Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude
International audience We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s d...
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ftccsdartic:oai:HAL:hal-00316255v1 2023-11-12T04:16:36+01:00 Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude Malnes, E. Bjørnå, N. Hansen, T. L. The Auroral Observatory University of Tromsø (UiT) 1996 https://hal.science/hal-00316255 https://hal.science/hal-00316255/document https://hal.science/hal-00316255/file/angeo-14-1328-1996.pdf en eng HAL CCSD European Geosciences Union hal-00316255 https://hal.science/hal-00316255 https://hal.science/hal-00316255/document https://hal.science/hal-00316255/file/angeo-14-1328-1996.pdf info:eu-repo/semantics/OpenAccess ISSN: 0992-7689 EISSN: 1432-0576 Annales Geophysicae https://hal.science/hal-00316255 Annales Geophysicae, 1996, 14 (12), pp.1328-1342 [SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere [SDU.STU]Sciences of the Universe [physics]/Earth Sciences info:eu-repo/semantics/article Journal articles 1996 ftccsdartic 2023-10-21T23:08:39Z International audience We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s data dumps. They are offset by 125?130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental. Article in Journal/Newspaper EISCAT Archive ouverte HAL (Hyper Article en Ligne, CCSD - Centre pour la Communication Scientifique Directe) |
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Open Polar |
collection |
Archive ouverte HAL (Hyper Article en Ligne, CCSD - Centre pour la Communication Scientifique Directe) |
op_collection_id |
ftccsdartic |
language |
English |
topic |
[SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere [SDU.STU]Sciences of the Universe [physics]/Earth Sciences |
spellingShingle |
[SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere [SDU.STU]Sciences of the Universe [physics]/Earth Sciences Malnes, E. Bjørnå, N. Hansen, T. L. Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
topic_facet |
[SDU.OCEAN]Sciences of the Universe [physics]/Ocean Atmosphere [SDU.STU]Sciences of the Universe [physics]/Earth Sciences |
description |
International audience We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are short-lived and occur in single 2- or 10-s data dumps. They are offset by 125?130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental. |
author2 |
The Auroral Observatory University of Tromsø (UiT) |
format |
Article in Journal/Newspaper |
author |
Malnes, E. Bjørnå, N. Hansen, T. L. |
author_facet |
Malnes, E. Bjørnå, N. Hansen, T. L. |
author_sort |
Malnes, E. |
title |
Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
title_short |
Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
title_full |
Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
title_fullStr |
Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
title_full_unstemmed |
Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude |
title_sort |
anomalous echoes observed with the eiscat uhf radar at 100-km altitude |
publisher |
HAL CCSD |
publishDate |
1996 |
url |
https://hal.science/hal-00316255 https://hal.science/hal-00316255/document https://hal.science/hal-00316255/file/angeo-14-1328-1996.pdf |
genre |
EISCAT |
genre_facet |
EISCAT |
op_source |
ISSN: 0992-7689 EISSN: 1432-0576 Annales Geophysicae https://hal.science/hal-00316255 Annales Geophysicae, 1996, 14 (12), pp.1328-1342 |
op_relation |
hal-00316255 https://hal.science/hal-00316255 https://hal.science/hal-00316255/document https://hal.science/hal-00316255/file/angeo-14-1328-1996.pdf |
op_rights |
info:eu-repo/semantics/OpenAccess |
_version_ |
1782333675501780992 |