Velocity Fields in the Solar Atmosphere

Velocity fields in the solar atmosphere have been detected and measured by an adaptation of a technique previously used for measuring magnetic fields. Data obtained during the summers of 1960 and 1961 have been partially analyzed and yield the following principal results:

1. Large “cells” of horizontally moving material are distributed roughly uniformly over the entire solar surface. The motions within each cell suggest a (horizontal) outward flow from a source inside the cell. Typical diameters are 1.6 × 10⁴ km; spacings between centers, 3 × 10⁴ km (~5 × 10³ cells over the solar surface); r.m.s. velocities of outflow, 0.5 km sec⁻¹; lifetimes, 10⁴ sec. There is a similarity in appearance to the Ca⁺ network. The appearance and properties of these cells suggest that they are a surface manifestation of a “supergranulation” pattern of convective currents which come from relatively great depths inside the sun.

2. A distinct correlation is observed between local brightness fluctuations and vertical velocities: bright elements tend to move upward, at the levels at which the lines Fe λ 6102 and Ca λ 6103 are formed. In the line Ca λ 6103, the correlation coefficient is ~0.5. This correlation appears to reverse in sign in the height range spanned by the Doppler wings of the Na D₁ line and remains reversed at levels up to that of Ca⁺ λ 8542. At the level of Ca λ 6103, an estimate of the mechanical energy transport yields the rather large value 2 W cm⁻².

3. The characteristic “cell size” of the vertical velocities appears to increase with height from ~1700 km at the level of Fe λ 6102 to ~3500 km at that of Na λ 5896. The r.m.s. vertical velocity of ~0.4 km sec⁻¹ appears nearly constant over this height range.

4. The vertical velocities exhibit a striking repetitive time correlation, with a period T = 296 ± 3 sec. This quasi-sinusoidal motion has been followed for three full periods in the line Ca λ 6103, and is also clearly present in Fe λ 6102, Na λ 5896, and other lines. The energy contained in this oscillatory motion is about 160 J cm⁻²; the “losses” can apparently be compensated for by the energy transport (2).

5. A similar repetitive time correlation, with nearly the same period, seems to be present in the brightness fluctuations observed on ordinary spectroheliograms taken at the center of the Na D₁ line. We believe that we are observing the transformation of potential energy into wave energy through the brightness-velocity correlation in the photosphere, the upward propagation of this energy by waves of rather well-defined frequency, and its dissipation into heat in the lower chromosphere.

6. Doppler velocities have been observed at various heights in the upper chromosphere by means of the Ha line. At great heights one finds a granular structure with a mean size of about 3600 km, but at lower levels one finds predominantly downward motions, which are concentrated in “tunnels” which presumably follow magnetic lines of force and are geometrically related to the Ca⁺ network. The Doppler field changes its appearance very rapidly at higher levels, typical lifetimes being about 30 seconds.