Gerald A. Lehmacher

Helium in Jupiter's atmosphere: Results from the Galileo probe Helium Interferometer Experiment

On December 7, 1995, the NASA Galileo probe provided the first in situ measurements of the helium abundance in the atmosphere of Jupiter. Our Jamin interferometer measured precisely the refractive index of the Jovian atmosphere in the pressure region from 2 to 12 bars. From these measurements, we derive the atmospheric helium mole fraction to be 0.1359±0.0027. The corresponding helium mass fraction matches closely, but accidentally, the current helium abundance of the atmosphere of the Sun. However, both the Jovian and the solar value fall somewhat below the protosolar value. This suggests that in both Jupiter and the Sun processes are active which separate helium from hydrogen.

Structure of the Atmosphere of Jupiter: Galileo Probe Measurements

Temperatures and pressures measured by the Galileo probe during parachute descent into Jupiters atmosphere essentially followed the dry adiabat between 0.41 and 24 bars, consistent with the absence of a deep water cloud and with the low water content found by the mass spectrometer. From 5 to 15 bars, lapse rates were slightly stable relative to the adiabat calculated for the observed H2/He ratio, which suggests that upward heat transport in that range is not attributable to simple radial convection. In the upper atmosphere, temperatures of >1000 kelvin at the 0.01-microbar level confirmed the hot exosphere that had been inferred from Voyager occultations. The thermal gradient increased sharply to 5 kelvin per kilometer at a reconstructed altitude of 350 kilometers, as was recently predicted. Densities at 1000 kilometers were 100 times those in the pre-encounter engineering model.

Experiments revealing small impact of turbulence on the energy budget of the mesosphere and lower thermosphere

We have measured a total of 17 in situ profiles of small-scale density fluctuations (typical resolution: meters) in the lower thermosphere and upper mesosphere, which are used to derive turbulent parameters, such as the turbulent energy dissipation rate e, the turbulent diffusion coefficient K, and the mean turbulent velocity wturb. The accuracy of the absolute numbers is unprecedented thanks to the very high spatial resolution and a recently improved data analysis procedure. Concentrating on the 12 flights which were performed during winter conditions at high latitudes (69°N), we find mean energy dissipation rates of 1–2 mW kg−1 in the lower mesosphere (<75 km) and 10–20 mW kg−1 in the upper mesosphere and lower thermosphere (<100 km). The corresponding heating rates are approximately 0.1 and 1 K d−l, respectively. These values are at least 1 order of magnitude smaller than most of the previous measurements and are also significantly smaller than typical values assumed in models. Our observations suggest that the heating effect of turbulence is negligible compared to the most prevailing terms of the heat budget. It can be shown by theoretical considerations involving the turbulent energy budget equation that cooling by turbulent heat conduction is also negligible if e is small.

First in‐situ observations of neutral and plasma density fluctuations within a PMSE layer

The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, CUPRI and EISCAT observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.

Intercomparison of density and temperature profiles obtained by lidar, ionization gauges, falling spheres, datasondes and radiosondes during the DYANA campaign

Abstract During the course of the DYANA campaign in early 1990, various techniques to measure densities and temperatures from the ground up to the lower thermosphere were employed. Some of these measurements were performed near simultaneously (maximum allowed time difference: 1 h) and at the same location, and therefore offered the unique chance of intercomparison of different techniques. In this study, we will report on intercomparisons of data from ground-based instruments (Rayleigh- and sodium-lidar), balloon-borne methods (datasondes and radiosondes) and rocket-borne techniques (falling spheres and ionization gauges). The main result is that there is good agreement between the various measurements when considering the error bars. Only occasionally did we notice small but systematic differences (e.g. for the datasondes above 65 km). The most extensive intercomparison was possible between the Rayleigh lidar and the falling sphere technique, both employed in Biscarrosse (44°N,1°W). Concerning densities, excellent agreement was found below 63 km: the mean of the deviations is less than 1 % and the root mean square (RMS) is ~ 3%. Systematic differences of the order of 5% were noticed around 67 km and above 80 km. The former can be accounted for by an instrumental effect of the falling sphere (Ma = 1 transition; Ma = Mach number), whereas the latter is tentatively explained by the presence of Mie scatterers in the upper mesosphere. Concerning temperatures, the agreement is excellent between 35 and 65 km: the mean of the deviations is less than ± 3 K and the variability is ± 5 K. The two systematic density differences mentioned above also affect the temperatures: between 65 and 80 km, the Rayleigh lidar temperatures are systematically lower than the falling sphere values by ~ 5 K.

