The Sun is our closest star and we have been studying it for a long time. Yet, it has many associated puzzles that are unexplained. A significant advancement has been made by an international team of solar physicists led by those from Peking University, China, and National Center for Atmospheric Research of the United States. The group has measured the global magnetic field of the Sun’s corona, or outer atmosphere, for the very first time. This research has been published in the journal Science .
The solar puzzles
There are two main puzzles about the Sun which this advancement will help address. First is the coronal heating problem. Though the core of the Sun is at a temperature of about 15 million degrees, its outer layer, the photosphere is a mere 5700 degrees hot. However, its corona or outer atmosphere, which stretches up to several million kilometres beyond its surface, is much, much hotter than the surface. It is at a temperature of one million degrees or more. What causes the atmosphere of the Sun (corona) to heat up again, though the surface (photosphere) is cooler than the interior? That is the question which has baffled solar physicists. Popular attempts to explain this puzzle invoke the magnetic field of the corona. Hence the present work will help understand and verify these theories better.
The other set of questions pertain to the mechanisms of eruptions of the Sun, such as solar flares and coronal mass ejections. These are driven by magnetic reconnections happening in the Sun’s corona. “Magnetic reconnection is a process where oppositely polarity magnetic field lines connect and some of the magnetic energy is converted to heat energy and also kinetic energy which leads to the generation of heating, solar flares, solar jets, etc,” says Tanmoy Samanta, a postdoctoral research fellow working at George Mason University, and Johns Hopkins University Applied Physics Laboratory of the U.S. He is one of the authors of the paper.
Image courtesy: SDO/CoMP
The team used a technique known as coronal seismology or magnetoseismology to measure the coronal magnetic field which has been known for a few decades. This method requires the measurement of the properties of magnetohydrodynamic (MHD) waves and the density of the corona simultaneously. “In the past, these techniques were occasionally used in small regions of the corona, or some coronal loops due to limitations of our instruments/and proper data analysis techniques,” explains Dr. Samanta, in an email to The Hindu .
Sharp instrument
The team used the improved measurements of the Coronal Multi-channel Polarimeter (CoMP) and advanced data analysis to measure the coronal magnetic field. CoMP is an instrument operated by High Altitude Observatory, of the U.S. It is located at Mauna Loa Solar Observatory, near the summit of that volcano on the big island of Hawaii.
It is very important to measure the corneal magnetic fields regularly since the solar corona is highly dynamic and varies within seconds to a minute time scale. [While photospheric magnetic fields are measured regularly from space] “the measurement of global coronal magnetic fields was missing in the past since the coronal magnetic fields are very weak. We plan to regularly measure coronal magnetic fields using our sophisticated techniques to understand the physical processes of the highly dynamic corona,” says Dr. Samanta.
He adds that while ground-based measurements pose challenges, India’s first solar mission, Aditya-L1 satellite will aim to measure the solar coronal magnetic fields regularly. This will help understand the spectacular solar eruptions and predictions of space weather and many more things.
Waves in magnetic environments
The properties of waves depend on the medium in which they travel. By measuring certain wave properties and doing a reverse calculation, some of the properties of the medium through which they have travelled may be, in principle, obtained. Waves can be longitudinal waves (for example, sound waves) or transverse waves (for example, ripples on a lake surface).
The waves that propagate through a magnetic plasma are called magnetohydrodynamic (MHD) waves. There are different types of MHD waves, and one of these is the transverse MHD wave. From the theoretical calculation, it can be shown that the properties of the transverse MHD wave are directly related to the strength of magnetic fields and the density of the corona. “Once we measure the wave properties and the density of the corona, then by using the mathematical formula (derived from MHD theory), we can compute the magnetic fields in the corona. And the CoMP instrument is uniquely designed to measure these quantities and hence the magnetic fields,” says Dr. Samanta
Published - August 07, 2020 04:49 pm IST
