During a coordinated observing campaign (Solar and Heliospheric Observatory, SOHO JOP 139), we obtained simultaneous spectroheliograms of a solar active region in several spectral lines, sampling levels from the chromosphere to the corona. Ground-based spectroheliograms were acquired at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak in four chromospheric lines, while the coronal diagnostic spectrograph on board SOHO was used to obtain rasters of the active region in transition region (TR) and coronal lines. Such a complete data set allowed us to compare the development of intensity and velocity fields during a small two-ribbon flare in the whole atmosphere. In particular, we obtained for the first time quasi-simultaneous and spatially resolved observations of velocity fields during the impulsive phase of a flare, in both the chromosphere and upper atmosphere. In this phase, strong downflows (up to 40 km s À1 ) following the shape of the developing ribbons are measured at chromospheric levels, while strong upward motions are instead measured in TR (up to À100 km s À1 ) and coronal lines (À160 km s À1 ). The spatial pattern of these velocities have a common area about 10 00 wide. This is the first time that opposite-directed flows at different atmospheric levels are observed in the same spatial location during a flare. These signatures are highly suggestive of the chromospheric evaporation scenario predicted in theoretical models of flares.

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We analyze neutrino mixing and oscillations within the framework of extended theories of gravity. In particular, by relying on the covariant reformulation of Pontecorvo's formalism, we evaluate the oscillation probability of neutrinos propagating in several static spacetimes described by gravitational actions quadratic in the curvature invariants. We show that neutrino oscillation phase is sensitive to the violation of the strong equivalence principle. This connection may be in principle explained via the appearance of the Eddington-Robertson-Schiff parameter in the neutrino Hamiltonian. The above studies are then specialized to different extended models in order both to quantify such a violation and to understand how the characteristic free parameters of the theories could affect the standard result. The possibility to fix new bounds on such parameters and to constrain extended theories of gravity is also discussed.

In the context of quantum field theory, we derive flavor-energy uncertainty relations for neutrino oscillations. By identifying the non-conserved flavor charges with the "clock observables", we arrive at the Mandelstam-Tamm version of time-energy uncertainty relations. In the ultra-relativistic limit these relations yield the well known condition for neutrino oscillations. Ensuing non-relativistic corrections to the latter are explicitly evaluated. The analogy among flavor states and unstable particles and a novel interpretation of our uncertainty relations, based on the unitary inequivalence of Fock spaces for flavor and massive neutrinos, are also discussed.Introduction. -Neutrino mixing and oscillations represent one of the most pressing challenges of modern theoretical and experimental particle physics. They were first introduced by Pontecorvo [1] in a close analogy with the phenomenon of Kaon oscillations [2], and subsequently confirmed in a number of experimental settings [3]. While the quantum mechanical (QM) description [4][5][6] is quite successful in tackling high-energy features of neutrino oscillations, the corresponding quantum field theoretical (QFT) description (which could tackle also the lowenergy behavior) is still controversial [6][7][8][9].