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Coronal Transients in UV Spectral Lines

 

Coronal transients are the main evolutionary dynamical structures in the extended solar corona. Such phenomena are associated with a variation of the physical parameters (temperature, density and velocity) characterizing the coronal plasma, occurring over typical time scales ranging from tens of minutes to hours. Coronal transients are often related to the expansion of dense plasma clouds into the interplanetary medium, with outward velocities extending from less than 100 km s to more than 1000 km s, and average values around 500 km s: in this case, they are referred as Coronal Mass Ejections (CMEs). Typical total masses and kinetic energies of CMEs are 10 - 10 gr and 10 - 10 erg, respectively (Gosling et al. 1976, Hundhausen et al. 1984, Dryer 1982, Howard et al. 1985).

Presently, knowledge about coronal transients is mainly based on broad--band white-light observations from satellite instruments (e.g., OSO-7 and Skylab coronagraphs during solar cycle 20, P 78-1 Solwind, Solar Maximum Mission C/P during solar cycle 21) in the range 1.6 - 10 solar radii (R), and on ground-based instruments in the range 1.2 - 2.2 R (e.g., HAO Mauna Loa K coronameter). While such observations allow investigation of the location of the transient with respect to the solar disk, its morphology and the temporal variation of the height above solar limb, they give little information on the physical parameters of the perturbed coronal region, which are actually poorly understood.

Detailed spectroscopic observations of coronal transients are needed in order to understand the physical properties of the plasma inside the perturbed coronal region, evaluate the influence of the propagation of the transient on the unperturbed coronal plasma (shock formation due to transient propagation, etc.), investigate the association between coronal transients and other forms of solar activity (e.g., prominence eruption and solar flares), and discriminate among the different primary mechanisms proposed to cause coronal transients, with particular reference to the role played by processes involving magnetic field restructuring in the corona, such as magnetic reconnection.

The UVCS primary FOV (141 arcmin x 42 arcmin) allows coverage of the extended corona from 1.2 to 10 R: this range allows the study of coronal transient phenomena at different heights in the solar atmosphere and/or at different evolutionary stages. Observations will be performed in both the HI Ly (1130 - 1300 Å) and O VI (930 - 1068 Å) channels, in order to investigate systematically the dynamical and thermal properties of coronal mass ejections and, in general, of coronal transients. >From the analysis of line intensities and profiles, information concerning plasma temperature and velocity can be obtained. From the line widths the ``kinetic" temperature of the ions considered can be derived, while information on the plasma velocity is obtained from the Doppler dimming of the resonantly scattered line component. In turn, temperature also affects line intensity, since a change in the thermal status of the plasma produces a corresponding variation in the ionization balance.

A detailed study of the CME phenomenon requires the use of a combination of several SOHO instruments (e.g., SUMER, CDS, UVCS, LASCO, EIT) according to the Joint Observing Program designed by the SOHO SWT (coordinator R.A. Harrison).

Coronal Transients in UV Spectral Lines

(1) estimate based on CME angular span
(2) from ground based WL coronagraphs
(3) from space WL coronagraphs



next up previous contents
Next: Observations of Plasmoids Up: Examples of UVCS Previous: Stationary Coronal Streamers



Peter Smith
Fri Jan 17 12:11:15 EST 1997