SUN’S CORONAL MAGNETIC FIELD MEASURED; ANSWER TO SEVERAL PUZZLES ABOUT SOLAR ATMOSPHERE

-Basundhara Biswas

Chemistry Department, UG2

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. Like, the first measurement of the sun’s magnetic field has done before a century, but we don’t have a precise knowledge of the magnetic field in the upper solar atmosphere, especially the corona. At last, a team of solar physicists led by Peking University and the National Centre for Atmospheric Research cracked it; the group measured the global magnetic field of the sun’s corona for the very first time, using CoMP and coronal seismology.

There are two main puzzles about the sun which this advancement will help address. The first one is the coronal heating problem. Though the core of the sun is at a temperature of about 15 million degrees, it’s the outer layer, the Photosphere is a mere 5700 degrees hot. However, it’s the corona, which stretches up to several million kilometers beyond its surface, is much hotter than the surface, at a temperature of one million degrees or more. What causes the corona to heat up again, though the photosphere is cooler than the interior. That’s the question that has baffled solar physicists. Now, the present work will help understand and verify these theories better.

Fig: Corona or the outer atmosphere of the sun

The other set of questions concerns the mechanism 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 polarized magnetic lines connect and some of the magnetic energy is converted to heat energy and kinetic energy resulting generation of heating, solar flares, solar jets, etc.

The team used a technology known as Coronal Seismology or Magneto Seismology to measure the coronal magnetic field. They also used the improved measurements of the Coronal Multi-channel Polarimeter or CoMP (an instrument operated by high altitude observatory of the U.S.) and advanced data to analysis the measurement. The data collected from CoMP reveals that the sun’s corona is full of Alfven waves, that are observed to travel along the magnetic field and also provides us the best available view of them.

Fig: coronal magnetic field line obtained from PFSS model.

The process is almost similar to how seismologists use earthquakes to find out what the interior of the earth looks like.

The simultaneously measured magnetic field will provide critical information to understand how the magnetic field couples the different layers of the sun’s atmosphere as well as the mechanics responsible for solar eruption and solar cycle. So, it is very important to measure the coronal magnetic field regularly since the solar corona is highly dynamic and varies within seconds to minute time scale. With help of this, physicists will get to know about an upcoming solar eruption.