Ezequiel Echer

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.

Relativistic electron acceleration during high‐intensity, long‐duration, continuous AE activity (HILDCAA) events: Solar cycle phase dependences

High-intensity, long-duration, continuous AE activity (HILDCAA) intervals during solar cycle 23 (1995–2008) have been studied by a superposed epoch analysis. It was found that HILDCAA intervals order the solar wind velocity, temperature and density (characteristic of high-speed solar wind intervals), the polar cap potential, and various other geomagnetic indices well. The interplanetary magnetic field Bz is generally negative, and the Newell solar wind coupling function is high during HILDCAA events. The HILDCAA intervals are well correlated with an enhancement of magnetospheric relativistic (E > 2 MeV) electron fluxes observed at geosynchronous orbit with a delay of ~1.5 days from the onset of the HILDCAAs. The response of the energetic electrons to HILDCAAs is found to vary with solar cycle phase. The initial electron fluxes are lower for events occurring during the ascending and solar maximum (AMAX) phases than for events occurring during the descending and solar minimum (DMIN) phases. The flux increases for the DMIN phase events are >50% larger than for the AMAX phase events. Although the solar wind speeds during the DMIN phases were slightly higher and lasted longer than during the AMAX phases, no other significant solar wind differences were noted. It is concluded that electrons are accelerated to relativistic energies most often and most efficiently during the DMIN phases of the solar cycle. We propose two possible solar UV mechanisms to explain this solar cycle effect.

Comparative study between four classical spectral analysis methods

This work presents a comparison between four classical spectral analyses: Fourier, multitaper, maximum entropy and iterative regression. Six 256-sample artificial series were generated by superposition of sine functions, long trends (of time scale greater than series length) and noise (generated by pseudo-random function). A spectral analysis of an observational time series (sunspot number) was also performed. Advantages and drawbacks of every method are described in this work.

Dynamics of coronal mass ejections in the interplanetary medium

Context. Coronal mass ejections (CMEs) are large plasma structures expelled from the low corona to the interplanetary space with a wide range of speeds. In the interplanetary medium CMEs suffer changes in their speeds because of interaction with the ambient solar wind. Aims. To understand the interplanetary CME (ICME) dynamics, we analyze the interaction between these structures and the ambient solar wind (SW), approaching the problem from the hydrodynamic point of view. Methods. We assume that the dynamics of the system is dominated by two kinds of drag-force dependence on speed (U), as ∼U and ∼U 2 . Furthermore, we propose a model that takes variations of the ICME radius (R) and SW density (ρsw) into account as a function of the distance (x )a sR(x) = x 0.78 and ρsw(x) = 1/x 2 , respectively. Then, we solve the equation of motion and present exact solutions Results. Considering CME speeds measured at a few solar radii and at one AU, we were able to constrain the values of the constants (viscosity and drag coefficient) for the linear (U) and quadratic (U 2 ) speed dependences, which seems to reproduce the ICME – SW system well. We found different solutions in which the concavity of the curves of the ICME speed profile changes, depending on the dominant factor, either the ICME radius or the SW density. Conclusions. This work shows that the macroscopic ICME propagation may be described by the hydrodynamic theory and that it is possible to find analytical solutions for the ICME-SW interaction.

RELATIVISTIC (E > 0.6, > 2.0, AND > 4.0 MeV) ELECTRON ACCELERATION AT GEOSYNCHRONOUS ORBIT DURING HIGH-INTENSITY, LONG-DURATION, CONTINUOUS AE ACTIVITY (HILDCAA) EVENTS

Radiation-belt relativistic (E > 0.6, > 2.0, and > 4.0?MeV) electron acceleration is studied for solar cycle 23 (1995-2008). High-intensity, long-duration, continuous AE activity (HILDCAA) events are considered as the basis of the analyses. All of the 35 HILDCAA events under study were found to be characterized by flux enhancements of magnetospheric relativistic electrons of all three energies compared to the pre-event flux levels. For the E > 2.0?MeV electron fluxes, enhancement of >50% occurred during 100% of HILDCAAs. Cluster-4 passes were examined for electromagnetic chorus waves in the 5 0.6, > 2.0, and > 4.0?MeV electrons occurred ~1.0?day, ~1.5?days, and ~2.5?days after the statistical HILDCAA onset, respectively. The statistical acceleration rates for the three energy ranges were ~1.8?? 105, 2.2?? 103, and 1.0?? 101 cm?2 s?1 sr?1 d?1, respectively. The relativistic electron-decay timescales were determined to be ~7.7, 5.5, and 4.0?days for the three energy ranges, respectively. The HILDCAAs were divided into short-duration (D ? 3?days) and long-duration (D > 3?days) events to study the dependence of relativistic electron variation on HILDCAA duration. For long-duration events, the flux enhancements during HILDCAAs with respect to pre-event fluxes were ~290%, 520%, and 82% for E > 0.6, > 2.0, and > 4.0?MeV electrons, respectively. The enhancements were ~250%, 400%, and 27% respectively, for short-duration events. The results are discussed with respect to the current understanding of radiation-belt dynamics.

