We present multiwavelength observations of a large-amplitude oscillation of a polar crown filamen... more We present multiwavelength observations of a large-amplitude oscillation of a polar crown filament on 15 October 2002. The oscillation occurred during the slow rise (about 1 km/s) of the filament. It completed three cycles before sudden acceleration and eruption. The oscillation and following eruption were clearly seen in observations recorded by the Extreme-Ultraviolet Imaging Telescope onboard SOHO. The oscillation was seen only in a part of the filament, and it appears to be a standing oscillation rather than a propagating wave. The period of oscillation was about two hours and did not change significantly during the oscillation. We also identified the oscillation as a "winking filament" in the H-alpha images taken by the Flare Monitoring Telescope, and as a spatial displacement in 17 GHz microwave images from Nobeyama Radio Heliograph (NoRH). The filament oscillation seems to be triggered by magnetic reconnection between a filament barb and nearby emerging magnetic flu...
By using the method presented by Isobe et al. (2002), the non-dimensional reconnection rate Vin/V... more By using the method presented by Isobe et al. (2002), the non-dimensional reconnection rate Vin/Va has been determined for the impulsive phase of three two-ribbon flares, where Vin is the velocity of the reconnection inflow and Va is the Alfven velocity. The non-dimensional reconnection rate is important to make a constraint on the theoretical models of magnetic reconnection. In order to reduce the uncertainty of the reconnection rate, it is important to determine the energy release rate of the flares from observational data as accurately as possible. To this end, we have carried out one dimensional hydrodynamic simulations of a flare loop and synthesized the count rate detected by the soft X-ray telescope (SXT) aboard Yohkoh satellite. We found that the time derivative of the thermal energy contents in a flare arcade derived from SXT data is smaller than the real energy release rate by a factor of 0.3 - 0.8, depending on the loop length and the energy release rate. The result of si...
The sun occasionally undergoes the so-called grand minima, in which its magnetic activity, measur... more The sun occasionally undergoes the so-called grand minima, in which its magnetic activity, measured by the number of sunspots, is suppressed for decades. The most prominent grand minima, since the beginning of telescopic observations of sunspots, is the Maunder minimum (1645-1715), when the sunspots became rather scarce. The mechanism underlying the grand minima remains poorly understood as there is little observational information of the solar magnetic field at that time. In this study, we examine the records of one candidate aurora display in China and Japan during the Maunder minimum. The presence of auroras in such mid magnetic latitudes indicates the occurrence of great geomagnetic storms that are usually produced by strong solar flares. However, the records of contemporary sunspot observations from Europe suggest that, at least for the likely aurora event, there was no large sunspot that could produce a strong flare. Through simple theoretical arguments, we show that this geom...
We report observation of a large-amplitude filament oscillation followed by an eruption. This is ... more We report observation of a large-amplitude filament oscillation followed by an eruption. This is used to probe the pre-eruption condition and the trigger mechanism of solar eruptions. We used the EUV images from the Extreme-Ultraviolet Imaging Telescope on board SOHO satellite and the H-alpha images from the Flare Monitoring Telescope at Hida Observatory. The observed event is a polar crown filament that erupted on 15 Oct. 2002. The filament clearly exhibited oscillatory motion in the slow-rising, pre-eruption phase. The amplitude of the oscillation was larger than 20 km/s, and the motion was predominantly horizontal. The period was about 2 hours and seemed to increase during the oscillation, indicating weakening of restoring force. These results strongly indicate that, even in the slow-rise phase before the eruption, the filament retained equilibrium and behaved as an oscillator, and the equilibrium is stable to nonlinear perturbation. Moreover, the transition from such nonlinear s...
The Astronomical Diaries from Babylonia (ADB) are an excellent source of information of natural p... more The Astronomical Diaries from Babylonia (ADB) are an excellent source of information of natural phenomena, including astronomical ones, in pre-Christ era because it contains the record of highly continuous and systematic observations. In this article we present results of a survey of aurora-like phenomena in ADB, spanning from BCE 652 to BCE 61. We have found 9 records of aurora-like phenomena. Philological and scientific examinations suggest 5 of them can be considered as likely candidate for aurora observations. They provide unique information about the solar and aurora activities in the first millennium BCE.
The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4 class flare on 2006 ... more The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4 class flare on 2006 December 13. Typical two-ribbon structure was observed, not only in the chromospheric CaII H line but also in G-band and FeI 6302A line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, the sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge followed by a diffuse inside, as well as previously known core-halo structure. The underlying structures such as umbral dots, penumbral filaments and granules were visible in the flare ribbons. Assuming that the sharp leading edge was directly heated by particle beam and the diffuse parts were heated by radiative back-warming, we estimate the depth of the diffuse flare emission using the intensity profile of the flare ribbon. We found that the depth of the diffuse emission is about 100 km or less from the height of the source of ra...
