Combing Through the Sun’s Corona for Dark Matter
Dark matter is an elusive but consequential substance. It accounts for 27% of the total energy content of the Universe and plays a crucial role in the formation of cosmic structures, acting as the skeleton for the “cosmic web” of galaxies [1]. However, its nongravitational interactions with known particles remain a mystery. Among the many types of dark-matter particles that have been proposed, a compelling candidate is the ultralight dark photon [2]. Just as the photon mediates the electromagnetic force between electrically charged particles, the dark photon would mediate interactions between a hypothetical set of dark particles. Researchers have previously looked for dark photons using lab-based particle detectors and Earth-bound telescopes. But now Haipeng An from Tsinghua University in China and his colleagues have utilized a unique vantage point next to the Sun to search for a dark-photon signal [3]. The team analyzed radio-frequency data from the Parker Solar Probe (PSP) but did not find any spectral distortions that would have been dark-photon evidence. The researchers thus place the strongest limits yet for dark photons with masses around 10−9eV/c2.
The dark photon is an appealing dark-matter candidate because it can arise naturally within certain string-inspired theories. However, these models do not fix the mass of the dark photon. As a result, searches have spanned many orders of magnitude in mass from 10−18 to 106 eV/c2. The ultralight dark photon (less than about 1 eV/c2) stands out, as it offers distinctive cosmological effects and novel experimental signatures.
Remarkably, we may not be completely blind to the dark photon, as it could interact with normal particles through a phenomenon known as kinetic mixing [4]. This feeble coupling effectively allows dark photons to “oscillate” into ordinary photons and vice versa, much like neutrinos oscillate between different flavors. This oscillation can be enhanced in certain environments. One such environment is the solar corona—a superheated, ionized gas extending millions of kilometers into space around the Sun. The electrons in this plasma interact with normal photons, causing them to behave as if they had a mass. The value of this effective mass depends on the corona’s electron density, which decreases with distance from the Sun. If a dark photon passes through the solar corona, and its mass is equal to the effective mass in a particular region of the plasma, then the dark-to-normal conversion will be dramatically enhanced through a resonant interaction. The resulting photon is highly monochromatic, with an energy equal to the dark photon’s mass.
Some of these converted photons will be absorbed or scattered in the plasma, but a fraction of them will escape, leading to an observable “bump” in the spectrum of the corona. Researchers have previously looked for this dark-photon signature using ground-based radio telescopes [5]. These observations have been limited to photon frequencies above 10 MHz, corresponding to dark-photon masses above 10−6 eV/c2. The reason for this limited range is that radio frequencies below approximately 10 MHz are reflected back into space by Earth’s ionosphere. Also, ground-based radio telescopes suffer from an attenuated signal because of the vast distance between the Sun and Earth.
To address these limitations, An and his colleagues have used the PSP as an in situ dark-matter detector. The PSP is a NASA mission designed to study the solar corona by flying closer to the Sun than any previous spacecraft (see Special Feature: To Touch the Sun). Launched in 2018, the probe reached its closest perihelion distance of approximately 10 solar radii in July 2022, but its highly elliptical orbit has allowed it to study up close the corona’s plasma over a wide range of radii—and thus over a wide range of electron densities. The PSP’s two radio receivers are capable of measuring frequencies ranging from about 20 kHz to 20 MHz. Using these radio data and the orbital path of the probe, An and colleagues could look for spectral bumps corresponding to dark-photon masses between 3 × 10−10 and 8 × 10−8 eV/c2. For smaller dark-photon masses (and equivalently smaller frequencies), the PSP has reduced sensitivity. Thus, in the low-frequency range, the researchers also considered data from the Solar Terrestrial Relations Observatory (STEREO), which maintains a fixed orbital distance of 1 astronomical unit from the Sun.
By comparing the spectral flux density received by the spacecrafts with the expected dark-photon-induced signal, An and colleagues were able to establish limits on the kinetic mixing parameter, which is a dimensionless parameter that describes the mixing strength between dark and normal photons. In the aforementioned mass range, they found upper bounds of approximately 10−13 to 10−14. These limits surpass the constraints from cosmic-microwave-background observations, in which the putative signal is a spectral distortion caused by dark-photon conversions in the early Universe [2]. These limits also go beyond the low-mass region probed by laboratory haloscopes, microwave cavities tuned to convert dark photon or axion dark matter into ordinary photons (see Viewpoint: Homing in on Axions?).
The search for dark-photon dark matter remains vigorous, having made significant progress over the past decade. The work of An and colleagues carves out a novel pathway, limiting lightweight dark photons to have either an extremely feeble kinetic mixing parameter or a very low mass (below 10−15 eV/c2), where the parameter space remains relatively open. An interesting way to improve the sensitivity to ultralight dark photons involves resonant LC circuits—sometimes called dark-matter radios. These experiments aim to detect the dark photon’s tiny, oscillating electric fields by tuning the circuit to the particle’s Compton frequency. Operated at cryogenic temperatures, these setups can amplify the signal, allowing dark photons to induce a faint but measurable current. Several such experiments are on the horizon, so stay tuned for potential discoveries.
References
- G. Bertone and D. Hooper, “History of dark matter,” Rev. Mod. Phys. 90, 045002 (2018).
- P. Arias et al., “WISPy cold dark matter,” J. Cosmol. Astropart. Phys. 2012, 013 (2012).
- H. An et al., “In situ measurements of dark photon dark matter using Parker Solar Probe: Going beyond the radio window,” Phys. Rev. Lett. 134, 171001 (2025).
- B. Holdom, “Two U(1)’s and charge shifts,” Phys. Lett. B 166, 196 (1986).
- H. An et al., “Searching for ultralight dark matter conversion in solar corona using Low Frequency Array data,” Nat. Commun. 15, 915 (2024).
- N. J. Fox et al., “The Solar Probe Plus Mission: Humanity’s first visit to our star,” Space Sci. Rev. 204, 7 (2015).