S. L. G. Dutra

Periodic variation in the geomagnetic activity: A study based on the Ap index

The monthly and daily samples of the Ap geomagnetic index for 51 years, 1932-1982, were investigated by means of the power spectrum technique. In general, the results confirm previous findings about possible periodicities in the geomagnetic activity. However, in our opinion the following aspects are either new or they are being interpreted somewhat differently than other authors have done. The period around 4 years in the monthly Ap power spectrum is associated to the double peak structure observed in the geomagnetic activity variation [Gonzalez et al., 1990]. Several of the peaks shown by the daily Ap spectrum are interpreted as harmonics of the 6-month period and other peaks as caused by the solar rotation periodicity, in such a way that the two series of Fourier sequences are consider to be juxtaposed. A strong solar cycle modulation is observed in these series, particularly in that related to the solar rotation period, which almost disappears for the solar maximum phase. The study of the seasonal variation was complemented by a superposed epoch analysis. The profiles resulting from this analysis seem to show a multiple origin of the 6-month periodicity, so that it does not seem realistic to search for a unique cause for this well-known seasonal variation. This conclusion is also supported by the histograms of the occurrence of storms above a given intensity level, taken over short duration intervals (i.e., 8 days). According to these histograms, for large data samples the dates with largest number of storms are spread out around those predicted by the different theoretical models, while for short intervals the semiannual periodicity may sometimes not even be present. Therefore these known mechanisms would combine to give a resulting modulation of the geomagnetic response to the randomly generated source of storms. It was also found that an additional seasonal peak seems to exist in July, with an amplitude comparable to those of the equinoctial peaks, for the range of the most intense storms (Ap ≥ 150 nT). A weak periodicity around 158 days, well correlated to that of about 155 days observed in the solar activity, has also been detected for some years during solar cycle 21.

Global Pc5 geomagnetic pulsations of March 24, 1991, as observed along the American Sector

Analysis of the globally coherent Pc5 geomagnetic pulsation event of March 24, 1991, from a chain of stations extending from the auroral oval to the equatorial region along the American sector, has shown unequivocal evidence on the enhancement of pulsation amplitude in the narrow equatorial band centered at the dip equator. The fine spatial structure of this equatorial enhancement, documented observationally for the first time, emphasizes the importance of an ionospheric component associated with enhanced Cowling conductivity. The presence of an ionospheric component is also indicated at mid-latitudes by the sharp increase in the amplitude of pulsations, more dominantly in the Y than in the X component, on the sunward side of the dawn terminator. The sunrise effect also causes a reversal of the sense of rotation of the wave polarization across the dawn terminator. Propagation of the magnetospheric inducted polar electric field to low and equatorial latitudes through the ionosphere is invoked to account for the equatorial enhancement and the sunrise effect seen in these Pc5 pulsations.

Atmospheric and ionospheric response to sudden stratospheric warming of January 2013

In this work, we examine the atmospheric and ionospheric responses to the January 2013 sudden stratospheric warming (SSW) event. To examine the atmospheric and ionospheric behavior during this event, three main parameters are used (1) Total Electron Content (TEC) collected from the International Global Positioning System and from the Brazilian Network of Continuous Monitoring stations, (2) daytime E × B vertical drift derived from the magnetometers located at the equatorial station Alta Floresta (9.9°S, 55.9°W, dip latitude 1.96°) and an off-equatorial station Cuiaba (15.3°S, 56.0°W, dip latitude 7.10°), both in the Brazilian sector, (3) the mesosphere and lower thermosphere (MLT) meridional and zonal wind components measured by the Meteor Radar located at the southern midlatitude Santa Maria (29.4°S, 53.3°W, dip latitude 17.8°). We identify the anomalous variation in E × B drift based on later local-time migration of peak value with SSW days. A novel feature of the present study is the identification of the similar migration pattern in the TEC anomaly, in spite that the simultaneous solar flux increases during the SSW event. Other novel features are the amplification of the 13–16 day period in the TEC anomaly during the SSW days and simultaneous amplification of this period in the meridional and zonal wind components in the MLT region, as far as 30°S. These aspects reveal the presence of coupled atmosphere-ionosphere dynamics during the SSW event and the amplification of the lunar and/or solar tidal component, a characteristic which is recently reported from the electrojet current measurements.

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´

Low‐latitude scintillation weakening during sudden stratospheric warming events

Global Positioning System (GPS) L1-frequency (1.575 GHz) amplitude scintillations at Sao Jose dos Campos (23.1°S, 45.8°W, dip latitude 17.3°S), located under the southern crest of the equatorial ionization anomaly, are analyzed during the Northern Hemisphere winter sudden stratospheric warming (SSW) events of 2001/2002, 2002/2003, and 2012/2013. The events occurred during a period when moderate to strong scintillations are normally observed in the Brazilian longitude sector. The selected SSW events were of moderate and major categories and under low Kp conditions. The most important result of the current study is the long-lasting (many weeks) weakening of scintillation amplitudes at this low-latitude station, compared to their pre-SSW periods. Ionosonde-derived evening vertical plasma drifts and meridional neutral wind effects inferred from total electron content measurements are consistent with the observed weakening of GPS scintillations during these SSW events. This work provides strong evidence of SSW effects on ionospheric scintillations and the potential consequences of such SSW events on Global Navigation Satellite System-based applications.

