Magnetic waves behind sun’s hot atmosphere
The sun’s atmosphere presents a rich tapestry, providing astrophysics with new, unexpected designs of great intricacy. For millennia we have based our views of the sun (and the universe) in the narrow visual window of the electromagnetic spectrum. Over the last 70 years or so, this has been extended to radio, ultraviolet, X-ray, gamma-ray and other parts of the spectrum. This article presents the unveiling of one of its outstanding mysteries.
Much like a flame coming out of an ice cube, the sun’s atmosphere (corona) is million-degrees hot atop its cool surface at 6,000 kelvin (a primary unit of temperature measurement in the physical sciences). How can this be? Heat flows from a hotter to a colder object in nature and not the other way, as per the laws of physics. It defies common sense.
This extraordinary mystery of the sun has puzzled scientists since its discovery in the early 1940s. Despite a rich wealth of observations available from ground- and spacebased observatories across the entire electromagnetic spectrum, this remains an outstanding problem in astrophysics. So, what’s going on there?
But that is how it works on the sun (and other similar stars). Why is the sun’s atmosphere 200 times hotter than its much cooler surface (6000 kelvin)? The outer atmosphere of the Sun’s hot corona becomes visible to the naked eye during a total eclipse. In 1943, Bengt Edlén identified the green coronal spectral line emission from Fe+13 (13-times ionized iron) and the red line emission from Fe+9 (9-times ionized iron) observed during the eclipses. This hinted at the existence of hot corona.
But then, an unanswered question remains, “Why does the corona exist and how is it heated?” A leading solar physicist Abhishek Srivastava at IIT (BHU) and his colleagues have now solved this riddle, which has puzzled scientists for over seven decades. For the first time, this unique finding resulted from clear observational evidence and computer simulation.
Nobel Laureate Hannes Alfvén discovered in 1941 that the sun’s corona is “heated to an extremely high temperature.” It was his prophetic contribution. In addition to the sun’s gravitation, its magnetic field interacting with charged particles plays a pivotal role. In 1942, Alfvén further proposed that waves can propagate through a magnetized plasma (ionized gas) under similar conditions as in the sun’s atmosphere.
Even after over seven decades of this Nobel-prize winning discovery, scientists have been figuring out whether these magnetic waves are capable of heating the solar corona and accelerating the solar wind. Observations of ultraviolet, X-rays and the wind from the Sun confirmed the prediction of a million kelvin hot solar atmosphere. This posed a challenge to the understanding of how the energy is transported from the sun’s subsurface to its upper layers.
The role of Alfvén waves (a type of oscillation of plasma particles consisting of transverse waves propagating along the magnetic field lines in a plasma) has always been emphasized. The magnetic fields, which connect the solar photosphere with the corona, may guide the waves from the surface layers upwards similar to the transverse waves generated in a guitar string. These waves may transport the energy of powerful photospheric motions into the corona leading to plasma heating. But there was no direct evidence of such high-frequency waves in the past. Abhishek and his colleagues have analyzed the ground-based observations of the Sun’s chromospheres (a reddish gaseous layer immediately above the photosphere) from the world’s biggest solar telescope (1-m Swedish Solar Telescope at La Palma). For the first time, the analysis of observational data vis-à-vis 3-D computer modelling have shown the ubiquitous presence of high frequency (12 - 42 mHz) torsional Alfven waves in a large number of oscillating magnetic tubes as observed on June 10, 2014.
They find a compelling body of evidence of oscillating magnetized plasma at highfrequency. The associated magnetic force restores it creating torsional oscillations. This is analogous to a mechanical shaft oscillating along its own axis. In the sun, however, this is a combined manifestation of dynamic plasma and magnetic field.
It’s magnetic waves that seem to solve the mystery of the hot corona. They find that these high frequency drivers transfer 103 W m-2 energy into the upper layers of the sun’s atmosphere. Thus, they discover that these oscillating tubes are the sources of the Alfvén wave generation, providing enough energy flux to heat and maintain the corona.