Earth experienced the strongest geomagnetic storm in 20 years over 10–13 May 2024. The Ap and Dst geomagnetic indices were 273 and −291.94 nT on 11 May. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite observed significant enhancement in thermospheric infrared emission at 15, 5.3, 4.3, and 1.27 μm. On 11 May the daily global power radiated by nitric oxide (NO) at 5.3 μm was 1.41 TW and by carbon dioxide (CO2) at 15 μm was 1.35 TW. These are the largest single day power values observed by SABER in 22 years and the first time the daily power radiated by NO exceeded that of CO2. The total infrared power (above background) radiated during the storm was 2.64 TW (2.28 × 1017 J). Significant enhancement in limb radiance observed at 4.3 μm (to 250 km tangent height) is likely indicative of NO + formation during the storm.

Abstract Time series of radiative cooling of the upper mesosphere and lower thermosphere (UMLT) by carbon dioxide (CO2) are examined for evidence of trends over 20 years. Radiative cooling rates in K day−1 provided by the SABER instrument are converted to time series of infrared power radiated from three distinct layers between 0.1 hPa and 0.0001 hPa (65–105 km). Linear regression against time and a predictor for solar variability provides estimates of the trend in exiting longwave radiation (ELR) from these layers. Trends in ELR are not significantly different from zero at 95% or 99% confidence in each layer. These results demonstrate energy conservation in the UMLT on decadal time scales and show that the UMLT continues to radiate the same amount of energy it receives despite cooling and contracting over two decades. These results are enabled by the long‐term stability of the SABER instrument calibration.

K-changes are step-like electrostatic field changes, which occur during the final part of cloud flashes or between the return strokes in cloud-to-ground discharges. We numerically solve the full set of Maxwell’s equations coupled to the electrostatic Poisson’s equation for a given thundercloud charge structure to model the K-changes. We simulate the K-changes by a sequential increase of conductivity of the decayed vertical channel. This process creates a current pulse which attenuates as it propagates downward. We show how the modeled linear charge densities and electric potentials connected to K-changes evolve in time. We successfully compare our model with the electric field measured by a flat-plate E-change antenna with a sensor having a decay time constant of 1 s, a bandwidth of 0.16 Hz –2.6 MHz, and a sampling rate of 5 MS/s. The experimental data used for comparison with our model were obtained at KSC Florida in 2011.

We analyze periodic features in the F10.7 cm solar radio flux and the Ap geomagnetic index to examine solar-terrestrial coupling in Earth’s atmosphere above 100 km. The coupling is indicated by the same periodic features observed in the global infrared power radiated by Nitric Oxide (NO) and Carbon Dioxide (CO2). The infrared power data have been measured by the SABER instrument on the NASA TIMED satellite since January 2002. A strong dominant feature for the length of the solar cycle (≈11 years) can be observed in all the datasets. The length of this periodicity can vary from 9.95 years to 12.25 years for different solar cycles. Periodic features larger than the length of the solar cycle are also observed in the F10.7 and Ap index (≈22 years and ≈44 years). A strong 180-day periodic feature can be observed in the Ap index which corresponds to the well-known Russell-McPherron effect. Additionally, the NO power and the Ap index exhibit statistically significant periodic features for shorter periods (e.g., 27, 13.5 and 9 days). This similarity indicates a strong link between the thermosphere’s infrared energy budget and the geomagnetic conditions of the upper atmosphere.

Abstract Twenty‐two years (2002–2023) of infrared radiative cooling rate data derived from the SABER instrument on the NASA TIMED satellite are presented. Global daily and global annual infrared power (Watts, W) emitted by nitric oxide (NO) and carbon dioxide (CO2) illustrate the variability of the geospace environment on timescales from days to decades. The 11‐year solar cycle (SC) is evident in the global power data and in vertical profiles of infrared cooling rates (nW/m3). The global annual power radiated by NO and CO2 are larger in 2023 than at any time since 2003 and 2002, respectively. The to‐date peak in NO infrared power in SC 25 is larger than in SC 24, is comparable to SC 20, but is less than in SCs 18–19 and 21–23. Two geomagnetic storms in 2023 radiated more than 1 TW and are in the top 10 strongest storms observed by SABER.

