Jung-Tsung Li

Research

Solar-disk gamma-ray emission:

The Sun is a bright gamma-ray source due to hadronic cosmic-ray interactions with solar gas. While it is known that incoming cosmic rays must generally first be reflected by solar magnetic fields to produce outgoing gamma rays, theoretical models have yet to reproduce the observed spectra.

We introduce a simplified model of the solar magnetic fields that captures the main elements relevant to gamma-ray production. These are a flux tube, representing the network elements, and a flux sheet, representing the intergranule sheets. Both the tube and sheet have a horizontal size of order 100 km and serve as sites where cosmic rays are reflected and gamma rays are produced. Our model produces a gamma-ray spectrum that reasonably matches both the hard spectrum seen by Fermi-LAT data at 1–200 GeV and the considerably softer spectrum seen by HAWC at near 1 TeV. Notably, the spectrum softening observed by HAWC results from the limited effectiveness of capturing and reflecting 10 TeV cosmic rays by the finite-sized intergranule sheets.

Our study is important for understanding cosmic-ray transport in the solar atmosphere and will lead to insights about small-scale magnetic fields in the quiet photosphere.

[1] J.-T. Li, J. F. Beacom, S. Griffith, and A. H. G. Peter, ApJ 961, 167 (2024)

Cosmic-ray transport in the inner heliosphere:

A key goal of heliophysics is to understand how cosmic rays propagate in the solar system’s complex, dynamic environment. One observable is solar modulation, i.e., how the flux and spectrum of cosmic rays changes as they propagate inward.

We construct an improved force-field model, taking advantage of new measurements of magnetic power spectral density by Parker Solar Probe to predict solar modulation within the Earth’s orbit. We find that modulation of cosmic rays between the Earth and Sun is modest, at least at solar minimum and in the ecliptic plane. Our results agree much better with the limited data on cosmic-ray radial gradients within Earth’s orbit than past treatments of the force-field model. Our predictions can be tested with forthcoming direct cosmic-ray measurements in the inner heliosphere by Parker Solar Probe and Solar Orbiter. They are also important for interpreting the gamma-ray emission from the Sun due to scattering of cosmic rays with solar matter and photons.

[1] J.-T. Li, J. F. Beacom, and A. H. G. Peter, ApJ 937, 27 (2022)

Dark photons & big bang bucleosynthesis:

Freeze-in dark photons decaying out of equilibrium during the weak-decoupling epoch results in an entropy flow between the neutrino and plasma sectors. In my work with G. M. Fuller and E. Groh[1], we trace the evolution of nucleosynthesis numerically from the beginning of weak decoupling with the presence of late-decay dark photons. Using the 1%-level primordial deuterium abundance measurements from quasar absorption lines, our result excludes a range of dark photon model parameters.

[1] J.-T. Li, G. M. Fuller, and E. Grohs, JCAP 12 (2020) 049

Dark matter & plasma instability:

Milli-charged dark matter (mDM) possesses fractional electric charge and allows DM to have electromagnetic interaction with baryons. In my work with T. Lin[2], we provide a mechanism for a collisionless mDM to scatter efficiently with Standard Model particles. Our work reveals that the supernova shocks could sweep up and thermalize the ambient mDM via plasma instability. However, these mDM particles return to having roughly the ambient DM velocity in the end due to the adiabatic decompression. Our result implies the detectability of terrestrial experiments to mDM is not strongly affected by supernova shocks.

[1] J.-T. Li and T. Lin, Phys. Rev. D 101, 103034 (2020)

Gravitational waves & supermassive stars:

Supermassive stars collapse under post-Newtonian instability and become black holes. During the collapses, they radiate a significant fraction of their rest mass in neutrinos. In my work with G. M. Fuller and C. T. Kishimoto[3], we investigate the gravitational wave signatures driven by the neutrino bursts, which create nearly unique “memory” gravitational waves that fall in the optimal frequency band of DECIGO and BBO. We show this route of supermassive black hole formation is potentially detectable to redshifts as high as 13, before the epoch of reionization.

[1] J.-T. Li, G. M. Fuller, and C. T. Kishimoto, Phys. Rev. D 98, 023002 (2018)