Tropospheric jet stream as a source of traveling ionospheric disturbances observed by GPS

The integrity and the reliability of real-time precise positioning applications with Global Positioning System (GPS) are affected by the ionospheric variability with time and space. As a consequence, scientific community aims at describing, explaining and forecasting the occurrence and the amplitude of ionospheric irregularities observed by GPS. The use of the geometric-free combination of GPS dual frequency signals allows to retrieve the Total Electron Content (TEC) along the satellite-to-receiver path, which is the basic trans-ionospheric observable. Based on L1/L2 GPS phase measurements collected at a given station, the TEC high-frequency variability is isolated. A climatological study performed over 10 years in Western Europe shows that TEC irregularities are mostly observed daytime during quiet geomagnetic background in autumn and winter and correspond to classical Medium-Scale Traveling Ionospheric Disturbances (MSTIDs). The latter are generally understood as the ionospheric signature of Atmospheric Gravity Waves (AGWs), either generated in situ (solar terminator) or in the lower atmosphere and propagating upward. Because of its associated strong wind shears, the tropospheric jetstream, occurring mainly during autumn and winter months, constitutes an ideal candidate for AGW generation. This paper analyzes the spatial correlation between the presence of both MSTIDs and strong jetstream over Western Europe. This correlation is positive when the ionospheric pierce point of the satellite is located above regions of interest where wind shears are very large. In practice, we have selected regions for which wind speed is larger than 50 m/s. In addition, the propagation of AGWs up to the ionospheric layer is taken into account by assuming horizontal and vertical velocities of 200 and 50 m/s respectively. It comes that the region of interest of the correlation study is computed using an isotropic slant propagation of the AGW, which is supposed to be generated at a tropospheric level.Based on 30s GPS data collected over several stations in Belgium and on European Centre for Medium-Range Weather Forecasts (ECMWF) wind velocity maps, the correlation study covers a period ranging from January 2002 to December 2011. Preliminary results based on a limited number of cases show that large amplitude MSTIDs are generally observed during periods of strong wind speeds at an altitude corresponding to a pressure level of 250hPa (about 10 km)

Detection of medium scale traveling ionospheric disturbances with TIMED/GUVI limb observations at mid and low latitude regions

Medium-scale traveling ionospheric disturbances (MSTIDs) are the most recurrent type of ionospheric irregularities at mid-latitudes but also occur in low-latitude regions. Whether they are due to the propagation of atmospheric gravity waves originating from the lower atmosphere or related to sporadic E layers, their harmonic signature is a common feature that allows them to be easily identified. MSTIDs have been extensively studied and characterized during the last two decades, mainly using GNSS measurements, ground-based all-sky imagers, radars or ionosondes. However, only few studies aimed to describe their vertical structure using remote sensing observations from space, which is helpful to understand their propagation and their dissipation processes. NASA’s TIMED mission was launched in December 2001 on a 74° inclination orbit at an altitude of 625 km, which allowed covering both low and high-latitude regions. The Global Ultraviolet Imager (GUVI) instrument aimed at remotely sense, among others, the ionospheric ion and electron densities. GUVI performed disk observations and limb scans in five FUV wavelength channels, making it an ideal tool to characterize the vertical structure of the ionosphere as well as to contextualize the study. The purpose of this work is to use GUVI limb scans to characterize MSTIDs preliminary detected by GNSS before December 2007, until the limb scanning mode failed. We first select a few MSTID cases during maximum background conditions of the Total Electron Content (TEC) computed by GNSS ground stations. Then, coincidental GUVI limb scans of the OI-135.6 nm emission are analyzed to characterize the vertical structure of the MSTIDs. The comparison is completed by the analysis of ionosonde profiles collected in the vicinity of the region where the MSTIDs have been previously detected by GNSS and GUVI.Combining airglow, GNSS and ionosonde data to study ionospheric irregularities over low latitude

Potential of TIMED/GUVI limb observations for medium-scale traveling ionospheric disturbances study at mid-latitudes

At mid-latitudes, medium-scale traveling ionospheric disturbances (MSTIDs) are the most recurrent type of ionospheric irregularities. During daytime, the common source of MSTIDs is the propagation of atmospheric gravity waves whose origin is generally found in the lower atmosphere. In the nighttime hours, the Perkins instability induces another type of MSTIDs that is correlated with the appearance of sporadic E layers, sometimes leading to spread-F signatures in ionograms. MSTIDs climatology and characterization have been extensively described during the last two decades, mainly using GNSS measurements. However, only few studies are devoted to the description of their vertical structure and the monitoring of their propagation into the ionosphere, which is helpful to understand their dissipation processes and their physical origin. The NASA’s TIMED mission was launched in December 2001 on a 74° inclination low-Earth orbit at an altitude of 625 km, which allowed to cover both low and high-latitude regions. The Global Ultraviolet Imager (GUVI) instrument aimed at remotely sense, among others, the ionospheric ion and electron densities. GUVI performs disk observations and limb scans in five FUV wavelength channels, making it an ideal tool to characterize the vertical structure of the ionosphere as well as to contextualize the study. The purpose of this work is to use GUVI limb scans to characterize MSTIDs preliminary detected by GNSS in mid-latitudes before December 2007, after which the instrument exclusively supplied disk observations. We first select a few MSTID cases during solar maximum conditions that were observed in the Total Electron Content (TEC) by GNSS ground stations. Then, we combine our dataset with GUVI limb observation of the OI-135.6 nm emission to characterize the vertical structure of the MSTIDs. At last, concurrent observations from ionosondes located in the vicinity of the region where the GNSS and GUVI data were obtained will also provide an interesting cross-comparison dataset

