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- ItemThe stratorotational instability of Taylor-Couette flows with moderate Reynolds numbers(London [u.a.] : Taylor and Francis, 2017) Rüdiger, G.; Seelig, T.; Schultz, M.; Gellert, M.; Egbers, C.; Harlander, U.In view of new experimental data the instability against adiabatic nonaxisymmetric perturbations of a Taylor-Couette flow with an axial density stratification is considered in dependence of the Reynolds number (Re) of rotation and the Brunt-Väisälä number (Rn) of the stratification. The flows at and beyond the Rayleigh limit become unstable between a lower and an upper Reynolds number (for fixed Rn). The rotation can thus be too slow or too fast for the stratorotational instability. The upper Reynolds number above which the instability decays, has its maximum value for the potential flow (driven by cylinders rotating according to the Rayleigh limit) and decreases strongly for flatter rotation profiles finally leaving only isolated islands of instability in the (Rn/Re) map. The maximal possible rotation ratio μmax only slightly exceeds the shear value of the quasi-uniform flow with Uφ≃const. Along and between the lines of neutral stability the wave numbers of the instability patterns for all rotation laws beyond the Rayleigh limit are mainly determined by the Froude number Fr which is defined by the ratio between Re and Rn. The cells are highly prolate for Fr > 1 so that measurements for too high Reynolds numbers become difficult for axially bounded containers. The instability patterns migrate azimuthally slightly faster than the outer cylinder rotates.
- ItemLOFAR Observations of Substructure Within a Traveling Ionospheric Disturbance at Mid-Latitude([New York] : Wiley, 2023) Dorrian, Gareth; Fallows, Richard; Wood, Alan; Themens, David R.; Boyde, Ben; Krankowski, Andrzej; Bisi, Mario; Dąbrowski, Bartosz; Vocks, ChristianThe large scale morphology and finer sub-structure within a slowly propagating traveling ionospheric disturbance (TID) are studied using wide band trans-ionospheric radio observations with the LOw Frequency ARray (LOFAR; van Haarlem et al., 2013, https://doi.org/10.1051/0004-6361/201220873). The observations were made under geomagnetically quiet conditions, between 0400 and 0800 on 7 January 2019, over the UK. In combination with ionograms and Global Navigation Satellite System Total Electron Content anomaly data we estimate the TID velocity to ∼60 ms−1, in a North-westerly direction. Clearly defined substructures with oscillation periods of ∼300 s were identified within the TID, corresponding to scale sizes of 20 km. At the geometries and observing wavelengths involved, the Fresnel scale is between 3 and 4 km, hence these substructures contribute significant refractive scattering to the received LOFAR signal. The refractive scattering is strongly coherent across the LOFAR bandwidth used here (25–64 MHz). The size of these structures distinguishes them from previously identified ionospheric scintillation with LOFAR in Fallows et al. (2020), https://doi.org/10.1051/swsc/2020010, where the scale sizes of the plasma structure varied from ∼500 m to 5 km.
- ItemA LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances(Les Ulis : EDP Sciences, 2020) Fallows, Richard A.; Forte, Biagio; Astin, Ivan; Allbrook, Tom; Arnold, Alex; Wood, Alan; Dorrian, Gareth; Mevius, Maaijke; Rothkaeh, Hanna; Matyjasiak, Barbara; Krankowski, Andrzej; Anderson, James M.; Asgekar, Ashish; Avruch, I. Max; Bentum, Mark; Bisi, Mario M.; Butcher, Harvey R; Ciardi, Benedetta; Dabrowski, Bartosz; Damstra, Sieds; de Gasperin, Francesco; Duscha, Sven; Eislöffel, Jochen; Franzen, Thomas M.O.; Garrett, Michael A.; Griessmeier, Jean-Matthias; Gunst, Andre W.; Hoeft, Matthias; Horandel, Jorg R.; Iacobelli, Marco; Intema, Huib T.; Koopmans, Leon V.E.; Maat, Peter; Mann, Gottfried; Nelles, Anna; Paas, Harm; Pandey, Vishambhar N.; Reich, Wolfgang; Rowlinson, Antonia; Ruiter, Mark; Schwarz, Dominik J.; Serylak, Maciej; Shulevski, Aleksander; Smirnov, Oleg M.; Soida, Marian; Steinmetz, Matthias; Thoudam, Satyendra; Toribio, M. Carmen; van Ardenne, Arnold; van Bemmel, Ilse M.; van der Wiel, Matthijs H.D.; van Haarlem, Michiel P.; Vermeulen, Rene C.; Vocks, Christian; Wijers, Ralph A.M.J.; Wucknitz, Olaf; Zarka, Philippe; Zucca, PietroThis paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18–19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10–80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR “core” reveals two different velocities in the scintillation pattern: a primary velocity of ~20–40 ms−1 with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110 ms−1 with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.
