Source Altitude Research Articles

Source Altitude of Energetic In‐Cloud Pulses Inside Thunderstorms and Implication for the Intrinsic Brightness of Terrestrial Gamma‐Ray Flashes

AbstractUpward Terrestrial Gamma‐Ray Flashes (TGFs) are mainly produced during the upward propagating negative leaders inside thunderclouds. The exact source position of TGFs, which is crucial to understanding TGF source properties, is still unclear. The link between positive energetic in‐cloud pulses (+EIPs) and TGFs provides us with a potential target to aim at. In this study, the low‐frequency radio emissions of 75 +EIPs are analyzed to retrieve the source altitudes with an improved ray theory model. Furthermore, the meteorology contexts of +EIPs derived from the ground‐based weather radars and satellite‐based infrared cloud top temperature measurements are investigated. +EIPs are produced at 8.8–13.7 km, with an average of 11.3 km inside thunderclouds, and at an average of ∼2.5 km below cloud tops. These altitudes indicate that a total number of 1.7 × 1016 to 2.6 × 1018 gamma ray photons with energy greater than 1 MeV are required for an EIP‐TGF to be measured by spaceborne detectors.

A theoretical study of a radiofrequency wave propagation through a realistic turbulent marine atmospheric boundary layer based on large eddy simulation

This study aims at modeling and investigating the impact of realistic turbulence inhomogeneity on radiowave propagation by utilizing large eddy simulations (LES). An up-to-date version of the X-LES method for generating realistic turbulent phase screens is first introduced. It combines atmospheric simulations with the classical Tatarski statistical modeling. This method naturally incorporates vertical turbulence inhomogeneity into phase screens at scales resolved by LES. It is then extended to sub-grid scales by weighting statistically generated phase variations with the vertical profile of the turbulent structure constant extracted from atmospheric data. This method is applied to replicate turbulence of a classical tropical marine atmospheric boundary layer. The impact of the generated medium on the propagation of a 10 GHz spherical wave emanating from a Gaussian aperture is analyzed through a statistical study of log-amplitude profiles performed for three different source altitudes. Results first show that contrary to a classical homogeneous turbulence modeling, log-amplitude profiles resulting from the propagation into inhomogeneous turbulence exhibit a statistical heterogeneity strongly dependent of source altitude. Furthermore, classical stochastic phase screen generation from a homogeneous Von-Kàrmàn Kolmogorov spectrum seems to give a statistically significant underestimation of the actual impact of turbulence compared to the X-LES method.

Dependence of Lunar Pickup Ion Flux on Source Location: ARTEMIS Observations

The Moon is enveloped in an exosphere, which is comprised of a variety of neutral atoms and molecules. Once exospheric neutrals are ionized by photons, protons, or electrons from the Sun, the resulting ions are accelerated by the electromagnetic fields of their surroundings and can thereby travel away from their source locations. These ions are the so-called pickup ions and are frequently observed by the two Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft. In this study, we identify 115 events from an 11 yr period of ARTEMIS observations, which contain a total of 11,987 samples for our statistics. By using analytical ion trajectory calculations, we trace the source location of each pickup ion observation. Most pickup ion trajectories originate near the subsolar point, consistent with the efficiency of sputtering. We find that the flux of pickup ions strongly anticorrelates with the source altitude, providing indirect evidence of decreasing exospheric ion flux with increasing altitude. We also find that the flux of pickup ions does not show a significant relationship with the crustal magnetic field intensity. This implies that a depression of sputtering efficiency or the trapping of near-surface freshly born ions by a crustal magnetic anomaly may not reduce the subsequent pickup ion flux as effectively as expected. In summary, the present paper provides a statistical view of lunar pickup ion fluxes in association with the altitude, local time, and local crustal magnetic field of their source locations.

