Andrew J. Gerrard

An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts

Early observations indicated that the Earth’s Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep ‘slot’ region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth’s intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave–particle pitch angle scattering deep inside the Earth’s plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.

A study of the role of ion-molecule chemistry in the formation of sporadic sodium layers

Over two campaigns in 1998 and 1999, multiple sporadic sodium events were observed by the Arecibo Observatory sodium density lidar while simultaneously monitoring the plasma density using the incoherent scatter radar. In this paper, we test the theoretical explanation proposed by Cox and Plane (1998) where Na + in a plasma layer is neutralized via an ion–molecule mechanism to form a sporadic sodium layer. A particular challenge is to interpret observations made in a Eulerian frame of observation where the spatial and temporal characteristics of events cannot easily be separated. The reaction scheme in the original mechanism is modi=ed to include the reactions NaO + +N2 → Na + ·N2 +O and NaO + +O2 → Na + +O3, following the results of theoretical quantum calculations. Six unique case studies of sporadic sodium layers are presented here, and excellent agreement between simulation and observations was obtained for =ve of them. c � 2002 Published by Elsevier Science Ltd.

Gravity-wave influences on Arctic mesospheric clouds as determined by a Rayleigh lidar at Sondrestrom, Greenland

common peak volume backscatter coefficient as 20.0 � 10 � 11 m � 1 sr � 1 . The FWHM is noticeably thinner than determined by other lidar observations of NLCs in Norway and the South Pole. We found the mean backscatter strength to increase and the FWHM to decrease with decreasing cloud height. In addition, the cloud slopes with time are greater for the thicker weaker clouds at higher altitudes than the thinner stronger clouds at lower altitudes. Gravity-wave signatures are routinely observed in the cloud detections. Upon estimating stratospheric wave activity in the data, we observed stronger cloud backscatter during low gravity-wave activity and weak cloud backscatter during high gravity-wave activity. To help support these results, simulations from a microphysical cloud model were performed under summer mesospheric conditions with and without gravity-wave activity. Upon including short-period (� 2–3 hours) gravity-wave activity, the model simulation reproduced the behavior observed in the ensemble cloud properties by producing a broader altitude distribution, weaker backscatter strength, and thinner clouds. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0669 Electromagnetics: Scattering and diffraction; 1655 Global Change: Water cycles (1836); KEYWORDS: noctilucent clouds, gravity waves, Rayleigh lidar, volume backscatter coefficient, polar mesosphere

All-sky imaging observations of mesospheric fronts in OI 557.7 nm and broadband OH airglow emissions: Analysis of frontal structure, atmospheric background conditions, and potential sourcing mechanisms

[1] All-sky imaging observations of distinct, large horizontal transient mesospheric structures with a spatial scale of ∼100 km detected in images of broadband OH and OI 557.7 nm airglow emissions were made over Clemson, South Carolina, on the night of 14-15 October 2001. We designate these structures as mesospheric fronts and present a detailed summary of this night series of observations, paying particular attention to the details of the different frontal structures, the wave-like activity seen throughout the night, and the background atmospheric conditions. These data are compared to other observations of similar mesospheric fronts found in the literature, and we seek to understand them in relation to mesospheric bores, ducted gravity waves, mesospheric wall events, and nonlinear gravity wave interactions. We find that the observed frontal characteristics and the atmospheric background structure exhibit a close resemblance to previous observations of mesospheric bores. Owing to this similarity, and supported by gravity wave ray-tracing experiments, we propose a sequence of events that generated the mesospheric fronts observed in the airglow emission. Furthermore, we note this similarity in atmospheric structure suggests a potential means of predicting the occurrence of such mesospheric phenomena.

Noctilucent clouds and wave dynamics: Observations at Sondrestrom, Greenland

Rayleigh Lidar measurements of the arctic summer stratosphere and mesosphere have been conducted routinely since 1994 at the Sondrestrom atmospheric research facility near Kangerlussuaq, Greenland (67.0N, 50.9W). Between 1994 and 1996, seventeen separate events of noctilucent clouds (NLCs), as well as corresponding stratospheric wave structure, have been observed. We present in this study two main results: 1) the occurrence of persistent quasi-monochromatic wave structure in the stratosphere having observed vertical wavelengths of 8 to 12 km and observed temporal periods of approximately 2–3 hours, and 2) evidence that the strength of such lower altitude waves may be associated with the overall strength of the observed NLC volume backscatter coefficient.

