We observed AB Doradus , a young and active late type star ( K0 - K2 IV-V , P = 0.514 d ) with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with the time and spectral resolutions of 27 s and 15 km s ^ { -1 } , respectively ( November 14.08 - 14.30 , 1994 ( UT ) ) .
The wavelength band ( 1531 - 1565 Å ) included the strong CIV doublet ( 1548.202 and 1550.774 Å , formed in the transition region at 10 ^ { 5 } K ) , the chromospheric SiII 1533.432 Å line and the blend of SiI , CI and FeII lines at 1561 Å .
The mean quiescent CIV flux state was characterized by F _ { CIV } = ( 7.80 \pm { 0.34 } ) \times 10 ^ { 5 } ergs cm ^ { -2 } s ^ { -1 } , close to the saturated value and 100 times the solar one .
The line profile ( after removing the rotational and instrumental profiles ) is bimodal consisting of two Gaussians , narrow ( FWHM = 70 km s ^ { -1 } ) and broad ( FWHM = 330 km s ^ { -1 } ) .
This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base .
It is possible that TR transient events ( random multiple velocities ) , with a large surface coverage , give rise to the broadening of the narrow component , while true microflaring is responsible for the broad one as suggested by Wood , Linsky and Ayres ( 1997 ) .

The transition region was observed to flare frequently on different time scales and magnitudes .
The largest impulsive flare seen in the CIV 1549 Å emission at day 14.22 reached in less than one minute the peak differential emission measure N _ { e } ^ { 2 } V ( 10 ^ { 4.85 } K – 10 ^ { 5.15 } K ) = 10 ^ { 51.2 } cm ^ { -3 } and returned exponentially in 5 minutes to the 7 times lower quiescent level .
The 3 min average line profile of the flare was blue-shifted ( –190 km s ^ { -1 } ) and broadened ( FWHM = 800 km s ^ { -1 } ) .
This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam , accelerated ( by reconnection ) at the apex of a coronal loop .