TEPA 2013 workshop was devoted to high-energy phenomena
in the earth’s atmosphere
The discussions on the most intriguing problems of the newly emerging topic of highenergy physics in the atmosphere cover the following subjects:
TGEs and TGFs - what we can learn by comparisons of both?
Thunderstorm Ground Enhancements (TGEs, [Chilingarian et al., 2010] originating from
the lower dipole between the main negatively charged layer in the middle of the
thundercloud and the transient Lower positive charge region (LPCR, [Chilingarian and
Mkrtchyan, 2012] ) in the lower portion of the thundercloud. The lower dipole is the
expected acceleration region for electrons from the ambient population of secondary
cosmic rays (CR) downward. The electric field effectively transfers energy to the
electrons MOdifying their energy Spectra (MOS process, [Chilingarian, Mailyan and
Vanyan, 2012]), extending their lifetime and consequently their probability to radiate
gamma rays by bremsstrahlung. As thunderclouds at mountain altitudes usually are very
close to the Earth’s surface, the electron and gamma ray fluxes escaping from
thunderclouds do not attenuate in the atmosphere and reach earth’s surface enhancing
rather stable CR background by several percent in the energy range up to 100 MeV.
If the electric field strength exceeds the critical value, the Relativistic Runaway
Avalanches (RREA, [Wilson, 1925, Gurevich et al., 1992, Babich et al., 1998]) may be
unleashed, enlarging the electron and gamma ray fluxes several times. RREA avalanches,
called Extensive Cloud Showers (ECS, [Chilingarian et al., 2011]), are systematically
different from the Extensive Air Showers (EASs) originating from the galaxy or from
high-energy solar cosmic rays incident on the Earth’s atmosphere.The near-surface
location of thunderclouds at Aragats allows the direct observation of the RREA
avalanches and the “switching on” of the rather rare RREA mechanism. The simulation
of the TGEs with the GEANT4 code with the application of a plausible electric field and
thundercloud parameters (strength ~ 1.8 kV/m at 5000 m, elongation ~ 1 km, height
above earth’s surface 50-200 m), and making use of seed electrons from the ambient
population of CRs, this model successfully reproduces the observed Aragats TGE events,
including electron, gamma ray and neutron fluxes.
Terrestrial Gamma flashes (TGFs,
Fishman et al., 1994, Briggs et al., 2010]]) are believed to originate from electrons
accelerated in the upper dipole between the main negative and main positive layer in the
upper part of the thundercloud. Gamma rays emitted by accelerated upward electrons
propagate in space (possibly generating electron-positron pairs) and reach gamma ray
spectrometers in orbit several hundred kilometers above the Earth’s surface. The spacebased gamma ray observatories are intended primarily to detect gamma bursts and other
energetic processes in the Cosmos. Modified triggers of gamma ray events allow copious
detection of the TGFs mostly from equatorial thunderstorms. However, the distant
locations for the fast moving particle detectors lead to several difficulties in the
development of the TGF model:
• The required number of seed electrons greatly exceeds the available electrons in
secondary CRs; the proposed mechanism of “cold runaway” – acceleration of
electrons by the strong electric fields in front of lightning leaders—is still not
observed;
• Due to scarcity of detected particles, only cumulative energy spectra from all detected
events are available for analysis and comparison with simulations; too few
detected photons from each event are not enough for energy spectra recovering;
millions of gamma ray photons detected from several TGEs allows detailed
analysis of energy spectra evolution with time for individual events.
Direct measurements of the intense particle fluxes at the Earth’s surface may be used for
tuning the parameters of TGF models. The spatial and energetic characteristic of the
ECSs, and measured energy spectra of the TGE gamma rays and electrons, may be used
for checking characteristics of the particle fluxes obtained in the TGF simulations.
Particle fluxes and atmospheric discharges - any causal relation?
The observed relation of lightning occurrences detected during TGFs and TGEs do not
support claims on their causal relations. Most likely TGFs and TGEs are linked with two
types of the intracloud discharges: IC+ - corresponds to TGF and IC- - to TGEs. During
TGEs the cloud-to-ground (CG-) lightning flashes are highly suppressed [Chilingarian
and Mkrtchyan, 2012]. It is interesting to investigate short bipolar pulses (pulse trains)
during TGEs before CG lightning flashes. Possibly short bipolar pulses detected during
thunderstorms (see [Gurevich and Karashtin, 2013] manifested discharges of free
electrons of CR on polarized and stretched hydrometeors. If such a pulses precedes TGE
initiation it will be prove that LPCR is sited on the polarized droplets in the bottom of the
thundercloud.
Data bases of TGEs and TGFs: Are they available for community? Presentation how to
access the data bases of Cosmic ray division.
NASA maintains TGF data correspondent to different hierarchy of triggers. GBM TGF
web pages, including a triggered TGF table and individual light curves are available from
link: http://gammaray.nsstc.nasa.gov/gbm/science/tgf/. On-line data on particle fluxes,
electrical and geomagnetic fields, lightning occurrences and meteorological information
is included in the MSQL data bases of the Aragats Space Environmental Center (ASEC).
These multiple one-minute and one-second time series contain information on hundreds
of TGEs. On-line data and the 10 year data archive are freely accessible with a possibility
of multivariate visualization and statistical analysis. However, additional instruction
should be prepared and user-friendly TGE selection procedures should be added to make
the data available to the research community.
Transient energetic events in in the Earth' s atmosphere ( TGF, TGE, TLE, particle
precipitation... ) can they all be explained in one theoretical framework?
While the Earth's magnetic field is very good at trapping charged particles, it is not
perfect. There are several ways in which radiation belt particles can escape. Radiation
belt electrons can penetrate to an altitude of about 45 km [e.g. Solomon et al, 2012]
ionizing upper atmosphere and increasing its electrical conductivity. Magnetospherically
reflected whistlers originating in lightning can resonantly interact with radiation belt
electrons (in the energy-range of 1 to 2.6 MeV) and force their precipitation ]RistićDjurović, Bell, and Inan, 1998]. Thus, we can speculate that a feed back mechanism may
operate between thundercloud and radiation belt influencing high-energy processes in the
atmosphere.
The participants of the Workshop agreed that the research on high-energy phenomena in
thunderclouds is entering an intensive development stage. New satellite and balloon
missions are being prepared exclusively for the detection of optical, radio and gamma ray
emissions from thunderclouds. New research groups from countries worldwide are
installing surface-based particle detectors for TGE detection. New models aimed to
explain TGF and TGE events are currently being developed and tested. A vast amount of
experimental evidence on TGE, TGF and TLE is available for tuning the models.
During the Workshop a lecture on frontiers of very high-energy gamma-ray astronomy
was presented by Razmik Mirzuyan, MAGIC collaboration, MPI Munich, GE. In
addition to the scientific program, conference participants visited the Aragats research
station of YerPhI located at 3200 m altitude.
The excursion program included the 8th century Amberd Fortress, the museum of ancient
manuscripts in Yerevan, a cathedral in Echmiadzin, and several archeological
excavations in the nearby Aragats mountains.
The slides of presentations and videos of discussions are available on the conference
website: http://crd.yerphi.am/Conferences/tepa2013/home
Figure 1 Aragats station, near the Vishap stone erected in the vicinity of Kare Lake. The
highest peak north of Aragats (4090 m) is seen in the background to the right of the stone.
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