Solar wind‐magnetosphere energy coupling efficiency and partitioning: HILDCAAs and preceding CIR storms during solar cycle 23

A quantitative study on the energetics of the solar wind-magnetosphere-ionosphere system during High-Intensity, Long-Duration, Continuous AE Activity (HILDCAA) events for solar cycle 23 (from 1995 through 2008) is presented. For all HILDCAAs, the average energy transferred to the magnetospheric/ionospheric system was ~6.3 ×1016 J, and the ram kinetic energy of the incident solar wind was ~7.1 ×1018 J. For individual HILDCAA events the coupling efficiency, defined as the ratio of the solar wind energy input to the solar wind kinetic energy, varied between 0.3% and 2.8%, with an average value of ~0.9%. The solar wind coupling efficiency for corotating interaction region (CIR)-driven storms prior to the HILDCAA events was found to vary from ~1% to 5%, with an average value of ~2%. Both of these values are lower than the> 5% coupling efficiency noted for interplanetary coronal mass ejection (and sheath)-driven magnetic storms. During HILDCAAs, ~67% of the solar wind energy input went into Joule heating, ~22% into auroral precipitation, and ~11% into the ring current energy. The CIR-storm Joule heating (~49%) was noticeably less than that during HILDCAAs, while the ring current energies were comparable for the two. Joule dissipation was higher for HILDCAAs that followed CIR-storms (88%) than for isolated HILDCAAs (~60%). Possible physical interpretations for the statistical results obtained in this paper are discussed.

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.

Comment on “Comment on the abundances of rotational and tangential discontinuities in the solar wind” by M. Neugebauer

[1] Neugebauer [2006] has very nicely reviewed the current status of work done on identifying the abundance of rotational discontinuities (RDs) and tangential discontinuities (TDs) occurring in interplanetary space. This has been a topic of great interest and heated debate since the 1970s [Smith, 1973; Belcher and Solodyna, 1975; Burlaga et al., 1977; Lepping and Behannon, 1980] (see also discussion by Neugebauer et al. [1984]). Neugebauer [2006] has also reexamined jump conditions across discontinuities and has ended up with inconclusive answers. [2] We wish to make some suggestions that may help clarify the apparently conflicting results of the RD/TD occurrence ratio existing in the literature. We will argue that in many cases discontinuities are ‘‘contaminated’’ by overlying plasma and induced magnetic fields (see also discussion by Sonnerup and Scheible [1998], Horbury et al. [2001], and Knetter et al. [2004] concerning contamination by electromagnetic plasma waves), leading to errors (in interpretation) of results using the Sonnerup and Cahill [1967] minimum variance method (MVA). We also will argue that the establishment of pure (or nearly pure) solar wind convection of discontinuities does not necessarily lead to the conclusion that they are TDs. [3] Tsurutani et al. [1994] have argued that interplanetary discontinuities are (often) the phase steepened edges of nonlinear Alfvén waves as Neugebauer [2006] notes. Another feature detected in interplanetary space are decreases in the interplanetary magnetic field magnitude. These have been given the name magnetic holes (MHs), magnetic decreases (MDs) and other descriptive names in the literature [Turner et al., 1977; Winterhalter et al., 1994, 2000; Tsurutani and Ho, 1999]. These magnetic field magnitude (pressure) decreases are supplanted by enhanced, anisotropic plasma [Fränz et al., 2000; Neugebauer et al., 2001]. The total pressure is constant across these structures, to first order [Winterhalter et al., 1994]. It has recently been shown that these MHs/MDs are (often) collocated with the discontinuities/phase steepened edges of Alfvén waves [Tsurutani et al., 2002a, 2002b]. Dasgupta et al. [2003] and Tsurutani et al. [2002b, 2005a] have argued that the ponderomotive force associated with the steepened Alfvén wave edges (the discontinuities) accelerate solar wind ions (and electrons) perpendicular to the ambient magnetic field and thus create the MHs/MDs by plasma diamagnetic effects. In this scenario, the plasma blobs and their resultant magnetic decreases are external features to the discontinuities and not parts of the discontinuities/Alfvén waves themselves. From this viewpoint, MHs/MDs can thus be thought of as byproducts of the Alfvén wave dissipation process. [4] We view MH/MD plasma and induced field decreases which are collocated with the discontinuities as possible contaminants to the intrinsic discontinuity structures. In some cases the MHs/MDs can appear to be bounded by a pair of TD-like structures as well [Tsurutani and Ho, 1999]. Minimum variance analysis results of this very complex region of multiple discontinuities will be difficult, if not impossible to interpret. [5] Tsurutani et al. [2005b] have examined several (7) events from the Knetter [2005] Cluster discontinuity data set where the discontinuities were collocated with MHs/MDs. All of the discontinuities were associated with Alfvén waves. The same discontinuities were identified at ACE, 0.01 AU upstream of Cluster, by their similar field rotational characteristics. The time delay from detection at ACE to that at Cluster was measured. It was found that the discontinuities/MHs/MDs propagated at almost the solar wind convection speed (determined by plasma measurements), within measurement uncertainties. Tsurutani et al. [2005b] speculated that the low wave propagation speed relative to the ambient solar wind was due to a ‘‘slowing’’ of the wave phase speed through the high-density plasma (the MHs/MDs), oblique wave propagation, or a combination of both factors. [6] As to why most directional discontinuities (DDs) in the solar wind have small values of BN [Knetter et al., 2004; JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A03101, doi:10.1029/2006JA011973, 2007

