Search

The AGILE Satellite [8] [9], launched the 23rd April 2007, is an italian mission devoted to high energy gamma‐ray astrophysics in the 30 MeV–50 GeV range, with a window in the hard‐X domain 18–60 keV. One of the on‐board detectors, the Mini‐Calorimeter (MCAL), was also designed to work as all sky transient monitor in the energy range 0.33–100 MeV. Though the main purpose of MCAL is the detection of cosmological Gamma‐Ray‐Bursts (GRB), MCAL detected transient events with a very brief duration, few msec, that does not match timing and spectral features of known cosmic GRB. These events more likely agree with the features of typical Terrestrial Gamma‐Ray Flashes as described in the literature and previously detected by the BATSE instrument onboard the Compton Gamma Ray Observatory and the RHESSI satellite. The characteristics of the detected TGF candidate events, as well as their geographical distribution will be presented and discussed.

This paper deals with the rare high intensity electromagnetic pulses associated with earthquakes, whose spectrum signature differs from that of atmospherics produced by lightning discharges. On the basis of actual data records, cases of the generation of anomalous seismo‐electromagnetic emissions are described. These natural sub‐millisecond electromagnetic pulses were associated with the passage of seismic waves from earthquakes to Moscow, the place where the electromagnetic field observations were made. Space‐time coupling has been revealed between exact seismic waves from the earthquakes, lightning triggering and Transient Luminous Events triggering.

More than one hundred different transient luminous events (TLEs) i.e. sprites, sprite halos and one blue jet‐like emission have been observed in Central Europe at a new optical observation site in Sopron, Hungary, during the summer of 2007. The recently developed LINET lightning location network supplied information about the lightning activity in the region. This was the first time that data from this network was used to support TLE observations and to help interpret the results. TLE‐triggering lightning flashes were identified by their radiation in the ELF (Extremely Low Frequency) band, appearing as coherent transient signals with enhanced amplitudes in the monitored field components of the atmospheric electromagnetic field at NCK station in Hungary. Individual lightning flashes could be associated with 92% of the optically observed events. No ELF transient could be associated with the blue jet‐like emission, and for one sprite neither a corresponding transient nor any causative lightning flash could be found. Positive polarity lightning flashes were associated with almost all TLEs. In one exceptional case, a negative polarity parent lightning flash was associated with an ELF transient indicating a positive polarity source. In the present paper, the new optical observation site is introduced, the method of observation is described, and the potential role of LINET is discussed in connection with TLE research.

Fingers from ionization fronts for a hydrodynamic plasma model result from a balance between impact ionization and electron diffusion in a non‐attaching gas. An analytical estimation of the size of the fingers and its dependence on both the electric field and electron diffusion coefficient can be done when the diffusion is low and the electric field is strong.

We give a brief overview of our present understanding of the global circuit formed by the atmosphere, the ionosphere and the solid and ocean surface of the Earth. The circuit is driven by currents (∼1 kA) above both thunderstorms and electrified shower clouds up to the ionosphere, which is presumed to be an equipotential surface (∼250 kV with respect to the Earth). This large scale circuit is completed by currents (∼2 pA/m2) flowing down through the fair weather atmosphere to the Earth’s surface and up into the atmosphere below the electrically active clouds. Using a realistic profile of the atmospheric electric conductivity, we have generated a model of this equivalent circuit using the PSpice electrical engineering software package. The effects of model lightning discharges on the circuit have been estimated numerically. Model sprites exhibiting mesospheric discharge processes have been constructed, first for negative cloud‐to‐ground discharges (−CGs), then for discharges transferring positive charge to...

We report observations of lightning‐induced transient emissions (LITEs) in or below the OH airglow layer observed from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT‐II satellite. The ISUAL team conducted a nine‐day campaign in January 2007 using both the broadband and narrowband filters on the CCD imager. It is found that LITEs occur more frequently in the OH airglow layer than below. It is also found that the observations of LITEs below the OH airglow layer by the narrow‐band filter are rare. The observations of LITEs by the narrowband filter are the first of its kind in existence. Our data analysis shows that there is a significant intensity increase in the OH airglow layer when there is lightning and a residual intensity increase post the lightning activity. We propose that LITEs in the OH airglow layer could be due to OH species.

Calibrated measurements of TLE radiances are required for understanding the processes occurring at various altitudes and in order to map the spatial and temporal evolution of the optical emissions. We present a procedure for calculating the radiance of TLEs, based on observations with a calibrated Watec 902H2 Ultimate CCD camera. During the ILAN winter sprite campaign we measured the brightness of TLEs in the vicinity of Israel and the eastern Mediterranean. Calibration setup, image analysis and preliminary results are discussed.

