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Träfflista för sökning "WFRF:(Opgenoorth H.J) "

Sökning: WFRF:(Opgenoorth H.J)

  • Resultat 1-16 av 16
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2.
  • Borälv, E., et al. (författare)
  • Correlation between ground-based observations of substorm signatures and magnetotail dynamics
  • 2005
  • Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 23, s. 997-1011
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a substorm event study combining Cluster and ground-based instrumentation. For this event ground-based magnetograms show a substorm onset and two separate substorm intensifications over Scandinavia, at the time located in the pre-midnight sector. During the substorm Cluster is located in the southern plasma sheet at a downtail distance of 18.5 Re. For all the substorm signatures seen on ground, corresponding plasma sheet drop-outs and re-entries of all or individual spacecraft of the Cluster constellation are observed. In general, plasma sheet drop-outs are assumed to be due to plasma sheet thinning/thickening and/or to magnetotail flapping. However, in the literature there has been some disagreement on both spatial and temporal characteristics of plasma sheet thinning and thickening during substorms. We therefore investigate the causes for the plasma sheet drop-outs for this event, which at first glance appears to show plasma sheet thinning at substorm onset, contradictory to the present standpoint in the literature.
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3.
  • Borälv, E., et al. (författare)
  • The dawn and dusk electrojet response to substorm onset
  • 2000
  • Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 18:9, s. 1097-1107
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the time delay between substorm onset and related reactions in the dawn and dusk ionospheric electrojets, clearly separated from the nightside located substorm current wedge by several hours in MLT. We looked for substorm onsets occurring over Greenland, where the onset was identified by a LANL satellite and DMI magnetometers located on Greenland. With this setup the MARIA magnetometer network was located at dusk, monitoring the eastward electrojet, and the IMAGE chain at dawn, for the westward jet. In the first few minutes following substorm onset, sudden enhancements of the electrojets were identified by looking for rapid changes in magnetograms. These results show that the speed of information transfer between the region of onset and the dawn and dusk ionosphere is very high. A number of events where the reaction seemed to preceed the onset were explained by either unfavorable instrument locations, preventing proper onset timing, or by the inner magnetosphere's reaction to the Earthward fast flows from the near-Earth neutral line model. Case studies with ionospheric coherent (SuperDARN) and incoherent (EISCAT) radars have been performed to see whether a convection-induced electric field or enhanced conductivity is the main agent for the reactions in the electrojets. The results indicate an imposed electric field enhancement.
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  • Kaurisitie, K., Syrjsuo, M., Amm, O., Viljanen, A., Pulkkinen, T.I. and Opgenoorth, H.J. (författare)
  • A statistical study of evening sectorarcs and electrojets.
  • 2001
  • Ingår i: Advances in Space Research. ; 28, s. 16045-1610
  • Tidskriftsartikel (refereegranskat)abstract
    • We present results of a statistical study of evening sector auroral arcs associated with electrojets. The study (including similar to 1000 all-sky camera (ASC) frames) is based on data of the MIRACLE instrument network. An automatic search engine is used
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  • Palin, Laurianne, et al. (författare)
  • Three-dimensional current systems and ionospheric effects associated with small dipolarization fronts
  • 2015
  • Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:5, s. 3739-3757
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a case study of eight successive plasma sheet (PS) activations (usually referred to as bursty bulk flows or dipolarization fronts), associated with small individual B-ZGSM increases on 31 March 2009 (0200-0900 UT), observed by the Time History of Events and Macroscale Interactions During Substorms mission. This series of events happens during very quiet solar wind conditions, over a period of 7 h preceding a substorm onset at 1230 UT. The amplitude of the dipolarizations increases with time. The low-amplitude dipolarization fronts are associated with few (1 or 2) rapid flux transport events (RFT, E-h > 2 mV/m), whereas the large-amplitude ones encompass many more RFT events. All PS activations are associated with small and localized substorm current wedge (SCW)-like current system signatures, which seems to be the consequence of RFT arrival in the near tail. The associated ground magnetic perturbations affect a larger part of the contracted auroral oval when, in the magnetotail, more RFT are embedded in PS activations (> 5). Dipolarization fronts with very low amplitude, a type usually not included in statistical studies, are of particular interest because we found even those to be associated with clear small SCW-like current system and particle injections at geosynchronous orbit. This exceptional data set highlights the role of flow bursts in the magnetotail and leads to the conclusion that we may be observing the smallest form of a substorm or rather its smallest element. This study also highlights the gradual evolution of the ionospheric current disturbance as the plasma sheet is observed to heat up.
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11.
  • Rosenqvist, Lisa, et al. (författare)
  • Comparison of local energy conversion estimates from Cluster with global MHD simulations
  • 2008
  • Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 35:21
  • Tidskriftsartikel (refereegranskat)abstract
    • The local energy conversion across the magnetopause has been estimated with Cluster for two magnetopause crossings. A load region, conversion from magnetic to particle energy, was identified on the dayside high-latitude magnetopause during south/dawnward IMF. Another crossing of the dawn flank magnetotail during dominantly duskward IMF was identified as a generator region where the magnetosphere is loaded with magnetic energy. The observations have been compared to results of the BATS-R-US global MHD simulation based on observed IMF conditions. BATS-R-US reproduced the magnetopause regions crossed by Cluster as a load and a generator region, correspondingly. The magnitude of the estimated energy conversion from Cluster and the model are in quite good agreement. BATS-R-US cannot reproduce the observed sharp magnetopause and some topological differences between the observations and the model occur.
