GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 16, PAGES 3091-3094, AUGUST 15, 2001
Mars Global Surveyor Radio Science electron density profiles : Neutral atmosphere implications Stephen W. Bougher and Steffi Engel Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona
David P. Hinson Stanford University, Stanford, California
Jeffrey M. Forbes University of Colorado, Boulder, Colorado
Abstract. The Mars Global Surveyor (MGS) Radio Science (RS) experiment permits retrieval of electron density profiles versus height (∼90-200 km) from occultation measurements. An initial set of electron profiles is examined spanning high northern latitudes, early morning solar local times and high solar zenith angles (78 to 81◦ ) near aphelion. Sampling for these 32-profiles is well distributed over longitude. The height of the photochemically driven ionospheric peak is observed to respond to the background neutral density structure, with a mean height during this season at this location of ∼134.4 km. Strong wave-3 oscillations about this mean are clearly observed as a function of longitude, and correspond to neutral density variations measured by the MGS Accelerometer (ACC) experiment. The wave-3 tidal pattern implicated by both the RS and ACC datasets is consistent with a semi-diurnal wave frequency. Clearly, the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying Mars atmosphere. This ionospheric peak height can be used as a proxy of the longitude specific non-migrating tidal variations present in the Mars lower thermosphere.
1. Introduction to the MGS Radio Science Data The Mars Global Surveyor (MGS) Radio Science (RS) experiment permits retrieval of neutral temperature profiles (0-50 km) as well as electron density profiles (∼90-200 km), providing a means to monitor the characteristics of the martian lower and upper atmospheres. A recent paper [Hinson et al., 2001] reviews the occultation technique and describes a second series of neutral temperature profiles obtained from RS measurements conducted in late 1998 during Phase 2 of MGS aerobraking. This paper examines the corresponding set of electron density profiles for this same observational period (Dec. 24-31, 1998). We focus on the basic features of this initial set of 32-electron density profiles [Hinson et al., 2000] and their implications for the neutral atmospheric structure. This RS dataset has been submitted to the Planetary Data System (PDS). Copyright 2001 by the American Geophysical Union. Paper number 2001GL012884. 0094-8276/01/2001GL012884$05.00
This suite of radio occultation profiles was obtained near northern summer solstice (Ls = 74-77) at aphelion. The solar activity during this period most closely corresponds to solar moderate (F10.7 = 130) conditions. The MGS spacecraft provided sampling at high northern latitudes (64.7 to 67.3 N), early morning solar local times (0400 to 0300) and high solar zenith angles (78 to 81◦ ). These observations are essentially limited to a single latitude, solar local time, and solar zenith angle (SZA). However, sampling for these 32profiles is well distributed over longitude, with a separation between successive measurements of ∼55◦ . Two sample electron density profiles are chosen for presentation in Figure 1. These profiles are identified from the archive as (a) 8363G10A and (b) 8365F58A. Several features are noteworthy. Primary electron density peak magnitudes of 8 and 8.6 ×10 4 cm−3 are observed at heights of (a) 134 and (b) 136.6 km, respectively. These altitudes are typical of the entire RS dataset. The appearance of a secondary electron peak is quite pronounced in profile (b) at ∼110 km. Profile (a) reveals a subdued shoulder or ledge at roughly 115 km of approximately the same magnitude (4.0 × 10 4 cm−3 ) as profile (b). Finally, there is a rather large (factor of 2.5) change in electron densities at 200 km between these two profiles. This variation may reflect the solar wind interaction with the ionosphere under highly variable solar wind dynamic pressure conditions [Zhang et al., 1990].
2. Variations of Primary and Secondary Peak Heights Previous martian ionospheric measurements and modeling exercises confirm that the dayside ionosphere below ∼180 km is not subject to vertical or horizontal transport of ions [Zhang et al., 1990; Fox, 1997]. Most importantly, the primary electron density peak (F1-peak) is under the local control of photochemical processes. The location of the martian ionospheric F1-peak height is thus determined by how deeply into the atmosphere the solar EUV radiation penetrates, and therefore is driven by the neutral atmospheric structure. According to pre-MGS radio occultation data, the martian F1-peak rises with increasing SZA, consistent with a larger slant path [Zhang et al., 1990]. F1-peak heights also rise and fall with the advance of the martian seasons [Bougher et al., 2000], revealing a variation of as much as ∼15 km from aphelion to perihelion. Finally, the devel-
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Figure 1. Electron density [NE] profiles taken from the RS observing period : (8363G10A) LON = 328.4E; (8365F58A) LON = 346.7E.
