Over the past 5 years, there has been a significant interest in employing terahertz (THz) technology, spectroscopy and imaging for security applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation; (b) target compounds such as explosives and illicit drugs have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. In this paper, stand-off interferometric imaging and sensing for the detection of explosives, weapons and drugs is emphasized. Future prospects of THz technology are discussed.

Radio emission from solar flares offers a number of unique diagnostic tools to address long-standing questions about energy release, plasma heating, particle acceleration, and particle transport in magnetized plasmas. At millimeter and centimeter wavelengths, incoherent gyrosynchrotron emission from electrons with energies of tens of kilo electron volts to several mega electron volts plays a dominant role. These electrons carry a significant fraction of the energy released during the impulsive phase of flares. At decimeter and meter wavelengths, coherent plasma radiation can play a dominant role. Particularly important are type III and type III-like radio bursts, which are due to upward-and downwarddirected beams of nonthermal electrons, presumed to originate in the energy release site. With the launch of Yohkoh and the Compton Gamma-Ray Observatory, the relationship between radio emission and energetic photon emissions has been clarified. In this review, recent progress on our understanding of radio emission from impulsive flares and its relation to X-ray emission is discussed, as well as energy release in flare-like phenomena (microflares, nanoflares) and their bearing on coronal heating.

An observational relationship has been well established among magnetic reconnection, high-energy flare emissions and the rising motion of erupting flux ropes. In this paper, we verify that the rate of magnetic reconnection in the low corona is temporally correlated with the evolution of flare nonthermal emissions in hard X-rays and microwaves, all reaching their peak values during the rising phase of the soft X-ray emission. In addition, however, our new observations reveal a temporal correlation between the magnetic reconnection rate and the directly observed acceleration of the accompanying coronal mass ejection (CME) and filament in the low corona, thus establishing a correlation with the rising flux rope. These results are obtained by examining two well-observed two-ribbon flare events, for which we have good measurements of the rise motion of filament eruption and CMEs associated with the flares. By measuring the magnetic flux swept through by flare ribbons as they separate in the lower atmosphere, we infer the magnetic reconnection rate in terms of the reconnection electric field E rec inside the reconnecting current sheet (RCS) and the rate of magnetic flux convected into the diffusion region. For the X1.6 flare event, the inferred E rec is~5.8 V cm À1 and the peak mass acceleration is 3 km s À2 , while for the M1.0 flare event E rec is~0.5 V cm À1 and the peak mass acceleration is 0.2-0.4 km s À2 .

A systematic motion of Ha kernels during solar Ñares can be regarded as the chromospheric signature of progressive magnetic reconnection in the corona, in that the magnetic Ðeld lines swept through by the kernel motion are those connected to the di †usion region at the reconnection point. In this paper, we present high-cadence and high-resolution Ha[1.3 observations of an impulsive Ñare that exhibits a A systematic kernel motion and relate them to the reconnecting current sheet (RCS) in the corona. Through analyses of X-ray and microwave observations, we further examine the role of the macroscopic electric Ðeld inside the RCS in accelerating electrons. We measure the velocity of the kernel motion to be 20[100 km s~1. This is used together with the longitudinal magnetic Ðeld to infer an electric Ðeld as high as 90 V cm~1 at the Ñare maximum. This event shows a special magnetic Ðeld conÐguration and motion pattern of Ha kernels, in that a light bridge divides a Ñare kernel into two parts that move in di †erent manners : one moving into the stronger magnetic Ðeld and the other moving along the isogauss contour of the longitudinal magnetic Ðeld. The temporal variation of the electric Ðeld inferred from the former type of kernel motion is found to be correlated with 20È85 keV hard X-ray light curves during the rise of the major impulsive phase. This would support the scenario of magnetic energy release via current dissipation inside the RCS, along with the hypothesis of the DC electric Ðeld acceleration of X-rayÈemitting electrons below 100 keV. However, there is no good temporal correlation between the hard X-ray emission and the inferred electric Ðeld from the other motion pattern. Furthermore, the microwave emission, which supposedly comes from higher energy electrons, shows a time proÐle and electron spectrum that di †ers from those of the X-ray bursts. We conclude that either the twodimensional magnetic reconnection theory related to the Ha kernel motion is applicable to only some part of the Ñare region due to its special magnetic geometry, or the electron acceleration is dominated by other mechanisms depending on the electron energy.

We report the first science results from the newly completed Expanded Owens Valley Solar Array (EOVSA), which obtained excellent microwave imaging spectroscopy observations of SOL2017-09-10, a classic partially-occulted solar limb flare associated with an erupting flux rope. This event is also well-covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present an overview of this event focusing on microwave and HXR data, both associated with high-energy nonthermal electrons, and discuss them within the context of the flare geometry and evolution revealed by extreme ultraviolet (EUV) observations from the Atmospheric Imaging Assembly aboard the Solar Dynamics Observatory (SDO/AIA). The EOVSA and RHESSI data reveal the evolving spatial and energy distribution of high-energy electrons throughout the entire flaring region. The results suggest that the microwave and HXR sources largely arise from a common nonthermal electron population, although the microwave imaging spectroscopy provides information over a much larger volume of the corona.