Lunar and solar daily variations of the geomagnetic field at Italian stations

Mean hourly values of magnetic declination D, horizontal intensity H and vertical intensity Z observed at Italian stations have been analyzed to determine solar and luni-solar diurnal components, together with the corresponding terms O₁ and N₂ of the lunar tidal potential.The results, showing the variations of the first four harmonic components with season, degree of magnetic activity and annual sunspot number, are tabulated and discussed. Differences between the dependence of S and L on season and sunspot number are considered and tentative explanations offered. The oceanic tidal effect has been determined and it is apparent that this is more likely to show the influence of the Atlantic Ocean rather than the Mediterranean Sea.     

The vernal-autumnal asymmetry in the seasonal variation of geomagnetic activity

In intervals in which the polarity of the main solar dipole field is stabilized, a 12 month wave occurs in geomagnetic activity (indices aa, Ap, Dst) with its maximum in one of the equinoctial periods. Whether the vernal or the autumnal maximum is greater depends on the polarity of the main solar dipole; the existence of the wave may be explained by the north-south asymmetry in the main solar dipole field. The results favour the southward component of the interplanetary magnetic field as the decisive factor for geomagnetic activity.     

A new aspect of daily variations of the geomagnetic field at low latitude

Data from the unique network of low latitude geomagnetic observatories in India extending from the dip equator to the northern focus of the Sq current system have shown a new type of Sq current distribution different from those associated with the normal or the counter electrojet currents. On 3 December 1985 both the horizontal as well as the vertical components of the geomagnetic field at Annamalainagar showed maximum values around the midday hours. The abnormal feature described seems to be rather a rare phenomenon. The solar daily range of H field is found to be fairly constant from the dip equator up to about 12° dip latitude, suggesting the complete absence of the equatorial enhancement of ΔH, typical of the equatorial electrojet. The cancellation of the equatorial electrojet is suggested to be caused by a westward flowing current system much wider than the conventional equatorial electrojet. This additional current system could be due to the excitation of certain tidal modes at low latitudes on such abnormal days.     

The response of geomagnetic spectral composition to solar wind during storm time

In this paper 16 geomagnetic storms in 1968–1978 recorded at 8 magnetic observatories located from polar to equatorial regions in the λ= 120°E longitudinal zone and its vicinity have been analysed. The horizontal component H traces of 27 h intervals have been sampled once every 1.5 min. The time sequences of the data thus obtained have been processed by the method of digital filtering and maximum entropy spectral analysis (MESA).The results of the analysis are compared with the associated solar wind parameters. It confirms that the geomagnetic disturbances are controlled by the solar wind in several ways, i.e. geomagnetic disturbances respond differently to various solar wind parameters or to the different ranges of them. The north-south component of the interplanetary magnetic field (IMF) B_z., the IMF latitude θ and the solar wind velocity V play the most important part in inducing geomagnetic storms.     

Simulation of ionospheric currents and geomagnetic field variations of Sq for different solar activity

Variations of ionospheric Sq electric currents and fields caused by changes in electric conductivity due to changes in solar activity are studied using the International Reference Ionosphere (IRI) model. Calculations are made for R (sunspot number) = 35 and 200 on the assumption of constant (1, −2)mode tidal winds. It is shown that electric fields grow when solar activity is high, because the ratio of the conductivity in the F-region to that in the E-region increases. Currents in the F-region become stronger than those in the E-region, and nocturnal currents are not negligible when solar activity becomes high. F-region currents also play an important role in the westward currents on the high latitude side of the current vortex. The calculated geomagneticH component at the equator has a depression around 1600 LT for R = 35, while it decreases smoothly from 1100 LT to 1900 LT for R = 200. This difference is consistent with the observed geomagnetic field variation. The ratio of total Sq currents obtained by our simulation is about 3.5, which is a little larger than is found in the observed results.     

The future of geomagnetic storm predictions: implications from recent solar and interplanetary observations

Within the last 7–8 years, there has been a substantial growth in our knowledge of the solar and interplanetary causes of geomagnetic storms at Earth. This review article will not attempt to cover all of the work done during this period. This can be found elsewhere. Our emphasis here will be on recent efforts that expose important, presently unanswered questions that must be addressed and solved before true predictability of storms can be possible. Hopefully, this article will encourage some readers to join this effort and perhaps make major contributions to the field.     

