Title: ============== Acoustic wave propagation in chromosphere and transition region Lead Investigator: ======= P. Gouttebroze (I.A.S.) E-Mail Address: ============== goutte@medoc-ias.u-psud.fr Team Members: ================== J.-C. Vial (I.A.S.) K. Bocchialini (I.A.S.) J.W. Leibacher (NOAO) Local Coordinator: ================== Ted Tarbell Lead Programmer: ===== 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: (i) 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. (ii) Overstability at the base of the chromosphere, due to the dissociation and recombination of CO molecules (Hasan and Kneer 1986). (iii) "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. Proposal Text: ======================= For SUMER, this is basically the POP 38 program (not JOP!). One uses a fixed slit position on the solar disk and the full profiles are recorded. The dwell time is relatively long (35 s) so that no compression of data is necessary in this case. Lines selected: N V : 1238.82, Si I : 1256.49, Si II : 1260.42, C I : 1266.42. For CDS, this is basically the JOP58 program. For MDI: High resolution magnetograms, Dopplergrams and heliograms. Quiet Sun joint observations have been performed between SUMER, CDS and MDI on January 10, between 00.00 and 16.00 UT. A separate analysis of SUMER data has been done. We need the MDI high/res b, V and continuum data for this time span. See Also: ========= Ando, H., Osaki, Y. 1977: PASP, 29, 221 Gouttebroze, P. 1988: in J. Christensen-Dalsgaard and S. Frandsen (eds.), Advances in Helio- and Asteroseismology}, IAU Symposium 123, p.421 Hasan, S. S., Kneer, F. 1986: A&A, 158, 288 Jensen, E., Orrall, F. Q. 1963: Ap. J. 138, 252