1,721,090 research outputs found
Long-term Determination of Energetic Electron Precipitation into the Atmosphere Using Subionospheric VLF Perturbations
Get PDFEnergetic electron precipitation (EEP) is an important loss mechanism in the dynamic radiation belts. Obtaining accurate precipitating flux measurements is necessary for the understanding, modelling, and analysis of the spatial and temporal belt dynamics, their impact on the atmosphere, and ultimately climate. In this study we analyse observations of subionospherically propagating very low frequency (VLF) radio waves to determine EEP flux magnitudes from the outer radiation belt through their influence on the lower ionosphere. We analyse data from a radio wave receiver located in Sodankyla, Finland (SGO), part of the Antarctic-Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK), which observes VLF radio signals from the US high-power narrow-band communication transmitter with call sign NAA located in Culter, Maine. We use a near-continuous dataset from November 2004 until March 2013 to determine long time period EEP into the atmosphere along this path which spans 3-8 L, i.e., under where these outer radiation belt processes occur.
We determine quiet day curves (QDC) over the entire long time period and use these to identify propagation disturbances caused by EEP. Modelling of LWPC radio wave propagation is used to estimate the electron fluxes precipitating into the atmosphere from the observed amplitude disturbances. Correlation is preformed with other EEP measurements, geomagnetic indices, and chorus wave intensity to examine the link between geomagnetic indices, plasma wave occurrence and EEP flux magnitudes. We find that using a dynamically varying energy spectral gradient for precipitating fluxes in the modelling gives improvements in the extracted EEP flux magnitudes compared to the fixed gradient used in Clilverd et al. [2010]. Our method performs well during the summer months when the day-lit ionosphere is the most stable. However our approach is unusable during the winter-time as it grossly over-exaggerates precipitating fluxes because of the higher variability in the received signal amplitudes. During the summer months only we have obtained 611 days worth of reasonable NAA-SGO fluxes over the 2004-2013 period. These fluxes agree well with POES BLC measurements during EEP events. Our method of EEP detection is also sensitive to measuring flux magnitudes below the noise floor of the POES instruments. A case study is also performed contrasting the NAA-SGO extracted EEP fluxes presented here to EEP measurements from a different AARDDVARK path recently published in Simon Wedlund et al. [2014]
Long-term Determination of Energetic Electron Precipitation into the Atmosphere Using Subionospheric VLF Perturbations
No full textEnergetic electron precipitation (EEP) is an important loss mechanism in the dynamic radiation belts. Obtaining accurate precipitating flux measurements is necessary for the understanding, modelling, and analysis of the spatial and temporal belt dynamics, their impact on the atmosphere, and ultimately climate. In this study we analyse observations of subionospherically propagating very low frequency (VLF) radio waves to determine EEP flux magnitudes from the outer radiation belt through their influence on the lower ionosphere. We analyse data from a radio wave receiver located in Sodankyla, Finland (SGO), part of the Antarctic-Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK), which observes VLF radio signals from the US high-power narrow-band communication transmitter with call sign NAA located in Culter, Maine. We use a near-continuous dataset from November 2004 until March 2013 to determine long time period EEP into the atmosphere along this path which spans 3-8 L, i.e., under where these outer radiation belt processes occur.
We determine quiet day curves (QDC) over the entire long time period and use these to identify propagation disturbances caused by EEP. Modelling of LWPC radio wave propagation is used to estimate the electron fluxes precipitating into the atmosphere from the observed amplitude disturbances. Correlation is preformed with other EEP measurements, geomagnetic indices, and chorus wave intensity to examine the link between geomagnetic indices, plasma wave occurrence and EEP flux magnitudes. We find that using a dynamically varying energy spectral gradient for precipitating fluxes in the modelling gives improvements in the extracted EEP flux magnitudes compared to the fixed gradient used in Clilverd et al. [2010]. Our method performs well during the summer months when the day-lit ionosphere is the most stable. However our approach is unusable during the winter-time as it grossly over-exaggerates precipitating fluxes because of the higher variability in the received signal amplitudes. During the summer months only we have obtained 611 days worth of reasonable NAA-SGO fluxes over the 2004-2013 period. These fluxes agree well with POES BLC measurements during EEP events. Our method of EEP detection is also sensitive to measuring flux magnitudes below the noise floor of the POES instruments. A case study is also performed contrasting the NAA-SGO extracted EEP fluxes presented here to EEP measurements from a different AARDDVARK path recently published in Simon Wedlund et al. [2014]
Techniques to Determine Quiet Day Curves for Subionospheric VLF Observations
No full textThe ionization rate of the upper atmosphere can be significantly increased by space weather events, examples being solar proton events (SPE), solar flares, and energetic electron precipitation from the radiation belts. An increase in the ionization rate leads to a lowering of the lower edge of the ionospheric D-region. To study the effect of space weather events on our atmosphere it is important 1) to be able to detect the events and also 2) to have some way of determining changes in the height of the D-region. Very low frequency (VLF) radio waves propagate in the waveguide between the surface of the Earth and the lower edge of the ionosphere (D-region). Changes in the height of the D-region lead to changes in the amplitude and phase of the VLF signal received at an antenna. To gain an accurate indication of the size of these changes we need to know what the undisturbed signal, known as a Quiet Day Curve (QDC), would have been if no space weather event had taken place.
