X. Moussas

On signal–noise decomposition of time-series using the continuous wavelet transform: application to sunspot index

We show that the continuous wavelet transform can provide a unique decomposition of a time-series into ‘signal-like’ and ‘noise-like’ components. From the overall wavelet spectrum, two mutually independent skeleton spectra can be extracted, allowing the separate detection and monitoring in even non-stationary time-series of the evolution of (i) both stable but also transient, evolving periodicities, such as the output of low-dimensional dynamical systems, and (ii) scale-invariant structures, such as discontinuities, self-similar structures or noise. The idea of the method is to keep from the overall wavelet expansion of the time-series only the wavelet components of locally maximal amplitude at any given time or scale, thus obtaining the instantly maximal and scale maximal wavelet skeleton spectrum, respectively. The scale maximal spectrum was previously proposed for studying possible multifractal scaling properties of time-series. The instantly maximal spectrum proposed here exhibits clearer spectral peaks and reduced noise, as compared to the overall wavelet spectrum. An indicative application to the monthly-averaged sunspot index reveals, apart from the well-known 11-yr periodicity, three of its harmonics, the 2-yr periodicity (quasi-biennial oscillation, QBO) and several more (some of which have been detected previously in various solar, Earth–solar connection and climate indices), here proposed as just harmonics of the QBO, in all supporting the double-cycle solar magnetic dynamo model. The scale maximal spectrum reveals the presence of 1/f fluctuations with time-scales up to 1 yr in the sunspot number, indicating that the solar magnetic configurations involved in the transient solar activity phenomena with those characteristic time-scales are in a self-organized critical state, as previously proposed for the solar flare occurrence.

A shock associated (SA) radio event and related phenomena observed from the base of the solar corona to 1 AU

We present for the rst time an almost com- plete frequency coverage of a Shock Associated (SA) radio event and related phenomena observed on May 6, 1996 at 9:27 UT. It is observed from the base of the solar corona up to almost 1 Astronomical Unit (AU) from the Sun by the following radio astronomical instruments: the Ond rejov spectrometer operating between 4.5 GHz and 1 GHz (radi- ation produced near the chromosphere); the Thermopyles Artemis-IV spectrograph operating between 600 MHz and 110 MHz (distance range about 1.1-1.4R from sun center); the Nan cay Decameter Array operating between 75 and 25 MHz (distance range about 1.4-2 R); and the RAD2 and RAD1 radio receivers on the WIND spacecraft covering the range from 14 MHz to about 20 kHz (distance range be- tween 3 R and about 1 AU). Observations at the Nan cay Decameter Array clearly show that the SA event starts from a coronal type II radio burst which traces the progression of a shock wave through the corona above 1.8 R-2 R from the sun center. This SA event has no associated radio emis- sion in the decimetric-metric range, thus there is no evidence for electron injection in the low/middle corona. The SA event enigma: What does SA stand for? Type II and type III solar radio bursts result from the interaction of a disturbing agent {a beam of energetic elec- trons or a shock wave{ with the ambient plasma (Wild and Smerd, 1972). Radiation is produced near the fundamen- tal of the local plasma frequency f p (kHz) =9 n 1 = 2 e (cm 3 ) and/or its second harmonic through various plasma mech- anisms (see e.g. Robinson, 1997). The observed frequency can be converted into an altitude in the corona, assuming a density model and the radiated mode. Dierent frequency drifts reflect dierent velocities along the density gradient in the corona and interplanetary medium, helping us to charac- terize the nature of the exciter: 0.05-0.3c electron beam for

Wavelet Analysis on Solar Wind Parameters and Geomagnetic Indices

The geomagnetic activity is the result of the solar wind–magnetosphere interaction. It varies following the basic 11-year solar cycle; yet shorter time-scale variations appear intermittently. We study the quasi-periodic behavior of the characteristics of solar wind (speed, temperature, pressure, density) and the interplanetary magnetic field (Bx, By, Bz, β, Alfvén Mach number) and the variations of the geomagnetic activity indices (DST, AE, Ap and Kp). In the analysis of the corresponding 14 time series, which span four solar cycles (1966 – 2010), we use both a wavelet expansion and the Lomb/Scargle periodograms. Our results verify intermittent periodicities in our time-series data, which correspond to already known solar activity variations on timescales shorter than the sunspot cycle; some of these are shared between the solar wind parameters and geomagnetic indices.

