The largely unexplored decametre radio band (10–30 MHz) provides a unique window for studying a range of astronomical topics, such as auroral emission from exoplanets, inefficient cosmic ray acceleration mechanisms and fossil radio plasma. The scarcity of low-frequency studies is mainly due to severe ionospheric corruption. Here we present a calibration strategy to correct for the ionosphere in the decametre band. We apply this to an observation from the Low-Frequency Array (LOFAR) between 16 and 30 MHz. The resulting image covers 330 square degrees of sky at a resolution of 45″, reaching a sensitivity of 12 mJy per beam, which is an improvement by an order of magnitude in terms of sensitivity and resolution compared to previous decametre observations. Residual ionospheric effects cause additional blurring between 60″ and 100″. We have identified four fossil plasma sources in the surveyed region. These sources probably harbour rejuvenated radio plasma from past active galactic nuclei outbursts. Three are near the centre of low-mass galaxy clusters. Notably, two of these sources display the steepest radio spectral index among all the sources detected at 23 MHz. This indicates that fossil plasma sources constitute the primary population of steep-spectrum sources at these frequencies, emphasizing the large discovery potential of ground-based decametre observations.

很大程度上未开发的十米无线电频段（10-30 MHz）为研究一系列天文学主题提供了一个独特的窗口，例如系外行星的极光发射、低效的宇宙射线加速机制和化石射电等离子体。低频研究的缺乏主要是由于严重的电离层腐败造成的。在这里，我们提出了一种校准策略来校正十米波段的电离层。我们将其应用于 16 至 30 MHz 之间的低频阵列 (LOFAR) 的观测。所得图像以 45 英寸的分辨率覆盖 330 平方度的天空，达到每束 12 mJy 的灵敏度，与之前的十米观测相比，灵敏度和分辨率提高了一个数量级。残留的电离层效应会导致 60 英寸到 100 英寸之间的额外模糊。我们在调查区域确定了四个化石等离子体源。这些源可能含有来自过去活跃星系核爆发的再生射电等离子体。其中三个位于低质量星系团的中心附近。值得注意的是，其中两个源显示出在 23 MHz 检测到的所有源中最陡的无线电频谱指数。这表明化石等离子体源构成了这些频率下陡光谱源的主要群体，强调了地面十米观测的巨大发现潜力。