Magnetohydrodynamic Simulation of a Solar Flare with Chromospheric Evaporation Effect Based on the Magnetic Reconnection Model
Magnetohydrodynamic Simulation of a Solar Flare with Chromospheric Evaporation Effect Based on the Magnetic Reconnection Model
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DOI：
10.1086/319440
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发表时间：
2001-03
期刊：
影响因子：
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通讯作者：
T. Yokoyama;K. Shibata
中科院分区：
文献类型：
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作者：
T. Yokoyama;K. Shibata
基于磁重联模型，对一次太阳耀斑进行了包括各向异性热传导效应和色球蒸发效应在内的二维磁流体动力学模拟。在模拟模型中，日冕磁能通过磁重联转化为等离子体的热能。这种能量沿着磁力线通过热传导传递到色球，导致色球等离子体的温度和压力增加。压力梯度力驱使等离子体向日冕向上运动，即色球蒸发。这增强了日冕重新连接的耀斑环的密度，这种蒸发的等离子体被认为是观测到的耀斑软x射线发射的来源。结果表明，其温度分布与Yohkoh卫星上的软x射线望远镜观测到的长持续事件（LDE）耀斑的尖状结构相似。模拟结果可以用一个简单的耀斑温度标度定律来理解，其中top、B、ρ和κ0分别为耀斑环顶温度、日冕磁场强度、日冕密度和热传导系数。在模拟中对B、κ0、L进行了广泛的参数调查，证实了这一公式。能量释放率随时间线性增加：|dEm/dt|≈B2/(4π)VinCAt≈B2/(4π)0.1Ct，其中Em为磁能，Vin为流入速度，CA为alfv<s:1>速度。因此，二阶导数为|d2Em/dt2|∝B4。我们还发现，重联流入区的主要特征是从磁中性点向外传播的膨胀波。这种膨胀的等离子体具有非常低的发射量，比耀斑中最亮的特征小4个数量级。这就解释了与耀斑有关的变暗现象。
Two-dimensional magnetohydrodynamic (MHD) simulation of a solar flare including the effect of anisotropic heat conduction and chromospheric evaporation based on the magnetic reconnection model is performed. In the simulation model, the coronal magnetic energy is converted to the thermal energy of plasma by magnetic reconnection. This energy is transported to the chromosphere by heat conduction along magnetic field lines and causes an increase in temperature and pressure of the chromospheric plasma. The pressure gradient force drives upward motion of the plasma toward the corona, i.e., chromospheric evaporation. This enhances the density of the coronal reconnected flare loops, and such evaporated plasma is considered to be the source of the observed soft X-ray emission of a flare. The results show that the temperature distribution is similar to the cusp-shaped structure of long-duration-event (LDE) flares observed by the soft X-ray telescope aboard the Yohkoh satellite. The simulation results are understood by a simple scaling law for the flare temperature described as where Ttop, B, ρ, and κ0 are the temperature at the flare loop top, coronal magnetic field strength, coronal density, and heat conduction coefficient, respectively. This formula is confirmed by the extensive parameter survey about B, κ0, and L in the simulation. The energy release rate is found to be described as a linearly increasing function of time: |dEm/dt| ≈ B2/(4π)VinCAt ≈ B2/(4π)0.1Ct, where Em is the magnetic energy, Vin is the inflow velocity, and CA is the Alfvén velocity. Thus, the second time derivative is found to be |d2Em/dt2| ∝ B4. We also find that the major feature of the reconnection inflow region is the expansion wave propagating outward from the magnetic neutral point. This expanded plasma has very low emission measure, which is 4 orders of magnitude smaller than that of the brightest feature in a flare. This explains the dimming phenomena associated with flares.