The dissipation of solar wind energy in the Earth?s upper atmosphere remains one of the most challenging topics in space physics. Not only does this energy transfer have a profound influence on the global morphology of the upper atmosphere, it is also of practical interest since satellite ephemeris predictions and transionospheric radio communications may be severely degraded by this energy addition. In this tutorial lecture, different aspects of this fascinating phenomenon will be considered. After a more general introduction, three examples of space weather effects will be presented. First, disturbed radio communication due to ionospheric absorption; second, increased upper atmospheric air drag which affects the motion of satellites; and third, ionospheric storms which - among others - modify GPS signals. A review of the publication statistics in this field is next, and - time permitting - a brief discussion of open questions.

This presentation focuses on space weather effects where solar events and upper atmosphere dynamic phenomena combine to produce deterioration of navigation systems or communication systems. Electron density enhancements created by particle precipitation change the refractive index of the ionosphere as a propagation medium for radio waves - even at high frequencies - and therefore change the apparent distance from the GNSS satellite transmitter to the receiver. Modern receivers easily correct this modification by using several frequencies. However, any turbulence in the region of steep electron density gradients creates a chaotic propagation medium for the radio waves and leads to scintillations of the signal. Weak scintillations lead to apparent random jumps of a navigation system receiver even when at rest. Strong scintillations may lead to a loss of signal synchronization, making the navigation receiver useless for certain periods of time. Such effects are known to occur near the geomagnetic equator in connection with plasma bubbles and spread-F, near the auroral oval and near patches of ionization (electron clouds) within the auroral ovals. The presentation aims to explain what we know about these phenomena, give indications of where, when, and how often they occur, and point to open questions that require further research.

This paper illustrates the importance of understanding and accounting for the presence of various structural features in the northerly ionosphere, i.e. the electron depletion and irregularities associated with the sub-auroral trough, patches and arcs of enhanced electron density within the polar cap, and irregularities within the auroral zone when planning and operating HF radio links. These features often result in radiowaves propagating over paths well displaced from the great circle direction and have their most obvious relevance in the operation of HF radiolocation systems where deviations from the great circle path may result in significant (sometimes inter-continental) triangulation errors. However, the impact of these propagation effects is much wider than this particular application, extending to almost any HF communications system where the signal impacts on the ionosphere within and polewards of the sub-auroral trough.

For terrestrial HF radio systems, the electron density depletion in the trough region reduces the maximum frequency that can be reflected by the ionosphere along the GCP. For long paths, the signal is often received via a ground / sea-scatter mechanism to the side of the GCP [1. For shorter paths, gradients in electron density associated with the trough walls and embedded ionospheric irregularities often result in propagation in which the signal path is well displaced from the great circle direction, with directions of arrival at the receiver offset by up to 100° [2]. Furthermore, the Doppler and multi-mode delay spread characteristics of the signal are also affected when propagation is via scatter / reflection from irregularities in or close to the north wall of the trough [3].

Measurements have been made of the direction of arrival (DOA) and amplitude as a function of time of flight (TOF) of HF signals received over two paths oriented along the trough between Uppsala, Sweden (2001, close to sunspot maximum, and from August 2006 to January 2008, close to sunspot minimum) and Helsinki, Finland (since December 2006) and Bruntingthorpe, near to Leicester. Examination of these observations has led to the identification of five characteristic types of off great circle propagation events (see [4] for full details). Recently the positions of the electron density minimum and the poleward and equatorward edges of the trough have been determined with measurements made by the DEMETER satellite using a hybrid method based on wave diagnostics across the whole frequency band together with electron and density temperature measurements. The HF measurements have been examined in conjunction with the satellite observations of the electron density structure. The DEMETER measurements indicate that, as well as the expected reduction in electron density, there are strong filamentary electron density structures (i.e. irregularities) inside the main ionospheric trough, and we suggest that scattering from these irregularities is the main mechanism responsible for one type off great circle propagation within this region.

