Gamma Ray Glow Observations at 20-km Altitude

Abstract

In the spring of 2017 an ER-2 aircraft campaign was undertaken over continental United States to observe energetic radiation from thunderstorms and lightning. The payload consisted of a suite of instruments designed to detect optical signals, electric fields, and gamma rays from lightning. Starting from Georgia, USA, 16 flights were performed, for a total of about 70 flight hours at a cruise altitude of 20 km. Of these, 45 flight hours were over thunderstorm regions. An analysis of two gamma ray glow events that were observed over Colorado at 21:47 UT on 8 May 2017 is presented. We explore the charge structure of the cloud system, as well as possible mechanisms that can produce the gamma ray glows. The thundercloud system we passed during the gamma ray glow observation had strong convection in the core of the cloud system. Electric field measurements combined with radar and radio measurements suggest an inverted charge structure, with an upper negative charge layer and a lower positive charge layer. Based on modeling results, we were not able to unambiguously determine the production mechanism. Possible mechanisms are either an enhancement of cosmic background locally (above or below 20 km) by an electric field below the local threshold or an enhancement of the cosmic background inside the cloud but then with normal polarity and an electric field well above the Relativistic Runaway Electron Avalanche threshold.

Keywords: atmospheric electricity; gamma ray glow; lightning; thundercloud.

Figure 1

(a) Trajectory of ER‐2 flying over two convection cells in Colorado just north and northwest of Denver. The cloud images provided by the National Oceanic and Atmospheric Administration GOES is accumulated over 7 min centered around 21:45 UT. Black solid line marks the time of observations studied in this paper, and light gray segments indicate when the gamma glow was observed. No gamma glow was observed during the overpass of the convective system in the northwest. Red dots mark lightning activity detected by WWLLN ±5 min around 21:47 UT. (b) The inset shows the gamma ray observation along the path (time is running to the left). (c) Flight altitude (dashed line) and the upper cloud layer (red and blue) as measured by Cloud Physics Lidar. (d) The gamma ray observations (UIB‐BGO >300 keV) during the entire flight on 8 May and the time of glow observation is marked with red vertical line.

Figure 2

Measurement of quasi‐static electric field and gamma rays at 20 km. () E _X, (b) E _Y, and (c) E _Z measured by Lightning Instrument Package with 0.1‐s resolution. Red line is a 20‐s running average. (d) Flying altitude and cloud top measured by Cloud Physics Lidar (5‐s resolution). Two horizontal dashed lines are added to point out the altitude of cloud top during the glow and between the glow. (e) UIB‐BGO (>300 keV) and (f) the two large plastic detectors in iSTORM (∼100 keV to ∼8 MeV), both with 1‐s resolution. Time intervals of glow observation are marked with black horizontal lines in all panels. LIP = Lightning Instrument Package; ALOFT = Airborne Lightning Observatory for FEGS and TGFs; iSTORM = in Situ Thunderstorm Observer for Radiation Mechanism.

Figure 3

Altitude distribution (2–14 km) of lightning activity detected by the Colorado Lightning Mapping Array in selected intervals from flight trajectory ±2 min around the peak of the glow. (a) −5 to 0 km (to the left and south of the trajectory). (b) 0 to 5 km to the right (north) of trajectory. (c) 5 to 10 km (right/north) from trajectory. (d) Cloud tops from Cloud Physics Lidar. Time intervals of glow observations are shown as two thick black horizontal lines between (a) and (b) and in (d).

Figure 4

(a) The National Lightning Detection Network data ±3 min around 21:49:30 UT spanning the time interval of glow observations showing the polarity and peak currents of intracloud (IC) and polarity of cloud‐to‐ground (CG) lightning. Blue circles are IC− bringing negative charges downward, and red circles are IC+ bringing negative charges upward. Size of circle of IC indicates the peak current magnitude ranging from 2 to 20 kA. Light blue small dots are CG−, and magenta small dots are CG+. (b) The distribution of peak current magnitudes for IC− and IC+ (left) and for CG− and CG+ (right).

Figure 5

(a) The E _XZ vectors from Lightning Instrument Package data. Notice that time is running from right to left. Black vectors are every tenth of the 0.1‐s resolution data, and red vectors are every tenth of the 20‐s running average. (b) A possible charge structure of the cloud in the X Z plane just underneath the aircraft. X and Z is defined in (a). Red large arrows show the strong convection. Minus and plus show the two main negative and positive charge regions, while the small plus indicates the positive screening layer on both sides of the convective core. Blue arrows illustrate the lightning activity.

Figure 6

The energy spectra observed before and during the glow. (a) Background spectrum before: (21:30–21:38 UT) and during the glow (21:46.30–21:48.10 UT). (b) The background subtracted spectrum during the glow.

Figure 7

Variation of the flux at 20‐km altitude as function of the electric field at the same altitude (E _ac) for (a) photons, (b) electrons, and (c) positrons. The different flux scales on the left side of the panels reflect the relative number of photons (∼91.7%), electrons (∼6.0%), and positrons (∼2.3%) above 300 keV for the background values (E _ac=0). Unit on the right side is the increase (%) relative to the background values (when E _ac=0). Positive (negative) is a downward (upward) electric field at 20 km (E _ac). Blue (red) lines are particles entering 20 km from above (from below), while yellow lines are all particles passing 20 km. The dashed vertical gray lines (panels a and b) indicate the 10% and 45% flux increases. The red dashed vertical line in panel (b) is electric field strength needed for increasing the total photon flux by 10% (see text for explanation). The photon flux (panel a) increases by 10% for E _ac=−8.8 kV/m and E _ac=9.5 kV/m and by 45% for E _ac=−13.2 kV/m and for E _ac=17.3 kV/m. Increases of the electron flux (panel b) by 10% and 45% are obtained for field strengths of −1.8 and −6.0 kV/m, respectively.

Figure 8

Reduced χ ² values of the simulated photon spectra fits to the measured spectrum. The critical value ${\chi }_{\text{red},c}^2=1.52$ assuming a 5% acceptance level is highlighted by a dashed red line. A ${\chi }_{\text{red}}^2$ value below ${\chi }_{\text{red},c}^2$ indicates a compatible fit. The required field strengths for 10% and 45% increase of photon flux are marked by black dashed lines for both downward (positive) and upward (negative) fields.

Figure 9

Model B simulation results of acceptable ${\chi }_{\text{red}}^2$ values for both the spectral fit and the enhanced fluxes as function of potential (ΔU) and altitude (H). The blue area indicates the altitude‐potential range where the reduced ${\chi }_{\text{red}}^2$ of the spectral fit of the simulation to the measurement is below 1.53, which is the critical value for the 5% level for 23° of freedom. The gray hatched area denotes the altitude‐potential range where the photon flux increase (at 20‐km altitude and between 300 and 30 MeV) with respect to background is between 10% and 45%. A constant electric field over 2 km is assumed, and the values are shown for the middle of this region; for example, 11 km is for an electric field from 10 to 12 km. Black line is the RREA threshold. RREA = Relativistic Runaway Electron Avalanche.

Figure 10

Spectrum comparison between the measurement (background subtracted) and fits obtained from Geant4 modeling. (a) Model A photon spectra for electric field strengths required at 20 km for a 10% increase in photon flux. (b) Model B photon spectra for two potential values between 8 and 10 km. LIP = Lightning Instrument Package; ALOFT = Airborne Lightning Observatory for FEGS and TGFs; iSTORM = in Situ Thunderstorm Observer for Radiation Mechanism.