Rapid Cadence EUNIS‐06 Observations of a HeiiTransient Brightening in the Quiet Sun

Abstract

We observed a transient brightening in the quiet Sun at rapid cadence (2.10 s) with the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS-06) sounding rocket instrument on 2006 April 12. The transient was visible only in He II at 303.78 A (T ≈ 5 × 104 K), and its maximum temperature T was <4 × 105 K. Taking its linear extent along the EUNIS slit to be the diameter of a circular feature, the transient's solar surface area was 7.8 × 107 km2. EUNIS observed the brightening to begin at 18:12:52 and peak at 18:13:29 UT; coordinated observations with SOHO's EIT confirm that EUNIS observed the onset of the brightening. EUNIS spectra yield maximum and average He II intensity enhancements of 2.09 and 1.46, respectively, relative to the pre-event quiet Sun. He II line profiles from EUNIS reveal that relative upflows were persistent during the transient (with a maximum speed around 20 km s−1) and that the upflow speed and intensity were positively correlated. Variations in the observed He II intensity and relative Doppler velocity were neither abrupt not impulsive, but occurred slowly compared to the EUNIS cadence. The local photospheric longitudinal magnetic field strength measured with SOHO's MDI revealed no significant variability. The transient's measured properties are consistent with its identification as a blinker or an elementary blinker, and its observed behavior suggests a formation mechanism involving gentle chromospheric evaporation.