tgfs as a laboratory for acceleration - fermi gamma-ray ...of 28 tgf observations • hard • 7 mev...

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

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of 2831 Image Credit: Celestin et al., 2012b

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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