EXTREME-ULTRAVIOLET EXPLORER SPECTRA OF THE 1993 MARCH FLARES ON AD LEONIS - THE DIFFERENTIAL EMISSION MEASURE AND IMPLICATIONS FOR CORONALSTRUCTURE

The flare star AD Leonis was observed by the Extreme Ultraviolet Explo rer (EUVE) from 1993 March 1 to 3 UT. Two flares were detected by the EUVE Deep Survey detector and spectrometer and also seen in optical ph otometry on 1993 March 2 UT. The DS Lexan/boron-band and optical resul ts have been discussed in the previous paper by Hawley et al. In this paper, we describe the spectra observed by EUVE during quiescence, the peaks of the flares, and the decay phase following the first flare an d analyze the spectra to investigate the stellar atmospheric structure during these time periods. The spectra show that the observed EUV emi ssion from AD Leo is dominated by iron lines from a hot coronal plasma . Two methods were used to estimate the differential emission measure distribution (DEM) of the stellar corona. In the ''Pottasch'' method, we fitted Gaussian line profiles to the strongest lines in the spectra and estimated the DEM at the formation temperature of those lines. Up per limits to the DEM were obtained in the case of no detection. We al so used a regularized inversion technique, together with a weighting s cheme based on information contained in the plasma-emission model and on the signal-to-noise ratio of the data, to find the DEM. The weighti ng was designed to prevent the noisy pixels in our low-signal-to-noise ratio data from dominating the solution. The results produced by the two methods are consistent in the temperature regimes where strong lin es are present. The inversion method provides additional information w here no strong single lines dominate the spectra. The ability to use l ines from the entire wavelength region covered by the spectra allowed us to investigate the hydrogen column N-H and iron abundance [Fe/H]. W e found that [Fe/H] in the corona of AD Leo was essentially unconstrai ned by our data, but N, was well determined, yielding N-H similar to ( 3 +/- 1) x 10(18) cm(-2). We assumed both a solar-coronal value of [Fe /H] and a value one tenth of this and computed the DEM distribution of the stellar corona for both cases. The DEM of the quiescent corona is dominated by a broad plateau of emission ranging from 10(6.8) to 10(7 .2) K, With the DEM of plasma near 10(6.2) K about an order of magnitu de less. We interpret the plateau of the DEM in terms of a broad distr ibution of loops with differing peak temperatures. We discuss and comp are these results with those of Giampapa et al., who analyzed ROSAT so ft X-ray data from AD Leo taken during a different time period. The DE M of the flare plasma is strongly peaked at temperatures greater than 10(7) K, indicative of hot flare loops, while that of the decay phase consists of a smaller peak at temperatures less than 10(7) K, as might be expected from the cooling and condensation of previously heated fl are loops. These results are consistent with a flare model that includ es strong evaporation and condensation as in our previous paper. The E UVE spectral analysis leads to lower peak flare temperatures than thos e used in our previous paper, but the basic conclusion reached-that th e dominant flaring emission originates from long loops with L similar to R and with peak flare densities ranging from 10(9) to 10(11) cm(-3 )-remains unchanged. This conclusion is not qualitatively affected by the value of [Fe/H] used in our DEM analysis.