Lightning and Thunderstorms
Lightning is the most powerful radiator on Earth. A lightning stroke emits an electromagnetic pulse (EMP) strong enough to be measured on the opposite side of the Earth. Recently, it has been discovered that during the lightning initiation process, radiation including x-rays, gamma rays (see section below), and even positrons - antimatter! - are created in the thunderstorm.
Despite its ubiquity and direct impact on society, there is a huge amount that we do not understand about lightning and thunderstorms. How the thunderstorm becomes electrified in the first place; how, where, and when lightning initiates; what is the connection between lightning and other severe weather phenomena associated with thunderstorms; and any ability to predict when and where a lightning discharge will occur - these and other questions keep the lightning research community busy. |
Our Work
Our lightning research in the LAIR exploits the use of several remote sensing instruments, as well as modeling techniques, to address the above science questions about thunderstorm electrification. Some of these instruments have been invented or modernized by the LAIR group, including: (1) the Electric Field Mill (EFM) instrument for measuring ambient electric fields, and thus capable of proving the electric field structure in and around thunderstorms when deployed as an instrument array; and (2) the Low-Frequency Autonomous Magnetic Field Sensors (LFAMS) radio instrument for capturing a lightning stroke’s radio emission in the VLF/LF frequency regime (3—300 kHz), used in studying the originating lightning stroke and its generating environment.
In late 2018, the LAIR was part of the lightning research group of the RELAMPAGO field campaign, for which it deployed an array of EFM and LFAMS instruments. This campaign saw international collaboration from a multitude of institutions, and was conducted in west central Argentina, a region known for the most intense storms on Earth. Data products derived from our instrumentation in RELAMPAGO are to be released on the EOL data archive. Following the release of the data products, our primary research focus is to study the population of energetic lightning (classified by their LF radio emissions) in different storm contexts and their association to thunderstorm severe weather production.
In late 2018, the LAIR was part of the lightning research group of the RELAMPAGO field campaign, for which it deployed an array of EFM and LFAMS instruments. This campaign saw international collaboration from a multitude of institutions, and was conducted in west central Argentina, a region known for the most intense storms on Earth. Data products derived from our instrumentation in RELAMPAGO are to be released on the EOL data archive. Following the release of the data products, our primary research focus is to study the population of energetic lightning (classified by their LF radio emissions) in different storm contexts and their association to thunderstorm severe weather production.
Terrestrial Gamma-Ray Flashes
Above: Illustration of TGF production, as well as the fluorescence light resulting from the radiative relaxation of excited species produced by the large fluxes of TGF-induced energetic electrons.
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Thunderstorms occasionally behave as powerful particle accelerators and produce gamma-ray bursts with energies as high as a few tens of MeVs named terrestrial gamma- ray flashes (TGFs) [Fishman et al., 1994]. This high-energy phenomenon has been extensively observed by low-orbit satellites: the Compton Gamma-Ray Observatory [Fishman et al., 1994], the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) [Smith et al., 2005], the Fermi Gamma-ray Space Telescope [Briggs et al., 2010], and most recently by the Astrorivelatore Gamma a Immagini Leggero (AGILE) satellite [Marisaldi et al., 2010]. Space-based measurements have revealed some temporal and spectral features of TGFs: they typically last from a fraction of to a few milliseconds, have a typical fluence slightly weaker than 1 photon/cm2 when observed from a low Earth orbit, and exhibit a hard energy spectrum extending up to a few tens of MeVs [e.g., Smith et al., 2005; Briggs et al., 2010; Dwyer et al., 2012]. In addition, detailed studies of TGF-associated lightning sferics have correlated this phenomenon with normal polarity intra-cloud lightning (+IC) that acts to transport negative charge upward within the cloud [e.g., Stanley et al., 2006; Lu et al., 2010; Shao et al., 2010], more precisely with the initial development stages [e.g., Marshall et al., 2013; Lyu and Cummer, 2018]. It has also been proposed that the large number of runaway and secondary electrons involved in TGFs also radiate energetic radio signals [e.g., Connaughton et al., 2013; Dwyer and Cummer, 2013], with amplitudes comparable with conventional lightning discharges.
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Our Work
In our group, we have predicted a novel type of transient luminous event produced by TGFs via interaction with the Earth’s atmosphere [Xu et al., 2017]. Moreover, we have investigated the bursts of X-ray emission during natural cloud-to-ground lightning discharges [Xu et al., 2017] and quantified the effects of Compton scattering on the temporal and spectral properties of TGFs [Xu et al., 2019].
