Sub-THz radiation mechanisms in solar flares

Observations in the sub-THz range of large solar flares have revealed a mysterious spectral component increasing with frequency and hence distinct from the microwave component commonly accepted to be produced by gyrosynchrotron (GS) emission from accelerated electrons. Evidently, having a distinct sub-THz component requires either a distinct emission mechanism (compared to the GS one), or different properties of electrons and location, or both. We find, however, that the list of possible emission mechanisms is incomplete. This Letter proposes a more complete list of emission mechanisms, capable of producing a sub-THz component, both well-known and new in this context and calculates a representative set of their spectra produced by a) free-free emission, b) gyrosynchrotron emission, c) synchrotron emission from relativistic positrons/electrons, d) diffusive radiation, and e) Cherenkov emission. We discuss the possible role of the mechanisms in forming the sub-THz emission and emphasize their diagnostics potential for flares.Comment: Submitted to ApJL, 5 figures, minor revision to match resubmitted versio

Local SiC photoluminescence evidence of non-mutualistic hot spot formation and sub-THz coherent emission from a rectangular Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} mesa

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature $T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, the local surface temperature $T({\bm r})$ was directly measured during simultaneous sub-THz emission from the $N\sim10^3$ intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents $I$ and low bath temperatures $T_{\rm bath}\lesssim~35$ K, the center of a large elliptical hot spot with $T({\bm r})> T_c$ jumps dramatically with little current-voltage characteristic changes. The hot spot doesn't alter the ubiquitous primary and secondary emission conditions: the ac Josephson relation and the electromagnetic cavity resonance excitation, respectively. Since the intense sub-THz emission was observed for high $T_{\rm bath}\gtrsim~50$ K in the low $I$ bias regime where hot spots are absent, hot spots can not provide the primary mechanisms for increasing the output power, the tunability, or for promoting the synchronization of the $N$ IJJs for the sub-THz emission, but can at best coexist non-mutualistically with the emission. No $T({\bm r})$ standing waves were observed

Sub-THz Ray Tracing Simulation and Experimental Validation for Indoor Scenarios

Sub-terahertz (THz) communication is envisioned as one of the key components for 6G because of the abundantly available spectrum resource. Accurate and efficient channel models are prerequisites for developing sub-THz communicationsystems. Due to the sparsity and more ray optics propagation characteristics of the sub-THz channel, deterministic Ray-Tracing (RT) has attracted much attention for sub-THz channel modeling, which shows the potential of reducing the simulation complexity yet maintaining the accuracy. This paper presents an implementation of RT for sub-THz channel modeling and demonstrates its performance based on sub-THz channel measurements. A virtual massive multiple-inputmultiple-output (MIMO) channel operating at 100 GHz anda double-directional 300 GHz channel are considered in the RT implementation, where the RT achieves a high similarity compared to the channel measurements in terms of channel impulse response and power angular spectrum. Besides, thenear-field and spatial non-stationary properties of the sub-THz massive MIMO channel and the dominant multipaths of the 300 GHz channel are accurately reconstructed in the RT simulation. This work can provide insights into deterministic sub-THz channel modeling research from the implementation,evaluation, and challenges perspectives

Sub-terahertz, microwaves and high energy emissions during the December 6, 2006 flare, at 18:40 UT

The presence of a solar burst spectral component with flux density increasing with frequency in the sub-terahertz range, spectrally separated from the well-known microwave spectral component, bring new possibilities to explore the flaring physical processes, both observational and theoretical. The solar event of 6 December 2006, starting at about 18:30 UT, exhibited a particularly well-defined double spectral structure, with the sub-THz spectral component detected at 212 and 405 GHz by SST and microwaves (1-18 GHz) observed by the Owens Valley Solar Array (OVSA). Emissions obtained by instruments in satellites are discussed with emphasis to ultra-violet (UV) obtained by the Transition Region And Coronal Explorer (TRACE), soft X-rays from the Geostationary Operational Environmental Satellites (GOES) and X- and gamma-rays from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz impulsive component had its closer temporal counterpart only in the higher energy X- and gamma-rays ranges. The spatial positions of the centers of emission at 212 GHz for the first flux enhancement were clearly displaced by more than one arc-minute from positions at the following phases. The observed sub-THz fluxes and burst source plasma parameters were found difficult to be reconciled to a purely thermal emission component. We discuss possible mechanisms to explain the double spectral components at microwaves and in the THz ranges.Comment: Accepted version for publication in Solar Physic

