Spatially-resolved Energetic Electron Properties for the 21 May 2004 Flare from Radio Observations and 3D Simulations

We investigate in detail the 21 May 2004 flare using simultaneous observations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters, Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar and Heliospheric Observatory (SOHO). The flare images in different spectral ranges reveal the presence of a well-defined single flaring loop in this event. We have simulated the gyrosynchrotron microwave emission using the recently developed interactive IDL tool GX Simulator. By comparing the simulation results with the observations, we have deduced the spatial and spectral properties of the non-thermal electron distribution. The microwave emission has been found to be produced by the high-energy electrons ($>100$ keV) with a relatively hard spectrum ($\delta\simeq 2$); the electrons were strongly concentrated near the loop top. At the same time, the number of high-energy electrons near the footpoints was too low to be detected in the RHESSI images and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope images and the low-frequency microwave spectra suggest the presence of an extended "envelope" of the loop with lower magnetic field. Most likely, the energetic electron distribution in the considered flare reflects the localized (near the loop top) particle acceleration (injection) process accompanied by trapping and scattering.Comment: Accepted for publication in Solar Physic

Solar science with the Atacama Large Millimeter/submillimeter Array - A new view of our Sun

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere - a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases.Comment: 73 pages, 21 figures ; Space Science Reviews (accepted December 10th, 2015); accepted versio

Particle acceleration

Data is compiled from Solar Maximum Mission and Hinothori satellites, particle detectors in several satellites, ground based instruments, and balloon flights in order to answer fundamental questions relating to: (1) the requirements for the coronal magnetic field structure in the vicinity of the energization source; (2) the height (above the photosphere) of the energization source; (3) the time of energization; (4) transistion between coronal heating and flares; (5) evidence for purely thermal, purely nonthermal and hybrid type flares; (6) the time characteristics of the energization source; (7) whether every flare accelerates protons; (8) the location of the interaction site of the ions and relativistic electrons; (9) the energy spectra for ions and relativistic electrons; (10) the relationship between particles at the Sun and interplanetary space; (11) evidence for more than one acceleration mechanism; (12) whether there is single mechanism that will accelerate particles to all energies and also heat the plasma; and (13) how fast the existing mechanisms accelerate electrons up to several MeV and ions to 1 GeV

Study of the solar corona using radio and space observations

The physics of coronal transients, the characteristics of radiation and accelerated particles at the time of flares, and the density/temperature structure of the transition region and corona and the coronal magnetic field are investigated

Millimeter-wave and terahertz imaging techniques

This thesis presents the development and assessment of imaging techniques in the millimeterwave (mmW) and terahertz frequency bands. In the first part of the thesis, the development of a 94 GHz passive screener based on a total-power radiometer (TPR) with mechanical beamscanning is presented. Several images have been acquired with the TPR screener demonstrator, either in indoor and outdoor environments, serving as a testbed to acquire the know-how required to perform the research presented in the following parts of the thesis. In the second part of the thesis, a theoretical research on the performance of near-field passive screeners is described. This part stands out the tradeoff between spatial and radiometric resolutions taking into account the image distortion produced by placing the scenario in the near-field range of the radiometer array. In addition, the impact of the decorrelation effect in the image has been also studied simulating the reconstruction technique of a synthetic aperture radiometer. Guidelines to choose the proper radiometer depending on the application, the scenario, the acquisition speed and the tolerated image distortion are given in this part. In the third part of the thesis, the development of a correlation technique with optical processing applicable to millimeter-wave interferometric radiometers is described. The technique is capable of correlating wide-bandwidth signals in the optical domain with no loss of radiometric sensitivity. The theoretical development of the method as well as measurements validating the suitability to correlate radiometric signals are presented in this part. In the final part of the thesis, the frequency band of the imaging problem is increased to frequencies beyond 100 GHz, covering the THz band. In this case the research is centered in tomographic techniques that include spectral information of the samples in the reconstructed images. The tomographic algorithm can provide detection and identification of chemical compounds that present a certain spectral footprint in the THz frequency band.Postprint (published version

Potential of millimeter- and submillimeter-wave satellite observations for hydrometeor studies