Relativistic electron acceleration during HILDCAA events: are precursor CIR magnetic storms important?

We present a comparative study of high-intensity long-duration continuous AE activity (HILDCAA) events, both isolated and those occurring in the “recovery phase” of geomagnetic storms induced by corotating interaction regions (CIRs). The aim of this study is to determine the difference, if any, in relativistic electron acceleration and magnetospheric energy deposition. All HILDCAA events in solar cycle 23 (from 1995 through 2008) are used in this study. Isolated HILDCAA events are characterized by enhanced fluxes of relativistic electrons compared to the pre-event flux levels. CIR magnetic storms followed by HILDCAA events show almost the same relativistic electron signatures. Cluster 1 spacecraft showed the presence of intense whistler-mode chorus waves in the outer magnetosphere during all HILDCAA intervals (when Cluster data were available). The storm-related HILDCAA events are characterized by slightly lower solar wind input energy and larger magnetospheric/ionospheric dissipation energy compared with the isolated events. A quantitative assessment shows that the mean ring current dissipation is ~34 % higher for the storm-related events relative to the isolated events, whereas Joule heating and auroral precipitation display no (statistically) distinguishable differences. On the average, the isolated events are found to be comparatively weaker and shorter than the storm-related events, although the geomagnetic characteristics of both classes of events bear no statistically significant difference. It is concluded that the CIR storms preceding the HILDCAAs have little to do with the acceleration of relativistic electrons. Our hypothesis is that ~10–100-keV electrons are sporadically injected into the magnetosphere during HILDCAA events, the anisotropic electrons continuously generate electromagnetic chorus plasma waves, and the chorus then continuously accelerates the high-energy portion of this electron spectrum to MeV energies.

Solar wind effects on Jupiter non-Io DAM emissions during Ulysses distant encounter (2003-2004)

Aims. We analyze solar wind data from the Ulysses spacecraft during the distant Jupiter encounter from 2003 November to 2004 March as well as Nancay decametric (DAM) radio observations non-controlled by Io. Methods. The Ulysses solar wind data are balistically propagated towards Jupiter and are correlated with Nancay non-Io DAM emissions. Results. It is found that the average solar wind dynamic pressure around the time of DAM emissions is 1.7 times higher than its average value during the Ulysses encounter. The occurrence of fast forward (FS) and reverse (RS) interplanetary shocks and heliospheric current sheet crossings (HCS) is correlated with the occurrence of non-Io DAM emissions. We note an enhanced probability of occurrence of non-Io DAM emissions after the expected arrival of FS, RS and HCS at Jupiter. However, about half emissions (54%) did not seem to be associated with these interplanetary structures. We also note that the average duration and power of non-Io DAM emissions are enhanced during periods associated with those interplanetary structures. Conclusions. From the results obtained in this work it seems that non-Io DAM emissions occur during intervals of enhanced solar wind dynamic pressure. Yet, there is no direct correlation between the non-Io DAM emissions duration or power versus the solar wind pressure values and the interplanetary shock Mach number.