G. M. Keating, ^* S. W. Bougher, R. W. Zurek, R. H. Tolson, G. J. Cancro, S. N. Noll, J. S. Parker, T. J. Schellenberg, R. W. Shane, B. L. Wilkerson, J. R. Murphy, J. L. Hollingsworth, R. M. Haberle, M. Joshi, J. C. Pearl, B. J. Conrath, M. D. Smith, R. T. Clancy, R. C. Blanchard, R. G. Wilmoth, D. F. Rault, T. Z. Martin, D. T. Lyons, P. B. Esposito, M. D. Johnston, C. W. Whetzel, C. G. Justus, J. M. Babicke

The Mars Global Surveyor (MGS) z-axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermospheric response at mid-northern latitudes, increasing the altitude of thermospheric pressure surfaces there by as much as 8 kilometers and indicating a strong global thermospheric response to a regional dust storm. Throughout the MGS mission, thermospheric density bulges have been detected on opposite sides of the planet near 90°E and 90°W, in the vicinity of maximum terrain heights. This wave 2 pattern may be caused by topographically-forced planetary waves propagating up from the lower atmosphere.

G. M. Keating, R. H. Tolson, G. J. Cancro, S. N. Noll, J. S. Parker, T. J. Schellenberg, R. W. Shane, B. L. Wilkerson, The George Washington University at NASA Langley, MS 269, Hampton, VA 23681, USA.
S. W. Bougher and J. M. Babicke, University of Arizona, Tucson, AZ 85721, USA.
R. W. Zurek, T. Z. Martin, D. T. Lyons, P. B. Esposito, M. D. Johnston, C. W. Whetzel, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
J. R. Murphy, J. L. Hollingsworth, R. M. Haberle, M. Joshi, NASA Ames Research Center, Moffett Field, CA 94035, USA.
J. C. Pearl, B. J. Conrath, M. D. Smith, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
R. T. Clancy, Space Science Institute, Boulder, CO 80309, USA.
R. C. Blanchard, R. G. Wilmoth, D. F. Rault, NASA Langley Research Center, Hampton, VA 23681, USA.
C. G. Justus, Computer Sciences Corporation, Huntsville, AL 35824, USA.
^*   To whom correspondence should be addressed.

Previous in situ measurements from Viking Landers 1 and 2 and Mars Pathfinder (5), and remote sensing measurements of air glow, plasma and neutral density scale heights, and monitoring of the altitude of the F1-peak in electron density (1, 6) have only partially characterized these thermospheric variations because of the limited coverage in space and time. Now, measurements from the z-axis accelerometer aboard MGS (7) have yielded to date more than 200 vertical structures of thermospheric density and derived temperature and pressure. These data have been obtained at successive MGS periapses over a 5-month period spanning northern fall on Mars (Ls = 180° to 270°) (8), from September 1997 to February 1998, as MGS continues aerobraking into its mapping orbit (9). During the mission the spacecraft periapsis has moved from 32°N to 50°N and from a local solar time (LST) of 6 p.m. to noon, with data acquired from 170 km to as low as 110 km.

The MGS z-axis accelerometer, aligned closely to the spacecraft velocity vector (v), measures the drag force acceleration (a), related to atmospheric density () by the classical relation:

(1)

with spacecraft mass (m), cross-sectional area (s) relative to v, and drag coefficient (C_D). The z-accelerometer (10) is 38 times more sensitive (0.332 mm/s per count) than the Viking entry accelerometers (11) allowing measurements up to at least 170 km. Accelerometer measurements sensitive to the thermosphere are generally obtained from 200 s before periapsis to 200 s after periapsis, a span of about 30° in latitude. Additional accelerometer measurements are obtained before and after this period to determine accelerometer bias for each pass. Measurements are obtained every 0.1 s. Inbound atmospheric measurements were north of periapsis, and outbound to the south. Extraneous accelerations associated with spacecraft thruster activity and angular accelerations were removed. Using spacecraft pointing knowledge, C_D was determined through an iterative process that accounts for the transitional regime between free molecular and continuum flow (12). Densities and scale heights were determined over 6-s running means to filter out 6-s oscillations of the spacecraft or 40-s running means to filter out atmospheric wave activity to better estimate mean structure. Accelerations of one micro-g were detected. Overall, random errors in density were estimated to be less than 3% at altitudes below 140 km. At higher altitudes uncertainties in accelerometer measurement bias dominate error estimates. Systematic uncertainties in C_D of less than 10% do not significantly affect measurements of relative variations, such as wave activity.

