|
|
|
|
A preliminary discussion of the needs and possible formulations of a model ionosphere for Mars occurred at the COSPAR meeting in Paris in 2004 [1]. That effort then became part of the COSPAR sub-group dealing with reference atmospheres and ionospheres in the Solar System. Since that time, a remarkable amount of new observations became available from Mars Global Surveyor (MGS), Mars Express (MEX), Mars Reconnaissance Orbiter (MRO) and Mars Atmosphere and Volatile EvolutioN (MAVEN). This website offers the current status of the COSPAR-inspired attempt to have a reference ionosphere for Mars. This is not an official COSPAR Mars International Reference Ionosphere and, for that reason, is called the Mars Initial Reference Ionosphere (MIRI). Stages of development span several years [2], [3],[4], with the current version called MIRI-2018[5].
The Mars Initial Reference Ionosphere (MIRI-2018) is a semi-empirical model that links the photo-chemical-equilibrium equation to observations of maximum electron density (Nmax) obtained from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express (MEX) spacecraft. Over 200,000 values of peak electron density from the years 2005-2015 were used to create parameterizations for a semi-empirical MIRI-2018. The model gives a primary output parameter: the maximum electron density (Nmax) of the Martian ionosphere versus solar zenith angle (SZA).
In addition, MIRI-2018 generates the full electron density profile, Ne(h), and its integral, called the total electron content (TEC). The updated analysis method relates Nmax to solar flux* and solar zenith angle, resulting in coverage that extends past SZA = 90o for a single post-sunset estimate of nighttime values. The resultant Nmax value is used to calibrate Ne(h) profiles for every 10-degrees of SZA, with the topside ionosphere coming from MARSIS [3] and the bottomside ionosphere from a theoretical model [4].
Finally, MIRI-2018 includes a separate module to calculate TEC, using over 100,000 TEC observations over the years 2006-2014 from the SHAllow RADar (SHARAD) instrument on the Mars Reconnaissance Orbiter (MRO) spacecraft. Validation of the MIRI-SHARAD-TEC module with data from the Radio Occultation Experiment on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission was successful [5].
To obtain a run of MIRI, the user selects a date from an interactive calendar. If a specific solar zenith angle (SZA) is desired, the user can input that value. MIRI uses its archive of Mars' distances from the Sun and solar radio fluxes (F10.7) spanning the years 1964-2019 to compute Nmax vs. SZAs (between 0o and 90o and a single value for SZA ≥ 90o) at Mars for that date. Uncertainty levels are given for the model values based on the variability patterns in the data used in its formulation. A graphical output shows the date requested, the distance of Mars from the Sun, its orbital longitude, the "rotated Sun date", and solar radio flux measured at 1 AU that is applied at Mars (together with its contextual 3-month mean). The model can be used for dates in the past (back to 1964) and well as for the future (to 2019). Forecasts involve use of predicted solar flux conditions. Tabulated values are provided.
To obtain electron density profiles between 80km and 400km vs. SZAs, Tabulated and Graphical links can be followed to access values and plots respectively. Lastly, a table of total electron content vs. SZAs is generated for the MIRI-Profile-Integration module as well as the MIRI-SHARAD-TEC module.
Give it a try! We welcome comments and suggestions.
*Prior to the MAVEN mission, solar fluxes were only measured from 1 AU. To apply a solar flux measured at Earth to Mars, one has to take into account both radial distance effects (1/d2), as well as the difference in their orbital longitudes. Thus, the hemisphere of the Sun facing Mars on a given day faced Earth on a different day (except for the dates when Mars was in opposition phase). Correcting for this effect is accomplished by determination of the “rotated Sun date”—a correction that can be ±14 days from the date of interest at Mars. MIRI-Mark-2 determines the rotated-Sun-date and specifies it as an output parameter. A daily value of solar radio flux (F10.7) can show a significant difference from it prior day or subsequent day value. There is a sluggishness to the response of the martian ionosphere to such effects, and thus an “effective flux” parameter (Feff) is used in MIRI. This blends the daily value with its three-solar rotation average:
[1] Approaches to a Mars International Reference Ionosphere, M. Mendillo, P. Withers and the MIRI Team, Paper presented at the 35th COSPAR Meeting, Paris, 22 July 2004.
[2] A New Semi-empirical Model of the Peak Electron Density of the Martian Ionosphere, M. Mendillo, A. Marusiak, P. Withers, D. Morgan and D. Gurnett, Geophysical Research Letters, 40, 1-5, doi:10.1002/2013GL057631, 2013.
[3] The equivalent slab thickness of Mars' ionosphere: Implications for thermospheric temperature, M. Mendillo, C. Narvaez, G. Lawler, W. Kofman, J. Mouginot, D. Morgan, and D. Gurnett, Geophys. Res. Lett., 42, 3560–3568. doi: 10.1002/2015GL063096, 2015.
[4] MAVEN and the total electron content of the Martian ionosphere, M. Mendillo,et al., J. Geophys. Res. Space Physics, 122, 3526–3537, doi:10.1002/2016JA023474, 2017.
[5] Mars Initial Reference Ionosphere (MIRI) Model: Updates and Validations Using MAVEN, MEX, and MRO Data Sets, M. Mendillo,et al., Journal of Geophysical Research: Space Physics, 123, doi:10.1029/2018JA025263, 2018.