Very high frequency radiation emitted by negative narrow bipolar events occurred over malacca strait

In this paper, Very-High Frequency (VHF) radiation pulses associated with 11 negative Narrow Bipolar Events (NBEs) produced by a tropical storm over Malacca Strait are examined. The lightning data were recorded from a measurement station (ST) which consisted of a fast antenna (FA) and three VHF sensors (two 5 m perpendicular baselines interferometer). The average rise time (RT), average zero-crossing time (ZCT), average pulse duration (PD), and range of peak currents of the negative NBEs were 1.4 ± 0.4 μs, 2.7 ± 1.0 μs, 12.0 ± 6.9 μs, and −10 to −64 kA, respectively. The key finding is that all VHF radiation pulses have been found to precede the negative NBEs with an average lead time of 0.7 ± 0.3 μs. An interferometer map for one negative NBE (labelled as NBE10) detected at 35.7 km from ST has shown a characteristic of mixed propagation direction of fast streamers. The first VHF radiation source was detected at 12.4 ± 0.4 km above sea level. The total length and estimated velocity of the main propagation of the VHF radiation sources were 2.2 ± 0.7 km and between 1.4 × 108 and 2.8 × 108 ms−1, respectively. Moreover, based on the Himawari satellite image, the maximum extent of the cloud top height was estimated to be around 20.9 km over sea level (over Malacca Strait). All the VHF radiation sources associated with NBE10 were suggested to be detected above the main negative charge region (6 km altitude that corresponds to −10 °C). Thus, it could be suggested that NBE10 was initiated most likely in the environment of the ice crystals alone, based on the first altitude of the VHF radiation source and maximum extent of cloud top height.

Determining a Lower Limit of Luminosity for the First Satellite Observation of a Reverse Beam Terrestrial Gamma Ray Flash Associated With a Cloud to Ground Lightning Leader

AbstractWe provide an updated analysis of the gamma ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma ray Burst Monitor first reported by Pu et al. (2020, https://doi.org/10.1029/2020GL089427). A TGF produced 3 ms prior to a negative cloud‐to‐ground return stroke was close to simultaneous with an isolated low‐frequency radio pulse during the leader’s propagation, with a polarity indicating downward moving negative charge. In previous observations, this “slow” low‐frequency signal has been strongly correlated with upward‐directed (opposite polarity) TGF events (Pu et al., 2019, https://doi.org/10.1029/2019GL082743; Cummer et al., 2011, https://doi.org/10.1029/2011GL048099), leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward‐directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined that the most likely source altitude of the TGF reverse beam was 7.5 km ± 2.6 km, just below an estimated negative charge center at 8 km. At that altitude, the Monte Carlo simulations indicate a lower luminosity limit of 2 × 1018 photons above 1 MeV for the main downward beam of the TGF, making the reverse beam detectable by the Fermi Gamma ray Burst Monitor.

Tracing the geographical origin of bottled water collected in South Korea via isotopic and elemental analyses.

The production of bottled water requires a forensic discriminant technique that enables the identification of the brands or accidents caused by intended contaminants. The bottled water poisoning crimes have drawn much attention, and such crimes may recur in the future. The water is colorless and odorless, and thus it is difficult to detect contaminated water through visual observation. Thus, bottled water can be easily exploited for poisoning, and a method for tracing their origin is currently required. In this study, a total of 27 brands of bottled water samples were analyzed to determine stable oxygen isotopes, strontium isotopes, major and trace elements. The geographical origin of the water was traced based on the climatic and geographical characteristics of the location from where water was sourced, which was assumed to be reflected in the bottled water. Furthermore, we investigated whether this method can be applied to identify bottled water products. The results demonstrated that the characteristics of the bottled water, including the oxygen stable isotope ratios, reflect the latitude and altitude of bottled water source in South Korea, from the high-latitude region to the coastal regions. In addition, the results indicated that excellent discrimination was achieved using strontium isotopes to identify source areas with different types of bedrock, complex underlying lithologies, and ocean areas in South Korea. A statistical method based on discriminant analysis was applied to measure trace elements, and the results effectively reflected the characteristics of water-rock interactions (cross-validated classification probability: ≥92%). These data suggest that the geographical characteristics of the source area are well reflected in commercial bottled water in South Korea. The proposed analytical methods can be utilized to trace the geographical origin of different bottled water samples and identify bottled water products used in poisoning crimes.

