Keywords

We analyze electron acceleration by a large-scale electric field E in a collisional hydrogen plasma under the solar flare coronal conditions based on approaches proposed by Dreicer and Spitzer for the dynamic friction force of electrons. The Dreicer electric field E_Dr is determined as a critical electric field at which the entire electron population runs away. Two regimes of strong (E ≲ E_Dr) and weak (E ≪ E_Dr) electric field are discussed. It is shown that the commonly used formal definition of the Dreicer field leads to an overestimation of its value by about five times. The critical velocity at which the electrons of the "tail" of the Maxwell distribution become runaway under the action of the sub-Dreiser electric fields turns out to be underestimated by $\sqrt{3}$ times in some works because the Coulomb collisions between runaway and thermal electrons are not taken into account. The electron acceleration by sub-Dreicer electric fields generated in the solar corona faces difficulties.

Precise measurements of energy spectra of different cosmic ray (CR) species have been obtained in recent years, by particularly the AMS-02 experiment on the International Space Station. It has been shown that apparent differences exist in different groups of the primary CRs. However, it is not straightforward to conclude that the source spectra of different particle groups are different since they will experience different propagation processes (e.g., energy losses and fragmentations) either. In this work, we study the injection spectra of different nuclear species using the measurements from Voyager-1 outside the solar system, and ACR-CRIS and AMS-02 on the top of atmosphere, in a physical framework of CR transportation. Two types of injection spectra are assumed, the broken power-law (BPL) form and the non-parametric spline interpolation form. The non-parametric form fits the data better than the BPL form, implying that potential structures beyond the constrained spectral shape of BPL may exist. For different nuclei the injection spectra are overall similar in shape but do show some differences among each other. For the non-parametric spectral form, the helium injection spectrum is the softest at low energies and the hardest at high energies. For both spectral shapes, the low-energy injection spectrum of neon is the hardest among all these species, and the carbon and oxygen spectra have more prominent bumps in 1–10 GV in the ${R}^{2}{dN}/{dR}$ presentation. Such differences suggest the existence of differences in the sources or acceleration processes of various nuclei of CRs.

The growing observed evidence shows that the long- and short-duration gamma-ray bursts (GRBs) originate from massive star core-collapse and the merger of compact stars, respectively. GRB 201221D is a short-duration GRB lasting ∼0.1 s without extended emission at high redshift z = 1.046. By analyzing data observed with the Swift/BAT and Fermi/GBM, we find that a cutoff power-law model can adequately fit the spectrum with a soft ${E}_{{\rm{p}}}={113}_{-7}^{+9}$ keV, and isotropic energy ${E}_{\gamma ,\mathrm{iso}}={1.36}_{-0.14}^{+0.17}\times {10}^{51}\,\mathrm{erg}$. In order to reveal the possible physical origin of GRB 201221D, we adopted multi-wavelength criteria (e.g., Amati relation, ε-parameter, amplitude parameter, local event rate density, luminosity function, and properties of the host galaxy), and find that most of the observations of GRB 201221D favor a compact star merger origin. Moreover, we find that $\hat{\alpha }$ is larger than $2+\hat{\beta }$ in the prompt emission phase which suggests that the emission region is possibly undergoing acceleration during the prompt emission phase with a Poynting-flux-dominated jet.

Backward-propagating or reverse fluctuations in Alfvénic turbulence are shown to produce magnetic-field-aligned (MFA) electric fields capable of highly intermittent acceleration of particles along the local mean magnetic field. Probability distribution functions (PDFs) for the angles $\chi -{\chi }_{e}$ between magnetic and electric local mean fields in the plane perpendicular to the background magnetic field are calculated both analytically and through Monte Carlo simulations as functions of the fraction $\varepsilon$ of reverse fluctuations. The PDFs peak at $| \chi -{\chi }_{e}| =\pi /2$ but quickly broaden as $\varepsilon$ increases, up to the limit of a uniform PDF for $\varepsilon =0.5$ or zero cross-helicity. Energy from a mixture of forward- and backward-propagating Alfvén waves can easily be transferred to the plasma, through the intermittent MFA electric fields, on a timescale much shorter than the Kolmogorov timescale for turbulence cascade. In such a mixture, for typical 1 au solar wind turbulence parameters, nonresonant interaction through the MFA electric fields rather than gyroresonance controls the energy exchanges between turbulent fields and particles. Possible consequences of the nonresonant interaction through the MFA fields are further suggested, from the observed fast variations of solar wind speed and resulting ${\boldsymbol{v}}$ spectral flattening above 10⁻² Hz, and the turbulence level variability/intermittency near 1 au, to the powering of chromospheric jets/spicules in the upper chromosphere and heating of the chromosphere, transition region, and corona, due to the high reflection rate of Alfvén waves in the upper chromosphere. Conditions for the direct proton acceleration (jet formation) in the chromosphere include a temperature ≤10⁴ K and a magnetic field between about 10 and 100 G.

