GLOW News Blog

News about the German Long Wavelength Consortium and radio astronomy in Germany

Am 9. September 2015 wurde die internationale Station DE 609 in Norderstedt feierlich eröffnet. Sie ist die 47. Station des LOFAR Radiointerferometers. Über 50 Gäste besichtigten zunächst die Station selbst, geführt von Dr. D. Engels (U Hamburg) und Dr. J. Künsemöller (U Bielefeld). In einem Grusswort an die Gäste schrieb die Wissenschaftssenatorin der Freien und Hansestadt Hamburg Katharina Fegebank: "Technisch gesehen ist das Radiointerferometer eine Art Digitalkamera für Radiostrahlung aus dem All mit der Größe von ganz Nordwesteuropa. Wissenschaftspolitisch gesehen ist es ein Beispiel für die gelungene Kooperation von Hochschulen und Forschungseinrichtungen in bisher sechs europäischen Ländern." Diese zentrale Bedeutung der Kooperation stand auch im Mittelpunkt der Grussworte von M. Hecht (Kanzler, U Hamburg), Prof. Dr. H. Graener (Dekan, Min. Fakultät, U Hamburg) und Prof. Dr. M. Egelhaaf (Prorektor, U Bielefeld) zu Beginn des Festaktes im Feuerwehr-Museum Norderstedt. Dr. R. Vermeulen (Direktor des International LOFAR Telescope) und Prof. Dr. H. Falcke (Vorsitzender des ILT Aufsichtsrates) stellten in ihren Vorträgen den Stand des Ausbaus des Interferometers und die faszinierenden ersten Beobachtungsergebnisse vor. Der Vorsitzende des Zusammenschlusses der deutschen LOFAR-Stationen (GLOW), Prof. R. Dettmar, sah mit der Einweihung der neuen Station eine weitere Stärkung der Position Deutschlands in der modernen Radioastronomie einhergehen. Der Oberbürgermeister Norderstedts H.-J. Grote begrüsste die Entscheidung der Universitäten, die Station hier zu bauen. Sie würde dem Profil der aufstrebenden Stadt am Nordrand Hamburgs als Technologiestandort eine weitere Facette hinzufügen. Die Bedeutung der Station für die Weiterentwicklung der Forschung und Lehre in der Physik machten die beiden Projektleiter der Station Prof. Dr. M. Brüggen (U Hamburg) und Prof. Dr. D. Schwarz (U Bielefeld) deutlich.  Einen Vorgeschmack auf die neuen Forschungsfelder, die sich für die Wissenschaftler an den beiden Universitäten jetzt eröffnen, gab Dr. S. Oslowski in seinem Beitrag zu den Einflüssen des interstellaren Mediums auf die Signale von extrem schnell rotierenden Neutronensterne, sogenannten Pulsaren.

From University Groningen

How is lightning initiated in thunderclouds? This is difficult to answer – how do you measure electric fields inside large, dangerously charged clouds? It was discovered, more or less by coincidence, that cosmic rays provide suitable probes to measure electric fields within thunderclouds. This surprising finding is published in Physical Review Letters on April 24nd. The measurements were performed with the LOFAR radio telescope located in the Netherlands.

"We used to throw away LOFAR measurements taken during thunderstorms. They were too messy." says astronomer Pim Schellart. "Well, we didn't actually throw them away of course, we just didn't analyze them." Schellart, who completed his PhD in March this year at Radboud University in Nijmegen and is supervised by Prof. Heino Falcke, is interested in cosmic rays. These high-energy particles, originating from exploding stars and other astrophysical sources, continuously bombard Earth from space. High in the atmosphere these particles strike atmospheric molecules and create 'showers' of elementary particles. These showers can also be measured from the radio emission that is generated when their constituent particles are deflected by the magnetic field of the Earth. The radio emission also gives information about the original particles. These measurements are routinely conducted with LOFAR at ASTRON in Dwingeloo, but not during thunderstorms...

 

Modeling

That changed when the data were examined in a collaborative effort with astrophysicist Gia Trinh, Prof. Olaf Scholten from Groningen University and lightning expert Ute Ebert from the Centrum Wiskunde & Informatica in Amsterdam. "We modeled how the electric field in thunderstorms can explain the different measurements. This worked very well. How the radio emission changes gives us a lot of information about the electric fields in thunderstorms. We could even determine the strength of the electric field at a certain height in the cloud." says Schellart. This field can be as strong as 50 kV/m. This translates into a voltage of hundreds of millions of volts over a distance of multiple kilometers: a thundercloud contains enormous amounts of energy.

 

Dangerous charge

Lightning is a highly unpredictable natural phenomenon that inflicts damage to infrastructure and claims victims around the world. This new method to measure electric fields in thunderclouds will contribute to a better understanding and ultimately better predictions of lightning activity. Current measurement methods from planes, balloons or little rockets are dangerous and too localized. Most importantly the presence of the measurement equipment influences the measurements. Cosmic rays probe the thunderclouds from top to bottom. Moving at almost the speed of light they provide a near instantaneous 'picture' of the electric fields in the cloud. Moreover, they are created by nature and are freely available. 'This research is an exemplary form of interdisciplinary collaboration between astronomers, particle physicists and geophysicists', says Heino Falcke. 'We hope to develop the model further to ultimately answer the question: how is lightning initiated within thunderclouds?'

 

Probing atmospheric electric fields in thunderstorms through radio emission from cosmic-ray induced air showers Schellart e.a

Physical Review Letters 24 april 2015 

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