opment of dust storm events (both regional and global) is observed to inflate the entire martian atmosphere, resulting in the dramatic increase of the F1-peak height (8 to 20 km) [Zhang et al., 1990; Keating et al., 1998]. Clearly, the ionospheric peak height tracks the local expansion/contraction of the lower atmosphere of Mars. Conversely, the F1-peak density is sensitive to the solar ionizing fluxes. The dominant ion near the martian F1-peak is O+ 2 , which almost always recombines dissociatively. Under these photochemical equilibrium conditions, the electron density near the F1-peak varies approximately as the square root of the production rate (i.e. as the square root of the solar flux intensity). Thus, the magnitude of the F1-peak can be used as a first-order monitor of relative changes in the solar EUV ionizing fluxes arriving at Mars. A secondary electron density ledge or peak (near 110 km) was also observed by pre-MGS radio occultation measurements; recently, this feature was modeled [Fox et al., 1995]. + ions comprise this secondary peak, reBoth O+ 2 and NO + quiring partial O2 loss via reactions with N and NO. It is suggested that this ledge or peak is produced by soft Xray ionization; the responsible Xray fluxes (1.8-20.0 nm) vary more strongly with the solar cycle than longer wavelength EUV fluxes. This implies that secondary peak densities should be more prominent at high solar activity. The 32-profiles comprising our initial RS dataset are averaged together, washing out all SZA and longitude variations, to yield a mean F1-peak density of 8.1×10 4 cm−3 at a mean height of 134.4 km. These values are consistent with those observed by the Mariner 9 (extended mission) radio occultation measurements [Zhang et al., 1990], taken also during similar solar cycle (F10.7 = 130), seasonal (near aphelion), and SZA (70-80◦ ) conditions. This comparison places the MGS/RS profiles in the context of previous ionospheric measurements, enabling photochemical control to be confirmed near the primary peak. In addition, the MGS observed variation of F1-peak heights (1-σ = 3.5 km) suggests an underlying neutral density variation of nearly a factor of ± 1.6, assuming a 7.5 km scale height. Likewise, the standard deviation of F1-peak densities implies a mean solar flux variation of ±15% over this observing period. The heights of both the primary and secondary electron density peaks are regulated by the underlying neutral atmospheric structure (column densities). Therefore, local max-
ima and minima of neutral densities at a constant height will translate into corresponding electron density peaks and troughs for a constant thermospheric temperature. This expected behavior is generally illustrated in Figure 2 as a function of E. longitude for (a) F1-peak heights and (b) secondary peak heights, respectively. For each plot, a leastsquares spectral fit through wave-3 is applied to the RS data. The set of distinct (prominent) secondary peaks is smaller than the 32-primary peak heights, resulting in larger data deviations from the wave-3 fit (Figure 2b). Nevertheless, wave-3 features persist for both panels in the lower ionosphere. The primary F1-peak heights show maxima at 100150E, 220-260E, and 340-30E about a statistical mean of 134.4 km (see section 2); the secondary peak heights (∼20 km lower) reveal maxima that are shifted westward by ∼30◦ . Do these observed longitude variations in electron density peak heights correspond to measured neutral density variations?
3. Comparisons to MGS Neutral Atmosphere Data The MGS Accelerometer (ACC) obtained neutral density measurements on aeropasses spanning 110 to 160 km during Phase 2 of aerobraking [Keating et al., 2000a, b]. Early during Phase 2 (Ls∼30), the ACC sampled high northern latitudes (60-65N) on the dayside from outbound aeropasses.
Figure 2. The heights of both the : (a) primary and (b) secondary [NE] peaks are presented as a function of longitude. For each plot, a least-squares spectral fit using harmonics through wave-3 (solid curve) is applied to the RS data, with corresponding 1-sigma errors (dotted curves).
BOUGHER ET AL.: RADIO SCIENCE ELECTRON DENSITY PROFILES
Figure 3. MGS Accelerometer (ACC) measurements were made at Ls∼30, yielding densities near 60-65N latitude on outbound aeropasses. A least-squares spectral fit through wave-3 (solid curve) is applied to 130 km altitude density data (asteriks), with corresponding 1-sigma errors (dotted curves).
