Marisa Storini

On the origins of solar EIT waves

Abstract : Approximately half of the large-scale coronal waves identified in images obtained by the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory from 1997 March to 1998 June were associated with small solar flares with soft X-ray intensities below C class. The probability of a given flare of this intensity having an associated EIT wave is low. For example, of ~8,000 B-class flares occurring during this 15 month period, only 1% were linked to EIT waves. These results indicate the need for a special condition that distinguishes flares with EIT waves from the vast majority of flares that lack wave association. Various lines of evidence, including the fact that EIT waves have recently been shown to be highly associated with coronal mass ejections (CMEs), suggest that this special condition is a CME. A CME is not a sufficient condition for a detectable EIT wave, however, because we calculate that 5 times as many front-side CMEs as EIT waves occurred during this period, after taking the various visibility factors for both phenomena into account. In general, EIT wave association increases with CME speed and width.

Cosmic Rays in Relation to Space Weather

A review of selected experimental results relevant for the use of cosmic ray records in Space Weather research is presented. Interplanetary perturbations, initiated in the solar atmosphere, affect galactic cosmic rays. In some cases their influence on the cosmic ray intensity results in data signatures that can possibly be used to predict geomagnetic storm onsets. Case studies illustrating the complexity of the cosmic ray effects and related geomagnetic activity precursors are discussed. It is shown that some indices for cosmic ray activity are good tools for testing the reliability of cosmic ray characteristics for Space Weather forecasts. A brief summary of the influence of cosmic rays on the ozone layer is also given. The use of cosmic ray data for Space Weather purposes is still in its infant stage, but suggestions for both case and statistical studies are made.

The GNEVYSHEV gap: A review for space weather

Abstract The ability to forecast long-term Space Weather effects needs a good knowledge of solar activity behaviour along the 11-year sunspot cycle. In this paper we concentrate our attention on the so-called maximum activity phase and we discuss the role of the Gnevyshev gap (a time interval in which a decrease in solar atmospheric activity is observed) in the time history of Space Weather parameters.

Structure of the maximum phase of solar cycles 21 and 22

A distinctive peak and gap structure in a number of solar indices was observed in the maximum phase of solar cycles 21 and 22. The effect became even more prominent after separating the northern and southern solar hemispheres. In cycle 21 the multi-peaked structures observed in the two solar hemispheres were not synchronous and their sum resulted in the rather shallow two-peaked solar maximum for the parameters taken over the whole solar disk. In cycle 22 there were only double peaks in each hemisphere which were rather synchronous. Examination of solar activity in the northern and southern hemispheres has shown that the structured maximum appears to be due to the superposition of two quasi-oscillating processes with characteristic time-scales of 11 years and of 1–3 years (quasi-biennial oscillations). The absolute amplitude of the quasi-biennial oscillations depends on the 11-year cycle phase and reaches its maximum at the maximum of the 11-year cycle. This explains the occurrence of a double- or triple-peak structure in the solar maximum phase.

QUASI-BIENNIAL MODULATION OF SOLAR NEUTRINO FLUX AND SOLAR AND GALACTIC COSMIC RAYS BY SOLAR CYCLIC ACTIVITY

Using some solar activity indicators such as sunspot areas and green-line coronal emission during the period 1974-2001, we find that the quasi-biennial periodicity is a fundamental mode of solar variability. We provide evidence for the quasi-biennial modulation of the solar neutrino flux, thus supporting the hypothesis of a connection between solar neutrinos and solar magnetic fields, probably through direct interaction with the neutrino magnetic moment. The same periodic modulation has been detected when fluxes of solar energetic protons and galactic cosmic rays are investigated. These modulation results significantly correlate to that of the neutrino flux. Finally, the superposition of the quasi-biennial cycle to the eleven-year cycle can explain the Gnevyshev Gap phenomenon.

