# Solar Submillimeter Telescope next generation

C. Guillermo Giménez de Castro*unodos  Jean-Pierre Raulinuno  Adriana Valiouno
Emilia Correiaunotres  Paulo J. A. Simõesunocuatro and Sérgio Szpigeluno
unoCentro de Rádio Astronomia e Astrofísica Mackenzie, EE, UPM, São Paulo,
Brazil; e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS>dosInstituto de
Astronomía y Física del Espacio, UBA / CONICET, Buenos Aires, Argentina
tresInstituto Nacional de Pesquisas Espaciais, INPE, São José dos Campos,
Brazil cuatroSUPA School of Physics and Astronomy, University of Glasgow,
Glasgow, G12 8QQ, UK

###### Abstract

The Solar Submillimeter Telescope (SST) is an unique instrument that has been
observing the Sun daily since 2001 bringing a wealth of information and
raising new questions about the particle acceleration and transport, and
emission mechanisms during flares. We are now designing its successor, the
SSTng, that will expand the scientific goals of the instrument, including non-
solar source observations.

##  1 Introduction

Submillimeter-wave (submm) observations, here considered for
$0.3\leq\lambda\leq 3$ mm, allow us to study the solar low atmospheric layers,
from the Transition Region to the Chromosphere [1]. During flares, the submm
emission might be originated by synchrotron emission from relativistic
particles [2]. Therefore, we can track the energy transport from the
acceleration to the emission sites. Moreover, Kaufmann et al. [3] have shown
that some flares have a second spectral submm component (Figure 1) with a
still unknown origin [4].

Even though the immense wealth of information that submm observations may
bring to the understanding of the solar atmosphere and its dynamics, there is
a lack of regular observations to cover this wavelength range. First efforts
were carried on with the James Clerk Maxwell Telescope (JCMT) [5]. The Swiss
KOSMA telescope, also observed the Sun a few times in 2003/2004 before it was
decommissioned [6, 7]. More recently, the Atacama Large Millimeter Array
(ALMA) is revealing fine details of the quiet and quiescent solar behavior
[8]. However, JCMT and KOSMA observed the Sun just a couple of times, and ALMA
allocates a small portion of its observing time to the Sun and it is not the
best instrument to catch fast transient phenomena, like solar flares.

Figure 1: Left: Time profiles of the SOL2003-11-04T1945 solar event at 212 GHz
(blue) and 405 GHz (red) observed by the SST. Spectra obtained at two
different instants of the event (see vertical dashed lines on the left panel).
Microwave data were obtained by the OVSA array. Observations at 44 GHz were
carried out at Pierre Kaufmann Radio Observatory (ROPK) with its 14-m single
dish antenna. This was the first event to show a second spectral submm
component [3].

Since 1999, the only solar dedicated submm instrument is the Solar
Submillimeter Telescope (SST) [9], a single dish telescope with room
temperature receivers operating at 212 GHz ($\lambda=1.4$ mm) and 405 GHz
($\lambda=0.7$ mm). After more than 20 years of excellent service, SST has to
be updated in order to provide answers to the questions its observations have
raised: what is the emission mechanism that creates a second spectral
component $>100$ GHz during flares? Does this component existe in “weak”
flares? It may also bring more information about the 3–5 minute p-mode
oscillations, the time evolution of the large scale chromospheric structures
and its relationship with the magnetic field, the “slow” components at these
frequencies, among other.

In the following lines we will present the general characteristics of the SST
and introduce the SST next generation (SSTng) which is being designed at the
Center for Radio Astronomy and Astrophysics Mackenzie (CRRAM) in São Paulo
(Brazil).

##  2 SST

Figure 2: The SST with the radome open for maintenance.

SST (Figure 2) is a product of state-of-art technologies of the 1990s. It has
a 1.5 m, $f/D=8$, radome-enclosed single-dish aluminum reflector built at the
Steward Observatory, University of Arizona, Tucson, USA. Its frontend has six
room temperature radiometers that operate simultaneously: four receivers
operate at 212 GHz and two at 405 GHz, with nominal beam sizes of 4 and 2 arc
minutes, respectively. The six beams form two arrays separated by
approximately 6 arc-minutes. The first array has three 212 GHz beams arranged
in an equilateral triangle, in the center of this triangle, there is one 405
GHz beam. The receiver horns have a tapper that allows the beam intersection
at 50% level (-3 dB). Although the tapper reduces the efficiency and increases
the spill over, it allows the use of the multibeam method to instantly
localize the emission centroid of point-like sources and to correct the flux
for offset pointing [10, 11]. The second array has one 212 GHz and one 405 GHz
beam with the same center and is used for reference. The radiometers have a
$\Delta\nu=8.5$ GHz bandwidth, temperatures of around 2000–3000 K and were
custom made by RPG-Radiometer GmbH, Meckenheim, Germany. The backend output
signal is converted to 2-byte integer numbers. SST has an Alt-Azimuth mount
with 3.6 milliarcsec resolution and maximum speed of $3^{\circ}\
\mathrm{s}^{-1}$. The output data is recorded in three different file
structures: sub-integrated with 5 ms time resolution, integrated with 40 ms
time resolution and auxiliary with 1 s time resolution.

