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- Kastinen, Daniel, et al.
(författare)
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Resolving the ambiguous direction of arrival of weak meteor radar trail echoes
- 2021
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Ingår i: Atmospheric Measurement Techniques. - : Copernicus GmbH. - 1867-1381 .- 1867-8548. ; 14:5, s. 3583-3596
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Tidskriftsartikel (refereegranskat)abstract
- Meteor phenomena cause ionized plasmas that can be roughly divided into two distinctly different regimes: a dense and transient plasma region co-moving with the ablating meteoroid and a trail of diffusing plasma left in the atmosphere and moving with the neutral wind. Interferometric radar systems are used to observe the meteor trails and determine their positions and drift velocities. Depending on the spatial configuration of the receiving antennas and their individual gain patterns, the voltage response can be the same for several different plane wave directions of arrival (DOAs), thereby making it impossible to determine the correct direction. A low signal-to-noise ratio (SNR) can create the same effect probabilistically even if the system contains no theoretical ambiguities. Such is the case for the standard meteor trail echo data products of the Sodankyl Geophysical Observatory SKiYMET all-sky interferometric meteor radar. Meteor trails drift slowly enough in the atmosphere and allow for temporal integration, while meteor head echo targets move too fast. Temporal integration is a common method to increase the SNR of radar signals. For meteor head echoes, we instead propose to use direct Monte Carlo (DMC) simulations to validate DOA measurements. We have implemented two separate temporal integration methods and applied them to 2222 events measured by the Sodankyl meteor radar to simultaneously test the usefulness of such DMC simulations on cases where temporal integration is possible, validate the temporal integration methods, and resolve the ambiguous SKiYMET data products. The two methods are the temporal integration of the signal spatial correlations and matchedfilter integration of the individual radar channel signals. The results are compared to Bayesian inference using the DMC simulations and the standard SkiYMET data products. In the examined data set, 13% of the events were indicated as ambiguous. Out of these, 13% contained anomalous signals. In 95% of all ambiguous cases with a nominal signal, the three methods found one and the same output DOA, which was also listed as one of the ambiguous possibilities in the SkiYMET analysis. In all unambiguous cases, the results from all methods concurred.
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- Pitkänen, Timo, et al.
(författare)
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Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm
- 2009
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Ingår i: Annales Geophysicae. - : Copernicus Publications. - 0992-7689 .- 1432-0576. ; 27, s. 2157-2171
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of the polar cap and the auroral oval are examined in the evening sector during a substorm period on 25 November 2000 by using measurements of the EISCAT incoherent scatter radars, the north-south chain of the MIRACLE magnetometer network, and the Polar UV Imager.The location of the polar cap boundary (PCB) is estimated from electron temperature measurements by the mainland low-elevation EISCAT VHF radar and the 42m antenna of the EISCAT Svalbard radar. A comparison to the poleward auroral emission (PAE) boundary by the Polar UV Imager shows that in this event the PAE boundary is typically located 0.7 of magnetic latitude poleward of the PCB by EISCAT. The convection reversal boundary (CRB) is determined from the 2-D plasma drift velocity extracted from the dual-beam VHF data. The CRB is located 0.5–1 equatorward of the PCB indicating the existence of viscous-driven antisunward convection on closed field lines.East-west equivalent electrojets are calculated from the MIRACLE magnetometer data by the 1-D upward continuation method. In the substorm growth phase, electrojets together with the polar cap boundary move gradually equatorwards. During the substorm expansion phase, the Harang discontinuity (HD) region expands to the MLT sector of EISCAT. In the recovery phase the PCB follows the poleward edge of the westward electrojet.The local ionospheric reconnection electric field is calculated by using the measured plasma velocities in the vicinity of the polar cap boundary. During the substorm growth phase, values between 0 and 10 mV/m are found. During the late expansion and recovery phase, the reconnection electric field has temporal variations with periods of 7–27 min and values from 0 to 40 mV/m. It is shown quantitatively, for the first time to our knowledge, that intensifications in the local reconnection electric field correlate with appearance of auroral poleward boundary intensifications (PBIs) in the same MLT sector. The results suggest that PBIs (typically 1.5 h MLT wide) are a consequence of temporarily enhanced longitudinally localized magnetic flux closure in the magnetotail.
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