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  • 1
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Rapid fading of auroral activity a few minutes before substorm breakup has earlier been analyzed in case-studies. Here we report on a study in which all-sky camera (ASC) and magnetic data over 3 years were examined to find breakups that were accompanied by a preceding fading. To illustrate typical features of the fading effect we analyze three events in detail and discuss seven other events to find the spatial and temporal behavior of the fading and the global conditions favoring this phenomenon, which is not associated with every breakup. In these ten events the precipitation diminished typically for about 2 min and a local breakup followed after 2–3 min. Usually the arc which broke up had faded earlier. Comparison with geostationary electron flux recordings shows that in many cases the global onset had already taken place when the fading was recorded at a different longitude. Thus fading is not just a growth-phase phenomenon as often thought, but can also appear as a precursor of the approaching auroral bulge. The AE index and solar-wind data reveal that the fading has a tendency to take place during magnetically disturbed conditions caused by continuous energy input from the solar wind. Furthermore, while a widely recognized phenomenon, we have found that the fading prior to breakup is not a very common feature in the spatio-temporal scale of auroral ASC recordings. In many cases the deepness of the fading had a longitudinal dependence, which leads to the suggestion that this phenomenon is related to azimuthal gradients in the tail magnetic field and/or plasma pressure. Possible scenarios causing fading both before and after the onset are discussed based on a few previously presented theoretical auroral-arc models.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The AE indices are generally used for monitoring the level of magnetic activity in the auroral oval region. In some cases, however, the oval is either so expanded or contracted that the latitudinal coverage of the AE magnetometer chain is not adequate. Then, a longitudinal chain in the key region would give more information of the real situation, but, of course, only during some limited UT-period. In order to find out the UT coverage of a single meridional chain, we have compared the global AL and AU indices with corresponding local indices determined using data from the meridional part of the EISCAT Magnetometer Cross during the years 1985–1987. A statistical study shows that the local indices are close (within relative error of 0.2) to the global AU and AL during periods 1500–2000 UT (∼ 1730–2230 MLT) and 2130−0130 UT (∼000–0400 MLT), respectively. In the middle of these optimal MLT-sectors the EISCAT Cross sees more than 70% of the cases when the global AE chain records activity. Then, also the correlation between the local and global indices is generally good (〉0.7). Thus we conclude that five to six evenly located meridional chains are needed for covering all the UT-periods. On the other hand, already the combination of IMAGE, CANOPUS, and the Greenland chains catches ∼50% of the substorms. Case-studies show that usually during 2130 − 1100 UT the AL achieved from these chains reproduces the real AL with good timing, although it does not follow all transient variations.
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 2011-08-24
    Description: The International Sun-Earth Explorer 3 (ISEE-3) magnetic field and plasma electron data from Jan - March 1983 have been searched to study thin current sheets in the deep tail region. 33 events were selected where the spacecraft crossed through the current sheet from lobe to lobe within 15 minutes. The average thickness of the observed current sheets was 2.45 R(sub E), and in 24 cases the current sheet was thinner than 3.0 R(sub E); 6 very thin current sheets (thickness lambda less than 0.5 R(sub E) were found. The electron data show that the very thin current sheets are associated with considerable temperature anisotropy. On average, the electron gradient current was about 17% of the total current, whereas the current arising from the electron temperature anisotropy varied between 8-45% of the total current determined from the lobe field magnitude.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 22; p. 2427-2430
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  • 4
    Publication Date: 2011-08-24
    Description: The particle scattering and current sheet stability features in the geomagnetic tail during the phase of substorm growth were investigated using Tsyganenko's (1989) magnetic field model. In a study of four substorm events which were observed both in the high-altitude nightside tail and in the auroral ionosphere, the model magnetic field was adjusted to each case so as to represent the global field development during the growth phase of the substorms. The model results suggest that the auroral brightenings are connected with processes taking place in the near-earth region inside about 15 earth radii. The results also suggest that there is a connection between the chaotization of the electrons and the auroral brightenings at substorm onset.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; A12; p. 19,283-19,297.
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  • 5
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Theoretical pressure balance arguments have implied that steady convection is hardly possible in the terrestrial magnetotail and that steady energy input necessarily generates a cyclic loading-unloading sequence, i.e., repetitive substorms. However, observations have revealed that enhanced solar wind energy input to the magnetospheric system may either lead to substorm activity or enhanced but steady convection. This topic is reviewed with emphasis on several recent case studies of the Steady Magnetospheric Convection (SMC) events. In these cases extensive data sets from both satellite and ground-based instruments from various magnetospheric and ionospheric regions were available. Accurate distinction of the spatial and temporal scales of the magnetospheric processes is vital for correct interpretation of the observations during SMC periods. We show that on the large scale, the magnetospheric configuration and plasma convection are stable during SMC events, but that both reveal considerable differences from their quiet-time assemblies. On a shorter time scale, there are numerous transient activations which are similar to those found during substorms, but which presumably originate from a more distant tail reconnection process, and map to the poleward boundary of the auroral oval. The available observations and the unresolved questions are summarized here. The tail magnetic field during SMC events resembles both substorm growth and recovery phases in the neartail and midtail, respectively, but this configuration may remain stable for up to ten hours. Based on observations and model results we discuss how the magnetospheric system avoids pressure balance problems when the plasma convects earthward. Finally, the importance of further coordinated studies of SMC events is emphasized. Such studies may shed more light on the substorm dynamics and help to verify quantitatively the theoretical models of the convecting magnetosphere.