This latitude coverage overlaps that from the present RS data. However, the ACC local time (LT) coverage (1600) is about 12-hours offset from RS data (0300-0400). In addition, datasets were obtained early (ACC) and late (RS) during northern spring. The seasonal discrepancy between these datasets is not significant; however the local time differences are crucial. A detailed comparison of these RS and ACC datasets is made, keeping in mind the 12-hour discrepancy in local time. Figure 3 shows 130 km ACC densities as a function of E. longitude. A least squares spectral fit through wave-3 is applied to the ACC data spanning 6065N latitude. Wave-3 features are clearly indicated, with maxima at 100-140E, 220-260E, and 340-20E. The phasing of these neutral density maxima and minima closely matches the F1-peak height maxima and minima illustrated in Figure 2a. The longitude region near 260-300E is rather poorly sampled (i.e. large 1-σ errors) in both datasets. Nevertheless, a wave-3 pattern is clearly visible. This consistency between neutral (ACC) and ionospheric (RS) datasets makes sense if a semi-diurnal wave frequency (σ=2) is responsible for the longitude features observed. The most likely candidate is the eastward propagating (s = -1) semidiurnal oscillation generated by modulation of the migrating (sun-synchronous) semidiurnal forcing by the wave three (m = 3) component of Mars topography [Forbes and Hagan, 2000; Wilson, 2000]. Lower in the martian atmosphere (∼25 km), a wave-3 tidal pattern is also implicated in Thermal Emission Spectrometer (TES) temperature data obtained at 60N latitude. However, semi-diurnal waves could not be confirmed using TES data obtained during mapping from the final sun synchronous (2AM/2PM) orbit [Wilson, 2000]. New diurnal sampling of lower atmosphere temperatures is needed to confirm the presence of semi-diurnal waves at high northern latitudes during this season.
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put fields are obtained on 33-pressure levels (above 1.32 micro-bar), corresponding to ∼70-300 km with a 5◦ latitude and longitude resolution. At present, a photochemical iono+ sphere is formulated for the MTGCM including O+ 2 , CO2 , + + O , NO below 200 km. Key ion-neutral reactions and rates are taken from [Fox et al., 1995]; empirical electron and ion temperatures are adopted from the Viking mission. In order to properly account for the Mars lower to upper atmospheric coupling that has been observed [Keating et al., 1998], the NASA Ames Mars General Circulation Model (MGCM) [Haberle et al., 1999] and the MTGCM are being run sequentially and their fields exchanged at an appropriate model interface (1.32 micro-bar level) [Bougher et al., 1999a] enabling zonal average coupling to be realized. MTGCM electron density profiles are presented in Figure 4, appropriate to Mars and MGS conditions during Dec. 24-31, 1998. These “zonally coupled” MTGCM profiles can be compared with the statistically averaged RS profiles. An F1-peak density (solid curve) of 1.0×10 5 cm−3 at a height of 135 km is calculated by the MTGCM at 62.5N latitude and 0400 LT, in substantial agreement with RS data. This F1peak height confirms that the basic features (climate) of the lower to upper atmosphere coupling are being captured by this MTGCM simulation. The electron density peak magnitude confirms that the solar fluxes during this time period correspond roughly to solar moderate conditions (F10.7 = 130 at Earth). The topside (200 km) electron density is calculated to be 2.3×10 4 cm−3 , which is typically larger than observed by the RS profiles examined. This implies that photochemical processes are not sufficient to describe the Mars ionosphere near 200 km during this observational period. Dynamical processes (vertical diffusion or solar wind induced ion flow yielding dayside removal) may dominate instead. A secondary ledge or shoulder is visible (solid curve) in the MTGCM electron density profile over 105-110 km. To test the role of solar Xray fluxes in regulating this secondary peak, an additional (dashed) MTGCM electron profile is presented. Here, soft Xray fluxes (1.8-15.0 nm) are enhanced by a factor of 10, leaving other fluxes unchanged. Notice the factor of ∼3 increase in the secondary ledge density, while the F1-peak density near 135 km only increases slightly. This confirms that the magnitude of the secondary peak
4. MTGCM Simulations The Mars Thermospheric General Circulation Model (MTGCM) is a finite difference primitive equation model that self-consistently solves for neutral thermospheric tempera- Figure 4. Two MTGCM simulations were conducted for the tures, neutral-ion densities, and 3-component neutral winds RS observing period, yielding electron density profiles at LAT = over the globe [Bougher et al., 1999b, 2000]. MTGCM out- 62.5N and 0400 LT.
