Anomalous Dawn‐Side O + Density Enhancement in the Martian Ionosphere: MAVEN Observations and Photochemical Modeling

The dominant ions in the dayside Martian ionosphere are O 2 + , O + , and CO 2 + , with NO + becoming increasingly important on the nightside. Generally, ion densities are higher during the daytime than at nighttime and are typically enhanced on the dusk‐side relative to the dawn‐side. This behavior is primarily attributed to strong dayside photoionization, followed by continuous plasma depletion via recombination and day‐to‐night transport as the ionosphere rotates into darkness. In this study, observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft reveal a unique feature during Martian Year (MY) 37. During this period, a pronounced enhancement in O + density is observed on the dawn‐side compared to the dusk‐side, a behavior not seen in O 2 + and CO 2 + ions. Further analysis reveals that the O/CO 2 density ratio is significantly elevated, indicating a concurrent reduction in CO 2 density and an increase in atomic oxygen. Photochemical model calculations demonstrate that although O + production rate on the dawn‐side is ∼0.60 times the dayside value, its chemical loss frequency is approximately four times smaller. This reduced loss frequency substantially increases the chemical lifetime of O + , allowing ions to accumulate despite weaker production. The enhanced atomic oxygen abundance is likely driven by increased EUV‐induced photodissociation of CO 2 on the dayside, producing CO and O, followed by the transport of O atoms toward the midnight and dawn sectors through global thermospheric circulation. The enhanced O abundance and together with reduced CO 2 availability, substantially limits CO 2 ‐driven O + loss. This mechanism explains the anomalous dawn‐side O + enhancement in the Martian ionosphere.