Post on 19-Jun-2020
TGFs as a Laboratory for understanding particle accelerationGer Fitzpatrick for the Fermi GBM TGF-Team
Image Credit: Reuters and NASA
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High Energy Atmospheric Phenomena
2 Image Credit: Collier et al., 2011
• Larger family of energetic atmospheric phenomena
• x-rays, gamma-ray flashes, gamma-ray glows
• Short intense flashes of gamma-rays
• Associated with lightning activity in thunderstorms
• Observed by gamma-ray detectors in low-earth orbits
• Discovered in 1994
• BATSE, RHESSI, AGILE, FERMI LAT/GBM
Terrestrial Gamma-ray Flashes
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TGF Observations• Hard
• 7 MeV average energy
• Beamed
• 45 deg cone (half-angle)
• Rapid variability
• Altitude
• < 15 km
• Source - thunderstorms
• Correlates well with lightning
• Intra-Cloud Lightning
• Exact connection disputed
• Meteorological conditions
• Not well understood
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TGF Observations• Hard
• 7 MeV average energy
• Beamed
• 45 deg cone (half-angle)
• Rapid variability
• Altitude
• < 15 km
• Source - thunderstorms
• Correlates well with lightning
• Intra-Cloud Lightning
• Exact connection disputed
• Meteorological conditions
• Not well understood
4 Image Credit: Foley et al., 2014
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TGF Observations• Hard
• 7 MeV average energy
• Beamed
• 45 deg cone (half-angle)
• Rapid variability
• Altitude
• < 15 km
• Source - thunderstorms
• Correlates well with lightning
• Intra-Cloud Lightning
• Exact connection disputed
• Meteorological conditions
• Not well understood
5 Image Credit: Chronis et al., 2015
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How common are they?• What kind of storms?
• ~750 observed per year
• How many occur in total?
• Instrument & calculation dependent
• 400,000 per year [Briggs et al., 2013]
• 2 million per year [Ostgaard et al., 2012]
6 Image Credit: NASA
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How common are they?• What kind of storms?
• ~750 observed per year
• How many occur in total?
• Instrument & calculation dependent
• 400,000 per year [Briggs et al., 2013]
• 2 million per year [Ostgaard et al., 2012]
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Multi-Wavelength observations keyFacilitates studies into:
• Production mechanisms [e.g. Connaughton et al., 2012]
• Location [e.g. Briggs et al., 2103, Chronis et al., 2015]
• Altitude [e.g. Cummer et al., 2012, 2015]
• Duration [e.g. Fitzpatrick et al., 2014]
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Production Mechanisms
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Relativistic Runaway Electron Avalanche
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Relativistic Runaway Electron Avalanche
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Relativistic Runaway Electron Avalanche
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cold runaway
runaway threshold
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Spectra
13 Image Credit: Dwyer & Smith, 2005
• Avalanche process
• Acceleration in a medium
• multiplication of secondaries
• Canonical photon spectrum at the source is:
• Spectra at S/C altitudes will be modified by passage through the atmosphere
• Simulations imply that ~1017 energetic electrons required at the source
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Two theories - two locations
14 Image Credit: S. Celestin
RREA with relativistic feedback
• Strong ambient electric field (~300+ kV/m)
• Large scale (~100 m to ~1 km)
Cold runaway in lightning leaders
• Very strong local field (>10 MV/m)
• Small scale
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Relativistic Feedback• Seed electrons
• Relativistic Feedback
• positrons
• back-scatter x-rays
• Explains:
• Spectra
• Duration
• Intensities
• Radio detections
15 Image Credit: Dwyer 2007
t < 0.5 us
t < 2 us
t < 10 us
e-
e+
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Terrestrial Electron Beams• Primary electrons absorbed, secondary
leptons not (entirely)
• Distribution of pitch angles -> temporal dispersion
• Magnetic Mirroring
• positron fraction ~18 %
16 Image Credit: Dwyer 2008, NASA
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Terrestrial Electron Beams• Primary electrons absorbed, secondary
leptons not (entirely)
• Distribution of pitch angles -> temporal dispersion
• Magnetic Mirroring
• positron fraction ~18 %
17 Image Credit: Dwyer 2008, NASA
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Terrestrial Electron Beams
18 Image Credit: Briggs et al., 2012
• Primary electrons absorbed, secondary leptons not (entirely)
• Distribution of pitch angles -> temporal dispersion
• Magnetic Mirroring
• positron fraction ~18 %
Fermi Highlights
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LAT ObservationsGeolocating TGFs directly from gamma-ray observations
• Calorimeter casts shadow on tracker for photons coming from nadir
• TGFs are bright - complicates analysis
• high multiplicity
• tens or hundreds of photons per single LAT “event”
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Poster: Terrestrial Gamma ray Flashes as Seen by the Fermi LAT J. E. Grove et al.