Relation between Northern Hemisphere winter temperatures and geomagnetic or solar activity at different QBO phases

In our previous work a dominant coupling was shown between solar and geomagnetic activity, and surface air temperature of the QBO-west and east phases, respectively. The aim of the present study is to define ‘typical’ regions of positive and negative deviations from the long-term temperature average, to determine their magnitude and to find the conditions under which such a ‘typical’ distribution of temperature fields occurs.     

Recipe for predicting the IMF Bz polarity's change of direction following solar disturbances and at the onset of geomagnetic storms

A three-dimensional, time-dependent, MHD model of solar-disturbance-caused storms (Wu, 1993; Wu et al., 1996a) is used to predict the turning direction of the interplanetary magnetic field (IMF) at Earth. More explicitly, we examine the polarity of B_z caused by solar disturbances on the Sun. Three manifestations of solar disturbances, as studied by previous workers, are examined. Firstly, twenty-nine kilometric Type II events, associated (Cane, 1985) with geomagnetic storms, are studied within the context of our three-dimensional model. Then, an additional eleven long-duration X-ray events (LDEs) with radio fluxes greater than 100 solar flux units were examined; these events were not associated with interplanetary Type II events but were also associated (Cane, 1985) with geomagnetic storms. Finally, in situ interplanetary phenomena that caused ten large (Dst < −100 nT, the intensification of the storm) geomagnetic storm episodes (Tsurutani et al., 1988) near solar maximum are also studied via the B_z predictions of our 3D MHD model. The accuracy of these B_z turning-direction-predictions is found to be as follows: (1) for the kilometric Type II events, the model's prediction was successful for 26 of the 29 events studied; (2) 10/11 for the LDE events; and (3) 7/9 for the major geomagnetic storm events. The overall prediction accuracy of these three independent data sets is 43/49. Thus, consideration of these three independent data sets strongly suggests that the recipe proposed by the basic 3D MHD model may be valid for a zero-th order prediction scheme.     

Dependence of seasonal variation of absorption on solar activity in the equatorial ionosphere

From data for the absorption of radio waves at oblique incidence in Lagos, and at vertical incidence at Colombo, the seasonal variation of absorption at the two sites are examined. It is shown that, if subsolar absorption be assumed to depend upon sunspot number, the cos X law gives the same index for both the diurnal and the seasonal variation of absorption.     

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.     

The trouble with thermospheric vertical winds: geomagnetic,seasonal and solar cycle dependence at high latitudes

The vertical wind component is frequently used to determine the zero-velocity baseline for measurements of thermospheric winds by Fabry-Perot and other interferometers. For many of the upper atmospheric emission lines from which Doppler shifts are determined, for example for the OI 630 nm emission, available laboratory sources are not convenient for long-term use at remote automatic observatories. Therefore, the assumption that the long-term average vertical wind is zero is frequently used to create a baseline from which the Doppler shifts corresponding with the line-of-sight wind from other observing directions can then be calculated. A data base consisting of 1242 nights of thermospheric wind measurements from Kiruna (68°N, 20°E), a high-latitude site, has been analysed. There are many interesting short-term fluctuations of the vertical wind which will be discussed in future papers. However, the mean vertical wind at Kiruna also has a systematic variation dependent on geomagnetic activity, season and solar cycle. This means that the assumption that the average value of the vertical wind is zero over the observing period cannot be used in isolation to determine the instrument reference or baseline. Despite this note of caution, even within the auroral oval, the assumption of a zero mean vertical wind can be used to derive a baseline which is probably valid within 5 ms⁻¹ during periods of quiet geomagnetic activity (K_p < 2), near winter solstice. During other seasons, and during periods of elevated geomagnetic activity, a systematic error in excess of 10 ms⁻¹ may occur.     

The seasonal variation of the diurnal thermospheric winds over Millstone Hill during solar cycle maximum