- ItemThe flare likelihood and region eruption forecasting (FLARECAST) project: flare forecasting in the big data & machine learning era(Les Ulis : EDP Sciences, 2021) Georgoulis, Manolis K.; Bloomfield, D. Shaun; Piana, Michele; Massone, Anna Maria; Soldati, Marco; Gallagher, Peter T.; Pariat, Etienne; Vilmer, Nicole; Buchlin, Eric; Baudin, Frederic; Csillaghy, Andre; Sathiapal, Hanna; Jackson, David R.; Alingery, Pablo; Benvenuto, Federico; Campi, Cristina; Florios, Konstantinos; Gontikakis, Constantinos; Guennou, Chloe; Guerra, Jordan A.; Kontogiannis, Ioannis; Latorre, Vittorio; Murray, Sophie A.; Park, Sung-Hong; Stachelski, Samuel von; Torbica, Aleksandar; Vischi, Dario; Worsfold, MarkThe European Union funded the FLARECAST project, that ran from January 2015 until February 2018. FLARECAST had a research-to-operations (R2O) focus, and accordingly introduced several innovations into the discipline of solar flare forecasting. FLARECAST innovations were: first, the treatment of hundreds of physical properties viewed as promising flare predictors on equal footing, extending multiple previous works; second, the use of fourteen (14) different machine learning techniques, also on equal footing, to optimize the immense Big Data parameter space created by these many predictors; third, the establishment of a robust, three-pronged communication effort oriented toward policy makers, space-weather stakeholders and the wider public. FLARECAST pledged to make all its data, codes and infrastructure openly available worldwide. The combined use of 170+ properties (a total of 209 predictors are now available) in multiple machine-learning algorithms, some of which were designed exclusively for the project, gave rise to changing sets of best-performing predictors for the forecasting of different flaring levels, at least for major flares. At the same time, FLARECAST reaffirmed the importance of rigorous training and testing practices to avoid overly optimistic pre-operational prediction performance. In addition, the project has (a) tested new and revisited physically intuitive flare predictors and (b) provided meaningful clues toward the transition from flares to eruptive flares, namely, events associated with coronal mass ejections (CMEs). These leads, along with the FLARECAST data, algorithms and infrastructure, could help facilitate integrated space-weather forecasting efforts that take steps to avoid effort duplication. In spite of being one of the most intensive and systematic flare forecasting efforts to-date, FLARECAST has not managed to convincingly lift the barrier of stochasticity in solar flare occurrence and forecasting: solar flare prediction thus remains inherently probabilistic.
- ItemMeasuring the cosmic web(Vilnius : Lithuanian Astronomical Union, 2011) Müller, V.; Hoffmann, K.; Nuza, S.E.A quantitative study of the clustering properties of the cosmic web as a function of absolute magnitude and color is presented using the SDSS Data Release 7 galaxy survey. Mark correlations are included in the analysis. We compare our results with mock galaxy samples obtained with four different semi-analytical models of galaxy formation imposed on the merger trees of the Millenium simulation. The clustering of both red and blue galaxies is studied separately.
- ItemLITOS - A new balloon-borne instrument for fine-scale turbulence soundings in the stratosphere(Göttingen : Copernicus, 2011) Theuerkauf, A.; Gerding, M.; Lübken, F.-J.We have developed a new compact balloon payload called LITOS (Leibniz-Institute Turbulence Observations in the Stratosphere) for high resolution wind turbulence soundings in the stratosphere up to 35 km altitude. The wind measurements are performed using a constant temperature anemometer (CTA) with a vertical resolution of ∼2.5 mm, i.e. 2 kHz sampling rate at 5 m/s ascent speed. Thereby, for the first time, it is possible to study the entire turbulence spectrum down to the viscous subrange in the stratosphere. Including telemetry, housekeeping, batteries and recovery unit, the payload weighs less than 5 kg and can be launched from any radiosonde station. Since autumn 2007, LITOS has been successfully launched several times from the Leibniz-Institute of Atmospheric Physics (IAP) in Kühlungsborn, Germany (54° N, 12° E). Two additional soundings were carried out in 2008 and 2009 in Kiruna, Sweden (67° N, 21° E) as part of the BEXUS program (Balloon-borne EXperiments for University Students). We describe here the basic principle of CTA measurements and prove the validity of this method in the stratosphere. A first case study allows a clear distinction between non-turbulent regions and a turbulent layer with a thickness of some tens of meters. Since our measurements cover the transition between the inertial and viscous subrange, energy dissipation rates can be calculated with high reliability.
