A. Marun

New telescopes for ground-based solar observations at submillimeter and mid-infrared

The solar submillimeter-wave telescope (SST) is the only one of its kind dedicated to solar continuous observations. Two radiometers at 0.740 mm (405 GHz), and four at 1.415 mm (212 GHz) are placed in the Cassegrain focal plane of the 1.5-m dish at El Leoncito high altitude site, San Juan, Argentina. The aperture efficiencies are close to design predictions: 20% and 35% for 2 and 4 arcminutes beam sizes at 405 and 212 GHz, respectively. The positioner absolute pointing accuracy is 10 arcseconds. Spectral coverage is complemented by ground-based mid-infrared telescopes developed for high cadence observations in the continuum 10 micron band (30 THz), using small apertures and room-temperature microbolometer cameras. Using the system, a new solar burst spectral component was discovered, exhibiting fluxes increasing for smaller wavelengths, separated from the well known microwave component. Rapid sub-second pulsations are common for all bursts. The pulsations onset times of appear to be connected to the launch times of CMEs. Active regions are brighter for shorter submillimeter-waves. Mid-IR bright regions are found closely associated with calcium plages and magnetic structures near the solar photosphere. Intense and rapid 10 micron brightening was detected on active centers in association with weak flares. These results raise challenging difficulties for interpretation.

Solar Submillimeter and Gamma-Ray Burst Emission

Solar are emission was measured at 212 GHz in the submillimeter range by the Submillimeter Solar Telescope in the 1.2-18 GHz microwave range by the Owens Valley Solar Array and in the gamma-ray energy range (continuum) by experiments on board the Yohkoh ( > 1. 2 MeV) and Shenzhou 2 ( > 0.2 MeV) satellites. At the burst onset, the submillimeter and microwave time profiles were well correlated with gamma rays to the limit of the temporal resolution (less than or equal to10 s). At 212 GHz, fast pulses ( 1. 2 MeV), attaining nearly 50 pulses per minute at the maximum. These results suggest that gamma rays might be the response to multiple rapid pulses at 212 GHz and might be produced at different sites within the flaring region.

BRIGHT 30 THz IMPULSIVE SOLAR BURSTS

Impulsive 30 THz continuum bursts have been recently observed in solar flares, utilizing small telescopes with a unique and relatively simple optical setup concept. The most intense burst was observed together with a GOES X2 class event on October 27, 2014, also detected at two sub-THz frequencies, RHESSI X-rays and SDO/HMI and EUV. It exhibits strikingly good correlation in time and in space with white light flare emission. It is likely that this association may prove to be very common. All three 30 THz events recently observed exhibited intense fluxes in the range of 10 4 solar flux units, considerably larger than those measured for the same events at microwave and sub-mm wavelengths. The 30 THz burst emission might be part of the same spectral burst component found at sub-THz frequencies. The 30 THz solar bursts open a promising new window for the study of flares at their origin.

Sub-THz and Hα activity during the preflare and main phases of a Goes class M2 event

Radio and optical observations of the evolution of flare-associated phenomena have shown an initial and rapid burst at 0.4?THz only followed subsequently by a localized chromospheric heating producing an H? brightening with later heating of the whole active region. A major instability occurred several minutes later producing one impulsive burst at microwaves only, associated with an M2.0 GOES X-ray flare that exhibited the main H? brightening at the same site as the first flash.The possible association between long-enduring time profiles at soft X-rays, microwaves, H?, and sub-THz wavelengths is discussed. In the decay phase, the H? movie shows a disrupting magnetic arch structure ejecting dark, presumably chromospheric, material upward. The time sequence of events suggests genuine interdependent and possibly non-thermal instabilities triggering phenomena, with concurrent active region plasma heating and material ejection.

Photometry of THz radiation using Golay cell detector

The measurement of THz radiation in the continuum presents new technical challenges concerning materials, frequency filters and detection devices. We present the first results of a radiometric system using Golay cell as a detector, for the whole > 15 THz range, and at discrete frequencies centered at 2, 10 and 30 THz. The system was designed to measure solar THz radiation. It is capable to detect small solar bursts, with a large dynamic range to be able to detect larger events.

Uncooled detectors of continuum terahertz radiation

THz continuum spectral photometry has new and unique applications in different civil and military areas presenting a number of distinctive advantages on the well known microwaves or mid- to near-infrared technologies. THz sensing is essential to investigate the emission mechanisms by high energy particle acceleration processes. Technical challenges appear to diagnose radiation produced by solar flare burst emissions measured from space as well as radiation produced by high energy electrons in laboratory accelerators. THz filters and detectors have been investigated for the construction of solar flare high cadence radiometers to operate outside the terrestrial atmosphere. Experimental setups have been assembled for testing THz continuum radiation response from distinct detectors: adapted commercial microbolometer array, pyroelectric module, and opto-acoustic (Golay cell). The results permitted the final design of a THz double radiometer using Golay cells to be flown in stratosphere balloon missions.

