Colloque Lemaire, Orsay, November 2004

1. Colloque Lemaire, Orsay, November 2004 • The « Doppler Dimming » technique • using SUMER • Alan Gabriel • IAS Orsay work in association with: Françoise Bely-Dubau Philippe Lemaire

2. The Doppler Dimming method • Uses observations beyond the solar limb of spectral lines subject to excitation in the corona, by resonant scattering of bright disk illumination • Method developed by Withbroe, Noci, Kohl, … in preparation for the use of UV coronagraph/spectrometers, Spartan and UVCS/SOHO • The resulting resonance scattering is reduced (« dimmed ») from its otherwise expected intensity, due to a Doppler shift between the illuminating radiation and the outward streaming coronal wind • Unique amongst Doppler techniques the method measures the velocity perpendicular to the line of sight, crucial for the solar wind • To « calibrate », (or determine dimming to compared to what??) it is necessary to have a reference. The usual technique is to use a multiplet of OVI, 1032/1038 A, since the two lines have different sensitivities and their ratio gives the calibrated dimming

3. Doppler DimmingLimitations • The measured mulplet ratio has first order dependence on outflow velocity and density • In order to determine outflow, it is in general necessary to have precise density information (see below, where the different density curves are separated by only a factor of 1.1 in density) • However, around 100 to 150 km/s velocity, the dependence on density vanishes. In the absence of precise density data, it is this property that has yielded the most valuable results from the UVCS observations, but over this limited velocity range. • But, now we note from the curves that a different limit can also be exploited. At low velocities (<20 km/s) the multiplet ratio depends not on velocity but only on density. • This region, not accessible from UVCS, is readily observed with SUMER and leads to a precise determination of density, low in the corona

4. Exploitation of wide height range • We have shown that SUMER observations at low altitude serve to determine the local density with precision • In many case, the physics of the situation allows a coupling of this level with the larger heights, eg if we assume conservation of mass flow and an appropriate geometry of flow; by for example assuming Munro and Jackson coronal hole structure • The figure shows the situation for 9 models of a coronal hole: three different base densities (factors of 3) with three different mass outflow rates. At low altitude, the density is determined; using this value, at greater heights the flow rate is determined • Observations, shown here by the shorter dashed curve, can be fitted unambiguously to a single density and flow rate.

5. Conclusions • We have shown how UVCS and SUMER can be used together, in combination with mass-flow conservation principles, to determine density and mass-flow rates over a wide height range. The SUMER part of this has already been used to determine plume and interplume flow velocities (Gabriel, Bely-Dubau and Lemaire, ApJ, 2003). • This method could be applied to new space instrumentation projects in order to shed important light on the global outflow structures in the solar wind. It is necessary to ensure above-limb UV spectral information on the OVI doublet, over a height range from 1.1 Ro, up to 2.0 Ro or beyond. • Spectral line-profile information is not required although, if present, this can add useful additional information on ion kinetic energies and heating. We might think of developments to the requirement specification of future projects such as Lyot,  Vol en Formation or the IFTSUV, to exploit this unique possibility. • In conclusion, we have to recognise that, in developing the SUMER concept, Philippe Lemaire has made an unplanned contribution to the exploitation, now and in the future, of the Doppler Dimming technique for measuring transverse flow velocities.