TOTAL: a rocket-borne instrument for high resolution measurements of neutral air turbulence during DYANA

Abstract An improved version of a rocket-borne instrument (‘TOTAL’), optimized for high resolution measurements of relative density variations, was successfully employed during the DYANA campaign in winter 1990. Both the inertial-convective subrange and the viscous-diffusive subrange of turbulence were observed in the power spectra derived from density fluctuations. An extended spectral model which comprises both subranges has been used to analyse the data. In this paper we present altitude profiles of turbulent parameters, such as turbulent energy dissipation rates e and turbulent diffusion coefficients K , which were derived from a total of eight successfully launched instruments at high (Andoya, 69°N) and middle (Biscarosse, 44°N) latitudes. The limitations of the measurement technique as well as instrumental errors are discussed. The results mainly show small values of e and K throughout the whole campaign period. The turbopause was found at an altitude of 95 ± 3 km.

Geostrophic wind fields in the stratosphere and mesosphere from satellite data

[1]xa0Daily maps of horizontally resolved zonal and meridional geostrophic wind fields in the altitude range of 20–90 km were obtained from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment during November 1994 and August 1997. Results are compared with zonal mean zonal winds from climatological sources and with assimilated data from standard analyses. Correlative data from ground-based and balloon- and rocket-borne measurements are used to validate the CRISTA winds. The comparisons show that geostrophic wind fields from high spatial resolution satellite-borne temperature measurements are a good approximation for the winds in the middle atmosphere up to mesopause heights. Therefore, they may be used as input for further modeling efforts particularly in the mesosphere, where no wind data from standard analyses are available.

Rocket and incoherent scatter radar common‐volume electron measurements of the equatorial lower ionosphere

[1]xa0A coordinated rocket and radar investigation has enabled the comparison of electron density measurements in essentially a common volume of the equatorial lower ionosphere. The rocket instrumentation included a Faraday rotation experiment, uniquely adapted for measuring electrons at low latitude, and fixed-bias Langmuir probes. The ALTAIR tracking radar was configured in an incoherent scatter mode. For the 75–92 km region, we note very good agreement between Faraday rotation and radar electron density measurements. Langmuir probe current measurements, calibrated to the Faraday rotation data, provided information about fine-scale electron density vertical structure and extended the rocket-radar comparisons of electron density to 130 km. Good overall agreement was also observed at higher altitudes, with evidence of electron density layering - including sporadic E - detected by both measurement techniques. The outcomes from our study support this unique Faraday rotation approach and the IS radar for measuring electron density in the equatorial lower ionosphere.

Morphology and sources of turbulence in the mesosphere during DYANA

Abstract During the DYANA campaign in early 1990 turbulent parameters were measured at various places by means of in situ and ground supported techniques. Rocket borne instruments detected small-scale fluctuations of neutral (TOTAL instrument) and ion (PIP instrument) number densities in the mesosphere and lower thermosphere. A total of six flights was successfully performed in Andoya (69°N) and two in Biscarosse (44°N). Altitude profiles of turbulent parameters, such as turbulent energy dissipation rates e. and turbulent diffusion coefficients K were derived from the fluctuations. Thanks to improvements in the instrumental capabilities, the reliability of the absolute values is unprecedented. The mean turbulent energy dissipation rates measured both by TOTAL and PIP in Andoya show very low values ( Nearly simultaneously with the sounding rocket flights, temperatures and winds were measured by meteorological rockets and by lidars. This allows study of the relationship between the occurrence of turbulence and the atmospheric stability, parameterized by the Richardson number. In 14 out of 18 cases strong turbulent layers were accompanied by low Richardson numbers. Both wind shear and convectively generated instabilities were observed. Given a particular altitude in the mesosphere, the TOTAL and PIP instrument detected turbulence in approximately 40–60% of all flights. In addition to the in situ measurements, energy dissipation rates were measured around 75 km by the chaff dispersion technique at Heiss Island (81°N) and at Volgograd (48°N). Much higher e values were observed (typically 100 mW/kg) using this procedure than those values obtained by in situ measurements. However, due to non-turbulent dispersion processes, such as small-scale gravity waves and wind shears, the absolute e values from this technique are considered upper limits.

Intense turbulence observed above a mesospheric temperature inversion at equatorial latitude

[1] Results from a sounding rocket experiment launched on September 19, 2004 from Kwajalein Atoll, Marshall Islands are reported. A large modulation of the temperature profile in the upper mesosphere was observed with a local maximum at 92 km, 40 K warmer than 2 km below. The temperature gradient between 92 and 102 km was near-adiabatic, suggesting strong mixing. Turbulence was observed in the lower part of the mixed layer, as evidenced by neutral and plasma density fluctuations on both the upleg and downleg portions of the flight. The plasma density gradient was less steep in the mixed region. The turbulent energy dissipation rate was found to be 170 mW/kg. The thermal structure can be described as an upper mesospheric inversion layer, possibly caused by enhanced wave breaking or turbulent heat transport.