Reading SDO and SOHO Solar Images: What Each Wavelength Shows

The Two Observatories Every Operator Should Know

Two spacecraft provide the foundational imagery for tracking solar activity that affects HF propagation. Understanding what each instrument shows — and what to look for — takes perhaps 20 minutes to learn and will serve you for decades of radio operating.

SDO (Solar Dynamics Observatory): NASA mission launched February 2010 into a geosynchronous orbit. Carries the Atmospheric Imaging Assembly (AIA), which photographs the entire solar disk in 10 different wavelength channels simultaneously, every 12 seconds. Images are transmitted at a data rate of 1.5 terabytes per day — the highest sustained data rate of any NASA mission at launch. AIA wavelengths range from visible light to extreme ultraviolet (EUV) and reveal plasma at temperatures from a few thousand kelvin to over 10 million kelvin (NASA SDO mission documentation).

SOHO (Solar and Heliospheric Observatory): NASA/ESA mission launched December 1995, stationed at the L1 Lagrange point 1.5 million km sunward of Earth. Carries the LASCO coronagraph (Large Angle and Spectrometric Coronagraph), which blocks the solar disk with an occulting disk to photograph the much fainter corona extending from 2 to 32 solar radii. SOHO is the primary instrument for detecting and characterizing CMEs in near-real-time. Despite exceeding its designed lifetime by nearly 30 years, SOHO remains the workhorse of operational CME monitoring as of 2026.

Together, these two observatories give operators a complete picture: SDO shows what is happening on the solar disk (flares, eruptions, active region complexity, filaments, coronal holes), while SOHO shows what has been launched into space (CMEs, their speed, direction, and angular width).

SDO AIA Wavelengths: A Practical Guide for Operators

AIA images the Sun in seven EUV channels, two UV channels, and one visible-light channel. Each EUV wavelength is sensitive to plasma at a specific temperature range, determined by the dominant ion emission lines at that wavelength. For HF operators, six channels are routinely useful.

171 Å — Quiet Corona and Coronal Loops

Temperature sensitivity: ~1 million K. Dominant emission: Fe IX (nine-times-ionized iron).

Appearance: Golden-yellow false-color in standard AIA color scheme. Shows the quiet corona in extraordinary detail: coronal loops arc between opposite-polarity regions in active areas, and the overall disk structure reflects the global magnetic field topology.

What to look for:

• Coronal loop complexity above active regions — dense, bright, interlocking loops indicate stressed magnetic configurations
• Prominence structure — large arcs extending at the solar limb in profile
• The overall "activity level" of the disk — a quiet 171 Å image with sparse loops means low flare probability

Radio relevance: 171 Å is primarily a structural overview. It is less diagnostically specific than 193 Å or 304 Å for space weather purposes but gives excellent context for active region development.

193 Å — Coronal Holes and Space Weather Planning

Temperature sensitivity: ~1.5 million K (corona) and ~20 million K (flare plasma). Dominant emission: Fe XII and Fe XXIV.

Appearance: Teal/green false-color. The most commonly displayed AIA wavelength in space weather contexts.

What to look for:

Coronal holes are the most radio-relevant feature in 193 Å. They appear as distinctly dark regions — significantly darker than the surrounding corona — because coronal hole plasma is cooler, less dense, and less emissive at this wavelength. A large coronal hole near Sun-center (disk center) will be facing Earth and will pump a fast solar wind stream (600–800 km/s) toward Earth 3–4 days after it crosses central meridian.

The size, position, and apparent lifetime of a coronal hole determine the probable geomagnetic impact:

• A small high-latitude coronal hole may produce Kp 3–4 for 1–2 days
• A large equatorial coronal hole can produce G1–G2 conditions for 3–5 days
• A persistent coronal hole that recurs over multiple solar rotations will produce repeated geomagnetic disturbances every ~27 days

During flares: 193 Å also shows hot flare plasma because Fe XXIV emission at ~18 MK becomes dominant. Flare ribbons appear as bright elongated regions in the active area during the impulsive phase. Post-flare arcade loops appear bright in the decay phase.

304 Å — Filaments, Prominences, and CME Launches

Temperature sensitivity: ~50,000 K. Dominant emission: He II (singly-ionized helium).

Appearance: Red/orange false-color. Images the chromosphere and transition region — the layer between the visible solar surface and the hot corona.

What to look for:

Filaments appear as dark elongated lanes running along polarity inversion lines — the boundaries between regions of opposite-polarity magnetic field. Filaments are the same material as prominences (cool, dense chromospheric plasma suspended in the corona by magnetic tension), viewed from above rather than at the limb. A long, dark, sinuous filament running across the disk is a potential CME source: when the magnetic configuration destabilizes, the filament erupts outward as part of a CME.

