The Impact of Sudden Stratospheric Warmings and Elevated Stratopause Events on the VLF signal in high latitudes

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in wind, temperatures and vertical mass transport, especially in stratospheric and mesospheric altitudes. Likely, SSW and ES induced changes also cause modifications in the sensitive D-region ionization (~60-90 km). This bottom side of the Ionosphere behaves together with the Earth-surface as a reflection boundary for the Very Low Frequency Transmission, used for long distance communication. Here we want to study the impact of SSW/ES events on the VLF signal in high latitudes. For the identification of SSW/ES induced perturbations of the VLF signal, the typical seasonal variation and outlier caused by noise, technical adjustments or solar events need to be removed. A quiet time curve, representing the seasonal VLF signal variation under undisturbed conditions, was developed with a polynomial fit of the composite. In preparation for the composite, the VLF data needed to be leveled due to artificial amplitude steps with technical origin in the timeseries. The leveling was done with help of the Pruned Exact Linear Time method. Additionally, outlier have been removed using the Median Absolute Deviation, a method from robust statistics. With help of the developed quiet time curve, VLF signal perturbations could be identified, caused by the SSW and ES events. Here we want to describe and discuss those VLF signal perturbations for multiple links in high latitudes, considering the different pathways between Transmitter and Receiver as the ES events vary strongly with longitude

The VLF network GIFDS for a ground-based monitoring of solar disturbances

It has been observed that solar flares reflect in the amplitude and phase of radio measurements. As these bursts of X-ray radiation may be harmful, the German Aerospace Center (DLR) operates the Global Ionospheric Flare Detection System (GIFDS) to continuously monitor from the ground the current state of the ionosphere's D region, which is the linking layer affected from the outside in the relevant solar spectral range and in turn taking effect on steadily available VLF signals. A major issue in flare recognition based on the radio signal changes is the non-constant progression of the quiet level over the day. That's why a flare may yield different pictures depending on the time it hits the ionosphere. Taking care of the diurnal variation is crucial for setting up a system capable of raising alerts. In addition, investigating seasonal changes is important also for understanding Earth-originating anomalies like the "October effect", a sharp decrease in signal strength during fall in mid-latitude regions (subject to the project "Analysis of the MEsosphere and Lower Ionosphere fall Effect", AMELIE). Proper flare detection and size estimation rely on identifying the normal amplitude level. As the measurements could suffer also from small-scale oscillations, the instruments were improved and harmonized to suppress noise, resulting in the compact GIFDS receiver

Findings on the October Effect

Very Low Frequency (VLF) radio signals provide a unique possibility of continuously monitoring the lower ionosphere and their dynamics since these signals are reflected at the ionospheric D region between 60-90 km. Recent investigations have shown a very sharp decrease in signal amplitude at the beginning of October which deviates from the actual symmetric course of solar zenith angle variation over the year. The effect is developed differently depending on latitude, longitude and frequency, as we will present. In investigation for the cause of this phenomenon, first comparisons suggest a close correlation with the sudden reversal from easterly to westerly zonal flow, the asymmetric peak in semidiurnal solar tide S2, and the progression of the lower mesospheric temperature. Independent of the solar zenith angle mostly in high latitudes, a strong warming of the lower mesosphere during fall can be observed, confirming dominating atmospheric inner dynamics. Further studies are ongoing

A new plasmapause model based on IMAGE RPI and Van-Allen-Probe data via automatic detection

The plasmapause, i.e. the outer boundary of the plasmasphere, is characterised by a sharp electron density gradient. The Neustrelitz ESOC PlasmaPause Model (NEPPM) is a newly developed model of the plasmapause location Lpp. The actual plasmapause positions are derived from the electron density measurements recorded onboard the IMAGE satellite between 2000 and 2005 and the Van Allen probes between 2012 and 2018. An automatic algorithm is developed for detecting plasmapause location along electron density versus altitude profile. The NEPPM model functions are fitted to the Lpp measurements in a least squares sense and model parameters are determined. In our NEPPM approach an ellipse is assumed to describe the principal plasmapause shape in the geomagnetic equatorial plane. This is aligned with the bulge that follows the level of solar activity. Embedded into a 3D approach, the NEPPM allows non-dipole B vectors, providing 3D positions on the plasmapause torus for given latitude, longitude, epoch and Dst. The underlying fitting procedures recreate the varying Lpp as a function of the Dst index and magnetic local time, which gives a better conformity than the GCPM (Global Core Plasma Model). We thank ESOC (ESA/ESOC/OPS-GN) for their support in developing the model

Spring-fall asymmetry in VLF amplitudes recorded in the North Atlantic region: The fall-effect

A spring-fall asymmetry is observed in daytime amplitude values of very low frequency (VLF) radio wave signals propagating over the North Atlantic during 2011-2019. We explore the processes behind this asymmetry by comparing against mesospheric mean temperatures and the semidiurnal solar tide (S2) in mesospheric winds. The solar radiation influence on VLF subionospheric propagation was removed from the daytime VLF amplitude values, isolating the fall-effect. Similarly, the symmetric background level was removed from mesospheric mean temperatures undertaking comparable analysis. During fall, all three analyzed parameters experience significant deviation from their background levels. The VLF amplitude variation during spring is explained by the seasonal variation in solar illumination conditions, while the fall-effect can be interpreted as a mean zonal wind reversal associated with both a S2 enhancement, and temperature reductions. Decreases in temperature can produce decreases in collision frequency, reducing VLF signal absorption, driving the observed VLF asymmetry