We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar ... more We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. Magnetic flux emergence is the origin of sunspots and active regions, and often associated with solar flares and coronal mass ejections. Owing to the high-resolution achieved by the Earth Simulator, we could simulate the formation of turbulent, fine structures in the emerging flux region as a result of global dynamics. Our simulation results naturally explain many observed features such as arch filament system and intermittent coronal heating.
The interplanetary magnetic flux rope (IFR) has been a subject of extensive research activity sin... more The interplanetary magnetic flux rope (IFR) has been a subject of extensive research activity since its discovery in 1981 as a key structure in the solar wind that provide important information on the solar eruption phenomena and on how the southward magnetic fields are carried from the Sun to the Earth. In this review, we discuss solar-cycle variation of occurrence frequency of IFRs that still remains unsettled, based on our own results. First, we have found more than 500 IFRs in the time period from 1995 to 2009, whereas the survey by Lepping et al. (AnnGeo, 2006) identified 82 IFRs during 1995-2003. The difference mainly comes from the fact that their survey was not successful in identifying IFRs when the spacecraft passed only near the surface of IFRs. Our result indicates that the rate of IFR occurrence to the ICMEs should be much higher than those which were suggested by previous evaluation. Secondly, the following trend is clearly seen: namely, the occurrence rate of IFRs inc...
The launch of SOT on the Hinode satellite, with it’s previously unprecedented high resolution, hi... more The launch of SOT on the Hinode satellite, with it’s previously unprecedented high resolution, high cadence images of solar prominences, led to the discovery of small scale, highly dynamic flows in quiescent prominences. Berger et al. (2008) reported dark upflows that propagated from the base of the prominence through a height of approximately 10 Mm before ballooning into the familiar mushroom shape often associated with the Rayleigh-Taylor instability. Whether such phenomena can be driven by instabilities and, if so, how the instability evolve is yet to be fully investigated.
We have been developing UFCORIN, an automated space weather prediction system based on machine-le... more We have been developing UFCORIN, an automated space weather prediction system based on machine-learning technologies. Our aim is twofold: one is to provide real-time space weather forecast that thoroughly utilize the huge amount of solar observation data available today. The other is to discover the observational flare-triggering features, by analyzing the big data with the clear goal of predicting the solar flares. UFCORIN stands for Universal Forecast Constructor by Optimized Regression of INputs. As the name suggests, UFCORIN is designed as a generic time-series predictor, which can be set to predict arbitrary time series from arbitrary numbers and kinds of input time series. Using our system we predict maximum of GOES X-ray flux for 24-hour period in the future. As inputs to the predictor, we use wavelet powers of the full disk line-of-sight magnetogram obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). We also use the total magn...
We present the results of two-dimensional magnetohydrodynamic simulations of magnetic reconnectio... more We present the results of two-dimensional magnetohydrodynamic simulations of magnetic reconnection with the effect of ambipolar diffusion, the diffusion of magnetic field due to the ion-neutral collision. It is found that the spatial profile of the current sheet thinning by ambipolar diffusion has a decisive effect on the structure and dynamics of magnetic reconnection. When the scale of the ambiplar diffusion is large, Sweet-Parker like reconnection with an elongated current sheet appears at first, followed by plasmoid instability and intermittent reconnection. When the scale of the ambiplar diffusion is small, Petschek-like reconnection with a localized diffusion region is realized even though the resistivity is uniform. We also found field-aligned outflows from outside the current sheet driven by the gas pressure gradient that is generated by the localized heating of ambipolar diffusion.
Some important observations have shown that the strong correlation between emerging flux and erup... more Some important observations have shown that the strong correlation between emerging flux and eruptions of quiescent filaments (Feynman & Martin, 1995; Wang & Sheeley, 1999). Chen & Shibata (2000) performed twodimensional simulations including a flux rope in the corona, and their results suggest that the eruption process is triggered by the emerging flux through the reconnection. Our purpose in this paper is to investigate that “how a filament is produced and how an eruption process can be initiated by the emerging flux and what effects of three-dimensionality appear in the process of eruption”. For that purpose, we performed three-dimensional numerical simulations of the emerging flux model. From our results, a filamentary structure is produced from the coronal arcade field by the reconnection process, and when the reconnection process proceeds effectively, the produced structure is ejected by the magnetic force. These processes can thought to be a new mechanism of the eruption whic...
We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar ... more We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. Magnetic flux emergence is the origin of sunspots and active regions, and often associated with solar flares and coronal mass ejections. Owing to the high-resolution achieved by the Earth Simulator, we could simulate the formation of turbulent, fine structures in the emerging flux region as a result of global dynamics. Our simulation results naturally explain many observed features such as arch filament system and intermittent coronal heating.