We study high-energy neutrino emissions from tidal disruption remnants (TDRs) around supermassive black holes. The neutrinos are produced by the decay of charged pions originating in ultrarelativistic protons that are accelerated there. In the standard theory of tidal disruption events (TDEs), there are four distinct phases from the debris circularization of stellar debris to super- and sub-Eddington to radiatively inefficient accretion flows (RIAFs). In addition, we consider the magnetically arrested disk (MAD) state in both the super-Eddington accretion and RIAF phases. We find that there are three promising cases to produce neutrino emissions: the super-Eddington accretion phase of the MAD state and the RIAF phases of both the non-MAD and MAD states. In the super-Eddington MAD state, the enhanced magnetic field makes it possible to accelerate the protons to ${E}_{p,\max }\sim 0.35\,\mathrm{PeV}{({M}_{\mathrm{bh}}/{10}^{7.7}{M}_{\odot })}^{41/48}$ with the other given appropriate parameters. The neutrino energy is then ${E}_{\nu ,\mathrm{pk}}\sim 67\,\mathrm{TeV}{({M}_{\mathrm{bh}}/{10}^{7.7}{M}_{\odot })}^{41/48}$ at the peak of the energy spectrum. For M_bh ≳ 10^7.7 M_⊙, the neutrino light curve is proportional to ${t}^{-65/24}$, while it follows the standard ${t}^{-5/3}$ decay rate for ${M}_{\mathrm{bh}}\lt {10}^{7.7}\,{M}_{\odot }$. In both cases, the large luminosity and characteristic light curves diagnose the super-Eddington MAD state in TDEs. In the RIAF phase of the non-MAD state, we find ${E}_{p,\max }\sim 0.45\,\mathrm{PeV}{({M}_{\mathrm{bh}}/{10}^{7}{M}_{\odot })}^{5/3}$ and ${E}_{\nu ,\mathrm{pk}}\sim 0.35\,\mathrm{PeV}{({M}_{\mathrm{bh}}/{10}^{7}{M}_{\odot })}^{5/3}$, and its light curve is proportional to ${t}^{-10/3}$. This indicates that one can identify whether the existing RIAFs are the TDE origin or not. TDRs are potentially a population of hidden neutrino sources invisible in gamma-rays.

It has been proposed that ultra-high-energy cosmic rays (UHECRs) up to 10²⁰ eV could be produced in the relativistic jets of powerful active galactic nuclei (AGNs) via a one-shot reacceleration of lower-energy CRs produced in supernova remnants (the espresso mechanism). We test this theory by propagating particles in realistic 3D magnetohydrodynamic simulations of ultrarelativistic jets and find that about 10% of the CRs entering the jet are boosted by at least a factor of ∼Γ² in energy, where Γ is the jet's effective Lorentz factor, in agreement with the analytical expectations. Furthermore, about 0.1% of the CRs undergo two or more shots and achieve boosts well in excess of Γ². Particles are typically accelerated up to the Hillas limit, suggesting that the espresso mechanism may promote galactic-like CRs to UHECRs even in AGN jets with moderate Lorentz factors, and not in powerful blazars only. Finally, we find that the sign of the toroidal magnetic field in the jet and in the cocoon controls the angular distribution of the reaccelerated particles, leading to a UHECR emission that may be either quasi-isotropic or beamed along the jet axis. These findings strongly support the idea that espresso acceleration in AGN jets can account for the UHECR spectra, chemical composition, and arrival directions measured by Auger and Telescope Array.

