Dispersive changes in magnetic background noise polarization at 0.1 to 6Hz during sunset and sunrise at L=1.3

Polarization properties of the magnetic background noise (MBN) and the spectral resonance structure (SRS) of the ionospheric Alfvén resonator (IAR) below the first Schumann resonance but above 0.1 Hz are measured by a sensitive pulsation magnetometer at the island of Crete (<i>L</i>=1.3) and analyzed using the existing SRS theory by Belyaev et al. (1989b). The focus of the paper is on the systematic changes in the MBN and SRS properties associated with the transition from a sunlit to a dark ionosphere (sunset) and vice versa (sunrise). We are able to pinpoint in observations an E-region and F-region terminator effect and to simulate it by means of a simple ionosphere model, implying the formalism given by Belyaev et al. (1989b). The E-region terminator effect is associated with an apparent control for the SRS presence or absence with no clear frequency dispersion in polarization properties, whereas the F-region terminator effect exhibits strong frequency dispersion, especially in the low frequency range. This yields a change in the ellipticity of MBN, starting as early as 2 to 3h ahead of the "zero-line" of the terminator. In a 24h presentation of the ellipticity versus frequency and time, the sunrise/sunset effect produces a sharp, dispersive boundary between night and day (day and night). Only inside this boundary, during the night hours, is SRS observed, at times accompanied by a large quasi-periodic long period modulation in the azimuthal angle of the major axis of the polarization ellipse. Attention is also paid to peculiarities in the low frequency range (~0.1Hz), where especially large changes in the polarization properties occur in association with the passage of the terminator. The F-region effect is very distinct and well reproduced by our simple model. Changes in the azimuth associated with the E-region terminator effect are of the order of 20&deg

Durchgängigkeit für Tiere in Fließgewässern : Leitfaden ; Teil 3., Hochwasserrückhaltebecken und Talsperren

Die Durchgängigkeit für Tiere in Fließgewässern hat eine herausragende Bedeutung für die Erhaltung und Wiederherstellung von naturnahen Verhältnissen mit artenreichen und gewässertypischen Lebensgemeinschaften. Die Vielzahl der in diesem Zusammenhang zu beantwortenden Fragen kann nicht in einem einzigen Leitfaden behandelt werden, daher wird die Thematik in einer fünfteiligen Leitfadenreihe aufgearbeitet. Dabei werden alle Aspekte von der Aufstiegshilfe bis zum Abstieg, die Durchgängigkeit in Längsrichtung der Gewässer, die Vernetzung mit den Auen und Zuflüssen sowie zwischen Sohle und fließender Welle aufgegriffen. Der vorliegende Teil 3 der Leitfadenreihe stellt eine Arbeitshilfe dar zu Fragen der Durchgängigkeit für Tiere in Fließgewässern bei Hochwasserrückhaltebecken und Talsperren. Er gilt sowohl bei Neubauten als auch bei Sanierungen. Neben der Darstellung der Grundlagen wird die Bedeutung der Durchgängigkeit für Tiere in Fließgewässern hervorgehoben und die gesetzlichen und fachlichen Regeln dargestellt. Es wird erläutert, warum die Durchgängigkeit wegen dieser Vorgaben grundsätzlich zu gewährleisten ist. Die Durchgängigkeit ist ein wichtiges Kriterium in der europäischen Wasserrahmenrichtlinie und der im Bundes- und Landesrecht umgesetzten Vorgaben, z. B. in der Bewässerbeurteilungsverordnung vom 30.8.2004. Die Wirkung der Bauwerke auf die Durchgängigkeit der Gewässer wird aufgezeigt und ein Anforderungskatalog abgeleitet. Neben den Fischen als Indikatororganismen für die Bewertung des ökologischen Zustandes von Wasserkörpern nach der EU-Wasserrahmenrichtlinie (WRRL) werden auch die Anforderungen der Benthosorganismen sowie der Landtiere und der Tiere der Wasserwechselzone erläutert. Der Leitfaden enthält Abwägungskriterien, bei deren Berücksichtigung die Anforderungen an die Durchgängigkeit unter Umständen eingeschränkt werden können. Vorgaben und Lösungsmöglichkeiten für den Neubau von Hochwasserrückhaltebecken und Talsperren und für die Umgestaltung bestehender Anlagen werden dargestellt. Anhand von positiven Beispielen werden die bisherigen Erfahrungen bei der Abwägung und bei der baulichen Umsetzung aufgezeigt

Two satellite study of substorm expansion near geosynchronous orbit

During several time intervals in 1979–1980 the satellites GEOS-2 and SCATHA were situated relatively close on the nightside of the Earth at geosynchronous distances. Several substorm events were identified during these periods. The event considered in this paper was recorded on 22 May 1979, when the satellites were separated by less than 30min in local time around 21:00 LT. The observed 45 to 60 s delay of magnetic signatures observed at the two s/c indicates a westward expansion of ~7.7°/min. At the two s/c, the magnetic signatures are, in particular for the azimuthal magnetic field components, quite different. At GEOS-2, being close to the magnetic equator, the dominant feature is a dipolarization with a weak field-aligned current signature corresponding to a symmetric current which cancels at the equator. On SCATHA, however, being close to the current sheet boundary, the azimuthal magnetic field indicates a strong field-aligned Birkeland current structure. On both s/c the first indication of an approaching substorm was an increase in the high energy ion flux followed by a reduction in the flux intensity of energetic electrons and a further tailward stretching of the magnetic field, starting ~2min before the onset of the magnetic field dipolarization. The tailward stretching, the observed variations of the magnetic field components, and the subsequent dipolarization are interpreted in terms of an azimuthally tilted field-aligned current system passing the s/c on the tailward side from east to west. The westward expansion and dipolarization observed at the two s/c are consistent with the propagation of a Rayleigh-Taylor type instability. The increased radial ion flux corresponds to the <i><b>E</b></i>x<i><b>B</b></i>-drift due to the substorm associated electric field.<br><br> <b>Key words.</b> Magnetospheric physics (storms and substorms; plasma waves and instabilities; current systems

