L. E. A. Vieira

Solar and interplanetary causes of very intense geomagnetic storms

Abstract The dominant interplanetary phenomena causing intense magnetic storms are the interplanetary manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of such class of storms, involving an intense and long duration B s component of the IMF: the sheath region just behind the forward shock, and the CME ejecta itself. Frequently, these structures lead to the development of intense storms with two-step growth in their main phases. These structures also lead sometimes to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structure accompanying another stream. The second stream can also compress the primary cloud, intensifying the B s field, and bringing with it an additional B s structure. Thus, at times very intense storms are associated with three or more B s structures. We also discuss evidence that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus implying large amplitude interplanetary electric fields that can drive very intense storms.

RECONSTRUCTION OF WOLF SUNSPOT NUMBERS ON THE BASIS OF SPECTRAL CHARACTERISTICS AND ESTIMATES OF ASSOCIATED RADIO FLUX AND SOLAR WIND PARAMETERS FOR THE LAST MILLENNIUM

A reconstruction of sunspot numbers for the last 1000 years was obtained using a sum of sine waves derived from spectral analysis of the time series of sunspot number Rz for the period 1700–1999. The time series was decomposed in frequency levels using the wavelet transform, and an iterative regression model (ARIST) was used to identify the amplitude and phase of the main periodicities. The 1000-year reconstructed sunspot number reproduces well the great maximums and minimums in solar activity, identified in cosmonuclides variation records, and, specifically, the epochs of the Oort, Wolf, Spörer, Maunder, and Dalton Minimums as well the Medieval and Modern Maximums. The average sunspot number activity in each anomalous period was used in linear equations to obtain estimates of the solar radio flux F10.7, solar wind velocity, and the southward component of the interplanetary magnetic field.

Compression of magnetic clouds in interplanetary space and increase in their geoeffectiveness

Abstract Using a set of 54 magnetic clouds observed in the period from 1965 to 1997, we found that the magnetic cloud field and velocity strength relationship proposed by Gonzalez et al. (Geophysical Research Letters 25 (7) (1998) 963–966) is generally followed. However, for a few events, the field strength is fairly higher than that predicted by the work of Gonzalez et al. We examined some of these events and found that compression of these magnetic clouds might have occurred, due to the follow-up presence of a higher speed stream, which led to a stronger magnetic field strength at the back region of the cloud. This intensification of the magnetic field strength is closely related to the occurrence of intense geomagnetic storms when the field has a north–south polarity. Such interplanetary compression of magnetic clouds should be considered in space weather forecasting models.

A study of magnetic storms development in two or more steps and its association with the polarity of magnetic clouds

Abstract The study of the response of the magnetosphere, measured by the Dst index, to different interplanetary magnetic field configurations observed in magnetic clouds has led us to conclude that about 20% of intense magnetic storms have the growth of their main phases in three or more steps. We also have found that the magnetic clouds with south-to-north magnetic field rotation tend to lead to moderate or intense magnetic storms with a two-step main phase development due to the closer southward magnetic fields in the sheath and in the cloud. On the other hand, magnetic clouds with a north-to-south magnetic field rotation mostly seem to lead to magnetic storms with one-step main phase development, due to the larger separation between the southward magnetic fields in the sheath and in the cloud. Furthermore, the magnetic clouds with a substantial tilt to the ecliptic plane appear to lead to intense magnetic storms when they have a prolonged southward axial field.

Interplanetary shock parameters during solar activity maximum (2000) and minimum (1995-1996)

Interplanetary shock parameters are analyzed for solar maximum (year 2000) and solar minimum (years 1995-1996) activity. Fast forward shocks are the most usual type of shock observed in the interplanetary medium near Earths orbit, and they are 88% of the identified shocks in 2000 and 60% in 1995-1996. Average plasma and magnetic field parameters for upstream and downstream sides of the shocks were calculated, and the parameter variations through the shock were determined. Applications of the Rankine-Hugoniot equations were made, obtaining shock speeds and Alfvenic Mach number. Static and dynamic pressures variations through the shocks were also calculated. Every parameter have larger variation through the shock in solar maximum than in solar minimum, with exception of the proton density. The intensity of shocks relative to the interplanetary medium, quantified by the Alfvenic Mach Number, is observed to be similar in solar maximum and minimum. It could be explained because, during solar maximum, in despite of the higher shock speeds, the Alfvenic speed of the interplanetary medium is higher than in solar minimum.

