Geomagnetic storms,the Dst ring-current myth and lognormal distributions

The definition of geomagnetic storms dates back to the turn of the century when researchers recognized the unique shape of the H-component field change upon averaging storms recorded at low latitude observatories. A generally accepted modeling of the storm field sources as a magnetospheric ring current was settled about 30 years ago at the start of space exploration and the discovery of the Van Allen belt of particles encircling the Earth. The Dst global ‘ring-current’ index of geomagnetic disturbances, formulated in that period, is still taken to be the definitive representation for geomagnetic storms. Dst indices, or data from many world observatories processed in a fashion paralleling the index, are used widely by researchers relying on the assumption of such a magnetospheric current-ring depiction. Recent in situ measurements by satellites passing through the ring-current region and computations with disturbed magnetosphere models show that the Dst storm is not solely a main-phase, growth to disintegration, of a massive current encircling the Earth. Although a ring current certainly exists during a storm, there are many other field contributions at the middle-and low-latitude observatories that are summed to show the ‘storm’ characteristic behavior in Dst at these observatories. One characteristic of the storm field form at middle and low latitudes is that Dst exhibits a lognormal distribution shape when plotted as the hourly value amplitude in each time range. Such distributions, common in nature, arise when there are many contributors to a measurement or when the measurement is a result of a connected series of statistical processes. The amplitude-time displays of Dst are thought to occur because the many time-series processes that are added to form Dst all have their own characteristic distribution in time. By transforming the Dst time display into the equivalent normal distribution, it is shown that a storm recovery can be predicted with remarkable accuracy from measurements made during the Dst growth phase. In the lognormal formulation, the mean, standard deviation and field count within standard deviation limits become definitive Dst storm parameters.     

Scintillation at high latitudes during winter magnetic storms

Scintillation observations are described which were made at Kiruna in northern Sweden during three magnetic storm periods in the winter of 1984–1985. The results were obtained using transmissions from the multisatellite NNSS system, so that it has been possible to chart the development of scintillation activity over some 20° of geomagnetic latitude as a function of time for several days throughout each storm. A region of strong scintillation at the highest latitudes near magnetic noon is a common feature on all but the quietest days. This feature, probably associated with soft particle precipitation into the cusp, shows an abrupt boundary which moves equatorwards as the disturbance develops. In the magnetic midnight sector two latitudinally separate zones of scintillation are found, patchy at high latitudes although more sustained in the auroral zone. An absence of auroral scintillations around midnight UT can be followed by prolonged intense scintillation activity at auroral latitudes during the early morning hours on some disturbed days.     

The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.     

Response of night-time equatorial F-region to magnetic disturbances

The response of the equatorial night-time F-region to magnetic stormtime disturbances has been examined using mainly ionograms recorded at Trivandrum and magnetograms recorded at high, middle and low latitudes during the magnetic storm of 23–26 November 1986. The analysis revealed a close coupling between the equatorial F-region and high latitude magnetic field disturbances originating in solar wind-magnetosphere interactions. The presence of spread-F on ionograms during this period is found to be consistent with the Rayleigh-Taylor instability mechanism for the growth of the irregularities.     

Time-weighted magnetic indices as predictors of ionospheric behaviour

A time-weighted accumulation of the ap index, ap(τ) (Wrenn, 1987; Wrenn et al., 1987, 1989), together with other similar indices, was explored as a predictor of ionospheric behaviour, using ƒ₀F2 data for a selection of locations in Australia and Europe for September and October 1989. All the time accumulated indices showed improved linear correlations, indicative of a response time of the order of about 15 hours. The response time could be decomposed into a lag between respective time series and a persistence time, although the decomposition appeared unnecessary as the persistence time carried the same information. Of the individual indices investigated, aa(τ) appeared best and the auroral oval equatorward edge index (AI index) was poorest, although the differences were not statistically significant. Comparisons between the aa, ap and Kp indices, plus comparisons between different ionospheric parameters showed that forecasting may be improved using different transformations of the data. While these results appear good, further studies using other stations and seasons are warranted to confirm their utility for forecasting.     