A nonlinear RLC solar cycle model

A simplified, monoparametric model, based on the Van der Pol nonlinear RLC electric oscillator, is found capable of describing the shape and related morphological properties (such as the Waldmeier effect) of the sunspot cycles. The model can also exhibit long periods of sunspot inactivity of the Maunder Minimum type. According to the model, the significant rise-to-fall time asymmetry of the most recent cycles suggests that it is unlikely that another cycle suppression will occur in the forthcoming decades. The complete sunspot record and the systems attractor are successfully emulated, given the sunspot number at cycle maxima.

The improved ARTEMIS IV multichannel solar radio spectrograph of the University of Athens

We present the improved solar radio spectrograph of the University of Athens operating at the Thermopylae Satellite Telecommunication Station. Observations now cover the frequency range from 20 to 650 MHz. The spectrograph has a 7-meter moving parabola fed by a log-periodic antenna for 100–650 MHz and a stationary inverted V fat dipole antenna for the 20–100 MHz range. Two receivers are operating in parallel, one swept frequency for the whole range (10 spectrums/sec, 630 channels/spectrum) and one acousto-optical receiver for the range 270 to 450 MHz (100 spectrums/sec, 128 channels/spectrum). The data acquisition system consists of two PCs (equipped with 12 bit, 225 ksamples/sec ADC, one for each receiver). Sensitivity is about 3 SFU and 30 SFU in the 20–100 MHz and 100–650 MHz range respectively. The daily operation is fully automated: receiving universal time from a GPS, pointing the antenna to the sun, system calibration, starting and stopping the observations at preset times, data acquisition, and archiving on DVD. We can also control the whole system through modem or Internet. The instrument can be used either by itself or in conjunction with other instruments to study the onset and evolution of solar radio bursts and associated interplanetary phenomena.

Thermal and Chemical Structure Variations in Titan's Stratosphere during the Cassini Mission

We have developed a line-by-line Atmospheric Radiative Transfer for Titan code that includes the most recent laboratory spectroscopic data and haze descriptions relative to Titans stratosphere. We use this code to model Cassini Composite Infrared Spectrometer data taken during the numerous Titan flybys from 2006 to 2012 at surface-intercepting geometry in the 600-1500 cm–1 range for latitudes from 50°S to 50°N. We report variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE). We find indication for a weakening of the temperature gradient with warming of the stratosphere and cooling of the lower mesosphere. In addition, we infer precise concentrations for the trace gases and their main isotopologues and find that the chemical composition in Titans stratosphere varies significantly with latitude during the 6 years investigated here, with increased mixing ratios toward the northern latitudes. In particular, we monitor and quantify the amplitude of a maximum enhancement of several gases observed at northern latitudes up to 50°N around mid-2009, at the time of the NSE. We find that this rise is followed by a rapid decrease in chemical inventory in 2010 probably due to a weakening north polar vortex with reduced lateral mixing across the vortex boundary.

Solar type II and type IV radio bursts observed during 1998–2000 with the ARTEMIS-IV radiospectrograph

A catalogue of the type II and type IV solar radio bursts in the 110-687 MHz range, observed with the radio spectrograph ARTEMIS-IV operated by the University of Athens at Thermopylae, Greece from 1998-2000 is presented. These observations are compared with the LASCO archives of Coronal Mass Ejections and the Solar Geophysical Reports of solar flares (Ha & SXR) and examined for possible associations. The main results are: - 68% of the catalogue events were associated with CMEs. - 67% of the type II events were associated with CMEs, in accordance with previous results. This percentage rises to 79% in the case of composite type II/IV events. - 77% of the type IV continua were associated with CMEs, which is higher that the CME-type II association probability. - The type II associated CMEs had an average velocity of (835 ± 380) km s -1 , while the CMEs not associated with type IIs had an average velocity of (500 ± 150) km s -1 . - All events, but one, were well associated with Ha and/or SXR flares. - Most of the CME launch times precede by 5-60 min (30 min on average) the associated SXR flare peak; an important fraction (72%) precede the flare onset as well.

