Seasonal variability of atmospheric tides in the mesosphere and lower thermosphere: Meteor radar data and simulations

Thermal tides play an important role in the global atmospheric dynamics and provide a key mechanism for the forcing of thermosphere–ionosphere dynamics from below. A method for extracting tidal contributions, based on the adaptive filtering, is applied to analyse multi-year observations of mesospheric winds from ground-based meteor radars located in northern Germany and Norway. The observed seasonal variability of tides is compared to simulations with the Kühlungsborn Mechanistic Circulation Model (KMCM). It is demonstrated that the model provides reasonable representation of the tidal amplitudes, though substantial differences from observations are also noticed. The limitations of applying a conventionally coarse-resolution model in combination with parametrisation of gravity waves are discussed. The work is aimed towards the development of an ionospheric model driven by the dynamics of the KMCM

Polar tongue of ionisation during geomagnetic superstorm

During the main phase of geomagnetic storms, large positive ionospheric plasma density anomalies arise at middle and polar latitudes. A prominent example is the tongue of ionisation (TOI), which extends poleward from the dayside storm-enhanced density (SED) anomaly, often crossing the polar cap and streaming with the plasma convection flow into the nightside ionosphere. A fragmentation of the TOI anomaly contributes to the formation of polar plasma patches partially responsible for the scintillations of satellite positioning signals at high latitudes. To investigate this intense plasma anomaly, numerical simulations of plasma and neutral dynamics during the geomagnetic superstorm of 20 November 2003 are performed using the Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIE-GCM) coupled with the statistical parameterisation of high-latitude plasma convection. The simulation results reproduce the TOI features consistently with observations of total electron content and with the results of ionospheric tomography, published previously by the authors. It is demonstrated that the fast plasma uplift, due to the electric plasma convection expanded to subauroral mid-latitudes, serves as a primary feeding mechanism for the TOI anomaly, while a complex interplay between electrodynamic and neutral wind transports is shown to contribute to the formation of a mid-latitude SED anomaly. This contrasts with published simulations of relatively smaller geomagnetic storms, where the impact of neutral dynamics on the TOI formation appears more pronounced. It is suggested that better representation of the high-latitude plasma convection during superstorms is needed. The results are discussed in the context of space weather modelling.</p

Polar cap plasma transport during geomagnetic superstorm

Positive plasma anomalies appear during the main phase of geomagnetic storms at (sub)auroral latitudes, extending across the polar cap as tongue of ionisation (TOI). Physical mechanisms of TOI, including electrodynamic plasma transport and neutral wind forcing, are simulated with TIE-GCM during the superstorm of Nov. 2003. The simulations are compared with TEC observations and GNSS tomography. The electrodynamic transport (vertical ExB component in particular) is identified as the main mechanism controlling TOI anomaly during great storms (Dst < -300 nT). This makes the choice of high-latitude convection model critical for simulations

Ion-Neutral Coupling in the Ionospheric Dynamo Region - Measurements, Application and Impact on Ionospheric Variability

Ionospheric conductivities transverse to the magnetic field lines maximize in the ionospheric dynamo region at about 80 - 150 km altitude. Due to the plasma-neutral interactions, neutral winds contribute to the generation of currents and electric fields in this region. Atmospheric oscillations, e.g. gravity waves and tides, at these altitudes, are therefore important for the variability of the ionosphere. Measurements of atmospheric dynamics at these altitudes are extremely difficult due to the limited number of instruments that cover the dynamo region. Incoherent Scatter Radars (ISRs) can measure basic plasma parameters at dynamo region altitudes and neutral dynamics can be inferred from that due to the coupling of ionosphere and neutral atmosphere. We present measurements of tidal oscillations and atmospheric gravity waves in the thermosphere derived from ISR observations of ionospheric parameters. Both types of waves can be forced either in situ in the thermosphere or propagate upward from the lower and middle atmosphere. In combination with horizontally resolved measurements from the Nordic Meteor Radar Cluster, gravity wave parameters derived from ISR electron density measurements can be applied to infer neutral wind velocities in the thermosphere. We demonstrate this on multiple detections of gravity waves. The coupling of atmospheric waves into the ionospheric plasma strongly depends on the ion-neutral collision frequency. Direct measurement of the ion-neutral collision frequency is only possible with simultaneous measurements using two ISRs with significantly different transmission frequencies. We demonstrate the difference spectrum method which allows us to infer ion-neutral collision frequencies from dual-frequency measurements with the EISCAT UHF and VHF ISRs. Since this method is based on the standard EISCAT analysis software GUISDAP, it is more suited for the general application than previously reported methods

