Comparison of Transient Network Brightenings and Explosive Events in the Solar Transition Region.

The relation between transient network brightenings, known as blinkers, and explosive events is examined based on coordinated quiet Sun observations in the transition region line O v lambda630 recorded by the Coronal Diagnostic Spectrometer (CDS), in the transition region line Si iv lambda1402 recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument, and in photospheric magnetograms taken by the Big Bear Solar Observatory videomagnetograph. From these observations, we find that (1) explosive events, which are traditionally defined as features with very broad UV line profiles, tend to keep away from the centers of network brightenings and are mostly located at the edges of such brightenings, (2) CDS blinkers consist of many small-scale, short-lived SUMER "unit brightening events" with a size of a few arcseconds and a lifetime of a few minutes, and most importantly (3) each SUMER unit brightening event is characterized by a UV line profile that is not as broad as those of explosive events, but still has significantly enhanced wings. Our results imply that, like explosive events, individual unit brightening events involve high velocities, and, hence, blinkers may have the same physical origin as explosive events. It is likely that transient network brightenings and explosive events are both due to magnetic reconnection-but with different magnetic geometries.

The Seismic Structure of the Sun

Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the sun's structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.