Loops are fundamental structures in the magnetized atmosphere of the Sun. Their physical properties are crucial for understanding the nature of the solar atmosphere. Transition region loops are relatively dynamic and their physical properties have not yet been fully understood. With spectral data of the line pair of O iv 1399.8 Å and 1401.2 Å ( Tmax=1.4×105 K) of 23 transition region loops obtained by IRIS, we carry out the first systematic analyses to their loop lengths (L), electron densities (n _e ), and effective temperatures. We found electron densities, loop lengths, and effective temperatures of these loops are in the ranges of 8.9 × 10⁹–3.5 × 10¹¹ cm⁻³, 8–30 Mm, and 1.9 × 10⁵–1.3 × 10⁶ K, respectively. At a significant level of 90%, regression analyses show that the relationship between electron densities and loop lengths is n _e [cm⁻³] ∝ (L[Mm])^{−0.78±0.42}, while the dependences of electron densities on effective temperatures and that on the line intensities are not obvious. These observations demonstrate that transition region loops are significantly different than their coronal counterparts. Further studies on the theoretical aspect based on the physical parameters obtained here are of significance for understanding the nature of transition region loops.

中文翻译：

---

从 IRIS O iv 光谱数据导出的过渡区环的电子密度

环是太阳磁化大气中的基本结构。它们的物理特性对于理解太阳大气的性质至关重要。过渡区环路是相对动态的，其物理特性尚未完全了解。具有 O 线对的光谱数据四号1399.8 埃和 1401.2 埃（ 时间最大限度=1.4×105 在IRIS获得的23个过渡区环中，我们对它们的环长度进行了第一次系统分析（L), 电子密度 (n _e ）和有效温度。我们发现这些环的电子密度、环长度和有效温度分别在8.9 × 10 ⁹ –3.5 × 10 ¹¹ cm ^-3、8–30 Mm和1.9 × 10 ⁵ –1.3 × 10 ⁶ K范围内。在 90% 的显着水平上，回归分析表明电子密度和环长度之间的关系为n _e [cm ⁻³ ] ∝ (L[Mm]) ^-0.78±0.42，而电子密度对有效温度和线强度的依赖性并不明显。这些观察结果表明，过渡区环与其冠状区环明显不同。基于这里获得的物理参数的理论方面的进一步研究对于理解过渡区环路的本质具有重要意义。