CME Expansion as the Driver of Metric Type II Shock Emission as Revealed by Self-consistent Analysis of High-Cadence EUV Images and Radio Spectrograms

On 13 June 2010, an eruptive event occurred near the solar limb. It included a small filament eruption and the onset of a relatively narrow coronal mass ejection (CME) surrounded by an extreme ultraviolet (EUV) wave front recorded by the Solar Dynamics Observatory’s (SDO) Atmospheric Imaging Assembly (AIA) at high cadence. The ejection was accompanied by a GOES M1.0 soft X-ray flare and a Type-II radio burst; high-resolution dynamic spectra of the latter were obtained by the Appareil de Routine pour le Traitement et l’Enregistrement Magnetique de l’Information Spectral (ARTEMIS IV) radio spectrograph. The combined observations enabled a study of the evolution of the ejecta and the EUV wave front and its relationship with the coronal shock manifesting itself as metric Type-II burst. By introducing a novel technique, which deduces a proxy of the EUV compression ratio from AIA imaging data and compares it with the compression ratio deduced from the band-split of the Type-II metric radio burst, we are able to infer the potential source locations of the radio emission of the shock on that AIA images. Our results indicate that the expansion of the CME ejecta is the source for both EUV and radio shock emissions. Early in the CME expansion phase, the Type-II burst seems to originate in the sheath region between the EUV bubble and the EUV shock front in both radial and lateral directions. This suggests that both the nose and the flanks of the expanding bubble could have driven the shock.

Surface albedo spectral properties of geologically interesting areas on Titan

We investigate the nature and possible formation processes of three areas on Titans surface which have been suggested as geologically interesting: Hotei Regio, Tui Regio, and Sotra Patera. We also reanalyze the spectral characteristics of the Huygens Landing Site. We apply a statistical Principal Component Analysis (PCA) and a radiative transfer (RT) method on the Visual and Infrared Mapping Spectrometer Datacubes in order to retrieve the surface albedo of distinct spectral units in the near infrared. We have been able to exploit only a subset of the currently available Hotei Regio data, which are, in general, not optimal in terms of geometry for an analysis with a plane-parallel RT code. Our inferred surface albedos present generally higher values from 1 to 2 µm and lower ones at 0.94 and in the 2.6–5 µm region. The Regions of Interest (RoIs) within Hotei Regio, Tui Regio, and Sotra Patera are always significantly brighter than the surrounding areas. The largest variations are found longward of 2 µm and mainly at 5 µm. This higher surface albedo with respect to the surrounding area and, in general, the fact that the spectral behavior is different for each of these areas, is probably indicative of diverse chemical compositions and origins. We compare the spectral albedos with some suggested surface candidates on Titan (such as H2O, CO2, and CH4 ices, as well as tholin) and discuss possible chemical composition variations as well as other interpretations.

The correlation of fractal structures in the photospheric and the coronal magnetic field

Context. This work examines the relation between the fractal properties of the photospheric magnetic patterns and those of the coronal magnetic fields in solar active regions. Aims. We investigate whether there is any correlation between the fractal dimensions of the photospheric structures and the magnetic discontinuities formed in the corona. Methods. To investigate the connection between the photospheric and coronal complexity, we used a nonlinear force-free extrapolation method that reconstructs the 3d magnetic fields using 2d observed vector magnetograms as boundary conditions. We then located the magnetic discontinuities, which are considered as spatial proxies of reconnection-related instabilities. These discontinuities form well-defined volumes, called here unstable volumes. We calculated the fractal dimensions of these unstable volumes and compared them to the fractal dimensions of the boundary vector magnetograms. Results. Our results show no correlation between the fractal dimensions of the observed 2d photospheric structures and the extrapolated unstable volumes in the corona, when nonlinear force-free extrapolation is used. This result is independent of efforts to (1) bring the photospheric magnetic fields closer to a nonlinear force-free equilibrium and (2) omit the lower part of the modeled magnetic field volume that is almost completely filled by unstable volumes. A significant correlation between the fractal dimensions of the photospheric and coronal magnetic features is only observed at the zero level (lower limit) of approximation of a current-free (potential) magnetic field extrapolation. Conclusions. We conclude that the complicated transition from photospheric non-force-free fields to coronal force-free ones hampers any direct correlation between the fractal dimensions of the 2d photospheric patterns and their 3d counterparts in the corona at the nonlinear force-free limit, which can be considered as a second level of approximation in this study. Correspondingly, in the zero and first levels of approximation, namely, the potential and linear force-free extrapolation, respectively, we reveal a significant correlation between the fractal dimensions of the photospheric and coronal structures, which can be attributed to the lack of electric currents or to their purely field-aligned orientation.