E. Echer

Interplanetary conditions leading to superintense geomagnetic storms (Dst ≤ −250 nT) during solar cycle 23

[1] The interplanetary causes of superintense geomagnetic storms (superstorms, Dst < -250 nT) that occurred during solar cycle 23 are studied. Eleven superstorms occurred during the cycle, five close to solar maximum (2000-2001) and six in the post-maximum/declining phase (2003-2004). About 1/3 of the superstorms were caused by magnetic clouds (MCs), 1/3 by a combination of sheath and MC fields, and 1/3 by sheath fields alone. The interplanetary parameter best correlated with peak Dst was the time-integrated E y during the storm main phase (in contrast with peak B s and/or peak Ey for less intense geomagnetic storms). The range of peak Dst for these storms was -263 to -422 nT. The storm main phase durations had a range of 3-33 h. We conclude from this study that: (1) only MCs and/or interplanetary sheaths had fields intense enough and with long enough durations to cause superstorms; (2) superstorms occurred only in the maximum and declining phases; (3) the total energy transferred from the solar wind to the magnetosphere is best correlated with the time-integrated solar wind Ey parameter.

SOLAR VARIABILITY EFFECTS STUDIED BY TREE-RING DATA WAVELET ANALYSIS

Abstract The global change approach to study the Sun-Earth system gives a growing amount of evidences that climate dynamics is affected by a large number of factors. The solar variability is very likely to be among them. Natural records, such as tree ring data, can be investigated to study the past global and regional climate, which was influenced by the solar radiative output variations, associated to solar activity. Wavelet transform analysis was applied to sunspot number and tree ring width time series from 1837 to 1996 at Concordia, Brazil. The amplitude and cross-amplitude spectral representation in the time-frequency domain allowed us to detect the occurrence of predominant periodicities and the relationship between the sunspot number and the tree ring time series. The Morlet complex wavelet analysis was used to study the most important variability factors on time scales ranging from from 2 to 100 years, and their stability in time, which is shown in both time series studied. We also applied the cross-wavelet spectral analysis to evaluate time delay among different tree ring time series, and between tree ring and sunspot number time series.

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.

Heliospheric plasma sheet (HPS) impingement onto the magnetosphere as a cause of relativistic electron dropouts (REDs) via coherent EMIC wave scattering with possible consequences for climate change mechanisms

A new scenario is presented for the cause of magnetospheric relativistic electron decreases (REDs) and potential effects in the atmosphere and on climate. High density solar wind heliospheric plasmasheet (HPS) events impinge onto the magnetosphere, compressing it along with remnant noon-sector outer-zone magnetospheric ~10-100 keV protons. The betatron accelerated protons generate coherent EMIC waves through a temperature anisotropy (T┴/T|| > 1) instability. The waves in turn interact with relativistic electrons and cause the rapid loss of these particles to a small region of the atmosphere. A peak total energy deposition of ~3 x 1020 ergs is derived for the precipitating electrons. Maximum energy deposition and creation of electron-ion pairs at 30-50 km and at < 30 km altitude are quantified. We focus the readers’ attention on the relevance of this present work to two climate change mechanisms. Wilcox et al. [1973] noted a correlation between solar wind heliospheric current sheet (HCS) crossings and high atmospheric vorticity centers at 300 mb altitude. Tinsley et al. [1994] has constructed a global circuit model which depends on particle precipitation into the atmosphere. Other possible scenarios potentially affecting weather/climate change are also discussed.

Magnetospheric Energetics During HILDCAAs

Some aspects of High-Intensity, Long-Duration, Continuous AE activity (HILDCAA for short) events are discussed in the context of storm and substorm energy flux and dynamics. The issue of energy transferred from the solar wind to the magnetosphere during HILDCAAs is of particular interest. We make estimates of the latter under the assumption of a summation of short magnetopause reconnection events associated with the quasiperiodic behavior of the IMF Bz component (part of interplanetary Alfven wave trains). A global magnetospheric MHD simulation is used to illustrate the response of the near-Earth plasma sheet to the changing behavior of IMF Bz during idealized HILDCAA events. In conclusion, it is argued that HILDCAAs are a new type of geomagnetic activity, substantially different than either substorms or storms. The relationship between HILDCAAs and substorms from both the interplanetary driver perspective and magnetospheric energy are discussed.

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.

Extremely low geomagnetic activity during the recent deep solar cycle minimum

The recent solar minimum (2008-2009) was extreme in several aspects: the sunspot number, Rz , interplanetary magnetic field (IMF) magnitude Bo and solar wind speed Vsw were the lowest during the space era. Furthermore, the variance of the IMF southward Bz component was low. As a consequence of these exceedingly low solar wind parameters, there was a minimum in the energy transfer from solar wind to the magnetosphere, and the geomagnetic activity ap index reached extremely low levels. The minimum in geomagnetic activity was delayed in relation to sunspot cycle minimum. We compare the solar wind and geomagnetic activity observed in this recent minimum with previous solar cycle values during the space era (1964-2010). Moreover, the geomagnetic activity conditions during the current minimum are compared with long term variability during the period of available geomagnetic observations. The extremely low geomagnetic activity observed in this solar minimum was previously recorded only at the end of XIX century and at the beginning of the XX century, and this might be related to the Gleissberg (80-100 years) solar cycle.

The interplanetary magnetic decrease automatic detection (IMDAD) code

A new code, called the interplanetary magnetic decrease (MD) automatic detection (IMDAD) code, has been developed to enable researchers in the field to rapidly identify MD events for further analyses. The criterion used for MD selection is Bmin < XB0, where X is a variable value and B0 is the ambient magnetic field magnitude. The code can be applied to data sets from different instruments/missions located in different space plasma environments in the heliosphere. The code has been tested during slow solar wind, fast solar wind and CIR intervals at ∼5 AU (from November 28 to December 03, 1992). For this test, we used a sliding window with a width of 300 seconds applied to 1-second high-resolution magnetic field data. The events identified by the code have been confirmed by hand analyses. The routine was able to identify 57 of the 118 MDs identified by hand (∼50%). The selection criteria for IMDAD and hand-analyses MDs were not exactly the same, accounting for the different rates of occurrence. What is particularly encouraging is that IMDAD did not falsely identify any events. The discrepancies between the two methods are discussed in the text. This code will be made available to the general public.

A study of the solar cycle signal on total ozone over low-latitude Brazilian observation stations

Abstract Observations of total ozone at low latitudes in Brazil have been made using Dobson spectrophotometers since 1974 for Cachoeira Paulista (23.1° S, 45° W) and since 1978 for Natal (5.8° S, 35.2° W). Annual averages, 12 months and 36 months running averages have been analyzed. Spectral analyses of the data revealed that the most important periods found (confidence level > 90%) were: for Natal, 2.5 years (93.1%, quasi-biennial oscillation-QBO) and 10 years (98,2%, possibly the solar cycle signal); for Cachoeira Paulista, 2.4 years (96.8%, QBO) and 8 years (99.6%). The difference in total ozone between maximum and minimum solar cycles were estimated, using yearly averages of total ozone. For solar cycle 21, 1.16% and 1.26% for Natal and Cachoeira Paulista were found; for solar cycle 22, a larger difference of 3.8% for Natal and 4.1% for Cachoeira Paulista were found. The corresponding variation in UV-B at 300 nm, using Beers law, is 8–10% for C. Paulista and 4–5% for Natal, with maxima occurring during the minimum of the solar cycle.