On the origin of 140 GHz emission from the 4 July 2012 solar flare

The sub-THz event observed on the 4 July 2012 with the Bauman Moscow State Technical University Radio Telescope RT-7.5 at 93 and 140~GHz as well as Kislovodsk and Mets\"ahovi radio telescopes, Radio Solar Telescope Network (RSTN), GOES, RHESSI, and SDO orbital stations is analyzed. The spectral flux between 93 and 140 GHz has been observed increasing with frequency. On the basis of the SDO/AIA data the differential emission measure has been calculated. It is shown that the thermal coronal plasma with the temperature above 0.5~MK cannot be responsible for the observed sub-THz flare emission. The non-thermal gyrosynchrotron mechanism can be responsible for the microwave emission near $10$~GHz but the observed millimeter spectral characteristics are likely to be produced by the thermal bremsstrahlung emission from plasma with a temperature of about 0.1~MK.Comment: 18 pages, 6 figure

High-resolution antenna near-field imaging and sub-THz measurements with a small atomic vapor-cell sensing element

Atomic sensing and measurement of millimeter-wave (mmW) and THz electric fields using quantum-optical EIT spectroscopy of Rydberg states in atomic vapors has garnered significant interest in recent years towards the development of atomic electric-field standards and sensor technologies. Here we describe recent work employing small atomic vapor cell sensing elements for near-field imaging of the radiation pattern of a K$_u$-band horn antenna at 13.49 GHz. We image fields at a spatial resolution of $\lambda/10$ and measure over a 72 to 240 V/m field range using off-resonance AC-Stark shifts of a Rydberg resonance. The same atomic sensing element is used to measure sub-THz electric fields at 255 GHz, an increase in mmW-frequency by more than one order of magnitude. The sub-THz field is measured over a continuous $\pm$100 MHz frequency band using a near-resonant mmW atomic transition

Dynamic Sub-THz Radio Channel Emulation: Principle, Challenges, and Experimental Validation

Sub-terahertz (Sub-THz) technology, as one of the key candidates for the six generation (6G) systems, has attracted increasing attention from academia and industry, due to its promise to unleash vast amounts of new frequency spectrum. Sub-THz system designs pose unique and more challenging circumstances compared to traditional communication systems. These challenges arise from the demanding propagation conditions, limited availability of commercial radio frequency (RF) components, the need for high-gain and beam-steerable antennas that are highly integrated at both ends of the communication link, short-range communication scenarios, and the requirement for extreme data rates. Therefore, it is crucial to assess the performance of radio devices in realistic propagation channels in sub-THz communication systems. In this work, we present the concept, challenges, and enabling solutions for achieving sub-THz radio channel emulation. Moreover, we experimentally demonstrated the reconstruction of the measured propagation channels at 140 GHz with a commercial radio channel emulator in the laboratory. The developed dynamic fading channel replay concept and experimental validation procedure allows initial tests of future sub-THz communication devices

Massive MIMO Channel Sounder for Sub-THz Based on Virtual Array Concept

The sub-THz frequency spectrum ranging from 90 GHz to 300 GHz offers huge untapped frequency band, up to several tens of GHz, which is highly promising forachieving extremely high data rate transmission. Therefore, sub-THz technology is considered as one of the key technology candidates for beyond 5G communication systems. However, sub-THz communication relies on dominant propagation path in the environments. It is essential to identify and track thedominant paths in the channel to maintain the communication link. Understanding the spatial profiles of the channel is therefore essential. In this work, we present a long range sub-THz channel sounder we developed at Aalborg University Denmark, which is capable of capturing the spatial profile of the channel with high spatial resolution based on the virtual array concept. The channel sounder is experimentally validated in field measurements

Observation of phase noise reduction in photonically synthesized sub-THz signals using a passively mode-locked laser diode and highly selective optical filtering

A Continuous Wave (CW) sub-THz photonic synthesis setup based on a single Passively Mode-Locked Laser Diode (PMLLD) acting as a monolithic Optical Frequency Comb Generator (OFCG) and highly selective optical filtering has been implemented to evaluate the phase noise performance of the generated sub-THz signals. The analysis of the synthesized sub-THz signals up to 120 GHz gives as a result an effective reduction of the electrical linewidth when compared to direct harmonic generation that begins at 50 GHz and becomes greater as the frequency increases. The phase noise reduction offered by the setup, along with its integration potential, cost and bandwidth, make it a promising candidate to the development of an integrated and high performance low phase noise local oscillator in the sub-THz range.Work supported by the Spanish Ministry of Science and Technology through the project TEC2009-14525-C02-02 and by the European Commission FP7 iPHOS Project. The work by A.R. Criado has been supported by the Spanish Ministry of Science and Technology under the FPI Program, Grant# BES2010-030290.Publicad