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We study the H$\alpha$ and Ca II 8542 \r{A} line spectra of four typicalEllerman bombs (EBs) in active region NOAA 11765 on 2013 June 6, observed withthe Fast Imaging Solar Spectrograph installed at the 1.6 meter New SolarTelescope at Big Bear Solar Observatory. Considering that EBs may occur in arestricted region in the lower atmosphere, and that their spectral lines showparticular features, we propose a two-cloud model to fit the observed lineprofiles. The lower cloud can account for the wing emission, and the uppercloud is mainly responsible for the absorption at line center. After choosingcarefully the free parameters, we get satisfactory fitting results. Asexpected, the lower cloud shows an increase of the source function,corresponding to a temperature increase of 400--1000 K in EBs relative to thequiet Sun. This is consistent with previous results deduced from semi-empiricalmodels and confirms that a local heating occurs in the lower atmosphere duringthe appearance of EBs. We also find that the optical depths can increase tosome extent in both the lower and upper clouds, which may result from either adirect heating in the lower cloud, or illumination by an enhanced radiation onthe upper cloud. The velocities derived from this method, however, aredifferent from those obtained using the traditional bisector method, implyingthat one should be cautious when interpreting this parameter. The two-cloudmodel can thus be used as an efficient method to deduce the basic physicalparameters of EBs.

Spectral observations of Ellerman bombs and fitting with a two-cloud model