A terrestrial gamma-ray flash and ionospheric ultraviolet emissions powered by lightning

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Science  10 Jan 2020:
Vol. 367, Issue 6474, pp. 183-186
DOI: 10.1126/science.aax3872

Gamma-ray flash from a lightning leader

Terrestrial gamma-ray flashes (TGFs) are millisecond pulses of gamma rays produced by thunderstorms. Neubert et al. observed a TGF from above, using instruments on the International Space Station. High-speed photometry in optical, ultraviolet, x-ray, and gamma-ray bands allowed them to determine the sequence of events that produced the TGF. Emission from an intracloud lightning leader was followed within a millisecond by the TGF. The subsequent lightning flash produced an electromagnetic pulse, which induced expanding waves of ultraviolet emission in the ionosphere above the thunderstorm, called an elve. The authors conclude that high electric fields produced within the lightning leader generated the TGF.

Science, this issue p. 183


Terrestrial gamma-ray flashes (TGFs) are transient gamma-ray emissions from thunderstorms, generated by electrons accelerated to relativistic energies in electric fields. Elves are ultraviolet and optical emissions excited in the lower ionosphere by electromagnetic waves radiated from lightning current pulses. We observed a TGF and an associated elve using the Atmosphere-Space Interactions Monitor on the International Space Station. The TGF occurred at the onset of a lightning current pulse that generated an elve, in the early stage of a lightning flash. Our measurements suggest that the current onset is fast and has a high amplitude—a prerequisite for elves—and that the TGF is generated in the electric fields associated with the lightning leader.

Transient luminous emissions (TLEs), such as sprites, jets, and elves, occur above thunderstorm clouds (1). Terrestrial gamma-ray flashes (TGFs) are brief (less than a few milliseconds) emissions of photons, reaching energies of tens of MeV, observed by astrophysical satellites passing over thunderstorms (25). TGFs were initially thought to originate from high-altitude TLEs, where low atmospheric absorption would allow the TGF photons to escape, but their source was later determined to be within the thunderstorm clouds (68). It is now understood that TGFs are produced by bremsstrahlung radiation from electrons with energies in the runaway regime, where the cross sections for interactions decrease with further acceleration (9). The generation of TGFs requires seed electrons in this regime that start a runaway electron avalanche. The seeds can be electrons released by cosmic ray interactions with the atmosphere, born with high energies (10), or cold electrons in streamers that are accelerated into the runaway regime in the electric fields of lightning leader tips (11). The relativistic electron flux must then be amplified for the bremsstrahlung to reach the measured flux levels, as in relativistic runaway electron avalanches, possibly enhanced by feedback effects from positrons created by pair production and from ionization by backscattered bremsstrahlung (9). The role of the large-scale electric field in a cloud relative to the enhanced field at a lightning leader tip, however, is still debated (911).

We present observations with the Atmosphere-Space Interactions Monitor (ASIM), onboard the International Space Station (ISS), of a TGF produced in the initial stage of a lightning flash. The TGF was observed east of the island of Sulawesi in Indonesia on 10 October 2018, at 13:01:33.100080 Coordinated Universal Time (UTC). ASIM has two x-ray and gamma-ray detectors, three ultraviolet (UV) and optical photometers, and two optical imaging cameras (1214). The instruments are pointed toward the nadir (directly downward) to minimize gamma-ray flux losses due to atmospheric absorption. Figure 1, A and B, shows the location of the TGF, the surrounding cloud top altitudes derived from observations by the Himawari weather satellite (15), and the full and cropped field of view of the ASIM optical instruments. Figure 1C shows the projection of the ASIM camera image and the location of a coincident lightning event detected by the World Wide Lightning Location Network (WWLLN) (15).

Fig. 1 Location of the TGF event observed on 10 October 2018.

(A) Cloud top altitudes (15) are shown according to the color scale to the right of (B). The yellow curve shows the ISS orbit; the red dot marks its position at the onset time of the TGF at 13:01:33.100080 UTC. The white dot marks the most probable TGF location, with white contours outlining the 68% and 95.4% confidence regions. The black curves outline the southern limit of the full field of view of the optical instruments, the white dash-lined box outlines the cropped segment of the image downlinked from the ISS, and the red cross is the location of a coincident lightning event detected by WWLLN (15). (B) The same view zoomed in to the active cloud region. A single thundercloud partially overlaps with the TGF 95% confidence region. (C) The TGF position overlain with a projection of the ASIM camera image in the 337-nm filter; exposure time, 83 ms. The attitude of the ASIM instruments is calibrated to align the WWLLN lightning location with the maximum optical activity of the ASIM image.

