English
norskBlar i forfatter "Brekke, Asgeir"
Viser treff 1-18 av 18
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A case study of gravity wave dissipation in the polar MLT region using sodium LIDAR and radar data
(Journal article; Tidsskriftartikkel; Peer reviewed, 2014) -
A case study on generation mechanisms of a sporadic sodium layer above Tromsø (69.6° N) during a night of high auroral activity
(Journal article; Tidsskriftartikkel; Peer reviewed, 2015-08-05)We have quantitatively evaluated generation mechanisms of a sporadic sodium layer (SSL) based on observational data obtained by multiple instruments at a high-latitude station: Ramfjordmoen, Tromsø, Norway (69.6° N, 19.2° E). The sodium lidar observed an SSL at 21:18 UT on 22 January 2012. The SSL was observed for 18 min, with a maximum sodium density of about 1.9 × 1010 m−3 at 93 km with a 1.1 km ... -
Climatic trends in E-region critical frequency and virtual height above Tromsø (70 degrees N, 10 degrees E)
(Journal article; Tidsskriftartikkel; Peer reviewed, 2007-11-29)We have examined the long time series of observations of E-region virtual height (1948–2006) and critical frequency (1935-2006) hitherto made by the Tromsø ionosonde at 70 degrees N, 19 degrees E. Combining a simplistic trend analysis with a rigorous treatment of errors we identify a negative trend in critical frequency. While a similar analysis of the virtual height h'E also suggests a negative ... -
Comparison of the orientation of small-scale electron density irregularities and F region plasma flow direction.
(Journal article; Tidsskriftartikkel; Peer reviewed, 2000)Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic ... -
Det historiske nordlys
(Journal article; Tidsskriftartikkel, 1980)Nordlyset har vært omgitt av både mystikk og beundring. Det har inspirert kunstnere til betagende kunstverk og skremt andre til fortvilelse. Det er umulig å fastslå når den første beskrivelsen av nordlys har funnet sted. I det følgende kapitlet skal vi gå historiens vei og presentere noen inntrykk av de tidligste forestillingene om fenomenet, slike som en kan finne spør av i den muntlige tradisjon, ... -
Directional features of the downshifted peak observed in HF-induced stimulated electromagnetic emission spectra obtained using an interferometer.
(Journal article; Peer reviewed; Tidsskriftartikkel, 2006-08-09)A high frequency (HF) ionospheric modification experiment was carried out between 25 September and 8 October 2004, using the EISCAT HF transmitter located near Tromsø, Norway. During this experiment the spectra of the stimulated HF sideband waves (stimulated electromagnetic emission or SEE) induced by the HF pump were observed using an interferometer consisting of three spaced receiving antennas ... -
Evaluation of a method to derive ionospheric conductivities using two auroral emissions (428 and 630 nm) measured with a photometer at Tromsø (69.6°N)
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-07-03)This paper mainly aims at evaluating capabilities of derivation of ionospheric conductivities using two principal auroral emissions (427.8 and 630 nm). We have evaluated a photometric method of derivation of ionospheric conductivities based on simultaneous observations of a photometer (feld of view = ~1.2°), a digital camera, and the EISCAT UHF radar (feld of view = ~0.7°) operated at Tromsø, ... -
Fysikk i den polare atmosfæren og nordlyset
(Research report; Forskningsrapport, 2008)Den polare atmosfæren har fått mer oppmerksomhet i den senere tid enn den vanligvis har vært forunt. Dette henger sammen med tanken om at den er så ren og upåvirket av eventuelle forurensninger som vi pumper ut i atmosfæren i de mer urbane strøk. Dersom atmosfæren tar skade av denne aktiviteten, forventer en å se skadevirkningene først og klarest i den polare atmosfæren. Svekkelsen av ozonlaget ... -
Generation of the lower-thermospheric vertical wind estimated with the EISCAT KST radar at high latitudes during periods of moderate geomagnetic disturbance
(Journal article; Tidsskriftartikkel; Peer reviewed, 2008-06-11)Lower-thermospheric winds at high latitudes during moderately-disturbed geomagnetic conditions were studied using data obtained with the European Incoherent Scatter (EISCAT) Kiruna-Sodankylä-Tromsø (KST) ultrahigh frequency (UHF) radar system on 9–10 September 2004. The antenna-beam configuration was newly designed to minimize the estimated measurement error of the vertical neutralwind speed in the ... -
An imaging interferometry capability for the EISCAT Svalbard Radar.
