The Sun
in a New Light
The SMA is the world's only submillimeter interferometer delivering high-resolution, broadband solar imaging — revealing the chromosphere and corona in wavelengths no other ground-based array can reach.
A Chromospheric Blind Spot — Closed
The solar chromosphere sits between the photosphere and the corona, a turbulent boundary layer where magnetic energy is converted into heat and explosive outflows. It has long been one of the most poorly constrained regions of the Sun — and for a simple reason: few observatories can see it directly.
Millimeter and submillimeter radiation is unique in probing this layer. Unlike optical or EUV emission, mm/submm brightness temperature traces the chromospheric plasma directly through free-free emission — a nearly linear diagnostic of electron temperature. The SMA is only the second facility on Earth, alongside ALMA, capable of interferometric imaging at these wavelengths. And unlike ALMA, the SMA runs dedicated, high-cadence solar campaigns on a routine observing schedule.
The 2020 Astronomy Decadal Survey (Pathways to Discovery) specifically identified chromospheric and coronal radio observations as a priority for understanding energy release and space weather precursors. The SMA is positioned to answer that call — now, at solar maximum.
The Only Observatory of Its Kind
Resolving fine solar structure at millimeter wavelengths demands both high angular resolution and high cadence — two requirements that push the limits of any single-dish telescope. Interferometry is the only way to achieve sub-arcsecond resolution from the ground, and it requires multiple antennas, specialized solar-tracking modes, and real-time calibration against a source that fills the primary beam.
The SMA has developed all of these capabilities. With eight 6-meter antennas on baselines up to 509 meters, operating from 190 to 420 GHz, the array achieves 0.8 arcsecond synthesized beams — resolving features smaller than a typical active region at submillimeter wavelengths. The SWARM correlator delivers 48 GHz of simultaneous bandwidth, enabling spectral index mapping across the chromosphere in a single observation.
Solar SMA campaigns are coordinated with DKIST, IRIS, Hinode, and the Expanded Owens Valley Solar Array (EOVSA) for panchromatic context from the photosphere to the corona.
Four Frontiers of
Solar Millimeter Science
From the mystery of sub-THz flare emission to the fine structure of chromospheric magnetic loops — the SMA opens windows that no other observatory can.
Solar Flares & Active Regions
The submillimeter spectral index of solar flares remains one of the most contested puzzles in high-energy solar physics. Between 100 GHz and 400 GHz, observations defy simple gyrosynchrotron models — suggesting a distinct sub-THz component whose origin (coherent emission? trapped electrons? hot thermal plasma?) is still unknown. The SMA's 48 GHz bandwidth maps this spectral transition in real time, with simultaneous imaging across multiple receiver bands during M- and X-class events. Combined with EOVSA microwave data and DKIST optical context, SMA flare campaigns probe the particle acceleration and energy release chain from first principles.
Chromospheric Structure & Magnetic Diagnostics
Free-free emission at submillimeter wavelengths provides a direct, model-independent temperature diagnostic of the chromosphere. Active regions, plage, filament channels, and quiet Sun all imprint distinct mm/submm brightness temperature signatures that trace the thermal and magnetic structure of the chromospheric network. The SMA's high-cadence imaging resolves individual chromospheric loops and fibrils, tracking the evolution of temperature gradients across the solar cycle. Spectropolarimetric modes (Stokes I, Q, U, V) constrain longitudinal magnetic fields via free-free Faraday rotation — a capability unique to interferometric facilities at these wavelengths.
Prominences, Filaments & Coronal Rain
Solar filaments — dense, cool plasma suspended in the hot corona by magnetic tension — are the immediate precursors to many eruptive events. Coronal rain, the condensation and fall of chromospheric plasma along coronal loop field lines, traces the energetics of post-flare and non-flare heating. Both phenomena are nearly invisible at optical wavelengths but emit strongly at millimeter frequencies. The SMA is uniquely placed to image their thermal structure at sub-arcsecond resolution, resolving the fine strands of filament bodies and the spatial distribution of rain clumps — crucial inputs for MHD models of coronal heating and CME initiation.