The distribution of hydrometeors is highly variable in space and time, since it is the result of a complex chain of processes with scales from microphysical (1e-6 m) to synoptical (1e3 m). It is a challenging task to observe these highly variable atmospheric constituents on a global scale with a temporal and spatial resolution sufficient for numerical weather prediction (NWP) and hydrological purposes. This study investigates the potential of the millimeter- and submillimeter-wavelength range on space-borne sensors for hydrometeor and surface precipitation rate observations. The approach is based on simulations with cloud resolving models (CRMs) coupled to a radiative transfer (RT) model. The simulations are performed for mid-latitude cases covering a broad band of precipitation events such as heavy convective and light stratiform winter precipitation. Realistic atmospheric conditions were simulated with two mesoscale CRMs: the Meso-scale NonHydrostatic model (Meso-NH) on a 10 km and the COSMO-DE (COnsortium for Small-scale MOdeling-DEutschland) on a 2.8 km horizontal resolution. When calculating brightness temperatures for satellite observations with the one-dimensional radiative transfer model MWMOD (MicroWave MODel), the detailed cloud microphysics and the three-dimensional fields of temperature, humidity, and pressure of the CRMs are considered in the calculation of the interaction parameters. The model framework has been evaluated by comparing the simulated brightness temperature fields to observations of the Special Sensor Microwave Imager (SSM/I) as well as to those of the Advanced Microwave Sounding Unit-B (AMSU-B). The results show a good agreement as long as the CRMs capture the atmospheric situation correctly. Consequently, by coupling the radiative transfer model for microwave radiation to CRMs it is possible to evaluate these models through comparison to microwave satellite observations. Brightness temperatures for frequencies between 50 and 428 GHz at nine observation angles have been simulated for five mid-latitude cases at two time steps. In combination with the vertically integrated hydrometeor contents, these brightness temperature simulations have been used to set up a database. On the basis of this database simple retrieval algorithms have been developed to estimate the potential of the millimeter- and submillimeter-wavelength region for precipitation and hydrometeor observations. The results show, that especially for snow and graupel, the total column content can be retrieved accurately with relative errors smaller than 20% in stratiform precipitation cases over land and ocean surfaces. The performance for rain water path is similar to the one for graupel and snow in light precipitation cases. For the cases with higher precipitation amounts, the relative errors for rain water path are larger especially over land. The same behavior can be seen in the surface rain rate retrieval with the difference that the relative errors are doubled in comparison to the rain water path. Algorithms with a reduced number of frequencies show that window channels at higher frequencies are important for the surface rain rate retrieval. These are sensitive to the scattering in the ice phase related to the rain below. For the frozen hydrometeor retrieval, good results can be achieved by retrieval algorithms based only on frequencies at 150 GHz and above which are suitable for geostationary applications due to their reduced demands concerning the antenna size

Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that utilizes the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions we derive quiet-Sun values at disk center of 7300 K at lambda=3 mm and 5900 K at lambda=1.3 mm. These values have statistical uncertainties of order 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of order 25 arcsec, the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range.Comment: Solar Physics, accepted: 24 pages, 13 figure

Diagnostics of energy release in solar flares with radio dynamic imaging spectroscopy

