We provide an updated analysis of the gamma ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma ray Burst Monitor first reported by Pu et al. (2020,
In 2015, Bowers et al. (2018,
- Award ID(s):
- 1613028
- NSF-PAR ID:
- 10457807
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 125
- Issue:
- 6
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract https://doi.org/10.1029/2020GL089427 ). A TGF produced 3 ms prior to a negative cloud‐to‐ground return stroke was close to simultaneous with an isolated low‐frequency radio pulse during the leader’s propagation, with a polarity indicating downward moving negative charge. In previous observations, this “slow” low‐frequency signal has been strongly correlated with upward‐directed (opposite polarity) TGF events (Pu et al., 2019,https://doi.org/10.1029/2019GL082743 ; Cummer et al., 2011,https://doi.org/10.1029/2011GL048099 ), leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward‐directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined that the most likely source altitude of the TGF reverse beam was 7.5 km ± 2.6 km, just below an estimated negative charge center at 8 km. At that altitude, the Monte Carlo simulations indicate a lower luminosity limit of 2 × 1018photons above 1 MeV for the main downward beam of the TGF, making the reverse beam detectable by the Fermi Gamma ray Burst Monitor. -
Abstract We report on the mountain top observation of three terrestrial gamma‐ray flashes (TGFs) that occurred during the summer storm season of 2021. To our knowledge, these are the first TGFs observed in a mountaintop environment and the first published European TGFs observed from the ground. A gamma‐ray sensitive detector was located at the base of the Säntis Tower in Switzerland and observed three unique TGF events with coincident radio sferic data characteristic of TGFs seen from space. We will show an example of a “slow pulse” radio signature (Cummer et al., 2011,
https://doi.org/10.1029/2011GL048099 ; Lu et al., 2011,https://doi.org/10.1029/2010JA016141 ; Pu et al., 2019,https://doi.org/10.1029/2019GL082743 ; Pu et al., 2020,https://doi.org/10.1029/2020GL089427 ), a −EIP (Lyu et al., 2016,https://doi.org/10.1002/2016GL070154 ; Lyu et al., 2021,https://doi.org/10.1029/2021GL093627 ; Wada et al., 2020,https://doi.org/10.1029/2019JD031730 ), and a double peak TGF associated with an extraordinarily powerful and complicated positive‐polarity sferic, where each TGF peak is possibly preceded by a short burst of stepped leader emission. -
Abstract Recent work has indicated the presence of a nitric oxide (NO) product channel in the reaction between the higher vibrational levels of the first electronically excited state of molecular nitrogen, N2(A
), and atomic oxygen. A steady‐state model for the N2(A) vibrational distribution in the terrestrial thermosphere is here described and validated by comparison with N2A‐X, Vegard‐Kaplan dayglow spectra from the Ionospheric Spectroscopy and Atmospheric Chemistry spectrograph. A computationally cheaper method is needed for implementation of the N2(A) chemistry into time‐dependent thermospheric models. It is shown that by scaling the photoelectron impact production of ionized N2by a Gaussian centered near 100 km, the level‐specific N2(A) production rates between 100 and 200 km can be reproduced to within an average of 5%. This scaling, and thus the N2electron impact ionization/excitation ratio, is nearly independent of existing uncertainties in the 2–20 nm solar soft X‐ray irradiance. To investigate this independence, the N2electron‐impact excitation cross sections in the GLOW photoelectron model are replaced with the results of Johnson et al. (2005, https://doi.org/10.1029/2005JA011295 ) and the multipart work of Malone et al. (2009https://doi.org/10.1103/PhysRevA.79.032704 ) (Malone, Johnson, Young, et al., 2009,https://doi.org/10.1088/0953-4075/42/22/225202 ; Malone, Johnson, Kanik, et al., 2009,https://doi.org/10.1103/PhysRevA.79.032705 ; Malone et al., 2009,https://doi.org/10.1103/PhysRevA.79.032704 ), together denotedJ 05M 09. Upon updating these cross sections it is found that (1) the total N2triplet excitation rate remains nearly constant; (2) the steady state N2(A) vibrational distribution is shifted to higher levels; (3) the total N2singlet excitation rate responsible for the Lyman‐Birge‐Hopfield emission is reduced by 33%. It is argued that adopting theJ 05M 09 cross sections supports (1) the larger X‐ray fluxes measured by the Student Nitric Oxide Explorer (SNOE) and (2) a temperature‐independent N2(A)+O reaction rate coefficient. -
Abstract Using Magnetospheric Multiscale (MMS) observations and combined MHD/test particle simulations, we further explore characteristic ion velocity distributions in the plasma sheet boundary layer. The observations are characterized by earthward beams, which at a slightly later time are accompanied by weaker but faster tailward beams. Two events are presented showing different histories. The first event happens at entry from the lobe into the plasma sheet. Energy‐time dispersion indicates a source region about 25
tailward of the satellite. The second event follows the passage of a dipolarization front closer to Earth. In contrast to earlier MHD simulations, but in better qualitative agreement with the first observation, reconnection in the present simulation was initiated near . Simulated distributions right at the boundary are characterized by a single crescent‐shaped earthward beam, as discussed earlier (Birn, Hesse, et al., 2015, https://doi.org/10.1002/2015JA021573 ). Farther inside, or at a later time, the distributions now also show a simple reflected beam, evolving toward a more ring‐like distribution. The simulations provide insight into the acceleration sites: The innermost edges of the direct and reflected beams consist of ions accelerated in the vicinity of the reconnection site. This supports the validity of estimating the acceleration location based on a time‐of‐flight analysis (after Onsager et al., 1990,https://doi.org/10.1029/GL017i011p01837 ). However, this assumption becomes invalid at later times when the acceleration becomes dominated by the earthward propagating dipolarization electric field, such that earthward and tailward reflected beams are no longer accelerated at the same location and the same time. -
Abstract Atomic oxygen (O) in the mesosphere and lower thermosphere (MLT) results from a balance between production via photo‐dissociation in the lower thermosphere and chemical loss by recombination in the upper mesosphere. The transport of O downward from the lower thermosphere into the mesosphere is preferentially driven by the eddy diffusion process that results from dissipating gravity waves and instabilities. The motivation here is to probe the intra‐annual variability of the eddy diffusion coefficient (k
zz ) and eddy velocity in the MLT based on the climatology of the region, initially accomplished by Garcia and Solomon (1985,https://doi.org/10.1029/JD090iD02p03850 ). In the current study, the intra‐annual cycle was divided into 26 two‐week periods for each of three zones: the northern hemisphere (NH), southern hemisphere (SH), and equatorial (EQ). Both 16 years of SABER (2002–2018) and 10 years of SCIAMACHY (2002–2012) O density measurements, along with NRLMSIS®2.0 were used for calculation of atomic oxygen eddy diffusion velocities and fluxes. Our prominent findings include a dominant annual oscillation below 87 km in the NH and SH zones, with a factor of 3–4 variation between winter and summer at 83 km, and a dominant semiannual oscillation at all altitudes in the EQ zone. The measured global average kzz at 96 km lacks the intra‐annual variability of upper atmosphere density data deduced by Qian et al. (2009,https://doi.org/10.1029/2008JA013643 ). The very large seasonal (and hemispherical) variations in kzz and O densities are important to separate and isolate in satellite analysis and to incorporate in MLT models.