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Physics of Shock Waves

 

This observing program aims to measure the effects of collisionless shocks in the solar corona. Several theoretical studies have shown that it might be possible for standing and propagating shocks to form in the inner corona, below 4 , as a result of impulses, or changes in the divergence of the flow tubes (Habbal and Rosner 1984, Leer and Holzer 1990). More recently Esser and Habbal (1990) have shown that these shocks produce measurable changes in the Ly line intensity and the polarization brightness.

It will be possible, but difficult, to measure the jump in density, velocity, proton and ion temperatures across a strong collisionless shock by measuring the Ly profile. Changes in the O VI and other ion abundances will be related to . The velocity distribution just behind the shock will be a mixture of pre-shock and post-shock velocity distributions, corresponding to neutrals which have or have not undergone charge transfer with post-shock ions. It may be possible to see the effects of the shock precursor as well. This will be useful for studies of non-thermal particle acceleration in shocks, electron-ion temperature equilibration, and the energetics of CMEs.

When studying shocks, the difficulties are their unpredictability, their short duration, and the time lag for the neutral H distribution to respond to changes in the proton distribution. A fast (1000 km/s) shock will cross the UVCS slit in about 12 seconds (for a 0.05 mm slit). The precursor is likely to be in the slit for only about 10-20 seconds as well. The charge transfer time is about seconds. During that time the neutral population at any position includes two populations; one with the pre-shock ion distribution and one with the post-shock ion velocity and temperature. If we catch an event in the quiet corona at 1.5 , the pre-shock density is around , and the charge transfer relaxation time is seconds. The electron temperature will probably increase by a modest amount at the shock, then slowly ( seconds) approach the post-shock ion temperature. The electron-ion equilibration is one of the major uncertainties in the physics of strong collisionless shocks. On the equilibration timescale, the increased electron temperature will reduce the and O VI concentrations and increase Si XII. The count rates in all lines will be complicated functions of time which reflect the shock compression, the increasing electron temperature, and the large bulk and thermal velocities of the ions which affect the Doppler dimming. The estimates in the tables pertain to a 700 km/s shock at 1.5 , but they are extremely uncertain.

Because these events are unpredictable, much of this analysis may be done on data serendipitously taken. We will want to have very wide spectral coverage to look for low ionization material driving the shock. The observing program relative to CME's might be adequate for this purpose.

For studies of standing and propagating shocks in general in the corona, observations can be made between 1.5 and 4 , in spatial steps of 0.1 . The dwell time could be 1 min below 2 , and increase to 5 min at larger distances. Such an observing sequence should be repeated for different azimuthal directions within a coronal hole, or a quiet region. Ideally it would be best to coordinate this observing plan with LASCO.

The first column refers to a 700 km/s shock at 1.5 , but the predicted intensities are extremely uncertain. For the operational mode in this case we refer to the CME's JOP. The second column is conceived to observe in general the shock phenomenon.

Physics of Shock Waves



next up previous contents
Next: Rotation of the Up: Examples of UVCS Previous: Contribution of ARs



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