High power narrow-band communications transmitters operated by multiple nations provide the VLF radio signals used in this technique. In this study we use VLF radio wave observations from the Antarctic-Arctic Radiation- belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK) receivers located at Edmonton, Canada and Scott Base, Antarctica.
The purpose of this study is to develop a technique for the automatic calculation of QDCs for long-period experimental subionospheric VLF data sets. To enable the quantitative evaluation of how well our QDC finding techniques identify the true QDC of a data set, we have created a suite of synthetic data layers with a known QDC and imposed perturbations similar to those seen in real VLF data. We present this evaluation and comparison between the techniques to allow determination of the best QDC finding technique from those developed.
We evaluate two techniques for determining a long-period QDC by algorithm. These are Principal Component Analysis (PCA) and 2-dimensional Discrete Fourier Transforms (DFT). We also evaluate an averaging technique that finds a combined daily curve as a baseline comparison to our techniques. We further evaluate several adjustments to these techniques, endeavouring to improve the resulting QDC. We determine that the best QDC technique for data sets longer than two years is an adjustment to the DFT technique, while, for data sets shorter than two years, the best technique is PCA applied to a smoothed data set.
We judge the success of our adjusted DFT technique from the finding that the typical difference between the QDC and the synthetic data background is 0.13~dB for day and 0.17~dB for night. These values are smaller than typical experimentally observed noise levels. We therefore conclude that this QDC finding technique is successful. The pre-smoothed PCA technique gives a typical difference between the QDC and the synthetic data background of 0.38~dB for day, 0.47~dB for night. We therefore conclude that this QDC finding technique is fairly successful, although not as conclusively as the DFT based technique is.
We then apply our chosen QDC finding techniques, according to the length of the data sets, to the VLF observations. We apply the adjusted DFT technique to our Scott Base data sets, which span 4 years of observations, i.e., 2,103,840 minutes. We find that the QDC finding technique appears to qualitatively extract the QDC from these real data sets. In particular during solar flares, the extraction looks sensible. We provide examples of the difference between the received VLF signal and the QDC for an example day during which 10 solar flares occurred. We apply the pre-smoothed PCA technique to our Edmonton data sets, which span 20 months of usable data, i.e., 915,840 minutes. We find that this technique appears to be working, but have less confidence in its ability to extract perturbations lasting longer than a few hours
Techniques to Determine Quiet Day Curves for Subionospheric VLF Observations
No full textThe ionization rate of the upper atmosphere can be significantly increased by space weather events, examples being solar proton events (SPE), solar flares, and energetic electron precipitation from the radiation belts. An increase in the ionization rate leads to a lowering of the lower edge of the ionospheric D-region. To study the effect of space weather events on our atmosphere it is important 1) to be able to detect the events and also 2) to have some way of determining changes in the height of the D-region. Very low frequency (VLF) radio waves propagate in the waveguide between the surface of the Earth and the lower edge of the ionosphere (D-region). Changes in the height of the D-region lead to changes in the amplitude and phase of the VLF signal received at an antenna. To gain an accurate indication of the size of these changes we need to know what the undisturbed signal, known as a Quiet Day Curve (QDC), would have been if no space weather event had taken place.
High power narrow-band communications transmitters operated by multiple nations provide the VLF radio signals used in this technique. In this study we use VLF radio wave observations from the Antarctic-Arctic Radiation- belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK) receivers located at Edmonton, Canada and Scott Base, Antarctica.
The purpose of this study is to develop a technique for the automatic calculation of QDCs for long-period experimental subionospheric VLF data sets. To enable the quantitative evaluation of how well our QDC finding techniques identify the true QDC of a data set, we have created a suite of synthetic data layers with a known QDC and imposed perturbations similar to those seen in real VLF data. We present this evaluation and comparison between the techniques to allow determination of the best QDC finding technique from those developed.
We evaluate two techniques for determining a long-period QDC by algorithm. These are Principal Component Analysis (PCA) and 2-dimensional Discrete Fourier Transforms (DFT). We also evaluate an averaging technique that finds a combined daily curve as a baseline comparison to our techniques. We further evaluate several adjustments to these techniques, endeavouring to improve the resulting QDC. We determine that the best QDC technique for data sets longer than two years is an adjustment to the DFT technique, while, for data sets shorter than two years, the best technique is PCA applied to a smoothed data set.