Ground-level enhancement (GLE) events are often associated with large gradual solar events such as fast coronal mass ejections (CMEs), but not all fast CMEs lead to GLE events. Is there a type of coordinated CME that could produce GLEs with larger intensity and higher energies than those in the normal fast isolated CMEs? Here we propose a twin-shock scenario driven by the twin CME coordinately, in which the posterior shock catches up with the preceding shock and has a pileup collision. In the present study, we chose the first GLE event of the solar cycle 24 occurring on 2012 May 17 as an example to investigate the probable association with the twin-shock scenario. We use a dynamic Monte Carlo method to examine the energy spectrum with relevance to the GLE event. In the twin-shock scenario, the seed energetic particles produced by the normal preceding shock can be injected into the posterior shock for reacceleration efficiently. As a result, we obtain the detailed energy spectrum of the solar energetic particles (SEPs) with different behaviors at the related episodes of the twin-shock evolution. Therefore, we predict that the pileup collision of the twin shock would dominate a concave energy spectral slope in the 2012 May 17 SEP event.

The behavior of energetic particles in interplanetary coronal mass ejections (ICMEs) is of great interest. In general, due to the relatively closed magnetic structures of ICMEs, the energetic-particle intensities are usually depressed in them. However, previous studies have found some counterexamples. In this work, using protons with energies form ∼200 keV to ∼7 MeV observed by Wind/3dp as a measure, we check the proton intensity signatures of the 487 ICMEs between 1995 and 2017. A total of 12 ICMEs with extraordinary energetic-particle enhancements have been found, 9 of which are shock-interplanetary coronal mass ejection complex structures (S-ICMEs) and 3 that are isolated interplanetary coronal mass ejections (I-ICMEs). Comparing the two kinds of ICMEs, we find that energetic-particle intensities increase more in the S-ICMEs than in the I-ICMEs in all energy channels, especially in the high-energy channels. In addition, shocks inside energetic-particle-enhanced S-ICMEs are relatively fast and strong. These results indicate that shock-ICME interaction may be an effective local acceleration mechanism.

Early and late multiwavelength observations play an important role in determining the nature of the progenitor, circumburst medium, physical processes, and emitting regions associated with the spectral and temporal features of bursts. GRB 180720B is a long and powerful burst detected by a large number of observatories at multiple wavelengths that range from radio bands to sub-TeV gamma-rays. The simultaneous multiwavelength observations were presented over multiple periods of time beginning just after the trigger time and extending to more than 30 days. The temporal and spectral analysis of Fermi Large Area Telescope (LAT) observations suggests that it presents similar characteristics to other bursts detected by this instrument. Coupled with X-ray and optical observations, the standard external shock model in a homogeneous medium is favored by this analysis. The X-ray flare is consistent with the synchrotron self-Compton (SSC) model from the reverse-shock region evolving in a thin shell and previous LAT, X-ray, and optical data with the standard synchrotron forward-shock model. The best-fit parameters derived with Markov chain Monte Carlo simulations indicate that the outflow is endowed with magnetic fields and that the radio observations are in the self-absorption regime. The SSC forward-shock model with our parameters can explain the LAT photons beyond the synchrotron limit as well as the emission recently reported by the HESS Collaboration.

This is the fourth in a series of companion papers showing that when an efficient dynamo can be maintained by accretion disks around supermassive black holes in active galactic nuclei, it will lead to the formation of a powerful, magnetically collimated helix that could explain the observed jet/radiolobe structures on very large scales. Here, we present a hyper-resistive kinetic theory that shows how different instabilities can cause the slowly evolving magnetically collimated jets to accelerate ions and electrons in different regions of jets and lobes. In particular, we propose that the Drift Cyclotron Loss Cone instability at the ends of jets can accelerate ions up to the observed ultra-high-energy cosmic rays with energies ≈10²⁰ eV. Based on this jet/lobe structure model and its associated acceleration processes, we estimate cosmic-ray intensities and likely radiative signatures and compare them with observations.