# Terrestrial effects of moderately nearby supernovae [EPA]

Recent data indicate one or more moderately nearby supernovae in the early Pleistocene, with additional events likely in the Miocene. This has motivated more detailed computations, using new information about the nature of supernovae and the distances of these events to describe in more detail the sorts of effects that are indicated at the Earth. This short communication/review is designed to describe some of these effects so that they may possibly be related to changes in the biota around these times.

A. Melott and B. Thomas
Fri, 8 Dec 17

Comments: 2 figures; to be published in Lethaia

# Critical frequencies of the ionospheric $F_1$ and $F_2$ layers during the last four solar cycles: sunspot group type dependencies [SSA]

The long term solar activity dependencies of ionospheric F$_1$ and F$_2$ regions’ critical frequencies ($f_0F_1$ and $f_0F_2$) are analyzed for the last four solar cycles (1976–2015). We show that the ionospheric F$_1$ and F$_2$ regions have different solar activity dependencies in terms of the sunspot group (SG) numbers: F$_1$ region critical frequency ($f_0F_1$) peaks at the same time with the small SG numbers, while the $f_0F_2$ reaches its maximum at the same time with the large SG numbers, especially during the solar cycle 23. The observed differences in the sensitivity of ionospheric critical frequencies to sunspot group (SG) numbers provide a new insight into the solar activity effects on the ionosphere and space weather. While the F$_1$ layer is influenced by the slow solar wind, which is largely associated with small SGs, the ionospheric F$_2$ layer is more sensitive to Coronal Mass Ejections (CMEs) and fast solar winds, which are mainly produced by large SGs and coronal holes. The SG numbers maximize during of peak of the solar cycle and the number of coronal holes peaks during the sunspot declining phase. During solar minimum there are relatively less large SGs, hence reduced CME and flare activity. These results provide a new perspective for assessing how the different regions of the ionosphere respond to space weather effects.

E. Yigit, A. Kilcik, A. Elias, et. al.
Fri, 1 Dec 17
11/68

Comments: Accepted for publication in Journal of Atmospheric and Solar-Terrestrial Physics

# Stratosphere circulation on tidally locked ExoEarths [CL]

Stratosphere circulation is important to interpret abundances of photo-chemically produced compounds like ozone that we aim to observe to assess habitability of exoplanets. We thus investigate a tidally locked ExoEarth scenario for TRAPPIST-1b, TRAPPIST-1d, Proxima Centauri~b and GJ 667 C~f with a simplified 3D atmosphere model and for different stratospheric wind breaking assumptions.
These planets are representatives for different circulation regimes for orbital periods: $P_{orb}=1-100$~days. The circulation of exoplanets with $P_{orb} \leq$ 25~days can be dominated by the standing tropical Rossby wave in the troposphere and also in the stratosphere: It leads to a strong equatorial eastward wind jet and to ‘Anti-Brewer-Dobson’-circulation that confines air masses to the stratospheric equatorial region. Thus, the distribution of photo-chemically produced species and aerosols may be limited to an ‘equatorial transport belt’. In contrast, planets with $P_{orb}>25$~days, like GJ~667~C~f, exhibit efficient thermally driven circulation in the stratosphere that allows for a day side-wide distribution of air masses.
The influence of the standing tropical Rossby waves on tidally locked ExoEarths with $P_{orb} \leq 25$~days can, however, be circumvented with deep stratospheric wind breaking alone – allowing for equator-to-pole transport like on Earth. For planets with $3 \leq P_{orb} \leq 6$~days, the extratropical Rossby wave acts as an additional safe-guard against the tropical Rossby wave in case of shallow wind breaking. Therefore, TRAPPIST-1d is less prone to have an equatorial transport belt in the stratosphere than Proxima~Centauri~b.
Even our Earth model shows an equatorial wind jet, if stratosphere wind breaking is inefficient.

L. Carone, R. Keppens, L. Decin, et. al.
Fri, 1 Dec 17
19/68

# Photonuclear Reactions in Lightning Discovered from Detection of Positrons and Neutrons [HEAP]

Lightning and thundercloud are the most dramatic natural particle accelerators on the Earth. Relativistic electrons accelerated by electric fields therein emit bremsstrahlung gamma rays, which have been detected at ground observations, by airborne detectors, and as terrestrial gamma-ray flashes (TGFs) from space. The energy of the gamma rays is sufficiently high to potentially invoke atmospheric photonuclear reactions 14N(gamma, n)13N, which would produce neutrons and eventually positrons via beta-plus decay of generated unstable radioactive isotopes, especially 13N. However, no clear observational evidence for the reaction has been reported to date. Here we report the first detection of neutron and positron signals from lightning with a ground observation. During a thunderstorm on 6 February 2017 in Japan, a TGF-like intense flash (within 1 ms) was detected at our monitoring sites 0.5-1.7 km away from the lightning. The subsequent initial burst quickly subsided with an exponential decay constant of 40-60 ms, followed by a prolonged line emission at about 0.511 megaelectronvolt (MeV), lasting for a minute. The observed decay timescale and spectral cutoff at about 10 MeV of the initial emission are well explained with de-excitation gamma rays from the nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to the electron-positron annihilation, and hence is the conclusive indication of positrons produced after the lightning. Our detection of neutrons and positrons is unequivocal evidence that natural lightning triggers photonuclear reactions. No other natural event on the Earth is known to trigger photonuclear reactions. This discovery places lightning as only the second known natural channel on the Earth after the atmospheric cosmic-ray interaction, in which isotopes, such as 13C, 14C, and 15N, are produced.

