We have studied the variation of the time profile of X-ray emission of solar flares that occurred during the second half of solar cycle 23 (SC 23) and for about the full solar cycle 24 (SC 24) (2002–2018). We define a new index, called the “ratio index” (R_f), for all X-ray solar flares. This index is defined as the ratio of the flare’s rising time interval by its total duration period. According to the ratio index, the X-ray solar flares are classified into two types: (1) sudden flares [R_f < 0.5], and (2) gradual flares [R_f > 0.5]. The sudden flare type, with fast-rising and slow recovery, is more common and represents most of the flares that happen most of the time during the solar cycles but are less common during the minimum solar activity years. On the other hand, the gradual flare type (or R_f > 0.5) is less common but predominates during the minimum solar activity epochs. Sudden flares tend to be strong, large, and numerous in the polar regions, while gradual flares are weak, short, and countable in the latitude range between 50 and 70, both for northern and southern latitudes. However, both types appear to happen in the lower latitudes and the solar equatorial regions.

中文翻译：

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X射线太阳耀斑时间分布的变化

我们研究了在太阳周期 23 (SC 23) 的后半期和整个太阳周期 24 (SC 24) (2002-2018) 期间发生的太阳耀斑 X 射线发射的时间分布的变化。我们为所有 X 射线太阳耀斑定义了一个新的指数，称为“比率指数”（R _{f ）。}该指数被定义为耀斑的上升时间间隔与其总持续时间的比率。根据比率指数，X射线太阳耀斑分为两种类型：（1）突然耀斑[ R _f < 0.5]，和（2）渐进耀斑[ R _f> 0.5]。具有快速上升和缓慢恢复的突然耀斑类型更为常见，代表了太阳周期中大部分时间发生的大多数耀斑，但在太阳活动最小年期间不太常见。另一方面，渐进式耀斑（或R _f > 0.5）不太常见，但在最小太阳活动时期占主导地位。在极地地区，突然耀斑往往强烈、大且数量多，而在北纬和南纬 50 至 70 纬度范围内，渐进耀斑则弱、短且可数。然而，这两种类型似乎都发生在低纬度地区和太阳赤道地区。