A simple, autonomous, non-linear inversion method for the analysis of occultation observation of the dusty atmosphere of Mars.

editorial reviewedOzone (O3) is an important atmospheric specie of planet Mars, capable of absorbing ultraviolet (UV) radiation. Occultation of solar (or stellar) radiation and measurement of the extinction of UV photons by the atmosphere is a standard O3 remote sensing method. Both O3 and carbon dioxide (CO2) absorb UV photons in the 200 – 300 nm range, the O3 Hartley absorption band peaking near 250 nm. Dusts also contribute to, and sometimes dominate, the UV extinction by the atmosphere of Mars. The wavelength-dependent dust extinction coefficient (k) is often described using a power law k=k0 (λ0/ λ)α with reference value k0 at wavelength λ0. The ad-hoc α exponent stems from the properties of the dusts. We develop a simple autonomous, nonlinear method to retrieve the vertical profiles of CO2, O3 and dust properties from solar occultation profiles, under a spherical symmetry assumption. The gas concentration and dust reference extinction (k0) are represented using a combination of triangle functions of the radial distance (r), producing a piecewise linear profile. The α parameter is represented similarly using triangle functions of log(r). Slant line-of-sight optical thickness results from the Abel transform of these profiles, producing hypergeometric 2F1 functions for the dusts. The different parameters are retrieved by inverse Abel transform using a least squares minimization, which depends linearly on the CO2, O3 and k0 profiles, and non-linearly on α. The linear parameters are considered as functions of the α, reducing the fitting to a non-linear minimization over the α parameter profile only. This drastically reduces the number of dimensions of the parameter space. We show that this method allows efficient retrieval of all the parameters. Noise is however expected to be present when analyzing occultation data from the NOMAD-TGO instrument, which can reduce the ability to retrieve the minimization parameters. The k0 and O3 profiles can, nevertheless, be expected to be retrieved over about two orders of magnitude, while the CO2 density profile can be expected to be fairly retrieved at relatively low altitude

Comparison of ICON O+ density profiles with electron density profiles provided by COSMIC-2 and ground-based ionosondes

In October 2019, NASA-ICON was launched to observe the low-latitude ionosphere using in-situ and remote sensing instruments, from a LEO circular orbit at about 575 km altitude. The six satellites of the radio-occultation program COSMIC-2 were also successfully launched and currently provide up to 3000 electron density profiles on a daily basis since October 1, 2019. Besides, the network of ground-based ionosondes is constantly growing and allows retrieving very accurate measurements of the electron density profile up to the peak altitude. These three sources of scientific observation of the Earth ionosphere therefore provide a very complementary set of data. We compare O+ density profiles provided during nighttime by the ICON-FUV instrument and during daytime by the ICON-EUV instrument against electron density profiles measured by COSMIC-2 and ionosondes. Co-located and simultaneous observations are compared on statistical grounds, and the differences between the several methods are investigated. Particular attention is given to the most important variables, such as the altitude and the density of the F-peak, hmF2 and NmF2. The time interval considered in this study covers the whole ICON data availability period, which started on November 16, 2019. Manual screening and scaling of ionograms is performed to ensure reliable ionosonde data, while COSMIC-2 data are carefully selected using an automatic quality control algorithm. A particular attention has been brought to the geometry of the observation, because the line-of-sight integration of both airglow and radio-occultation measurements assimilates horizontal and vertical gradients. As a consequence, the local density profiles obtained by inversion of the ICON and COSMIC-2 observation cannot be exactly assimilated to vertical measurements, such as vertical incidence soundings from ionosondes. This slightly limits the reach of the interpretation of the comparison between data of different origin. However, using similar observing geometries, the comparison of ICON and COSMIC-2 data does nevertheless provide very reliable and valuable comparisons.Combining airglow, GNSS and ionosonde data to study ionospheric irregularities over low latitude

GENESIS: Co-location of Geodetic Techniques in Space

Improving and homogenizing time and space reference systems on Earth and, more directly, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1mm and a long-term stability of 0.1mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation for predicting natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities which has enunciated geodesy requirements for Earth sciences. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, proposed as a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this white paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology.Comment: 31 pages, 9 figures, submitted to Earth, Planets and Space (EPS