Space Weather Impact of Three Solar Flares Observed at Millimeter Wavelengths
| Nugget | |
|---|---|
| Number: | 527 |
| 1st Author: | Adriana VALIO et al. |
| 2nd Author: | |
| Published: | May 25, 2026 |
| Next Nugget: | TBD |
| Previous Nugget: | X-ray Log Letters |
Introduction
Solar flares are among the most energetic events in the solar system, releasing vast amounts of energy across the electromagnetic spectrum (from radio waves to gamma-rays) in a matter of minutes to hours. Long-duration events are especially important because their prolonged magnetic reconnection continuously accelerates particles, pumping energy into the heliosphere long after the initial impulsive phase has ended. Understanding the link between the properties of these flares, the coronal mass ejections (CMEs) launched simultaneously, and the disturbances caused at Earth is a central challenge of space weather science.
One key area of observation of these phenomena -- millimeter radio waves -- has not been so well exploited. The POEMAS telescope fills this key gap by making systematic solar observations at 45 and 90 Ghz (6.6 and 3.3 mm), crucial to understanding the gyrosynchrotron radiation fundamental to flare development.
We used this facility to carry out a detailed multiwavelength analysis (Ref. [1]) of three long-duration solar flares during the solar maximum period of 2012 (Ref. [1]):
SOL2012-03-02 (M3.3; m4.80) SOL2012-06-06 (M2.1; m3.07) SOL2012-07-12 (X1.4; x1.85)
The adjusted ABCMX classes come from [1] the previous Nugget] as an experiment here; by either class scheme these events span a factor of 40 in magnitude. The POEMAS high-frequency observations include measurements of circular polarization, a key attribute not otherwise available.
Radio Bursts and Gyrosynchrotron Modelling
Each flare was observed in both millimeter emission (POEMAS at 45 and 90 GHz) and microwaves (the Radio_Solar_Telescope_Network at 1–15 GHz). Combined radio spectra spanning 5–90 GHz were assembled at four time intervals during each burst and fitted with a gyrosynchrotron model using a Markov Chain Monte Carlo (MCMC) algorithm. The fits yield four key physical parameters: the magnetic field strength Bmag, the total number of accelerated electrons Ne, the source size a, and the spectral index of the electron energy distribution δ.
A main result is that all three events return remarkably similar magnetic field strengths of about 160–180 G, despite the enormous differences in overall flare energy. This suggests a common coronal magnetic configuration for these gyrosynchrotron-emitting sources, and that what distinguishes powerful from weaker events is primarily the number of electrons accelerated and the spatial extent of the emitting region, not the field strength itself. SOL2012-07-12 (X1.4; Figure 1) had a source diameter roughly 3–6 times larger than the other events and accelerated 6–13 times more electrons. The spectral index δ was about 2.8 across all events, harder than the typical microwave range of 3–5. This reflects a bias toward energetic events at millimeter wavelengths, where only hard-spectrum flares produce detectable flux at 90 GHz. In all three cases δ hardened further as each burst progressed, reaching values near 2 by the end of the event. The June and July flares also showed flux at 90 GHz exceeding that at 45 GHz, a "sub-THz component" previously reported by Ref. [2] and indicating an additional emission mechanism at the highest frequencies, distinct from the standard gyrosynchrotron component.
CMEs and Space Weather
Each of the three flares was accompanied by a coronal mass ejection (CME) detected by LASCO/SOHO, and in all three cases the CME was launched essentially simultaneously with the radio burst onset. Two of the three events led to geomagnetic storms, as measured by the Dst index. Notably the interaction of the July ICME with the magnetosphere also triggered aurora australis, photographed by the crew of the International Space Station on 15 July 2012.
Conclusions
The central finding of this study is a consistent positive correlation between the peak radio flux measured at 45 GHz and a suite of space weather indicators: GOES flare class, SXR energy, CME velocity and kinetic energy, ionospheric D-region depression, and geomagnetic storm severity as quantified by Dst and Kp (Figure 2). The X1.4 event on 12 July 2012 sat at the high end of all these scales simultaneously producing the brightest millimeter burst (116 SFU at 45 GHz), the fastest and most energetic CME (884 km/s, 2.7 × 1031 ergs), the deepest ionospheric depression (9 km), and the strongest geomagnetic storm (Dst = −139 nT) of the three events.
The magnetic field strength in the gyrosynchrotron-emitting source (~160–180 G) was essentially the same in all three flares, pointing to a common magnetic topology. What differentiated the events was the number of accelerated electrons and the source volume. Moreover, the spectral index δ, consistently hard (~2.8, hardening toward 2 as each burst progressed), is characteristic of the millimeter-emitting population and harder than what is typically inferred from microwave-only studies, reflecting a selection effect at the highest frequencies.
High-frequency millimeter radio observations thus appear to be a promising tool for early space weather assessment: they are sensitive to the most energetic particle populations, they respond on the same timescale as the flare itself (allowing prediction hours or even days before a CME arrives), and they correlate well with both the CME properties and the ultimate geomagnetic impact. POEMAS recorded 49 solar radio bursts during the 2012–2013 solar maximum, at least half associated with halo or partial halo CMEs, offering a valuable archive for extending this type of analysis to a larger statistical sample.
Acknowledgment
The authors of Ref. [1] and this Nugget are A. Valio, K.F. Lopez, J.L. Gamonal Valenzuela, R.M. Romero Ramírez, and D.F. da Silva.
References
[1] "Space weather impacts of three solar flares observed by the POEMAS telescope at 45 and 90 GHz"
[2] "A new solar burst spectral component emitting only in the terahertz range:
[3] "New circular polarization solar telescopes at two millimeter wavelength ranges"