(Journal article; Peer reviewed; Tidsskriftartikkel, 2005-01-31)Interferometric imaging (aperture synthesis imaging) is a technique used by radio astronomers to achieve angular resolution that far surpasses what is possible with a single large aperture. A similar technique has been used for radar imaging studies of equatorial ionospheric phenomena at the Jicamarca Radio Observatory. We present plans for adding an interferometric imaging capability to the ... -
An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra.
(Journal article; Peer reviewed; Tidsskriftartikkel, 2005-01-31)When the Earth's ionosphere is irradiated by a radiofrequency (RF) electromagnetic wave of sufficiently high power density and tuned to match a natural E- or F-region plasma frequency, ionospheric magnetoionic wave modes may be excited and may generate RF electromagnetic sideband waves via nonlinear interactions. These secondary emissions, which may then escape from the ionosphere, have been termed ... -
Internasjonale forpliktelser
(Journal article; Tidsskriftartikkel, 1980)I forbindelse med at Nordlysobservatoriet har eksistert i 50 år kan det være på sin plass å spørre om det fortsatt er behov for virksomheten. Svaret er at observatoriet fyller en viktig oppgave i internasjonal forskning. -
Irregular structures of the F layer at high latitudes during ionospheric heating.
(Journal article; Peer reviewed; Tidsskriftartikkel, 2000)Results on heating the ionospheric F region above Tromsø, Norway are presented. The ionosphere was monitored by satellite tomography and amplitude scintillation methods as well as the EISCAT incoherent scatter radar. No effect of heating was observed in the daytime. In the evening and in the pre-midnight sector, noticeable tilts of the F region were observed during heating periods. The tilts overlapped ... -
The legacy of the Haldde Observatory
(Journal article; Tidsskriftartikkel; Peer reviewed, 2021-01-14)A historical review of the establishment of the Haldde Observatory in Bossekop, Kaafjord, Finnmark, in northern Norway is presented together with some of the scientific outcomes of the efforts and the aftermath of this enterprise that led to the establishment of the University of Tromsø in 1968 and finally the inauguration of the Haldde Observatory as a historic site by the European Physical Society ... -
Nordlysobservatoriet 50 år
(Book; Bok, 1980)Nordlysobservatoriet i Tromsø ble høytidelig innviet 6. august 1930. Et 25-års jubileum ble markert med en tilstelning 30. juli 1955. Ved 25-års jubileet i 1955 ble det utarbeidet et festskrift - men det var ikke penger til å få det trykt. Desto gledeligere er det at vi til 50-års jubileet får anledning til et spesialhefte av OTTAR for å beskrive Nordlysobservatoriets historie og virksomhet, et hefte ... -
Nordområdeprosjektet EISCAT
(Chronicle; Kronikk, 2011) -
Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency
(Journal article; Tidsskriftartikkel; Peer reviewed, 2009-01-08)Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency fH was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency ... -
The relationship between small-scale and large-scale ionospheric electron density irregularities generated by powerful HF electromagnetic waves at high latitudes
(Journal article; Tidsskriftartikkel; Peer reviewed, 2006-11-21)Satellite radio beacons were used in June 2001 to probe the ionosphere modified by a radio beam produced by the EISCAT high-power, high-frequency (HF) transmitter located near Tromsø (Norway). Amplitude scintillations and variations of the phase of 150- and 400-MHz signals from Russian navigational satellites passing over the modified region were observed at three receiver sites. In several papers ...