We judge the success of our adjusted DFT technique from the finding that the typical difference between the QDC and the synthetic data background is 0.13~dB for day and 0.17~dB for night. These values are smaller than typical experimentally observed noise levels. We therefore conclude that this QDC finding technique is successful. The pre-smoothed PCA technique gives a typical difference between the QDC and the synthetic data background of 0.38~dB for day, 0.47~dB for night. We therefore conclude that this QDC finding technique is fairly successful, although not as conclusively as the DFT based technique is.
We then apply our chosen QDC finding techniques, according to the length of the data sets, to the VLF observations. We apply the adjusted DFT technique to our Scott Base data sets, which span 4 years of observations, i.e., 2,103,840 minutes. We find that the QDC finding technique appears to qualitatively extract the QDC from these real data sets. In particular during solar flares, the extraction looks sensible. We provide examples of the difference between the received VLF signal and the QDC for an example day during which 10 solar flares occurred. We apply the pre-smoothed PCA technique to our Edmonton data sets, which span 20 months of usable data, i.e., 915,840 minutes. We find that this technique appears to be working, but have less confidence in its ability to extract perturbations lasting longer than a few hours
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Experimental evidence and properties of EMIC wave driven electron precipitation
No full textOne of the key drivers of electron losses from the radiation belts is the interaction between radiation belt electrons and the electromagnetic plasma waves that populate the magnetosphere. In particular, electromagnetic ion cyclotron (EMIC) waves have been touted as a potential sources of significant electron loss from the radiation belts. However, until recently there has been a lack of experimental evidence for this precipitation occurring. Because of this, there is little experimental evidence for the properties of the precipitation, in particular the lower energy limit of EMIC interactions with radiation belt electrons.
The main focus of this thesis is investigating a 17 year database of proton precipitation-associated relativistic electron precipitation events detected by the POES satellite constellation, believed to be driven by interactions with EMIC waves. This database represents an unheralded opportunity for in-depth study of EMIC waves and their interactions with energetic electrons. Unfortunately, the utility of this database has been limited due to the lack of accompanying wave observations; without direct evidence of EMIC wave activity, there remains significant doubt as to the true driver of the observed precipitation. In this thesis, we initially present two in-depth case studies of events from the precipitation database, showing clear evidence of concurrent EMIC wave activity and the observed precipitation. We follow up these studies with a broad statistical analysis of the precipitation database, comparing the event locations to ground-based magnetometers. We show a remarkable correlation between the precipitation events and EMIC waves observed on the ground, with as many as 90% of precipitation events occurring during periods of EMIC wave activity. We show that this correlation cannot be due to random chance, establishing a strong link between the precipitation events and EMIC wave activity. Finally, we also show that while our precipitation events imply wave activity, wave activity does not necessarily imply electron precipitation.
Given the results of these studies, we have significant confidence that our database represents EMIC-wave scattered electron precipitation. We present two further case studies, investigating in-depth the energy and intensity characteristics of two events from the precipitation database. Through comparison with the DEMETER satellite, we are able derive electron energy spectra for these events
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Experimental evidence and properties of EMIC wave driven electron precipitation
Get PDFOne of the key drivers of electron losses from the radiation belts is the interaction between radiation belt electrons and the electromagnetic plasma waves that populate the magnetosphere. In particular, electromagnetic ion cyclotron (EMIC) waves have been touted as a potential sources of significant electron loss from the radiation belts. However, until recently there has been a lack of experimental evidence for this precipitation occurring. Because of this, there is little experimental evidence for the properties of the precipitation, in particular the lower energy limit of EMIC interactions with radiation belt electrons.
The main focus of this thesis is investigating a 17 year database of proton precipitation-associated relativistic electron precipitation events detected by the POES satellite constellation, believed to be driven by interactions with EMIC waves. This database represents an unheralded opportunity for in-depth study of EMIC waves and their interactions with energetic electrons. Unfortunately, the utility of this database has been limited due to the lack of accompanying wave observations; without direct evidence of EMIC wave activity, there remains significant doubt as to the true driver of the observed precipitation. In this thesis, we initially present two in-depth case studies of events from the precipitation database, showing clear evidence of concurrent EMIC wave activity and the observed precipitation. We follow up these studies with a broad statistical analysis of the precipitation database, comparing the event locations to ground-based magnetometers. We show a remarkable correlation between the precipitation events and EMIC waves observed on the ground, with as many as 90% of precipitation events occurring during periods of EMIC wave activity. We show that this correlation cannot be due to random chance, establishing a strong link between the precipitation events and EMIC wave activity. Finally, we also show that while our precipitation events imply wave activity, wave activity does not necessarily imply electron precipitation.
Given the results of these studies, we have significant confidence that our database represents EMIC-wave scattered electron precipitation. We present two further case studies, investigating in-depth the energy and intensity characteristics of two events from the precipitation database. Through comparison with the DEMETER satellite, we are able derive electron energy spectra for these events
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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