Hinode-3: The 3rd Hinode Science Meeting
ASP Conference Series, Vol. 454
T. Sekii, T. Watanabe, and T. Sakurai eds.
c 2012 Astronomical Society of the PacificSurvey of Accelerated Particles in a Solar Active Region Using
Hinode/XRT and Ground-Based Type-I Radio Burst Observations
Kazumasa Iwai,1 Hiroaki Misawa,1 Fuminori Tsuchiya,1 Akira Morioka,1Satoshi Masuda,2 and Yoshizumi Miyoshi2
1 Planetary Plasma and Atmospheric Research Center, Tohoku University
2 Solar-Terrestrial Environment Laboratory, Nagoya University

Abstract. The relationships between solar radio type-I bursts and soft X-ray activities were investigated using Hinode/XRT and a ground-based radio telescope belonging
to Tohoku University. Although a type-I burst is thought to be generated by high energy
non-thermal electrons in the solar corona, the counterpart of this radio burst in X-rays
or EUV have yet to be identified. In this study, we found some small scale soft X-ray
activities on the XRT images around the onset time of the type-I burst when 10 percentof the soft X-ray flux enhancement around the onset time of the radio burst is defined
as a burst-related activity. However, the causal relationship between the observed soft
X-ray activities and the onset of the type-I burst are unclear, and more simultaneous observations of radio bursts and X-rays are needed to investigate the coronal counterpartof the type-I burst.

1. Introduction
Type-I is one of the solar radio phenomena that has been frequently observed in the metric range. It has generally been believed that non-thermal electrons generated throughsome physical process are trapped in a closed magnetic field and generate Langmuirwaves. Then, the Langmuir waves are converted into O-mode radio waves and finallyobserved as type-I (Benz & Wentzel 1981). The flux density of type-I is weak comparedto other solar radio bursts (Elgaroy 1977), and therefore, it is believed that the emission is caused by some small scale particle acceleration processes in the solar corona.
However, the acceleration processes of the non-thermal electrons are not yet fully understood. One of the major reasons is a lack of coronal phenomena that are associatedwith the appearance of a type-I burst (Krucker et al. 1995). Bentley et al. (2000) andWilson (2005) tried to detect the coronal counterpart of a type-I burst using magnetograms and EUV images. However, not enough information was collected in thesestudies to find a definite correlation between them.
Some non-thermal electrons can reach the chromosphere and could enhance softX-ray or EUV flux through chromospheric evaporation (Aschwanden 2006). The X-ray
Telescope (XRT) onboard the Hinode spacecraft has the highest spatial resolution in thesoft X-ray wavelength range and it is an efficient tool for investigating small scale solar
soft X-ray phenomena. Thus, we investigated the relationships between type-I observedby using ground-based radio telescope and soft X-ray activities observed by the Hinodespacecraft for this study.
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2. Observations
Iitate Planetary Radio Telescope (IPRT) was used to observe type-I. The IPRT is aground-based radio telescope developed by Tohoku University (Misawa et al. 2003;
Tsuchiya et al. 2009). The physical aperture of IPRT is 1023 square meters, so it realize
very high-sensitivity observations. The solar radio observation system in IPRT allows
for continuous spectral observations of the Sun within a frequency range of 230 to 400
MHz. The time and frequency resolutions are 270 ms and 1 MHz, respectively. The
minimum detectable sensitivity of this system is better than 1 Solar Flux Unit in themeter wavelength range. An IPRT observation was carried out from December 2006 toJanuary 2007.
The data from the X-ray Telescope (XRT) onboard the Hinode spacecraft are used
to identify the solar soft X-ray phenomena associated with radio bursts. During theIPRT observation period, XRT mainly observed the active region NOAA 10933, which
was expected to be a source region of radio bursts. XRT observed the active region
using only an Al-Poly filter at an exposure time of 0.1-0.5 s. The images were recorded
every 60 s during the observation time. IPRT and XRT made simultaneous observationsfor 1-3 hours a day.

