spectra of the thunderstorm correlated electron and gamma-ray measured at aragats

Spectra of the Thunderstorm Correlated Electron and Gamma-Ray Measured at Aragats

Bagrat Mailyan and Ashot Chilingarian

TEPA - 2010, September 6 - 11, 2010, Nor Amberd, Armenia

Particle acceleration in atmospheric electric fieldsRunaway Relativistic Electron Avalanche (RREA)


Detectors of Aragats Space-Environmental Center

Detectors located outdoor and indoor correspond to the various energy thresholds of measured particles. Indoor detectors also located under roof materials with different thicknesses. These detectors, sensitive to various energies, allow us to construct the energy spectrum of the additional particles.


Recently 3 layers of 1 cm and 1layer of 3cm scintillating detectors are installed outdoors.


Aragats Solar Neutron Telescope (ASNT)

ASNT detects not only particle count rate, but also energy releases spectra in

scintillators!


Light attenuation in thick scintillators

Chilingarian et al., 2007

Scintillator layers Light output (%)

1 89.3

2 73.96

3 61.31

4 50.1

5 40.96

6 33.64

7 27.56

8 22.56

9 18.92

10 15.6

11 11.5


Reconstruction of the energy spectra of incidentgammas

The unfolding of the gamma ray spectrum above the roof of the MAKET building at altitude of 3250 m was made in the following way:

• A power spectrum with initial parameters randomly chosen from predetermined interval is generated;

• This spectrum is used to simulate traversal of gamma rays via roof and ASNT detector components to finally obtain simulated energy release spectrum;

• The obtained simulated spectrum is compared with experimental one; the discrepancy (quality function) and initial spectrum parameters are stored;

• If number of iterations is not fulfilled go to step 1.


Reconstruction of the gamma spectrum of September 19, 2009 event

The minimum of quality function corresponds to power index -2.96.

Electron contamination of gamma spectrum is taken into account.


Electron contamination of gamma spectra and gamma contamination of electron spectra


Construction of electron spectra of September 19, 2009

We correct the electron energy spectrum for the gamma-ray contamination. After the electron number correction for gamma contamination,59472, 28268, 22036 and 10982 electrons wereestimated to be detected by MAKET outdoor, MAKET indoor and SEVAN detectors respectively.

According to calculations and simulations 4 types of detectors have energies approximately equal to 9, 12, 15, 18 MeV. Errors about 2 MeV, reflecting uncertainties in determination of the energy thresholds, are taken due to the complicated structure of roof substance

To compare electron and gamma spectra, gamma differential spectrum was integrated.


Electron and gamma ray spectra of September 19, 2009 event


Reconstruction of the electron spectrum just below the thundercloud electric field

We can assume that maximal energy of the RREA electrons can reach 40-50 MeV. (Marisaldi et al., 2010)

Taking into account the maximal energy of detected electrons and assuming that electrons lose 200 keV per meter of air, thundercloud altitude was estimated to be ~100-150 m! By trying different trial spectra, evaluating the avalanche from 3380 till 3250 m and comparing each obtained spectrum with experimental one we have found the RREA spectrum, just below the thundercloud.

The exponential index was found to be ~ -0.152.


Reconstruction of the electron spectrum just below the thundercloud electric field


Parameters of the fit for September 19 event


Electron and gamma spectra of September 19, 2009 event


Comparison with other events

• We have perform the same operations as for September 19 event to reconstruct the gamma spectra and obtain electron spectra of three smaller thunderstorm events.

• Gamma contamination of electron spectra are taken into account.


May 21 event: More gammas than electrons


July 9 event: No electrons only gammas


August 26, 2010 event

The energy thresholds were estimated to be 1.5, 4.5 and 7.5 MeV for upper, middle and lower detectors respectively.


Comparison of the differential gamma spectra of the three events

Intensity [#/sq.m.min.] for >7MeV gammas Power law index

May 21 25000 -2.62

July 9 90000 -4.23

September 19 90000 -2.96

August 26 10000 -1.68


Comparison with gamma spectra calculated and measured with other groups

• According to simulations by J.R.Dwyer (2008), the RREA spectrum is independent on thundercloud electric field. The mean electron energy is ~7.3 MeV.

• Using Baksan data, Lidvansky and Khaerdinov (2009) assume 1000 , 2000, 3000 m cloud height, to obtain 3 different values of the power spectra index from to -2.57 to -4.36, for various events discussed in the paper. For instance, at October 11, 2003 the spectral indexes were estimated to be -3.78, -4.09, -4.36 for 1000, 2000, 3000 m respectively.

• Tsuchiya et al., (2009) have simultaneously observed electrons and gammas spectrum extending up to 10 MeV on the top of the mountain Norikura, 2700 m above sea level. The RREA electrons source distance is estimated to be ~90m and spectral index of the gamma spectrum is -1.15 0.09; no energy spectrum of electrons was presented.

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Conclusion

For the first time we present the energy spectra of RREA electrons and gamma rays,using the variety of ASEC detectors including the enhanced possibilities of ASNT to measure energy releases in the thick scintillator. Gamma ray spectra are described bypower law and electron spectra by exponential law.The intensities of additional particle fluxes for the discussed events are different. Electron/gamma ratio also varies for different thunderstorm events. There is an event without electron detections >9 MeV. Probably this is connected with thundercloud height.


Thank you for attention!

Acknowledgements

The work was partially supported by the grant

No. CRDF-NFSAT-SCS MES RA ECSP-69/A-27

spectra of the thunderstorm correlated electron and gamma-ray measured at aragats

Documents

electron energy spectrum

gamma ray spectrum

power spectrum

gammaray contamination

generatedthis spectrum

initial spectrum parameters

obtained simulated spectrum

indoor detectors