We present multiwavelength observations of a large-amplitude oscillation of a polar crown filamen... more We present multiwavelength observations of a large-amplitude oscillation of a polar crown filament on 15 October 2002. The oscillation occurred during the slow rise (about 1 km/s) of the filament. It completed three cycles before sudden acceleration and eruption. The oscillation and following eruption were clearly seen in observations recorded by the Extreme-Ultraviolet Imaging Telescope onboard SOHO. The oscillation was seen only in a part of the filament, and it appears to be a standing oscillation rather than a propagating wave. The period of oscillation was about two hours and did not change significantly during the oscillation. We also identified the oscillation as a "winking filament" in the H-alpha images taken by the Flare Monitoring Telescope, and as a spatial displacement in 17 GHz microwave images from Nobeyama Radio Heliograph (NoRH). The filament oscillation seems to be triggered by magnetic reconnection between a filament barb and nearby emerging magnetic flu...
By using the method presented by Isobe et al. (2002), the non-dimensional reconnection rate Vin/V... more By using the method presented by Isobe et al. (2002), the non-dimensional reconnection rate Vin/Va has been determined for the impulsive phase of three two-ribbon flares, where Vin is the velocity of the reconnection inflow and Va is the Alfven velocity. The non-dimensional reconnection rate is important to make a constraint on the theoretical models of magnetic reconnection. In order to reduce the uncertainty of the reconnection rate, it is important to determine the energy release rate of the flares from observational data as accurately as possible. To this end, we have carried out one dimensional hydrodynamic simulations of a flare loop and synthesized the count rate detected by the soft X-ray telescope (SXT) aboard Yohkoh satellite. We found that the time derivative of the thermal energy contents in a flare arcade derived from SXT data is smaller than the real energy release rate by a factor of 0.3 - 0.8, depending on the loop length and the energy release rate. The result of si...
The sun occasionally undergoes the so-called grand minima, in which its magnetic activity, measur... more The sun occasionally undergoes the so-called grand minima, in which its magnetic activity, measured by the number of sunspots, is suppressed for decades. The most prominent grand minima, since the beginning of telescopic observations of sunspots, is the Maunder minimum (1645-1715), when the sunspots became rather scarce. The mechanism underlying the grand minima remains poorly understood as there is little observational information of the solar magnetic field at that time. In this study, we examine the records of one candidate aurora display in China and Japan during the Maunder minimum. The presence of auroras in such mid magnetic latitudes indicates the occurrence of great geomagnetic storms that are usually produced by strong solar flares. However, the records of contemporary sunspot observations from Europe suggest that, at least for the likely aurora event, there was no large sunspot that could produce a strong flare. Through simple theoretical arguments, we show that this geom...
We report observation of a large-amplitude filament oscillation followed by an eruption. This is ... more We report observation of a large-amplitude filament oscillation followed by an eruption. This is used to probe the pre-eruption condition and the trigger mechanism of solar eruptions. We used the EUV images from the Extreme-Ultraviolet Imaging Telescope on board SOHO satellite and the H-alpha images from the Flare Monitoring Telescope at Hida Observatory. The observed event is a polar crown filament that erupted on 15 Oct. 2002. The filament clearly exhibited oscillatory motion in the slow-rising, pre-eruption phase. The amplitude of the oscillation was larger than 20 km/s, and the motion was predominantly horizontal. The period was about 2 hours and seemed to increase during the oscillation, indicating weakening of restoring force. These results strongly indicate that, even in the slow-rise phase before the eruption, the filament retained equilibrium and behaved as an oscillator, and the equilibrium is stable to nonlinear perturbation. Moreover, the transition from such nonlinear s...
The Astronomical Diaries from Babylonia (ADB) are an excellent source of information of natural p... more The Astronomical Diaries from Babylonia (ADB) are an excellent source of information of natural phenomena, including astronomical ones, in pre-Christ era because it contains the record of highly continuous and systematic observations. In this article we present results of a survey of aurora-like phenomena in ADB, spanning from BCE 652 to BCE 61. We have found 9 records of aurora-like phenomena. Philological and scientific examinations suggest 5 of them can be considered as likely candidate for aurora observations. They provide unique information about the solar and aurora activities in the first millennium BCE.
The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4 class flare on 2006 ... more The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4 class flare on 2006 December 13. Typical two-ribbon structure was observed, not only in the chromospheric CaII H line but also in G-band and FeI 6302A line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, the sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge followed by a diffuse inside, as well as previously known core-halo structure. The underlying structures such as umbral dots, penumbral filaments and granules were visible in the flare ribbons. Assuming that the sharp leading edge was directly heated by particle beam and the diffuse parts were heated by radiative back-warming, we estimate the depth of the diffuse flare emission using the intensity profile of the flare ribbon. We found that the depth of the diffuse emission is about 100 km or less from the height of the source of ra...