Great geomagnetic storms in the rise and maximum of solar cycle 23

Geomagnetic storms are intervals of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial injection of energy into the magnetosphere-ionosphere system, to an intensified ring current, strong enough to exceed some key threshold of the quantifying storm time Dst index. We have studied all the 9 great magnetic storms (peak Dst < -200 nT) observed during the rise and maximum of solar cycle 23 (from 1997 to early 2001), in order to identify their solar and interplanetary causes. Apart of one storm occurred during the period without observations from the Solar and Heliospheric Observatory (SOHO), all of them were related to coronal mass ejections observed by the Large Angle and Spectroscopic Coronagraph (LASCO). The sources of interplanetary southward magnetic field, Bs, responsible for the occurrence of the storms were related to the intensified shock/sheath field, interplanetary magnetic clouds field, or the combination of sheath-cloud or sheath-ejecta field. It called our attention the fact that one of the events was related to a slow CME, with CME expansion speed not greater than 550 km/s. The purpose of this paper is to address the main sources of large geomagnetic disturbances using the current satellite capability available. As a general conclusion, we found that shock/sheath compressed fields are the most important interplanetary causes of great magnetic storms during this period.

WAVELET ANALYSIS OF SOLAR-ENSO IMPRINTS IN TREE RING DATA FROM SOUTHERN BRAZIL IN THE LAST CENTURY

In order to study the imprint of solar and ENSO signals on terrestrial archives, the wavelet spectrum analysis was applied to solar-geophysical indices and tree ring data. Time series of Sunspot Number (SSN), southern oscillation index (SOI) and tree-ring indices from Southern Brazil, for the period 1876–1991, were used in this work. The 11-year solar cycle was present during the whole period in tree ring data, being more intense during 1930–1980, in agreement with an earlier study that was performed for thesame region but a different time range (1836–1996). ENSO effects on treering data from Southern Brazil were studied by the first time in this work using wavelet analysis. Short-term variations, between 2–5 years, arealso present in tree ring data. This represents the signature of ENSO events and was also observed in the SOI, as expected. The cross-wavelet spectrum analysis shows that both solar and climatic factors are recorded in tree ring data.

A study of the latitudinal dependence of the quasi-biennial oscillation in Total Ozone Mapping Spectrometer total ozone

A wavelet multiresolution analysis of the quasi-biennial oscillation (QBO) latitudinal structure in total ozone is performed for the period 1979–1992 (Nimbus-7 TOMS data). It has been found that ozone is nearly in phase with the QBO signal in the equatorial region (0°—5° and 5°—10°), and it is out of phase (lag ~ +15 and —15 months in north and south) in the 10°—15° to 55°—60° latitudinal bands. The cross-correlation coefficient between total ozone and zonal wind index is high (>0.7) at latitudes lower than 10°, decreases in the transition region 10°—15° (r ~ 0.4) and it has a non-linear profile at high latitudes, with a maximum near the 25°—30° band. The correlation is observed to be higher in the Southern Hemisphere latitudinal bands. Spectral analysis was performed for each latitudinal range, and QBO period and amplitude profiles were obtained. A detailed latitudinal profile of the QBO signal in total ozone is obtained from the present analysis.

Outer radiation belt dropout dynamics following the arrival of two interplanetary coronal mass ejections

Magnetopause shadowing and wave-particle interactions are recognized as the two primary mechanisms for losses of electrons from the outer radiation belt. We investigate these mechanisms, sing satellite observations both in interplanetary space and within the magnetosphere and particle drift modeling. Two interplanetary shocks sheaths impinged upon the magnetopause causing a relativistic electron flux dropout. The magnetic cloud (C) and interplanetary structure sunward of the MC had primarily northward magnetic field, perhaps leading to a concomitant lack of substorm activity and a 10 day long quiescent period. The arrival of two shocks caused an unusual electron flux dropout. Test-particle simulations have shown 2 to 5 MeV energy, equatorially mirroring electrons with initial values of L 5.5can be lost to the magnetosheath via magnetopause shadowing alone. For electron losses at lower L-shells, coherent chorus wave-driven pitch angle scattering and ULF wave-driven radial transport have been shownto be viable mechanisms.

Sunspot number, solar activity index

On ´ umero de manchas solares ´ondice mais antigo da atividade solar, com observac ¸˜ oes de manchas solares estendendo-se desde 1611. Ele serve como uma medida da atividade magnetica geral do Sol. Neste trabalho apresentamos uma descric ¸˜ ao das caracter´osticas f´ osicas das manchas solares, a definic ¸˜ ao do numero de manchas solares, uma analise espectral do mesmo e uma reconstruc ¸˜ ao de 1000 anos. The sunspot number is the longest solar activity index available with observations since 1611. It is a measure of the general state of solar magnetic activity. In this work we present a desciption of sunspot physical caracte- ristics, the definition of the sunspot number, a spectral analysis and a reconstruction for the last 1000 years of the sunspot number.