SSP 2022 White Paper Frequency Agile Solar Radiotelescope A Next-Generation Radio Telescope for Solar Physics and Space Weather

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SSP 2022 White Paper

Frequency Agile Solar Radiotelescope

A Next-Generation Radio Telescope for Solar Physics and Space Weather

Principal Author: Dale E. Gary, New Jersey Institute of Technology; Email: dgary@njit.edu;

Phone: (973) 642-7878;

Co-authors: Bin Chen, NJIT; James F. Drake, UMD; Gregory D. Fleishman, NJIT; Lindsay

Glesener, UM; Pascal Saint-Hilaire, UC/Berkeley; Stephen M. White, AFRL; and many others

on the accompanying spreadsheet.

Executive Summary:

The Frequency Agile Solar Radiotelescope (FASR) has been strongly endorsed as a top com-

munity priority by both Astronomy & Astrophysics Decadal Surveys and Solar & Space Physics

Decadal Surveys in the past two decades. Although it was developed to a high state of readiness

in previous years (it went through a CATE analysis and was declared “doable now”), the NSF has

not had the funding mechanisms in place to fund this mid-scale program. Now it does, and the

community must seize this opportunity to modernize the FASR design and build the instrument in

this decade. The concept and its science potential have been abundantly proven by the pathﬁnd-

ing Expanded Owens Valley Solar Array (EOVSA), which has demonstrated a small subset of

FASR’s key capabilities such as dynamically measuring the evolving magnetic ﬁeld in eruptive

ﬂares, the temporal and spatial evolution of the electron energy distribution in ﬂares, and the ex-

tensive coupling among dynamic components (ﬂare, ﬂux rope, current sheet). The FASR concept,

which is orders of magnitude more powerful than EOVSA, is low-risk and extremely high reward,

exploiting a fundamentally new research domain in solar and space weather physics. Utilizing

dynamic broadband imaging spectropolarimetry at radio wavelengths, with its unique sensi-

tivity to coronal magnetic ﬁelds and to both thermal plasma and nonthermal electrons from large

ﬂares to extremely weak transients, the ground-based FASR will make synoptic measurements of

the coronal magnetic ﬁeld and map emissions from the chromosphere to the middle corona in 3D.

With its high spatial, spectral, and temporal resolution, as well as its superior imaging ﬁdelity and

dynamic range, FASR is poised to provide a system-wide perspective on myriad coupled phenom-

ena. FASR will be a highly complementary and synergistic component of solar and heliospheric

observing capabilities that is critically needed to support the next generation of solar science.

arXiv:2210.10827v1 [astro-ph.IM] 19 Oct 2022

1 Introduction

Radio observations of the Sun provide a unifying perspective among multi-wavelength observa-

tions, because of their sensitivity to both thermal plasma and nonthermal particles and their unique

sensitivity to solar magnetic ﬁelds—the energy source of solar activity. The spatially-resolved

radio spectrum provides a powerful source of diagnostic information with the potential for trans-

formational insights into solar activity and its terrestrial impacts.

The need to develop the necessary ground-based infrastructure and analysis tools to fully ex-

ploit radio observations of the Sun has long been recognized but is so far largely unrealized. The

Frequency Agile Solar Radiotelescope (FASR) meets this need. FASR is a next-generation solar-

dedicated radio telescope (a radioheliograph) combining superior “snapshot” imaging capability

and ultra-broad frequency coverage to address a wide range of science goals.

The value of FASR to the solar, heliophysics, and space weather science communities – as

well as the astronomy and astrophysics community – has been recognized by four (!) previous

decadal surveys, two in Astronomy and Astrophysics (NRC 2001,2010) and two in Solar and

Space Physics (NRC 2003,2013). Although the most recent Astronomy and Astrophysics survey

(Astro2020; NASEM 2021) elected not to prioritize solar ground-based facilities, it did reafﬁrm

the need for FASR and described it as a “missed opportunity.” FASR was ranked as the number

one ground-based project in the previous Solar and Space Physics decadals, while the Astro2010

decadal radio-millimeter-submillimeter panel ranked FASR second, behind HERA. Astro2010 per-

formed a CATE analysis of FASR, described it as “doable now” and singled it out as an exemplar

of an ideal mid-scale program. While individual sections of NSF developed mid-scale infras-

tructure budget lines, it was only recently that it became available as an agency-wide “big idea”.

Therefore, opportunities to convert these strong recommendations into investments in FASR de-

velopment have been lacking. The one outstanding exception has been an NSF MRI-R2 grant for

the Expanded Owens Valley Solar Array (EOVSA), a 13-antenna pathﬁnder array that leveraged

and validated previous FASR design work. Some important EOVSA results are discussed in the

next section.

This white paper outlines steps necessary to restart the FASR initiative to bring this unique and

powerful instrument to fruition.

