The Mars advanced radar for subsurface and ionospheric sounding (MARSIS) on Mars Express is the first high-frequency sounding radar operating from orbital altitudes since the Apollo 17 Lunar Sounder flown in 1972. The radar operates from a highly elliptical orbit but acquires data only from altitudes lower than 1200km. The periapsis altitude is 250km. This radar has been succesfully operating since August 2005. The radar is a dual channel low-frequency sounder, operates between 1.3 and 5.5MHz (MegaHertz) with wavelengths between 230 and 55m in free space for subsurface sounding and between 0.1 and 5.5MHz (wavelengths between 3000 and 55m) for ionospheric sounding. The subsurface sounder can operate at one or two-frequency bands out of four available bands at either like or cross polarization. The subsurface sounding radar transmits radio frequency (RF) pulses of 250μs duration through a 40m dipole antenna. The return echoes are then converted to digital form and temporarily stored on board for some digital processing. A second antenna, a monopole, provides reception for the cross-polarized return and its data are processed by a second channel. This processing reduces the data rate produced by the instrument to rates allowed by the spacecraft communications channel. These processed returns are then sent to Earth by the telecommunications system on the spacecraft. The advances in digital data acquisition and processing, since 1972, have enabled this technique to be used in a compact spacecraft science instrument.. This sounder has obtained returns from several kilometers below the surface of the Mars. The ionospheric sounder operates at altitudes greater than 800km in a mode that sweeps the entire 0.1–5.5MHz range. During ionospheric sounding, the transmitter sends a 91μs tone at 127 pulses per second rate. The frequency sweep takes 7.3s to complete the 0.1–5.5MHz range. Operational aspects of the instrument are described, including the selection of frequency bands and receive antenna selection, which are based on the expected solar zenith angle. The process of data take planning as well as data archiving are described. Results of both subsurface and ionospheric sounding are presented.

A probe was designed to investigate the moulins (melt water drainage channels on an ice cap) and ice-hydrology interaction in the Greenla nd Ice Cap.

The success of a scientific mission is determined by the quality of the scientific results. The prompt delivery of instrument and ancillary raw data to the instrument teams and of reduced and calibrated data to the scientific community is therefore a key element in the mission design. This chapter describes the flow of data from the spacecraft through the ground segment via the instrument teams to the final scientific archive. Several software tools and standards are used to support data dissemination. The functionality of the individual tools is explained, the interfaces to the individual groups are discussed, and examples of the graphical user interfaces are shown. Finally, the chapter provides a brief introduction to each of the currently available datasets.

The low-frequency radar, Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board the Mars Express spacecraft is used to sound electron densities in the topside Martian ionosphere. The radar records the delay times to echoes of reflected radio waves as a function of frequency, yielding spectrograms with traces of radar echoes. At times, two traces are present in spectrograms of the Martian ionosphere. One of these traces corresponds to reflections from the direction to nadir. The other trace originates in a localized reflector in the ionosphere. The local reflectors can be associated with the cusplike regions of near-vertical crustal magnetic fields. The apparent nadir angle of reflection can occasionally increase to 90 degrees. This suggests that steep gradients of the altitude of the electron isodensity exist in the Martian ionosphere and indicates rapid horizontal spatial variations of vertical diffusion of Martian plasma. Such gradients may arise owing to preferential access of solar wind to the cusplike regions or to precipitation of energetic electrons from acceleration regions located on cusp magnetic field lines high above the ionosphere.

The Martian ionosphere has for the first time been probed by a low frequency topside radio wave sounder experiment (MARSIS) (Gurnett et al., 2005). The density profiles in the Martian ionosphere have for the first time been observed for solar zenith angles less than 48 degrees. The sounder spectrograms typically have a single trace of echoes, which are controlled by reflections from the ionosphere in the direction of nadir. With the local density at the spacecraft derived from the sounder measurements and using the lamination technique the spectrograms are inverted to electron density profiles. The measurements yield electron density profiles from the sub-solar region to past the terminator. The maximum density varies in time with the solar rotation period, indicating control of the densities by solar ionizing radiation. Electron density increases associated with solar flares were observed. The maximum electron density varies with solar zenith angle as predicted by theory. The altitude profile of electron densities between the maximum density and about 170m altitude is well approximated by a classic Chapman layer. The neutral scale height is close to 10 to 13 km. At altitudes above 180 km the densities deviate from and are larger than inferred by the Chapman layer. At altitudes above the exobase the density decrease was approximated by an exponential function with scale heights between 24 and 65 km. The densities in the top side ionosphere above the exobase tends to be larger than the densities extrapolated from the Chapman layer fitted to the measurements at lower altitudes, implying more efficient upward diffusion above the collision dominated photo equilibrium region.

