The magnetic field of coronal mass ejections (CMEs) determines their geoeffectiveness, but it is routinely measured only in situ, when the ejection is already near Earth. Remote methods, such as band splitting in type II radio bursts or circular polarization of type IV moving bursts, give only a line-of-sight averaged field. This work stands apart: nearby in the archive are studies of the solar corona, solar wind, and fast radio bursts, but there are no direct analogues. The closest neighbor, an express about a stellar magnet that holds back an ejection, only underscores how little is known about the magnetic structure of CMEs in the middle corona.
Observations were carried out with the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), an array of 352 antennas operating in the 13.4 to 86.9 MHz range. The standard imaging mode was used with a time resolution of 10 s and a bandwidth of 5 MHz. Ionospheric refraction was corrected manually using white-light images from LASCO C2. Spectra were extracted from a region that was bright in white light and ahead of the main CME body; the region size corresponded to the restoring beam at 39 MHz. To test the emission mechanism, models of gyrosynchrotron emission from an inhomogeneous source and thermal gyroresonance emission were constructed.
In the interval 23:34 to 00:16 UT on March 9 to 10, 2024, spectra in several regions of the CME showed a plateau in brightness temperature at low frequencies and a sharp drop at high frequencies, the classic shape of gyroresonance emission. The break frequency changed from about 57 MHz to about 43 MHz as the CME moved from 4.9 to 7.5 solar radii. Assuming the third harmonic of the gyrofrequency, this corresponds to a magnetic field of 7.9 to 5.6 G. The observed brightness temperature (about 0.1 MK) is an order of magnitude lower than expected for a filled source, indicating a small filling factor: the emission comes from compact magnetic islands with sizes on the order of 0.03 solar radii. A gyrosynchrotron model with an inhomogeneous field can also reproduce the spectra, but requires fine-tuning of parameters, whereas the thermal gyroresonance explanation is simpler and consistent with Occam's razor.
This is the first possible direct measurement of the total magnetic field of a CME in the middle corona, where previously only indirect estimates existed. Unlike neighboring works that use band splitting or polarization for an averaged field, here a local field is obtained in individual bright knots of the CME. The result confirms that the magnetic field of a CME is inhomogeneous and concentrated in small structures, which are visible in WISPR images and in simulations. The gyroresonance diagnostic method could become a routine tool for tracking the evolution of the CME magnetic field from the Sun to Earth.
With the commissioning of the spectropolarimetry mode on OVRO-LWA, it will become possible to determine the harmonic number of the gyrofrequency and hence the magnetic field without assumptions. Accumulating statistics on different CMEs will allow testing how the field decays with distance and how universal the dependence proposed in Koya et al. (2024) is. Joint observations with WISPR and Solar Orbiter will help link radio-emitting knots to specific magnetic structures in white light.
The method could be used for early warning of CME geoeffectiveness and for calibrating models of ejection propagation in the interplanetary medium.
Conduct spectropolarimetric observations of CMEs to determine the gyrofrequency harmonic and refine the magnetic field strength. Compare radio-emitting knots with small-scale structures in WISPR images.
This work directly addresses the unresolved problem of the origin and evolution of small-scale magnetic structures inside CMEs. The question remains open how these structures form and how their field relates to the global field of the ejection. It is also unclear how universal the law of magnetic field decay with distance is for different CMEs.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The analysis uses OVRO-LWA observations of the coronal mass ejection (CME) on March 9, 2024. From spectra in several regions of the ejecta, a gyroresonance break is identified, and its frequency yields magnetic field estimates from 7.9 to 5.6 G at distances of 4.9–7.5 solar radii. The emission is shown to originate from compact magnetic islands with a small filling factor.
Where its strength is
This method allows measuring the local magnetic field in individual CME nodes in the middle corona, where previously only indirect estimates were available. It paves the way for routine remote diagnostics of the field's evolution from the Sun to Earth.
Where it could go next
Worth trying
Why this may matter
Direct measurements of the CME magnetic field in the middle corona could fill a critical gap in models of ejecta propagation and improve forecasts of geoeffectiveness.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
A magnetic field of 7.9 G at 4.9 solar radii is about 200 times weaker than the field at Earth's surface, yet such fields govern the motion of billions of tons of plasma.
In Arthur C. Clarke's novel 'A Solar Storm', a coronal mass ejection threatens Earth and the heroes try to predict its consequences. The new method for measuring the CME magnetic field in the middle corona is a step toward making such predictions a reality.