Long-term climate change in the D-region

Controversy exists over the potential effects of long-term increases in greenhouse gas concentrations on the ionospheric D-region at 60-90 km altitudes. Techniques involving in-situ rocket measurements, remote optical observations, and radio wave reflection experiments have produced conflicting results. This study reports a novel technique that analyses long-distance subionospheric very low frequency radiowave observations of the NAA 24.0 kHz transmitter, Cutler, Maine, made from Halley Station, Antarctica, over the period 1971-2016. The analysis is insensitive to any changes in the output power of the transmitter, compensates for the use of different data logging equipment, and can confirm the accuracy of the timing systems operated over the 45 year long record. A ~10% reduction in the scale size of the transmitter nighttime interference fringe pattern has been determined, taking into account the quasi-11 year solar cycle. Subionospheric radiowave propagation modeling suggests that the contraction of the interference fringe pattern about the mid-latitude NAA transmitter is due to a 3 km reduction in the effective height of the nighttime ionospheric D-region over the last 45 years. This is consistent with the effect of enhanced infra-red cooling by increasing greenhouse gases

The atmospheric implications of radiation belt remediation

High altitude nuclear explosions (HANEs) and geomagnetic storms can produce large scale injections of relativistic particles into the inner radiation belts. It is recognised that these large increases in >1 MeV trapped electron fluxes can shorten the operational lifetime of low Earth orbiting satellites, threatening a large, valuable population. Therefore, studies are being undertaken to bring about practical human control of the radiation belts, termed "Radiation Belt Remediation" (RBR). Here we consider the upper atmospheric consequences of an RBR system operating over either 1 or 10 days. The RBR-forced neutral chemistry changes, leading to NOx enhancements and Ox depletions, are significant during the timescale of the precipitation but are generally not long-lasting. The magnitudes, time-scales, and altitudes of these changes are no more significant than those observed during large solar proton events. In contrast, RBR-operation will lead to unusually intense HF blackouts for about the first half of the operation time, producing large scale disruptions to radio communication and navigation systems. While the neutral atmosphere changes are not particularly important, HF disruptions could be an important area for policy makers to consider, particularly for the remediation of natural injections

Characteristics of relativistic microburst intensity from SAMPEX observations

Relativistic electron microbursts are an important electron loss process from the radiation belts into the atmosphere. These precipitation events have been shown to significantly impact the radiation belt fluxes and atmospheric chemistry. In this study we address a lack of knowledge about the relativistic microburst intensity using measurements of 21,746 microbursts from the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX). We find that the relativistic microburst intensity increases as we move inward in L, with a higher proportion of low‐intensity microbursts (2,250 [MeV cm2 sr s]−1) in the 03–11 magnetic local time region increases as geomagnetic activity increases, consistent with changes in the whistler mode chorus wave activity. Comparisons between relativistic microburst properties and trapped fluxes suggest that the microburst intensities are not limited by the trapped flux present alongside the scattering processes. However, microburst activity appears to correspond to the changing trapped flux; more microbursts occur when the trapped fluxes are enhancing, suggesting that microbursts are linked to processes causing the increased trapped fluxes. Finally, modeling of the impact of a published microburst spectra on a flux tube shows that microbursts are capable of depleting <500‐keV electrons within 1 hr and depleting higher‐energy electrons in 1–23 hr

Evidence of sub‐MeV EMIC‐driven trapped electron flux dropouts from GPS observations

For many years, it was believed that resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons were restricted to electron energies > 1 − 2 MeV. In recent years, however, a growing body of experimental evidence has shown that EMIC waves can cause the scattering loss of electrons down to sub‐MeV energies. Using measurements of trapped electron flux from the GPS satellite constellation, we investigate the ability of EMIC waves to cause significant depletions of radiation belt electron populations between 4 ≤ L* ≤ 5. For the first time, we present statistical evidence demonstrating global decreases in sub‐MeV trapped electron flux in response to EMIC wave activity. Although we find that electron losses extend down to sub‐MeV energies, we also show strong statistical support for the ability of EMIC waves to preferentially cause substantial depletions of ultra‐relativistic electrons in the radiation belts

Impact of EMIC‐wave driven electron precipitation on the radiation belts and the atmosphere

In recent years there has been a growing body of direct experimental evidence demonstrating electromagnetic ion cyclotron (EMIC) waves driving energetic electron precipitation (EEP) at unexpectedly low, sub‐MeV energies — as low as only a few hundred keV. EMIC wave driven scattering at these energies has important ramifications for our understanding of not only radiation belt electron dynamics, but also the importance of EMIC‐driven EEP to the chemical balance of the Earth’s atmosphere. In this study, we use three experimentally derived EMIC‐driven EEP flux spectra to investigate the impact of this precipitation on trapped radiation belt fluxes. In doing so, we resolve an apparent contradiction with earlier results derived from trapped electron flux populations that suggested EMIC waves only caused significant scattering at ultra‐relativistic energies. We show that strong sub‐MeV EEP measurements are not necessarily mutually exclusive with a strongly relativistic‐only trapped flux response, as the sub‐MEV peak precipitation is comparatively much smaller than the trapped population at those energies. Using a further six EEP spectra, we also demonstrate that EMIC‐driven EEP can generate significant ionisation of the Earth’s atmosphere above 40km, leading to the loss of mesospheric ozone. We find poor correlation between EMIC‐driven EEP fluxes and geomagnetic activity proxies, such that EMIC‐driven EEP is likely to be poorly specified in the forcing factors of modern coupled‐climate models