Erupting filaments are one of the most reliable CME precursors available in disk imagery. In 304 Å, an eruption appears as:

1. The dark filament lane brightening and becoming diffuse
2. The brightened material rising and expanding visibly over a sequence of images
3. Brightening of the surrounding chromosphere (flare ribbons may appear in the vicinity if a flare accompanies the eruption)

The key question for Earth-impact assessment: where is the eruption located? Eruptions near Sun-center (within ~30° of central meridian) carry the highest probability of Earth-directed CMEs. Eruptions near the solar limb produce CMEs whose axis is directed well away from the Sun-Earth line — typically only grazing impacts or misses.

131 Å — X-Class Flare Identification

Temperature sensitivity: ~10 million K and ~0.4 million K. Dominant emission: Fe VIII and Fe XXI.

Appearance: Blue false-color. In quiet conditions, this channel shows relatively faint emission. During major flares, it lights up dramatically.

What to look for:

131 Å is the best channel for confirming that a major flare is in progress. X-class flare plasma reaches temperatures of 10–20 MK, which produces intense Fe XXI emission at 131 Å. The flare site appears as a brilliant blue-white region that can completely dominate the image. The time evolution in 131 Å shows the impulsive phase peak very clearly.

For operators monitoring for immediate HF blackout risk: when 131 Å shows a bright flaring region coinciding with GOES X-ray data showing a rising X-ray flux above M1 level, a radio blackout (R-scale event) is in progress or imminent on the sunlit hemisphere.

1600 Å and 1700 Å — Active Region Structure

Temperature sensitivity: ~10,000 K (1700 Å, upper photosphere) and ~100,000 K (1600 Å, transition region plus C IV ion emission).

Appearance: Yellow/cream false-color. Shows sunspot umbra, penumbra, and plage regions.

What to look for:

These UV channels are most useful for active region classification. The detailed structure of sunspot groups — their size, complexity, and the magnetic configuration of penumbral regions — is visible at these wavelengths with higher contrast than in white-light imagery. Large, magnetically complex active regions (NOAA spot classification Dβγδ or similar) are the source of major flares and CMEs.

AIA 1600 Å also shows flare ribbons clearly during events — the two parallel bright stripes that appear in the chromosphere on either side of the polarity inversion line as a flare's reconnection front sweeps outward.

HMI Continuum — Sunspot Groups in White Light

The Helioseismic and Magnetic Imager (HMI) aboard SDO provides visible-light continuum images showing the solar photosphere with high spatial resolution. This is the closest analogue to a traditional white-light solar telescope image.

What to look for:

• Sunspot group size, shape, and complexity — the raw material of NOAA active region designation and flare probability forecasting
• New emerging active regions — fresh flux emergence appears as small spots that grow over days
• Sunspot group area — expressed in millionths of a solar hemisphere (MSH). Groups above 500 MSH are statistically associated with significant flare activity (per NOAA SWPC flare probability products)
• Polarity separation — if two large umbral groups of opposite polarity are close together with a complex shared penumbra, that is a δ-type configuration with the highest X-class flare probability

HMI also produces line-of-sight magnetograms — false-color images of the magnetic field polarity and strength at the solar surface. These are the most direct view of the magnetic complexity that drives space weather.

SOHO LASCO: Watching CMEs in Flight

LASCO blocks the solar disk with a circular occulting disk and photographs the much fainter corona in white light (scattered sunlight). Two cameras cover different angular ranges:

LASCO C2: Field of view from 2 to 6 solar radii. Images the inner corona and captures CME initiation and early acceleration. CME leading edges, bright cores, and the characteristic three-part structure (bright leading front, dark void, bright core) are visible. Temporal cadence is approximately 20 minutes per image in standard operations.

LASCO C3: Field of view from 3.7 to 32 solar radii. CMEs are visible for hours as they expand outward. The CME speed is measured from the change in position across successive C3 images. Angular width (the half-angle of the CME cone) is measured in C3. These parameters feed directly into DONKI cataloguing and ENLIL model runs.

Identifying a Halo CME

A halo CME is the most critical signature for ham radio operators. When a CME propagates directly toward Earth (or directly away), the expanding front appears as a ring or partial ring encircling the entire occulting disk — a halo. This occurs because the CME front, which is a sphere expanding in three dimensions, projects as a limb-brightened ring when viewed along the propagation axis.

