# Solar Radio Emissions and Ultralight Dark Matter

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## Abstract

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## 1. Introduction

**Figure 1.**(

**a**) Quiet-Sun electron density ${n}_{e}$ and temperature T profiles in the solar atmosphere above the photosphere. Figure adapted from [67] based on the result of [68]. (

**b**) ${n}_{e}$ and T (thermal temperature of electrons) profiles in the solar-wind plasma with the distance extending up to about 1 AU. The ${n}_{e}$ profile is based on the in situ measurements of the Parker solar probe [69] and then fitted by the relation ${n}_{e}\left(r\right)\sim \left[3.3\times {10}^{5}{(r/{R}_{\odot})}^{-2}+4.1\times {10}^{6}{(r/{R}_{\odot})}^{-4}+8.0\times {10}^{7}{(r/{R}_{\odot})}^{-6}\right]\phantom{\rule{3.33333pt}{0ex}}{\mathrm{cm}}^{-3}$ given in [70]. The T profile is the relation $T\left(r\right)\sim 418\phantom{\rule{3.33333pt}{0ex}}\mathrm{eV}\times {(r/{R}_{\odot})}^{-0.74}$ [69] based on the in situ measurements.

## 2. Ultralight Dark Matter

## 3. Conversion in Solar Plasma

## 4. Propagation of the Converted Photons

**Refraction effect**—The behaviour of converted photons propagating in the solar plasma obeys the rule of refraction:

**Absorption effect**—In addition to the refraction effect discussed above, the converted photons could be absorbed by interacting with the plasma along the propagation. The absorption effect is mainly due to the inverse bremsstrahlung process, the rate of which is estimated as [67,72]

**Scattering effect**—The converted photons also interact with plasma via Compton scattering, the rate of which is

## 5. Detection

**Figure 2.**The projected sensitivity of the parameter space of the dark photon dark matter based on the radio telescopes SKA phase 1 and LOFAR, with the observation time assumed to be 1 and 100 h, respectively. The plot also shows the existing constraints from CMB [8,86], the cavity experiment WISPDMX [87], and the multiple haloscope-type experiments [8,100,101,102,103,104]. Figure adapted from [67].

**Figure 3.**The projected sensitivity of the parameter space of the axion photon dark matter based on the radio telescopes SKA phase 1 and LOFAR, with the observation time assumed to be 1 and 100 h, respectively. The results are translated from Figure 2 via the relation in Equation (21) with $|{\mathit{B}}_{T}|$ taken as 1 Gauss [88]. Together we show the existing constraints, which can be classified into four categories: the astrophysical searches for axions in various environments, including white dwarfs, neutron stars and pulsars, quasars and blazars, the radio galaxy NGC 1275, globular clusters, etc. [57,92,93,94,95,96,97,98]; various haloscope experiments aiming to detect axions in the galactic dark matter halo [100,102,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125]; the helioscope, CAST, aiming to detect axions emitted by the Sun; and light-shining-through-a-wall (LSW) experiments including CROWS, ALPS, and OSQAR [89,90,91]. The existing constraints are also nicely summarized in [126].

## 6. Summary

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**MDPI and ACS Style**

An, H.; Ge, S.; Liu, J.
Solar Radio Emissions and Ultralight Dark Matter. *Universe* **2023**, *9*, 142.
https://doi.org/10.3390/universe9030142

**AMA Style**

An H, Ge S, Liu J.
Solar Radio Emissions and Ultralight Dark Matter. *Universe*. 2023; 9(3):142.
https://doi.org/10.3390/universe9030142

**Chicago/Turabian Style**

An, Haipeng, Shuailiang Ge, and Jia Liu.
2023. "Solar Radio Emissions and Ultralight Dark Matter" *Universe* 9, no. 3: 142.
https://doi.org/10.3390/universe9030142