early results showed that two poststarburst galaxies host large H2reservoirs despite their low inferred star formation rates (SFRs). Here we expand this analysis to a larger statistical sample of 13 galaxies. Six of the primary targets (45%) are detected, withMH2109M. Given their high stellar masses, this mass limit corresponds to an average gas fraction offH2MH2/M*7%or ∼14% using lower stellar masses estimates derived from analytic, exponentially declining star formation histories. The gas fraction correlates with theDn4000 spectral index, suggesting that the cold gas reservoirs decrease with time since burst, as found in local K+A galaxies. Star formation histories derived from flexible stellar population synthesis modeling support thismore »empirical finding: galaxies that quenched ≲150 Myr prior to observation host detectable CO(2–1) emission, while older poststarburst galaxies are undetected. The large H2reservoirs and low SFRs in the sample imply that the quenching of star formation precedes the disappearance of the cold gas reservoirs. However, within the following 100–200 Myr, theSQuIGGLEgalaxies require the additional and efficient heating or removal of cold gas to bring their low SFRs in line with standard H2scaling relations.

    « less
  3. Abstract

    State transitions in black hole X-ray binaries are likely caused by gas evaporation from a thin accretion disk into a hot corona. We present a height-integrated version of this process, which is suitable for analytical and numerical studies. With radiusrscaled to Schwarzschild units and coronal mass accretion rateṁcto Eddington units, the results of the model are independent of black hole mass. State transitions should thus be similar in X-ray binaries and an active galactic nucleus. The corona solution consists of two power-law segments separated at a break radiusrb∼ 103(α/0.3)−2, whereαis the viscosity parameter. Gas evaporates from the disk to the corona forr>rb, and condenses back forr<rb. Atrb,ṁcreaches its maximum,ṁc,max0.02(α/0.3)3. If atrrbthe thin disk accretes withṁd<ṁc,max, then the disk evaporates fully before reachingrb, giving the hard state. Otherwise, the disk survives at all radii, giving the thermal state. While the basic model considers only bremsstrahlung cooling and viscous heating, we also discuss a more realistic model that includes Compton cooling and direct coronal heating by energy transport from the disk. Solutions are again independent of black hole mass, andrbremainsmore »unchanged. This model predicts strong coronal winds forr>rb, and aT∼ 5 × 108K Compton-cooled corona forr<rb. Two-temperature effects are ignored, but may be important at small radii.

    « less
  4. Abstract

    We perform particle-in-cell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electron-positron pairs. Various external magnetizationsσ≲ 10−4and pair-loading factorsZ±≲ 10 are studied, whereZ±is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσexceeds a critical valueσLthat decreases withZ±. AtσσLthe shock is mediated by particle scattering in the self-generated microturbulent fields, the strength and scale of which decrease withZ±, leading to lowerσL. (2) The energy fraction carried by the post-shock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per post-shock electron scales asE¯eZ±+11. (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low asσ≈ 5 × 10−6. The ions then become essentially thermal with mean energyE¯i, while electrons form a nonthermal tail, extending fromEZ±+11E¯itoE¯i. Whenσ= 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here,more »the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gamma-ray burst afterglows.

    « less
  5. 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 powermore »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.

    « less