OBSERVATIONS OF THE THERMAL AND DYNAMIC EVOLUTION OF A SOLAR MICROFLARE
OBSERVATIONS OF THE THERMAL AND DYNAMIC EVOLUTION OF A SOLAR MICROFLARE
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DOI：
10.1088/0004-637x/692/1/492
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发表时间：
2009-02
期刊：
影响因子：
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通讯作者：
J. Brosius;G. Holman
中科院分区：
文献类型：
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作者：
J. Brosius;G. Holman
我们利用SOHO航天器上的三种仪器（日冕诊断光谱仪（CDS）、极紫外成像望远镜（EIT）和迈克尔逊多普勒成像仪（MDI））、TRACE （1600 Å）、GOES和RHESSI在很宽的温度范围内观测到了太阳微耀斑。微耀斑的性质和行为是经历温和色球蒸发的微型耀斑，可能是由非热电子驱动的。在快速节奏（9.8 s）下，用CDS在瞪眼模式下获得了极紫外光谱，其中包括来自色球（形成温度Tm≈1 × 104 K）和过渡区（TR）到日冕和耀斑（Tm≈8 × 106 K）温度的发射谱线。从CDS光谱和TRACE图像（以可变节奏≈34 s获得）得出的光曲线显示，微耀斑发生前有两个前驱体增亮。在前驱体之后，色球和TR发射首先增加，这与非热电子的能量沉积一致。在最初的缓慢上升之后，色球和TR线出现了短暂的（20 s）脉冲EUV爆发，在此期间，日冕和热耀斑发射逐渐开始增加。CDS测量的相对多普勒速度呈上升趋势，在第二次前驱期间和脉冲峰值前不久，其最大值约为20 km s−1，表明色球蒸发缓慢。由oiv线强度比（Tm≈1.6 × 105 K）得到的电子密度从静止时的2.6 × 1010 cm−3增加到脉冲峰时的5.2 × 1011 cm−3。RHESSI观测到的x射线发射在色球和TR温度的脉冲峰之后达到峰值，没有发现非热电子发射的证据。与RHESSI数据的光谱拟合表明最高温度约为13 MK，与GOES数据推断的温度略低一致。MDI的磁图显示，微耀斑发生在一个嵌在大面积正磁场中的负磁极岛内及其周围。微耀斑结构紧凑，在195 Å的EIT图像中覆盖面积为4 × 107 km2，在RHESSI图像中以位于EIT源以西7″的点源出现。TRACE图像显示微耀斑充满了小圈。
We observed a solar microflare over a wide temperature range with three instruments aboard the SOHO spacecraft (Coronal Diagnostic Spectrometer (CDS), Extreme-ultraviolet Imaging Telescope (EIT), and Michelson Doppler Imager (MDI)), TRACE (1600 Å), GOES, and RHESSI. The microflare's properties and behavior are those of a miniature flare undergoing gentle chromospheric evaporation, likely driven by nonthermal electrons. Extreme-ultraviolet spectra were obtained at a rapid cadence (9.8 s) with CDS in stare mode that included emission lines originating from the chromosphere (temperature of formation Tm ≈ 1 × 104 K) and transition region (TR), to coronal and flare (Tm ≈ 8 × 106 K) temperatures. Light curves derived from the CDS spectra and TRACE images (obtained with a variable cadence ≈34 s) reveal two precursor brightenings before the microflare. After the precursors, chromospheric and TR emission are the first to increase, consistent with energy deposition by nonthermal electrons. The initial slow rise is followed by a brief (20 s) impulsive EUV burst in the chromospheric and TR lines, during which the coronal and hot flare emission gradually begin to increase. Relative Doppler velocities measured with CDS are directed upward with maximum values ≈20 km s−1 during the second precursor and shortly before the impulsive peak, indicating gentle chromospheric evaporation. Electron densities derived from an O iv line intensity ratio (Tm ≈ 1.6 × 105 K) increased from 2.6 × 1010 cm−3 during quiescent times to 5.2 × 1011 cm−3 at the impulsive peak. The X-ray emission observed by RHESSI peaked after the impulsive peak at chromospheric and TR temperatures and revealed no evidence of emission from nonthermal electrons. Spectral fits to the RHESSI data indicate a maximum temperature of ≈13 MK, consistent with a slightly lower temperature deduced from the GOES data. Magnetograms from MDI show that the microflare occurred in and around a growing island of negative magnetic polarity embedded in a large area of positive magnetic field. The microflare was compact, covering an area of 4 × 107 km2 in the EIT image at 195 Å, and appearing as a point source located 7″ west of the EIT source in the RHESSI image. TRACE images suggest that the microflare filled small loops.