Solar scientists review Hinode findings
Sen—Japan has a long tradition in solar physics and in 2006 launched one of the major space observatories – Hinode. For almost six years this satellite has been constantly monitoring our local star with a suite of three telescopes: the Solar Optical Telescope, X-ray Telescope and Extreme Ultraviolet Imaging Spectrometer (EIS). Together, they enable the study of how magnetic energy is generated and released in the atmosphere of our Sun.
This week in St. Andrews over 150 scientists from around the world gathered for the "Hinode 6" conference to celebrate what has been learnt using the Hinode satellite. Although launched and led by Japan, the satellite has major contributions from the UK, the USA and Norway.
Unexpectedly, St. Andrews has a connection to Hinode’s modern observing methods that dates back to the late 1600s. The Scottish mathematician James Gregory upon walking along the beach in St. Andrews, Scotland, picked up a feather and wondered what would happen if a beam of light were shone through it. Isaac Newton was conducing similar experiments with glass prisms in Cambridge. Back in his lab, Gregory saw that the feather split the light into its component colours in a process now known as diffraction – a simple technique that is used today in many solar telescopes as it allows us to measure the properties of sunlight and in turn learn about the star that emitted it.
My department at UCL led the development of the EIS telescope - a modern equivalent to the bird’s feather - which splits the ultraviolet light emitted from atmospheric gases into the component colours.
A major topic for discussion at the conference has been how magnetic fields that emanate up from the Sun’s surface into the atmosphere, create structures that glow in ultraviolet and X-rays and produce activity such as solar flares and coronal mass ejections.
A large X-class flare captured by the X-ray telescope on Hinode. Image credit: JAXA/Hinode
High-speed gas flows associated to solar flares have been observed, helping scientists understand the processes that convert energy stored in the magnetic fields into energy of gas motions. Computer models have been combined with observations to understand how currents surge along the magnetic structures, supported by the charged particles of the atmospheric gases, heating the atmospheric gases to very high temperatures.
On Friday I will be talking about the question that puzzles me most about the Sun – how immense magnetic structures carrying the mass of Mount Everest can be ejected into the Solar System at millions of kilometers per hour. These are the, now infamous, coronal mass ejections. Hinode watches how magnetic structures evolve in the days, hours and minutes before they erupt and is showing that a particular configuration of the magnetic field is important. The majority of the magnetic field that fills the solar atmosphere is shaped like an arch – a familiar concept to those who have played with iron filings and magnets. The magnetic lines of force arch from the north pole of the magnet to the south.
The above movie is taken by the X-ray Telescope onboard the Japanese Hinode satellite. It shows a clear S shaped structure that shines brightly in X-rays and then erupts to produce what is known as a coronal mass ejection (the eruption starts about 9 seconds into the movie). Coronal mass ejections are eruptions of magnetic field and electrically charged gas that race out into the Solar System and produce 'space weather' when they reach the Earth. These S shaped features have answered a long-standing mystery in solar physics - what is the configuration of the magnetic field just before it erupts? Using observations like this it has been realised that the magnetic field evolves to becomes a twisted bundle of magnetic field lines, like a rope. These magnetic ropes can then be studied, using observations and computer models, to understand the physical processes that lead to the eruption.
I’ll be talking about how the Sun’s magnetic fields are not static and can change their shape over time. In the right conditions they twist themselves up forming something that resembles a rope. Field lines spiral around the edge of the rope making S shapes, signaling the immanent eruption. Although this has been suspected for several years, Hinode’s detailed observations have given us the ability to track this evolution and see a pattern emerging for the first time.