We found a match
Your institution may have access to this item. Find your institution then sign in to continue.
- Title
MAVEN/NGIMS Dayside Exospheric Temperatures Over Solar Cycle and Seasons: Role of Dayside Thermal Balances in Regulating Temperatures.
- Authors
Bougher, S. W.; Benna, M.; Elrod, M.; Roeten, K.; Thiemann, E.
- Abstract
Several important processes of the Mars upper atmosphere are regulated by exospheric temperature (Texo) variations, including atmospheric escape rates. From the MAVEN mission, significant and largely periodic variability of Mars dayside Texo is now revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected throughout solar cycle 24. Two complementary methods are utilized to extract temperatures from NGIMS data sets. These NGIMS dayside mean exospheric temperatures are shown to vary by ∼80 K (∼180–260 K) over solar cycle 24. This corresponds to a ΔTexo/ΔEUV sensitivity of ∼38 K m2 mW−1, where Lyman‐α is the solar index. Previous Mars Global Surveyor derived Texo values for solar cycle 23 yielded a sensitivity of ∼45 in the same units. This close correspondence suggests that the underlying dayside energy balances are similar yet slightly different over these two solar cycles. Corresponding Mars Global Ionosphere‐Thermosphere Model simulations show that molecular thermal conduction largely balances EUV heating for the Mars dayside thermosphere, while CO2 15‐µm cooling is secondary in importance, along with global winds. It is valuable to compare this Mars sensitivity to that computed for the dayside thermosphere of Venus. Pioneer Venus dayside data sets imply a sensitivity of ∼17.5 K m2 mW−1 units for solar cycle 21, a factor of ∼2.2 smaller than computed for Mars. This is because Venus CO2 cooling strongly balances EUV heating near its peak, thereby serving as a more efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars. Plain Language Summary: Modern Earth global warming of its lower atmosphere is complemented by enhanced CO2 15‐μm cooling in its upper atmosphere. This CO2 cooling process also has a role in the CO2 dominated upper atmospheres of Venus and Mars. From the MAVEN mission, significant and largely periodic variability of Mars dayside exospheric temperatures is revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected during the declining phase of the solar cycle (2014–2019). These NGIMS dayside mean exospheric temperatures vary by ∼80 K (180–260 K) over this weak (low sunspot number) solar cycle 24. Corresponding Global Climate Model (GCM) simulations of the Mars upper atmosphere reproduce this 80 K variability, revealing Extreme Ultraviolet heating is largely conducted downward to lower thermosphere altitudes providing net cooling, while CO2 15‐μm cooling is secondary in importance. By contrast, Pioneer Venus Orbiter dayside data sets (1978–1992) also imply a similar ∼80 K variability at Venus, now for the very strong (∼3‐times larger sunspot number) solar cycle 21. GCM simulations for Venus determine that CO2 cooling strongly balances EUV heating near its peak, thereby serving as an efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars, yielding larger solar cycle variations in exospheric temperatures. Key Points: MAVEN dayside temperatures are shown to vary by 80 K (∼180–260 K) over solar cycle 24, reduced from the Mars Global Surveyor (MGS) value of 120 KMars temperature versus EUV sensitivity is similar for MAVEN and MGS exospheric measurements at 38 and 45 K m2 mW−1Mars thermal conduction balances EUV heating (160–180 km) for solar maximum conditions, while CO2 cooling is secondary
- Subjects
SOLAR cycle; THERMOSPHERE; CLIMATE change models; THERMOSTAT; MARTIAN atmosphere; SOLAR temperature; VENUSIAN atmosphere
- Publication
Journal of Geophysical Research. Planets, 2023, Vol 128, Issue 1, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2022JE007475