The telescope is installed in the El Leoncito Astronomical Complex (CASLEO, in
Spanish) at 2550 m above sea level in the Argentinean Andes, Province of San
Juan. First light was in July 1999 and since April 2001 does daily
observations. During the past 20+ years we refined the measurement of the
atmospheric optical depth with different techniques and gathered a large
statistics to understand the atmospheric transmission on the site at both
frequencies [12, 13]. Median values of the opacity are 0.16 and 1.1 for 212
and 405 GHz, respectively [14]. That means that for more than 50% of the time,
the atmosphere is nominally optically thick at 405 GHz.

Figure 3: Top panel: Observed atmospheric opacity histogram at 212 GHz and
expected opacity at 150 GHz. Bottom panel: Observed opacity histogram at 405
GHz and expected opacity for 300 GHz. The observed opacities were determined
with skydips between 2008 and 2012.

##  3 SSTng

The SSTng will be more sensitive. Indeed, the SST noise flux density, when
observing the Sun, is 1 and 7 SFU111Solar Flux Unit $\equiv 10^{4}$ Jy. for
212 and 405 GHz, respectively, considering 40 ms integration time, the median
values of the atmospheric opacities and a mean elevation angle of 60∘. With
this sensitivity, the weakest flares we have observed are of GOES class M. By
changing the receiver frequencies to 150 and 300 GHz we gain a factor $>2$ in
opacity: from our statistics and using the relationship obtained in [13] we
derive median values of 0.07 and 0.4 for the atmospheric opacities at 150 and
300 GHz, respectively. In Figure 3 we show the histograms of the observed
zenith opacities obtained between 2008 and 2012 using the skydip method. The
same figure presents the expected histograms for 150 and 300 GHz showing an
expressive reduction.

We also want to keep the same beam sizes, therefore we plan to substitute the
present reflector by a new one of 3-m diameter. Moreover, today receivers have
lower temperatures and larger bandwidths. For the present work we assume
temperatures around 1000 and 2500 K for 150, and 300 GHz, respectively, and
$\Delta\nu=16$ GHz for both frequency bands. Everything combined, lower
opacities, larger reflector surface and receiver bandwidth, and smaller
temperature, shall yield noise fluxes of 0.06 and 0.12 SFU, i.e. SSTng will be
15 and 55 times more sensitive when compared with SST 212 and 405 GHz
observations. In terms of flares, this gain means that events of GOES class C,
and maybe, class B, will be detected, dramatically increasing the number of
events to analyze. In terms of quiet sun behavior, it will certainly be
possible to detect the 3-5 minute oscillations, and faint structures.

Polarization is key to discriminate the origin of the emission and to study
the ambient magnetic field, however it was not yet explored at submm
wavelengths during flares. SSTng will be the first solar telescope to have
circular polarization detectors for both frequency bands. And we plan to have
a spectrometer to make studies of the yet to be observed large $n$ Rydberg
hydrogen lines at these frequencies. On the other hand the multibeam system
will be maintained with three receivers at 300 GHz and one at 150 GHz in a
triangular array similar to SST.

SSTng will be able to make observations of non-solar objects like H ii regions
and QSOs. Indeed, for 1-min integration time, the noise flux density of SSTng
will be 3 and 12 Jy for 150 and 300 GHz, respectively, making possible night
surveys.

Figure 4: Provisional project schedule.

##  4 Final remarks

As we said above, SSTng is more than an update of the 1990s technology. It
intends to be a new instrument, based on our experience in this frequency
range that will enlarge the original scientific goals. At the present time we
are finishing the scientific requirements, afterwards we will start to
identify possible contractors for the different subsystems. By the end of 2023
we will submit projects to our funding agencies to obtain financial support.
Construction should start early 2025 and by 2027 it should have its first
light, starting the commissioning and the scientific observations (Figure 4).

## Acknowledgements

We acknowledge FAPESP and CAPES funding agencies through grants 2013/24155-3
and 88887.310385/2018-00, respectively, for their support to this scientific
project.

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