Ionospheric plasma transport across polar cap

Large-scale ionospheric plasma anomalies appear at high latitudes, extending across the polar cap as a tongue of ionisation and polar patches. Physical mechanisms responsible for the plasma uplifts and transport are investigated using global ionospheric circulation models driven by parameterised highlatitude plasma convection models. Various convection models are considered, including the models based on satellite data, radar data, and data assimilation models. Contributions from electrodynamic plasma transport and neutral wind forcing are assessed. Mechanisms responsible for the energy dissipation, including frictional heating and Joule dissipation, are investigated. The numerical simulations are compared with plasma density measurements by positioning GNSS satellites and ground radar observations. The results are discussed in the context of space weather modelling and GNSS signal scintillation modelling at polar latitudes

Difference spectrum fitting of the ion-neutral collision frequency from dual-frequency EISCAT measurements

The plasma-neutral coupling in the mesosphere-lower thermosphere strongly depends on the ion-neutral collision frequency across that region. Most commonly, the collision frequency profile is calculated from the climatologies of atmospheric models. However, previous measurements indicated that the collision frequency can deviate notably from the climatological average. Direct measurement of the ion-neutral collision frequency with multifrequency incoherent scatter radar (ISR) measurements has been discussed before, though actual measurements have been rare. The previously applied multifrequency analysis method requires a special simultaneous fit of the two incoherent scatter spectra, which is not possible with standard ISR analysis software. The difference spectrum method allows us to infer ion-neutral collision frequency profiles from multifrequency ISR measurements based on standard incoherent scatter analysis software, such as the Grand Unified Incoherent Scatter Design and Analysis Package (GUISDAP) software. In this work, we present the first results by applying the difference spectrum method. Ion-neutral collision frequency profiles obtained from several multifrequency EISCAT ISR campaigns are presented. The profiles obtained with the difference spectrum method are compared to previous collision frequency measurements, both from multifrequency ISR and other measurements, as well as results from empirical and comprehensive atmosphere models. Ion-neutral collision frequency measurements can be applied to improve first-principle ionospheric model

Vertical coupling and transport in high-latitude ionosphere

Various plasma anomalies appear at auroral latitudes, extending across the polar cap as a tongue of ionisation (TOI) and polar patches (PPs). Mechanisms responsible for horizontal and vertical plasma transport include the forcing from above (solar irradiance and geomagnetic activity) and from below (atmospheric waves and tides). The forcing mechanisms balance each other in lower thermosphere, forming the ionospheric transition region. At higher altitudes (the F layer), plasma irregularities can be transported for hours, while in the E layer and below the irregularities recombine within seconds. Incoherent (ISR) and coherent scatter radars can observe vertical coupling and dynamics at various horizontal scales, providing a tool for analysing the transition region. Modelling with GCMs provides a basis for mitigating/forecasting the space weather. Vertical coupling and transport will be addressed in connection to the drifting polar cap anomalies (TOI and PPs). Examples of the vertical coupling and transport with the EISCAT ISR will show relative contributions of the forcing from above and below. Examples of plasma irregularities seen by GNSS receiver networks and SAR radars will be also shown

Vertical coupling and transport in high-latitude ionosphere

Various ionospheric plasma anomalies appear at auroral latitudes, extending across the polar cap as a tongue of ionisation (TOI) and/or polar patches (PPs). Mechanisms responsible for horizontal and vertical plasma transport include the forcing from above (solar irradiance and geomagnetic activity) and from below (atmospheric waves and tides). The forcing mechanisms balance each other in mesosphere/lower thermosphere, forming the ionospheric transition region. At higher altitudes (in the F layer), plasma irregularities can be transported for hours, while below (in the E layer) the irregularities recombine within seconds. Incoherent (ISR) and coherent scatter radars can observe vertical coupling and dynamics at various horizontal scales, providing a tool for analysing the transition region. Modelling with GCMs provides a basis for mitigating/forecasting the space weather. Vertical coupling and transport will be addressed in connection to the drifting polar cap anomalies (TOI and PPs). Examples of the vertical coupling and transport with the EISCAT ISR, coherent scatter radars, and GNSS receiver networks will show relative contributions of the forcing from above and below