Inner Solar System & Cometary Science
The Sun-grazing comets and inner-solar-system bodies that pass through the SMA's field of regard offer a bonus science stream from the same instrumental configuration. Thermal emission from cometary nuclei and outgassing coma at submillimeter wavelengths constrains surface composition and volatile depletion rates. Several historically significant comets (e.g. C/2012 S1 ISON, C/2020 F3 NEOWISE) have been detected at mm/submm wavelengths. The SMA's combination of resolution, sensitivity, and solar proximity tracking makes it a natural platform for cometary science that complements dedicated planetary defense observations of near-Earth objects.
"The SMA delivers broadband, high-cadence chromospheric imaging at mm/submm wavelengths — answering the 2020 Decadal Survey's call for new solar radio diagnostics and positioning the facility as the world's premier ground-based observatory for submillimeter solar physics at solar maximum."
— Solar SMA Science Case, NSF ATI Program 2025
What the SMA
Brings to the Sun
Real-Time Flare Imaging
Dedicated solar-mode software triggers snapshot imaging within seconds of a GOES X-ray event flag. Sub-arcsecond maps of the mm/submm burst source location, morphology, and spectral index are produced on a cadence matched to EOVSA microwave frames.
Dual-Band Full Polarimetry
Two simultaneous receiver pairs cover the 190–280 GHz and 280–420 GHz windows, delivering full Stokes (I, Q, U, V) at each band. Faraday rotation measurements directly constrain line-of-sight magnetic fields in the chromosphere — a unique solar diagnostic.
2× ALMA Field of View
At 230 GHz, the SMA's 6-meter dishes deliver a 52 arcsecond primary beam — twice the FoV of ALMA's 12-meter antennas — covering entire active regions in a single pointing. Mosaic mode extends this to full-disk mapping campaigns.
Panchromatic Network Access
Solar SMA campaigns are pre-coordinated with DKIST, IRIS, Hinode, SDO/AIA, and EOVSA, providing simultaneous coverage from the photosphere through the corona. The SMA fills the critical chromospheric mm/submm gap that no other network partner can provide.
Premier High-Altitude Site
At 4,080 meters on Maunakea, the SMA operates above the bulk of atmospheric water vapor, minimizing precipitable water vapor (PWV) absorption that plagues sea-level observatories. Median PWV < 3 mm enables routine 345 GHz and 400 GHz solar imaging that is simply not achievable at lower-elevation sites.
Dedicated Campaign Scheduling
Unlike general-purpose facilities, the SMA allocates 300+ hours per year to solar science through dedicated campaign semesters. Proposals for targeted active region monitoring, flare patrol, and synoptic programs are accepted twice yearly through the standard SAO and ASIAA time allocation processes.
Solar Observing
At a Glance
Synthesized beam at 230 GHz — sub-arcsecond solar imaging
Spanning the submm continuum from chromospheric to coronal emission
SWARM correlator — simultaneous spectral index mapping
Resolves fine spectral structure in burst emission
2× the ALMA field of view; covers full active regions
Sub-second cadence for impulsive flare light curves
The Solar Multiwavelength
Network
Coordinated campaigns with leading solar facilities deliver panchromatic coverage from the photosphere to the high corona — with the SMA filling the critical submillimeter chromospheric window.
DKIST
Daniel K. Inouye Solar Telescope
World's largest solar telescope. Photospheric and chromospheric magnetic field maps at optical/IR wavelengths. Co-located on Haleakalā, Maui — ideal for joint Hawai'i campaigns with the SMA.
EOVSA
Expanded Owens Valley Solar Array
Microwave imaging spectroscopy from 1–18 GHz. Constrains the non-thermal electron spectrum in flares. EOVSA + SMA together span three decades of radio frequency across a single event.
IRIS
Interface Region Imaging Spectrograph
NASA SMEX satellite imaging the chromosphere–corona transition region in UV/FUV. IRIS spectra of C II, Mg II, and Si IV complement SMA free-free temperature maps at the same atmospheric layer.
Hinode
Solar-B / Hinode (JAXA/NASA/ESA)
X-ray, UV, and optical polarimetry from solar orbit. The Solar Optical Telescope (SOT) delivers photospheric magnetograms and chromospheric Ca H imaging that anchor the thermal structure of SMA-targeted active regions.
— Open for Proposals —
Observe the Sun
With the SMA
Proposal calls are open twice yearly. Solar observing time is available to the entire community through SAO and ASIAA time allocation. No prior submillimeter experience required.