Multi-stage volcanic events: A statistical investigation

Volcanic events comprising multiple eruptive stages are common in the historical and geological record and display activity of variable intensity ranging, in some cases, through to several centuries. To better understand the characteristics of such events globally, this study explores a database of historical events having Volcanic Explosivity Indices 4 or greater. The database was compiled for the most part from published and unpublished material provided by the Smithsonian Institution. The database was examined to find common statistical relationships, within and between the variables characterising multi-stage explosive events.     

Russian studies of Antarctic auroras: A review

The studies of auroras at Russian Antarctic observatories in the Southern Hemisphere began in 1957 during the second Complex Antarctic expedition and performed almost continuously up to 1993 during more than 30 years. Many observers of auroras and scientists that analyzed obtained results participated in these studies. Members of the Arctic and Antarctic Research Institute (AANII), Russian Committee on Hydrometeorology (Rosgidromet); Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Russian Academy of Sciences (IZMIRAN); Vernov Institute of Nuclear Physics, Moscow State University (NIIYaF MGU); Polar Geophysical Institute, Russian Academy of Sciences (PGI); St. Petersburg State University (SPbGU); Schmidt Institute of Physics of the Earth (IFZ); Shafer Institute of Cosmophysical Research and Aeronomy, Siberian Branch, Russian Academy of Sciences (IKFIA SB RAN); and other institutions made an enormous contribution to the studies of Antarctic auroras. The main results of the studies of Antarctic auroras, obtained by Russian scientists, are reviewed in this work.     

A search for coastline effects on the auroras

A camera on board the Polar spacecraft has provided an unique opportunity to search for coastline effects on the spatial distributions of auroral emissions. This study is motivated by a ground-based report of such shoreline effects on auroral emissions during the Russian Polar Expedition of almost a century ago. This Polar camera, the Low-Resolution Visible Camera of the Visible Imaging System, is capable of obtaining auroral images of the Northern Hemisphere with high spatial and temporal resolutions for extended continuous periods of time, i.e., tens of km, one to several minutes, and many hours, respectively. The entire set of 8588 auroral images at OI 557.7 nm which were gathered during January 1997 were examined for three types of coastal effects, (1) diversion of the auroral arc at the shore, (2) increase of intensity as the arc is followed from sea to land, and (3) decrease of intensity from sea to land. A null test of the results of this auroral survey was conducted by a search of the entire set of images for random coincidences of auroral features in rotated maps of the coastlines. This null test provides support for the occasional transient existence of coastline effects during the onset and early expansion phases of auroral substorms and some auroral intensifications.     

Nighttime subvisual high-latitude auroras

Special methods for processing TV images have been used to study the characteristics of nighttime auroras based on the observations at high-latitude observatories on Spitsbergen. Weak subvisual auroras (SVAs), originating 3°–4° north of brighter auroras in the auroral oval, have been detected in the interval 1900-0400 MLT. The average lifetime of SVAs is approximately 7 min, and the average velocity of the equatorward shift is ～0.6 km/s. SVAs were observed during relatively quiet periods, when the IMF B _z component is mainly positive. However, SVAs are not polar-cap auroras since they are oriented from east to west rather than toward the Sun. The optical observations indicate that the SVA intensity is 0.2–0.5 and 0.1–0.3 kR in the 630 and 557.7 nm emissions, respectively. The average ratio of the emission intensities (I ₅₅₇₇/I ₆₃₀₀) is about 0.5. According to the direct satellite observations, the SVA electron spectrum has a maximum at 0.4–1.0 keV. In this case the energy flux of precipitating electrons is approximately an order of magnitude as low as such a flux in brighter auroral arcs in the auroral oval.     