Within the polar cap, the most important causes of off great circle propagation are the presence of convecting patches or sun-aligned arcs of enhanced ionisation. The simulation of the impact of these features on HF propagation has been the subject of significant developments [5]. A unique ray-tracing model has been developed that accurately reproduces many of the features observed in experimental measurements of propagation both within the polar cap [6] and within the trough to a level well beyond that which we originally anticipated would be possible. The simulations make use of a numerical ray tracing code [5] to estimate the ray paths through a realistic ionosphere based partly on measurements of foF2. Localised, time varying, perturbations in the electron density are then applied to the background model.

A major outcome of these ray-tracing simulations is that paths other than those that have been the subject of experimental investigation can readily be assessed. In particular from an HF communications point of view, the area coverage to be expected from a transmitter at a given location can be estimated.

1 STOCKER, A.J., E.M. WARRINGTON and T.B. JONES (2003), Radio Science, 38, (3), 1045, doi:10.1029/2002RS002781.
2 ROGERS, N.C., E.M. WARRINGTON and T.B. JONES (1997), IEE Proceedings on Microwaves, Antennas and Propagation, 144, (2), 91-96.
3 WARRINGTON, E.M. and A.J. STOCKER (2003), Radio Science 38, (5), 1080, doi:10.1029/2002RS002815.
4 STOCKER, A.J., N.Y. ZAALOV, E.M. WARRINGTON and D.R. SIDDLE (2009), Advances in Space Research, 44, (6), 677-684, 2009.
5 ZAALOV, N.Y., E.M. WARRINGTON and A.J. STOCKER (2005), Radio Science, 40, (4), RS4006, doi:10.1029/2004RS003183.
6 WARRINGTON, E.M., N.C. ROGERS AND T.B. JONES (1997), IEE Proceedings on Microwaves, Antennas and Propagation, 144, (4), 241-249.

Under specific operational conditions, intense solar radio bursts (SRBs) have been observed to interfere wireless communications. In particular, outstanding solar radio events have been identified as primary sources of significant GPS synchronization failures lasting for more than 10 minutes.

Apart from the maximum radio flux density level and predominant polarization sense, a full characterization of SRBs in terms of energetics and timing requires a high time resolution analysis of the event profile. In fact, a key factor in the evaluation of the interfering potentiality is represented by the statistics of the attention threshold exceeding and polarization time evolution.

In this work we suggest a scheme for SRB characterization in the single-frequency radio flux-time domain and we present its application on a selection of outstanding SRBs detected by the Trieste Solar Radio System (TSRS) in the period 2001-2006. Specifically, we comment on the relevancy of this approach to the evaluation of wireless communication interference risk in space weather applications.

Magnetic drivers bring essential energetic input to the ionosphere, so synthetic information gathered in magnetic indices is often used. They present the quantification of the ionospheric changes. Redistribution of the electron concentration in the lower part of ionosphere during quiet magnetic circumstances has been studied by means of the eta magnetic index. Local ionospheric drifts has been shown and discussed.

We analysed data of ionospheric characteristics (foE, foEs, h'E, h'F2) during 30-day long quiet day conditions (Kp = 0-2) in 2004. We find correlations between high local disturbances in ionosphere during very quiet days and high values of magnetic index eta. The most sensitive to magnetic influence - ionospheric E layer data (foE characteristic) - reaches median deviations up to (+1.6 MHz and -1.1 MHz) during very low magnetic activity (Kp = 0-1). Additional observation is connected with correlation in time of the high eta value with the negative median deviations of h'F2 (in some cases up to -90 km).

The analysis was based on one-minute data recorded at each of 20 European Magnetic Observatories working in the INTERMAGNET network and from 18 ionosondes.

European magnetic and ionospheric observatories are located at different positions. Ionospheric data are sparse in time and in space in opposite to the magnetic data. Usually measured magnetic indices used in ionospheric modeling represent the magnetic activity at the position of the magnetic observatories which could not represent the magnetic properties at the positions of the ionospheric observatories. The map of the magnetic indices can suggest the behaviour of ionospheric characteristics in the areas where we have no data.

