摘要
我们用新的时间分辨率短至８ｍｓ 的２ ．６ ～３ ．８ＧＨｚ 微波动态频谱仪在１９９８ 年４ 月１５ 日爆发和１９９７ 年１１ 月３ 日爆发中发现了微Ｕ 型爆发。其顶部频率为３ ．２ ～３ ．４ＧＨｚ（ 相应的等离子体密度：基波：１ ．２ ×１０１１ － １ ．４ ×１０１１／ｃｍ ３ ，二次谐波：３ ．２ ×１０１０ － ３ ．５ ×１０１０／ｃｍ ３） ；根部频率为３ ．４ ～３ ．６ＧＨｚ；单个Ｕ 型爆发的频率范围为６０ ～２２０ ＭＨｚ ；上升段频漂率为７ ～２８ＧＨｚ／ｓ；上升段持续时间为８ ～２４ｍｓ；寿命为１６ ～４８ｍｓ；偏振度大于８０ ％ 。在活动区为偶极子磁场的假设下，估计源区高度约为１ ．３ ×１０４ 公里，单一环的高度为２５０ ～８００ 公里。由此得出结论：１ ．由于高频漂率和高偏振度，似乎发现的微Ｕ 型爆发不是Ⅲ型爆发形成，而是尖峰辐射（Ｓｐｉｋｅ） 形成。２ ．我们发现的是小尺度的微磁环，其尺度与Ｓｐｉｋｅ 辐射的寿命相当。我们在１９９７ 年１１ 月２ 日的爆发中发现平均周期为数十ｍｓ 的准周期振荡群。在高密度流管的磁声波ＭＨＤ 振荡条件下，可取得磁环半径约９０ 公里的结果。
The mini U bursts have been found with the 2.6～3.8 GHz microwave radiospectrograph with time resolution of as short as 8 msec in the microwave bursts on April 15, 1998 and Nov.3, 1997. The top (Maximum) frequencies are 3.2～3.4 GHz, corresponding to a range of plasma density: fundamental -1.2×10 11 to 1.4×10 11 /cm 3; second harmonic -3.2×10 10 to 3.5×10 10 /cm 3. The root (minimum) frequencies are 3.6～3.6 GHz. The frequency ranges for dividual mini U bursts are 60～200MHz. The frequency drift rates in rising branch are 7～28 GHz/s. The durations of rising branch are 8～24 msec. The full durations are 16～48 msec. The polarization degree is more than 80%. Assuming that the magnetic field of active region is a vertical diople below photosphere, we estimated that the altitudes of the sources are about 1.3×10 4 km, and the heights of single loop are about 500～800km. This study demonstrates, for the first time: 1. According to high frequency drift rate and high polarization degree, it seems likely that the nature of mini U bursts prefers spike emissions rather than type Ⅲ bursts. 2. These phenomena are mini scale loops, compatible with the duration of spike emissions. A quasi period pulsation cluster with the average period of 37 msec was found in the microwave burst on Nov.2, 1997. For magnetoacoustic MHD oscillations of dense flux tubes, it can be deduced that the radius of the loops is about 90 km. Then a physical pattern of mini magnetic loops forms.
