{"title":"Exobase and homopause altitudes in the Martian upper atmosphere: Solar cycle variability and response to the 2018 global dust storm","authors":"N.V. Rao , V. Leelavathi , Neha Gupta","doi":"10.1016/j.icarus.2025.116565","DOIUrl":null,"url":null,"abstract":"<div><div>Understanding the composition of volatile gases in the Martian upper atmosphere and the mechanisms governing their escape into outer space is crucial for unraveling planetary evolution. Two critical neutral atmospheric boundaries, the exobase and the homopause, play key roles in this context. In this study, we conducted a detailed analysis of the exobase and homopause altitudes derived from measurements by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. The exobase and homopause altitudes observed in this study range from 140 to 200 km and 90–135 km, respectively, broadly agreeing with values reported in previous studies. Using extended observations, this study reveals significant interannual variability in these altitudes. Both the exobase and homopause were found to be higher during moderate solar activity compared to solar minimum. During moderate solar activity, these two altitudes rise and fall nearly synchronously, while this coupling weakens during solar minimum. Seasonal trends are also pronounced, with elevated altitudes observed during perihelion and in the summer hemisphere, particularly during the southern summer. These findings suggest that enhanced solar activity during moderate solar periods increases atmospheric heating, elevating atmospheric scale heights and causing the exobase and homopause to rise and fall in-phase. In contrast, during solar minimum, weaker thermal forcing reduces the influence of atmospheric expansion, allowing gravity wave activity to dominate the variability of the homopause. During the 2018 global dust storm (GDS), exobase altitudes were elevated with reduced sinusoidal amplitude, while homopause altitudes exhibited significant fluctuations. These results indicate that thermospheric expansion raises the altitudes of both boundaries during the GDS, but enhanced gravity wave activity introduces substantial fluctuations in the homopause altitude, disrupting its coupling with the exobase and resulting in a phase shift between the variations of these two critical altitudes.</div></div>","PeriodicalId":13199,"journal":{"name":"Icarus","volume":"435 ","pages":"Article 116565"},"PeriodicalIF":2.5000,"publicationDate":"2025-03-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Icarus","FirstCategoryId":"101","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0019103525001125","RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"ASTRONOMY & ASTROPHYSICS","Score":null,"Total":0}
引用次数: 0
Abstract
Understanding the composition of volatile gases in the Martian upper atmosphere and the mechanisms governing their escape into outer space is crucial for unraveling planetary evolution. Two critical neutral atmospheric boundaries, the exobase and the homopause, play key roles in this context. In this study, we conducted a detailed analysis of the exobase and homopause altitudes derived from measurements by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. The exobase and homopause altitudes observed in this study range from 140 to 200 km and 90–135 km, respectively, broadly agreeing with values reported in previous studies. Using extended observations, this study reveals significant interannual variability in these altitudes. Both the exobase and homopause were found to be higher during moderate solar activity compared to solar minimum. During moderate solar activity, these two altitudes rise and fall nearly synchronously, while this coupling weakens during solar minimum. Seasonal trends are also pronounced, with elevated altitudes observed during perihelion and in the summer hemisphere, particularly during the southern summer. These findings suggest that enhanced solar activity during moderate solar periods increases atmospheric heating, elevating atmospheric scale heights and causing the exobase and homopause to rise and fall in-phase. In contrast, during solar minimum, weaker thermal forcing reduces the influence of atmospheric expansion, allowing gravity wave activity to dominate the variability of the homopause. During the 2018 global dust storm (GDS), exobase altitudes were elevated with reduced sinusoidal amplitude, while homopause altitudes exhibited significant fluctuations. These results indicate that thermospheric expansion raises the altitudes of both boundaries during the GDS, but enhanced gravity wave activity introduces substantial fluctuations in the homopause altitude, disrupting its coupling with the exobase and resulting in a phase shift between the variations of these two critical altitudes.
期刊介绍:
Icarus is devoted to the publication of original contributions in the field of Solar System studies. Manuscripts reporting the results of new research - observational, experimental, or theoretical - concerning the astronomy, geology, meteorology, physics, chemistry, biology, and other scientific aspects of our Solar System or extrasolar systems are welcome. The journal generally does not publish papers devoted exclusively to the Sun, the Earth, celestial mechanics, meteoritics, or astrophysics. Icarus does not publish papers that provide "improved" versions of Bode''s law, or other numerical relations, without a sound physical basis. Icarus does not publish meeting announcements or general notices. Reviews, historical papers, and manuscripts describing spacecraft instrumentation may be considered, but only with prior approval of the editor. An entire issue of the journal is occasionally devoted to a single subject, usually arising from a conference on the same topic. The language of publication is English. American or British usage is accepted, but not a mixture of these.