Aditya-L1 and Europe's Proba-3 Watched the Same Two Solar Eruptions on 14 and 16 September 2025, and a Bengaluru-Led Team Has Published the First Joint Analysis of Both Spacecraft's Data

On 15 October 2026 ISRO announced that scientists at the Indian Institute of Astrophysics (IIA) in Bengaluru had, for the first time, jointly analysed observations of the same solar eruptions from India’s Aditya-L1 and Europe’s Proba-3. Both spacecraft watched two coronal mass ejections, clouds of magnetised plasma (electrically charged gas) thrown off the Sun, on 14 and 16 September 2025. ISRO’s release says the mass and thermal-energy estimates from the two instruments agree well, and that the study has been published in the journal Astronomy & Astrophysics.

Who did the work

Anjali Agarwal of IIA and Pondicherry University is the first author, according to the paper’s author list. Posted on arXiv on 6 September 2026, the preprint names 19 authors at 12 institutions in 10 countries: India, Belgium, the United States, Italy, the Czech Republic, France, Romania, Germany, Poland and Greece. ISRO’s release describes the study as led by IIA researchers together with European and American solar physicists.

One of the co-authors is Andrei Zhukov of the Royal Observatory of Belgium, whom ESA names as the principal investigator of ASPIICS, the Proba-3 coronagraph used in the study. When posted, the preprint was marked as accepted for publication, and ISRO’s release of 15 October gives it as published.

One patch of corona, two kinds of instrument

A coronagraph blocks the Sun’s bright disc so the faint corona, the Sun’s outer atmosphere, becomes visible. Each of the two used here does a different job.

VELC, the Visible Emission Line Coronagraph on Aditya-L1, was developed by IIA in close collaboration with ISRO. It splits the light of the corona into a spectrum, and ISRO’s release says it covers the region from 1.05 to 1.5 solar radii (distances measured from the Sun’s centre, in units of the Sun’s radius) in the 5303 Angstrom green coronal line. That line comes from gas at about two million degrees. In the preprint, the authors describe VELC as the first space-based instrument able to observe such eruptions routinely in that line at these low heights.

ASPIICS takes pictures. It images the corona from about 1.1 to 3 solar radii, in white light and in the same green line. Proba-3, the European Space Agency (ESA) mission that carries it, flies two spacecraft about 150 metres apart, and one casts a shadow on the other to make an artificial eclipse. ISRO’s release calls it the world’s first precision formation-flying mission.

Indian rockets launched both spacecraft. A PSLV-C57 put Aditya-L1 into orbit on 2 September 2023, and the spacecraft reached its halo orbit around the Sun-Earth L1 point on 6 January 2024, according to ISRO. A PSLV-C59 carried Proba-3 from Sriharikota on 5 December 2024 as a dedicated commercial mission of NewSpace India Limited, and ISRO’s page says it placed ESA’s satellites in their designated orbit with precision. Our Aditya-L1 explainer covers all seven of its instruments, and the piece on PSLV-C37 covers the rocket’s record with foreign payloads.

An earlier VELC result appeared in The Astrophysical Journal Letters in November 2024. It used the same green line to catch a coronal mass ejection starting on 16 July 2024.

Only two events lined up

From ASPIICS’s first six months of routine observing, July to December 2025, the team listed more than ten eruptions. VELC and ASPIICS did not always observe at the same times, so only two eruptions, on 14 and 16 September, were recorded by both.

VELC caught both early. For the 14 September eruption, the paper reports a signature in VELC data at 08:56 UT (universal time), before ASPIICS’s earliest available image of that eruption, at about 09:22 UT. Meanwhile the catalogue of the SOHO spacecraft’s LASCO coronagraph, which the paper cites, first lists that eruption at 09:12 UT, 16 minutes after VELC’s signature.

What the two eruptions did

ISRO’s release gives the 14 September eruption a speed of about 645 kilometres a second and the 16 September one about 145 kilometres a second. Electron density in each cloud fell as it moved outward, according to the preprint. It fell with distance r from the Sun’s centre as r to the power of minus 3.3 for the fast eruption and r to the power of minus 6 for the slow one. They read the steeper fall in the 16 September cloud as stronger expansion early in its life.

Energy was divided differently too. In the 14 September eruption, ISRO says, thermal energy was about 1 per cent of the total. In the 16 September eruption it grew until it matched the kinetic (motion) energy. They estimate the magnetic energy in both as comparable to or greater than the kinetic energy, and the preprint describes those figures as lower limits.

For the mass of the 14 September cloud, the preprint’s Table 1 gives about 300 trillion grams from ASPIICS and 360 trillion grams from VELC, roughly 300 million and 360 million tonnes. For the 16 September cloud the figures are about 108 trillion and 160 trillion grams, roughly 108 million and 160 million tonnes. The conversion to tonnes is ours.

Why the pairing helps

ISRO’s release says earlier space coronagraphs only partly succeeded in observing eruptions close to the Sun and could not follow the early development of faint ones. It adds that how an eruption’s energy is shared among motion, heat and magnetic fields in the inner corona has remained poorly constrained.

Per the release, ASPIICS’s pictures give the shape, movement and mass of an eruption, while VELC’s spectra give an independent estimate of its density and thermal energy. It adds that measurements near the Sun are valuable for understanding the physical processes that shape an eruption and its later evolution, and that eruptions aimed at Earth can disturb the near-Earth space environment.

What the authors say limits the result

Several limits are listed in the paper’s discussion section. Both eruptions were relatively weak. The paper says there are no direct measurements of an eruption’s magnetic field strength or temperature at these heights, so the thermal and magnetic energies rest on stated assumptions, and the authors give uncertainties of up to about 35 per cent for the magnetic energy and 30 per cent for the thermal energy.

Volumes and densities from ASPIICS, extrapolated back to VELC’s heights, differ from VELC’s by a factor of about 3. Those differences largely cancel in the mass and thermal-energy figures, the authors say, which is why those two agree. VELC also sees only gas that glows in the green line, so its thermal energy may not cover the whole cloud.

For the 16 September eruption, the speed in the green line was about 90 kilometres a second against about 145 in white light. The authors say the two channels may be following different plasma. They add that the difference in energy split between the two eruptions may be real or may come from the methods, and that it should be read with caution. A larger sample is the next step they name.

The Proba-3 side, in 2026

ESA reported that an anomaly on the weekend of 14 and 15 February 2026 made Proba-3’s Coronagraph spacecraft lose its orientation and go silent. ESA’s mission page dates to 19 March 2026 its report that the spacecraft had re-established contact, and on 9 June 2026 ESA said the spacecraft and ASPIICS were healthy and ready to resume formation flying.

Before that, ESA reported on 13 April 2026 that Proba-3 had made 57 artificial eclipses since July 2025 and recorded more than 250 hours of coronal video. The first results, led by the Royal Observatory of Belgium and published in The Astrophysical Journal Letters, found small-scale flows in the region where the slow solar wind forms, moving at 250 to 500 kilometres a second against the 100 expected.

What comes next for Aditya-L1

On 16 October 2026, a day after the joint study, ISRO opened the fourth Announcement of Opportunity for Aditya-L1. It invites Indian and international researchers to propose VELC and SUIT observations, the second being the mission’s ultraviolet telescope. ISRO’s page says more than 37 TB of Aditya-L1 data is already in the public domain.

The announcement’s PDF says approved observations will run from January to March 2027, that there is no proprietary period, and that data will be released through the Indian Space Science Data Centre as soon as they are generated. The deadline for proposals is 20 November 2026.