Magnetic field of the magnetospheric ring current and its dynamics during magnetic storms

This review examines models existing in the literature which describe the magnetic field produced by the ring current (DR) at the Earth's surface based on the energy balance equation. The parameters of this equation, the injection function F and decay parameter τ are considered to depend on parameters of the interplanetary medium and the DR intensity. The existing models are shown to be able to describe the DR variations with sufficient accuracy (r.m.s. deviation δ between the experimental and modelled values of DR for 170 magnetic storms is 5 < δ < 15 nT and the correlation coefficient between the two is 0.85 <r<1). The models describe that part of the geomagnetic field variation at low latitudes during a magnetic storm that is controlled by the geoeffective characteristics of the interplanetary medium and which thus responds immediately to its variations (the driven part).The values of τ are significantly less during the main phase of a magnetic storm than during the recovery phase. This reflects the difference in the main mechanisms of ion loss from the ring current during the two phases of the storm. These are the interaction of ions with hydromagnetic waves during the main phase of the storm with its intervals of intense plasma injection into the inner magnetosphere and charge exchange with the cold hydrogen geocorona during the recovery phase.     

Response of the equatorial thermosphere to magnetic activity analysed with accelerometer total density data. Asymmetrical structure

A great deal of accurate total density data from the CACTUS accelerometer experiment has been collected in equatorial regions during the last minimum of the solar cycle. Using these data, an analysis of the magnetically disturbed thermosphere has been performed with an improved resolution. Time delay of the response, latitudinal and longitudinal variations for solstice, day-time and night-time conditions have been studied. An attempt is made to interpret the behaviour of the thermosphere in the light of existing theories. An asymmetrical heating in latitude and a heating depending on longitude are needed in order to interpret the results.     

Some peculiarities of altitudinal distribution of atomic oxygen at low latitudes during magnetic storms

Several storms were examined during the period of the ESRO-4 mission to study the altitudinal behaviour of atomic oxygen and molecular nitrogen at low latitudes. A special method of data reduction was applied to distinguish composition changes due to thermal expansion and those which resulted from some non-thermal process. It was shown that in the majority of cases studied there exists some latitudinal interval where changes of the N₂/O ratio from quiet to storm days are small compared with that observed in the purely thermal case. This latitudinal interval was found to be displaced to the winter hemisphere (not always across the equator) during solstice cases and the corresponding altitudinal interval was sometimes equal to the whole height range covered by the satellite. The effective temperature of atomic oxygen turned out to be greater than that deduced from the altitudinal distribution of N₂. This implies that there exists a non-thermal component of the [O] increase on a storm day at low latitudes and thus [O] is distributed in a non-barometric way. A possible mechanism responsible for this behaviour is downwelling due to the disturbed global thermospheric circulation.     

Adaptive separation of regular and irregular magnetic activity for K indices

A numerical data adaptive separation method is developed for the discrimination of quasi-regular magnetic variations from the irregular variations for the determination of the K indices. The method is based on a pattern comparison and discrimination technique using harmonic analyses and is more objective than the hand-scaling method. The wave characteristics of a quiet regular or reference day are compared with those of the disturbed day being examined. The harmonic components that are consistent with those of the reference day are determined by a cosine weighting function. The Fourier coefficients are calculated from these modified harmonics and are then transformed back to the time domain. This ‘regular variation’ is subtracted from the data before the ranges are determined for the semi-logarithmic K scale. The method is tested on digital data from the Canadian magnetic observatories Victoria, Meanook and Ottawa over a 4 month period. The data are at a 10 s sampling interval and encompass the full range of K values (0–9). These K values are compared and checked with hand-scaled values, which were carefully and independently determined from reconstructed magnetograms from the same data. The two methods were found to agree ∼ 97% of the time, with most errors occurring during very irregular periods. This is comparable with values carefully determined by highly skilled observers. It is proposed that the method is suitable for use on a regular basis.     

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earth's horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst < −150 nT have been found to be most effective in producing the above scintillation activity. From the present observations, a threshold value of the maximum negative excursion of Dst for producing scintillation may be obtained, Dst < −75 nT being significantly associated with the post-midnight scintillation occurrences. The results are interpreted in terms of the reversal of the equatorial horizontal electric field, under magnetically disturbed conditions, due to a coupling of the high latitude and magnetospheric current systems with the equatorial electric field.     

Winds,wind-shears and plasma densities during the initial phase of a magnetic storm from equatorial latitudes

Results of a sodium vapour release experiment carried out from SHAR (India), an equatorial rocket launching station, immediately after (⩽ 2 h) a storm sudden commencement (SSC) during the initial phase of a magnetic storm, followed by electron density measurements are presented. Many of the relevant atmospheric parameters, namely, neutral winds and their altitude variation, the magnitude of the shears in them, the neutral temperature with altitude by spectroscopic methods, diffusion measurements on the released trail, clues on the turbopause level and the electron densities including the structures (irregularities) in them were obtained. The results of the temperature measurements carried out independently on the sodium trail by means of a ground-based Fabry-Perot spectrometer, operating on the sodium D 1 line, resonantly scattered by the trail have already been reported by us (Ranjan Guptaet al., 1986). In this paper the effects of the excess temperature reported earlier and the rest of the measured parameters on the electron density profiles are evaluated using models and compared with the measurements.The formation of sharp layers of ionization have been explained on the basis of the electro-dynamical processes associated with the wind shears at a location, close to the edge of the equatorial electrojet belt. The significance of the changes in the neutral composition due to the enhanced neutral temperature and the low turbopause level, in increasing the base-level plasma densities by a factor of 3–5 are demonstrated and the possible role of plasma dynamics discussed.     

