Studies of solar coronal X-ray jets Studies of Solar Coronal X-ray Jets

博士論文

The soft X-ray telescope (SXT: Tsuneta et al. 1991) aboard Yohkoh (Ogawara et al. 1991) has discovered X-ray jets in the solar corona (Shibata et al. 1992, 1994, Strong et al. 1992, Shibata, Yokoyama and Shimojo, 1996). According to Shibata et al. (1992), these X-ray jets are transitory X-ray enhancements with an apparent collimated motion, and have the following observed characteristics: 1) The typical size of the jet is 5 × 10<SUP>3</SUP> - 4 × 10<SUP>5</SUP> km, and the apparent velocity is 30 - 300 km s<SUP>-1</SUP>. 2) The kinetic energy is estimated to be 10<SUP>25</SUP> - 10<SUP>28</SUP> erg. 3) Many jets are associated with small flares in X-ray bright points (XBPs), emerging flux regions (EFRs) or active regions (ARs). On the other hand, Shibata et al. (1992) and Yokoyama and Shibata (1995, 1996) proposed that the X-ray jets are produced by the magnetic reconnection. The two-dimensional MHD numerical simulations of this scenario were performed by Yokoyama and Shibata (1995, 1996) and they succeeded in reproducing observed characteristics of the X-ray jet and the Hα surge. These results, however, were obtained from one month data taken during November 1991 (～ 20 jets). The purpose of this thesis is to elucidate the statistical properties of solar X- ray jets, to clarify physical conditions of X-ray jets, and to understand the formation mechanism of X-ray jets using many envents and high resolution data. Thus, this thesis is organized into 6 chapters. The extended abstracts of the subsequent chapters follow: Chapter 1. Introductory Review . In this chapter, we briefly mention the properties of X-ray jets which have been found by previous studies, and summarized characteristics of other jet phenomena in the solar atmosphere since these are very important to understand X-ray jets. Chapter 2. Statistical Study of Solar X-Ray Jets 　 We have found 100 X-ray jets in the database of full Sun images taken with the Soft X-ray Telescope (SXT) aboard Yohkoh during the period from 1991 November through 1992 April. A statistical study for these jets results in the following characteristics: 1) Most are associated with small flares (microflares - subflares) at their footpoints. 2) The lengths lie in the range of a few × 10<SUP>4</SUP> - 4 × 10<SUP>5</SUP> km. 3) The widths are 5 × 10<SUP>3</SUP> - 10<SUP>5</SUP> km. 4) The apparent velocities are 10 - 1000 km s<SUP>-1</SUP> with an average velocity of about 200 km s<SUP>-1</SUP>. 5) The lifetime of the jet extends to - 10 hours and the distribution of the observed lifetime is a power law with an index of ～1.2. 6) 76% of the jets show constant or converging shapes; the width of the jet is constant or decreases with distance from the footpoint. The converging type tends to be generated with an energetic footpoint event and the constant type by a wide energy range of the footpoint event. 7) Many jets (～68%) appear in or near to active regions (AR). Among the jets ejected from bright-point like features in ARs, most (～86%) are observed to the west of the active region. 8) 27% of the jets show a gap (>10<SUP>4</SUP> km) between the exact footpoint of the jet and the brightest part of the associated flare. 9) The X-ray intensity distribution along an X-ray jet often shows an exponential decrease with distance from the footpoint. This exponential intensity distribution holds from the early phase to the decay phase. This chapter has been published by Shimojo et al. (1996). Chapter 3. Magnetic Field Properties of Solar X-Ray Jets. From a list of X-ray jets made by Shimojo et al. (1996), we selected events for which there were magnetic field data from NSO/ Kitt Peak. Using co-aligned SXT and magnetograms, we examined the magnetic field properties of X-ray jets. We found that 8% of studied jets occurred at a Single Pole (SP), 12% at a Bipole (BP), 24% in a Mixed Polarity (MP) and 48% in a Satellite Polarity (ST). If the satellite polarity region is the same as the mixed polarity region, 72% of jets occurred at the (general) mixed polarity region. We also investigated the magnetic evolution of jet-producing area in active regions NOAA 7067, NOAA 7270 and NOAA7858. It is found that X-ray jets favored regions of evolving magnetic flux (increasing or decreasing). This chapter has been published by Shimojo et al. (1998). Chapter 4. Physical Parameters of Solar X-ray Jets. We derived the physical parameters of X-ray jets and associated flares using the high resolution data (PFI) taken with the soft X-ray telescope (SXT) aboard Yohkoh. We analyzed 16 X-ray jets and found the following properties of the jet and flare: 1) the temperature is 3 - 8 MK ( Average : 5.6 MK ). 2) the density is 0.7 - 4.0 × 10<SUP>9</SUP> cm<SUP>-3</SUP> (Average 1.7 × 10<SUP>9</SUP>cm-3 ). 3) The temperature of the jet is similar to that of the flare. 4) The thermal energy is 10<SUP>27</SUP> - 10<SUP>29</SUP> ergs, which are 1/4 - 1/7 of that of the flare. 5) The apparent velocity is usually slower than sound speed. 6) There is a correlation between the temperature of the jet and the size of the flare. On the basis of these results, we find that the temperature of the jet and the flare are controlled by the balance between the heating flux and the conductive flux, and that the mass of the jet is comparable to the theoretical value based on the balance between the conductive flux and the enthalpy flux carried by the evaporation flow. Chapter 5. 1D & Pseudo 2D Hydrodynamic Simulation of Solar X-ray Jets. We present results of 1-dimensional hydrodynamic simulations of the chromospheric evaporation produced by a microflare in a large scale loop as a model of X-ray jets. The initial condition of the simulations is based on the actual observation of the X-ray jet and we deposit the thermal energy (～ 1 × 10<SUP>28</SUP> ergs) in the corona. This thermal energy is comparable to that of the microflare associated with the X-ray jet observed on 3 September, 1992. The deposited energy is rapidly transported to the chromosphere by heat conduction, heating dense plasma in the upper chromosphere. As a result, the gas pressure is enormously increased and drives strong upflow of dense hot plasma along magnetic loop. This upflow (i.e., evaporated flow) is identified as an X-ray jet. We computed soft X-ray intensity distributions of the simulated jets seen in bandpass filter of Yohkoh/Soft X-ray Telescope (SXT), and compare X-ray intensity distributions and the physical parameters (temperature, density, and thermal energy) of the simulated jets with those of the X-ray jet observed with Yohkoh/ SXT on 3 September 1992. We found that the evaporation flow in the simulations reproduced the observed properties of the X-ray jet. We also found that the multiple-loop model reproduced the observed properties of the X-rat jet better than the single loop model. Since the reconnection leads to successively heated multiple loops, these results suggest that X-ray jets are evaporation flows which are produced by magnetic reconnection. Chapter 6. Summary and Discussion. We summarized this thesis in this section and also discussed some of difference between X-ray jets and other jet phenomena in the solar atmosphere. In particular, we discussed some jet-phenomena which have been observed by new orbital solar observatory, SoHO and TRACE. Finally, we discussed the future direction of the study of X-ray jets.

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