Full halo (360° ring): High probability of Earth-directed CME (or anti-sunward CME — verify with disk imagery to distinguish front-side from back-side eruption). A full halo in C3 combined with an eruption visible on the frontside disk in SDO imagery = probable impacting CME.

Partial halo (>120° angular extent): Also warrants attention. The central axis may be 20–40° off the Sun-Earth line, which could still produce a glancing blow and G1–G2 storm.

CME speed measured in LASCO C3 is the primary input to arrival time prediction:

• < 400 km/s: Slow CME. Arrival in 3–4+ days if Earth-directed. Low storm probability unless Bz is particularly well-oriented.
• 400–800 km/s: Typical. 2–3 day transit time. Storm potential depends heavily on Bz in the magnetic cloud.
• 800–1,500 km/s: Fast. 1–2 day transit. Stronger shock → higher initial Kp potential.
• > 1,500 km/s: Extreme. Transit can be under 18 hours. Very strong shock; G4–G5 possible if Bz cooperates.

Connecting the Imagery: A Complete Space Weather Assessment

Effective use of solar imagery for radio purposes means reading the observatory outputs as a connected sequence:

Step 1 — SDO AIA 193 Å daily check: Any dark equatorial coronal holes facing Earth? If yes, mark 3–4 days out for elevated solar wind. Any new large bright active regions? If yes, elevate general alertness for flare and CME potential.

Step 2 — SDO AIA 304 Å for eruption monitoring: Check during elevated activity periods. Erupting filaments — visible as bright material lifting from the disk — precede CMEs by minutes to hours. An eruption near disk center that lifts clearly in 304 Å should be followed by a check on LASCO.

Step 3 — SDO AIA 131 Å for flare confirmation: During a GOES X-ray flux rise above M1, check 131 Å to confirm flare location and intensity. This gives you the source location for directional assessment and confirms whether an HF blackout is in progress on your hemisphere.

Step 4 — SOHO LASCO C3 for CME confirmation: After a significant disk eruption (SDO 304 Å), check LASCO C3 for a developing halo or directed CME. CME entry into the C3 field typically occurs 30–90 minutes after disk eruption. If a halo develops, check DONKI within 1–3 hours for the official trajectory and ENLIL arrival time.

Step 5 — DSCOVR Bz monitoring (L1 data): When DONKI predicted arrival time is approaching (±6 hours), switch attention from imagery to real-time L1 data. Solar wind speed jump + Bz turning southward = CME shock arrival. You now have 15–60 minutes before storm conditions begin at Earth.

DXRadar's solar imagery page links directly to current SDO and SOHO images, making this assessment workflow practical from a single browser tab during an operating session.

Frequently Asked Questions

What does SDO 193 angstrom imagery show?

SDO AIA 193 Å images the solar corona at approximately 1.5 million K. Coronal holes — regions of open magnetic field that emit fast solar wind — appear as dark areas. This wavelength is most useful for identifying coronal holes that will drive elevated solar wind and possible G1–G2 geomagnetic activity 3–4 days after a coronal hole faces Earth.

What does SDO 304 angstrom show for ham radio operators?

SDO AIA 304 Å images the chromosphere and transition region at ~50,000 K. Prominences and filaments appear as bright arcs or dark lanes. An erupting filament in 304 Å imagery — visible as a bright arc lifting and expanding — is a primary indicator of a CME launch. Eruptions near Sun-center carry the highest Earth-impact probability.

What is a halo CME in SOHO LASCO imagery?

A halo CME appears in SOHO LASCO C2 or C3 coronagraph images as an expanding bright ring surrounding the blocked solar disk in all directions. This 360° halo means the CME is propagating toward or away from Earth. A frontside halo (visible on AIA as an eruption on the near side of the Sun) indicates an Earth-directed CME and probable geomagnetic storm within 1–3 days.

What is the difference between LASCO C2 and C3?

LASCO C2 covers 2–6 solar radii from Sun-center, showing the inner corona and early CME development. LASCO C3 covers 3.7–32 solar radii, showing CME propagation well out into the heliosphere. C3 images are used to measure CME speed and angular width for DONKI cataloguing. CME halos are most clearly visible in C3.

Where can I access live SDO and SOHO images?

DXRadar's solar imagery page provides curated links to current SDO AIA and SOHO LASCO images. NASA's SDO website at sdo.gsfc.nasa.gov provides near-real-time AIA images in all wavelengths, updated every 12 seconds. SOHO LASCO images are available at soho.nascom.nasa.gov, typically with a 15–30 minute delay.