Abstract.

We describe the ionospheric modification by the SGR 1806-20 gamma flare (27 December 2004) seen in the global electromagnetic (Schumann) resonance.

The gamma rays lowered the ionosphere over the dayside of the globe and modified the Schumann resonance spectra.

We present the extremely low frequency (ELF) data monitored at the Moshiri observatory, Japan (44.365◦ N, 142.24◦ E).

Records are compared with the expected modifications, which facilitate detection of the simultaneous abrupt change in the dynamic resonance pattern of the experimental record.

The gamma flare modified the current of the global electric circuit and thus caused the “parametric” ELF transient.

Model results are compared with observations enabling evaluation of changes in the global electric circuit.

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1 Introduction

Monitoring of the global electromagnetic (Schumann) resonance allows for studying both the Earth–ionosphere cavity and the natural sources of radiation-lightning strokes (Nickolaenko and Hayakawa, 2002).

We compare the experimental and model results concerning the impact of the powerful gamma ray flare from SGR 1806-20 that occurred on 27 December 2004.

Since 1979 a couple of intense gamma ray flares took place arriving from the extra-terrestrial sources.

Records of remote VLF transmitters indicated the ionosphere depression caused by the gamma rays (Inan et al., 1999,2007; Terasawa et al., 2005; Tanaka et al., 2008), but an at-tempt was unsuccessful at finding any changes in the Schumann resonance records caused by the gamma flare from SGR 1900+14 on27August1998 (PriceandMushtak,2001).

In this paper we report a successful detection of changes in the Schumann resonance spectra during the intense gamma ray flare SGR 1806-20 (27 December 2004).

The records were performed at the Moshiri observatory, Japan (44.365 ◦N and 142.24 ◦E).

Experimental material is compared with the model predictions for the Schumann resonance background signal based on the “knee” ionosphere model (Mushtak and Williams, 2002; Williams, et al., 2006), and the partially uniform knee (PUK) ionosphere model (Pechony and Price,2004).

We also compare the experimental records at theMoshiri observatory with the model estimates for the para-metric ELF pulse generated by the gamma flare (Nicko-laenko, 2010, 2011; Nickolaenko and Schekotov, 2011a, b).

Detailed description of the gamma ray event can be found in the papers by Hurley et al. (2005), Terasawa et al. (2005),Inan et al. (1999, 2007), and Tanaka et al. (2011). We men-tion only the minimal information here.

The flare occurred around 21:30:26UT when the hard X/gamma rays arrived at the dayside of the Earth.

Radiation came from a neutron star 30-40 thousand light years away.

The peak flow exceeded the most intense solar flares by five orders of magnitude, and it was 100 times greater than the SGR 1900+14 gamma flare of 1998.

The disturbance was centered above the Pacific Ocean(146.2◦ W and 20.4◦S), i.e. at a distance of ≈450km from the center of the dayside hemisphere.

Monitoring the transpacific VLF transmissions at the“Palmer” Antarctic station allowed for the following inter-pretation of observations (Inan et al., 2007). The gamma flare lowered the dayside ionosphere by ∼20km, and modifica-tion lasted for more than 1h. It was detected at distances up to 60 ◦from the sub-flare point.

An abrupt drop of the iono-sphere height occurred in less than 0.02s (Inan et al., 2007), and we fit the temporal change of the height by the following function:

dH=−19·(9.6441t )−0.1501 (1)

Here t is the time since the gamma ray arrival. In our computations, we change the ionosphere height with a 10s step (Nickolaenko and Hayakawa, 2010

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