Acoustic wave propagation in chromosphere and transition region
Lead Investigator:
P. Gouttebroze (I.A.S.)
Team Members:
J.-C. Vial (I.A.S.)
K. Bocchialini (I.A.S.)
J.W. Leibacher (NOAO)
Technical Summary:
In 1963, Jensen and Orrall noticed that the central intensity in the K line
was oscillating with a period (170 s) definitely shorter than that observed
on other Fraunhofer lines (around 300 s). Many other observations followed
and showed that, when one observes with high angular resolution the temporal
variations of typically chromospheric lines, one obtains a spectrum of
oscillations or pseudo-oscillations in the [2 mn, 4mn] range. It was
long believed that these oscillations consisted in acoustic waves
trapped in the chromospheric temperature gradient (cf. Ulrich and Rhodes 1977,
Ando and Osaki 1977, Leibacher et al. 1982). However, the computation of
eigenfrequencies for this chromospheric mode with different classical
atmospheric models (Gouttebroze 1988) yields an almost unique period of
about 200 seconds, which does not agree with the rather large range
observed. Rather the p-mode ridges appear to continue uninterrupted to
higher frequency, with a shifting of power to higher frequencies for
chromospheric lines. So the question remains: is there a chromospheric
cavity, and if so, are oscillations excited within it?
Concerning this probem of driving, at least three different mechanisms
have been proposed:
- Coupling between two resonant cavities, the lower one in the convection
zone and photosphere, the upper one in the chromosphere (Leibacher et al.
1982). Oscillations are driven from the lower cavity by convective
turbulence.
- Overstability at the base of the chromosphere, due to
the dissociation and recombination of CO molecules (Hasan and Kneer 1986).
- "Shock overtaking" of short period ($< 40 s$) waves (Rammacher and
Ulmschneider 1992): short period waves travelling upwards form shocks in
the chromosphere and communicate their energy to the "3-minute"
oscillation.
To answer these questions, SUMER offers the opportunity of studying the
velocity field in the upper chromosphere and transition region, which
constitutes, according to theory, the upper reflecting layer which closes
the chromospheric cavity. The simultaneous measurement of dopplershifts
in different lines could be used to trace evanescent waves in the transition
region or low corona. The phase differences may be used to study propagation,
excitation and damping. In order to detect an eventual modal structure of
the oscillations, it will be necessary to observe the same solar area during
several hours (to have a sufficient frequency resolution). Two (at least)
different sequences, one close to the center and the other to the limb,
will be useful to study the distribution of velocities with respect to
the vertical axis. In addition, observations over active regions and
sunspots should be carried out.
In order to compare the chromospheric velocity field with the photospheric
one, some co-operation with MDI would be very useful. In this way, phenomena
of upward or downward propagation through the temperature minimum region
could be evidenced and, if so, the transformation of linear waves into
nonlinear ones could be studied.
Complete text of proposal.