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A New Solar Flare Scenario: High-beta Plasma Disruption  

Kiyoto Shibasaki   Submitted: 2003-01-14 23:02

A new solar flare scenario is proposed based on a high-beta plasma instability. Plasma confined in a curved magnetic field (magnetic loop) has free energy. This free energy will be converted to other forms explosively under certain conditions through localized quasi-interchange instability called ``ballooning instability". Relations between solar flare phenomena and ballooning instability phenomena are discussed.

Authors: Kiyoto Shibasaki
Projects:

Publication Status: ESA SP-506, 749 (2002)
Last Modified: 2003-01-14 23:02
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Signature of Energy Release and Particle Acceleration Observed by the Nobeyama Radioheliograph  

Kiyoto Shibasaki   Submitted: 2003-01-14 22:58

Microwave imaging observations of solar flares are presented and a new scenario for solar flares is proposed. Microwaves are effectively emitted by high-energy electrons gyrating in active region magnetic fields. Higher harmonics (10 - 100) of the gyro-frequency in active regions, excited by mildly relativistic electrons, correspond to microwaves. Imaging observations of strong microwave emission associated with solar flares make it possible to study where and how the high-energy electrons are created in solar flares, which is one of the long-standing questions of solar flares. Hot and dense plasma created by solar flares also emits microwaves by the free-free mechanism although usually weak compared to the non-thermal emission. It is shown that flares start in a small loop and also shown that hot plasmas and high-energy electrons are fed into a nearby larger loop from the small one. Based on these observations and other observations, it is proposed that ``high-beta disruption' is the cause of solar flare phenomena.

Authors: Kiyoto Shibasaki
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Publication Status: Lecture Note in Physics, 612, 94 (2003)
Last Modified: 2003-02-21 11:08
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Energy and Mass Supply in the Decay Phase of Long Duration Solar Flare Events  

Kiyoto Shibasaki   Submitted: 2002-01-29 19:38

A new mechanism is proposed to supply energy and mass in the decay phase of long duration solar flare events (LDEs). LDEs are known to be caused by filament eruptions and bright arcade structures continue for several hours or even a day. To overcome the short cooling time, continuous energy and mass supply is required. Continuous magnetic reconnection in the current sheet extended above the arcade as been proposed to supply energy and mass. However, no direct observations of the current sheet have been reported. The new mechanism that we are proposing is based on the recent observations of down flows onto the top of arcades along the vertical spiky features observed by {it Yohkoh} Soft X-ray Telescope. We interpret these down flows as free-falling plasma. Falling plasma from high altitude can convert its potential energy into thermal energy and heat itself with an average temperature of several million-degree Kelvin. Observations show that the down flows continue for several hours. Coronagraph observations by SOHO/LASCO also show many falling features after coronal mass ejections. We propose these falling plasmas as the cause of the energy and mass supply for LDE events in the long lasting decay phase instead of magnetic reconnection.

Authors: Kiyoto Shibasaki
Projects:

Publication Status: ApJL(in press)
Last Modified: 2002-01-29 19:38
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Periodic Acceleration of Electrons in the 1998 November 10 Solar Flare  

Kiyoto Shibasaki   Submitted: 2001-10-16 23:27

We present an examination of the multi-wavelength observation of a C7.9 flare which occurred on 1998 November 10. This is the first time of imaging observation of the quasi-periodic pulsations (QPPs). Four bursts were observed with the hard X-ray telescope aboard Yohkoh and the Nobeyama Radioheliograph during the impulsive phase of the flare. In the second burst, the hard X-ray and microwave time profiles clearly showed a QPP. We estimated the Alfvén transit time along the flare loop using the images of the soft X-ray telescope aboard Yohkoh and the photospheric magnetograms, and found that the trnasit time was almost equal to the period of the QPP. We therefore suggest, based on a shock acceleration model, that variations of macroscopic magnetic structures, such as oscillations of coronal loops, affect the efficiency of particle injection/acceleration.

Authors: A. Asai, M. Shimojo, H. Isobe, T. Morimoto, T. Yokoyama, K. Shibasaki, and H. Nakajima
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Publication Status: ApJL(in press)
Last Modified: 2001-10-16 23:27
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High-beta Disruption in the Solar Atmosphere  

Kiyoto Shibasaki   Submitted: 2001-05-10 11:48

The outer layers of X-ray loops in solar flares are known to be more active: they are hotter than the lower loops and above-the-loop-top hard X-ray sources are formed there. These phenomena are interpreted as the result of the reconnection above the loop, which convert magnetic energy into thermal and non-thermal energy of plasma. However, little direct evidence for the reconnection has been presented so far. This paper interprets the activity in the outer layer of flaring loops on a different scenario. Coronal loops filled with hot and dense plasma (high beta) or with fast plasma flow, surrounded by the low-beta corona, are unstable at their outer boundary where the curvature is convex outward and the density gradient is inward. The centrifugal force acting upward on the plasma in the loop can exceed that of gravity. This condition is favorable for localized interchange instability called "ballooning instability" and the plasma in the loop is ejected when the instability has developed into non-linear phase ("high-beta disruption"). This is a natural consequence of the high-beta (and/or the high velocity) plasma confined in a curved coronal loop. The high-beta disruption has many elements common to solar flares. In this paper, importance of the high-beta plasma is stressed, which is a neglected part of the solar activity so far.

Authors: Kiyoto Shibasaki
Projects:

Publication Status: ApJ (in press)
Last Modified: 2001-05-10 11:48
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Abstracts by Author
A New Solar Flare Scenario: High-beta Plasma Disruption
Signature of Energy Release and Particle Acceleration Observed by the Nobeyama Radioheliograph
Energy and Mass Supply in the Decay Phase of Long Duration Solar Flare Events
Periodic Acceleration of Electrons in the 1998 November 10 Solar Flare
High-beta Disruption in the Solar Atmosphere

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