The equatorial electrojet and associated currents as seen in Magsat data

Magsat data are re-examined with regard to the presence and character of fields due to the equatorial electrojet and meridional currents at dawn and dusk local times. Dip-latitude organized field variations at dawn are:

• 1.(1) extremely weak,
• 2.(2) extremely variable with longitude,
• 3.(3) inconsistent with the pattern expected from a line or narrow sheet current.

It is shown that the use of Magsat dusk data can ‘contaminate’ a main field model, introducing apparent equatorial electrojet effects into the dawn data.Fields due to the equatorial electrojet and (presumably) associated meridional currents are clearly present in the dusk data. They show a variation with longitude which is apparently associated with the longitudinal variation of the strength, or square of the strength, of the main field in the E-region. Also evident is a variation with time of the year, although data are available for only a six month period. The meridional currents are generally minimum during January and February and maximum either during November and December or March and April, depending upon longitude. The E-region horizontal currents are minimum in November and December and maximum in March and April, except for − 30° to −90° longitude when the maximum occurs in January and February.Assuming that field gradients in local time are considerably smaller than field gradients in dip-latitude, current densities are estimated to be 1–3.6μA/m² for the horizontal current at 110km and about 10–20 × 10⁻⁹ A/m² for the vertical currents at 400km altitude. These results confirm and extend earlier results of Takeda and Maeda.Most models of the electrojet system in the literature disagree severely with these measurements either because their scope is inadequate or because of the wind system they assume. Those models which best describe the data invoke an eastward wind and/or an eastward electric field at dusk local time.     

Solar activity effects on equatorial plasma bubble zonal velocity and its latitude gradient as measured by airglow scanning photometers

Using a new mode of scanning 630-nm photometer operation the zonal velocities of ionospheric plasma depletions were measured over Cachoeira Paulista in Brasil in two east-west planes tilted 30°N and 30° S with respect to zenith. The measurements cover a time period of approximately 2 years, from January 1988 to January 1990, a period marked by significant increase in solar activity of the ongoing cycle. The results have permitted a rather detailed evaluation of the local time and latitude variations in the zonal plasma bubble velocity as a function of solar activity. Although the mean trend in the velocity local time variation is a decrease from early evening to post-midnight hours, a strong tendency for velocity peaks is observed near 21 LT and midnight. The velocities as well as their height (latitude) gradients show perceivable increases with solar activity represented as sunspot numbers. The present results are compared with the ambient plasma velocities measured using the Jicamarca radar by Fejer el al. (1985), J. Geophys. Res. 90, 12249, with that measured on board the DE 2 satellite on the equatorial latitudes by Coley and Heelis (1989), J. geophys. Res. 94, 6751, and with various theoretical calculations, in an attempt to bring out the salient features of the plasma dynamics of the equatorial ionosphere.     

Refilling process in the plasmasphere and its relation to magnetic activity

When geomagnetic activity is moderate, the geosynchronous orbit crosses the plasmasphere bulge region in which the variations of plasma density from day to day can therefore be detected by geosynchronous satellites. The plasma density was measured by the Relaxation Sounder onboard ESA's GEOS-2 satellite. Variations of plasma density reflect the combined effects of refilling of particles from the ionosphere and loss of plasma by convection. The saturation level of the electron density at the geo-synchronous orbit and the refilling rate under different conditions of geomagnetic activity have been obtained and are found to be 70.5 cm⁻³ and 7–25 cm⁻³ day⁻¹, respectively. In this paper the refilling morphology and the relationship between the refilling process and magnetic activity (Dst index) are analysed. The refilling rate or refilling time constant inferred from the data, either directly on fairly well-defined refilling events, or indirectly through a simple model, are found to compare reasonably well with the refilling time constant expected by theory. The observed correlation of refilling rate with Dst index is interpreted as resulting from the modification of the composition of the topside ionosphere occurring after intense storms.     

The Amundsen expedition to the magnetic pole

AFRICA

Behind God's Back. By N. Farson. Illustrated. Demy 8vo. Pp. 448. London : Victor Gollancz Ltd., 1940. Price 10s. 6d.