The instrument data projections are to 12 km altitude, 1 km below the maximum altitude of the cloud top (Fig. 1C). The pointing direction of the sensors is determined from the ISS attitude and the nominal mounting of the ASIM platform. It is calibrated by assuming that the WWLLN lightning location is at the maximum optical activity of the ASIM image. This correction is ~1.4°, corresponding to 11.8 km at ground level, which also aligns the ASIM image of the cloud with its position in the Himawari data. The WWLLN lightning location has an uncertainty of ~5 km and occurred ~11 ms after the time estimated by the ASIM photometer pulses (15), which is within the ~20 ms uncertainty in the absolute timing signal received by ASIM from the ISS. The TGF source location is estimated from the angle of arrival of 70- to 350-keV photons to the ASIM low-energy x-ray detector (LED). This angle is determined from the shadow pattern cast by a coded mask onto the pixelated detector plane. The relative pointing direction between the LED and the optical sensors has not been calibrated, so we assume its nominal value (co-aligned). We conclude that the optical and x-ray measurements provide a consistent identification of this small (20 km2) convective cloud as the source of the lightning associated with the TGF.

The signals of the three ASIM photometers around the time of the event are shown in Fig. 2. The photometers measure continuum emission in the UV band (180 to 235 nm) covering part of the Lyman‐Birge‐Hopfield system of N2, as well as line emissions from N2 2P at 337 nm (filter width 5 nm) and O i at 777.4 nm (filter width 3 nm), all at sample frequencies of 100,000 Hz. Also shown in Fig. 2 are the observations of the TGF, in photon counts measured by the LED (50 to 350 keV) and the high-energy detector (HED, 300 keV to 30 MeV). The time resolution is 1 μs for the LED and 27 ns for the HED, and the relative timing accuracy between the optical sensors and LED/HED is ±5 μs.

Fig. 2 Light curves of the event.

The gamma-flash trigger time is at t = 0, which corresponds to 13:01:33.100080 UTC. (A) Photometer (left axis) and x-ray and gamma-ray (right axis) measurements around the time of the event. LED is the low-energy x-ray detector (50 to 350 keV) and HED the high-energy detector (300 keV to 30 MeV). The UV photometer measures 180 to 235 nm and is multiplied by 100 to show on the same scale as the optical photometers. All three photometers sample at 100 kHz. (B) The same data shown zoomed in further at the time of the TGF.

The optical activity began several milliseconds before the main optical pulses and intensified at a time t = –200 μs, measured with respect to the onset of the HED pulse, signifying an increase of the lightning current. The onset of the TGF coincided with the onset of the UV emissions (to within 10 μs), and an accelerated increase in the 337-nm and 777.4-nm signals appeared as a change in their slope 0 to 60 μs after the TGF trigger (15). The TGF lasted about 30 to 40 μs and had a weaker tail extending to ~80 μs for the high-energy photons and ~200 μs for the low-energy photons (detected by the HED and LED, respectively), consistent with delays expected from Compton scattering of x-rays and gamma rays in the atmosphere (16).

The emissions at 777.4 nm are from atomic oxygen and therefore from the current in a lightning leader channel, and the 337-nm emissions are mostly from the lightning discharge in the cloud (leader and streamers) (17) with negligible contributions from the ionosphere (i.e., an elve) (18). UV emissions, although strongly damped in the atmosphere, are seen here with an amplitude that rises more rapidly than the optical signals, which is the signature of an elve (18). Elves are quite common emissions (19), ~1 ms in duration, excited by the electromagnetic pulse (EMP) from the large current impulse of cloud-to-ground lightning (20) or intra-cloud lightning (21). The EMP energizes free electrons at the lower edge of the ionosphere at 80 to 90 km altitude, which excite the atmospheric constituents. The radiation pattern from the lightning current onset excites rings of elve emission centered above the lightning (for vertical lightning), which rapidly expand horizontally with detectable radii from ~50 to 400 km (22). The ASIM imaging cameras are not sufficiently sensitive to measure elves because of their short duration and large spatial extent when observed toward the nadir, so we only analyze data from the photometers.

The UV elve was detected with a delay corresponding to the travel times of the EMP to the ionosphere and of UV photons from the ionosphere to the instruments. The shortest combined path is approximately the direct path, considering the geometry of the observations (Fig. 3). We therefore consider the initiation of the UV pulse as coinciding with the initiation of the current pulse, which starts ≲10 μs after the onset of the TGF. The equivalent optical photons are scattered in the cloud, causing the optical pulse to broaden and the peak to be delayed by several hundred microseconds or more, depending on the optical depth of the source and on the cloud properties (23). This is consistent with the UV and optical pulses we observe (Fig. 2B). The characteristics of the optical pulses also resemble those of lightning emissions observed by the Fast On-orbit Rapid Recording of Transient Events (FORTE) satellite (24). This suggests that the current source lifetime is much shorter than the optical pulses, although we cannot further characterize the pulse current from optical measurements alone. The first optical photons to arrive at the sensors are those that have undergone the least scattering in the cloud. A pulse is first detected by ASIM when the flux levels are above the sensor sensitivity thresholds, and the onset of a pulse may therefore appear with a delay. For this event, the optical pulses are bright and rising out of pre-activity; this suggests that a delay from the limits of the sensor sensitivities and of cloud scattering are minor, and therefore that the optical pulses should start at approximately the same time as the UV pulse, as observed.

