1,721,006 research outputs found
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
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 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
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
Modelling of N21P emission rates in aurora using various cross sections for excitation
No full textMeasurements of N21P auroral emissions from the (4,1) and (5,2)
bands have been made at high temporal and spatial resolution in
the region of the magnetic zenith. The instrument used was the
auroral imager ASK, situated at Ramfjordmoen, Norway (69.6 N,
19.2 E) on 22 October 2006. Measurements from the European
Incoherent Scatter Radar (EISCAT) have been combined with the
optical measurements, and incorporated into an ionospheric model
to obtain height profiles of electron density and emission rates
of the N21P bands. The radar data provide essential
verification that the energy flux used in the model is correct.
One of the most important inputs to the model is the cross section
for excitation to the B3Πg electronic state, as well as
the cross sections to higher states from which cascading into the
B state occurs. The balance equations for production and loss of
the populations of all levels in each state are solved in order to
find the cascade contributions. Several sets of cross sections
have been considered, and selected cross sections have been used
to construct "emission" cross sections for the observed bands. The
resulting brightnesses are compared with those measured by ASK.
The importance of specific contributions from cascading is found,
with more than 50% of the total brightness resulting from
cascading. The cross sections used are found to produce a range of
brightnesses well within the uncertainty of both the modelled and
measured values
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
High resolution measurements and modeling of auroral hydrogen emission line profiles
Full text linkMeasurements in the visible wavelength range at
high spectral resolution (1.3 A° ) have been made at Longyearbyen,
Svalbard (15.8 E,78.2 N) during an interval of intense
proton precipitation. The shape and Doppler shift of hydrogen
Balmer beta line profiles have been compared with
model line profiles, using as input ion energy spectra from
almost coincident passes of the FAST and DMSP spacecraft.
The comparison shows that the simulation contains the important
physical processes that produce the profiles, and confirms
that measured changes in the shape and peak wavelength
of the hydrogen profiles are the result of changing
energy input. This combination of high resolution measurements
with modeling provides a method of estimating the
incoming energy and changes in flux of precipitating protons
over Svalbard, for given energy and pitch-angle distributions.
Whereas for electron precipitation, information on the incident
particles is derived from brightness and brightness ratios
which require at least two spectral windows, for proton precipitation
the Doppler profile of resulting hydrogen emission
is directly related to the energy and energy flux of the incident
energetic protons and can be used to gather information
about the source region. As well as the expected Doppler
shift to shorter wavelengths, the measured profiles have a significant
red-shifted component, the result of upward flowing
emitting hydrogen atoms
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/
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