Field line resonances as a trigger and a tracer for substorm onset

In this paper, we show that periodic auroral arc structures are seen at the location of one particular auroral substorm onset for the 15 min preceding onset, suggesting that field line resonances should be considered a strong candidate for triggering substorm onset. Irrespective of whether this field line resonance is coincidentally or causally linked to this substorm onset, the characteristics of the field line resonance can be used to remote sense the characteristics of the geomagnetic field line that supports substorm onset. In this instance, the eigenfrequency of this resonance is around 12 mHz. Interestingly, however, there is no evidence of this field line resonance in a seven satellite major Time History of Events and Macroscale Interactions during Substorms (THEMIS)-GOES conjunction, ranging from geosynchronous orbit to ~30 RE. However, using space-based cross-phase measurements of the local field line eigenfrequency at the inner THEMIS locations, we find that the local field line eigenfrequency is 6–10 mHz. Hence, we can reliably say that this 12 mHz Field Line Resonance (FLR) must lie inside of THEMIS locations. Our conclusion is that a high-m field line resonance can both represent a strong candidate for a trigger for substorm onset, as first proposed by Samson et al. (1992), and that its characteristics can provide invaluable information as to where substorm onset occurs in the magnetosphere

Timing and localization of ionospheric signatures associated with substorm expansion phase onset

In this paper, we present case studies of the optical and magnetic signatures of the characteristics of the first minute of substorm expansion phase onset observed in the ionosphere. We find that for two isolated substorms, the onset of magnetic pulsations in the 24–96 s period wavelet band are colocated in time and space with the formation and development of small-scale optical undulations along the most equatorward preexisting auroral arc prior to auroral breakup. These undulations undergo an inverse spatial cascade into vortices prior to the release of the westward traveling surge. We also present a case study of a multiple activation substorm, whereby discrete onsets of ULF wave power above a predetermined quiet time threshold are shown to be associated with specific optical intensifications and brightenings. Moreover, in the multiple activation substorm event, we show that neither the formation of the small-scale undulations nor the formation of similar structures along a north–south aligned arc is sufficient to produce auroral breakup associated with expansion phase onset. It is only ∼10 min after these two disparate activation regions initiate that auroral breakup and the subsequent formation of a westward traveling surge occur. We discuss the implications of these results in terms of the triggering mechanisms likely to be occurring during these specific events

Observation of Ionospheric Alfven Resonances at 1--30 Hz and their superposition with the Schumann Resonances

Long-term measurements of the high-frequency magnetic field (0.1--100 Hz) have been made at Eskdalemuir Observatory in the UK since September 2012. We analyze five years of dynamic spectrograms to examine the occurrence and behavior of the Schumann and Ionospheric Alfven Resonances (IAR), and Pc1 pulsations. The resonances, observed as diffuse bands, arise from reflections of energy both within the earth-ionosphere cavity and from the non-linear conductivity gradient of the ionosphere. Schumann Resonances (SR) occur continuously but IAR are observed to arise at local night time in ~50% of days in the dataset. Typically, IAR are found at frequencies of 1--8 Hz, but we find them extending out to 30 Hz and strongly superimposing over the first three Schumann Resonances around 9% of the time. These phenomena include constructive and destructive interference, non-linear frequency changes over the span of several hours and polarity enhancements. In addition, the magnitude of the IAR do not decline rapidly with frequency as often proposed. We find the IAR and their superposition with SR are strongly controlled by season and geomagnetic activity. We compare six days with the most unusual IAR behavior in the dataset to ionosonde measurements of f0F2, a proxy for ionospheric conductivity, but find little correlation. We suggest that, as current theoretical modelling does not account for these observations, further work is needed to understand how they arise

On the nature of ULF wave power during nightside auroral activations and substorms: 2. temporal evolution

We present a statistical analysis of the time evolution of ground magnetic fluctuations in three (12–48 s, 24–96 s and 48–192 s) period bands during nightside auroral activations. We use an independently derived auroral activation list composed of both substorms and pseudo-breakups to provide an estimate of the activation times of nightside aurora during periods with comprehensive ground magnetometer coverage. One hundred eighty-one events in total are studied to demonstrate the statistical nature of the time evolution of magnetic wave power during the ∼30 min surrounding auroral activations. We find that the magnetic wave power is approximately constant before an auroral activation, starts to grow up to 90 s prior to the optical onset time, maximizes a few minutes after the auroral activation, then decays slightly to a new, and higher, constant level. Importantly, magnetic ULF wave power always remains elevated after an auroral activation, whether it is a substorm or a pseudo-breakup. We subsequently divide the auroral activation list into events that formed part of ongoing auroral activity and events that had little preceding geomagnetic activity. We find that the evolution of wave power in the ∼10–200 s period band essentially behaves in the same manner through auroral onset, regardless of event type. The absolute power across ULF wave bands, however, displays a power law-like dependency throughout a 30 min period centered on auroral onset time. We also find evidence of a secondary maximum in wave power at high latitudes ∼10 min following isolated substorm activations. Most significantly, we demonstrate that magnetic wave power levels persist after auroral activations for ∼10 min, which is consistent with recent findings of wave-driven auroral precipitation during substorms. This suggests that magnetic wave power and auroral particle precipitation are intimately linked and key components of the substorm onset process