EUV and radio observations of an equatorial coronal hole

Abstract

EUV and radio data of an equatorial coronal hole, observed in October 1996 at its central meridian transit, are compared. EUV lines were observed by the CDS instrument onboard SOHO and the radio emission by the Nancay Radioheliograph (France) in the frequency range 164-410 MHz. Using the Differential Emission Measure (DEM), derived from EUV line intensities, we have computed the radio brightness temperature T_b, leaving the coronal temperature (upper limit of the DEM definition) and pressure as free parameters, to be determined from the comparison with the observations. This analysis has shown that radio data, contrarily to EUV line intensities, cannot be fitted without the presence of isothermal plasma above the region where the DEM is defined, independently of the choice of parameters. A model, nicely fitting both sets of data, is derived, in which the coronal temperature and density are T_c <= 9 x 10(5) K and N_e(0) =~ 3x 10(8 cm(-3)) respectively. The assumption of hydrostatic equilibrium with a scale height derived from the electron temperature is discussed.