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is controlled by soft Xray ionization. However, RS electron profiles don’t always reveal distinct (prominent) secondary peaks, owing to the changing level of EUV ionization just below the primary peak.
5. Conclusions This analysis of MGS/RS electron density data reveals wave-3 longitude-specific variations that follow the underlying neutral density oscillations observed by the MGS/ACC. The height of the main electron density peak is an excellent indicator of this coupling of the Mars lower and upper atmospheres. Non-migrating tidal forcing is implicated by these wave features, exhibiting a phasing that is consistent with semi-diurnal waves. Most importantly, electron density peak heights can be monitored beyond aerobraking as a proxy of the longitude variations of the neutral atmospheric structure over the Mars seasons. Acknowledgments. We thank the MGS Radio Science Team for making their ionospheric profile data available on a public website. The Editor would like to thank the reviewer of this manuscript.
References Bougher, S. W. et al., Mars Global Surveyor Aerobraking : Atmospheric Trends and Model Interpretation, Adv. in Space Research, 23(11), 1887-1897, 1999a. Bougher, S. W., S. Engel, R. G. Roble, and B. Foster, Comparative Terrestrial Planet Thermospheres : 2. Solar Cycle Variation of Global Structure and Winds at Equinox, J. Geophys. Res., 104, 16591-16611, 1999b. Bougher, S. W., S. Engel, R. G. Roble, and B. Foster, Comparative Terrestrial Planet Thermospheres : 3. Solar Cycle Variation of Global Structure and Winds at Solstices, J. Geophys. Res., 105, 17669-17692, 2000. Forbes, J. M. and M. Hagan, Diurnal Kelvin Wave in the Atmosphere of Mars: Towards an Understanding of ‘Stationary’ Density Structures Observed by the MGS Accelerometer, Geophys. Res. Lett., 27, 3563-3566, 2000. Fox, J. L., P. Zhou, and S. W. Bougher, The Martian Thermosphere/Ionosphere at High and Low Solar Activities, Adv.in Space Res., 17(11), 203-218, 1995.
Fox, J. L., Upper limits to the outflow of ions at Mars: Implications for atmospheric evolution, Geophys. Res. Lett., 24, 2901-2904, 1997. Haberle, R. M. et al., General Circulation Model Simulations of the Mars Pathfinder Atmospheric Structure Investigation/Meteorology Data, J. Geophys. Res., 104, 8957-8974, 1999. Hinson, D. P. et al., Public Access to MGS RS Standard Electron Density Profiles, nova.stanford.edu/projects/mgs/edspublic.html, 2000. Hinson, D. P, G. L. Tyler, J. L. Hollingsworth, and R. J. Wilson, Radio Occultation Measurements of Forced Atmospheric Waves on Mars, J. Geophys. Res., 106, 1463, 2001. Keating, G. M. et al., The Structure of the Upper Atmosphere of Mars : In-situ Accelerometer Measurements from Mars Global Surveyor, Science, 279, 1672, 1998. Keating, G. M. et al., Evidence for Large Global Diurnal Kelvin Wave in the Mars Upper Atmosphere, (abstract # 50.02), B. A. A. S. 32(3), 1091, 2000a. Keating, G. M. et al., MGS-M-ACCEL-5-PROFILE-V1.0, NASA Planetary Data System, 2000b. Wilson, R. J., Evidence for Diurnal Period Kelvin Waves in the Martian Atmosphere from Mars Global Surveyor TES Data, Geophys. Res. Lett., 27, 3889-3892, 2000. Zhang, M. H. G., J. G. Luhmann, A. J. Kliore,and J. Kim, A postPioneer Venus reassessment of the Martian dayside ionosphere as observed by radio occultation methods, J. Geophys. Res., 95, 14829-14839, 1990. S. W. Bougher, and S. Engel, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 87521. (e-mail:
[email protected]) D. P. Hinson, Department of Electrical Engineering, Stanford University, Stanford, CA 94305-9515, (e-mail: hinson@ nimbus.stanford.edu) J. M. Forbes, Department of Aerospace Engineering University of Colorado Boulder, CO 80309 (e-mail: forbes@ zeke.Colorado.EDU)
(Received January 22, 2001; revised May 4, 2001; accepted May 23, 2001.)
