ALBERT

Beschreibung: Post sunset equatorial ionospheric plasma irregularities are a unique study case to investigate ion-neutral coupling in the low-latitude ionosphere, additionally, they are known to disturb radio wave communication. We use high precision magnetometer data from 10 years of CHAMP (2000-2010) and the recent the Swarm (2013-today) LEO missions high precision magnetometer data to derive a global model of their occurrence rate in the F region ionosphere (300-500km). The model is derived by appropriate functions, e.g., parameterizing season, longitude, solar flux, local time and persistence to describe the significant probability distribution. The model shows very good agreement to earlier climatological investigations based on other satellite missions. Identification of the plasma irregularities is through correlation between the high resolution insitu magnetic and electron density time series. This relation tells about ionospheric pressure-gradient electric currents. We find a typical global distribution of positive and negative correlation values opening new questions on processes of local ionospheric energy balance.

Beschreibung: As a result of the coupling between the solar wind and the Earth’s magnetosphere-ionosphere system, plasma density irregularities are often developed in the polar ionosphere, which in turn can severely degrade satellite-based positioning and communication systems. High-latitude ionospheric irregularities are often associated with polar cap patches, blobs, particle precipitation, and the main ionospheric trough. These ionospheric phenomena can be studied by using in-situ data from polar orbiting satellites. Here, we present a statistical study based on observations from the European Space Agency’s Swarm mission. Swarm satellites are especially useful at high latitudes due to the good coverage in both hemispheres as compared to ground-based instruments. Two comprehensive datasets are used to detect ionospheric irregularities, i.e., electron density from the Langmuir probe and GPS TEC from the GPS receiver onboard Swarm satellites. We derive several parameters to describe ionospheric irregularities, e.g., the rate of change of density index (RODI) and electron density gradients (\nabla_Ne) from the electron density data, rate of change of TEC index (ROTI) from GPS data (these parameters are included in the Swarm IPIR product). The statistics show that electron density and plasma irregularities are modulated by the Earth’s magnetic field in both hemispheres. The climatology maps in magnetic coordinate show predominant plasma irregularities near the dayside cusp, polar cap, and nightside auroral oval. The spatial distribution of plasma irregularities are also modulated by the interplanetary magnetic field (IMF), e.g., when considering the IMF By component, the irregularity distribution in the cusp and polar cap shows asymmetry between the Northern (NH) and Southern hemispheres (SH), i.e., for negative IMF By, the irregularities are stronger in the dusk (dawn) sector in the NH (SH) and vice versa. This feature is consistent with high-latitude ionospheric convection pattern that is regulated by the IMF By. Plasma irregularities are also strongly controlled by solar activity within the current declining solar cycle. Ionospheric irregularities in polar caps show clear seasonal variation with higher values near equinoxes and lower values near solstices for both hemispheres.

Beschreibung: The LEO (Low Earth Orbit) satellite-based TEC (Total Electron Content) and ROTI (Rate Of change of TEC Index) are highly relevant for users in navigation and communications in view of strong plasma gradients causing GPS signal degradation and can lead to the losses of the signal. ESA’s Swarm constellation mission, consisting of three identical satellites, was launched on 22 November 2013. Among other instruments, each Swarm satellite carries a dual frequency GPS (Global Positioning System) Receiver (GPSR), which can be used to derive TEC and ROTI data. Swarm ROTI is defined as a standard deviation of Rate Of change of TEC (ROT) and it describes the small-scale variability of the line of sight electron content resulting from the ionosphere and plasmasphere. We present the climatology of TEC and ROTI using 5 years of Swarm constellation mission observations and investigate the seasonal and local time dependence at high and low latitudes. Also, the newly derived TEC and ROTI products for the ESA’s GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) mission, which flew from 2009 to 2013 at approximately 270 km altitude, are presented. We compare GOCE results to the Swarm observations for the same local times and seasons, taking into account similar solar conditions. Contrary to the Swarm satellite data, GOCE TEC observations contain a considerable part of the F region electron density. The comparison between both data sets enables the estimation of the electron content variation in the upper atmosphere.

Beschreibung: Gravity fields derived from GPS tracking of the three Swarm satellites have shown artifacts near the geomagnetic equator, where the carrier phase tracking on the L2 frequency is unable to follow rapid ionospheric path delay changes due to a limited tracking loop bandwidth of only 0.25 Hz in the early years of the mission. Based on the knowledge of the loop filter design, an analytical approach is developed to recover the original L2 signal from the observed carrier phase through inversion of the loop transfer function. Precise orbit determination and gravity field solutions are used to assess the quality of the correction. We show that the a posteriori RMS of the ionosphere-free GPS phase observations for a reduced-dynamic orbit determination can be reduced from 3 to 2 mm while keeping up to 7% more data in the outlier screening compared to uncorrected observations. We also show that artifacts in the kinematic orbit and gravity field solution near the geomagnetic equator can be substantially reduced. The analytical correction is able to mitigate the equatorial artifacts. However, the analytical correction is not as successful compared to the down-weighting of problematic GPS data used in earlier studies. In contrast to the weighting approaches, up to 9–10% more kinematic positions can be retained for the heavily disturbed month March 2015 and also stronger signals for gravity field estimation in the equatorial regions are obtained, as can be seen in the reduced error degree variances of the gravity field estimation. The presented approach may also be applied to other low earth orbit missions, provided that the GPS receivers offer a sufficiently high data rate compared to the tracking loop bandwidth, and provided that the basic loop-filter parameters are known.

Beschreibung: ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) orbited the Earth between 2009 and 2013 for the determination of the static part of Earth’s gravity field. The GPS-derived precise science orbits (PSOs) were operationally generated by the Astronomical Institute of the University of Bern (AIUB). Due to a significantly improved understanding of remaining artifacts after the end of the GOCE mission (especially in the GOCE gradiometry data), ESA initiated a reprocessing of the entire GOCE Level 1b data in 2018. In this framework, AIUB was commissioned to recompute the GOCE reduced- dynamic and kinematic PSOs. In this paper, we report on the employed precise orbit determination methods, with a focus on measures undertaken to mitigate ionosphere-induced artifacts in the kinematic orbits and thereof derived gravity field models. With respect to the PSOs computed during the operational phase of GOCE, the reprocessed PSOs show in average a 8–9% better consistency with GPS data, 31% smaller 3-dimensional reduced-dynamic orbit overlaps, an 8% better 3-dimensional consistency between reduced-dynamic and kinematic orbits, and a 3–7% reduction of satellite laser ranging residuals. In the second part of the paper, we present results from GPS-based gravity field determinations that highlight the strong benefit of the GOCE reprocessed kinematic PSOs. Due to the applied data weighting strategy, a substantially improved quality of gravity field coefficients between degree 10 and 40 is achieved, corresponding to a remarkable reduction of ionosphere-induced artifacts along the geomagnetic equator. For a static gravity field solution covering the entire mission period, geoid height differences with respect to a superior inter-satellite ranging solution are markedly reduced (43% in terms of global RMS, compared to previous GOCE GPS-based gravity fields). Furthermore, we demonstrate that the reprocessed GOCE PSOs allow to recover long-wavelength time-variable gravity field signals (up to degree 10), comparable to information derived from GPS data of dedicated satellite missions. To this end, it is essential to take into account the GOCE common-mode accelerometer data in the gravity field recovery.