We report here, for the —rst time, on spatial oscillations of coronal loops, which were detected in extreme-ultraviolet wavelengths (171 with the T ransition Region and Coronal Explorer, in the tem- Ae ) perature range of MK. The observed loop oscillations occurred during a —are that began at T e B 1.0¨1.5 1998 July 14, 12:55 UT and are most prominent during the —rst 20 minutes. The oscillating loops connect the penumbra of the leading sunspot to the —are site in the trailing portion. We identi—ed —ve oscillating loops with an average length of L \ 130,000 ^ 30,000 km. The transverse amplitude of the oscillations is A \ 4100 ^ 1300 km, and the mean period is T \ 280 ^ 30 s. The oscillation mode appears to be a standing wave mode (with —xed nodes at the footpoints). We investigate diUerent MHD wave modes and —nd that the fast kink mode with a period q \ 205(L /1010 cm~3)1@2 cm)(n e /109 (B/10 G)~1 s provides the best agreement with the observed period. We propose that the onset of loop oscillations in distant locations is triggered by a signal or disturbance that propagates from the central —are site with a radial speed of B700 km s~1. Because the observed loop oscillation periods are compa- rable to photospheric 5 minute oscillations, a resonant coupling between the two systems is possible. We further —nd evidence for global extreme-UV dimming in the entire active region possibly associated with a coronal mass ejection. Subject headings: Sun: coronaSun: —aresSun: oscillationsSun: UV radiation

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Coronal Loop Oscillations Observed with the Transition Region and Coronal Explorer

‘Pulsations’ include a wide range of phenomena from strictly sinusoidal oscillations up to quasiperiodic fine structures, observed in the radio, microwave and X-ray frequency range. The various versions of pulsation models are reviewed and classified in three groups according to their driver mechanisms: (1) Magnetic flux tube oscillations (the emissivity of trapped particles is modulated by a standing or propagating MHD wave), (2) cyclic self-organizing systems of plasma instabilities (wave-particle, wave-wave interactions), and (3) modulation of acceleration (acceleration/injection of particles into the source). Observational references illustrate the applicability of the models. In conclusion, discrimination criteria of models are discussed, in order to give a key for interpretation of observations.

Theory of Radio Pulsations in Coronal Loops

Pulsations in radio emission from the solar coronal plasma have been detected for over a decade1–9. The oscillations are quasiperiodic, with periods of typically a second or so. Recently, sub-second time structures have been found in hard X rays10, and simultaneously in hard X rays and microwaves11. Here we examine whether magnetohydrodynamical oscillations in a density enhancement, treated for simplicity as a straight magnetic slab, can explain the observed short periods. A dense region in the corona (for example, a loop) can act as a wave trap, and symmetrical oscillations within that trap must be of short wavelength with correspondingly short period. An impulsive source (such as a flare) naturally gives rise to a quasiperiodic disturbance. Such oscillations are closely akin to the Pekeris modes of oceanography, the Love waves of seismology and the dielectric waves of fibre optics.

Fast pulsations in the solar corona

A recent study by Nakariakov and coworkers pointed out that the dispersion relation of MHD sausage-mode oscillations has been incorrectly applied to coronal loops, neglecting the highly dispersive nature of the phase speed and the long-wavelength cutoff of the wavenumber. In the light of these new insights, we revisit previous observations that have been interpreted in terms of MHD sausage-mode oscillations in coronal loops and come to the following conclusions: (1) fast sausage MHD-mode oscillations require such a high electron density imposed by the wavenumber cutoff that they can only occur in flare loops, and (2) in the previously reported radio observations (ν ≈ 100 MHz to 1 GHz) with periods of P ≈ 0.5-5 s, the fast sausage MHD-mode oscillation is likely to be confined to a small segment (corresponding to a high harmonic node) near the apex of the loop, rather than involving a global oscillation over the entire loop length. The recent microwave and soft X-ray observations of fast periods (P ≈ 6-17 s) by Asai and coworkers and Melnikov and coworkers, however, are consistent with fast sausage MHD oscillations at the fundamental harmonic.

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Magnetohydrodynamic Sausage-Mode Oscillations in Coronal Loops

A CURIOUS but rare feature of the metre–wave continuum (type IV) radiation received from certain large solar flares is the pulsating structure which modulates the intensity in a periodic or quasi-periodic manner1,2. The modulation may persist for about l min or more and the period of the pulsation is typically of the order of 1 s. Of various explanations suggested, we consider the most satisfactory to be that of Rosenberg, in which the radiation is attributed to synchrotron radiation emitted by electrons in a magnetic flux tube embedded in the solar corona and the pulsations are attributed to modulation by standing magnetohydrodynamic (MHD) waves set up within the tube. We now present further evidence of this phenomenon and outline a model which accounts both for the acceleration of the emitting electrons and for the pulsations themselves. There is also evidence to suggest that energetic protons may be accelerated by the same process. We consider that the phenomenon affords a possible observational clue to a physical process by which solar cosmic rays can be generated high in the solar corona.

Regular Pulses from the Sun and a Possible Clue to the Origin of Solar Cosmic Rays

Three solar outbursts which show pulsating radio emissions at metric waves (239 MHz) are examined. The behaviour of the single frequency, high-time resolution records and the spectral diagrams seem to indicate that such phenomena are peculiar phases of type IV radiation, perhaps connected with absorptions in the solar corona. The spectral analysis of the low-frequency modulation of the emissions show a very definite spectral line with a period ranging from 1s.7 to 3s.1.

Pulsating radio emissions from the solar corona.

A study has been made of the behaviour in time of the positions, flux density and polarization for the radio emissions at frequencies of 237 and 408 MHz (from Trieste observations), taking into account also the published results at 169 and 160–200 MHz (from Nancay and Utrecht) connected with the solar flare which occurred on January 14, 1971, at 11h21m UT.

A. Abrami

Papers

Pulsating radio emissions from the solar corona.

The solar radio event of January 14, 1971.