Measuring the locations and properties of VHF sources emitted from an aircraft flying through high clouds

TL;DR

This study demonstrates that broad-band, pulsed VHF emissions from an aircraft flying through high clouds can be localized to specific spots on the airframe using LOFAR's near-field beamforming. By developing an adaptive windowing extension (ATRI-D) to the TRI-D imager, the authors achieve absolute localization around ~10 m and relative precision near ~50 cm for tail and engine sources, and they extract 3D polarization information to distinguish emission sites. The results show emissions originate at the engines and tail, with no observed activity from static wicks, offering insights into aircraft charging processes and informing improvements in lightning observation techniques. The work also outlines possible physical mechanisms—charging in clouds and onboard electronics—while underlining the need for additional observations to confirm the exact discharge processes and their link to VHF radiation.

Abstract

We show that it is possible to locate the few places on the body of an airplane, while it is flying through high clouds, from which broad-band, pulsed, radiation is emitted at Very High Frequency (VHF) radio frequencies. This serendipitous discovery was made whilst imaging a lightning flash using the Low-Frequency Array (LOFAR). This observation provides insights into the way the airplane sheds the electrical charge it acquires when flying through clouds. Furthermore, this observation allowed us to test and improve the precision and accuracy for our lightning observation techniques. Our new results indicate that with the improved procedure the location precision for strong pulses is better than 50~cm, with the orientation of linear polarization being accurate to within 25$^\circ$. For the present case of a Boeing 777-300ER, VHF emissions were observed exclusively associated with the two engines, as well as a specific spot on the tail. Despite the aircraft flying through clouds at an altitude of 8~km, we did not detect any emissions from electrostatic wicks.

Figures (7)

• Figure 1: The Boeing 777-306 (ER) named "Grand Canyon National Park" we have observed with LOFAR (photo by Suparat Chairatprasert).
• Figure 2: Side (a) provides an overview of LOFAR recorded lightning from flash 19A-1. The little specs at (N,E)=(23.2,-41.8) km, marked by the square box, are the recorded events coming from the airplane. On side (b) the open circles give a zoom-in of the sources in this little spec, where the stars show the positions of the airplane in 1 second intervals as determined from flight data, and discussed in Section \ref{['sec:Results']}.
• Figure 3: The two panels display a schematic map of the antenna layout to show the importance of properly adjusting the beamforming time window. Next to the antenna locations, indicated by $\bigotimes$ symbols, the insets show a schematic time trace where, in green and red the time-windows are denoted for the TRI-D (top panel) and ATRI-D (bottom panel) procedures. In green (red) the window for the correct (mislocated) source positions are given, respectively.
• Figure 4: (a) The flight path of the plane based on historical flight data from The OpenSky Network cite(Schaefer:2014) where the circle indicates the section of the flight path observed by LOFAR (see Fig. \ref{['fig:19A-1']}). The background in this figure shows the parallax-corrected cloud-top heights as measured by MSG cite(Finkensieper:2016, Meirink:2022). (b) Banking angle of the aircraft as a function of time in seconds where $t=0$ corresponds to the start of the flash.
• Figure 5: The imaged VHF-sources transformed into a frame that is co-moving with the plane. The 'Along'-axis is pointing in the flight direction of the plane. A bank angle of -14$^\circ$ has been accounted for. The size of the wagon wheels indicates intensity of each source. The color coding of the events is following the position on the airplane where 'rEa', 'rEb', 'T', 'lEb', 'lEa' label right engine front, right engine back, tail, left engine back, and left engine front respectively. Only analyzed data between t=-325 and -45 ms is shown in the figure. The drawing of the airplane, a Boeing 777-300ER is taken from Ref. cite(Boeing777-300ER).