Using electric field change data and VHF data of 201 positive Narrow Bipolar Events (+NBEs) detected at close range (< 80 km) at seven sensor sites, three aspects of +NBEs are investigated: the VHF "Development Time" (∆T0), the detailed VHF waveform, and the VHF "Major Trailing Pulses" (MTPs) that follow some +NBEs. The +NBE Development Time study used 137 + NBEs divided into two groups: 72 INBEs (+NBEs that initiate a full lightning flash) and 65 Not-INBEs. These 137 + NBEs had VHF peak powers ranging from 0.1 to 66 kW and ∆T0 values ranging from <1 μs to 23.4 μs with an average of 4.2 μs. The +NBEs with higher peak power (> 20 kW) were all found to have short ∆T0 (< 4 μs), while lower power NBEs (< 7 kW) exhibit the full range of ∆T0 values. The detailed study of VHF waveforms of 201 + NBEs shows that the shape of each +NBE is significantly different at the multiple sensors sites. This finding supports mechanisms in which an NBE is caused by numerous streamers occurring within a few μs. Among 201 + NBEs, 67 + NBEs (with VHF powers of 400–37,500 W) were followed, within 30 μs, by at least one MTP; 39 of these 67 NBEs had a second MTP and 16 of those 39 NBEs had a third MTP. All but one MTP had VHF peak power between 5 W and 270 W; the exceptional case, a single MTP occurring after an NBE with peak VHF power of 6.5 kW, had VHF power of 3.3 kW. MTPs were found to occur from 4.9 to 32 μs after the +NBE or preceding MTP, and they are hypothesized to be evidence of re-radiation in the previously ionized NBE region that is (repeatedly) excited by reflection from an above-cloud ion layer.

The ability of satellite instruments to accurately observe long-term changes in atmospheric temperature depends on many factors including the absolute accuracy of the measurement, the stability of the calibration of the instrument, the stability of the satellite orbit, and the stability of the numerical algorithm that produces the temperature data. We present an example of algorithm instability recently discovered in the temperature dataset from the SABER instrument on the NASA TIMED satellite. The instability resulted in derived temperatures that were substantially colder than anticipated from mid-December 2019 to mid-2022. This algorithm-induced change in temperature over one to two years corresponded to the expected change over several decades from increasing anthropogenic CO2. This paper highlights the importance of algorithm stability in developing Geospace Data Records (GDRs) for Earth’s mesosphere and lower thermosphere. A corrected version (Version 2.08) of the temperatures from SABER is described.

A newly detected feature of some positive Narrow Bipolar Events (+NBEs) is investigated, with the goal of using the new information to help determine the mechanism of NBEs. The investigations use data collected with a seven-station lightning sensor array spread over 60 km × 30 km near Oxford, Mississippi, USA. Each station was equipped with four electromagnetic sensors, including Fast antenna (FA, with 10 ms decay time), Slow antenna (1.0 s decay time), electric field derivative sensor (dE/dt), and LogRF antenna (VHF bandwidth 186-192 MHz).NBEs are typically considered to be short duration, isolated events on a time scale of ~ 10 -30 µs, although some NBEs apparently initiate a full lightning flash that begins within 10 ms of the NBE. However, we found that 67 of 201 +NBEs were followed by 1-3 VHF pulses (herein termed Major Trailing Pulses or MTPs); these MTPs occurred within 100 µs after the NBE. The VHF powers of the 67 NBEs with MTPs averaged 7700 W with a range of 370 – 37,000 W. For 66 of the 67 NBEs with MTPs, the MTPs had VHF powers between 4 W and 270 W. The other NBE (with a power of 6500 W) had a single MTP with VHF power of 3300 W.In this presentation examples of +NBEs with Major Trailing Pulses are presented, and possible causes for these features are discussed.

Optical data are presented for two intracloud flashes in which there were five events having large amplitude, bipolar electric field change (E-change) pulses and strong VHF emissions. These bright events occurred 18.98-67.33 ms after the initiating event and 10-59 ms after the end of the initial breakdown stage. The three largest events were coincident with WorldWide Lightning Location Network detections of the sort previously associated with terrestrial gamma-ray flashes (TGFs); these pulses had range-normalized E-change amplitudes of 8.73, 8.33, and 3.81 V/m and estimated peak current magnitudes of 262, 250, and 114 kA. The other two events were 3.20 and 1.62 V/m (96 and 49 kA). All five events have bright enhanced luminosity (> 10% above back-ground) for durations of 0.7-1.20 ms, similar to durations of their moderate-to-strong VHF emissions. Full-frame peak intensities are factors of 1.82-4.46 times the background level. Maximum cumulative intensity in the camera frame occurs 84-98 mu s after the E-change peak in three cases and ranges from-10 mu s (before) to +115 mu s (after) for all cases. Notably, in all five events the luminosity starts increasing when the E-change and VHF sensors begin detecting oscillations, 135-550 mu s before the E-change peak. Pulse locations of each event extend through 3.6 to 4.3 km depth, starting below and ending above the altitude of flash initiation, and the linear path-length of activity with these events is estimated at 8.9-11.3 km within radar reflectivities of 20-40 dBZ. Although these events are not initial breakdown pulses, their luminosity could be visible from satellite-borne instruments and they may be associated with TGFs.