Features of morning-time auroras during SC

The optical observations on Heiss Island and the ion drift measurements on the DMSP F8 satellite were used to study the aurora characteristics and ionospheric convection before and after SC registered at 2330 UT on January 13, 1988. It has been indicated that two zones of luminosity can be distinguished in morning-time auroras during the quiet period before SC: the soft zone with auroral arcs and the harder diffuse auroral zone (equatorward of the first zone). After SC, a gradual smooth activation of auroras in both zones was followed (4–5 min later) by a more abrupt intensification of diffuse luminosity and by the appearance of numerous bright discrete auroras throughout the sky. In the diffuse auroral zone, the variations in the luminosity intensity with a period of 6–7 min were observed after SC. Auroral and geomagnetic field pulsations are closely correlated. During the quiet period before SC, sunward convection was concentrated in the soft precipitation region in the form of jets located in the vicinity of auroral arcs. After SC, considerable sunward convection was observed in the diffuse auroral zone. Peaks of the upward ion drift velocity were registered in the vicinity of auroral arcs.     

Ground-based observations of auroras under the sunlit conditions

The results of the ground-based optical observations of sunlit auroras, performed at Lovozero and Apatity observatories on April 10 (event 1) and April 27, 2007(event 2), are presented. The observations were performed in the (OI) 557.7 nm emission, using a new equipment based on a Fabry-Pérot interferometer connected to a PhotonMAX CCD camera. During event 1, the observations were performed in the Harang discontinuity region at a low magnetic disturbance. It has been indicated that an auroral arc was located in the polar part of the eastward electrojet, and the arc position coincides with the equatorward boundary of structured precipitation (b2e). During event 2, auroras were observed within the average statistical boundaries of the auroral oval and the region of structured precipitation under the conditions of rather high geomagnetic activity. However, during the period of low geomagnetic activity, discrete auroras were registered at a geomagnetic latitude of ~64° on that day, which is 3°—4° equatorward of the structured precipitation region. Such a low latitudinal position of auroras can be explained by the effect of a high solar wind velocity, which was ~580 km/s during the period of observations.     

Interplanetary magnetic field By control of dayside auroras

The effect of the interplanetary magnetic field (IMF) By component on the dayside auroral oval from Viking UV measurements for March–November 1986 is studied. Observations of dayside auroras from Viking UV images for large positive (15 cases) and negative (22 cases) IMF By (∣By∣>4 nT), suggest that: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras; (2) for negative IMF By, the auroral oval tends to be narrow and continuous throughout the noon meridian without any noon gap or any strong undulation in the auroral distribution. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of auroral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upward field-aligned currents associated with the IMF By in the noon sector.     

Spatial-temporal dynamics of auroras during the magnetic storm main phase

The structure and dynamics of auroras in the midnight sector during substorms, which develop during the magnetic storm main phase as compared to the characteristics of a typical auroral substorm, have been studied using the ground-based and satellite observations. It has been found out that a difference from the classical substorm is observed in auroras during the magnetic storm main phase. At the beginning of the storm main phase, the series of pseudobreakups with the most pronounced jump-like motion toward the equator shifts to lower latitudes. The substorm expansion phase can be observed not only as arc jumps to higher latitudes but also as an explosive expansion of a bright diffuse luminosity in all directions. During the magnetic storm main phase, auroras are mainly characterized by the presence of stable extensive rayed structures and by the simultaneous existence of different auroral forms, typical of different substorm phases, in the TV camera field of view.     

A statistical study of the interplanetary magnetic field control of sporadic E-layer occurrence in the southern polar cap ionosphere

The influence of the interplanetary magnetic field (IMF) on the occurrence of sporadic E (Es)-layers in the southern polar cap ionosphere has been investigated. We statistically analysed ionogram and Doppler velocity observations made using a HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E; 81°S magnetic latitude) during the two summer campaign intervals 1 January to 18 February, and 1 November to 31 December 1997. The ionogram and Doppler velocity measurements were used to determine the Es-occurrence and electric field vectors (assuming E×B/B² drift), respectively. Concurrent IMF data were obtained from measurements made on board the Wind spacecraft. First, the gross properties of the IMF dependence of Es-formation were obtained: the occurrence rate was higher for negative B_y and/or positive B_z, and lower for positive B_y and/or negative B_z. To reconcile these gross properties with the electric field theory of Es-layer formation, the detailed diurnal variation of both Es-occurrence and the ionospheric electric field were obtained for different orientations of the IMF. The main statistical results are that: (1) the B_y component mainly controls the occurrence of the midnight Es-layers through its influence on the corresponding South West electric field; and (2) the B_z component mainly controls the occurrence of the evening Es-layers. However, the change in the occurrence rate for evening Es-layers was not related to the strength of the associated North West and North East electric fields. The total occurrence of Es-layers depended more on B_y than on B_z, owing to the dominance of B_y-controlled midnight Es-layers in the occurrence distribution. Nevertheless, the dependence of Es-occurrence on B_z was important. We suggest that the increase in Es-occurrence for positive B_z might be explained by the intermittent production of lower F-region ionisation by polar showers and squalls, which also increase in frequency and intensity for positive B_z. The importance of metallic ion transport within the ionosphere is also considered.     