With data from the Atmosphere-Space Interactions Monitor (ASIM) onboard the International Space Station and from an array of three LFAMS instruments deployed in Colorado in the summer of 2019, we are also extending our research on energetic lightning, from the previous section, to include TGF production.
With data from the Atmosphere-Space Interactions Monitor (ASIM) onboard the International Space Station and from an array of three LFAMS instruments deployed in Colorado in the summer of 2019, we are also extending our research on energetic lightning, from the previous section, to include TGF production.
Lightning Generated Whistlers
Above: Green et al., 2005 illustration of whistler mode wave propagation.
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Lightning has far reaching effects in space. When lightning strikes, it releases an electromagnetic (EM) pulse. Most of the released energy is trapped within the Earth-ionosphere wave-guide, but a fraction is injected into the Earth’s magnetosphere where it propagates as a whistler-mode wave. Many spacecraft, including the Van Allen Probes, measure these waves, which are called lightning generated whistlers (LGWs) due to the whistling sound they make when shifted to audible frequencies. LGWs can interact with particles in the radiation belts, causing them to precipitate into the atmosphere where they trigger a chain of chemical reactions. Studying LGWs is important for understanding how energy and particles transfer between Earth and space.
Plasma ducts are regions with much higher or lower density than the surrounding plasma. Ducts can act as wave guides for LGWs, determining where an LGW’s energy travels, how long it persists in the magnetosphere, and what effect it has on the radiation belt population. While k-vectors of non-ducted LGWs become oblique during propagation, ducted LGWs remain close to field-aligned. The figure to the left illustrates ducted and non-ducted rays in the magnetosphere. |
Our Work
We are currently analyzing a large set of burst wave data from the Van Allen Probes (RBSP) mission that contain LGWs. Our goal is to assess the prevalence of ducted vs. non-ducted LGWs using wave normal angles calculated from spectral analysis of the RBSP EFW magnetic search coil data. The figure below on the left shows an example analysis for a single 4-second burst. First, we compute the spectrogram. Second, we filter the data based on the noise curve of the magnetic search coil. Next we use spectral analysis to calculate the ellipticity and wave normal angle of the waves. Finally, we use a clustering method to group whistlers so that each group can be assigned a single average wave normal angle. The polar plots in the figure below to the right show initial results from two years of data analyzed thus far. Of these events, 20.0% of bursts contain LGWs. Of the bursts containing LGWs, we identified 20,000 LGW clusters. 15.2% of the LGW clusters may be ducted based on their average wave normal angle. After completing our identification of LGWs in the full EFW burst data set, we will release a database of whistlers identified in the RBSP data. For all potentially ducted events, we will calculate the local plasma density surrounding the burst time and report any measured plasma density ducts.
Left: Example burst event analysis showing (from top to bottom) magnetic spectrogram, filtered magnetic spectrogram, ellipticity, wave normal angle, and clustering. Right: Results of burst analysis thus far showing (A) L vs. MLT histogram of all bursts; (B) L vs. MLT histogram of only bursts containing LGWs; (C) mean wave amplitude in nT for each L-MLT bin calculated from the RMS wave amplitudes of filtered bursts containing LGWs; (D) mean WNA in degrees for each L-MLT bin calculated from the mean WNA of each LGW cluster; (E) L vs. MLT histogram of LGW clusters with cluster mean WNAs above L-dependent threshold; (F) L vs. MLT histogram of LGW clusters with cluster mean WNAs below L-dependent threshold. |
Our next steps in this project will focus on connecting LGWs in space to lightning strikes on Earth. We will use the GLD360 lightning database to match RBSP measured LGWs to potential source lightning strikes and globally extrapolate LGW energy injected into the magnetosphere. Left: Total lightning density map from the Global Lightning Detection Network GLD360 |
References
- Inan, U. S., S. A. Cummer, and R. A. Marshall (2010), A survey of ELF and VLF research on lightning ionosphere interactions and causative discharges, J. Geophys. Res. Space Physics, 115(A6), doi: 10.1029/2009ja014775.
- Schultz, C. J, W. A. Petersen, and L. D. Carey (2011), Lightning and severe weather: A comparison between total and cloud-to-ground lightning trends. Weather and Forecasting, 26(5):744–755, doi: 10.1175/waf-d-10-05026.1.
