Q. Could you describe HINODE's major achievements so far?
HINODE is the world's most advanced orbital solar observatory. It was developed by JAXA (the Japan Aerospace Exploration Agency) with the National Astronomical Observatory of Japan (NAOJ) as domestic partner and research institutes in the United States (NASA) and the United Kingdom (STFC) as international partners. The mission is operated by these agencies in corporation with the European Space Agency (ESA) and Norwegian Space Center, who support data downlinks at Svalbard station. We also accept observation proposals from researchers over the world and the data are open to any researchers for data analysis as soon as they are acquired by HINODE.
The Sun constantly ejects plasma into interplanetary space - a phenomenon called solar wind. These huge plasma clouds are released when massive explosions, known as flares, occur on the solar surface, although plasma outflow also occurs without flaring. Solar wind disturbs Earth's magnetic field, sometimes causing problems such as the malfunctioning of electronic equipments on spacecraft orbiting the planet. Because solar wind affects our planet, it is critical to understand its origin.
For the first time in history, HINODE successfully captured solar-wind outflows on X-ray movies. High-speed solar wind, moving at up to 900 kilometers per second, has been known to emanate from polar coronal holes, which are regions of low coronal density. However, the origins of low-speed solar wind (400 to 600 kilometers per second) had never been detected. The X-Ray Telescope (XRT) on HINODE has now discovered a new source of low-speed solar wind. Coronal gas was observed to flow up along magnetic field lines at the boundary between a coronal hole and an active region, with many sunspots and strong magnetic fields, which are located at low latitudes. Unlike most solar magnetic field lines that return to the solar surface, we found that the magnetic field lines with gas outflows expand into interplanetary space. That is, coronal gas, emanating along magnetic field lines into space, is filling our solar system.
The ultimate question about solar wind is "what is its velocity mechanism?" In other words, where and how does solar wind accelerate on its way to Earth? The velocity measured near the solar surface in HINODE's X-ray observation was 140 kilometers per second. However, in general, when approaching Earth, the speed increases, at its fastest, to about 900 kilometers per second, and, on average, to 500 or 600 kilometers per second. Understanding the velocity mechanism of solar wind is an important research task.
Before HINODE, it was extremely difficult to observe the dynamic motions of magnetic field lines on the solar surface. The unprecedented resolution of HINODE's Solar Optical Telescope is around 0.2 arcseconds, which can analyze a magnetic field structure of 100 to 200 kilometers on the Sun's surface. Magnetic field lines on the solar surface are quantized as magnetic flux tubes at about 0.2 arcsecond, which have kilo gauss order of magnetic field strength. HINODE has captured video images of the formation of magnetic flux tubes, as well as dynamical evolutions of these tubes in continuous turbulent convection on the solar surface. And, while researchers had assumed that most magnetic field lines existed as magnetic flux tubes, HINODE has revealed that magnetic field lines that lie almost parallel to the solar surface outnumber magnetic field lines in magnetic flux tubes. They float and sink in accordance with the convection on the Sun's surface. We have also discovered that, in regions where magnetic field lines are moving dynamically, gas on the surface is jetting at near sonic velocity or greater.
Another new discovery was the observation of continuous microjets and plasma eruptions in the chromosphere, which is just above the photosphere, into the upper atmosphere. HINODE has recorded video that captures the vicinity of the limb of the Sun, using a filter to catch the light of chromospheric plasma, and producing a brilliant visual of chromospheric plasma jetting up towards the corona along invisible magnetic lines. Scientists will now be able to investigate the physical process of the interaction between the dynamic magnetic field lines and the gas.