Thermospheric structure. Thermospheric densities measured by the MGS accelerometer at the evening terminator near the start of aerobraking were close to the late afternoon Viking 1 densities (Fig. 1), but at 130 km were a factor of 4.5 higher than the corresponding pre-dawn Mars Pathfinder values (5). Seasonal and diurnal variations should contribute to this difference (13). Simulations by a coupled Mars General Circulation Model (MGCM) and Mars Thermospheric Global Circulation Model (MTGCM) (14) indicate that the density difference between MGS and Mars Pathfinder can largely be accounted for by a factor of 2 increase in density due to seasonal variation (Ls = 143° to 190°), and a doubling due to diurnal variation within the thermosphere (3 a.m. to 6 p.m.), assuming a visible dust opacity level between 0.3 and 1.0. Using the MGCM-MTGCM model to adjust the Viking 1 thermospheric data to the MGS periapsis latitude, LST, solar activity level, Ls (8), and Mars-sun distance, the thermospheric densities in the Viking 1 era are estimated to be more than a factor of 2 higher than these MGS values. This is primarily due to the large differences in the Mars-sun distance between the two thermospheric data sets. Thus, the close overlap of these two thermospheric density profiles (Fig. 1) may be associated with long-term climate variations (1977 to 1997), differences in opacity (15), natural variability, or a combination of all three sources.

The MTGCM simulations reflect a boundary forcing based on the MGCM simulations of the lower atmosphere for specified dust opacities. Currently, only zonally averaged height and temperature fields and specific (semi-diurnal) components of the LST variation of the geopotential field are included. In absolute terms, densities obtained from the MGS accelerometer generally exceeded the unadjusted MTGCM values by 50 to 100%. These underestimates of densities are equivalent to a 3- to 5-km offset of the model and actual thermosphere heights. To remove this bias, given the current inability to validate the model temperature fields between about 70 km and periapsis altitudes, the MGCM zonally-averaged heights used at the MTGCM lower boundary (1.32-µbar level) are offset up to 5 km to obtain the MGS accelerometer measured height of the 1.26-nbar level in the thermosphere (16).

MGS-derived temperatures above 150 km from accelerometer-derived scale heights appear to asymptote to values near 200 K. The vertical temperature structures, derived from scale heights and estimates of mean molecular weight, are in general accord with MTGCM temperatures of the upper thermosphere for visible dust opacities of 0.3 and 1.0. However, observed scale height derived temperatures at 130 km are colder than the MTGCM model predictions (120 K versus 150 K), even with the altitude adjustment described previously. These colder temperatures appear to be a general characteristic of the Mars lower thermosphere. The MTGCM takes into account the present low solar activity, but may underestimate CO₂ 15-µm cooling (2, 17). Specifically, a larger O/CO₂ mixing ratio would lead to an enhancement of CO₂ cooling, as for Venus. This O/CO₂ ratio is presently uncertain for Mars by at least a factor of 2. Gravity wave processes may also not be represented adequately in the models and could contribute to the temperature discrepancy.

MGS accelerometer measurements of atmospheric density obtained in the upper thermosphere at 160 km (November 1997 to January 1998) (Fig. 2) are different on inbound and outbound. Densities to the south exceed those to the north in every case. On the average, the densities during this northern fall season increase from 57°N to 33°N by 75%. Comparable values simulated by the MTGCM are about 66%, in agreement with observations. The latitudinal variations in the upper thermosphere may be largely due to changing solar extreme ultraviolet radiation (EUV) input with solar zenith angle and changing conditions at lower altitudes. In the lower thermosphere at 130 km, densities are occasionally higher to the north.