Fig. 3 Our proposed scenario.

An intra-cloud (IC) lightning event generates a TGF and electromagnetic pulse (EMP). The EMP excites expanding waves of UV emission in the lower ionosphere (elve). TGF and UV emissions are observed by ASIM on the ISS (arrows).

The photometer measurements are shown on a logarithmic scale in Fig. 4. The TGF and the associated high-amplitude pulses occur ~5 ms after the beginning of the lightning flash (Fig. 4B), consistent with studies of radio emissions from TGF-producing flashes (25). The flash fades over ~280 ms, terminating with two smaller optical pulses (Fig. 4A). The onset of the emission at t = –5 ms could reflect the time when an upward-propagating lightning leader first becomes detectable in the cloud, or it may reflect the onset of the formation of the leader. The start of the leader emission is modulated by two larger oscillations (Fig. 4B), possibly related to the leader propagation, and the continuation after the TGF could reflect the continued propagation of the leader upward/horizontally into a positive-charge layer, as was previously proposed (26). However, cloud scattering is too strong to identify the spatial propagation of the leader from the camera images.

Fig. 4 Analysis of the lightning flash.

(A) The same data as Fig. 2 but on a logarithmic scale and smoothed by a Gaussian filter with width σ = 10 samples. Two additional optical pulses occur 200 to 300 ms after the initial flash. (B) The same data zoomed in to the start of the flash smoothed with σ = 2 samples. Optical emission begins 5 ms prior to UV emission and the TGF.

Although TGF-type emissions have been observed from leaders propagating to the ground (27), we reject that scenario as an explanation of this event because of the strong photon absorption that would occur in the lower atmosphere (9). Instead, we interpret this event as an intra-cloud lightning flash of positive polarity (Fig. 3) where the electric field ahead of the leader plays a role in producing the TGF.

The elve requires a high-amplitude lightning current pulse with a fast rise time, which suggests the formation of, or access to, a large charge reservoir within the cloud that is rapidly drained. The reservoir could be formed by the ionizing avalanche of the relativistic electrons or by the formation of an extended streamer corona, as suggested by radio signal studies of TGFs (2830). It is likely that further current amplification is required to produce an elve. Pulses with high currents have been identified in radio observations and have been associated with TGFs (31). They are termed positive energetic in-cloud pulses (+EIPs); they last ~50 μs and may reach hundreds of thousands of amperes (31). +EIPs occur typically in the upper regions of clouds after a few milliseconds of ascending negative leader activity, and it has been suggested that they can generate TGFs and elves simultaneously (31). The TGF in this event is relatively short in duration relative to past observations (25). A short duration was predicted for TGFs accompanied by elves, because elves require lightning source currents that vary more rapidly (31). The optical pulses we observe may then be the counterpart of radio +EIPs.

Our observations illustrate the temporal sequence of emissions in optical, UV, x-ray, and gamma-ray bands with a time resolution of 10 μs and simultaneous imaging of a TGF in hard x-rays and optical images. The observations provide evidence that there is a connection between TLEs and TGFs after all.

Supplementary Materials

Materials and Methods

Supplementary Text

Fig. S1

Data S1 to S5

References (3235)

References and Notes

  1. See supplementary materials.
Acknowledgments: We thank M. Stolzenburg and M. Rycroft for valuable comments, and the World Wide Lightning Location Network for the lightning location data used in this paper. Funding: ASIM is a mission of the European Space Agency (ESA), funded by ESA and national grants of Denmark, Norway, and Spain. ESA PRODEX contracts C 4000115884 (DTU) and 4000123438 support the ASIM Science Data Centre (Bergen). The science analysis is supported by ESA Topical Team contract 4200019920/06/NL/VJ; the European Commission, Innovative Training Network SAINT, project grant 722337-SAINT; European Research Council grant AdG-FP7/2007-2013: n 320839; Research Council of Norway contracts 223252/F50 and 208028/F50 (CoE/BCSS); and Ministerio Ciencia, Innovacion y Universidades grant ESP 2017-86263-C4. Author contributions: T.N. leads the ASIM project; N.Ø. leads the LED and HED instrument consortia, supported by C.B.-J. and I.K.; V.R. leads the LED imaging analysis, supported by J.N.-G. and P.H.C.; O.C. leads the optical instrument consortium, supported by M.H. and K.D.; F.C. performed in-orbit LED health analysis; I.L.R. performed in-flight calibration of the photometers; K.U., G.G., S.Y., P.K., and C.J.E. conducted LED and HED in-orbit commissioning; and M.M. and A.M. performed LED and HED data analysis. Competing interests: The authors declare no competing interests. Data and materials availability: The ASIM data for this event are provided in data S1 to S5. The WWLLN lightning data, cloud data, and ISS attitude data are provided in the supplementary materials.

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