Terahertz Photometer to Observe Solar Flares in Continuum

Abstract

Solar observations at sub-THz frequencies detected a new flare spectral component peaking in the THz range, simultaneously with 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 THz photometer system was developed to observe outside the terrestrial atmosphere on stratospheric balloons or satellites, or at exceptionally transparent ground stations. The telescope was designed to observe the whole solar disk detecting small relative changes in input temperature caused by flares at localized positions. A Golay cell detector is preceded by low-pass filters to suppress visible and near IR radiation, a band-pass filter, and a chopper. A prototype was assembled to demonstrate the new concept and the system performance. It can detect temperature variations smaller than 1 K for data sampled at a rate of 10/s, smoothed for intervals larger than 4 s. For a 76 mm aperture, this corresponds to small solar burst intensities at THz frequencies. A system with 3 and 7 THz photometers is being built for solar flare observations on board of stratospheric balloon missions.

THz photometers for solar flare observations from space

“…Two known independent telescope properties were combined in order to obtain a sufficiently large gain to detect small bursts that may pop up at any place on the solar disk, while observing the whole field containing the solar disc [13,14]. Figure 1 illustrates the ray tracings for solar disk image formed, with size d, at the focal plane of a reflector aperture with diameter D. The same geometry is applicable for a lens aperture.…”

“…The system output is in voltages proportional to power 3 The THz photometers construction Figure 3 illustrates the SOLAR-T design conception. The final system has been derived from several characterizations and tests of materials, filters, sensors and the performance of actual complete operational prototypes [14][15][16][17]. The two 76 mm diameter Cassegrain telescopes have focal length of 580 mm.…”

“…The practical problem to observe the whole solar disk with enough sensitivity to detect flares subtended by much smaller angular sizes was solved with an innovative photon concentrator [13,14] that combines the formation of a full solar disk image at the focal plane with size smaller than the size of the surface of the detecting element (or of the photon concentrator aperture), with the physical size of telescope primary aperture, as described below. It becomes possible to use apertures large enough to detect small solar bursts without the need to point at a particular location on the solar disk with narrow beams, as in usual coherent optical configurations.…”

The performance of THz photometers for solar flare observations from space

“…A double THz photometers system to measure solar transient temperature bursts in the continuum at 3 and 7 THz has been built, based on concept, materials, filters and detector characterized by various previous laboratory developments leading to the setup of a prototype model [4][5][6]. The practical problem to observe the whole solar disk with enough sensitivity to detect flares subtended by much smaller angular sizes was solved with an innovative photon concentrator [7] that combines the formation a full solar disk image, with the physical size of the detecting surface at the focal plane, independently from the aperture size, with the relationship THz to visible radiation.…”