- ItemCombined wind measurements by two different lidar instruments in the Arctic middle atmosphere(Göttingen : Copernicus, 2012) Hildebrand, J.; Baumgarten, G.; Fiedler, J.; Hoppe, U.-P.; Kaifler, B.; Lübken, F.-J.; Williams, B.P.During a joint campaign in January 2009, the Rayleigh/Mie/Raman (RMR) lidar and the sodium lidar at the ALOMAR Observatory (69 N, 16 E) in Northern Norway were operated simultaneously for more than 40 h, collecting data for wind measurements in the middle atmosphere from 30 up to 110 km altitude. As both lidars share the same receiving telescopes, the upper altitude range of the RMR lidar and the lower altitude range of the sodium lidar overlap in the altitude region of ≈80-85 km. For this overlap region we are thus able to present the first simultaneous wind measurements derived from two different lidar instruments. The comparison of winds derived by RMR and sodium lidar is excellent for long integration times of 10 h as well as shorter ones of 1 h. Combination of data from both lidars allows identifying wavy structures between 30 and 110 km altitude, whose amplitudes increase with height. We have also performed vertical wind measurements and measurements of the same horizontal wind component using two independent lasers and telescopes of the RMR lidar and show how to use this data to calibrate and validate the wind retrieval. For the latter configuration we found a good agreement of the results but also identified inhomogeneities in the horizontal wind at about 55 km altitude of up to 20 ms-1 for an integration time of nearly 4 h. Such small-scale inhomogeneities in the horizontal wind field are an essential challenge when comparing data from different instruments.
- ItemBF Cyg during ITS current outburst(Vilnius : Lithuanian Astronomical Union, 2012) Siviero, A.; Tamajo, E.; Lutz, J.; Wallerstein, G.We are intensively monitoring the current outburst on BF Cyg, both spectroscopically (high and low resolution modes) and photometrically (so far 450 BVRCRIC measurements have been collected). The outburst is photometrically reminiscent of the major event BF Cyg experienced in 1890 when it rose by 4 mag in the blue. In this contribution we present the data and describe the plans to investigate this object.
- ItemCoincident measurements of PMSE and NLC above ALOMAR (69° N, 16° E) by radar and lidar from 1999-2008(Göttingen : Copernicus, 2011) Kaifler, N.; Baumgarten, G.; Fiedler, J.; Latteck, R.; Lübken, F.-J.; Rapp, M.Polar Mesosphere Summer Echoes (PMSE) and Noctilucent Clouds (NLC) have been routinely measured at the ALOMAR research facility in Northern Norway (69° N, 16° E) by lidar and radar, respectively. 2900 h of lidar measurements by the ALOMAR Rayleigh/Mie/Raman lidar were combined with almost 18 000 h of radar measurements by the ALWIN VHF radar, all taken during the years 1999 to 2008, to study simultaneous and common-volume observations of both phenomena. PMSE and NLC are known from both theory and observations to be positively linked. We quantify the occurrences of PMSE and/or NLC and relations in altitude, especially with respect to the lower layer boundaries. The PMSE occurrence rate is with 75.3% considerably higher than the NLC occurrence rate of 19.5%. For overlapping PMSE and NLC observations, we confirm the coincidence of the lower boundaries and find a standard deviation of 1.26 km, hinting at very fast sublimation rates. However, 10.1% of all NLC measurements occur without accompanying PMSE. Comparison of occurrence rates with solar zenith angle reveals that NLC without PMSE mostly occur around midnight indicating that the ice particles were not detected by the radar due to the reduced electron density.
- ItemQuantification of waves in lidar observations of noctilucent clouds at scales from seconds to minutes(Göttingen : Copernicus, 2013) Kaifler, N.; Baumgarten, G.; Fiedler, J.We present small-scale structures and waves observed in noctilucent clouds (NLC) by lidar at an unprecedented temporal resolution of 30 s or less. The measurements were taken with the Rayleigh/Mie/Raman lidar at the ALOMAR observatory in northern Norway (69 N) in the years 2008-2011. We find multiple layer NLC in 7.9% of the time for a brightness threshold of δ β 12 × 10-10 m-1 sr-1. In comparison to 10 min averaged data, the 30 s dataset shows considerably more structure. For limited periods, quasi-monochromatic waves in NLC altitude variations are common, in accord with ground-based NLC imagery. For the combined dataset, on the other hand, we do not find preferred periods but rather significant periods at all timescales observed (1 min to 1 h). Typical wave amplitudes in the layer vertical displacements are 0.2 km with maximum amplitudes up to 2.3 km. Average spectral slopes of temporal altitude and brightness variations are-2.01 ± 0.25 for centroid altitude,-1.41 ± 0.24 for peak brightness and-1.73 ± 0.25 for integrated brightness. Evaluating a new single-pulse detection system, we observe altitude variations of 70 s period and spectral slopes down to a scale of 10 s. We evaluate the suitability of NLC parameters as tracers for gravity waves.