In Russia several space missions are now planned to study transient luminous events in the atmosphere and high energy charged particles at satellite altitudes. The experimental goal is to investigate the origin of the high energy electrons and gamma‐ray quanta for specific transient luminous events (TLEs) and their role in the ionosphere‐magnetosphere system. Simultaneous measurements of electrons at the orbit of the satellite and TLE atmospheric radiation in many wavelength bands will be performed in two missions, Tatiana‐2 and RELEC. In the TUS mission UV transient event detection will be accompanied by measurements of the weak UV emission from the “seed” electrons of extensive air showers of extremely high primary energies.

Detectors on the “Universitetsky‐Tatiana” satellite measured a smoothly varying intensity of UV radiation from the night‐time atmosphere in the nadir direction and the intensity of the energetic electron flux at the orbit. At high latitudes the UV intensity in the auroral oval is interpreted as being due to electrons penetrating into the atmosphere. At middle latitudes the UV intensity is an order of magnitude less and more data are needed to reveal the origin of this radiation. Millisecond flashes of UV radiation were observed. The flashes’ energy, temporal profile and global distribution are similar to these parameters for Transient Luminous Events (TLEs). These studies will be continued aboard the next satellite “Tatiana‐2”.

High‐altitude discharges driven by runaway electron avalanches were simulated for moderate or even small thundercloud dipole moments, and the characteristics of the terrestrial γ‐ray and photo‐neutron flashes were calculated. The γ‐ray numbers and spectra fit observational data obtained from satellites. The calculated neutron numbers are large enough for detection by satellites in low Earth orbit.

Terrestrial gamma‐ray flash (TGF) emissions are predicted to be accompanied by energetic electron emissions. These electrons have been suggested to explain several anomalous TGF observations by the RHESSI and BATSE spacecraft. These electrons additionally provide a new window for study of TGFs as they carry additional information about the source through their pitch angle / arrival time distributions and confinement by the geomagnetic field. This information could prove very useful for analysis of TGF source mechanisms.We present results of simulations of electron and photon emissions and describe the implications of these results for inferences about the TGF source. We also present the results of a search of data from the SAMPEX satellite for these electrons and discuss in particular two events with multiple strong pulses of electrons.

Based on the first four years of the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) experiment onboard the FORMOSAT‐2 satellite, ELVEs are found to be the most abundant type (∼80%) of TLEs (transient luminous events), whereas sprites and halos only combine to account for ∼20%. The distribution of sprites and halos closely resemble that of the cloud‐to‐ground lightning. However, nearly 60% of ELVEs occurred over the ocean, a feature indicating that the high peak current lightning is more abundant over the ocean. The global TLE occurrence rates are inferred to be 72, 3.7, and ∼1 events/minute, respectively, for ELVEs, halos, and sprites. Comparing with the results from the first three years of the ISUAL experiment reported by Chen et al. [1], the global occurrence rates for ELVEs and halos are higher due to the adoption of different correction factors. Using these updated TLE rates, the free electron content over an ELVE hot zone is estimated to be elevated by more than 10%. From analyzing ISUAL ...

High energy radiation from tens of keV to tens of MeV in energy, the transition region from x‐rays to gamma rays, has been observed in association with high electric fields in air in many contexts: from the ground, associated with thunderstorms and with individual flashes of natural and rocket‐triggered lightning [1, 2, 3, 4, 5, 6, 7, 8] from sparks in the laboratory [9, 10, 11, 12], from Earth orbit in the form of millisecond Terrestrial Gamma‐ray Flashes (TGFs) [13, 14], and at intermediate altitudes from aircraft and balloons [15, 16, 17, 18, 19]. The gamma radiation in all cases is almost certainly bremsstrahlung generated by the collision of accelerated electrons with nuclei in the atmosphere. The penetrating gamma‐rays can provide a mechanism for coupling between the troposphere, mesosphere, ionosphere and magnetosphere in the form of energetic particles and ionization.

An experimental Very Low Frequency (VLF) World‐Wide Lightning Location Network (WWLLN) is being developed through collaborations with research institutions across the globe. In this paper we report on the steady improvement in the Detection Efficiency (DE) of the WWLLN due to increasing station number, which led to a doubling in locations provided from 2003–2007. In addition, a new algorithm has recently been implemented which leads to DE improvements of 63%.

Sprites, jets and elves, also referred to as “Transient Luminous Events (TLEs)”, are phenomena well‐suited for educational purposes as well as for public outreach activities. This paper gives an introduction to what is meant by “Education and Public Outreach (EPO)”, and emphasizes the usefulness and effectiveness of such activities in scientific projects. In 2002 the “Coupling of Atmospheric Layers (CAL)” research training project was funded by the European Commission to study TLEs. CAL consisted of ten scientific work‐packages and two work‐packages specifically dedicated to the training and outreach program of the project. CAL is an example and inspiration of how a scientific research project can be a unique opportunity to provide EPO learning activities to both young and senior scientists.