Monitoring the daily variation of Sun-Earth magnetic fields using galactic cosmic rays

Public summary

*Daily Sun's shadows on very high-energy galactic cosmic rays were observed for the first time using LHAASO.

*This work therefore proposes a novel method for monitoring the Sun-Earth IMF.

*Compared with the near-Earth spacecraft, the Sun's shadow can provide 3.3-day earlier predictions for the IMF.

*The timing advance significantly deviated from the predictions of current IMF models.

*These findings may provide valuable insights into the IMF structure, thus improving space weather research.

Abstract

The interplanetary magnetic field (IMF) between the Sun and Earth is an extension of the solar magnetic field carried by the solar wind into interplanetary space. Monitoring variations in the IMF upstream of the Earth would provide very important information for the prediction of space weather effects, such as effects of solar storms and the solar wind, on human activity. In this study, the IMF between the Sun and Earth was measured daily for the first time using a cosmic-ray observatory. Cosmic rays mainly consist of charged particles that are deflected as they pass through a magnetic field. Therefore, the cosmic-ray Sun shadow, caused by high-energy charged cosmic rays blocked by the Sun and deflected by the magnetic field, can be used to explore the transverse IMF between the Sun and Earth. By employing the powerful kilometer-square array at the Large High Altitude Air Shower Observatory, the cosmic-ray Sun shadows were observed daily with high significance for the first time. The displacement of the Sun shadow measured in 2021 correlates well with the transverse IMF component measured in situ by spacecraft near the Earth, with a time lag of 3.31±0.12 days. The displacement of the Sun shadow was also simulated using Parker’s classic IMF model, yielding a time lag of 2.06±0.04 days. This deviation may provide valuable insights into the magnetic field structure, which can improve space weather research.