Motions in solar magnetic tubes

Abstract

The line-centre magnetogram technique has been used to study velocities at spatial scales ≃5″ in several umbras without interference from light scattered from the surrounding non-magnetic photosphere. In addition, more traditional velocity observations of one sunspot were also analysed. The velocities are highly variable within any spot and from one spot to another. Rms velocities in Hα, b₁, 5233 and 5166 Å were typically ±0.6, ±0.2₀, ±0.1₀, and ±0.1₇ km s⁻¹ during present observations, but factors of 2 smaller or larger are not uncommon. In 5166 and 5233 Å these velocities are about two-thirds of those in non-magnetic photospheric regions.

In Hα there are umbral intensity variations whose maxima at Δλ = ±0.39 Å occur simultaneously with downward velocity maxima measured at the same Δλ. Intensity maxima at line centre lag by 12 s, demonstrating that there are waves propagating upwards. The corresponding height difference, 60– 70 km at the velocity of sound, requires excitation of Hα at much higher levels in the umbral atmosphere than if in LTE.

For about one-quarter of the observations it is possible to match the Hα and 5233 Å waveforms; velocities in the latter lead those in Hα by 27 s on average, also indicating outward wave propagation and, at the velocity of sound, a difference in the mean heights of formation of about 150 km. There is no significant phase difference between the 5166 Å and b₁ lines.

The velocity spectrum is variable in all lines, but in general has maxima at shorter periods in Hα (∽200 s) than at lower levels.

The estimated outward wave flux for the spatial scales studied is ≪3 × 10⁶ erg cm⁻² s⁻¹ at the 5233 Å level, much lower than previous estimates and far too low to produce any significant cooling. This conclusion is believed valid for all periods above about 7 s.