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The Forefront of Space Science

Advancement of Solar Research by HINODE Satellite
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Introduction

By the time this article is published online, JAXAís new rocket will have been launched. Epsilon is JAXAís first solid-propellant rocket to be launched in seven years. The last one launched in 2006 by the final launch of M-V, JAXAís previous solid-propellant rocket, was the solar-observation satellite HINODE. It carries three types of telescope: Solar Optical Telescope (SOT), X-Ray Telescope (XRT); and Extreme-ultraviolet Imaging Spectrometer (EIS). In the period since its launch, the satellite has continuously provided us with never-before-seen solar images.

This article introduces some recent results provided by the satellite.

Change in distribution of magnetic fields in the sunís polar regions

In past observations from the ground, it was thought that only weak, extended magnetic fields existed in the sunís polar regions. One significant result accomplished by HINODE was to dramatically alter our image of the polar regions. SOT is a visible-light telescope with a high resolution of 0.2 arc sec, together with another strong point that it can simultaneously obtain 3D information of the magnetic fields (i.e., vector magnetic-field information) of the solar surface. In the past, when viewing the surface of the sunís polar regions from the earth, our view is almost parallel with it and, thus, we have been unable to gain a clear image. Despite this, SOT utilized its high-spatial resolution to successfully compile a precise magnetic-field map of the sunís polar regions (Fig. 1a) in the form of a birdís-eye view. Looking at the map, we were surprised to find that unipolar magnetic-field patches with a strength of 1,000G (gauss) nearly equivalent to a sunspot are distributed everywhere in the polar region. Magnetic-field lines rooted in these ďstrong magnetic-field patchesĀEgrow almost vertically to the solar surface. From the ground, it is difficult to measure precisely the constituents of the magnetic fields perpendicular to the direction of sight. The distribution status of these magnetic fields was first revealed by SOT, with its high-spatial resolution and ability to precisely measure the three components of a vector magnetic field.


Figure 1
Figure 1. Polarity reversal process of polar magnetic fields observed by HINODE
a: Status of polarity reversal around the solar north pole observed by HINODEís SOT. In 2007, strong magnetic-field patches of S polarity (orange color: shown as negative pole in the figure) were found everywhere in the polar region above Lat. 70 deg. N while in 2012 the patches of N polarity (blue color: shown as positive pole in the figure) and S polarity were mixed.
b: Yearly change in mean magnetic-field strength of the polar regions, which is obtained by using the strong magnetic field patches of more than 1018 Mx in magnetic flux. Blue shows N polarity and orange S polarity.


Incidentally, it is already known that the magnetic-field polarity of the sunís north and south polar regions reverses every 11-year cycle of solar activity peak. For example, the north pole showing magnetic S polarity will change to N polarity at some time. Solar activity was believed to enter the peak of its cycle in 2013. HINODE observes the polar areas every month and carefully monitors the process of polarity reversal: the behavior of the strong unipolar magnetic field patches, and the polarity reversal of the whole polar regions. Soon after HINODEís launch, most of the magnetic-field patches in the north pole area was S polarity. However, N polarity patches started to emerge initially from the low latitude area, and at present, the two polarities are present almost evenly in the whole region (Fig. 1a).

Fig. 1b shows the yearly change in the mean magnetic-field strength in the sunís polar regions, which was calculated by selecting the magnetic-field patches above 1018Mx (Maxwell, 1Mx is 10 - 8Wb (weber)) in magnetic flux (i.e. magnetic-field strength). Around 2008, S polarity was dominant at the north pole. After that, the mean magnetic-field strength of the S polarity patches has continued to decline (because of a decrease in number of the strong patches). It is expected that the polarity in the north polar region will reverse around 2014. Meanwhile, at the south pole, although the mean magnetic-field strength of the N polarity patches has slowly fallen since the start of HINODE observation, N polarity still prevails. If the current trend continues, it should take several years for the polarity in the north polar area to reverse. It is likely, however, that both north and south poles will have N polarity in the near future. If so, the effect of the solar-magnetic field on the entire solar system may also change considerably.

Since the launch of HINODE, solar activity remained low for a long time and its last activity cycle (until 2009) casted for 13 years span contrary to the standard 11-year cycle. Some people say that activity will be stagnant in the long-term span. The magnetic fields in the polar region are also believed to be ďseedsĀEof the next solar activity cycle. Therefore, we need to keep our attention to the HINODE observation of the polar magnetic fields for the next few years.

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