A mechanism for increased fluxes of high energy particles detected in Thunderstorm Ground Enhancements (TGEs) is suggested. When the electric field Eambient at the ground beneath thunderstorms reaches modest magnitudes (6 -12 kV/m), corona discharge occurs from the tips of some grounded conducting pointy objects (GCPOs). Past theory and modeling of prolate semi-ellipsoidal GCPOs suggest the corona threshold value for free electron availability via seed electron detachment is E = 67 V/m/Pa at the tip. The field enhancement, Etip/Eambient, at the GCPO tip depends only on its length:tip radius of curvature ratio. Thus, Etip at all GCPOs is substantially enhanced by Eambient, whether or not they are in corona. Cosmic ray electrons striking these pointy tips will gain MeV energies. These high energy electrons may be detected with instrumentation located nearby, or they may produce gamma rays via bremsstrahlung within the tips before being detected, thereby contributing to TGEs.

This study describes results from video observations of five intracloud flashes located ≤ 20 km from the camera and recorded with 6.1 µs exposure time and 6.66 µs frame intervals. Video data are supported with electric field change (E-change) and VHF measurements, with emphasis on the flash initiating event (IE) and initial breakdown (IB) stage. In four of the five flashes, the IE is accompanied by weak luminosity, ≤ 5% above background, lasting for 300–500 µs. Two of these four IEs were positive Narrow Bipolar Events (NBEs) with VHF powers of 43 and 990 W; these are the first (known) data showing visible light detected with a positive NBE. Two other IEs with weak luminosity had powers of 0.5 and 1 W, and the IE with no detected luminosity had a VHF power of 3 W. A typical IB cluster consists of several narrow pulses and one classic pulse in E-change data (along with many VHF pulses), and each example flash has 2–10 IB clusters in the first 5–50 ms. The luminosity of IB clusters was substantially greater than IE luminosity, ranging from 10 to 40% above background in four examples, while for one flash with 10 IB clusters, the luminosity range was 35–360% above background (average 190%). Luminosity durations of IB clusters were 520–1750 µs with average 1210 µs. For both IEs and IB clusters, increases in the detected luminosity were closely timed with substantial VHF emissions and decreased when VHF emissions weakened.

This investigation is focused on groups of Narrow Bipolar Events (NBEs), defined as NBEs that occurred within 10 km horizontally and ±660 ms of a located, large-amplitude NBE from a dataset of positive NBEs that occurred in Mississippi thunderstorms. In two months only 15 groups were found, with a total of 31 positive and 4 negative NBEs. Each group had 2 to 5 NBEs; four groups had both positive and negative polarity NBEs. About half of the NBEs had typical values for range-normalized fast antenna (FA) electric field change magnitudes (4–15 V/m) and typical VHF powers (1000–45,000 W), but 17 NBEs had FA magnitudes 0.2–2.5 V/m, and 17 NBEs had VHF powers 30–900 W. Seven weak NBEs had FA magnitudes of 0.2–1.0 V/m and VHF powers of 30–100 W. These findings indicate that weak NBEs are more common than previously thought. None of the NBEs in groups initiated a lightning flash, and (with one possible exception) none of the later NBEs in a group were initiated by earlier NBEs in the group. The data of the NBE groups are consistent with the turbulence-extensive air shower (EAS)/relativistic runaway electron avalanche (RREA) mechanism, which states that each NBE occurs in a separate 1-km3 volume containing many small regions with electric field ≥3 MV/(m∙atm); an EAS/RREA passing through the 1-km3 volume initiates the positive streamers that comprise the NBE. Relative to thunderstorm radar reflectivity, 23 NBEs occurred in or above the reflectivity core, 10 NBEs occurred high in the storm anvil, and 2 NBEs occurred beside the storm core. We speculate that the occurrence of many of the NBE groups was associated with dynamically intense convection.

