  SDO/EVE provides rich information of the thermodynamic processes of solar
activities, particularly of solar flares. Here, we develop a method to
construct thermodynamic spectrum (TDS) charts based on the EVE spectral lines.
This tool could be potentially useful to the EUV astronomy to learn the
eruptive activities on the distant astronomical objects. Through several cases,
we illustrate what we can learn from the TDS charts. Furthermore, we apply the
TDS method to 74 flares equal to or greater than M5.0-class, and reach the
following statistical results. First, EUV peaks are always behind the soft
X-ray (SXR) peaks and stronger flares tend to have a faster cooling rate. There
is a power law correlation between the peak delay times and the cooling rates,
suggesting a coherent cooling process of flares from SXR to EUV emissions.
Second, there are two distinct temperature drift patterns, called Type I and
Type II. For Type I flares, the enhanced emission drifts from high to low
temperature like a quadrilateral, whereas for Type II flares, the drift pattern
looks like a triangle. Statistical analysis suggests that Type II flares are
more impulsive than Type I flares. Third, for late-phase flares, the peak
intensity ratio of the late phase to the main phase is roughly correlated with
the flare class, and the flares with a strong late phase are all confined. We
believe that the re-deposition of the energy carried by a flux rope, that
unsuccessfully erupts out, into thermal emissions is responsible for the strong
late phase found in a confined flare. Besides, with some cases we illustrate
the signatures of the flare thermodynamic process in the chromosphere and
transition region in TDS charts. These results provide new clues to advance our
understanding of the thermodynamic processes of solar flares and associated
solar eruptions, e.g., coronal mass ejections.
