TLE Research Activity Report

Christos Haldoupis and Ágnes Mika

Department of Physics, University of Crete, Heraklion, Crete, Greece

Christos Haldoupis
Physics Department, University of Crete
Heraklion, Crete, 710 03, Greece
Phone: +30-2810-394222, 394242
Fax: +30-2810-394201
e-mail: chald@physics.uoc.gr
http://users.physics.uoc.gr/~chald/

Ágnes Mika
Physics Department, University of Crete
Heraklion, Crete, 710 03, Greece
Presently at : ARGOSS B. V. Markensse, The Netherlands, e-mail:
mika@argoss.nl

Last updated: 18 Oct 2010


List of Publications and Conference Presentations


The Ionospheric Physics Laboratory (IPL), at the Department of Physics of the University of Crete, was a member of the “Coupling of Atmospheric Layers” (CAL) Research Training Network (RTN), supported by the European Union. This was a four-year project that started in November 2002, with 11 participating groups from various European Research Institutes and Universities. The purpose of the CAL network was to pursue basic research and to train young scientists, both at the post- and pre-doctoral level, on the physics of thunderstorm-related phenomena observed in the upper atmosphere/lower ionosphere.

In the framework of the CAL project, and in collaboration with Prof. Umran Inan of STAR Lab at Stanford University, an automatic narrowband VLF receiver was installed in Heraklion, Crete. The Crete VLF station started its routine operation on 18 July 2003, in time for its participation in the EuroSprite 2003 campaign, organised by the Danish Space Research Institute. In addition, an OmniPAL type VLF receiver was also installed in collaboration with János Lichtenberger from the Hungarian VLF group at the Eötvös Loránd University of Sciences, which operates continuously since 25 August 2005.

Ágnes Mika, a national of Hungary, joined CAL in November 2003 and started her Ph.D. at the University of Crete, taking graduate courses and doing research on the topic of subionospheric VLF perturbations in association with transient luminous events (TLEs) in the upper atmosphere. (If you're not familiar with the subject you might want to read some introductory material on TLE research and the use of VLF recordings first.) René J. Steiner from Austria was a member of CAL and our group from September 2005 until June 2006. His research focused on studying broadband VLF recordings in relation to sprites. Ágnes received her PhD degree on 4 May 2007. You can find Ágnes' thesis, entitled “Very Low Frequency EM Wave Studies of Transient Luminous Events in the Lower Ionosphere” here.

The scientific output of our research can be summarised as follows:

1. A virtually one-to-one relationship between sprites and early VLF events was established. This constitutes evidence of electron density enhancements in the D region during sprites, corroborating theoretical predictions of lightning-induced air breakdown and ionisation production in the upper atmosphere. The relation between sprites and their causative positive cloud-to-ground (+CG) discharges was examined and revealed a 30% occurrence of long-delayed sprites lagging a preceding discharge by 30 to 220 ms. This time lag is much larger than in previous reports, suggesting that long-lasting continuing currents play a key role in the build-up of sprite-causative, strong quasi-electrostatic (QE) fields in the upper atmosphere/lower ionosphere.
2. In collaboration with Oscar van der Velde from the Université Paul Sabatier, Toulouse, France, the role of intracloud lightning in the sprite generation process was studied further by using broadband VLF recordings in addition to lightning detection data of intracould discharges. It was found that intracloud lightning does indeed play a role, apparently by influencing the charge redistribution and thus shaping the intensity and duration of the mesospheric QE fields that produce sprites. In particular, the presence of intracloud discharges was found to be decisive in sprite morphology, leading to the appearance of carrot rather than column type sprites, possibly because they cause the impacting QE fields in the upper atmosphere to exceed the breakdown threshold for longer times and larger areas in the former cases than the latter.
3. A D-region model, accounting for reactions between four types of charged particles and involving various electron loss mechanisms, was solved numerically to compute relaxation times for electron density enhancements produced by the action on the ionosphere of lightning-induced QE fields. Model results for different parameters were compared with early VLF event recoveries to infer estimates of sprite-related ionisation increases and the altitudes at which they occur. The model accounted for the observed recovery times from about 20 to more than 250 s, if electron density increases of 100 to 10000 times the ambient values were taken to occur in the altitude range from about 75 to 85 km. This endorsed the notion of VLF scattering from relatively large diffuse regions of electron ionisation enhancements located right below the night-time VLF reflection heights, in line also with existing simulations and observations of sprite halos.
4. Studies of sprite-related early VLF perturbations lead to the discovery of a new category of VLF events, termed early/slow, characterised by long onset durations up to 2.5 s in contrast to the well-known early/fast events reaching their maximum amplitude in less than 50 ms. A new mechanism was proposed to explain the slow growth of these events, postulating a process of gradual secondary ionisation build-up, caused by the effects on sprite-produced electrons of electromagnetic pulses (EMPs) radiated upwards by intracloud lightning channels. This was substantiated by the identification of burst-like sferic activity in conjunction with intracloud discharges occurring in the sprite-producing storms, accompanying the early/slow event amplitude growth.
5. An examination of elve observations, made by ground-based and satellite-borne imagers, along with simultaneous VLF recordings, led to the identification of elve-related early VLF perturbations for the first time. This is long-awaited experimental evidence in favour of ionisation produced in the upper D region by lightning-induced EMPs that also produce elves, which came in support of existing theories and simulation models.
6. In collaboration with Dr. Tilmann Bösinger from the University of Oulu, Finland, the ultra low frequency (ULF, f < 10 Hz) response to sprite-producing lightning discharges was studied statistically using data from a chain of high-resolution pulsation magnetometers. ULF signatures of sprite-producing and non sprite-related +CG discharges were compared. No signature was found unique to sprite-causative +CG discharges, in contrast with previous claims based on event studies.
7. In collaboration with David Nunn from Southampton University, UK, an existing numerical model was upgraded to be capable of simulating VLF scattering from a group of vertical plasma structures corresponding to sprite bodies. Under the action of an incident VLF wave, the scattering system was treated as an assembly of conducting columns divided into sequential radiating segments (dipoles). A novel feature was the inclusion of electromagnetic interaction between the radiating elements and realistic Earth-ionosphere waveguide propagation effects, which enables the simulation of a self-consistent scattering radiation pattern. First tests indicate that this model could be easily adapted to simulate early VLF perturbations at a receiver location.

The IPL was the organiser of the CAL Mid-term Review and Science Meeting which took place in Elounda, Crete, from 20 to 24 June 2005.