EUROPE

The Place‐Names of Nottinghamshire. By J. E. B. Gover, Allen Mawer, and F. M. Stenton. Demy 8vo. Pp. xlii+348. London : Cambridge University Press Ltd., 1940. Price 21s.

Prehistoric England. By Grahame Clarke. Illustrated. Pp. viii+120. London : B. T. Batsford Ltd., 1940. Price 8s. 6d.

Lincolnshire in the 17th and 18th Centuries. By Charles Brears. Demy 8vo. Pp. xv+192. London : A. Brown and Sons Ltd., 1940. Price 7s. 6d.

AMERICA

The American Empire. A Study of the Outlying Territories of the United States. Edited by William H. Hass. Illustrated. Pp. xi+408. University of Chicago Press, 1940. (Published in Great Britain by the Cambridge University Press Ltd.) Price 24s.

I Lived with the Eskimos. By S. Montague. Illustrated. Demy 8vo. Pp. ix+223. London : Jarrolds Ltd., 1940. Price 12s. 6d.

Arctic Pilot. By Walter Gilbert, as told to Kathleen Shackleton. Illustrated. Demy 8vo. Pp. 256. London : T. Nelson and Sons Ltd., 1940. Price 7s. 6d.

ASIA

Old Routes of Western Iran. By Sir Aurel Stein. Illustrated. Imperial 8vo. Pp. xxviii+432. London: Macmillan and Co. Ltd., 1940. Price 42s.

The New Burma. By W. J. Grant. Crown 8vo. Pp. 156. London: George Allen and Unwin Ltd., 1940. Price 7s. 6d.

AUSTRALASIA

Australia. By Griffith Taylor. Demy 8vo. Pp. xv+455. London : Methuen and Co. Ltd., 1940. Price 21s.

GENERAL

The Story of the Pacific. By H. van Loon. Illustrated. Demy 8vo. Pp.315. London : George G. Harrap and Co. Ltd., 1940. Price 8s. 6d.

Coconut Quest. By J. S. Phillips. Illustrated. Demy 8vo. Pp. 254. London : Jarrolds Ltd., 1940. Price 16s.

Ten Great Mountains. By R. L. G. Irvine. Illustrated. Demy 8vo. Pp. xii+213. London : J. M. Dent and Sons Ltd., 1940. Price 12s. 6d.

Helvellyn to Himalaya. By F. S. Chapman. Illustrated. Demy 8vo. Pp. xv+285. London : Chatto and Windus, 1940. Price 18s.

A Long Look Back. By Two Victorians. Illustrated. Demy 8vo. Pp. 256. London : Seeley Service Ltd., 1940. Price 12s. 6d.

I Married Adventure. By Osa Johnson. Illustrated. Demy 8vo. Pp. 376. London: Hutchinson and Co. Ltd., 1940. Price 10s. 6d.

Rolling Home. By Surgeon‐Commander A. G. Bee. Demy 8vo. Pp. xi+495. London: Bale, 1940.

Savoy ! Corsica ! Tunis ! By Bernard Newman. Illustrated. Demy 8vo. Pp. 282. London : Herbert Jenkins Ltd., 1940. Price 10s. 6d.

Courtship and Display among Birds. By C. D. Stonor. Illustrated. Demy 8vo. Pp.xv+139. London: Country Life Ltd., 1940. Price 8s. 6d.

Old Morningside. By Wm. Mair. Illustrated. Pp. 40. Edinburgh : John Menzies and Co. Ltd., 1940. Price 1s.

English Lakeland. By Doreen Wallace. Illustrated. Demy 8vo. Pp. viii+120. London : B. T. Batsford Ltd., 1940. Price 8s. 6d.

Chiltern Country. By H. J. Massingham. Illustrated. Demy 8vo. Pp. viii + 120. London : B. T. Batsford Ltd., 1940. Price 8s. 6d.

Middlesex. By Arthur Mee. Illustrated. Demy 8vo. Pp. ix+232. London: Hodder and Stoughton Ltd., 1940. Price 7s. 6d.

EDUCATIONAL

Map Approach to Modern Problems, tgig‐iggg. By W. E. Coysh and A. W. Brown. Small quarto. Pp. 64. London : University Tutorial Press, 1940. Price 2s. 3d.

Elements of Political Geography. By S. van Valkenburgh. With Maps. Imperial 8vo. Pp. xix+401. London: Pitmans Ltd., 1940. Price 18s.

Economic Geography. By C. Colby and A. Foster. Illustrated. Imperial 8vo. Boston (U.S.A.) : Ginn and Co., 1940. Price 10s. 6d.