3. Results
During the observation period, type-I bursts started to be observed when the activeregion NOAA 10933 appeared in the eastern limb. The burst activity was highest whenthat active region passed the central meridian. The radio burst ceased when the active
region went down into the western limb. These results indicate that type-I bursts emitted
from this active region. Figure 1 (c) shows the radio dynamic spectrum observed byusing IPRT on January 11. A clear onset of type-I bursts was observed at 1:40 and
3:13 UT. Figure 1 (a) shows the full disc image of the Sun observed by using XRTon the same day. The active region NOAA 10933 is located around the eastern limb
(surrounded by a blue box). The region inside the blue box is divided into a 16×16 pixel
square area. Then, the light curves of each area are obtained. We define a burst-related
activity as 10 percent of a soft X-ray flux enhancement within 10 minutes of the onsetof radio bursts. Figures 1 (b1) and (b2) show the soft X-ray images of the active region
when the type-I onsets were observed. The regions which showed burst-related activity
are shown by the red squares. Figures 1 (d1) - (d7) show the light curves of some of theselected regions that showed the burst-related activity.
At the first onset (01:40 UT), there were two areas that showed burst-related activity. If non-thermal electrons, which excite radio bursts, cause the soft X-ray enhancement, the soft X-ray enhancement should start almost simultaneously with or after theradio bursts. The X-ray flux in the (d1) region is, however, concluded not to relate to
the radio burst since it was enhanced before the radio burst onset. The X-ray flux inthe (d2) region was detected as a burst-related activity in this analysis, but it is difficult to find significant activity from the noise of the light curve. In the second onset(03:13 UT), there were several areas that showed burst-related activity. In some areas,for example in (d4) or (d5), X-ray enhanced almost simultaneously with the onset ofthe radio burst although there remains some ambiguity in distinguishing burst-related
activity from accidental coincidences.

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Figure 1. (a) Full disc image of the Sun observed by XRT on January 11. (b1-b2)
Soft X-ray images around the NOAA 10933 region at radio burst onset times. (c)Radio dynamic spectrum observed by IPRT on January 11. (d1- d7) Light curves ofareas showing burst-related activity. The intensities of these light curves are individually normalized, so absolute values have no meaning.

4. Summary and Discussion
We investigated the relationships between solar radio type-I bursts and soft X-ray activities. We found some small scale soft X-ray activities around the onset of the type-I
burst although some ambiguity still remains for concluding that there are cause-andeffect relationships between them. Simultaneous observations of radio and X-ray were
too short to analyze the onsets of the radio burst in detail in this study. In addition, thefollowing two points should also be discussed.

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First, there is the possibility that high energy particles didn’t reach the chromosphere. If high energy particles propagate only into the interplanetary space, they could
excite radio bursts without the presence of soft X-ray emission. Note that type-I iscorrelated with a micro type-III storm, suggesting that high energy particles propagate
to the interplanetary space (Bougeret, Fainberg, & Stone 1984). XRT can be used toobserve the temperature distribution of the corona using two filters, and a change inthe temperature distribution may occur in the particle acceleration region regardless of
the accelerated direction of the particles. Unfortunately, temperature information is notavailable from the data set of this study.
Second, during the observation period, type-I bursts were observed at around 230300 MHz, which is almost the lower limit of the observational frequency range of IPRT.
In fact, type-I is usually emitted more frequently around 100-200 MHz (Elgaroy 1977).
It might be suggested that the frequencies of type-I bursts are too low to be detected byusing IPRT.
As future work, more simultaneous observations of radio and soft X-ray are required. Our Hinode observation proposal (HOP129) will start soon. The time variationin the temperature distribution will be available in this observation. The radio observation system in IPRT has also recently been improved. This system enables us to observe
lower frequencies than the previous system. The collaboration of these facilities will
provide useful data to investigate the coronal counterpart phenomena of a type-I burst.
Acknowledgments. Hinode is a Japanese mission developed and launched by
ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC(Norway). This work was carried out by the joint research program of the SolarTerrestrial Environment Laboratory, Nagoya University. This work was supported by
JSPS Research Fellowships for Young Scientists.
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