Both the MARSIS ionospheric sounder and the charged particle instrument package ASPERA-3 are experiments on board the Mars Express spacecraft. Joint observations have shown that events of intense ionospheric electron density enhancements occur in the lower ionosphere of magnetic cusp regions, and that these enhancements are not associated with precipitation of charged particles above a few hundred electron volts (<300eV). To account for the enhancement by particle precipitation, electron fluxes are required with mean energy between 1 and 10keV. No ionizing radiation, neither energetic particles nor X-rays, could be identified, which could produce the observed density enhancement only in the spatially limited cusp regions. Actually, no increase in ionizing radiation, localized or not, was observed during these events. It is argued that the process causing the increase in density is controlled mainly by convection of ionosphere plasma driven by the interaction between the solar wind and crustal magnetic field lines leading to excitation of two-stream plasma waves in the cusp ionosphere. The result is to heat the plasma, reduce the electron–ion recombination coefficient and thereby increase the equilibrium electron density.

Snow deposition, accumulation, and melt on an ice sheet are key components of mass balance. Innovative algorithms using satellite scatterometer data have been developed to monitor snowmelt, ice layer extent, and snow accumulation on Greenland with verifications using in-situ data from the Greenland Climate Network (GC-Net). QuikSCAT/SeaWinds Scatterometer (QSCAT) has collected data over Greenland and Antarctic two times per day since July 1999. QSCAT data show the shortest melt season in 2004, verified by GCNet data at ETH/CU Camp, and detect peculiar snowmelt during wintertime in Greenland in 2005. QSCAT results reveal a record increase in the snow accumulation rate on the Greenland ice sheet including the west flank in January-March 2005 with an estimate of 565 kM3 of total snow accumulation volume. The record snow anomaly is verified by GC-Net snow measurements, showing the largest snow accumulation rate in the first half of 2005 ever recorded in the past decade since the inception of the GC-Net. The QSCAT algorithins developed for Greenland are adapted for Antarctica. QSCAT results show strong melt in 2002 and prolonged melt in 2005 at McMurdo. New extensive ice layers, created by refreezing of melt water in the fim layer, were identified by QSCAT along the Antarctica Walgreen, Bakutis, and Hobbs coasts extending well inland in 2005. Extensive regions of ice layering, evidence of preceding strong melt occurrence, were also found over the Rockefeller Plateau and along the Ross Ice Shelf adjacent to Queen Maud Mountains in 2005.

The analysis of the surface return echoes in the subsurface data extraction in North Mars Polar region has shown the possibility to utilize simplified surface models, at least, for the purpose of the evaluation of the penetration depth capability. The surface simulation, obtained starting from MOLA data, has been utilized during the planning activity in order to select the MARSIS operative sequence in order to optimize the amount of scientific data taking into account the data rate available and the scientific target to be investigated during the next part of the mission. Moreover a simulator, still in progress, utilizing the surface characteristics will perform the analysis of the real data in order to make available the required information expected, by the mission, in terms of detection and identification of dielectric constant on the surface and subsurface.

Our objective is to determine seasonal snow accumulation in the percolation zone of the Greenland ice sheet on the daily-weekly basis over the large scale. Our approach utilizes data from the Greenland Climate Network (GC-Net) and from the SeaWinds Scatterometer on the QuikSCAT satellite (QSCAT) to measure snow accumulation (SA) in the percolation zone of the Greenland ice sheet. GC-Net measurements provide crucial in-situ data to facilitate the interpretation of QSCAT backscatter signature for the development of an algorithm to map SA.