High‐speed video and electric field change data are used to describe the first 5 ms of a negative cloud‐to‐ground flash. These observations reveal an evolution in character of the luminosity and electric field change pulses as two branches of the leader separately transition from initial leader to propagating as a negative stepped leader (SL). For the first time reported, there is evidence of weak luminosity coincident with the initiating event, a weak bipolar pulse 60 μs prior to the first initial breakdown (IB) pulse. During the IB stage, the initial leader advances intermittently at intervals of 100–280 μs, in separate light bursts that are bright for a few 20‐μs frames and are time coincident with IB pulses. In the intervals between IB pulses, the initial leader is dim or invisible during the earliest 1.8 ms. Within 2 ms, the leader propagation begins transitioning to an early SL phase, in which the leader tip advances at more regular intervals of 40–80 μs during relatively dim and brief steps which are coincident with SL pulses having short duration, small amplitude, and typically unipolar waveform. These data indicate that when the entire initial leader length behind the lower end begins to remain illuminated between bursts, the propagation mode changes from IB bursts to SL steps, and the IB stage ends. The results support a hypothesis that the early initial leader development occurs in the absence of a continuously hot channel, thus the initial leader propagation is physically unlike the self‐propagating SL advance.

The properties of the first 5-12 classic initial breakdown pulses (IBPs) of three cloud-to-ground (CG) lightning flashes were determined using a modified transmission line model. As part of the modeling, the current with respect to time of each IBP was determined from the measured electric field changes at multiple sites using three theoretical methods called Hilbert transform, Hertzian dipole, and matrix inversion. In the transmission line modeling the length of each IBP was estimated from high-speed video data of the IBPs. The modeling provided the following properties of the larger classic IBPs in each flash: peak current, velocity, total charge, charge moment, radiated power, and total energy dissipated for successive IB pulses in three developing lightning flashes. For the main initial leader channel in the three CG flashes (and for one long branch), IBP peak current was largest for the first or second classic IBP and declined mostly monotonically with successive IBPs. For the same channels, IBP current velocity was smallest for the first classic IBP and increased mostly monotonically with successive IBPs. The smallest velocities were (2.0, 2.5, 2.5, 3.0) x 10(7) m/s, respectively, while the largest velocities were (9.2, 12.2, 11.5, 12.0) x 10(7) m/s, respectively. These data support earlier hypotheses that it is the classic IB pulses during initial leader of normal negative CG flashes that change the nonconductive air into an ionized path that is sufficiently long and conductive to start the stepped leader.

Time-correlated high-speed video and electric field change data for 139 natural, negative cloud-to-ground (CG)-lightning flashes reveal 615 return strokes (RSs) and 29 upward-illumination (UI)-type strokes. Among 121 multi-stroke flashes, 56% visibly connected to more than one ground location for either a RS or UI-type stroke. The number of separate ground-stroke connection locations per CG flash averaged 1.74, with maximum 6. This study examines the 88 subsequent strokes that involved a subsequent stepped leader (SSL), either reaching ground or intercepting a former leader to ground, in 61 flashes. Two basic modes by which these SSLs begin are described and are termed dart - then - stepped leaders herein. One inception mode occurs when a dart leader deflects from the prior main channel and begins propagating as a stepped leader to ground. In these ‘divert’ mode cases, the relevant interstroke time from the prior RS in the channel to the SSL inception from that path is long, ranging from 105 to 204 ms in four visible cases. The alternative mode of SSL inception occurs when a dart leader reaches the end of a prior unsuccessful branch—of an earlier competing dart leader, stepped leader, or initial leader—then begins advancing as a stepped leader toward ground. In this more common ‘branch’ mode (85% of visible cases), there may be no portion of the subsequent RS channel that is shared with a prior RS channel. These two inception modes, and variations among them, can occur in different subsequent strokes of the same flash.

Based on experimental results of recent years, this article presents a qualitative description of a possible mechanism (termed the Mechanism) covering the main stages of lightning initiation, starting before and including the initiating event, followed by the initial electric field change (IEC), followed by the first few initial breakdown pulses (IBPs). The Mechanism assumes initiation occurs in a region of ~1 km3 with average electric field E > 0.3 MV/(m·atm), which contains, because of turbulence, numerous small “Eth volumes” of ~10−4–10−3 m3 with E ≥ 3 MV/(m·atm). The Mechanism allows for lightning initiation by either of two observed types of events: a high‐power, very high frequency (VHF) event such as a Narrow Bipolar Event or a weak VHF event. According to the Mechanism, both types of initiating events are caused by a group of relativistic runaway electron avalanche particles (where the initial electrons are secondary particles of an extensive air shower) passing through many Eth volumes, thereby causing the nearly simultaneous launching of many positive streamer flashes. Due to ionization‐heating instability, unusual plasma formations (UPFs) appear along the streamers' trajectories. These UPFs combine into three‐dimensional (3‐D) networks of hot plasma channels during the IEC, resulting in its observed weak current flow. The subsequent development and combination of two (or more) of these 3‐D networks of hot plasma channels then causes the first IBP. Each subsequent IBP is caused when another 3‐D network of hot plasma channels combines with the chain of networks caused by earlier IBPs.

