[1] Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from X GSM = À30 R E to X GSM = À7 R E . How these DFs are formed is still poorly understood. Three possible mechanisms have been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near-Earth region (X GSM > À10 R E ) are primarily attributed to jet braking, while the DFs detected in the mid-or far-tail region (X GSM < À15 R E ) are primarily attributed to transient reconnection or spontaneous formation. In the jetbraking mechanism, the high-speed flow "pushes" the preexisting plasmas to produce the DF so that there is causality between high-speed flow and DF. In the transientreconnection mechanism, there is no causality between highspeed flow and DF, because the frozen-in condition is violated. Citation: Fu, H. S., et al. (2013), Dipolarization fronts as a consequence of transient reconnection: In situ evidence, Geophys. Res. Lett., 40,[6023][6024][6025][6026][6027]

[1] Terrestrial gamma-ray flashes (TGFs) are energetic photon bursts observed from satellites and associated with lightning activity. Comparison between calculations based on the model of relativistic runaway electron avalanches (RREA) in large-scale weak electric field in thunderstorms and satellite measurements usually shows that the photon spectrum is consistent with source altitudes around 15 km. However, recent observations have located intra-cloud lightning (IC) discharges responsible for TGFs much deeper in the atmosphere (at altitudes $10 km). In the present work, we show that the TGF spectrum as produced by acceleration of electrons in the strong electric field of stepping IC leaders is consistent with the lower altitudes recently discovered. This study reconciles observations and measurements by setting new altitudes for the TGF sources based on mechanism of direct acceleration of electrons in the lightning leader field. Moreover, the photon source beaming geometry is consistently determined from the geometry of electric field lines produced by the lightning leader. Citation: Xu, W., S. Celestin, and V. P. Pasko (2012), Source altitudes of terrestrial gamma-ray flashes produced by lightning leaders, Geophys.

[1] Using Monte Carlo models simulating energetic electrons in inhomogeneous electric field and the transport of energetic photons in the Earth's atmosphere, we show that the spectrum of bremsstrahlung photons generated by nonequilibrium energetic electrons produced during stepping of lightning leaders can deviate from the typical spectrum of relativistic runaway electron avalanches (RREA) developing in weak homogeneous electric fields. This deviation is especially pronounced in the high-energy tail of the electron energy distribution for lightning leaders possessing high voltages (several hundreds of megavolts) and extremely high fields around their tips. The photon spectrum obtained accurately reproduces the recently discovered high-energy tail (up to 100 MeV) of terrestrial gamma-ray flashes (TGFs). This analysis provides the first direct evidence that TGFs are produced by lightning and not over large distances in weak thunderstorm electric fields.

In this paper, we model the production and acceleration of thermal runaway electrons during negative corona flash stages of stepping lightning leaders and the corresponding terrestrial gamma ray flashes (TGFs) or negative cloud‐to‐ground (−CG) lightning‐produced X‐ray bursts in a unified fashion. We show how the source photon spectrum and fluence depend on the potential drop formed in the lightning leader tip region during corona flash and how the X‐ray burst spectrum progressively converges toward typical TGF spectrum as the potential drop increases. Additionally, we show that the number of streamers produced in a negative corona flash, the source electron energy distribution function, the corresponding number of photons, and the photon energy distribution and transport through the atmosphere up to low‐orbit satellite altitudes exhibit a very strong dependence on this potential drop. This leads to a threshold effect causing X‐rays produced by leaders with potentials lower than those producing typical TGFs extremely unlikely to be detected by low‐orbit satellites. Moreover, from the number of photons in X‐ray bursts produced by −CGs estimated from ground observations, we show that the proportionality between the number of thermal runaway electrons and the square of the potential drop in the leader tip region during negative corona flash proposed earlier leads to typical photon fluences on the order of 1 ph/cm2 at an altitude of 500 km and a radial distance of 200 km for intracloud lightning discharges producing 300 MV potential drops, which is consistent with observations of TGF fluences and spectra from satellites.

Accurate specification of ionization production by energetic electron precipitation is critical for atmospheric chemistry models to assess the resultant atmospheric effects. Recent model-observation comparison studies have increasingly highlighted the importance of considering precipitation fluxes in the full range of electron energy and pitch angle. However, previous parameterization methods were mostly proposed for isotropically precipitation electrons with energies up to 1 MeV, and the pitch angle dependence has not yet been parameterized. In this paper, we first characterize and tabulate the atmospheric ionization response to monoenergetic electrons with different pitch angles and energies between ∼3 keV and ∼33 MeV. A generalized method that fully accounts for the dependence of ionization production on background atmospheric conditions, electron energy, and pitch angle has been developed based on the parameterization method of Fang et al. (2010, https://doi.org/10.1029/2010GL045406). Moreover, we validate this method using 100 random atmospheric profiles and precipitation fluxes with monoenergetic and exponential energy distributions, and isotropic and sine pitch angle distributions. In a suite of 6,100 validation tests, the error in peak ionization altitude is found to be within 1 km in 91% of all the tests with a mean error of 2.7% in peak ionization rate and 1.9% in total ionization. This method therefore provides a reliable means to convert space-measured precipitation energy and pitch angle distributions into ionization inputs for atmospheric chemistry models.