Development of a plasmapause model derived from Van-Allen-Probe data and IMAGE RPI data via automatic detection

The outer boundary of the plasmasphere, the plasmapause, is characterised by a sharp electron density gradient which changes under varying space weather conditions. With NEPPM (Neustrelitz ESOC Plasmapause Model), we introduce a new model of the plasmapause location Lpp based on electron density measurements made by the Van Allen probes from 2012 to 2016 and the IMAGE satellite from 2000 to 2005 that were automatically processed, yielding an improved performance for plasmapause detection. A 2D model provides a simple elliptical approach in the equatorial plane determined by the semi-major axis, the eccentricity, and the orientation angle. The Lpp varies as a function of Dst index and magnetic local time (MLT), resulting in a tighter fit compared to the GCPM (Global Core Plasma Model). The distinctive bulge in the evening hours follows the level of solar activity. By extending the ellipse fitting from the equatorial plane to a 3D approach, the NEPPM also allows non-dipole B vectors, providing 3D positions on the plasmapause torus for given latitude, longitude, epoch and Dst

The GIFDS network and its use for modeling the lower ionosphere - A case study of the solar eclipse effects of 20 March 2015

The Global Ionospheric Flare Detection System (GIFDS) of the DLR consists of a ground-based network of VLF receivers, which provides amplitude and phase measurements of multiple frequency channels ranging from 10 to 100 kHz. One of the main objectives of GIFDS is the immediate and continuous detection of solar flares as a result of their impact on the lower ionosphere. This presentation focuses on changes of the D-region ionosphere using VLF amplitude measurements. On the example of the solar eclipse of 2015, VLF signal propagation was simulated according to the waveguide mode theory with the help of the Long Wavelength Propagation Capability Code (LWPC). Based on an exponential model of D-region electron densities, ionospheric changes in the lower ionosphere can be reconstructed with high correlation coefficients between measured and modelled VLF amplitudes

A deeper insight into the dynamics of the fall transition in the MLT

In autumn the prevailing wind in the middle atmosphere at mid and high latitudes changes from summer easterly to winter westerly. This process is not smooth but interrupted by the Hiccup of the fall transition with characteristics similar to a mini sudden stratospheric warming (SSW) which occurs in fall even though the zonal mean zonal wind does not reverse to easterly again. Combining global reanalysis data and satellite observations we improve our knowledge and understanding of the dynamics of the Hiccup of the fall transition in the middle atmosphere. The introduction of a new definition for the onset of the Hiccup focusing now on its core region in the lower mesosphere allows us the automatic detection of a Hiccup in almost every year and thus a deeper insight into its dynamics. For example, we found a latitudinal and altitudinal shift in the zonal wind regime during the Hiccup. We also investigate its 3D-structure and compare the characteristics of the Hiccup in the Northern hemisphere with those in the Southern hemisphere. We found that the latitudinal and altitudinal shift of the zonal wind regime occurs in both hemispheres but is more pronounced in the Northern hemisphere and smoother in the Southern hemisphere. Additionally, we discuss the possible impact of the Hiccup on the D-region

A deeper insight into the dynamics of the Hiccup of the fall transition

In autumn the prevailing wind in the middle atmosphere at mid and high latitudes changes from summer easterly to winter westerly. This process is not smooth but interrupted by the Hiccup of the fall transition with characteristics similar to a mini sudden stratospheric warming (SSW) occurring in fall even though the zonal wind does not reverse to easterly again. Combining global reanalysis data and satellite observations we improve our knowledge and understanding of the dynamics of the Hiccup of the fall transition in the middle atmosphere. The introduction of a new definition for the onset of the Hiccup focusing now on its core region in the lower mesosphere allows us the automatic detection of a Hiccup in almost every year and thus a deeper insight into its dynamics. For example, we found a latitudinal and altitudinal shift in the zonal wind regime during the Hiccup. Additionally, we investigate its 3D-structure and discuss its possible impact on the D-region

Ionosphären Wetterdienst ein Beispiel für Wissenschaftsanwendung im DLR mit wachsenden Anforderungen an die Speicherinfrastruktur

Das Weltraumwetter wird hauptsächlich durch die Sonne verursacht und wirkt über die Kette Magnetosphäre, Thermosphäre und Ionosphärein auf die Erdatmosphäre ein. Physik und Effekte des Weltraumwetters zeichnen sich durch hohe Dynamik aus. Von der systematischen Erforschung der komplexen Zusammenhänge des Weltraumwetters erwarten wir eine tieferes Verständnis der Beeinflussung unserer technologischen Umwelt und des menschlichen Lebens durch die Sonne und andere kosmische Quellen. Monitoring, Modellierung und Vorhersage des Weltraumwetters sind die Voraussetzungen zur Reduzierung potenzieller Gefährdungen der Funktionalität technischer Systeme oder des menschlichen Lebens