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Hiroaki Isobe   Submitted: 2007-12-04 21:51

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-1) of the filament. It completed three cycles before sudden acceleration and eruption. The oscillation and following eruption were clearly seen in observations recorded by SOHO/EIT. 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 amplitudes of velocity and spatial displacement of the oscillation in the plane of the sky were about 5 km s-1 and 15,000 km, respectively. The period of oscillation was about two hours and did not change significantly during the oscillation. The oscillation was also observed in Hα by the Flare Monitoring Telescope at Hida Observatory. We determine the three-dimensional motion of the oscillation from the Hα wing images. The maximum line-of-sight velocity was estimated to be a few tens of km s-1, though the uncertainty is large owing to the lack of the line-profile information. Furthermore, we also identified the spatial displacement of the oscillation 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 flux as was evident from the MDI magnetogram observations. No flare was observed to be associated with the onset of the oscillation. We also discuss possible implications of the oscillation as a diagnostic tool for the eruption mechanisms. We suggest that in the early phase of eruption a part of the filament lost its equilibrium first, while the remaining part was still in an equilibrium and oscillated.

Authors: H. Isobe, D. Tripathi, A. Asai, R. Jain
Projects: SoHO-EIT

Publication Status: Solar Physics, in press.
Last Modified: 2007-12-05 07:14
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Ellerman Bombs and Jets Associated with Resistive Flux Emergence  

Hiroaki Isobe   Submitted: 2007-01-30 00:17

Using two-dimensional magnetohydrodynamic simulations we study the effects of resistive processes in the dynamics of magnetic flux emergence and its relation to Ellerman bombs and other dynamic phenomena in the Sun. The widely accepted scenario of flux emergence is the formation and expansion of Omega-shaped loops due to the Parker instability. Since the Parker instability has the largest growth rate at finite wavelength lambda_p sim 10-20 H, where H is the scale height (approx 200 km in the solar photosphere), a number of magnetic loops may rise from the initial flux sheet if it is sufficiently long. This process is shown in our numerical simulations. The multiple emerging loops expand in the atmosphere and interact eath other, leading to magnetic reconnection. At first reconnection occurs in the lower atmosphere, which allows the sinking part of the flux sheet to emerge above the photosphere. This reconnection also causes local heating that may account for Ellerman bombs. In the later stage, reconnection between the expanding loops occurs at higher levels of the atmosphere and creates high temperature reconnection jets, and eventually a large (gg lambda_p) coronal loop is formed. Cool and dense plasma structures, which are similar to Hα surges, are also formed. This is not because of magnetic reconnection but due to the compression of the plasma in between the expanding loops.

Authors: H. Isobe, D. Tripathi, and V. Archontis
Projects: None

Publication Status: ApJL accepted.
Last Modified: 2007-01-30 08:14
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Three-Dimensional Simulation of Solar Emerging Flux Using the Earth Simulator I. Magnetic Rayleigh-Taylor Instability at the Top of the Emerging Flux as the Origin of Filamentary Structure  

Hiroaki Isobe   Submitted: 2006-04-24 11:13

We present the results of three-dimensional magnetohydrodynamic simulations of solar emerging flux and its interaction with pre-existing coronal field. In order to resolve the fine structures and the current sheets, we used high resolution grids with up to 800x400x620 point, and the calculation was carried out using the Earth Simulator. The model set up is an extension of previous two dimensional simulation by Yokoyama and Shibata (1995) to include the variation along the third direction. Based on the same simulation result we reported in our previous paper (Isobe et al. 2005): (1) Dense filaments similar to Hα arch filament system are spontaneously formed in the emerging flux by the magnetic Rayleigh-Taylor type instability. (2) Filamentary current sheets are created in the emerging flux due to the nonlinear development of the magnetic Rayleigh-Taylor instability, which may cause the intermittent, nonuiform heating of the corona. (3) Magnetic reconnection between the emerging flux and preexisting coronal field occurs in a spatially intermittent way. In this paper we describe the simulation model and discuss the origin and the properties of the magnetic Rayleigh-Taylor instabiity in detail. It is shown that the top-heavy configuration that causes the instability is formed by the intrinsic dynamics of emerging flux.

Authors: H. Isobe, T. Miyagoshi, K. Shibata, T. Yokoyama
Projects: None

Publication Status: PASJ, 58, 423 (2006)
Last Modified: 2006-04-24 14:29
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Large amplitude oscillation of a polar crown filament in the pre-eruption phase  

Hiroaki Isobe   Submitted: 2006-02-20 07:43

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 Hα 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-1, 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 stability to either instabilities or a loss of equilibrium that leads to the eruption occurred in the Alfvén time scale. This suggests that the onset of the eruption was triggered by a fast magnetic reconnection that stabilized the pre-eruption magnetic configuration, rather than by the slow shearing motion at the photosphere.

Authors: Hiroaki Isobe, Durgesh Tripathi
Projects:

Publication Status: A&A, 449, L17 (2006)
Last Modified: 2006-04-24 11:18
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Measurement of the Energy Release Rate and the Reconnection Rate in Solar Flares  

Hiroaki Isobe   Submitted: 2005-08-11 11:33

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 Alfvén 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 simulation is presented in the paper and used to calculate the reconnection rate. We found that reconnection rate is 0.047 for the X2.3 flare on 2000 November 24, 0.015 for the M3.7 flare on 2000 July 14, and 0.071 for the C8.9 flare on 2000 November 16. These values are similar to that derived from the direct observation of the reconnection inflow by Yokoyama et al. (2001), and consistent with the fast reconnection models such as that of Petschek (1964)

Authors: Hiroaki Isobe, Hiroyuki Takasaki, and Kazunari Shibata
Projects: Yohkoh-SXT

Publication Status: ApJ (in press)
Last Modified: 2005-08-11 11:33
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Abstracts by Author
Subject will be restored when possible
Ellerman Bombs and Jets Associated with Resistive Flux Emergence
Three-Dimensional Simulation of Solar Emerging Flux Using the Earth Simulator I. Magnetic Rayleigh-Taylor Instability at the Top of the Emerging Flux as the Origin of Filamentary Structure
Large amplitude oscillation of a polar crown filament in the pre-eruption phase
Measurement of the Energy Release Rate and the Reconnection Rate in Solar Flares

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