Geolocation of a known-altitude moving source using TDOA and FDOA in the presence of altitude error

Using precise source altitude prior information can significantly improve the source geolocation accuracy. However, in practice, the available source altitude may contain certain errors due to the movement of the source, which can affect the geolocation accuracy. This paper considers geolocating a moving source using time difference of arrival (TDOA) and frequency difference of arrival (FDOA) measurements in the presence of altitude error. The geolocation performance when the known altitude is assumed accurate but in fact has errors is investigated analytically. An efficient closed-form estimator is then developed for localization. The analysis results validate that the proposed closed-form estimator can attain the constrained Cramér-Rao lower bound (CCRLB) over small noise regions. Simulations show good agreement with the theoretical developments and demonstrate the good performance of the proposed geolocation method.

FORTE Measurements of Global Lightning Altitudes

AbstractWhile multiple lightning detection systems provide geographical locations of lightning events across the globe, robust lightning altitude measurements on a global scale have proven elusive. Space‐based platforms have an advantageous viewing geometry for making these measurements, but prior studies with the Fast On‐orbit Recording of Transient Events (FORTE) satellite were limited to a few thousand events. In this study, we apply the same technique for calculating source altitude from the previous efforts to a large catalog of hundreds of thousands of global FORTE in‐cloud lightning events that were coincident with flashes geolocated by its lightning imager between 1997 and 2003. We use this new data set to document global variations in lightning altitude. As in previous studies, we find that FORTE primarily resolves sources from the upper (positive) charge layer at ∼11 km altitude in normal thunderstorms. However, sources are also recorded from other charge layers in the storm and from leaders developing between layers. In particular, we note a pronounced increase in source altitude in the first 20 ms of FORTE flashes from the negative leader developing upward into the upper positive charge layer. Regions known for wintertime and/or stratiform lightning have increased contributions from low‐altitude sources, while tropical regions particularly around Panama and the Maritime Continent have the greatest concentrations of high‐altitude sources.

Estimate of the source parameters of terrestrial gamma-ray flashes observed at low-Earth-orbit satellites

A special attention has been paid in the past decades to studies of terrestrial gamma-ray flashes (TGFs) observed above active thunderstorms. The physical mechanism of the TGF generation and its source location in the atmosphere have not been firmly established. A numerical modelling, such as Monte Carlo simulation, is commonly used to analyze the problem having regard to the complexity of basic electrodynamic and transport equations. Here, in contrast to previous numerical studies, we have constructed a suitably idealized analytical model of a point source of gamma-rays in a vertically inhomogeneous atmosphere. An energy-distribution of gamma photons is assumed to be determined by the energy spectrum of electron bremsstrahlung resulted from the relativistic runaway electron avalanche in a strong electric field. The absorption of photon energy due to photoelectric effect, Compton scattering and the electron-positron pair production have been accounted for approximately with an effective coefficient of energy absorption. A photon mean free path in the atmosphere is assumed to be dependent on its energy and altitude. A spatiotemporal distribution of gamma-ray flux density and photon fluence at a low-Earth orbit (LEO) are estimated as functions of the TGF source altitude, total number of photons emitted by the source, and other parameters. The model matches the LEO observations indicating that the TGF source is located at an altitude about 10−14 km. The same model can be applied for the description of the recently found downward-directed TGF detected by the large-area Telescope Array cosmic ray observatory.

Geolocation a known‐altitude moving source by TDOA and FDOA measurements

Source altitude information, if known or can be measured by an altimeter, can greatly improve localisation performance. In this letter, a novel algebraic solution for known-altitude moving source geolocation using time difference of arrival and frequency difference of arrival measurements is developed. The localisation problem is solved via the successive unconstrained minimisation method. Simulation results demonstrate that the estimation accuracy of the source position and velocity can achieve the constrained Cramér–Rao lower bound at moderate noise levels.

The Relationship of Lightning Radio Pulse Amplitudes and Source Altitudes as Observed by LOFAR.

