Improved SuperDARN radar signal processing: A first principles statistical approach for reliable measurement uncertainties and enhanced data products

Ground-based radar systems are the best way to continuously monitor medium-to-large-scale features of the near-Earth space environment on a global scale. The Super Dual Auroral Radar Network (SuperDARN) radars are used to image the high-latitude ionospheric plasma circulation, which is produced by magnetosphere-ionosphere coupling processes generated by the interaction of both the solar and terrestrial magnetic fields. While investigating ways to expand the usable data products of SuperDARN to include electron density inferred using a multiple-frequency technique, it was determined that SuperDARN error estimates were lacking sufficient rigour. The method to calculate SuperDARN parameters was developed approximately 25 years ago when available computing resources were significantly less powerful, which required a number of simplifications to ensure both valid data and reasonable processing time. This resulted in very conservative criteria being applied to ensure valid data, but at the expense of both rigorous error analysis and the elimination of some otherwise valid data. With access to modern computing resources, the SuperDARN data processing methodology can be modernized to provide proper error estimates for the SuperDARN parameters (power, drift velocity, width). This research has resulted in 3 publications, which are presented here as Chapters 5, 6, and 7. The error analysis started with a first principles analysis of the self-clutter generated by the multiple-pulse technique that is used to probe the ionosphere (Chapter 5). Next, the statistical properties of voltage fluctuations as measured by SuperDARN were studied and the variance of these measurements were derived (Chapter 6). Finally, the statistical error analysis was propagated to the standard SuperDARN data products using a new First-Principles Fitting Methodology (Chapter 7). These results can be applied to all previously recorded SuperDARN data and have shown a practical increase in data of >50%. This has significant impact on the SuperDARN and space science communities with respect to, for example, global convection maps and their use in global modelling efforts. These results also enable quantitative experiment design facilitating research into using SuperDARN to provide electron density measurements, with a preliminary investigation using the new SuperDARN fitting methodology presented in Chapter 8

Effects of sudden commencement on the ionosphere: PFISR observations and global MHD simulation

Sudden commencement (SC) induced by solar wind pressure enhancement can produce significant global impact on the coupled magnetosphere‐ionosphere (MI) system, and its effects have been studied extensively using ground magnetometers and coherent scatter radars. However, very limited observations have been reported about the effects of SC on the ionospheric plasma. Here we report detailed Poker Flat Incoherent Scatter Radar (PFISR) observations of the ionospheric response to SC during the 17 March 2015 storm. PFISR observed lifting of the F region ionosphere, transient field‐aligned ion upflow, prompt but short‐lived ion temperature increase, subsequent F region density decrease, and persistent electron temperature increase. A global magnetohydrodynamic (MHD) simulation has been carried out to characterize the SC‐induced current, convection, and magnetic perturbations. Simulated magnetic perturbations at Poker Flat show a satisfactory agreement with observations. The simulation provides a global context for linking localized PFISR observations to large‐scale dynamic processes in the MI system.Key PointsPFISR‐observed ionospheric plasma responses to field‐aligned currents and ionospheric convection vortices formed during sudden commencementResponses include F region plasma lifting, field‐aligned ion upflow, density decrease, short‐lived Ti increase and long‐lasting Te increaseGlobal MHD simulation reproduced the magnetic perturbation on the ground and revealed SC‐related FACs and convection evolutionsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136741/1/grl55728_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136741/2/grl55728.pd

Statistical Study of Ion Upflow and Downflow Observed by PFISR

Ion upflow in the F region and topside ionosphere can greatly influence the ion density and fluxes at higher altitudes and thus has significant impact on ion outflow. We investigated the statistical characteristics of ion upflow and downflow using a 3‐year (2011–2013) data set from the Poker Flat Incoherent Scatter Radar (PFISR). Ion upflow is twice more likely to occur on the nightside than on the dayside in PFISR observations, while downflow events occur more often in the afternoon sector. Upflow and downflow on the dayside tend to occur at altitudes ~500 km, higher than those on the nightside. Both upflow and downflow occur more frequently as ion convection speed increases. Upflow observed from 16 to 6 magnetic local time through midnight is associated with temperature and density enhancements. Occurrence rates of upflow on the nightside and downflow on the dayside increase with geomagnetic activity level. On the nightside, occurrence rate of ion upflow increases with enhanced solar wind and interplanetary magnetic field (IMF) drivers as well as southwestward local magnetic perturbations. The lack of correlation of upflow on the dayside with the solar wind and IMF parameters is because PFISR is usually equatorward of the dayside auroral zone. Occurrence rate of downflow does not show strong dependence on the solar wind and IMF conditions. However, it occurs much more frequently on the dayside when the IMF By > 10 nT and the IMF Bz < −10 nT, which we suggest is associated with the decaying of the dayside storm‐enhanced density (SED) and the SED plume.Key PointsThe occurrence frequency of ion upflow increases with enhanced geomagnetic activity level and stronger solar wind and IMF drivingIon upflow at PFISR latitude is twice more likely to occur on the nightside than on the daysidePeak ion downflow occurrence rate reaches 30% on the dayside during strongly positive IMF By and negative Bz, associated with SED and plumePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163443/3/jgra56049-sup-0001-2020JA028179-SI.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163443/2/jgra56049.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163443/1/jgra56049_am.pd

