L. F. C. Rezende

Multi-technique investigations of storm-time ionospheric irregularities over the São Luís equatorial station in Brazil

On 11 April 2001, a large magnetic storm occurred with SSC at 13:43 UT, and Dst reached below 200 nT after two southward Bz excursions. The Kp index during this storm reached 8 and remained high (>4) for about 21 h, and the SLumagnetometer H component presented simultaneous oscillations and decreased substantially rela- tive to the previous magnetically quiet days. This storm trig- gered strong ionospheric irregularities, as observed by a re- cently installed 30 MHz coherent scatter radar, a digisonde, and a GPS scintillation receiver, all operating at the S ˜ ao Lu´

Survey and prediction of the ionospheric scintillation using data mining techniques

[1] Irregularly structured ionospheric regions may cause amplitude and phase fluctuations of radio signals. Such distortion is called ionospheric scintillation. These ionospheric irregularities occur as part of depleted plasma density regions that are generated at the magnetic equator after sunset by equatorial ionospheric plasma instability mechanism. Also known as ionospheric bubbles, they drift upward to high altitudes at the equator and extend/expand to low latitudes along the Earth magnetic field lines. Ionospheric irregularities affect the space weather since they present large variations with the solar cycle and during solar flares and coronal mass ejections. In general, navigation systems such as the Global Positioning System and telecommunications systems are also affected by the scintillation. The aim of this work is to apply data mining for the prediction of ionospheric scintillation. Data mining can be divided into two categories: descriptive or predictive. The first one describes a data set in a concise and summarized way, while the second one, used in this work, analyzes the data to build a model and tries to predict the behavior of a new data set. In this study we employed data series of ionospheric scintillation and other parameters such as the level of solar activity, vertical drift velocity of the plasma at the magnetic equator, and magnetic activity. The results show that prediction of the ionospheric scintillation occurrence during the analyzed period was possible regardless of the high variability of the ionospheric parameters that affect the generation of such irregularities.

Correlation analysis between the occurrence of ionospheric scintillation at the magnetic equator and at the southern peak of the Equatorial Ionization Anomaly

Ionospheric scintillation refers to amplitude and phase fluctuations in radio signals due to electron density irregularities associated to structures named ionospheric plasma bubbles. The phenomenon is more pronounced around the magnetic equator where, after sunset, plasma bubbles of varying sizes and density depletions are generated by plasma instability mechanisms. The bubble depletions are aligned along Earths magnetic field lines, and they develop vertically upward over the magnetic equator so that their extremities extend in latitude to north and south of the dip equator. Over Brazil, developing bubbles can extend to the southern peak of the Equatorial Ionization Anomaly, where high levels of ionospheric scintillation are common. Scintillation may seriously affect satellite navigation systems, such as the Global Navigation Satellite Systems. However, its effects may be mitigated by using a predictive model derived from a collection of extended databases on scintillation and its associated variables. This work proposes the use of a classification and regression decision tree to perform a study on the correlation between the occurrence of scintillation at the magnetic equator and that at the southern peak of the equatorial anomaly. Due to limited size of the original database, a novel resampling heuristic was applied to generate new training instances from the original ones in order to improve the accuracy of the decision tree. The correlation analysis presented in this work may serve as a starting point for the eventual development of a predictive model suitable for operational use.

Study of ionospheric irregularities during intense magnetic storms

The effects of two intense magnetic storms over ionospheric irregularities were analyzed using GPS signal scintillation data from the stations of Sao Luis (2.57oS, 44.21oW, dip latitude 1.73oS) in the equatorial region, Sao Jose dos Campos (23.07oS, 45.86oW, dip latitude 18.01oS) and Cachoeira Paulista (22.57oS, 45.07oW, dip latitude 18.12oS) both under the Equatorial Ionization Anomaly (EIA), and Sao Martinho da Serra (29.28oS, 53.82oW, dip latitude 18.57oS), located in the South of Brazil. Total Electron Content (TEC) data for Sao Luis and Sao Jose dos Campos, were also analyzed. The analyzed storms occurred on October 28-31, 2003 and on November 7-11, 2004. Both storm periods presented two main phases. In the nights of 29/30 and 30/31 of October, during the two storm main phase, it was observed that TEC over Sao Jose dos Campos reached higher values than the TEC for the magnetically quiet day of October 10, due to the effect of eastward electric field prompt penetration to magnetic equator that intensified the EIA. Compared to a quiet day (Oct 10), scintillation in the GPS signal amplitude due to ionospheric irregularity, quantified by the scintillation index S4, was stronger for Cachoeira Paulista (under EIA) during the night of 30/31but not for the night of 29/30 and for Sao Martinho da Serra was stronger during the nights of 29/30 and 30/31. Scintillation for the nights of 29/30 and 30/31 at these two stations lasted longer than on October 10, reaching the post midnight time sector. During the November 7-11 storm, TEC kept the behavior of a quiet day except during days 10 and 11 (up to 9 UT), when a large TEC decrease was observed. The GPS scintillation, compared to the quiet day November 19, was larger at the equatorial station of Sao Luis during the nights of 7/8 and 8/9 and it was completely inhibited for the Sao Luis and Sao Jose dos Campos stations during the nights of 9/10 and 10/11, probably due to action of westward disturbance dynamo electric field penetration to equator.

Influence of the geomagnetic activity on the GPS signal

The Global Positioning System has been largely used to indicate the position of a receiver. The stability of this system depends on the medium propagation characteristics. In the present work we study the influence of 2 geomagnetic storms on the variations of the amplitude (scintillation) of the GPS signal in different periods of the year. This scintillation in the GPS signal is due to the presence of ionospheric irregularities. We verified that on the April 2000 storm, that occurred in a period of low incidence of ionospheric irregularities, the geomagnetic storms led to large scintillations of the GPS signal, while during November 2003 when the scintillations are usually high, the geomagnetic storms lead them to fall down. The storm main phase time of occurrence, that was different for each analysed storm, is proposed to be the cause of this different response of the ionospheric irregularities to the magnetic storms.