Search for periodicities in the IMP 8 Charged Particle Measurement Experiment proton fluxes for the energy bands 0.50–0.96 MeV and 190–440 MeV

[1] Past studies revealed that the photospheric magnetic field, as well as many solar activity phenomena, undergoes both periodic and quasiperiodic variations on different time scales. Nevertheless, only a few attempts have been made so far to detect corresponding variations in the occurrence frequency of solar energetic particle events. Here we search for periodicities in the proton fluxes, measured in the interplanetary space, on time scales ranging from a few (>6) Bartels rotations (27 days) up to the Schwabe (~11 years) period. We apply the wavelet technique to the proton fluxes recorded by the Charged Particle Measurement Experiment (CPME) instrument aboard IMP 8 spacecraft, from 1974 to 2001, in the energy bands 0.50-0.96 MeV and 190-440 MeV. The reliability of the obtained results is tested by analyzing the wavelet response to suitable artificial functions. The ~9.8, ~3.8, and -1.7-2.2 year periods are the most significant found in the interplanetary proton flux. Shorter periods (such as ~1 year) are detected in some time intervals, but they are not significant in the whole sequence of data.

Coronal mass ejections, flares, and geomagnetic storms

We investigate the role of the coronal mass ejections (CMEs) (by using soft X ray solar emission as proxy data) in producing the nonrecurrent geomagnetic storms in the period 1969-1974. The linkage between these phenomena is confirmed: however, it turns out that CMEs associated with chromospheric flares, accompanied by type 4 radio emission, are the most effective in perturbing the geomagnetic field.

Diurnal modulation of the geomagnetic activity induced by the southward component of the interplanetary magnetic field

The diurnal (universal time) modulation of the geomagnetic activity is investigated, using the am index along with interplanetary plasma and magnetic field data collected by a variety of spacecraft from 1965 to 1987. A direct comparison with satellite measurements of the Bs component of the interplanetary magnetic field (IMF) is made. Our results confirm the Russell-McPherron (R-M) model, which attributes the above phenomenon to the varying southward component (Bs) of the IMF in the geocentric solar magnetospheric system. Moreover, we compared the contributions of the equinoctial and R-M mechanisms to the observed am modulation, showing that the latter effect is dominant.

DRIFT EFFECTS ON THE GALACTIC COSMIC RAY MODULATION

Cosmic ray (CR) modulation is driven by both solar activity and drift effects in the heliosphere, although their role is only qualitatively understood as it is difficult to connect the CR variations to their sources. In order to address this problem, the Empirical Mode Decomposition technique has been applied to the CR intensity, recorded by three neutron monitors at different rigidities (Climax, Rome, and Huancayo-Haleakala (HH)), the sunspot area, as a proxy for solar activity, the heliospheric magnetic field magnitude, directly related to CR propagation, and the tilt angle (TA) of the heliospheric current sheet (HCS), which characterizes drift effects on CRs. A prominent periodicity at ~six years is detected in all the analyzed CR data sets and it is found to be highly correlated with changes in the HCS inclination at the same timescale. In addition, this variation is found to be responsible for the main features of the CR modulation during periods of low solar activity, such as the flat (peaked) maximum in even (odd) solar cycles. The contribution of the drift effects to the global Galactic CR modulation has been estimated to be between 30% and 35%, depending on the CR particle energy. Nevertheless, the importance of the drift contribution is generally reduced in periods nearing the sunspot maximum. Finally, threshold values of ~40°, ~45°, and >55° have been derived for the TA, critical for the CR modulation at the Climax, Rome, and HH rigidity thresholds, respectively.

THE EMPIRICAL MODE DECOMPOSITION TO STUDY THE QUASI-BIENNIAL MODULATION OF SOLAR MAGNETIC ACTIVITY AND SOLAR NEUTRINO FLUX

The time variability of solar activity indices such as sunspot areas (SAs) and green-line coronal emission, fluxes of solar energetic protons and galactic cosmic rays (CRs) in the period 1974–2001 has been investigated through the empirical mode decomposition (EMD). We found that the quasi-biennial periodicity is a prominent time scale of solar variability, having the energetic particle indices an amplitude comparable with the 11 years one. Moreover, we provide evidence for the quasi-biennial modulation of the solar neutrino flux, which results to be also significantly correlated with the fluxes of solar energetic protons and galactic CRs. The significance of all the correlation has been tested by applying both bootstrap and Monte Carlo methods.