Stephen J. Keihm

Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s–1 in early June 2014 to 1.2 kg s–1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K–1 m–2 s–0.5), consistent with a thermally insulating powdered surface, is inferred.

TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphere range correction algorithm and pre-launch error budget

For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted. >

TOPEX/Poseidon Microwave Radiometer (TMR). I. Instrument description and antenna temperature calibration

The TOPEX/Poseidon microwave radiometer (TMR) is a three-frequency radiometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit. It operates at 18, 21, and 37 GHz in a nadir-only viewing direction which is co-aligned with the T/P radar altimeters. The TMR monitors and corrects for the propagation path delay of the altimeter radar signal due to water vapor and nonprecipitating liquid water in the atmosphere. The paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (T/sub A/) from the raw digital data. T/sub A/ precision of 0.4 K is predicted on orbit in all expected thermal environments, T/sub A/ accuracy of 0.5-0.6 K is expected following a post-launch field calibration campaign. These performance figures represent a significant improvement over those of the Seasat and Nimbus-G Scanning Multichannel Microwave Radiometer on which TMR is based. The improvements are the result of specific hardware design and calibration changes. Hardware changes include a redesigned feed horn, to reduce impedance mismatches, and the addition of radomes over the feed and sky horns, to reduce thermal variations. Calibration changes involve more extensive temperature cycling and data analysis during thermal/vacuum testing. >

TOPEX/POSEIDON microwave radiometer performance and in‐flight calibration

Results of the in-flight calibration and performance evaluation campaign for the TOPEX/POSEIDON microwave radiometer (TMR) are presented. Intercomparisons are made between TMR and various sources of ground truth, including ground-based microwave water vapor radiometers, radiosondes, global climatological models, special sensor microwave imager data over the Amazon rain forest, and models of clear, calm, subpolar ocean regions. After correcting for preflight errors in the processing of thermal/vacuum data, relative channel offsets in the open ocean TMR brightness temperatures were noted at the ≈1 K level for the three TMR frequencies. Larger absolute offsets of 6–9 K over the rain forest indicated a ≈5% gain error in the three channel calibrations. This was corrected by adjusting the antenna pattern correction (APC) algorithm. A 10% scale error in the TMR path delay estimates, relative to coincident radiosondes, was corrected in part by the APC adjustment and in part by a 5% modification to the value assumed for the 22.235 GHz water vapor line strength in the path delay retrieval algorithm. After all in-flight corrections to the calibration, TMR global retrieval accuracy for the wet tropospheric range correction is estimated at 1.1 cm RMS with consistent performance under clear, cloudy, and windy conditions.

TOPEX microwave radiometer performance evaluation, 1992-1998

The stability and accuracy of the TOPEX Microwave Radiometer (TMR) measurement of the atmospheric path delay due to water vapor is assessed over the interval from launch (August 1992) through June 1998. Detailed global comparisons are made with path delays derived from the special sensor microwave imager (SSM/I) instruments and a network of 15 island radiosondes. The results provide consistent evidence that the TMR path delay measurements included an instrument-related downward drift of 1.0-1.5 mm/yr between October 1992 and December of 1996. The four-year drift correlates with an upward drift seen in the coldest TMR 18-GHz brightness temperature time series and is further supported by independent comparisons of TMR with ERS-1 and 2, GPS, and the Harvest Platform water vapor radiometer measurements. From January 1997 through June 1998, no significant relative path delay drift between TMR and SSM/I is seen in the comparison data, although anomalies do appear in early 1998. In terms of accuracy, both the SSM/I and radiosonde comparisons indicate no significant (>2%) scale error in the TMR path delay. An overall bias of <10 mm mag be present, but the comparisons are not consistent in this determination.

Improved 20- to 32-GHz atmospheric absorption model

An improved model for the absorption of the atmosphere near the 22-GHz water vapor line is presented. The Van Vleck-Weisskopf line shape is used with a simple parameterized version of the model from Liebe et al. [1993] for the water vapor absorption spectra and a scaling of the model from Rosenkranz [1993] for the 20 to 32-GHz oxygen absorption. Radiometrie brightness temperature measurements from two sites of contrasting climatological properties, San Diego, California, and West Palm Beach, Florida, were used as ground truth for comparison with in situ radiosonde-derived brightness temperatures under clear-sky conditions. Estimation of the new models four parameters, related to water vapor line strength, line width and continuum absorption, and far-wing oxygen absorption, was performed using the Newton-Raphson inversion method. Improvements to the water vapor line strength and line width parameters are found to be statistically significant. The accuracy of the new absorption model is estimated to be 3% between 20 and 24 GHz, degrading to 8% near 32 GHz. In addition, the Hill line shape asymmetry ratio was evaluated on several currently used models to show the agreement of the data with Van Vleck-Weisskopf based models and to rule out water vapor absorption models near 22 GHz given by Waters [1976] and Ulaby et al. [1981], which are based on the Gross line shape.

