1,720,964 research outputs found
Spectral imaging of proton aurora and twilight at Tromsø, Norway
No full textAn imaging Echelle spectrograph designed for high-resolution studies of selected spectral features located in the visible spectrum was deployed from November 2001 until April 2003 in Tromsø, Norway. For moderately disturbed magnetic conditions, Tromsø is located on the equatorial edge of the evening auroral oval for several hours. Energetic protons are frequently the dominant particle energy source in this region. For this experiment, four spectral windows were selected, each around different emission features: H? (656.3 nm), H? (486.1 nm), N2+1NG 427.8 nm, and OI 777.4 nm. The 8° long slit of the spectrograph was centered on the magnetic zenith. This instrument provided simultaneous, high-resolution (~0.1 nm) spectra of H? and H? emissions, which offers a unique opportunity to investigate the H? to H? Balmer decrement in proton aurora. Information on the cloud cover and on the overall auroral activity was provided by a large field of view (70°) conventional imaging spectrograph that spans the 350–800 nm spectral range. In this paper we describe both instruments and demonstrate their capabilities for the study of the H Balmer emissions in twilight and during auroral activity. Our high-resolution spectra taken in twilight could be used to observe the variability of the geocoronal component over time and to compare the derived variability with midlatitude sites. We conclude that the 0.1 nm spectral resolution is sufficient to identify and take into account contaminating OH and N2 1PG features in H? emission profiles. Comparison of H? Doppler profiles observed at different locations (Tromsø, Poker Flat, Svalbard) in proton aurora is presented. Lummerzheim and Galand [2001] find that the shape of the violet wing of the Balmer profile is a more suitable indicator of the mean energy of the incident protons than the Doppler shift of the peak. Numerous uncertainties in measured and modeled H? and H? line profiles preclude using the Balmer decrement as an indicator of the precipitating proton flux
Observation of O+ (4P-4D0) lines in electron aurora over Svalbard
No full textThis work reports on observations of O+ lines in aurora over Svalbard, Norway. The Spectrographic Imaging Facility measures auroral spectra in three wavelength intervals (H, N+2 1N(0,2) and N+2 1N(1,3)). The oxygen ion 4P-4D0 multiplet (4639–4696 A° ) is blended with the N+2 1N(1,3) band. It is found that in electron aurora, the brightness of this multiplet, is on average, about 0.1 of the N+2 1N(0,2) total brightness. A joint optical and incoherent scatter radar study of an electron aurora event shows that the ratio is enhanced when the ionisation in the upper E-layer (140–190 km) is significant with respect to the E-layer peak below 130 km. Rayed arcs were observed on one such occasion, whereas on other occasions the auroral intensity was below the threshold of the imager. A one-dimensional electron transport model is used to estimate the cross section for production of the multiplet in electron collisions, yielding 0.18×10-18 cm2
Excitation of tall auroral rays by ohmic heating in field-aligned current filaments at F region heights
No full textThe formation of tall red rays in the ionosphere has been a longstanding unresolved problem of auroral physics. These rays are pencil-like structures which can extend from 150 km at their base to as high as 600 km. At these heights it is very difficult to deposit sufficient power in order to account for the luminosity of tall rays. This work examines ohmic heating by collisional processes in strong field-aligned current sheets to account for visible tall rays. The mechanism is demonstrated by two-dimensional simulation in a fully self-consistent treatment of the ionosphere and coupling to the magnetosphere. We find that a filamentary current density of about 600 µAm-2 over about ten seconds can pump sufficient energy into the ambient oxygen atoms to produce visible auroral red rays. The ohmic heating leads to an electron temperature in excess of 10,000 K in the upper F-region
Observation of O+ (4P-4D0) lines in proton aurora over Svalbard
Full text linkSpectra of a proton aurora event show lines of O+ 4P-4D0 multiplet (4639–4696 Å) enhanced relative to the N2+1N(0,2) compared to normal electron aurora. Conjugate satellite particle measurements are used as input to electron and proton transport models, to show that p/H precipitation is the dominant source of both the O+ and N2+1N emissions. The emission cross-section of the multiplet in p collisions with O and O2 estimated from published work does not explain the observed O+ brightness, suggesting a higher emission cross-section for low energy p impact on O
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Ohmic heating as evidence for strong field-aligned currents in filamentary aurora
No full textA large increase in electron temperature measured in filamentary aurora with the European incoherent scatter radar has been modeled with a one-dimensional electron transport and ion chemistry code. To account for the observed changes in electron temperature, while also reproducing the measured E region electron density profiles, a source of electron heating is required in addition to local heating from energy degradation of the precipitating electrons. We show that ohmic heating in a strong field-aligned current can account for the required heat source.<br/
Variations on the Author
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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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