Courtesy of J. E. Grove
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LAT Observations• More than 150 TGFs geolocated
from gamma-rays
• 19 have both good gamma and radio locations
• Gamma & radio spatially and temporally coincident
• Supports hypothesis that gamma and radio have a common origin
• Storms with modest lightning activity can produce bright TGFs
21 Courtesy of J. E. Grove
Poster: Terrestrial Gamma ray Flashes as Seen by the Fermi LAT J. E. Grove et al.
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TGF 140204
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35. 40. 45. 50.East Longitude
-25.
-20.
-15.
-10.
Latit
ude
TGF1402045812014-02-04 13:56:34.062001 UTC
Fermi: 44.2748 -18.3387ENTLN_strk
MirrorTEB
TGF TEB
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TGF 140204
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35. 40. 45. 50.East Longitude
-25.
-20.
-15.
-10.
Latit
ude
TGF1402045812014-02-04 13:56:34.062001 UTC
Fermi: 44.2748 -18.3387ENTLN_strk
MirrorTEB
TGF TEB
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TGF 140204: Simulations
24 Simulations courtesy of J. Dwyer @ UNH
e+/e-
gammas
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TGF 140204: Simulations
25 Simulations courtesy of J. Dwyer @ UNH
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Spectral Analysis• Source spectrum always has
the same shape
• Propagation effects
• Beam angle distribution
• Previous analyses have stacked multiple TGFs
• smearing of spectra
• Dead Time & Pulse Pile up are issues
26 Image Credit: Dwyer & Smith, 2005
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TGF 140204: Spectral Analysis
• Fermi observations are sufficiently bright that individual TGFs can be studied
• Dead Time & Pulse Pile Up
• Template fitting
• Can directly estimate the beam geometry and altitude of the source
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preliminary
Image Credit: B. Mailyan et al., in prep
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Summary• TGFs - novel particle
acceleration in our backyard
• Fermi provides an unique opportunity to facilitate multi-wavelength observations
• key to untangling source mechanisms
• Fermi continues to make new discoveries, e.g. TGF/TEB 140204, individual spectral analysis
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Acknowledgements
Key Papers • Briggs et al., 2013 • Dwyer et al., 2012 • Celestin & Pasko 2012 • Dwyer 2008 • Moss et al., 2006 • Dwyer & Smith 2005
Jacobs - M. Gibby & M. GilesUSRA - W. ClevelandWWLLN & ENTLN Collaborations
Back up slides
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Grove – T’storm accelerator
Transient Luminous Events (TLEs)
• TLEs are not directly related to TGFs
• Sprites • ~10 ms, luminous “jellyfish” sprays • Related to strong positive cloud-to-ground (+CG)
lightning • Elves
• ~1 ms, expanding luminous ring in ionosphere • Related to EMP from strong negative cloud-to-ground
(-CG) lightning? • Blue jets, gnomes
• ~300 ms, luminous jet from top of thunderhead • Not directly triggered by lightning discharges • Correlated with intense hail?
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