Could Neutrinos Tell Us About the Inside of the Sun?

Neutrinos generated through solar fusion reactions travel effortlessly through the Sun’s dense core. Each specific fusion process creates neutrinos with distinctive signatures, potentially providing a method to examine the Sun’s internal structure. Multiple neutrino detection observatories on Earth are now capturing these solar particles, which can be analysed alongside reactor-produced neutrinos with the data eventually enabling researchers to construct a detailed map of the interior of the Sun.

The Sun is a massive sphere of hot plasma at the centre of our solar system and provides the light and heat to make life on Earth possible. Composed mostly of hydrogen and helium, it generates energy through nuclear fusion, converting hydrogen into helium in its core. This process releases an enormous amount of energy which we perceive as heat and light. The Sun’s surface, or photosphere, is around 5,500°C, while its core reaches over 15 million°C. It influences everything from our climate to space weather, sending out solar wind and occasional bursts of radiation known as solar flares. As an average middle-aged star, the Sun is about 4.6 billion years old and will (hopefully) continue burning for another 5 billion years before evolving into a red giant and eventually becoming a white dwarf.

The standard solar model (SSM) is used to understand and predict the Sun’s internal structure and evolution, it’s how we work out what’s going on inside the Sun. It explains how, in the Sun’s core, different nuclear fusion reactions are constantly pumping out neutrinos – tiny, nearly massless particles that travel through almost anything. Each type of reaction creates neutrinos with their own properties. These neutrinos may help us to understand more about the interior of the Sun. Right now, we only know about its internal density structure from theoretical models based on the SSM, matched with what we can see on the Sun’s surface. The neutrinos may hold the information that will gives us more direct data about the solar interior. 

In a paper published by Peter B. Denton from the Brookhaven National Laboratory and Charles Gourley from Rensselaer Polytechnic Institute they show how solar neutrinos can help us to look inside the Sun and establish its density structure. In contrast, photons of light only tell us about the surface of the Sun as it is right now, and give us a little information about the Sun’s interior hundreds of thousands of years ago. This delay in photons exiting the Sun is because they bounce around the dense solar interior for centuries before escaping. Neutrinos on the other hand give us up to the minute information because they can zip straight through the Sun without getting stopped. 

It has long since been known that neutrinos change their flavour or type (electron neutrino, muon neutrino or tau neutrino) as they travel through matter and that depends on the local density. This is well documented as the Mikheyev-Smirnov-Wolfenstein effect and, by measuring the flux of the neutrino as observed at Earth, compared to unoscillating  predicted flux, the density where the neutrinos were produced can be calculated. Input is also required from independent measurements from neutrino oscillations  that have been created inside nuclear reactors. 

The team demonstrate that the approach does have its limitations  and that there are constraints on just how much density information can be gleaned from the SSM alone. Further data from projects like JUNO and DUNE are needed to further improve the solar internal density profile and give us a more realistic view of the internal workings of our local star.

Source : Determining the Density of the Sun with Neutrinos

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