Simultaneous observation of chorus and hiss near the plasmapause

On 4 August 2010 a moderate geomagnetic storm occurred with minimum Dst of −65 nT and maximum Kp of 7−. Shortly after the onset of this storm, VLF chorus was observed at Marion Island (L= 2.6). Over time the spectral structure of the chorus transformed into a hiss band spanning the same frequency range. The observation of overlapping chorus and hiss suggests that Marion Island was close to the plasmapause at the time of this event, and provides ground-based observational confirmation of the generation mechanism of plasmaspheric hiss from chorus waves outside of the plasmasphere. Chorus observations at Marion Island were not common during this period of the solar cycle and so this event was investigated in detail. The geomagnetic conditions are discussed and geosynchronous particle data and broadband data from two other stations are presented. Empirical models are employed to predict the location of the plasmapause, and its location is inferred from a knee whistler recorded at Dunedin, New Zealand. These show that Marion Island is in the vicinity of the plasmapause during the event. The event is also compared to chorus observed at similarL after the Halloween storms of 2003. The rarity of the chorus observation is quantified using DEMETER VLF data. The DEMETER data, along with the various ground based VLF measurements, allows us to infer temporal and spatial variations in the chorus source region

Spatial distributions of nitric oxide in the Antarctic winter‐time middle atmosphere during geomagnetic storms

Energetic electron precipitation leads to increased nitric oxide (NO) production in the mesosphere and lower thermosphere. NO distributions in the winter time, high‐latitude Southern hemisphere atmosphere during geomagnetic storms are investigated. NO partial columns in the upper mesosphere at altitudes 70–90 km and in the lower thermosphere at 90–110 km have been derived from observations made by the Solar Occultation For Ice Experiment (SOFIE) onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite. The SOFIE NO measurements during 17 geomagnetic storms in 2008–2014 have been binned into selected geomagnetic latitude and geographic latitude/longitude ranges. The regions above Antarctica showing the largest instantaneous NO increases coincide with high fluxes of 30–300 keV precipitating electrons from measurements by the second generation Space Environment Monitor (SEM‐2) Medium Energy Proton and Electron Detector instrument (MEPED) on the Polar orbiting Operational Environmental Satellites (POES). Significant NO increases over the Antarctic Peninsula are likely due to precipitation of >30 keV electrons from the radiation belt slot region. NO transport is estimated using Horizontal Wind Model (HWM14) calculations. In the upper mesosphere strong eastward winds (daily mean zonal wind speed ~20–30 ms‐1 at 80 km) during winter transport NO‐enriched air away from source regions 1–3 days following the storms. Mesospheric winds also introduce NO poor air into the source regions, quenching initial NO increases. Higher up, in the lower thermosphere, weaker eastward winds (~5–10 ms‐1 at 100 km) are less effective at redistributing NO zonally

A multi-instrument approach to determining the source‐region extent of EEP-driving EMIC Waves

Recent years have seen debate regarding the ability of electromagnetic ion cyclotron (EMIC) waves to drive EEP (energetic electron precipitation) into the Earth's atmosphere. Questions still remain regarding the energies and rates at which these waves are able to interact with electrons. Many studies have attempted to characterize these interactions using simulations; however, these are limited by a lack of precise information regarding the spatial scale size of EMIC activity regions. In this study we examine a fortuitous simultaneous observation of EMIC wave activity by the RBSP‐B and Arase satellites in conjunction with ground‐based observations of EEP by a subionospheric VLF network. We describe a simple method for determining the longitudinal extent of the EMIC source region based on these observations, calculating a width of 0.75 hr MLT and a drift rate of 0.67 MLT/hr. We describe how this may be applied to other similar EMIC wave events

Why Does the October Effect Not Occur at Night?

The October effect is known as a rapid and strong decrease in the signal amplitude of radio waves with very low frequency (VLF), reflected at the lowest edge of the ionosphere. This strong decrease can be observed only during the daytime. Although the October effect is long known, it is hardly investigated and its mechanism is still unknown. To get closer to a mechanism, we answer why the October effect does not occur during nighttime. Therefore, average characteristics of the October effect are obtained from different VLF transmitter-receiver combinations. The occurrence of the October effect is then compared with characteristics of the neutral atmosphere temperature at VLF reflection heights as it seems to act as a proxy for the unknown mechanism. The temperature shows an asymmetric seasonal behavior at daytime VLF reflection heights poleward of 50°N but not during the nighttime, resulting in the October effect