Temporal and Spatial Analyses of Spectral Indices of Nonthermal Emissions Derived from Hard X-Rays and Microwaves

We studied electron spectral indices of nonthermal emissions seen in hard X-rays (HXRs) and in microwaves. We analyzed 12 flares observed by the Hard X-ray Telescope aboard {\it Yohkoh}, Nobeyama Radio Polarimeters (NoRP), and the Nobeyama Radioheliograph (NoRH), and compared the spectral indices derived from total fluxes of hard X-rays and microwaves. Except for four events, which have very soft HXR spectra suffering from the thermal component, these flares show a gap $\Delta\delta$ between the electron spectral indices derived from hard X-rays $\delta_{X}$ and those from microwaves $\delta_{\mu}$ ($\Delta\delta = \delta_{X} - \delta_{\mu}$) of about 1.6. Furthermore, from the start to the peak times of the HXR bursts, the time profiles of the HXR spectral index $\delta_{X}$ evolve synchronously with those of the microwave spectral index $\delta_{\mu}$, keeping the constant gap. We also examined the spatially resolved distribution of the microwave spectral index by using NoRH data. The microwave spectral index $\delta_{\mu}$ tends to be larger, which means a softer spectrum, at HXR footpoint sources with stronger magnetic field than that at the loop tops. These results suggest that the electron spectra are bent at around several hundreds of keV, and become harder at the higher energy range that contributes the microwave gyrosynchrotron emission.Comment: 24 pages, 6 figures, accepted for publication in Ap

Numerical examination of plasmoid-induced reconnection model for solar flares: the relation between plasmoid velocity and reconnection rate

The plasmoid-induced-reconnection model explaining solar flares based on bursty reconnection produced by an ejecting plasmoid suggests a possible relation between the ejection velocity of a plasmoid and the rate of magnetic reconnection. In this study, we focus on the quantitative description of this relation. We performed magnetohydrodynamic (MHD) simulations of solar flares by changing the values of resistivity and the plasmoid velocity. The plasmoid velocity has been changed by applying an additional force to the plasmoid to see how the plasmoid velocity affects the reconnection rate. An important result is that the reconnection rate has a positive correlation with the plasmoid velocity, which is consistent with the plasmoid-induced-reconnection model for solar flares. We also discuss an observational result supporting this positive correlation.Comment: 27 pages, 12 figures, Accepted for publication in Ap

A 100 W-Class Water-Vapor Hall Thruster for Constellations and Space Explorations by SmallSats

The laboratory models of a water-vapor Hall thruster and LaB6 thermionic cathode were developed and tested. To optimize the thruster design to water-vapor propellant, the geometrical investigation was conducted. After testing six different models, the smallest thruster, with an outer diameter of 20 mm, was found to be the most suitable for 100-W class operation. This thruster was able to be operated less than 100 W at 200 V. In addition, the discharge power was suppressed to 200 W even at 300 V. Based on the plume diagnostics, the thrust force of 2.9 mN, specific impulse of 650 s, and anode efficiency of 4.6 % were obtained as a representative performance of this 300 V operating point. After the thruster operation was achieved, the cathode coupling test was conducted to demonstrate electron emission under water-vapor plasma existence. As a result of this experiment, the effective increase in electron current compared to the previous stand-alone tests was confirmed as well as the compatability to the water-vapor plasma plume. On the other hand, the electron emission current has not achieved 100 mA-class yet and the required heating power was predicted over 100 W; thus, further improvement is progressing