When a lightning flash is propagating in the atmosphere it is known that especially the negative leaders emit a large number of very high frequency (VHF) radio pulses. It is thought that this is due to streamer activity at the tip of the growing negative leader. In this work, we have investigated the dependence of the strength of this VHF emission on the altitude of such emission for two lightning flashes as observed by the Low Frequency ARray (LOFAR) radio telescope. We find for these two flashes that the extracted amplitude distributions are consistent with a power‐law, and that the amplitude of the radio emissions decreases very strongly with source altitude, by more than a factor of 2 from 1 km altitude up to 5 km altitude. In addition, we do not find any dependence on the extracted power‐law with altitude, and that the extracted power‐law slope has an average around 3, for both flashes.

The Illumination of Thunderclouds by Lightning: 3. Retrieving Optical Source Altitude.

Optical space‐based lightning sensors such as the Geostationary Lightning Mapper (GLM) detect and geolocate lightning by recording rapid changes in cloud top illumination. While lightning locations can be determined to within a pixel on the GLM imaging array, these instruments are not individually able to natively report lightning altitude. It has previously been shown that thunderclouds are illuminated differently based on the altitude of the optical source. In this study, we examine how altitude information can be extracted from the spatial distributions of GLM energy recorded from each optical pulse. We match GLM “groups” with Lightning Mapping Array (LMA) source data that accurately report the 3‐D positions of coincident Radio‐Frequency (RF) emitters. We then use machine learning methods to predict the mean LMA source altitudes matched to GLM groups using metrics from the optical data that describe the amplitude, breadth, and texture of the group spatial energy distribution. The resulting model can predict the LMA mean source altitude from GLM group data with a median absolute error of <1.5 km, which is sufficient to determine the location of the charge layer where the optical energy originated. This model is able to capture changes to the source altitude distribution in response to convective processes in the thunderstorm, and the GLM predictions can reveal the vertical structure of individual flashes ‐ enabling 3‐D flash geolocation with GLM for the first time. Future work will account for differences in thunderstorm charge/precipitation structures and viewing angle across the GLM Field of View.

The Illumination of Thunderclouds by Lightning: 4. Volumetric Thunderstorm Imagery

AbstractOptical instruments such as the Geostationary Lightning Mapper (GLM) detect lightning based on transient changes in cloud illumination. The horizontal location of lightning is determined from the coordinates of the pixels on the imaging array illuminated during the flash. However, the vertical position of the lightning pulses (approximated by GLM “groups”) below the cloud top cannot be routinely measured from a single space‐based instrument. In our prior work, we have developed a machine learning algorithm that can infer optical source altitude for a given pulse based on how the optical energy is distributed across the group footprint and the local Advanced Baseline Imager Cloud‐Top Height (CTH). In this fourth part of our thundercloud illumination study, we leverage these source altitudes to generate volumetric GLM imagery of a Colombia thunderstorm. We find that 3D versions of the current GLM meteorological imagery products (that describe thunderstorm kinematics) and thundercloud imagery products (that depict how the flashes appear from space) provide additional insights into lightning activity in the thunderstorm that are lost in the vertical integration used to generate the current 2D GLM gridded products. This new volumetric imaging capability provides a more comprehensive picture of where lightning occurs in the storm, how its physical characteristics vary across three‐dimensional space, and how its optical emissions interact with surrounding the cloud medium.