Synopsis of the Mineral Resources of Scotland. By M. Macgregor and Others. Special Reports on the Mineral Resources of Great Britain, Vol. xxxiii, 59 pages. One Text Figure. H.M. Stationery Office. Price 1s.

A Map Book of Africa and South America. By A. Ferriday. London: Macmillan and Co. Ltd., 1940. Price 1s. 9d.

Outlines of Structural Geology. By E. S. Hills. Illustrated. Crown 8vo. Pp. x+172. London : Methuen and Co. Ltd., 1940. Price 6s. 6d.

Asia and Australia. By A. Mamour. Illustrated. Demy 8vo. Pp. 348. London: Macmillan and Co. Ltd., 1940.

The World and Ourselves. By E. White. Illustrated. Demy 8vo. Pp. 207. Edinburgh: W. and A. K. Johnston Ltd., 1940. Price 2s. 9d.     

Spread-F occurrence in mid- and low-latitude regions related to various levels of geomagnetic activity

The occurrence of spread-F from seven latitude regions for ionosonde stations (78 in all) located from L-shell = 3.3 to 1.05 has been investigated (using the superposed-epoch technique) relative to four different levels of geomagnetic activity. Data for 14.5 years were used. For moderate, high and very-high geomagnetic activity a significant peak in spread-F occurrence is found for the four latitude regions closest to the auroral zone. These peaks are delayed (after the geomagnetic activity) by a matter of days, the delays being greater for the lower levels of activity and also greater for regions further from the auroral zone. Similarly, delayed dips in spread-F occurrence are found for very-low geomagnetic activity. Analyses for the remaining three regions (those closest to the equator) failed to show corresponding delayed peaks or dips in the occurrence of spread-F relative to the appropriate levels of geomagnetic activity. It is suggested that (for the three highest levels of geomagnetic activity) the mechanism which is responsible for the suppression of spread-F in equatorial regions may operate at these low latitudes and thus counterbalance the other mechanism which is responsible for the positive correlation found for the higher-latitude regions.     

The influence of sunspot number and magnetic activity on the diurnal variation of the geomagnetic field at mid to high latitudes

The variations of the diurnal range of the geomagnetic field with sunspot number and with magnetic activity was studied at mid and high latitude stations in the northern hemisphere at different seasons. The effect of increasing sunspot number is small at lower latitudes and increases with geomagnetic latitude, while the effect of increasing magnetic activity is to increase the range at all latitudes, very greatly at the higher geomagnetic latitudes.     

Biological differences related to cultural variability during the Neolithic in a micro-geographical area of the Iberian Peninsula

This paper presents dental morphological data of Neolithic, Chalcolithic and Bronze Age populations from the Catalan Pre-Pyrenean area. The Neolithic group, in particular, differs from those of surrounding areas in its funerary culture: the building of cists, which is not present in the Sepulcres de Fossa Culture. A minimum number of 118 individuals from this area were studied for this work, and the data were compared with those of other Iberian and European groups. The results indicate that the two micro-regional groups from the Catalan area (Pre-Pyrenean and Pre-Coastal) were biologically different during the Neolithic and the Chalcolithic, but not in the Bronze Age, when they also appeared to be more homogeneous culturally. In addition, both areas differ biologically from coetaneous Italian groups, although those closer to the coast show slightly smaller differences. Finally, the Bronze Age groups also present fewer differences with regard to the Italian Bronze Age’s group. Therefore, the results suggest that the Catalan Neolithic population had two separate origins, related to cultural patterns, and that differences between the groups decreased within time, probably due to trade-related activities. Moreover, the fact that the difference with Italian populations decreased during the Bronze Age suggests major population movements through the Mediterranean that would affect the biological composition of the human groups.     

The background to the proposition that plankton be used as food in the United Kingdom during the Second World War

Food shortages, particularly of proteins, in Britain during the Second World War led to the suggestion re-surfacing that marine plankton might be harvested on an industrial scale first as human food, then turning to its potential use as a supplement to stock and poultry feed. The notion emanated in the United Kingdom from Sir John Graham Kerr, at Glasgow University. He encouraged Alister Hardy, at Hull, to develop the idea and the natural testing ground was the Clyde Sea Area (given the extensive history of plankton research at Millport). Unpublished documents from the archives of the Scottish Association for Marine Science shed new light on the interactions behind the scenes of this project between Kerr, Hardy and the Millport Marine Station's then director, Richard Elmhirst. Elmhirst, who was sceptical about the feasibility of the plan from the outset, went along with it; not least as a way of attracting welcome research funding during lean times but also, doubtless, regarding it as his patriotic duty in case the proposal proved worthwhile.