Two increases in density were observed at 160 km, one maximum in density after periapsis 50 (P050) and the second maximum in density after P090 (Fig. 2). The first peak is apparently related to a local dust storm and the second feature may be caused by an increase in EUV from the sun. The 10.7 cm solar flux, which is routinely used as a proxy to estimate solar EUV radiation variations, reaches a local maximum, when viewed from Earth 17 days before P093, when 160 km densities maximized. With the 27-day rotation period of the sun, this face of the sun would be seen at Mars 17 days later near the time of P093. Such atmospheric responses may have been previously obscured by the dust storm. The response is not indicated in the lower thermosphere (Fig. 3), due to much weaker solar EUV effects at these lower altitudes. Such possible 27-day variations in the thermosphere, related to the 27-day rotation of the sun, have been detected previously in the Venus thermosphere (18).

Thermospheric storms. The MGS accelerometer observed two episodes of prolonged and substantial increase in lower thermospheric density. The first occurred on P015, with a 50% increase in atmospheric density at 110 km altitude. Measurements on P016 to P018, taken near 121 km, showed further increases of 60 to 90% as compared to earlier orbits near that altitude, indicating that the P015 event was not short-lived.

An even larger density increase occurred on P051 (130%), after detection by the MGS Thermal Emission Spectrometer (TES) (18) two orbits earlier (P049, Ls = 224°, 25 November 1997) of a fully developed regional dust storm in the southern hemisphere Noachis Terra region (initially centered near 40°S, 20°E). Past data indicate that such storms are likely to occur on Mars during southern spring and summer, particularly during Ls = 200° to 330° (4).

In both cases, density increases coincided with warming and hydrostatic expansion of the lower atmosphere. In Fig. 3, periapsis densities have been converted to pressure at a common reference altitude of 126 km using atmospheric scale heights derived from MGS accelerometer data. These values are compared to pressures at a reference level of 61 km, estimated by using vertical profiles of temperature (as a function of pressure) derived from microwave (MW) spectrometer measurements of thermal emission in the 230 GHz CO line (15). The ground-based MW spectrometers average over the whole disk of Mars visible to the Earth observatory and so are representative of widespread changes, while the accelerometer measurements are specific to latitudes near periapsis between 32° and 50°N. The microwave and accelerometer data sets show similar trends, especially the onset of the dust storm in late November followed by its gradual decline. The stronger response in the thermosphere to the dust storm may be due to additional warming in the 70- to 120-km region which is not directly observed. The delay of increase in periapsis density by 2 to 3 days following the onset of the Noachis storm probably reflects the remote northern location of the aerobraking periapses.

Rapid increases in atmospheric temperatures have been seen in microwave data acquired in previous Mars years, particularly above 30 km (15). Such increases are attributed to changes in atmospheric dust loading, based on previous spacecraft data, together with radiative and radiative-dynamical modeling of the effects of dust heating of the atmosphere (3, 4). However, no dust storm activity was detected during the P015-P018 period by TES (18) or the MGS Mars Orbiter Camera (MOC) (19), although their coverage was restricted largely to the Southern Hemisphere. Full disk views on 5 October (P014) and 9 (P016-017) by the Hubble Space Telescope (HST) (20) also showed no significant dust activity or change (21). HST images were obtained until mid-October when the planet moved too close to the sun for safe viewing by HST. Meanwhile, column opacity estimates from the Imager for Mars Pathfinder (22) had also stopped with that mission's end in September.

By contrast, TES and MOC observed the Noachis regional dust storm (18, 19), and TES observations showed that by P052, 3 days after the storm was detected, the dust opacity in northern mid-latitudes had doubled. This is consistent with the higher temperatures seen by TES (increases >10 K) in both hemispheres, the ground-based MW data (Fig. 3), and increased thermospheric densities measured by the MGS accelerometer. The storm is classified as "regional" because the area of high opacity (>1) remained confined to Noachis and adjacent regions.

MGS measured heights of the 1.26-nbar level showed a general upward trend as perihelion (near southern summer solstice) approached and sharp increases (up to 140 km) during the Noachis dust storm (Fig. 4). This height is a rough measure of the integrated thermal structure of the lower atmosphere and reflects changing seasonal IR and dust storm heating of the lower atmosphere (17).

