  Recent irradiance measurements from numerous heliophysics and astrophysics
missions including SDO, GOES, Kepler, TESS, Chandra, XMM-Newton, and NICER have
provided critical input in understanding the physics of the most powerful
transient events on the Sun and magnetically active stars, solar and stellar
flares. The light curves of flare events from the Sun and stars show remarkably
similar shapes, typically with a sharp rise and protracted decay phase. The
duration of solar and stellar flares has been found to be correlated with the
intensity of the event in some wavelengths, such as white light, but not in
other wavelengths, such as soft X-rays, but it is not evident why this is the
case. In this study, we use a radiative hydrodynamics code to examine factors
affecting the duration of flare emission at various wavelengths. The duration
of a light curve depends on the temperature of the plasma, the height in the
atmosphere at which the emission forms, and the relative importance of cooling
due to radiation, thermal conduction, and enthalpy flux. We find that there is
a clear distinction between emission that forms low in the atmosphere and
responds directly to heating, and emission that forms in the corona, indirectly
responding to heating-induced chromospheric evaporation, a facet of the Neupert
effect. We discuss the implications of our results to a wide range of flare
energies.
