Researchers have investigated the mystery of why the Sun’s poles appear much brighter than the rest of the solar disk when mapped at certain microwave and radio frequencies.
This study could provide valuable information about the magnetic field and temperature conditions in the Sun’s polar atmosphere.
The Sun’s polar regions play a crucial role in the evolution of its magnetic field and the generation of the solar wind. However , despite decades of observation , the cause of the increased radio emission seen near the Sun’s poles , called microwave aurorae , remains unclear.
Microwave aurora appear as an increase in radio brightness near the Sun’s poles. Previous studies have linked this brightness to features such as polar coronal holes , polar faculae, and magnetic fields. Coronal holes are dark regions in EUV images where magnetic field lines are primarily open , allowing intense solar wind to escape. However , the contribution of each of these features and their role in the long-term behavior of microwave aurorae is not fully understood.
A recent study conducted by Aryabhatta Research Institute of Observational Sciences ( ARIES ) , Nainital , an autonomous research institute under the Department of Science and Technology ( DST ), Government of India , in collaboration with researchers from Udaipur Solar Observatory ( USO )/Physical Research Laboratory ( PRL ) , IIT Delhi and IIT Roorkee , comprehensively investigated microwave aurorae using nearly three decades of observations from the Nobeyama Radioheliograph ( NoRH ) operating at 17 GHz.
The radio data were combined with extreme-ultraviolet observations from NASA’s Solar Dynamics Observatory and magnetic field measurements from the Wilcox Solar Observatory to investigate the origin and long-term evolution of this event.
The research team led by Mr. Rohan Bose , Senior Research Fellow, DST INSPIRE , affiliated with ARIES , Nainital and IIT Roorkee , along with Dr. Anshu Kumari ( USO / PRL ) , Dr. Vaibhav Pant (IIT Delhi) and other collaborators, has revealed the long-term evolution of microwave aurora and a clear north-south difference in the region of polar coronal holes.
To understand the role of coronal holes , researchers compared microwave observations with the areas of polar coronal holes identified from extreme-ultraviolet ( EUV ) images. The study found a strong correlation between microwave aurorae and the size of polar coronal holes. An even stronger correlation was observed between microwave aurorae and the strength of the Sun’s polar magnetic field. These findings indicate that long-term changes in microwave aurorae are closely linked to the magnetic structure of the Sun’s polar atmosphere.
The researchers also studied individual microwave bright spots by comparing the microwave observations with simultaneously taken extreme ultraviolet images. They found that many of these bright areas corresponded to small , bright spots known as coronal bright spots. These spots are produced by small-scale magnetic loops and are typically found within polar coronal holes. In many cases , the bright areas were also associated with coronal jets , indicating ongoing small-scale magnetic activity in the Sun’s polar atmosphere.
The findings, published in the Astrophysical Journal, show that both large-scale and small-scale magnetic processes influence microwave aurora. While variations in overall brightness correspond to the evolution of the polar magnetic field and coronal holes , local brightness is often associated with dense magnetic structures and jet activity within these regions. These findings provide strong evidence that microwave aurora is controlled by the evolution of the Sun’s polar magnetic environment and highlight the importance of radio observations for understanding solar polar activity and the origin of the solar wind.
