Energetics of Blue Jets/Starters

Blue jets and starters are upward propagating cones of blue emissions. At the altitudes of blue jets (below ~40 km), the N₂(1PG) upper state (N₂(B³PI_g)) is strongly quenched (e.g. the upper state is collisionally deactivated more rapidly than decay by spontaneous emission), so no red N₂(1PG) emission is observed in blue jets. The blue light is affected strongly by Rayleigh scattering at long path lengths, while the close proximity to the storm required for short path lengths (~150 km) make blue jet/starter observations a geometrical problem: the blue jets and starters occur over the active cores of storms, so large storm systems may have intervening clouds blocking viewing of blue jets/starters. For these reasons, the dearth of ground based observations of blue jets may not be truly indicative of their frequency of occurrence.

The blue emissions in jets are most likely either from N₂(2PG) or N₂⁺(1NG) emissions. Analysis of color camera observations of blue jets found emissions from the lower energy N₂(2PG) alone could not sufficiently account for the observations (Wescott et al.(1998b)), so N₂⁺(1NG) emission were suggested. No blue jets or starters were observed with the NUV/Blue spectrograph on EXL98, but several starters were clearly recorded by the 427.8 nm imager. Blue starters are similar to blue jets, propagating from cloud tops, but terminate abruptly after less than 10 km of upward travel. A blue starter observed July 17, 1998 during the EXL98 campaign is presented in Figure 5. The left panel is a view of the 340.7 nm (a neutral N₂(2PG) emission) filtered observation, the center panel is the 427.8 nm filtered image, and the right panel is the unfiltered narrow imager. Although the starter is bright in the 427.8 nm filtered camera, it extends over a much smaller region than the unfiltered or 340.7 nm images. This confirms the suggestion of ionized emissions in starters (and likely blue jets as well) although the ionization appears to be confiend to the central region. Blue jets possibly have the signature of higher total energy processes than either sprites or elves because of the ionized emissions, and blue jets last 10-100 times longer than other phenomena. (Wescott et al.(1998a)) estimate that the average blue jet or starter transfers about 10⁹J of energy to the stratosphere.

Figure 5. Observations of the July 17, 1998 starter from the EXL98 aircraft. The left image is from the 340.7 nm filtered imager. The 427.8 nm filtered image is the center image and the right image is from the unfiltered imager. The 340.7 nm and 427.8 nm images have been histogram equalized to stretch the dynamic range of the images. The 427.8 nm and unfiltered images are identical cameras with the same field of view, while the 340.7 nm image is from a camera with a different field of view. The emission at 427.8 nm from N₂⁺(1NG) indicates that electron energies of at least 18.6 eV are present in the processes causing blue starters. This emission appears to extend over a smaller region than the neutral (340.7 nm) emission.

In addition to blue emission, jets have been observed in the near-IR during EXL98. These emissions are most likely some combinations of N₂(1PG) and N₂⁺(Meinel). Considering the quenching heights in Table 1 N₂(1PG) and N₂⁺(Meinel) emission should be strongly quenched. However, energy transfer processes may enhance the population of the lower vibrational level of the N₂(1PG) and N₂⁺(Meinel) upper electronic states. Another possibility would be emission from atomic species (O and N) as is observed in lightning. The existence of atomic emissions would indicate a much more energetic process than is thought to occur in sprites and elves. This issue will best be resolved by spectroscopic observations of blue jets and starters.

Two models based on streamer-type phenomena have been proposed to explain blue jets (Sukhorukov et al.(1996),Pasko et al.(1996),Sukhorukov and Stubbe(1998),Yukhimuk et al.(1998)). Based on modeling efforts, a single jet has been postulated to cause local density perturbations of nitric oxide (10%) and ozone (0.5%) at 30 km altitude (Mishin(1997)). As mentioned above, if the processes leading to blue jets and sprites are energetic enough to create atomic species of N and O, the implications for other chemical effects is significantly increased.