Search for continuum solar flare radiation in the terahertz range

A new solar burst emission spectral component has been found showing sub-THz fluxes increasing with frequency, spectrally separated from the well known microwave component bringing serious constraints for the interpretation. The THz spectral component might be the evidence of synchrotron emission from high energy electrons peaking somewhere in the far IR range. The knowledge of THz continuum spectral shape is therefore essential to investigate the nature of the emission mechanisms involved. Sub-THz solar flare flux data are available from observations carried out at El Leoncito high altitude observatory, at two atmosphere transmission windows (0.2 and 0.4 THz). We describe the results from developments addressed to the construction of solar flare radiometers at two frequencies to operate outside the terrestrial atmosphere (i.e. at 3 and 7 THz). The system has been planned as part of the SOLAR-T mission on a stratosphere long-duration balloon flight over Russia sometime in 2013–2016.

“A New Setup for Ground-based Measurements of Solar Activity at 10 μm” (PASP, 118, 1558 [2006])

The mid-IR (10 μm) temperature scale adopted in the article was inadvertently derived from a calibration made with the camera nearly saturated. It provided a temperature input versus reading temperature ratio of about 36. New calibrations were repeated several times in the following campaigns, one year later, showing a temperature input versus reading temperature ratio of 2. Therefore the published mid-IR burst time profile and derived fluxes should be divided by a factor of 18.

SOLAR-T : Terahertz Photometers to Observe Solar Flare Emission on Stratospheric Balloon Flights

A new solar flare spectral component has been found with intensities increasing for larger sub-THz frequencies, spectrally separated from the well known microwaves component, bringing challenging constraints for interpretation. Higher THz frequencies observations are needed to understand the nature of the mechanisms occurring in flares. A twofrequency THz photometer system was developed to observe outside the terrestrial atmosphere on stratospheric balloons or satellites, or at exceptionally transparent ground stations. 76 mm diameter telescopes were designed to observe the whole solar disk detecting small relative changes in input temperature caused by flares at localized positions at 3 and 7 THz. Golay cell detectors are preceded by low-pass filters to suppress visible and near IR radiation, band-pass filters, and choppers. It can detect temperature variations smaller than 1 K with time resolution of a fraction of a second, corresponding to small burst intensities. The telescopes are being assembled in a thermal controlled box to which a data conditioning and acquisition unit is coupled. While all observations are stored on board, a telemetry system will forward solar activity compact data to the ground station. The experiment is planned to fly on board of long-duration stratospheric balloon flights some time in 2013-2015. One will be coupled to the GRIPS gamma-ray experiment in cooperation with University of California, Berkeley, USA. One engineering flight will be flown in the USA, and a 2 weeks flight is planned over Antarctica in southern hemisphere summer. Another long duration stratospheric balloon flight over Russia (one week) is planned in cooperation with the Lebedev Physics Institute, Moscow, in northern hemisphere summer.

Continuum terahertz radiation detection using membrane filters

Technology of remote sensing in the terahertz range (frequency interval arbitrarily set between 0.1 – 30 THz) is the object of considerable development efforts addressed to a number of new civilian and military applications. Technical challenges appear in the THz sensing of temperature differences above an existing hot surface target, such as radiation patterns produced by high energy electrons in laboratory accelerators, and thermal differentiated structures in the solar disk in space. The efficient suppression of radiation in the visible and near infrared (set arbitrarily for wavelengths ≪ 10 µm) is an essential requirement. An experimental setup has been prepared for testing at room temperature THz materials and detectors, aiming the detection of solar radiation. A custom-made detector consisted in a room-temperature micro-bolometer INO camera with HRFZ-Si window. The THz transmission of two “low-pass” membranes were tested for black body temperatures ranging 300–1000 K: Zitex G110G and TydexBlack. It has been demonstrated that both are effective suppressors of radiation at wavelengths ≪ 15 µm, with the first one exhibiting a small radiation excess, that may be attributed to small visible and NIR allowance. We describe optical setups prepared to detect solar radiation, consisting in a microbolometer camera preceded by a photon pipe, low-pass membrane and band-pass resonant metal mesh, placed at the focus of the 1.5 m reflector for submillimeter waves (SST) at El Leoncito, Argentina Andes.