Mapping high-latitude TEC fluctuations using GNSS I.I. SHAGIMURATOV (1), A. KRANKOWSKI (2), R. SIERADZKI (2), I.E. ZAKHARENKOVA (1,2), Yu.V. CHERNIAK (1), N. TEPENITZYNA (1) (1) West Department of Institute of Terrestrial Magnetism, Ionosphere and Radiowaves Propagation, Kaliningrad, Russia (2) Geodynamics Research Laboratory, University of Warmia and Mazury in Olsztyn, Poland (GRL / UWM) Data source – GNSS networks The large and dense network of GPS stations enables to monitor the ionosphere with high spatial and temporal resolution. GPS observations carried at the Arctic IGS/EPN stations were used to study the development of TEC fluctuations in the high latitude ionosphere. Disturbed ionospheric conditions can impact on GNSS performance. Irregular ionospheric gradients can complicate phase ambiguities resolving and as consequence will worse the accuracy of GPS positioning. Storm-time geomagnetic conditions can influence also on the estimation of satellite/ receiver biases. Geomagnetic situation The main phase of both storms started before midnight on July and November 7-8, respectively. The Dst index reached -190 nT for July 27 and -370 nT for November 8 respectively. Maximal sum of Kp reached 60 on July 27 and 56 on November 10. The time development of both storms was rather similar. July 2004 November 2004 Spatial and temporal changes of TEC during a storm. TEC maps over Europe for July The positive effect started before the beginning of the main phase of geomagnetic storms and lasted during driven phase of storm days. For summer at middle latitudes maximal effect took place after noon, the enhancement of TEC was about 30-40%. For winter storm the positive effect was pronounced in evening and night time, the TEC enhancement reached 100%.In both events the enhancement value decreased to lower latitudes. Against to summer, the positive effect in winter was shifted to night. It may be related with some features of developing geomagnetic storms. TEC maps over Europe for 6-7 November TEC maps for the quiet and disturbed day Strong modification of ionosphere during November storm took place on second phase of storm in November 9–10, In day time of 12 – 16 UT the enhancement of TEC was observed at latitudes lower than 55° N. After 13 UT it was begun the development of the ionosphere trough. After 16 UT the trough was well structured on longitudes. At the same time the narrow longitudes belt of increase ionization had been observed on latitudes 60°-65°N. The TEC enhancement was factor 1.5–2.0 against to quiet day. During summer storm the maximal modification of the ionosphere took place on second and third phase of the storm. The reaction of the ionosphere to summer storm was similar to winter storm. TEC maps for the quiet - 21 July 2004 and for disturbed day - 25 July 2004 Storm-time dynamics of TEC latitudinal profiles Dynamics of latitudinal TEC profiles during quiet (blue line) and disturbed days (red line) of summer storm (left) and winter one (right) over Europe at longitude 20 E. The latitudinal TEC profiles were constructed from TEC maps at one-hour interval. The pictures demonstrate the dynamics of TEC profiles after first stage of storms. During storm the TEC profiles are essentially changed. For summer one can see the TEC depression (negative effect) at all discussed latitudes. At the same time in winter the sign of storm effect can be changed on latitude. The behavior of TEC profiles depends on the trough location. Evolution of diurnal TEC variations Evolution of diurnal TEC variations over TROM, JOZE and MATE during July The quiet day of 21 July was used as reference. In summer, after initially phase of storms the strong negative effect was observed the whole day at all stations. Fig. Diurnal variations of TEC over Europe at different latitudes (from 35  N to 70  N) around a longitude of 20  E during storm of 22 – 28 July 2004 (b) and during storm of 7 – 13 November 2004 (a). Diurnal TEC variations Strong day-time positive short-term perturbations (surges) on November 9, 10 and 12, 2004 are recognized after main phase. Similar surges can be also seen in summer on the background of the TEC depression. Maximal effects were occurred at lower latitudes. Time location of surge TEC enhancement in depend on latitudes for summer and winter storms. a)b) The distinctive features of this perturbation - these surges were occurred at the same time at latitudes low than 60°N. The observed structure of spatial-temporal TEC perturbations was associated with a wave processes. The perturbations were apparently arrived from north but at middle latitudes they were represented as a standing wave. Temporal and spatial TEC fluctuations intensity As a measure of ionospheric activity we used also the Rate of TEC Index (ROTI) based on standard deviation of ROT (Pi et al., 1997): ROTI has been estimated at 10-min interval. As well as auroral oval the spatial and temporal occurrence of the irregularities can be visually presented below in coordinates - Geomagnetic local time and Corrected geomagnetic latitude for quiet and disturbed day for northern and southern stations. The intensity of fluctuations is indicated with following symbols: blue TECU/min, green , yellow , red - more than JOEN JOEN JOEN JOEN RESO RESO polar station quiet day disturbed day S W RESO RESO auroral station quiet day disturbed day NYAL NYAL NYAL NYAL subauroral station quiet day disturbed day It can be seen that at polar station during quiet days the weak and moderate TEC fluctuations were observed all day. During disturbed days the intensity of TEC fluctuations essentially increase. The time location of maximal intensity was occurred while Bz component of IMF was strong. One can see that intensity of TEC fluctuations in winter was higher than in summer. At auroral ionosphere (NYAL) the intensity of fluctuations was lower that over the polar station. At these stations the intensity of fluctuations as well as at polar ionosphere increased during disturbance. The seasonal effect was also observed, the intensity of fluctuations was higher in winter than in summer. During quiet geomagnetic conditions at subauroral station (JOEN) the intensity of TEC fluctuations was very low (less than 0.01 TECU/min). During disturbed days the intensity of TEC fluctuations was essentially increased. Maximal effect took place during large values of Bz component. The seasonal effect in intensity of TEC fluctuations was similar to the auroral ionosphere. Dynamic of the irregularity oval for quiet and disturbed days. The geomagnetic storms have 2-3 active phases; it appears to be defined features of ionosphere behavior during disturbed period. It was found that both ionospheric storms consisted of negative as well as positive disturbances. The positive effect was observed during the initial phase of storms. In summer time the maximal TEC enhancement was observed during daytime, in winter - maximal effect was in night time at all discussed latitudes. Day-time enhancement for summer reached 40%, for night-time one - reached about 100% relative to quiet day. Maximal changes in TEC distribution over Europe took place in the auroral and subaroral ionosphere. During both storms the ionospheric gradients were essentially increased on middle latitudes. Short-term positive TEC perturbations were found during recovery phase of storms against the general depressions of TEC which were observed in day-time and strictly marked at lower latitudes. The perturbations were associated with standing Poincare waves in the atmosphere. During storms the intensity of irregularities essentially increases. Maximal activity of TEC fluctuations took place when IMF Bz component was negative. The increasing the TEC fluctuation activity can be observed during the time of large positive Bz. Storm-time development of TEC fluctuations caused by ionospheric irregularities was controlled by UT. During storm the strong TEC fluctuations can be registered at subouroral ionosphere, (on latitudes lower than 55 CGL). The seasonal effect in these aria also took place. These all non-modelling ionospheric effects can lead to degradation during storms on the GNSS performance and its accuracy and reliability. SUMMARY