Examining the Auroral Ionosphere in Three Dimensions Using Reconstructed 2D Maps of Auroral Data to Drive the 3D GEMINI Model

We use the Geospace Environment Model of Ion-Neutral Interactions (GEMINI) to create three-dimensional, time-dependent simulations of auroral ionospheric parameters in the localized, several 100 km region surrounding auroral arcs observed during a winter 2017 sounding rocket campaign, resolving three-dimensional features of fine-scale (km) flow structures in the vicinity of an auroral arc. The three-dimensional calculations of GEMINI allow (with sufficient driving data) auroral current closure to be investigated without idealizing assumptions of sheet-like structures or height integrated ionospheres. Datamaps for two nearly sheet-like arcs are reconstructed from replications of the Isinglass sounding rocket campaign data, and combined with camera-based particle inversions into a set of driving inputs to run the 3D time-dependent model. Comparisons of model results to radar density profiles and to in situ magnetometry observations are presented. Slices of volumetric current, flow, and conductance structures from model outputs are used to interpret closure currents in an auroral arc region, and are compared to original in situ measurements for verification. The predominant source of return current region field aligned current closure for these slow time variation events is seen to be from the conductance gradients, including the Hall. The importance of the versus terms in the determination of the current structure provides a more complicated picture than a previous GEMINI study, which relied predominantly on the divergence of the electric field to determine current structure. Sensitivity of data-driven model results to details of replication and reconstruction processes are discussed, with improvements outlined for future work

Tropical limestone forest resilience during MIS-2: implications for Pleistocene foraging & modern conservation

In this paper we present a multi-proxy study of tropical limestone forest and its utilization by human groups during the major climatic and environmental upheavals of MIS-2 (29-11.7 ka BP). Our data are drawn from new field research within the Tràng An World Heritage property, on the edge of the Red River Delta, northern Vietnam. Key findings from this study include 1) that limestone forest formations were resilient to the large-scale landscape transformation and inundation of the Sunda continent at the end of the last glaciation; 2) that prehistoric human groups were probably present in this habitat through-out MIS-2; and 3) that the forested, almost insular, karst of Tràng An provided foragers with a stable resource-base in a wider changing landscape. These results have implications for our understanding of the prehistoric utilization of karst environments and resonance for conservation efforts in the face of climate and environmental change today

The Long Term Response of Birds to Climate Change: New Results from a Cold Stage Avifauna in Northern England

The early MIS 3 (55–40 Kyr BP associated with Middle Palaeolithic archaeology) bird remains from Pin Hole, Creswell Crags, Derbyshire, England are analysed in the context of the new dating of the site’s stratigraphy. The analysis is restricted to the material from the early MIS 3 level of the cave because the upper fauna is now known to include Holocene material as well as that from the Late Glacial. The results of the analysis confirm the presence of the taxa, possibly unexpected for a Late Pleistocene glacial deposit including records such as Alpine swift, demoiselle crane and long-legged buzzard with southern and/or eastern distributions today. These taxa are accompanied by more expected ones such as willow ptarmigan /red grouse and rock ptarmigan living today in northern and montane areas. Finally, there are temperate taxa normally requiring trees for nesting such as wood pigeon and grey heron. Therefore, the result of the analysis is that the avifauna of early MIS 3 in England included taxa whose ranges today do not overlap making it a non-analogue community similar to the many steppe-tundra mammalian faunas of the time. The inclusion of more temperate and woodland taxa is discussed in the light that parts of northern Europe may have acted as cryptic northern refugia for some such taxa during the last glacial. These records showing former ranges of taxa are considered in the light of modern phylogeographic studies as these often assume former ranges without considering the fossil record of those taxa. In addition to the anomalous combination of taxa during MIS 3 living in Derbyshire, the individuals of a number of the taxa are different in size and shape to members of the species today probably due to the high carrying capacity of the steppe-tundra

Supplement: "Localization and broadband follow-up of the gravitational-wave transient GW150914" (2016, ApJL, 826, L13)

This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Localization and broadband follow-up of the gravitational-wave transient GW150914

A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the GW data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network circulars, giving an overview of the participating facilities, the GW sky localization coverage, the timeline, and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the EM data and results of the EM follow-up campaign are being disseminated in papers by the individual teams

Localization and Broadband Follow-up of the Gravitational-wave Transient GW150914

A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the GW data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network circulars, giving an overview of the participating facilities, the GW sky localization coverage, the timeline, and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the EM data and results of the EM follow-up campaign are being disseminated in papers by the individual teams. </p