- Nag, A., and V. A. Rakov (2010), Compact intracloud lightning discharges: 1. mechanism of electromagnetic radiation and modeling, J. Geophys. Res., 115(D20), doi:10.1029/2010jd014235.
- Lyu, F., et al. (2015), Insights into high peak current in-cloud lightning events during thunderstorms, Geophys. Res. Lett., 42(16):6836–6843, doi: 10.1002/2015gl065047.
- Deierling, W., et al. (2019), Low Frequency Autonomous Magnetic Field Sensors (LFAMS) Level 1 Data, UCAR/NCAR - Earth Observing Laboratory, Version 1.0, doi:10.26023/3CNH-AMVJ-B0D. Accessed 16 Oct 2019.
- Blay, P., et al. (2014), Atmosphere-space interactions monitor (ASIM): State of the art. Acta Polytechnica CTU Proceedings, 1(1):303–306, doi: 10.14311/app.2014.01.0303.
- Briggs, M. S., et al. (2010), First results on terrestrial gamma ray flashes from the Fermi Gamma-ray Burst Monitor, J. Geophys. Res., 115, A07323, doi:10.1029/2009JA015242.
- Dwyer, J. R., and S. A. Cummer (2013), Radio emissions from terrestrial gamma-ray flashes, J. Geophys. Res. Space Physics, 118, 3769–3790, doi:10.1002/jgra.50188.
- Dwyer, J. R., D. M. Smith, and S. A. Cummer (2012), High-energy atmospheric physics: Terrestrial gamma-ray flashes and related phenomena, Space Sci. Rev., 173, 133–196, doi:10.1007/s11214-012-9894-0.
- Fishman, G. J., et al. (1994), Discovery of intense gamma-ray flashes of atmospheric origin, Science, 264(5163), 1313–1316.
- Lu, G., et al. (2010), Lightning mapping observation of a terrestrial gamma-ray flash, Geophys. Res. Lett., 37, L11806, doi:10.1029/2010GL043494.
- Lyu, F., and S. A. Cummer (2018), Energetic radio emissions and possible terrestrial gamma-ray flashes associated with downward propagating negative leaders, Geophys. Res. Lett., 45, 10,764–10,771, doi:10.1029/2018GL079424.
- Marisaldi, M., et al. (2010), Detection of terrestrial gamma ray flashes up to 40 MeV by the AGILE satellite, J. Geophys. Res., 115, A00E13, doi:10.1029/2009JA014502.
- Marshall, T., et al. (2013), Initial breakdown pulses in intracloud lightning flashes and their relation to terrestrial gamma ray flashes, J. Geophys. Res., 118, 10,907–10,925, doi:10.1002/jgrd.50866.
- Shao, X.-M., T. Hamlin, and D. M. Smith (2010), A closer examination of terrestrial gamma-ray flash-related lightning processes, J. Geophys. Res., 115, A00E30, doi: 10.1029/2009JA014835.
- Smith, D. M., L. I. Lopez, R. P. Lin, and C. P. Barrington-Leigh (2005), Terrestrial gamma-ray flashes observed up to 20 MeV, Science, 307 (5712), 1085–1088.
- Stanley, M. A., X.-M. Shao, D. M. Smith, L. I. Lopez, M. B. Pongratz, J. D. Harlin, M. Stock, and A. Regan (2006), A link between terrestrial gamma-ray flashes and intracloud lightning discharges, Geophys. Res. Lett., 33, L06803, doi:10.1029/2005GL025537.
- Xu, W., S. Celestin, V. P. Pasko, and R. A. Marshall (2017), A novel type of transient luminous event produced by terrestrial gamma-ray flashes, Geophys. Res. Lett., 44, 2571–2578, doi:10.1002/2016GL072400.
- Xu, W., R. A. Marshall, S. Celestin, and V. P. Pasko (2017), Modeling of X-ray Images and Energy Spectra Produced by Stepping Lightning Leaders, J. Geophys. Res. Atmosphere, 42, 122, 11776–11786, doi:10.1002/2016JD026410.
- Xu, W., S. Celestin, V. P. Pasko, and R. A. Marshall (2019), Compton Scattering Effects on the Spectral and Temporal Properties of Terrestrial Gammaray Flashes, J. Geophys. Res. Space Physics, 124, 7220–7230, doi:10.1029/2019JA026941.