We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar ... more We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. Magnetic flux emergence is the origin of sunspots and active regions, and often associated with solar flares and coronal mass ejections. Owing to the high-resolution achieved by the Earth Simulator, we could simulate the formation of turbulent, fine structures in the emerging flux region as a result of global dynamics. Our simulation results naturally explain many observed features such as arch filament system and intermittent coronal heating.
The interplanetary magnetic flux rope (IFR) has been a subject of extensive research activity sin... more The interplanetary magnetic flux rope (IFR) has been a subject of extensive research activity since its discovery in 1981 as a key structure in the solar wind that provide important information on the solar eruption phenomena and on how the southward magnetic fields are carried from the Sun to the Earth. In this review, we discuss solar-cycle variation of occurrence frequency of IFRs that still remains unsettled, based on our own results. First, we have found more than 500 IFRs in the time period from 1995 to 2009, whereas the survey by Lepping et al. (AnnGeo, 2006) identified 82 IFRs during 1995-2003. The difference mainly comes from the fact that their survey was not successful in identifying IFRs when the spacecraft passed only near the surface of IFRs. Our result indicates that the rate of IFR occurrence to the ICMEs should be much higher than those which were suggested by previous evaluation. Secondly, the following trend is clearly seen: namely, the occurrence rate of IFRs inc...
The launch of SOT on the Hinode satellite, with it’s previously unprecedented high resolution, hi... more The launch of SOT on the Hinode satellite, with it’s previously unprecedented high resolution, high cadence images of solar prominences, led to the discovery of small scale, highly dynamic flows in quiescent prominences. Berger et al. (2008) reported dark upflows that propagated from the base of the prominence through a height of approximately 10 Mm before ballooning into the familiar mushroom shape often associated with the Rayleigh-Taylor instability. Whether such phenomena can be driven by instabilities and, if so, how the instability evolve is yet to be fully investigated.
We have been developing UFCORIN, an automated space weather prediction system based on machine-le... more We have been developing UFCORIN, an automated space weather prediction system based on machine-learning technologies. Our aim is twofold: one is to provide real-time space weather forecast that thoroughly utilize the huge amount of solar observation data available today. The other is to discover the observational flare-triggering features, by analyzing the big data with the clear goal of predicting the solar flares. UFCORIN stands for Universal Forecast Constructor by Optimized Regression of INputs. As the name suggests, UFCORIN is designed as a generic time-series predictor, which can be set to predict arbitrary time series from arbitrary numbers and kinds of input time series. Using our system we predict maximum of GOES X-ray flux for 24-hour period in the future. As inputs to the predictor, we use wavelet powers of the full disk line-of-sight magnetogram obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). We also use the total magn...
We present the results of two-dimensional magnetohydrodynamic simulations of magnetic reconnectio... more We present the results of two-dimensional magnetohydrodynamic simulations of magnetic reconnection with the effect of ambipolar diffusion, the diffusion of magnetic field due to the ion-neutral collision. It is found that the spatial profile of the current sheet thinning by ambipolar diffusion has a decisive effect on the structure and dynamics of magnetic reconnection. When the scale of the ambiplar diffusion is large, Sweet-Parker like reconnection with an elongated current sheet appears at first, followed by plasmoid instability and intermittent reconnection. When the scale of the ambiplar diffusion is small, Petschek-like reconnection with a localized diffusion region is realized even though the resistivity is uniform. We also found field-aligned outflows from outside the current sheet driven by the gas pressure gradient that is generated by the localized heating of ambipolar diffusion.
Some important observations have shown that the strong correlation between emerging flux and erup... more Some important observations have shown that the strong correlation between emerging flux and eruptions of quiescent filaments (Feynman & Martin, 1995; Wang & Sheeley, 1999). Chen & Shibata (2000) performed twodimensional simulations including a flux rope in the corona, and their results suggest that the eruption process is triggered by the emerging flux through the reconnection. Our purpose in this paper is to investigate that “how a filament is produced and how an eruption process can be initiated by the emerging flux and what effects of three-dimensionality appear in the process of eruption”. For that purpose, we performed three-dimensional numerical simulations of the emerging flux model. From our results, a filamentary structure is produced from the coronal arcade field by the reconnection process, and when the reconnection process proceeds effectively, the produced structure is ejected by the magnetic force. These processes can thought to be a new mechanism of the eruption whic...
We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar ... more We present the result of magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. Magnetic flux emergence is the origin of sunspots and active regions, and often associated with solar flares and coronal mass ejections. Owing to the high-resolution achieved by the Earth Simulator, we could simulate the formation of turbulent, fine structures in the emerging flux region as a result of global dynamics. Our simulation results naturally explain many observed features such as arch filament system and intermittent coronal heating.
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