The Greenland ice sheet underwent record extensive melt in 2002 and prolonged melt in 2003. The severe melting created a significant and extensive ice layer over the Greenland ice sheet. An innovative approach is developed to detect the ice layer formation using data acquired by the SeaWinds scatterometer on the QuikSCAT satellite. QuikSCAT backscatter together with in situ data from automatic weather stations of the Greenland Climate Network are used to map the extent of ice layer formation. The results reveal areas of extensive ice layer formed by the 2002 melt, which is consistent with the maximum melt extent in 2002. Moreover, during freezing seasons, QuikSCAT data show a linear decrease in backscatter (in decibels or dB) that is related to the amount of snow accumulation in the ice layer formation region. This snow accumulation signature is caused by the attenuation of radar waves in the snow layer, accumulating since the last major melt event, whose thickness appears as an exponential function in relation to the backscatter signature. We use the Greenland Climate Network data to calibrate the QuikSCAT accumulation rate in order to estimate and map snow accumulation. QuikSCAT results capture the extreme snowfall in mid‐April 2003, which deposited more than 0.5 m of snow in a day as measured by the automated weather station at the NASA South East site. Large‐scale QuikSCAT results show an anomalous increase of snow accumulation over the southeast region of Greenland during the 2002–2003 freezing season.

The main features of the VENSIS radar in the VENUS EXPRESS mission are shown and the expected subsurface and ionosphere sounding performance have been discussed, according to the models of the Venus crust composition, geometric structure and ionosphere environment. VENSIS complements Magellan data of the surface and subsurface of Venus by using long wavelength sounding radar to probe the third dimension of the Venusian crust. Ionosphere sounding improves new unique data in the solar wind interactions with the planet, and is used to monitor the atmosphere for lightning.

Active and passive microwave satellite data are used to map snowmelt extent and duration on the Greenland ice sheet. The passive microwave (PM) data reveal the extreme melt extent of 690,000 km2 in 2002 as compared with an average extent of 455,000 km2 from 1979–2003. A statistical analysis of the melt time series affirms an increased melt extent earlier in the melt season. The QuikSCAT (QSCAT or AM) analysis for 1999 to 2004 confirms the extreme melt situation in 2002. QSCAT data show a significant increase in melt season length over several areas in 2003. SSM/I and QSCAT melt data reveal that the later detects melt earlier and is more sensitive to surface melt. QSCAT detects an earlier stage of melt and SSM/I detects a later stage of melt. The SSM/I XGPR melt extent is approximately confined to the QSCAT melt areas experiencing 2 weeks or more of melting time.

As a subsurface sounder, VENSIS would obtain fundamentally different kinds of geologic Information than Magellan. Mapping of Interfaces of geologic units (e.g. tessera, plains, lava flows, impact debris) could be extended into the third dimension. Reflectivity variations recorded at the surface by Magellan are likely to extend into subsurface, providing dielectric contrast at interfaces. The VENSIS sounder would also snow detailed characterization of the Venus Ionosphere using active sounding in a frequency range of 100 kHz to 6.5 MHz. In a passive mode, VENSIS can be used to detect lightning the presence of which remains both controversial and critical to understanding the behavior of the atmosphere and the possibility of present day Volcanism. In this paper the Scientific Objectives are described and the system performance am analyzed.

High order spectral techniques are one of the most powerful tools for the identification of non-linear processes in space plasma turbulence. Without such tools it is impossible to determine, for example, the occurrence of non-linear interactions between spectral components. However, since the observations made by an instrument involve some internal processing, it is possible that non-linearities may arise from within the instrument itself rather than in the plasma. The possibility of such effects are investigated using data resulting from tests carried out using the Cluster Wideband electric field instrument.

In order to get the maximum scientific return from available resources, the wave experimenters on Cluster established the Wave Experiment Consortium (WEC). The WEC's scientific objectives are described, together with its capability to achieve them in the course of the mission. The five experiments and the interfaces between them are shown in a general block diagram (Figure 1). WEC has organised technical coordination for experiment pre-delivery tests and spacecraft integration, and has also established associated working groups for data analysis and operations in orbit. All science operations aspects of WEC have been worked out in meetings with wide participation of investigators from the five WEC teams.