Low-latitude auroras: The great aurora of 4 February 1872

The aurora of 4 February 1872 was comparable to, and perhaps even greater than, that of 1/2 September 1859. In this paper we show that the aurora of 4 February 1872 was seen worldwide, and that in the Caribbean, Egypt, Southern Africa, the Indian Ocean, the Indian subcontinent, and China these observations extended as low as 20° magnetic latitude. Observations are also available in the north to as far as the vicinity of the north magnetic pole. This aurora is then comparable to, or greater than, in geographical extent, and in equatorward closeness, to that of 2 September 1859. Both must now be included as the only known members in the class of greatest auroras of the past few hundred years. For the 1859 aurora, however, there is no accepted observation at a lower magnetic latitude than about 20°. By contrast, several observations for the aurora of 4 February 1872 are reported at magnetic latitudes of the order of 10°, and one probable observation at an even lower magnetic latitude of about 3°. This paper presents a survey and discussion of these observations.     

Search for hidden periodicity in magnetometric measurements performed during auroras

A method for detecting discrete equidistant spectra in high-latitude magnetic observations is proposed. The method finds approximate solutions of the classical Schuster problem, which finds a hidden periodicity in observations with considerable noise. The usage of difference signals makes it possible to increase the latitudinal resolution of the magnetometric diagnostics. The equidistant spectra of two different types have been detected based on the proposed method. The spectra of the first type are interpreted as frequencies of coupled compressional-transverse magnetohydrodyncompressionalamic oscillations in the magnetospheric cavity. The origin of the second type of spectra can be related to a rotating source, i.e., a small-scale vortex of magnetospheric convection. Such an interpretation takes into consideration the Doppler frequency shift caused by entrainment of the ionospheric neutral gas by magnetospheric convection. The results confirm the conclusion that discrete auroral frequencies are stable under disturbed conditions.     

High-frequency loading effects by the Ligurian Sea — a statistical investigation

In the present work we analyzed the high-frequency band of crustal tilts at Genoa in order to investigate indirect loading effects by the quarter-diurnal oceanic tides, as well as by an observed oceanic oscillation of 3.66 h which was previously ascribed to the proper oscillation of the Ligurian Sea. With this aim two hourly series of tidal observation recorded by an horizontal pendulum along a NS direction have been submitted to spectral analyses (1 May 1965-28 February 1966; 1 June 1966-31 December 1966).Power spectra revealed two peaks near the periods of 6.15 and 3.66 h and highly resolved Fourier spectra allowed us to detect the loading effects by the oceanic tides M₄, MS₄ and MK₄. As regards to the oscillation of 3.66 h we have found a spreading of spectral lines and non-significant values for the phases. This result provides a decisive verification of the existence of a proper oscillation of the Ligurian Sea regarded as a wide oscillating system with an open boundary from Nice (Côte d'Azur) to Calvi (Corsica).Finally, a spectral analysis performed on sea-level observations at Genoa revealed a complete correspondence between ocean and earth oscillations in the quarter-diurnal band and at the frequency of the longitudinal free oscillation of the Ligurian basin, with a mean amplitude ratio of about 1 mseca/mm.     