Equatorial electrojet responses to intense solar flares under geomagnetic disturbance time electric fields

Large enhancement in the equatorial electrojet (EEJ) current can occur due to sudden increase in the E layer density arising from solar flare associated ionizing radiations, as also from background electric fields modified by magnetospheric disturbances when present before or during a solar flare. We investigate the EEJ responses at widely separated longitudes during two X-class flares that occurred at different activity phases surrounding the magnetic super storm sequences of 28–29 October 2003. During the 28 October flare we observed intense reverse electrojet under strong westward electric field in the sunrise sector over Jicamarca. Sources of westward disturbance electric fields driving large EEJ current are identified for the first time. Model calculations on the E layer density, with and without flare, and comparison of the results between Jicamarca and Sao Luis suggested enhanced westward electric field due to the flare occurring close to sunrise (over Jicamarca). During the flare on 29 October, which occurred during a rapid AE recovery, a strong overshielding electric field of westward polarity over Jicamarca delayed an expected EEJ eastward growth due to flare-induced ionization enhancement in the afternoon. This EEJ response yielded a measure of the overshielding decay time determined by the storm time Region 2 field-aligned current. This paper will present a detailed analysis of the EEJ responses during the two flares, including a quantitative evaluation of the flare-induced electron density enhancements and identification of electric field sources that played dominant roles in the large westward EEJ at the sunrise sector over Jicamarca.

Spectral and polarization analysis of geomagnetic pulsations data using a multitaper method

Abstract Spectral and polarization parameters of geomagnetic pulsations are computed from geomagnetic data recorded at ground station, using multitaper and singular value decomposition (SVD) analysis. These techniques were adapted to provide appropriate polarization parameters of pulsations. The spectra are comparable to those obtained from classical methods, but the dynamic spectra of degree of polarization present more localized spectral information, allowing easier identification of polarized pulsation events mixed to noise, which are associated by coherent sources of MHD waves. The time–frequency polarization parameters of polarized pulsations show different signatures to regular and irregular pulsation events, and the multiwavelet polarization method is used to compare with multitaper. The regular pulsation events showed to be more continuous on time with a narrow spectral frequency band, while the irregular events showed to be more localized on time with a wide spectral frequency band. The dynamic analysis of multitaper method is indicated to characterize regular pulsations and the time–frequency analysis of multiwavelet method efficiently detects irregular pulsations. The continuous multiwavelet analysis of ellipticity and azimuth is useful in progressively monitoring the propagations of MHD waves in the magnetosphere, mainly in the irregular pulsations as observed on the magnetic storm of March 24, 1991.

Geomagnetic field investigation on the polar microsatellite SACI-1

Abstract The first Brazilian Scientific Microsatellite, SACI-1, is scheduled to be launched in July 1999 in a near circular polar orbit at an inclination of 98.5° and an altitude of 750 km. It will carry a three-component fluxgate magnetometer constructed by IGPP/UCLA for measuring three components of the Earths Magnetic Field in space at a rate of 10 samples per second. The scientific objective is to study field-aligned currents in the auroral oval and the electrojet of the equatorial ionosphere. Together with the geomagnetic data from other polar orbiting spacecraft e.g. POLAR, FAST, OERSTED, it will be possible to obtain a truly global picture of a field-aligned current system. The experimental details and the plans of this project are presented here.

A study of geomagnetic variations with periods of four years, six months and 27 days

The monthly and daily samples of the Ap geomagnetic index for 51 years, in the 1932–1982 interval, were investigated by means of the power spectrum technique. Although in general the results confirm previous findings about possible periodicities in the geomagnetic activity, some aspects are either new or they are now interpreted somewhat differently than other authors have done. The period around 4 years in the monthly Ap power spectrum is associated to the dual peak-structure observed in the geomagnetic activity variation (Gonzalez et al., 1990). Several of the peaks shown by the daily Ap spectrum are interpreted as harmonics of the six-month period, and others as caused by the solar rotation periodicity, in such a way that the two series of Fourier sequences are considered to be juxtaposed. A strong solar cycle modulation is observed in these series, particularly in that related to the solar rotation period, which almost disappears for the solar maximum phase. Furthermore, a statistic analysis of the geomagnetic storm occurrence has confirmed the findings related to the dual peak distribution as well to the seasonal variation.

Downward mapping of quasi-static ionospheric electric fields at low latitudes during fair weather

Abstract The problem of downward mapping of equatorial ionospheric electric fields is studied in two dimensions employing the finite elements and finite differences numerical techniques. The solutions obtained for low latitudes are compared with known results for high latitudes. It is found that equatorial ionospheric electric fields of scale lengths of the order of 100 km or more reach balloon heights (30–40 km) without undergoing noticeable attenuation. However, in the case of equatorial ionospheric electric fields of scale lengths of a few tens of kilometers it is found that these fields reach balloon heights with severe attenuation. The corresponding attenuation factors are significantly larger than those known for high latitudes. It is also shown that the presence of mountains with a fairly large height as well as of a large-scale conductivity irregularity in the middle atmosphere, such as that expected in the South Atlantic Magnetic Anomaly (SAMA) region during energetic electron precipitation events, can considerably distort the mapped ionospheric electric fields at the middle atmosphere.