Unique observations of optical lightning and terrestrial gamma flash

An animated image of the event.

In a study by Nikolai Østgaard and Thomas Gjesteland at the Birkeland Centre for Space Science (BCSS), University of Bergen, together with researchers from USA and South Africa, the first simultaneous detection of a terrestrial gamma flash (TGF) and the optical signal from lightning is presented. This result may be a breakthrough in the understanding of the enigmatic terrestrial gamma flash that was discovered in the 90s. The study has recently been accepted for publication in Geophysical Research Letters.

"Our study shows that the TGF was produced deep in the thundercloud at the initial stage of an intracloud (IC) lightning before the leader reached the cloud top and extended horizontally" says professor Nikolai Østgaard, leader of BCSS. "A strong radio pulse was produced by the TGF itself. This is the first time the sequence of radio pulses, TGF and optical emissions have been identified" Østgaard adds.

By fortuitous coincidence two independent satellites passed less than 300 km from a thunderstorm that developed over Lake Maracaibo in Venezuala, October 27, 2006 and produced a TGF lasting only 70 ms . This is a region known for its powerful and frequent lightning. The RHESSI satellite detected a TGF that lasted only 70 ms and TRMM recorded optical emissions as well as the cloud temperature.

"Together with two independent measurements of radio emissions we have an unprecedented coverage of the event" says scientist Thomas Gjesteland. Both timing and location show clearly that the radio signals, the TGF and optical lightning came from the same IC discharge that lasted for about 300 ms.

It was recently suggested that TGFs could be dark lightning, that is, the gammas are produced by relativistic electrons that also make a strong electric current that again produces a strong radio pulse, and that this can occur without any following optical lightning to discharge the cloud. "Our results confirm that the TGF produces a strong radio signal, but the ´dark lightning´(TGF) is followed by a ´bright lightning´ and indicates that both phenomena are intrinsic processes in the lightning discharge" says Nikolai Østgaard.

Animations of the event can be found here.

Background:

Terrestrial gamma flashes are the most energetic photon phenomenon (>40 MeV) produced naturally on Earth. This phenomenon that was unknown to mankind before 1991 is produced in thunderstorms but we do not know how frequent they are or how they are produced. TGFs was first discovered by BATSE in 1991 and reported by Fishman et al., 1994. Several missions have observed TGFs since then (RHESSI, AGILE and Fermi), but none of these instruments were designed to detect TGFs. New missions like ASIM and TARANIS will carry instruments especially designed to detect both TGFs and the optical emissions from lightning. These missions will tell us whether ´dark lightning´ can occur alone or always is followed by a bright optical signal.

Acknowledgements:

The research leading to this result has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n. 320839. We also acknowledge the Birkeland Centre for Space Science, Research Council of Norway contract 223252/F50 as well as contracts 197638/V30, 208028/F50 and 216872/F50. We thank the RHESSI team and Dr. D. M. Smith for the use of RHESSI raw data and software. The LIS/OTD Lower Level Data and LIS/ODT software were provided by the LIS/OTD Science Team (Principal Investigator, Hugh J. Christian). We thank the institutions contributing to WWLLN (http://wwlln.net/). The Duke University component of this work was supported by the NSF Physical and Dynamical Meteorology Program

Animated image that demonstrates the sequence of radio pulses, TGF and optical emissions:

• (left): A leader forms and produces a weak radio pulse.
• (middle): Electrons with speed of light produce gamma rays (pink), electric current and a strong radio pulse.
• (right): The leader short-cuts the two charge layers, strong electric current, radio pulse and bright lightning are produced.