2 FASR Science Goals and Objectives

FASR is a radio interferometric array designed to perform Fourier-synthesis imaging. FASR is an

instrument designed and optimized for high-ﬁdelity radio spectral analysis over the extreme range

of ﬂux density and timescales presented by the Sun. The science FASR addresses is as broad as so-

lar physics itself, but FASR’s science goals cannot be adequately addressed by non-solar-dedicated,

general-purpose radio facilities. The major advances offered by FASR over previous generations

of solar radio instruments are its unique combination of ultra-wide frequency coverage, high

spectral and time resolution, and superior image quality. FASR measures the polarized bright-

ness temperature spectrum along every line of sight to the Sun as a function of time. The concept

proposed here would operate from 0.2 to 20 GHz, while yet lower frequencies (20–88 MHz) are

covered by the solar-dedicated upgrade to the Long Wavelength Array at the Owens Valley Radio

Observatory (OVRO-LWA), which is now in its commissioning stage. Radiation over this vast

wavelength range probes the solar atmosphere from the middle chromosphere to several solar radii

into the middle corona—the dynamic, magnetoactive, plasma environment in which a wealth of as-

trophysical and space weather processes occurs. By virtue of its broad frequency coverage, FASR

will image the entire solar atmosphere multiple times per second from the chromosphere through

the corona, while retaining the capability to image a selected frequency range with as little as 20

ms time resolution. FASR is sensitive to temperatures from <10,000 K to >30 MK, and nonther-

mal particle energies from ∼20 keV to >1MeV. Moreover, FASR’s panoramic view allows the

solar atmosphere and the physical phenomena therein, both thermal and nonthermal, to be studied

as a coupled system.

Here we summarize several main science goals of the proposed instrument while at the same

time emphasizing the fundamentally new observables enabled by FASR. With its unique and inno-

vative capabilities, FASR also has tremendous potential for new discoveries beyond those presently

anticipated.

2.1 The Nature and Evolution of Coronal Magnetic Fields

Quantitative knowledge of coronal magnetic ﬁelds is fundamental to understanding essentially

all solar phenomena above the photosphere, including the structure and evolution of solar ac-

tive regions, magnetic energy release, charged particle acceleration, ﬂares, coronal mass ejections

(CMEs), coronal heating, the solar wind and, ultimately, space weather and its impact on Earth.

Characterized as the solar and space physics community’s “dark energy” problem (Lin et al. 2004),

useful quantitative measurements of the coronal magnetic ﬁeld have been demonstrated with break-

through ﬂare observations by the FASR pathﬁnder EOVSA (e.g. Gary et al. 2018;Fleishman et al.

2020;Chen et al. 2020b) as well as broadband observations of solar magnetic active regions by

EOVSA and the Jansky Very Large Array (JVLA). Figure 2 illustrates the use of radio observa-

tions for measuring the coronal magnetic ﬁeld in active regions (ARs). See the white papers by

Gary et al. (2022b) and Chen et al. (2022a) for coronal magnetic ﬁeld measurements of ARs, coro-

nal cavities, and CMEs. Such measurements are complementary to numerical extrapolations of

the magnetic ﬁeld distribution at the photospheric or chromospheric level (De Rosa et al. 2009),

as well as ongoing efforts at O/IR wavelengths to make measurements of above-the-limb coronal

magnetic ﬁelds via the Hanle and Zeeman effects (e.g., with DKIST and COSMO; see discussion

in Gibson et al. 2021). Coordinating with radio observations (which is already underway with

EOVSA), DKIST and COSMO would enable us to derive the vector magnetic ﬁeld at the places

where various acceleration and transport mechanisms trigger or operate. The relation between the

inferred vector magnetic ﬁeld and energy spectra of accelerated particles will strongly constrain

the magnetic reconnection and acceleration mechanisms in transient energy release events such as

ﬂares, CMEs, and jets (e.g. Arnold et al. 2021). See the next subsection for more discussion.

2.2 The Physics of Flares

Outstanding problems in the physics of ﬂares include those of magnetic energy release (Drake

et al. 2014), particle acceleration, and particle transport. As ﬂare energy release requires the par-

ticipation of a large coronal volume sometimes comparable to the size of the entire active region

(Shibata & Magara 2011), one of the key challenges lies in the lack of measurements for key

physical parameters in a broad region around the ﬂare energy release site. At centimeter wave-

lengths, gyrosynchrotron emission – radiation from nonthermal electrons with energies of 10s of

keV to several MeV gyrating in a magnetic ﬁeld – illuminates any magnetic coronal loop to which

energetic electrons have access, showing when and where accelerated electrons are present. Inver-

sion of the gyrosynchrotron spectrum allows both the magnetic ﬁeld in the ﬂaring source and the

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SSP2022WhitePaperFrequencyAgileSolarRadiotelescopeANext-GenerationRadioTelescopeforSolarPhysicsandSpaceWeatherPrincipalAuthor:DaleE.Gary,NewJerseyInstituteofTechnology;Email:dgary@njit.edu;Phone:(973)642-7878;Co-authors:BinChen,NJIT;JamesF.Drake,UMD;GregoryD.Fleishman,NJIT;LindsayGlesener,UM;PascalS...

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SSP 2022 White Paper Frequency Agile Solar Radiotelescope A Next-Generation Radio Telescope for Solar Physics and Space Weather

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