High Spatial Resolution imaging for the Nobeyama Radioheliograph and Observations of Weak Activities Prior to Solar Flares

This paper is two-fold. Firstly, we present 'High Spatial Resolution Imaging for the Nobeyama Radioheliograph' which enable us to synthesize partial frame images of the Sun with a spatial resolution of 10 arcsec at 17 GHz, and secondly, results of 'Observations of Weak Activities Prior to Solar Flares' which have been obtained by using this new imaging software. Solar flares are processes in which a large amount of magnetic energy is slowly built up in the solar corona and explosively released. Knowledge of coronal conditions prior to solar flares is essential to understanding how energy is stored and then released in solar flares. To get information on such pre-flare coronal conditions we have analyzed in detail weak activities prior to flares for 32 events which were observed with the Nobeyama Radioheliograph. The Radioheliograph is a quite suitable instrument for statistical analysis of transient phenomena such as solar flares, since it can observe any activities on the Sun, with spatial and temporal resolutions of 10 arcsec and 1 sec respectively, during 8 hours every day. The Nobeyama Radioheliograph is a radio interferometer dedicated for solar observations. The array configuration of the Nobeyama Radioheliograph is specially designed only for solar observations and so very unique as compared with other radio interferometers for solar and non-solar observations. An imaging software with both high spatial resolution and high image quality is required in order to analyze weak activities prior to solar flares, though it is quite difficult to realize such a imaging software by usual imaging techniques in our unique array. We have succeeded to develop a new imaging software in which two requirements, i.e. high spatial resolution and high image quality, are fulfilled. (1)High Spatial Resolution Imaging for the Nobeyama Radioheliograph The Nobeyama Radioheliograph is a multiple, equally spaced T-array which consists of eighty-four 80-cm antennas arranged in the T-shaped baseline with 490 m in the east-west and 220 m in the north-south directions. Phase and gain errors mostly caused by atmospheric fluctuations can be calibrated by making use of redundancy of the antenna configuration, using the Sun itself as a calibrator. The absolute position of the synthesized image of the Sun can not be obtained in this self-calibration method, and therefore, is determined relative to the solar disk using the sharp limb of the high-quality image of the solar disk. In order to get the high-quality image of the solar disk, complex visibility measured by the Radioheliograph is heavily weighted on lower spatial frequencies. In the Nobeyama Radioheliograph, the spatial resolution of 10 arcsec can be expected at 17 GHz from the maximum antenna spacing. However, if the image is synthesized using the natural weighting, the synthesized main beam has extended wings around the sharp beam, and as a result, the spatial resolution of the synthesized image is very poor, almost double that expected from the maximum antenna spacing. In order to achieve the instrumental limit in spatial resolution, it is necessary to weight the sampled visibility so that the contribution from any part of visibility space is uniform (this is called the super uniform weighting). In this case, the synthesized beam (the point spread function) has extremely high sidelobes (about 90 % of the main beam level). This causes high probability of generation of spurious sources in case of imaging of the full Sun, since many radio sources are scattered on the solar disk. We develop a new algorithm for partial frame images of the Sun in order to overcome the above mentioned problem. The principle of the new algorithm is as follows. We consider the case that the radio sources to be synthesized are confined in the target region which is smaller than the interval of the high sidelobes. If we can separate visibility of radio sources in the target region from that outside the target region, the super uniform weighting can be safely applied to imaging of the target region (whose visibility is separated from that outside the target region) because no serious interaction between the high sidelobes and the radio sources can be expected in the target region. Then, we find that the natural weighting can be used to separate visibility of the radio sources between the inside and the outside of the target region because the synthesized beam (the point spread function) has lower sidelobes and many full disk maps have been successfully synthesized using the natural weighting. One of weak points of the CLEAN algorithm is avoided in the newly developed software, and as a result, the instrumental limit of spatial resolution, ～10 arcsecond, is achieved in the Nobeyama Radioheliograph at 17 GHz. This algorithm makes it possible to synthesize high spatial resolution maps not only of flares but also of relatively bright, active regions. The noise level on the map s ...

総研大甲第291号

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