Simultaneous Ground-based Measurements Research Articles

Estimation of soil and crop parameters for wheat from airborne radar backscattering data in C and X bands

The study of radar backscattering signatures of wheat fields was investigated, using data collected on the Orgeval agricultural watershed (France) by the airborne scatterometer ERASME in C and X bands, HH and VV polarizations, at incidence angles from 15° to 45°, during two years for different soil moisture conditions with simultaneous ground-based measurements. A simple parameterization as water-cloud model with two driving parameters (the surface soil moisture and the plant water content) gives satisfactory results to estimate radar cross sections of wheat for a wide range of frequencies (C and X bands) and incidence angles (20° and 40°) within 1 dB in CHH and XHH and 2 dB in CVV and XVV. At the lower frequency (C band) the attenuated soil backscattering by the vegetation is dominant. It is shown that simple linear relations in C band between radar cross section and soil moisture are insufficient. A correction term for the vegetation attenuation is needed and is determined. Low contrast between the backscattering of dry and wet soil (around 6 dB) for a given vegetation density leads to a relatively high error in the estimation of soil moisture by radar (0.06 cm 3 / cm 3). At the higher frequency (X band), the radar backscattering is negatively correlated to the vegetation water content with a saturation of the radar cross section as the plant grows (about 6 dB of dynamic range between low and fully grown canopy) with no dependence on the soil signal. The achievable accuracy in the estimation of crop water content is the same at 20° and 40° and higher in XHH (about 0.5 kg/m 2) than in XVV.

Equatorial mesospheric and F-region airglow emissions observed from latitude 4° south

Simultaneous ground-based measurements of the atmospheric airglow emissions OI 5577, 6300 and 7774 Å, NaD 5893 Å, OH (9, 4) band and O 2 atmospheric (0, 1) band at 8645 Å have been made at an equatorial station, Fortaleza (3.9°S, 38.4°W, geomag. 2.1°S), Brazil, since November 1986. A microprocessor controlled multichannel tilting filter type photometer was constructed for this purpose. Hydroxyl rotational temperatures in the range 190–210 K were obtained from the OH (9, 4) Q and R branch measurements. Surprisingly low OI 5577 Å and NaD intensities, about 40% and 25%, respectively, of those observed at low latitude (23°S), were observed. The OH (9, 4) band intensities, however, did not show any such difference. Time lagged nocturnal intensity variations of the mesospheric emissions observed provide evidence of dynamical effects in the equatorial upper atmosphere.

A case study of stably stratified airflow over the pennines using an instrumented glider

In this paper we present data gathered during a weak case of the Helm wind, an example of supercritical airflow, in the vicinity of Cross Fell in Northern England. Airborne measurements have been made using an instrumented glider with simultaneous ground-based measurements both on a hill summit and on the valley floor downwind of the hill.

Polar cleft structure and dynamics in the prenoon sector

Observations of electron and proton precipitation fluxes and Birkeland currents from spacecraft DMSP F7 and simultaneous ground-based optical measurements from Svalbard ( ∼75° geom.lat.) of dayside aurorae ( ∼ 09 MLT) are presented. In these cases, the auroral emissions within the field of view can be separated in different latitudinal zones with corresponding structures in the particle precipitation. The temporal evolution of these structures is monitored by the ground-based instruments, indicating the presence of both stationary cleft emissions produced by soft electron fluxes (energy few hundred eV) and multiple, transient, discrete arcs and arc-fragments corresponding to the precipitation of keV electrons. The latter are associated with narrow sheets of upward flowing Birkeland currents and reduced proton precipitation, suggesting the presence of field-aligned potential drops above the spacecraft.

Characteristics of a stable auroral red arc event

An intense (∼2.2 kR) stable auroral red (SAR) arc was observed on October 23, 1981 by the Dynamics Explorer 1 and 2 satellites as well as by a ground‐based scanning photometer operated by the Pacific Northwest Laboratory at Battelle Observatory in Richland, Washington, providing a unique series of nearly simultaneous measurements. The DE satellites provided in‐situ electric field and particle measurements at altitudes of about 850 km and 6000 km along approximately the same flux tube as well as in‐situ electron temperature and density measurements at 850 km for this red arc. These measurements along with ground‐based determination of arc position and intensity allowed a comprehensive analysis of the arc. Calculations of the electron heating rate and temperature which result from the observed precipitating electron flux at the arc location indicate that the heat required to establish the necessary and observed ionospheric electron temperatures is provided by the extrapolated low energy electron particle flux. The available data does not provide sufficient information to uniquely determine the energy source for this flux.

Large-scale neutral composition gravity waves in the thermosphere observed by Esro 4

Neutral composition waves with a wavelength of about 5000 km and a wave period of about 2.5 h were observed by ESRO 4 in the altitude region of 250 km. The amplitudes are of the order of 25% for Ar, 15% for N2, and roughly 10% for He and O at 25 deg geographic latitude. The Ar and N2 waves are almost in phase, whereas He is in antiphase, and O is in between. The wave amplitudes are seen to decrease toward lower latitudes, suggesting that the composition waves are launched by auroral sources. Simultaneous ground-based ionosonde measurements of the F2 layer critical frequency at middle and low latitudes show a wave period consistent with the satellite observations. From the relation between wavelength and wave period these waves are identified as gravity waves. The theoretical investigation with a multicomponent model shows that diffusion plays a major role in explaining the observed phase and amplitude relations between atmospheric constituents.

Correlation and spectrum properties of the potential gradient and space-charge density in the atmospheric surface layer

Correlation analysis of simultaneous ground-based measurements of the potential gradient and space-charge density, using antennae placed on a well-exposed site and separated by a horizontal distance of 27 m, shows that the two parameters possess the same ‘patterns’ and ‘scales’ of variation. ‘Self’ and mutual interaction rates of the two parameters are faster in high winds than in low winds, and under near-neutral than stable stratifications.

Electric currents and polarization fields at the base of the magnetosphere

A series of Ba-ion cloud measurements made by Föppl, Haerendel and collaborators and simultaneous ground-based measurements of the variation of the geomagnetic field have been investigated in detail. For undisturbed conditions these investigations lead to the conclusion that the main part of the electric polarization field at the base of the magnetosphere can be explained by the atmospheric dynamo theory, i.e. it is of ionospheric origin. Even at auroral zone latitudes, at most a small fraction of the field vector is due to processes acting in the magnetosphere and in the tail. For disturbed conditions the investigations prove that—even during small bay disturbances— a considerable part of the currents flowing in the ionosphere is due to polarization fields transferred from the magnetosphere.