Studies of the magnetic energy release and conversion process lie at the core of solar flare physics. Radio observations serve as a unique diagnostic method. In this dissertation, taking advantage of broadband radio dynamic imaging spectroscopy observations made by the Karl G. Jansky Very Large Array (VLA), studies are carried out on the flare energy release processes using different types of radio emissions. The VLA is a general-purpose radio observatory located in New Mexico, which provides high-quality radio dynamic imaging spectroscopic observations with an ultra-fast time cadence. In the first study, stochastic decimetric radio spike bursts are observed by the VLA during the extended energy release phase of an M8.4-class eruptive flare on March 10, 2012. Such short-lived and narrow-band spike bursts have been suggested as the signature of interactions between nonthermal electrons and density fluctuations at the termination shock front. VLA radio dynamic spectroscopic imaging of spikes centroids reveals an extended structure that resembles a shape consistent with a shock surface in projection. Using extreme-ultraviolet (EUV) imaging of the flare event from two different vantage views, combined with flare ribbons features from hard X-ray (HXR) and ultraviolet (UV) observations, a three-dimensional flare arcade is reconstructed. The radio spikes are located to be present at above the flare arcade, where the diffuse supra-arcade fan structure and plasma downflows exist. The source morphology and the reconstructed location suggest that the observed radio spikes are produced at a solar flare termination shock driven by fast plasma outflows impinging upon the flare arcade. The second study focuses on flare-associated quasi-periodic pulsations (QPPs), observed during the impulsive phase of a C1.8 flare. QPPs with a nonthermal nature are excellent probes for the energy release and transport processes during flares. However, a further understanding of their nature is limited owing to the paucity of spatially resolved observations with high temporal resolution. In this study, such a QPP event is observed during the impulsive phase of a C1.8-class eruptive solar flare. Radio imaging spectroscopy offered by the VLA shows three spatially distinct radio sources with very different physical characteristics. Two radio sources are located near the conjugate footpoints of the erupting magnetic flux rope with opposite senses of polarization, one of which displays a QPP behavior with a short, ?5-s period. The third radio source, located at the top of the post-flare arcade, coincides with the location of a looptop X-ray source and shares a similar period of ?25–45 s. It is further demonstrated that the two oppositely polarized radio sources are likely due to coherent electron cyclotron maser (ECM) emission from nonthermal electrons trapped near the footpoints of the erupting flux rope. On the other hand, the looptop QPP source, observed in both radio and X-rays, is consistent with incoherent gyrosynchrotron and bremsstrahlung emission, respectively, from nonthermal electrons trapped in the looptop region. It is concluded that the concurrent but spatially distinct QPP sources cannot be explained by a single modulation mechanism but must involve multiple mechanisms which operate in different magnetic loop systems and at different periods. The studies carried out in the dissertation address outstanding questions of flare energy release and the subsequent energy conversion and transport processes. They are mainly based on observations provided by the general-purpose Jansky VLA, which offers the capability of high-cadence, high-resolution dynamic radio imaging spectroscopy. They are also complemented by multi-wavelength data in EUV and X-rays obtained by space-borne solar telescopes observing from different perspectives as well as advanced emission modeling. These case studies further demonstrate the power of using high-cadence radio dynamic imaging spectroscopy to achieve an improved understanding of solar flare energy release. However, owing to the nature of these proposal-based, infrequent observations from a general-purpose telescope like the VLA, a systematic study is not possible. Future advance on this ground calls for the development of more powerful radio facilities dedicated to solar observing

Millimetre wave imaging for concealed target detection

PhDConcealed weapon detection (CWD) has been a hot topic as the concern about pub- lic safety increases. A variety of approaches for the detection of concealed objects on the human body based on earth magnetic ¯eld distortion, inductive magnetic ¯eld, acoustic and ultrasonic, electromagnetic resonance, MMW (millimetre wave), THz, Infrared, x-ray technologies have been suggested and developed. Among all of them, MMW holographic imaging is considered as a promising approach due to the relatively high penetration and high resolution that it can o®er. Typical concealed target detection methods are classi¯ed into 2 categories, the ¯rst one is a resonance based target identi¯cation technique, and the second one is an imaging based system. For the former, the complex natural resonance (CNR) frequencies associated with a certain target are extracted and used for identi¯cation, but this technique has an issue of high false alarm rate. The microwave/millimetre wave imaging systems can be categorized into two types: passive systems and active sys- tems. For the active microwave/millimetre wave imaging systems, the microwave holographic imaging approach was adopted in this thesis. Such a system can oper- ate at either a single frequency or multiple frequencies (wide band). An active, coherent, single frequency operation millimetre wave imaging system based on the theory of microwave holography was developed. Based on literature surveys and ¯rst hand experimental results, this thesis aims to provide system level parame- ter determination to aid the development of a target detection imager. The goal is approached step by step in 7 chapters, with topics and issues addressed rang- ing from reviewing the past work, ¯nding out the best candidate technology, i.e. the MMW holographic imaging combined with the resonance based target recog- i nition technique, the construction of the 94 GHz MMW holographic prototype imager, experimental trade-o® investigation of system parameters, imager per- formance evaluation, low pro¯le components and image enhancement techniques, feasibility investigation of resonance based technique, to system implementation based on the parameters and results achieved. The task set forth in the beginning is completed by coming up with an entire system design in the end.