A review of space helioseismology

Abstract

The helioseismology started in 1960 with the discovery of oscillations of the velocity field on the solar surface by Leighton, 1960. The mechanism producing these oscillations remained unexplained for the next ten years until Ulrich, 1970, and independently Leibacher and Stein, 1971, demonstrated that acoustic waves trapped beneath the solar surface could generate such oscillations. According to that theory the Sun should oscillate globally and not only locally as seen in the pionnering observations. First Deubner, 1975, and then Claverie et al., 1979, and Grec et al., 1980, demonstrated the existence of global oscillations of intermediate and low degrees. These observations analysed in terms of k-ω diagram and power spectrum led to the famous p-modes ridges and spectrum lines which are at the base of any inference of the solar structure. All these observations where carried out from the ground. Nevertheless ground-based observations at that time (early eighties) were suffering some limitations which could be over taken from space. This is the reason why the eighties saw the intensive development both of ground-based networks and space missions for helioseismology resulting in 1995 in the existence of four networks: BISON, IRIS, GONG and TON (see Pallé, 1996) and the launch of the SOHO satellite (Domingo et al., 1995). In the next section I review the limitations of ground-based observations and those of space missions. In section 3, I describe in a chronological order the past space helioseismology missions (from 1980 to 1994). Furthermore I discuss, in section 4, the present status of space helioseismology with SOHO and finally in section 5 the co-ordination of space missions and ground-based solar observations. In the last section the future and the prospects of space helioseismology are reviewed.