We analyse the fluctuations in the carrier frequency of the radio waves received on the ground, commonly referred to as ionospheric amplitude and phase scintillations, during the year 2008, which is characterized by very low solar activity. The investigation of such phenomena during the solar minimum is important, not only as background for the next solar maximum, expected in 2012, but also for a deeper understanding of the dynamics of the quiet ionosphere. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Institute of Engineering Surveying and Space Geodesy (IESSG) of the University of Nottingham manage the same kind of GISTM (GPS Ionospheric Scintillation and TEC Monitor) receivers over the European high and mid latitude regions. The GISTM receivers consist of NovAtel OEM4 dual-frequency receivers with special firmware specifically able to compute in near real time the amplitude and the phase scintillation from the GPS L1 frequency signals, and the ionospheric TEC (Total Electron Content) from the GPS L1 and L2 signals. It also computes the TEC rate of change based on the L1 and L2 carrier phase signals. From this ground-based network, we are able to capture the dynamics of ionospheric plasma in a wide latitudinal range. The results, obtained by statistically analysing a large data sample, show the effect of ionospheric disturbances on the GNSS signals, evidencing the different contributions of the auroral and the cusp/cap ionosphere and highlighting possible scintillation scenarios over North European regions.

Space weather and ionospheric effects remain one of the main factor which limit the precision and the reliability of many GNSS applications. Especially extreme severe solar events can cause extreme ionospheric effects .This contribution deals with changes in Earthes ionosphere during most extreme events of Solar activity ( with Dst > 200 nT) and its consequences in GNSS position accuracy. During October - November 2003, in descending phase 11- year Solar sunspot cycle, two periods of a suddenly enhanced solar activity were observed. The effects of these strong events, as were observed at Pruhonice observatory (49.59 N; 14.33E) and other European ionospheric stations, are main object of this paper. Two strong geomagnetic storms with Kp = 9 were observed during second half of 2003 year (28.10 - 5.11. and 19.11 -23.11.). These events were accompanied by sudden disturbances in Earth's ionosphere and strong ionospheric storms with duration of 4 - 5 days.
The first event (28.10. -5.11. 2003) is one of the strongest events which was observed during actual Solar cycle. Geomagnetic index Kp reached value 9 for two days. This geomagnetic event resulted strong ionospheric storm (with positive phase of the storm 28.10.2003), which was observed on many ionospheric stations and strongly effects ionospheric parameters (critical frequency of the ionospheric F2 layer during more than 5 days. The effects of this storm, as was observed on several european ionospheric stations, is reported.
Many interesting effects were observed during second storm 19.11.-23.11.2003. This November storm was weaker than October storm, but effects in the ionosphere (due to configurations of geomagnetic field) were very dramatic and unexpected. Quite extraordinary phenomenons (optical aurora and partical auroral ionospheric layers at midlatitudes, quick changes in electron density profiles, great chages in TEC) were observed. Extraordinary partical ionospheric layers (at 150 - 300 km heights interval) were observed in ionosphere at Pruhonice observatory 20.11.2003 between 15 and 22 hours UT. Electron density profiles, computed from these ionograms during this time interval, shows quick changes of the profiles with ionisations maximum about 3 -6.105 cm-3 at heights 150 - 200 km. These values are 3- 6 times higher than normal values of electron concentration at the same time. Similar increasing, 2 -3 times higher then quiet conditions value, in TEC was observed.
Final part of this paper deals with older observation of this type of storms (1986, 1989 years) and its frequency in history.

The enhanced emission of EUV- and X-rays, and fast coronal mass ejections in large solar storms severely affect the Earth's magnetosphere and ionosphere and perturb communication and navigation systems. To achieve a quantitative understanding of the impact of large solar storms on the Earth's ionosphere is thus a key subject of space weather research.
GPS-derived TEC maps computed at DLR Neustrelitz for the European and both polar areas with a time resolution of 10 minutes, are analyzed in conjunction with EUV- and X-ray measurements, coronagraphic observations and solar wind measurements for the October 2003 solar storms, the so-called Halloween events. Solar data are provided by the SOHO, GOES and ACE satellites. The results of this interdisciplinary research study contribute to a better understanding of the physical mechanisms and quantitative responses of the ionospheric TEC to huge solar storms and yield implications for their forecast.