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  • 6
    Publication Date: 2004-12-03
    Description: The thinning and intensification of the cross tail current sheet during the substorm growth phase are analyzed during the CDAW 6 substorm (22 Mar. 1979) using two complementary methods. The magnetic field and current sheet development are determined using data from two spacecraft and a global magnetic field model with several free parameters. These results are compared with the local calculation of the current sheet location and structure previously done by McPherron et al. Both methods lead to the conclusion that an extremely thin current sheet existed prior to the substorm onset, and the thicknesses estimated by the two methods at substorm onset agree relatively well. The plasma data from the ISEE 1 spacecraft at 13 R(sub E) show an anisotropy in the low energy electrons during the growth phase which disappears just before the substorm onset. The global magnetic model results suggest that the field is sufficiently stretched to scatter such low energy electrons. The strong stretching may improve the conditions for the growth of the ion tearing instability in the near Earth tail at substorm onset.
    Keywords: GEOPHYSICS
    Type: ESA, Substorms 1; p 131-135
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  • 7
    Publication Date: 2004-12-03
    Description: A pseudobreakup is a phenomenon similar to the substorm expansive phase onset, including an activation of an auroral arc, a burst of Pi2 micropulsations, and enhancement of the westward electrojet. However, these effects are weak and a pseudobreak is generally assumed to be very localized. The pseudobreakups are discussed based on simultaneous observations made in space and on the ground during the substorm growth phase. In the events studied the main features listed above are found, but the significance of the localization is unclear. The optical pseudobreakup, with associated magnetic perturbations, is highly localized, but simultaneously a wide local time sector of the auroral oval may be activated. The major differences between pseudobreakups and substorm expansive phase onsets are concluded to be the intensity and the development that follows. Careful study of pseudobreakups may help to determine phase initiation, and the role of the ionosphere-magnetosphere coupling in the substorm process.
    Keywords: GEOPHYSICS
    Type: ESA, Substorms 1; p 111-116
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  • 8
    Publication Date: 2004-12-03
    Description: The role of high-speed solar wind streams in driving relativistic electron acceleration within the earth's magnetosphere is discussed based on International Solar-Terrestrial Physics (ISTP) Observatory and related spacecraft observations. A 'recirculation' mechanism for electron acceleration and redistribution was invoked. Recently, an increase in the number of coronal mass ejections (CMEs) and related 'magnetic clouds' was seen at 1 AU. As these CME/cloud systems interact with the earth's magnetosphere, they are able to produce rapid enhancements in the magnetospheric electron population. The relativistic electron signatures observed by the POLAR, SAMPEX, and other spacecraft during recent magnetic cloud events, especially January 1997 and May 1997, were compared and contrasted. In these cases, there were large solar wind and IMF changes during the cloud passages and very rapid energetic electron acceleration was observed. The relative geoeffectiveness of these events is examined and 'space weather' predicatability is assessed.
    Keywords: Solar Physics
    Type: Proceedings of the 31st ESALB Symposium on Correlated Phenomena at the Sun, in the Heliosphere and in Geospace; 199-206; ESA-SP-415
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  • 9
    Publication Date: 2004-12-03
    Description: Empirical magnetic field models are discussed in terms of using models in multi-instrument data analysis. The variety of previous applications of field models are demonstrated. The problems found by using data based models are addressed and the prospects of their future development are outlined. Some issues related to time-dependency of the field configuration are presented.
    Keywords: Astrophysics
    Type: ; 293-317
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  • 10
    Publication Date: 2011-08-24
    Description: Previous observations have shown that during periods of steady magnetospheric convection (SMC) a large amount of magnetic flux crosses the plasma sheet (corresponding to approximately 10 deg wide auroral oval at the nightside) and that the magnetic configuration in the midtail is relaxed (the curent sheet is thick and contains enhanced B(sub Z). These signatures are typical for the substorm recovery phase. Using near-geostationary magnetic field data, magnetic field modeling and a noval diagostic technique (isotropic boundary algorithm), we show that in the near-Earth tail the magnetic confirguration is very stretched during the SMC events. This stretching is caused by an intense, thin westward current. Because of the srongly depressed B(sub Z), there is a large radial gradient in the near-tail magetic field. These signatures have been peviously associated only with the substorm growth phase. Our results indicate that during the SMC periods the magnetic configuration is very peculiar, with co-existing thin near-Earth current sheet and thick midtail plasma sheet. The deep local minimum of the equatorial B(sub Z) that devleops at R approximately 12 R(sub E) is consistent with steady, adiabatic, Earthward convection in the midtail. These results impose contraints on the existing substorm theories, and call for an explanation of how such a stressed configuration can persist for such a long time without tail current disruptions that occur at the end of a substorm growth phase.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A12; p. 23,571-23,582
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