In an article by Kostinskiy et al. (2019) proposed the mechanism of the origin and development of lightning from initiating event to initial breakdown pulses (termed the Mechanism). The Mechanism assumes initiation occurs in a region of a thundercloud of 1 km3 with electric field E > 0.4 MV/(m∙atm), which contains, because of turbulence, numerous small “Eth-volumes” of 0.001-0.0001 m3 with E ≥ 3 MV/(m∙atm). The Mechanism allows for lightning initiation by two observed types of initiating events: a high power VHF event called an NBE (narrow bipolar event or CID), or a weak VHF event. According to the Mechanism, both types of initiating events are caused by a group of relativistic runaway electron avalanche particles passing through many of the Eth-volumes, thereby causing the nearly simultaneous launching of many positive streamer flashes, Kostinskiy et al. (2019).In this report, based on the Meek’s criterion for the initiation of streamers (Raizer, 1991) at different heights of lightning initiation and taking into account the number of all background electrons, positrons and photons of cosmic rays with energy ε < 1012 eV (Sato, 2015) crossing Eth-volumes sizes of Eth-volumes are specified (3∙10-4-3∙10-5 m3). The report also showed that synchronous injection with a high probability of relativistic electrons into such small Eth-volumes requires of relativistic runaway electrons avalanches to be initiated by extensive air showers with energies ε > 1015 eV, which would supply (injected) 105-107 secondary electrons into a turbulent region of a thundercloud with a strong electric field.ReferencesKostinskiy, A. Yu., Marshall, T.C., Stolzenburg, M. (2019), The Mechanism of the Origin and Development of Lightning from Initiating Event to Initial Breakdown Pulses arXiv:1906.01033Raizer Yu. (1991), Gas Discharge Physics, Springer-Verlag, 449 p.Sato T. (2015), Analytical Model for Estimating Terrestrial Cosmic Ray Fluxes Nearly Anytime and Anywhere in the World: Extension of PARMA/EXPACS, PLOS ONE, 10(12): e0144679.

Positive Narrow Bipolar Events (+ NBEs) in Mississippi thunderstorms were studied using fast antennas (FA, bandwidth 16 Hz - 2.6 MHz) and VHF antennas (Log-RF, bandwidth 186-192 MHz). The waveform characteristics of 201 positive NBEs were determined using both sensors. The + NBEs were classified in two ways: by FA waveform into Types A-D and by + NBE occurrence relative to other lightning events into three groups called Isolated, Not-Isolated, and INBE. (An INBE initiates an intracloud flash.) The FA waveform properties of 188 positive NBEs were mainly in reasonable agreement with previous studies. The VHF waveform properties of + NBEs have not been studied previously. The VHF powers of 201 positive NBE ranged from 0.1-88.4 kW with an average value of 7.8 kW. Types C and D positive NBEs tended to be more energetic (average VHF powers of 9.2 and 13.2 kW) than Types A and B (average powers of 1.9 and 4.0 kW). The INBE group of +NBEs had a larger range of VHF powers (0.2-88.4 kW) than the combined power range of the Not-Isolated and Isolated groups (0.1-26.7 kW). The INBE group also had a larger average peak power (9.9 kW) than the Not-Isolated and Isolated Groups (4.0 and 8.7 kW, respectively). However, 53% of INBEs had peak VHF power < 5 kW, so an INBE does not require a large power to initiate an intracloud flash. For 90% of the 201 positive NBEs the magnitude of the time difference between the FA peak amplitude and the Log-RF peak power was <= 2 s, but the FA peak amplitude showed almost no correlation with the Log-RF peak power.

Abstract The SABER instrument on the National Aeronautics and Space Administration Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite continues to provide a long‐term record of Earth's stratosphere, mesosphere, and lower thermosphere. The SABER data are being used to examine long‐term changes and trends in temperature, water vapor, and carbon dioxide. A tacit, central assumption of these analyses is that the SABER instrument radiometric calibration is not changing with time; that is, the instrument is stable. SABER stratospheric temperatures and those derived from Global Positioning System Radio Occultation measurements are compared to examine SABER's stability. Global Positioning System Radio Occultation measurements are inherently stable due to the accuracy and traceability of the measured phase delay rate to the Système Internationale definition of the second. Differences in global annual mean SABER and COSMIC lower stratospheric temperatures show little significant change with time in the 11 years spanning 2007–2017. From this analysis we infer that SABER temperatures are stable to better than 0.1 to 0.2 K per decade.