Spectral Analysis of Individual Terrestrial Gamma‐Ray Flashes Detected by ASIM

AbstractThe Atmosphere‐Space Interactions Monitor (ASIM) is the first instrument in space specifically designed to observe terrestrial gamma‐ray flashes (TGFs). TGFs are high energy photons associated with lightning flashes and we perform the spectral analysis of 17 TGFs detected by ASIM. The TGF sample is carefully selected by rigorous selection criteria to keep a clean sample suitable for spectral analysis, that is, suitable count statistics, low instrumental effects, and reliable source location. Monte Carlo modeling of individual TGFs has been compared to the observed energy spectra to study the possible source altitudes and beaming geometries. A careful model of the instrumental effects has been developed and validated. Several combinations of source altitudes and beaming geometries are accepted by the statistical tests for all the TGFs in the sample resulting in a large uncertainty in the estimate of the intrinsic source luminosity. The analyzed TGFs show significant variations in observed fluence independent of the distance between source and ASIM. A lower limit on the maximum photon energy produced by TGFs is estimated to be 24 MeV for the analyzed TGFs. The intrinsic limitations of TGF spectral analysis from space are also investigated and it is found that the ability to constrain the source altitude and beaming geometries of TGFs strongly depends on the distance between source and satellite. With the current generation of instruments with effective areas in the range of few hundreds cm2, it is very difficult to constrain reliably the source properties without the help of simultaneous measurements in the radio band.

Isolated Peak of Oxygen Ion Fraction in the Post-Noon Equatorial F-Region: ICON and SAMI3/WACCM-X.

In the equatorial region, the fraction of oxygen ions (O+) in the topside ionosphere contains information on the source altitude of the plasma, which is controlled, in part, by the vertical plasma motion in the F-region. Previous studies on this topic are restricted by limited coverage of local time, latitude, and season, leaving a significant knowledge gap in the distribution of the topside ionospheric composition. In this study, we statistically investigate the O+ fraction measured by ICON/IVM over all the local time sectors and seasons at low/midlatitudes. For the first time, we have found that an isolated peak in the O+ fraction emerges in the post-noon equatorial region. The peak is most prominent during equinoxes, while during solstices it is connected to the O+ fraction bulges in the local summer midlatitudes. Simulations with SAMI3 coupled with thermospheric parameters from WACCM-X reproduce the peak of the O+ fraction. The post-noon equatorial peak can be explained by the net vertical motion of plasma consisting of transports either parallel or perpendicular to geomagnetic field lines.

Abnormal Phase Structure of Thermal Tides During Major Dust Storms on Mars: Implications for the Excitation Source of High‐altitude Water Ice Clouds

AbstractInteractions among the thermal tide, airborne dust, and water ice clouds significantly influence the Martian atmosphere. The absence of a stratosphere on Mars allows easier identification of the thermal tides from observational data than that on Earth, making Mars a natural laboratory to study the tidal excitation and propagation processes. However, the global tidal phase structure and its response to the tidal excitation sources on Mars have not been studied from observations. Here, we present a comprehensive analysis of the phase structure of migrating diurnal tide (DW1) using multi‐local‐time observations of the Mars Climate Sounder. The DW1 phase at low‐to‐mid latitudes shows a downward phase progression, which indicates upward tidal energy and momentum propagation throughout the year but with an apparent hemispheric asymmetry. The entire DW1 phase structure rises and falls with the dust height at the southern mid‐latitudes suggesting a modulation of vertical dust extent on the altitude of the excitation source. During major regional dust storms, the DW1 phase propagates anomalously upward from 10 to 1 Pa in the Southern hemisphere. The Hough analysis suggests that the DW1 in the dust layer can only propagate upward to 10 Pa while the downward propagating mode above 10 Pa may be excited from a high‐altitude source. Evidence suggests that the enhanced high‐altitude water ice clouds could be a potential excitation source for these downward propagating tidal modes. These findings demonstrate how thermal tides can locally behave like inertia‐gravity waves and the importance of water ice clouds to Martian atmospheric dynamics.

Possible Transient Luminous Events Observed in Jupiter's Upper Atmosphere

AbstractEleven transient bright flashes were detected in Jupiter's atmosphere using the ultraviolet spectrograph instrument on the Juno spacecraft. These bright flashes are only observed in a single spin of the spacecraft and their brightness decays exponentially with time, with a duration of ∼1.4 ms. The spectra are dominated by H2 Lyman band emission and based on the level of atmospheric absorption, we estimate a source altitude of 260 km above the 1‐bar level. Based on these characteristics, we suggest that these are observations of transient luminous events (TLEs) in Jupiter's upper atmosphere. In particular, we suggest that these are elves, sprites or sprite halos, three types of TLEs that occur in the Earth's upper atmosphere in response to tropospheric lightning strikes. This is supported by visible light imaging, which shows cloud features typical of lightning source regions at the locations of several of the bright flashes. TLEs have previously only been observed on Earth, although theoretical and experimental work has predicted that they should also be present on Jupiter.