TOPEX/Poseidon Microwave Radiometer (TMR). II. Antenna pattern correction and brightness temperature algorithm

For pt.I see ibid., vol.33, no.1, p.125-37 (1995). The calibrated antenna temperatures measured by the TOPEX Microwave Radiometer are used to derive radiometric brightness temperatures in the vicinity of the altimeter footprint. The basis for the procedure devised to do this-the antenna pattern correction and brightness temperature algorithm-is described in the paper, along with its associated uncertainties. The algorithm is based on knowledge of the antenna pattern, the ground-based measurements of which are presented along with their analyses. Using the results of these measurements, the authors perform an error analysis that yields the net uncertainties in the derived TMR footprint brightness temperatures. The net brightness temperature uncertainties range from 0.79 to 0.88 K for the three TMR frequencies, and include the radiometer calibration uncertainties which range from 0.54 to 0.57 K. the authors also derive an estimate of the uncertainty incurred by using brightness temperatures measured in the /spl sim/40 km TMR footprint to estimate path delay in the /spl sim/3 km altimeter footprint. The RMS difference in path delay averaged over the largest TMR footprint relative to that in the altimeter footprint is estimated to be about 0.3 cm. Finally, the authors discuss the error associated with using unequal beams at the three TMR frequencies to derive path delays, and describe an approach using along-track averaging of the algorithm brightness temperatures to reduce this error. >

Spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko observed from Rosetta/MIRO in August 2014

Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the (H2O)-O-16 rotational transition line at 556.936 GHz observed from Rosetta/MIRO in August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the (H2O)-O-16 outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7-9, 2014 and August 18-19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 x 1025 molecules s(-1) sr l to 3.0 x 10(25) molecules s(-1) sr, the terminal gas expansion velocity varies by 0.17 km s(-1) from 0.61 km s(-1) to 0.78 km s(-1), and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.

MIRO observations of subsurface temperatures of the nucleus of 67P/Churyumov-Gerasimenko

Observations of the nucleus of 67P/Churyumov-Gerasimenko in the millimeter-wave continuum have been obtained by the Microwave Instrument for the Rosetta Orbiter (MIRO). We present data obtained at wavelengths of 0.5 mm and 1.6 mm during September 2014 when the nucleus was at heliocentric distances between 3.45 and 3.27 AU. The data are fit to simple models of the nucleus thermal emission in order to characterize the observed behavior and make quantitative estimates of important physical parameters, including thermal inertia and absorption properties at the MIRO wavelengths. MIRO brightness temperatures on the irregular surface of 67P are strongly affected by the local solar illumination conditions, and there is a strong latitudinal dependence of the mean brightness temperature as a result of the seasonal orientation of the comet’s rotation axis with respect to the Sun. The MIRO emission exhibits strong diurnal variations, which indicate that it arises from within the thermally varying layer in the upper centimeters of the surface. The data are quantitatively consistent with very low thermal inertia values, between 10–30 J K -1 m -2 s -1/2 , with the 0.5 mm emission arising from 1 cm beneath the surface and the 1.6 mm emission from a depth of 4 cm. Although the data are generally consistent with simple, homogeneous models, it is difficult to match all of its features, suggesting that there may be some vertical structure within the upper few centimeters of the surface. The MIRO brightness temperatures at high northern latitudes are consistent with sublimation of ice playing an important role in setting the temperatures of these regions where, based on observations of gas and dust production, ice is known to be sublimating.

Microwave Radiometer Calibration on Decadal Time Scales Using On-Earth Brightness Temperature References: Application to the TOPEX Microwave Radiometer

Abstract A method is described to calibrate a satellite microwave radiometer operating near 18–37 GHz on decadal time scales for the purposes of climate studies. The method uses stable on-earth brightness temperature references over the full dynamic range of on-earth brightness temperatures to stabilize the radiometer calibration and is applied to the Ocean Topography Experiment (TOPEX) Microwave Radiometer (TMR). These references are a vicarious cold reference, which is a statistical lower bound on ocean surface brightness temperature, and heavily vegetated, pseudoblackbody regions in the Amazon rain forest. The sensitivity of the on-earth references to climate variability is assessed. No significant climate sensitivity is found in the cold reference, as it is not sensitive to a climate minimum (e.g., coldest sea surface temperature or driest atmosphere) but arises because of a minimum in the sea surface radio brightness that occurs in the middle of the climatic distribution of sea surface temperatures (...