Storm time response of the midlatitude thermosphere: Observations from a network of Fabry‐Perot interferometers

Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry-Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all-sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low-energy O+ ions from the magnetosphere. The presence of low-energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low-energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

Climatology of medium‐scale traveling ionospheric disturbances observed by the midlatitude Blackstone SuperDARN radar

A climatology of daytime midlatitude medium-scale traveling ionospheric disturbances (MSTIDs) observed by the Blackstone Super Dual Auroral Radar Network (SuperDARN) radar is presented. MSTIDs were observed primarily from fall through spring. Two populations were observed: a dominant population heading southeast (centered at 147° geographic azimuth, ranging from 100° to 210°) and a secondary population heading northwest (centered at −50° azimuth, ranging from −75° to −25°). Horizontal velocities ranged from 50 to 250 m s−1 with a distribution maximum between 100 and 150 m s−1. Horizontal wavelengths ranged from 100 to 500 km with a distribution peak at 250 km, and periods between 23 and 60 min, suggesting that the MSTIDs may be consistent with thermospheric gravity waves. A local time (LT) dependence was observed such that the dominant (southeastward) population decreased in number as the day progressed until a late afternoon increase. The secondary (northwestward) population appeared only in the afternoon, possibly indicative of neutral wind effects or variability of sources. LT dependence was not observed in other parameters. Possible solar-geomagnetic and tropospheric MSTID sources were considered. The auroral electrojet (AE) index showed a correlation with MSTID statistics. Reverse ray tracing with the HINDGRATS model indicates that the dominant population has source regions over the Great Lakes and near the geomagnetic cusp, while the secondary population source region is 100 km above the Atlantic Ocean east of the Carolinas. This suggests that the dominant population may come from a region favorable to either tropospheric or geomagnetic sources, while the secondary population originates from a region favorable to secondary waves generated via lower atmospheric convection.

Initial measurements of O‐ion and He‐ion decay rates observed from the Van Allen probes RBSPICE instrument

H-ion (∼45 keV to ∼600 keV), He-ion (∼65 keV to ∼520 keV), and O-ion (∼140 keV to ∼1130 keV) integral flux measurements, from the Radiation Belt Storm Probe Ion Composition Experiment (RBSPICE) instrument aboard the Van Allan Probes spacecraft B, are reported. These abundance data form a cohesive picture of ring current ions during the first 9 months of measurements. Furthermore, the data presented herein are used to show injection characteristics via the He-ion/H-ion abundance ratio and the O-ion/H-ion abundance ratio. Of unique interest to ring current dynamics are the spatial-temporal decay characteristics of the two injected populations. We observe that He-ions decay more quickly at lower L shells, on the order of ∼0.8 day at L shells of 3–4, and decay more slowly with higher L shell, on the order of ∼1.7 days at L shells of 5–6. Conversely, O-ions decay very rapidly (∼1.5 h) across all L shells. The He-ion decay time are consistent with previously measured and calculated lifetimes associated with charge exchange. The O-ion decay time is much faster than predicted and is attributed to the inclusion of higher-energy (> 500 keV) O-ions in our decay rate estimation. We note that these measurements demonstrate a compelling need for calculation of high-energy O-ion loss rates, which have not been adequately studied in the literature to date. Key Points We report initial observations of ring current ions We show that He-ion decay rates are consistent with theory We show that O-ions with energies greater than 500 keV decay very rapidly

Quiet time observations of He ions in the inner magnetosphere as observed from the RBSPICE instrument aboard the Van Allen Probes mission

He ions contribute to Earths ring current energy and species population density and are important in understanding ion transport and charge exchange processes in the inner magnetosphere. He ion flux measurements made by the Van Allen Probes Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument are presented in this paper. Particular focus is centered on geomagnetically quiet intervals in late 2012 and 2013 that show the flux, L-shell, and energy (65 keV to 518 keV) morphology of ring current He ions between geomagnetic storm injection events. The overall He ion abundance during the first nine months of RBSPICE observations, the appearance of a persistent high energy, low L-shell He ion population, and the temporal evolution of this population all provide new insights into trapped ring current energy He ions. These data provide a unique resource that will be used to provide verifications of, and improvements to, models of He ion transport and loss in Earths ring current region.

Investigation of a resonance Lidar for measurement of thermospheric metastable helium

Abstract Expected signal returns for a ground-based resonance Lidar system used for profiling metastable thermospheric helium are presented, scaleable to specific system configurations. The signal estimate is dependent on the calculation of the effective backscatter cross-section for the He 10 830 A resonant transition as well as an estimate of thermospheric metastable helium densities obtained from a recent model. The peak backscatter cross-section is found to be 2.7(±0.3) × 10 −16 m 2 with an effective backscatter cross-section (assuming a 1 GHz rms laser linewidth centered at a wavelength of 10 830.32 A) of 2.6(±0.3) × 10 −16 m 2 . Measurements using the metastable He 3188 A and 3889 A lines are evaluated. Challenges in experimental design (i.e., laser characteristics, near infrared single photon detection, and background noise), as well as potential operation from a space-borne platform, are also discussed. With current technology, profiles with relatively high temporal and vertical resolution are shown to be attainable under twilight conditions.