Blue Optical Observations of Narrow Bipolar Events by ASIM Suggest Corona Streamer Activity in Thunderstorms

AbstractWhile narrow bipolar events (NBEs) could be related with lightning initiation, their intrinsic physics remains in question. Here we report on optical measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station (ISS) of blue flashes associated with NBEs. They are observed in a narrow blue band centered at 337 nm, with no simultaneous activity at 777.4 nm, considered a strong lightning emission line. From radio waves measured from the ground, we find that 7 of 10 single‐pulse blue events can be identified as positive NBEs. The source altitudes estimated from optical and radio signals agree and indicate that the sources of the blue flashes are located between∼8.5 and∼14 km, in a cloud reaching 14–15 km altitude. The observations suggest that single‐pulse blue flashes are from cold ionization waves, so‐called streamers, and that positive NBEs are corona discharges formed by many streamers.

On the Accuracy of Ray-Theory Methods to Determine the Altitudes of Intracloud Electric Discharges and Ionospheric Reflections: Application to Narrow Bipolar Events.

Narrow bipolar events (NBEs) (also called narrow bipolar pulses [NBPs] or compact intracloud discharges [CIDs]) are energetic intracloud discharges characterized by narrow bipolar electromagnetic waveforms identified from ground‐based very low frequency (VLF)/low‐frequency (LF) observations. The simplified ray‐theory method proposed by Smith et al. (1999, https://doi.org/10.1029/1998JD200045; 2004, https://doi.org/10.1029/2002RS002790) is widely used to infer the altitude of intracloud lightning and the effective (or virtual) reflection height of the ionosphere from VLF/LF signals. However, due to the large amount of high‐frequency components in NBEs, the propagation effect of the electromagnetic fields for NBEs at large distance depends nontrivially on the geometry and the effective conductivity of the Earth‐ionosphere waveguide (EIWG). In this study, we investigate the propagation of NBEs by using a full‐wave Finite‐Difference Time‐Domain (FDTD) approach. The simulated results are compared with ground‐based measurements at different distances in Southern China, and we assess the accuracy of the simplified ray‐theory method in estimating the altitude of the NBE source and the effective reflection height of the ionosphere. It is noted that the evaluated NBE altitudes have a slight difference of about ±1 km when compared with the full‐wave FDTD results, while the evaluated ionospheric reflection heights are found to be bigger than those obtained from FDTD model by about 5 km.

On the Linear Polarization of TGFs and X‐Rays From Natural and Rocket‐Triggered Lightning and Its Association With Source Geometry

AbstractTerrestrial gamma‐ray flashes (TGFs) are bright bursts of gamma rays produced by thunderstorms, typically observed by spacecraft in the low‐Earth orbit. Unfortunately, it has been difficult to disentangle the source altitude and the width and direction of the gamma‐ray beam using single point spacecraft measurements, which has hampered attempts to constrain TGF models. Polarimetry of astrophysical sources has been of interest for many decades, which raises the question: Do TGFs and X‐rays from lightning have observable polarization, and if so, what would this polarization tell us about their source? REAM Monte Carlo code has been modified to record the linear polarization of X‐rays and gamma rays as a function of source altitude and beam geometry. It is found that polarization degree of a 20‐km narrow beam of TGF is substantially different from a 15‐km‐wide beam, which could be used to constrain the source geometry of TGFs. However, due to the low fluence of these events in space, detecting this level of polarization would be challenging. It is also found that low‐altitude TGFs (source at 3.5 km) produce polarizations up to about 8%; however, detectors need to be very close to the source region. Furthermore, very low altitude ground‐level TGFs and X‐rays showed a maximum polarization of 13% on the ground, of which the TGF's fluence was large enough for polarimetry. In addition, polarization reached its maximum further away from the z axis as the TGF's beam broadened. The dominant mechanism of the polarization was found to be Compton scattering.