The Sun also casts a shadow on IceCube

During the daylight season at the South Pole, as the Sun moves in circles above the horizon, IceCube can track the Sun’s position by the decrease in the number of high-energy particles that reach the detector from its direction. This is what scientists call a cosmic-ray shadow, because the decline in particles is due to cosmic rays being stopped by the Sun on their way from their sources to Earth.

The IceCube Collaboration has measured the Sun’s cosmic-ray shadow for the first time, from data covering a period of five years. The results, submitted today to The Astrophysical Journal, show a clear but different shadow pattern every year. When looking at the yearly variation, scientists have found that the shadow pattern follows changes in the solar activity, which we know are correlated with the strength of the Sun’s magnetic field. Thus, this study opens a new line of research for the Antarctic neutrino observatory: the study of the Sun’s magnetic field using IceCube cosmic-ray data.

Sun shadow
Seasonal results for the 2-D binned maps of the Sun shadow. Each map uses data from November through February of each season. Image: IceCube Collaboration

A few people get to spend a full year at the South Pole, accumulating countless stories that will be told over and over again after returning home. But only once during a year at the Pole does the Sun rise over the horizon, launching a dance of shadows that rotate each day around the clock.

Deep in the ice, IceCube detects a relativistic particle about three thousand times per second. In most cases, a million to one, it is a muon created by cosmic ray interactions in the Earth’s atmosphere. When the Sun is up, some of these cosmic rays are stopped by the Sun on their way to the Earth’s atmosphere, which results in a small drop in the number of muons coming from that direction, creating a Sun shadow in the detector. In a similar way, IceCube can also track the passage of the Moon during the several days each month that it is visible at the South Pole.

“With the data we analyzed—covering only about half of a solar cycle—we already see a clear variation of the Sun shadow. With more data, we will be able to prove in a statistically significant way that this variation actually correlates with solar activity,” explains Fabian Bos, who worked on this study as a PhD candidate at Ruhr Universität Bochum in Germany.

As expected, IceCube data confirms that cosmic rays are stopped by both the Sun and the Moon. But while the Moon shadow remains steady year after year, the Sun casts a different shadow each year. The reason for this disparity is related to the magnetic field of the Sun, and physicists would like to gather more data to learn every detail about it.

“The exciting news is that now we can compare our results to simulations of the Sun’s cosmic ray shadow,” says Frederik Tenholt, who is now also working on this topic for his PhD dissertation. “The solar magnetic field is a core ingredient of such simulations, which will enable the study of the solar magnetic field using IceCube data in a region where it is currently inaccessible to in situ measurements.”

More massive than electrons, muons hit Earth from all directions and will travel a few kilometers through matter before they are absorbed. Thus, only those created in the southern sky will make it to IceCube, adding up to 100 billion muons detected every year.

The reliability of the Moon shadow is proof that IceCube’s performance is very stable and that its angular resolution is well below one degree. Changes in the Sun shadow pattern are not yet well understood, but the shadow width seems to increase for years with increased solar activity, i.e., with a larger number of sunspots. In future analyses, and with data samples covering a longer period of time, IceCube plans to use these cosmic-ray muons to study in more detail the Sun’s magnetic field, which is known to change with a cycle of 22 years.

+ info “Detection of the temporal variation of the Sun’s cosmic ray shadow with the IceCube detector,” IceCube Collaboration: M. G. Aartsen et al. The Astrophysical Journal 872, (2019) 133,