  The hydrodynamic escape driven by external or internal energy sources sculpts
the population of low mass close-in planets. However, distinguishing between
the driving mechanisms responsible for the hydrodynamic escape of hydrogen-rich
atmospheres is a complex task due to the involvement of many physical factors.
My simulations show that the hydrodynamic escape can be driven solely by
thermal energy deposited in the lower layers of the atmosphere due to the heat
flux originating from the planetary core or bolometric heating from the star
even in the absence of other energy sources, as long as the planet's Jeans
parameter is below 3. Otherwise, stellar extreme ultraviolet irradiation or
tidal forces are necessary in driving the escape, which means that the Jeans
parameter is incapable of distinguishing the driving mechanisms, as it is only
related to the properties of planet. Here, an upgraded Jeans parameter that
takes into account tidal forces is introduced, which allows us to accurately
categorize the driving mechanisms. The results show that when the upgraded
Jeans parameter falls below 3 or exceeds 6, the atmospheric escape is primarily
driven by tidal forces or extreme ultraviolet radiation from the host star,
respectively. In the range of 3 to 6, both factors can trigger the escape of
the atmosphere. I find that planets with high gravitational potential and low
stellar irradiation are more likely to undergo subsonic escape, although
transonic escape is prevalent among most planets. Moreover, the ionization
status is significantly dependent on the gravitational potential. The upgraded
Jeans parameter, which is closely related to the underlying physics, provides a
concise method to categorize the driving mechanisms of hydrodynamic escape. The
results can be applied to planetary evolution calculations.
