CO 2 Thermal Infrared Signature Following a Sprite Event in the Mesosphere

Sprites are a potential thermal infrared radiation source in the stratosphere and mesosphere through molecular vibrational excitation. We developed a plasma‐chemical model to compute the vibrational kinetics induced by a sprite streamer in the 40‐ to 70‐km altitude range until several tens of seconds after the visible flash is over. Then, we computed the consecutive time‐dependent thermal infrared spectra that could be observed from the stratosphere (from a balloon platform), high troposphere (from an aircraft), and low troposphere (aircraft or altitude observatory) using a nonlocal thermodynamic equilibrium radiative transfer model. Our simulations predict a strong production of CO 2 in the (001) vibrational level which lasts at least 40 s before falling to background concentrations. This leads to enhanced emissions in the long‐wavelength infrared, around 1,000 cm −1 , and midwavelength infrared, around 2,300 cm −1 . The maximum sprite infrared signatures (sprite spectra minus background spectra) reach several 10 −7 W/sr/cm 2 /cm −1 after propagation through the mesosphere and stratosphere, to an observer located at 20–40 km of altitude. This maximum signal is about 1 order of magnitude lower if propagated until the troposphere. From the two spectral bands, the 1,000‐cm −1 one could be detected more easily than the 2,300‐cm −1 one, which is more affected by atmospheric absorption (CO 2 self‐trapping at all altitudes and H 2 O mostly in the troposphere). With a sufficiently sensitive instrumentation, mounted in an open stratospheric balloon platform for example, the 1,000‐cm −1 band could be detected from 20–40 km of altitude. Sprite thermal infrared signature could reach up to 10 −7 W/sr/cm 2 /cm −1 for an observer located in the stratosphere The maximum signal is in the long‐wavelength infrared, around 1,000 cm −1 This signature should be detectable from stratospheric balloons