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Diffusion coefficients of energetic charged particles in turbulent magnetic fields are a fundamental aspect of diffusive transport theory but remain incompletely understood. In this work, we use quasi-linear theory to evaluate the spatial variation of the parallel diffusion coefficient $\kappa_\parallel$ from the measured magnetic turbulence power spectra in the inner heliosphere. We consider the magnetic field and plasma velocity measurements from Parker Solar Probe made during Orbits 5-13. The parallel diffusion coefficient is calculated as a function of radial distance from 0.062 to 0.8 AU, and the particle energy from 100 keV to 1GeV. We find that $\kappa_\parallel$ increases exponentially with both heliocentric distance and energy of particles. The fluctuations in $\kappa_\parallel$ are related to the episodes of large-scale magnetic structures in the solar wind. By fitting the results, we also provide an empirical formula of $\kappa_{\parallel}=(5.16\pm1.22) \times 10^{18} \: r^{1.17\pm0.08} \: E^{0.71\pm 0.02} \; (cm^2/s)$ in the inner heliosphere which can be used as a reference in studying the transport and acceleration of solar energetic particles as well as the modulation of cosmic rays., Comment: Accepted to ApJ

The third law of thermodynamics has been verified experimentally, but how to perfectly express such a law in theory has become a cross-century problematic issue. It is found from the recent researches that by introducing an innovative method, the Nernst equation can be obtained directly from the experimental data of chemical reactions at low temperatures without the need for artificial additional assumptions appearing in textbooks, so that the Nernst theorem should be replaced by the Nernst statement. The heat capacity statement proposed recently can be also obtained from the experimental data of the heat capacity at low temperatures. The heat capacity statement and the Nernst statement are proved to be mutually derivable and the two are equivalent. The unattainability statement of absolute zero temperature is only a coloration of the Nernst statement or the heat capacity statement. Simultaneously, the defects and deficiencies related to the contents of the third law of thermodynamics appearing in textbooks are pointed out and corrected. The results obtained show clearly that the Nernst theorem and the unattainability statement of absolute zero temperature should be withdrawn from the statements of the third law of thermodynamics. It is important to find that the Nernst statement and the heat capacity statement are two equivalent statements of the third law of thermodynamics, which can solve the centennial debate problems of the third law of thermodynamics and supply the perfect statements for the third law of thermodynamics.

We present a 2D kinematic model to study the acceleration of solar energetic particles (SEPs) at a shock driven by a coronal mass ejection. The shock is assumed to be spherical about an origin that is offset from the center of the Sun. This leads to a spatial and temporal evolution of the angle between the magnetic field and shock normal direction ($\theta_{Bn}$) as it propagates through the Parker spiral magnetic field from the lower corona to 1 AU. We find that the high-energy SEP intensity varies significantly along the shock front due to the evolution of $\theta_{Bn}$. Generally, the west flank of the shock preferentially accelerates particles to high energies compared to the east flank and shock nose. This can be understood in terms of the rate of acceleration, which is higher at the west flank. Double power-law energy spectra are reproduced in our model as a consequence of the local acceleration and transport effects. These results will help better understand the evolution of SEP acceleration and provide new insights into large SEP events observed by multi-spacecraft, especially those close to the Sun, such as Parker Solar Probe and Solar Orbiter.

Heliophysics theory and modeling build understanding from fundamental principles to motivate, interpret, and predict observations. Together with observational analysis, they constitute a comprehensive scientific program in heliophysics. As observations and data analysis become increasingly detailed, it is critical that theory and modeling develop more quantitative predictions and iterate with observations. Advanced theory and modeling can inspire and greatly improve the design of new instruments and increase their chance of success. In addition, in order to build physics-based space weather forecast models, it is important to keep developing and testing new theories, and maintaining constant communications with theory and modeling. Maintaining a sustainable effort in theory and modeling is critically important to heliophysics. We recommend that all funding agencies join forces and consider expanding current and creating new theory and modeling programs--especially, 1. NASA should restore the HTMS program to its original support level to meet the critical needs of heliophysics science; 2. a Strategic Research Model program needs to be created to support model development for next-generation basic research codes; 3. new programs must be created for addressing mission-critical theory and modeling needs; and 4. enhanced programs are urgently required for training the next generation of theorists and modelers., Comment: White paper submitted to Heliophysics 2024 Decadal Survey

We propose a concentrated thermionic emission solar cell design, which demonstrates a high solar-to-electricity energy conversion efficiency larger than 10\% under 600 sun, by harnessing the exceptional electrical, thermal and radiative properties of the graphene as a collector electrode. By constructing an analytical model that explicitly takes into account the non-Richardson behavior of the thermionic emission current from graphene, space charge effect in vacuum gap, and the various irreversible energy losses within the subcomponents, we perform a detailed characterization on the conversion efficiency limit and electrical power output characteristics of the proposed system. We systematically model and compare the energy conversion efficiency of various configurations of graphene-graphene and graphene-diamond and diamond-diamond thermionic emitter, and show that utilizing diamond films as an emitter and graphene as a collector offers the highest maximum efficiency, thus revealing the important role of graphene in achieving high-performance thermionic emission solar cell. A maximum efficiency of 12.8\% under 800 sun has been revealed, which is significantly higher than several existing solid-state solar cell designs, such as the solar-driven thermoelectric and thermophotovoltaic converters. Our work thus opens up new avenues to advance the efficiency limit of thermionic solar energy conversion and the development of next-generation novel-nanomaterial-based solar energy harvesting technology., Comment: 12 pages, 7 figures, 1 table