# Solar Radio Emissions and Ultralight Dark Matter
###### Abstract  
Ultralight axions and dark photons are well-motivated dark matter candidates. Inside the plasma, once the mass of ultralight dark matter candidates equals the plasma frequency, they can resonantly convert into electromagnetic waves, due to the coupling between the ultralight dark matter particles and the standard model photons. The converted electromagnetic waves are monochromatic. In this article, we review the development of using radio detectors to search for ultralight dark matter conversions in the solar corona and solar wind plasma. solar radio emissions; ultralight dark matter; dark photon; axion; resonant conversion 00000-0000-0000-0000-0000-0  
dark photon field and the photon field. Thus, the once diagonalized fields can convert to each other again inside the plasma. Therefore the dark photon dark matter can also convert into electromagnetic waves in the region \(\omega_{p}=m_{\text{DM}}\). The energy of the converted photon equals the energy of the dark matter particle, which can be written as \(m_{\text{DM}}+E_{k\text{DM}}\), where \(E_{k\text{DM}}\) is the kinetic energy of each dark matter particle. In the galactic halo, we have \(E_{k\text{DM}}\)\(\sim\)\(10^{-6}m_{\text{DM}}\). Thus, we expect a monochromatic electromagnetic wave signal produced by ultralight dark matter conversions. To take advantage of the resonant conversion to search for ultralight dark matter, we need to first find a large plasma. Secondly, the region between the conversion region and the detector must be transparent for the converted electromagnetic wave to propagate to the detector. A perfect plasma that we can use is the Sun's corona. The distribution of temperature and plasma density in the Sun's corona is shown in Figure 1a, where one can calculate that \(\omega_{p}\) ranges from about \(10^{-6}\) to \(10^{-7}\) eV. Thus, if the mass of axion dark matter or dark photon dark matter falls in this region, it can produce monochromatic electromagnetic wave signals. The frequency in this region is about 10-1 GHz, which is in the radio wave band. There is already a lot of radio observation apparatus toward the Sun, such as the low-frequency array (LOFAR) [47], having collected plenty of solar radio data, which we can search for ultralight dark matter conversion signals. There will also be future projects, such as the square kilometre array (SKA) [48] and the upgraded five hundred metre aperture spherical radio telescope (FAST), which may provide more precise solar data in the future. The earth's ionosphere also provides the plasma for the ultralight dark matter conversion. However, the frequency range is about 4-6 MHz, which is polluted by human activities, and therefore cannot be used to search for ultralight dark matter. Furthermore, because of the screening effect of the earth's ionosphere, there are no terrestrial radio telescopes with frequency ranges below 10 MHz. Thus, if the frequency of the ultralight dark matter (\(m_{\text{DM}}/(2\pi)\)) is smaller than 10 MHz, we need to go outer space to search for the conversion signal. The reason the conversion can still happen is that solar wind provides charged particles in the inter-region between the Sun and the Earth. In Figure 1b, one can see that \(\omega_{p}\) in the region between the Sun and the Earth is all the way down to \(10^{-10}\) eV. Solar probe satellites with long-wave radio spectrometers, such as the STEREO satellites [49] and the Parker solar probe [50], have been sent out to observe solar activities, and have collected a lot of data. Thus, one can search for the conversion signal in these observation data. Similar ideas have been proposed to search for radio waves from axion resonant conversion in the mega magnetic field around neutron stars [51; 52; 53] and white dwarfs [54; 55; 56; 57]. On the other hand, solar radio emissions are also related to other dark matter models.