T. Enoto, Y. Wada, Y. Furuta, et. al.
Thu, 23 Nov 17
8/52

Comments: This manuscript was submitted to Nature Letter on July 30, 2017, and the original version that has not undergo the peer review process. See the accepted version at Nature website, published on the issue of November 23, 2017 with the revised title “photonuclear reaction triggered by lightning discharge”

# Exo-lightning radio emission: the case study of HAT-P-11b [EPA]

Lightning induced radio emission has been observed on solar system planets. Lecavelier des Etangs et al. [2013] carried out radio transit observations of the exoplanet HAT-P-11b, and suggested a tentative detection of a radio signal. Here, we explore the possibility of the radio emission having been produced by lightning activity on the exoplanet, following and expanding the work of Hodos\’an et al. [2016a]. After a summary of our previous work [Hodos\’an et al. 2016a], we extend it with a parameter study. The lightning activity of the hypothetical storm is largely dependent on the radio spectral roll-off, $n$, and the flash duration, $\tau_\mathrm{fl}$. The best-case scenario would require a flash density of the same order of magnitude as can be found during volcanic eruptions on Earth. On average, $3.8 \times 10^6$ times larger flash densities than the Earth-storms with the largest lightning activity is needed to produce the observed signal from HAT-P-11b. Combined with the results of Hodos\’an et al. [2016a] regarding the chemical effects of planet-wide thunderstorms, we conclude that future radio and infrared observations may lead to lightning detection on planets outside the solar system.

G. Hodosan, C. Helling and P. Rimmer
Thu, 23 Nov 17
19/52

Comments: Accepted to the Conference Proceedings of the 8th International Workshop on Planetary, Solar and Heliospheric Radio Emissions (PRE 8), held in Seggauberg near Leibnitz/Graz, Austria, October 25-27, 2016. 12 pages, 2 figures

# Exo-lightning radio emission: the case study of HAT-P-11b [EPA]

Lightning induced radio emission has been observed on solar system planets. Lecavelier des Etangs et al. [2013] carried out radio transit observations of the exoplanet HAT-P-11b, and suggested a tentative detection of a radio signal. Here, we explore the possibility of the radio emission having been produced by lightning activity on the exoplanet, following and expanding the work of Hodos\’an et al. [2016a]. After a summary of our previous work [Hodos\’an et al. 2016a], we extend it with a parameter study. The lightning activity of the hypothetical storm is largely dependent on the radio spectral roll-off, $n$, and the flash duration, $\tau_\mathrm{fl}$. The best-case scenario would require a flash density of the same order of magnitude as can be found during volcanic eruptions on Earth. On average, $3.8 \times 10^6$ times larger flash densities than the Earth-storms with the largest lightning activity is needed to produce the observed signal from HAT-P-11b. Combined with the results of Hodos\’an et al. [2016a] regarding the chemical effects of planet-wide thunderstorms, we conclude that future radio and infrared observations may lead to lightning detection on planets outside the solar system.

G. Hodosan, C. Helling and P. Rimmer
Thu, 23 Nov 17
19/52

Comments: Accepted to the Conference Proceedings of the 8th International Workshop on Planetary, Solar and Heliospheric Radio Emissions (PRE 8), held in Seggauberg near Leibnitz/Graz, Austria, October 25-27, 2016. 12 pages, 2 figures

# Photonuclear Reactions in Lightning Discovered from Detection of Positrons and Neutrons [HEAP]

Lightning and thundercloud are the most dramatic natural particle accelerators on the Earth. Relativistic electrons accelerated by electric fields therein emit bremsstrahlung gamma rays, which have been detected at ground observations, by airborne detectors, and as terrestrial gamma-ray flashes (TGFs) from space. The energy of the gamma rays is sufficiently high to potentially invoke atmospheric photonuclear reactions 14N(gamma, n)13N, which would produce neutrons and eventually positrons via beta-plus decay of generated unstable radioactive isotopes, especially 13N. However, no clear observational evidence for the reaction has been reported to date. Here we report the first detection of neutron and positron signals from lightning with a ground observation. During a thunderstorm on 6 February 2017 in Japan, a TGF-like intense flash (within 1 ms) was detected at our monitoring sites 0.5-1.7 km away from the lightning. The subsequent initial burst quickly subsided with an exponential decay constant of 40-60 ms, followed by a prolonged line emission at about 0.511 megaelectronvolt (MeV), lasting for a minute. The observed decay timescale and spectral cutoff at about 10 MeV of the initial emission are well explained with de-excitation gamma rays from the nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to the electron-positron annihilation, and hence is the conclusive indication of positrons produced after the lightning. Our detection of neutrons and positrons is unequivocal evidence that natural lightning triggers photonuclear reactions. No other natural event on the Earth is known to trigger photonuclear reactions. This discovery places lightning as only the second known natural channel on the Earth after the atmospheric cosmic-ray interaction, in which isotopes, such as 13C, 14C, and 15N, are produced.

T. Enoto, Y. Wada, Y. Furuta, et. al.
Thu, 23 Nov 17
37/52

Comments: This manuscript was submitted to Nature Letter on July 30, 2017, and the original version that has not undergo the peer review process. See the accepted version at Nature website, published on the issue of November 23, 2017 with the revised title “photonuclear reaction triggered by lightning discharge”