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The Forefront of Space Science

Mystery of Solar Coronal Heating Being Explored by Satellites
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As it is widely known, extremely strong magnetic fields exist within sunspots. It was thought that, as the magnetic fields are so powerful, convective motions are constrained and, therefore, inside the sunspot is quiet. However, HINODE SOT revealed that very active plasma phenomena occur there. In addition to its high-resolution capability, SOT also has a high-precision spectropolarimetric capability. Using this capability, we were able to observe directly how the jet phenomena in the chromosphere occur in what magnetic field structure. As a result, it became clear that magnetic fields horizontal and vertical to the surface are alternately present in the penumbra (called uncombed structure). It is thought that the structure is formed by the powerful interaction of strong magnetic fields and convection within the sunspot. Further, it is expected that a strong current layer is formed at the boundary between two types of magnetic fields. There is a high possibility that “magnetic reconnection,Ethe release of magnetic energy in a short time, occurs there to induce the jet-like brightening. We were able to watch the actual site where magnetic energy was released. This is because the strong magnetic fields in sunspots make it relatively easy to identify the magnetic field structure. It is thought, however, that similar magnetic field structures also exist outside sunspots and that they cause small-scale jet phenomena in various places.

The mystery of coronal heating is not yet solved. Compared to within the corona, the density and pressure of gas in the chromosphere are high. Thus, even if magnetic energy is released in the chromosphere, it is inefficient for heating. Nonetheless, one possibility is that, when waves (e.g. acoustic waves, Alfven waves) are excited by the magnetic reconnections, they are conveyed to the upper region along magnetic field lines and produce shock waves to heat the corona. This hypothesis is a hybrid of nanoflare heating and wave heating theories. With HINODE SOT, we also started detecting the cases where the oscillation of magnetic field lines is excited by jet phenomena. In combination with Extreme-Ultraviolet Imaging Spectrometer (EIS) and X-Ray Telescope (XRT), which are also onboard HINODE to observe high-temperature corona, it is expected that we can obtain decisive clues to understand the coronal heating mechanism.

Instrument development and science

When I was a graduate student, there was no HINODE data available (it had yet to be launched). I visited observatories abroad to conduct spectropolarimetric observations using ground telescopes. I proceeded with my research by merging ground telescope data and satellite data supplied by YOHKOH, SOHO, etc., and wrote my doctor thesis. Meanwhile, the development of an optical telescope that integrated cutting-edge optical technologies was not so easy. I spent much of my time in doctoral and postdoctoral courses on its development. I am certain that striking a balance between instrument development and science was a significant driving power for research after the launch of HINODE. Both were indispensable factors to my research. To experience again the indescribable excitement I felt when I first saw an image from HINODE, I am striving to realize a new observation instrument superior to HINODE in the near future.


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