A new diagnostic of stratospheric polar vortices

We studied the main climatological features of the Arctic and Antarctic stratospheric vortices, using a new approach based on defining the vortex edge as the 50 hPa geostrophic streamline of maximum average velocity at each hemisphere. Given the use of NCAR-NCEP reanalysis data, it was thought advisable to limit the study to the periods 1958–2004 for the Northern Hemisphere (NH) and 1979–2004 for the Southern Hemisphere (SH). After describing the method and testing sample results with those from other approaches, we analysed the climatological means and trends of the four most distinctive characteristics of the vortices: average latitude, strength, area, and temperature. In general terms, our results confirm most of what is already known about the stratospheric vortices from previous studies that used different data and approaches. In addition, the new methodology provides some interesting new quantifications of the dominant wavenumber and its interannual variability, as well as the principal variability modes through an empirical orthogonal function analysis that was performed directly over the vortex trajectories. The main drawbacks of the methodology, such as noticeable problems characterising highly disturbed stratospheric structures as multiple or off-pole vortices, are also identified.     

Simultaneous optical and satellite observations of auroras in the mantle: Case study

The all-sky camera data obtained in Barentsburg (Spitsbergen Archipelago) are compared with specific features of electron and ion precipitations on the DMSP F18 satellite during its flight within the camera field of view on December 15, 2012. Before arriving at the cusp from the mantle side, the satellite detects two outbursts of precipitating particles. The burst of mantle precipitations far from the cusp is observed simultaneously in both ionic and electronic components. In the ionosphere related to the satellite, no auroras are detected, which is likely due to the low intensity of the flux of precipitating electrons and their low energy (80 eV). Near the cusp, a more intensive burst of precipitations of higher-energy electrons (140 eV) is accompanied by an almost complete “locking” of ions. This burst of mantle precipitations is related to the faint luminous structure in the ionosphere. The ion locking is indicative of the accelerating potential difference in the force tube, which is based on the glowing region. The luminous structure is an element of the so-called “polewar moving auroral forms,” which is related in the literature to the reconnection in the daytime magnetopause. The possible relation of the observed phenomena to the reconnected magnetic force tubes, which drift from the cusp in the antisolar direction, is also confirmed by the dispersion of ionic precipitations, i.e., an increase in ion energy as the satellite approaches to the cusp.     

Behaviour of auroras during the Spörer minimum (1450-1550)

It is shown that such data that exist on auroral activity in the period between 1450 and 1550 do not provide conclusive evidence that some authors have claimed for the existence of the so-called “Spörer minimum” in solar activity during this interval.     

A statistical review of mudrock elastic anisotropy

Mudrocks, defined to be fine‐grained siliclastic sedimentary rocks such as siltstones, claystones, mudstones and shales, are often anisotropic due to lamination and microscopic alignments of clay platelets. The resulting elastic anisotropy is often non‐negligible for many applications in the earth sciences such as wellbore stability, well stimulation and seismic imaging. Anisotropic elastic properties reported in the open literature have been compiled and statistically analysed. Correlations between elastic parameters are observed, which will be useful in the typical case that limited information on a rock's elastic properties is known. For example, it is observed that the highest degree of correlation is between the horizontal elastic stiffnesses C₁₁ and C₆₆. The results of statistical analysis are generally consistent with prior observations. In particular, it is observed that Thomsen's ? and γ parameters are almost always positive, Thomsen's ? and γ parameters are well correlated, Thomsen's δ is most frequently small and Thomsen's ? is generally larger than Thomsen's δ. These observations suggest that the typical range for the elastic properties of mudrocks span a sub‐space less than the five elastic constants required to fully define a Vertical Transversel Isotropic medium. Principal component analysis confirms this and that four principal components can be used to span the space of observed elastic parameters.     

A statistical model of the geomagnetic field

Summary A statistical model of the geomagnetic field is derived, based on the assumption of an axial geocentric dipole field of strengthH _e at the equator perturbed by randomly directed components of constant magnitudeh. The model fits the dispersions found from an analysis of the 1945 field, and the ratioh/H _e obtained for this field and from the palaeomagnetic data both average to about 0.4. The model predicts that during reversal of the dipole field, the field intensity falls to between 0.2 and 0.4 of the steady field intensity, and this agrees with estimates made from the palaeomagnetic observations.