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12.
  • Rosenqvist, Lisa, et al. (författare)
  • Modulated reconnection rate and energy conversion at the magnetopause under steady IMF conditions
  • 2008
  • Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:8
  • Tidskriftsartikel (refereegranskat)abstract
    • We use the multi-spacecraft mission Cluster to make observational estimates of the local energy conversion across the dayside high-latitude magnetopause. The energy conversion is estimated during eleven complete magnetopause crossings under steady south-dawnward interplanetary magnetic field (IMF). We describe a new method to determine the reconnection rate from the magnitude of the local energy conversion. The reconnection rate as well as the energy conversion varies during the course of the eleven crossings and is typically much higher for the outbound crossings. This supports the previous interpretation that reconnection is continuous but its rate is modulated.
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13.
  • Rosenqvist, L., et al. (författare)
  • Timing of substorm onset signatures on the ground and at geostationary orbit
  • 2002
  • Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 29:12, s. 33(1)-33(4)
  • Tidskriftsartikel (refereegranskat)abstract
    • In order to study the relative timing of substorm onset signatures on the ground and at geostationary orbit we have used data of simultaneous dispersionless electron and proton injections from the LANL satellite 1991-80, located slightly westward of Scandinavia. Out of 9 years of data we have identified a number of events during which such injections occurred close to local magnetic midnight. By careful inspection of ground-based magnetograms from the Scandinavian magnetometer network, IMAGE, we then identified the location and time of the formation of a substorm current wedge (SCW) during these events. 40 clear cases of geostationary injections, which were clearly associated with the formation of SCWs, were found. A statistical study of these events reveals that there is a clear time delay of the order of several minutes in the occurrence of the substorm injection with respect to the first indication of the SCW measured on the ground.
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14.
  • Rosenqvist, Lisa, et al. (författare)
  • Titan ionospheric conductivities from Cassini measurements
  • 2009
  • Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 57:14-15, s. 1828-1833
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the first results of ionospheric conductivities at Titan based on measurements during 17 Titan flybys from the Cassini spacecraft. We identify an ionospheric region ranging from 1450 95 km (approximately the location of the exobase) to approximately 1000 km where electrical currents perpendicular to the magnetic field may become important. In this region the ionosphere is highly conductive with peak Pedersen conductivities of 0.002-0.05 S/m and peak Hall conductivities of 0.01-0.3 S/m depending on Solar illumination and magnetospheric conditions. Ionospheric conductivities are found to be typically higher on the sunlit side of Titan. However, Hall and Pedersen conductivities depend strongly on the magnetic field magnitude which is highly variable, both in altitude and with respect to the draping configuration of Saturn's magnetic field around Titan. Furthermore, a consistent double peak nature is found in the altitude profile of the Pedersen conductivity. A high altitude peak is found to be located between 1300 and 1400 km. A second and typically more conductive region is observed below 1000 km, where the magnetic field strength drops sharply while the electron density still remains high. This nature of the Pedersen conductivity profile may give rise to complicated ionospheric-atmospheric dynamics and may be expected also at other unmagnetized objects with a substantial atmosphere, such as e.g. Mars and Venus. Estimates of the total Pedersen conductance are found to range between 1300 and 22,000 S. The Pedersen conductance is always higher than the local Alfven conductance but the difference varies by two orders of magnitude (from a factor 4 to 100). Thus, the conditions for reflection or absorption of Alfven waves in Titans ionosphere are highly variable and depends strongly on the magnetic field strength.
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15.
  • Syrjäsuo, M., Pulkkinen, T.I., Pellinen, R.P., Janhunen, P., Kauristie, K., Viljanen, A., Opgenoorth, H.J., Karllson, P., Wallman, S., Eglitis, P., Nielsen, E. and Thomas, C. (författare)
  • Observations of substorm electrodynamics using the miracle network.
  • 2001
  • Ingår i: The 4th International Conference on Substorms, ICS4. - : Terra Scient. Publ. Comp. Tokyo, Japan. Kaje Hamana, Japan.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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16.
  • Welling, D. T., et al. (författare)
  • Recommendations for Next-Generation Ground Magnetic Perturbation Validation
  • 2018
  • Ingår i: Space Weather. - 1542-7390. ; 16:12, s. 1912-1920
  • Tidskriftsartikel (refereegranskat)abstract
    • Data-model validation of ground magnetic perturbation forecasts, specifically of the time rate of change of surface magnetic field, dB/dt, is a critical task for model development and for mitigation of geomagnetically induced current effects. While a current, community-accepted standard for dB/dt validation exists (Pulkkinen et al., 2013), it has several limitations that prevent more complete understanding of model capability. This work presents recommendations from the International Forum for Space Weather Capabilities Assessment Ground Magnetic Perturbation Working Team for creating a next-generation validation suite. Four recommendations are made to address the existing suite: greatly expand the number of ground observatories used, expand the number of events included in the suite from six to eight, generate metrics as a function of magnetic local time, and generate metrics as a function of activity type. For each of these, implementation details are explored. Limitations and future considerations are also discussed.
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