The trend from 128 to 132 km of the 1.26-nbar height before the dust storm (before P049) is close to that expected for this season (16). The orbit-to-orbit fluctuations seen reflect the MGS densities measured at thermospheric altitudes (Fig. 4). Post-dust storm heights are seen to dip below 132 km as the MGS periapsis moved northward. Thermospheric densities had largely returned to seasonal values by early January (Ls = 250°; P085), while dust visual opacities estimated from TES data had declined from values greater than 1 to typically 0.5 or less (18).

The passage of a dust storm event and its effects on the thermosphere were also monitored by tracking the thermospheric variability, measured by the MGS accelerometer, with time (Fig. 5). The ratio of the thermospheric density measured on any given pass to the density measured on the previous pass (adjusted for altitude using the observed scale heights) was used as an indicator of the atmospheric disturbance level. The "variability" parameter plotted is the 2- percent deviation of the logs of these ratios using the present ratio and the previous four ratios. Thermospheric disturbances sharply increased on P047 from 60% to over 100% before the initiation of the dust storm on P049. Maximum disturbances of 200% occurred on P051. The thermospheric disturbance level gradually decayed between P052 and P075 back toward pre-storm values of about 60%. These pre-storm and post-storm disturbance levels are consistent with prior estimates of short-term variability of the Mars thermosphere estimated using the Mariner 9 (M9) radio occultation data (16), which did not cover the onset of the 1971 global dust storm.

Planetary waves. A large part of the orbit-to-orbit variability observed with the MGS accelerometer is due to a stationary planetary-scale variation with longitude at thermospheric altitudes (110 to 170 km). Harmonic analysis of the longitudinal variations in densities were obtained from 14 accelerometer data sets, each with good longitudinal sampling (typically 10 orbits), and adjusted to a common altitude of 125 km (23). This analysis revealed a wave 2 pattern, which is a two-cycle wave over 360° of longitude. This wave 2 is essentially fixed in longitude, but varying in amplitude throughout the MGS aerobraking mission. One minimum of this wave 2 pattern always occurred near the central meridian, from 16°E to 18°W with an average phase of 2°E. Figure 6 shows the combined wave 1 and wave 2 variation during a non-dust storm time period (P090-P110). In most cases wave 2 was stronger than wave 1, and wave 1 was not stationary. During the initiation of the dust storm the wave 2 percent amplitude increased to ±39% compared to mission mean levels of ±22%. Through additional harmonic analyses, the standing wave 2 phenomena was also detected in the 1.26 nbar levels, 160 km densities, and conditions at 130 km.

The persistence of this wave 2 pattern with its nearly constant phase suggests a forcing continuous in time related to surface topographic forcing. MGS periapses have spanned the latitude range 32°N to 50°N during fall and early winter. Mars' surface topography exhibits significant longitudinal variability at the MGS periapsis latitudes (32°N to 50°N) (24) with a substantial wave 2 component (±2 km) due to the Tharsis and Arabia highlands. Low-level eastward winds at northern middle latitudes during northern autumn and winter can interact with this topography and excite vertically propagating planetary-scale waves. The ability of these waves to propagate all the way into the thermosphere depends on the presence of eastward zonal winds in the lower atmosphere. Such winds are inferred up to at least 50 km from the TES temperature cross-sections (18).

MGCM results suggest that waves 1 and 2 can be topographically forced during northern fall and winter and can propagate to above 80 km (25). Vertical propagation above these altitudes depends upon the winds at higher altitudes. The robust planetary-wave pattern seen in the thermosphere in the accelerometer data indicates that, unlike Earth, the eastward moving winds present in northern mid-latitude during the fall do not close off (that is, decrease to zero wind speed or reverse direction) below the thermosphere, thereby permitting strong planetary waves to propagate into the thermosphere.

These thermospheric measurements from the MGS accelerometer, coupled with simultaneous lower atmosphere measurements, provide a quantitative means to examine the coupling of these atmospheric regions. The physical mechanisms responsible for this coupling can be compared to those known to operate for Earth. These results may also benefit future Mars missions that will rely on aerobraking or aerocapture.

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29. We thank the Mars Surveyor Operations Project at Jet Propulsion Laboratory (JPL) and the MGS spacecraft operations team at Lockheed Martin Astronautics (Denver, CO) for their support in providing the MGS Accelerometer Team with the accelerometer and ancillary data used in this study. This work was funded in part by the JPL, Pasadena, CA.