Jasinski Jamie M, Cassidy Timothy A, Raines Jim M, Milillo Anna, Regoli Leonardo H, Dewey Ryan, Slavin James A, Mangano Valeria, Murphy Neil
NASA Jet Propulsion Laboratory California Institute of Technology Pasadena CA USA.
Laboratory of Atmospheric and Space Sciences University of Colorado Boulder Boulder CO USA.
Geophys Res Lett. 2021 Apr 28;48(8):e2021GL092980. doi: 10.1029/2021GL092980.
We present the first investigation and quantification of the photoionization loss process to Mercury's sodium exosphere from spacecraft and ground-based observations. We analyze plasma and neutral sodium measurements from NASA's MESSENGER spacecraft and the THEMIS telescope. We find that the sodium ion (Na) content and therefore the significance of photoionization varies with Mercury's orbit around the Sun (i.e., true anomaly angle: TAA). Na production is affected by the neutral sodium solar-radiation acceleration loss process. More Na was measured on the inbound leg of Mercury's orbit at 180°-360° TAA because less neutral sodium is lost downtail from radiation acceleration. Calculations using results from observations show that the photoionization loss process removes ∼10 atoms/s from the sodium exosphere (maxima of 4 × 10 atoms/s), showing that modeling efforts underestimate this loss process. This is an important result as it shows that photoionization is a significant loss process and larger than loss from radiation acceleration.
我们通过航天器和地面观测首次对水星钠逸散层的光电离损失过程进行了研究和量化。我们分析了美国国家航空航天局信使号航天器和THEMIS望远镜的等离子体和中性钠测量数据。我们发现，钠离子（Na⁺）含量以及光电离的重要性会随着水星绕太阳的轨道（即真近点角：TAA）而变化。钠的产生受到中性钠太阳辐射加速损失过程的影响。在水星轨道180° - 360° TAA的入轨段测量到了更多的钠，因为从辐射加速向下游损失的中性钠较少。利用观测结果进行的计算表明，光电离损失过程每秒从钠逸散层中移除约10个原子（最大值为4×10个原子/秒），这表明模型计算低估了这一损失过程。这是一个重要的结果，因为它表明光电离是一个显著的损失过程，且比辐射加速造成的损失更大。
