Supergranules, which are solar flow features with a lateral scale of 30,000–40,000 km and a lifetime of ~24 h, form a prominent component of the Sun’s convective spectrum. However, their internal flows, which can be probed only by helioseismology, are not well understood. We analyse dopplergrams recorded by the Solar Dynamics Observatory satellite to identify and characterize ~23,000 supergranules. We find that the vertical flows peak at a depth of ~10,000 km, and remain invariant over the full range of lateral supergranular scales, contrary to numerical predictions. We also infer that, within the local seismic resolution (≳5,000 km), downflows are ~40% weaker than upflows, indicating an apparent mass-flux imbalance. This may imply that the descending flows also comprise plumes, which maintain the mass balance but are simply too small to be detected by seismic waves. These results challenge the widely used mixing-length description of solar convection.

超颗粒是太阳流特征，横向尺度为 30,000-40,000 公里，寿命约为 24 小时，是太阳对流光谱的重要组成部分。然而，它们的内部流动只能通过日震学来探测，目前尚不清楚。我们分析太阳动力学观测站卫星记录的多普勒图，以识别和表征约 23,000 个超粒子。我们发现垂直流在深度约 10,000 公里处达到峰值，并且在横向超颗粒尺度的整个范围内保持不变，这与数值预测相反。我们还推断，在当地地震分辨率（≳5,000 km）内，下降流比上升流弱约 40%，表明存在明显的质量通量不平衡。这可能意味着下降流还包括羽流，羽流保持质量平衡，但太小而无法被地震波检测到。这些结果对广泛使用的太阳对流混合长度描述提出了挑战。