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density and vertical plasma drift are used to determine the diurnal variation of neutral winds at F-layer heights over Millstone Hill (42.6°N, 71.5°W) for different seasons. The technique to derive the thermospheric winds from incoherent scatter radar data and the results for equinox conditions have been discussed by Roble et al. (1974). Data for three summer and three winter days in 1969 and 1970 are examined for seasonal characteristics during geomagnetic quiet time at solar cycle maximum conditions. The derived diurnal variation of the neutral winds for summer months shows a later morning transition from equatorward to poleward winds and an earlier evening transition from poleward to equatorward winds than during winter months. These times of transition have a pronounced influence on the ionospheric structure and are partly responsible for the characteristic ‘summer’ and ‘winter’ type behavior of the ionosphere observed over Millstone Hill. The diurnally averaged values of the thermospheric winds at 300 km also have a seasonal variation. During summer, the zonal winds are westward at about 15 m⁻¹ and are eastward at about the same velocity during the winter. The diurnally averaged meridional winds show a strong equatorward flow during summer of about 50 m s⁻¹ whereas during winter months the winds are considerably weaker and irregular, with an average equatorward flow on some days and poleward flow on others. The zonally averaged values are consistent with a mean meridional circulation from the summer hemisphere to the winter hemisphere at F-layer heights. However the irregular meridional winds in the winter indicate a weakening of this circulation near Millstone Hill.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.     

Range indices of geomagnetic activity

The simplest index of geomagnetic activity is the range in nT from maximum to minimum value of the field in a given time interval. The hourly range R was recommended by IAGA for use at observatories at latitudes greater than 65°, but was superceded by AE. The most used geomagnetic index K is based on the range of activity in a 3 h interval corrected for the regular daily variation. In order to take advantage of real time data processing, now available at many observatories, it is proposed to introduce a 1 h range index and also a 3 h range index. Both will be computed hourly, i.e. each will have a series of 24 per day, the 3 h values overlapping. The new data will be available as the range (R) of activity in nT and also as a logarithmic index (I) of the range. The exponent relating index to range in nT is based closely on the scale used for computing K values.The new ranges and range indices are available, from June 1987, to users in real time and can be accessed by telephone connection or computer network. Their first year of production is regarded as a trial period during which their value to the scientific and commercial communities will be assessed, together with their potential as indicators of regional and global disturbances' and in which trials will be conducted into ways of eliminating excessive bias at quiet times due to the rate of change of the daily variation field.     

The components of the annual line in geomagnetic field elements

The annual cycle of the strength of the geomagnetic horizontal field shows a local time dependence. Recent explanations for this invoke a ring current or magnetospheric source for the night-time, and an ionospheric source for the daytime annual waves, but the relative importance of each source has been uncertain. Harmonic dials of the annual waves in H from harmonic analysis at several local times and observatories are presented. Comparison with corresponding analyses of observed ionospheric winds supports the two source model. It is shown that both the magnetospheric and ionospheric components in H are 180° out of phase across the equator. But because of differing amplitudes of the two components across the equator, the net annual waves are about 160° out of phase across the equator.     

On the equatorial electrojet influence on geomagnetic pulsation amplitudes

It is well known that several types of geomagnetic pulsations show a significant amplitude enhancement near the dip equator due to the daytime equatorial electrojet. In the present study, the dependence of this enhancement on the period and type of geomagnetic variations is examined. The results show that, in general, the amplitude enhancement appears to be more or less uniform, amounting to a factor of 2.0–2.5, over a wide range of periods. However, for pulsations, there is a fairly sharp cut-off of the equatorial enhancement around a 20 s period, the shorter period end of Pc3 pulsations. Further, shorter period pulsations (<20 s) sometimes suffer an attenuation at the dip equator near noon. These results are discussed in the light of the transmission characteristics of the ionosphere, including the possible relation to the equatorial anomaly in the ionospheric F-region.     

foF2 hysteresis variations and the semi-annual geomagnetic wave

Seasonal and solar cycle variations of the foF2 hysteresis magnitude are investigated. Data for the noon foF2 monthly medians for Slough (51.48°N, 0.57°W), the monthly means for the sunspot numbers, and for the geomagnetic activity index aa(N) for the northern hemisphere for the period 1933–1986, covering solar cycle from 17 to 21, are used. It is found that: (1) the greatest negative amplitudes of the foF2 hysteresis variation are near the equinoxes, and (2) the solar cycle average noon foF2 hysteresis magnitude is linearly correlated with the solar cycle average semi-annual geomagnetic amplitude of the aa-index. These results support the hypothesis that the foF2 hysteresis is due to the geomagnetic activity variation during the sunspot cycle.     

Solar origin of geomagnetic storms and predictions

Changes of the large-scale solar magnetic fields are described and related to the occurrence of solar coronal phenomena which are associated with geomagnetic storms. Only for the very largest geomagnetic storms is there agreement on the coronal origin. However, when and where coronal mass ejections occur are still very difficult questions to answer. Artificial neural networks have been used to forecast geomagnetic storms either from daily solar input data or from hourly solar wind data. With solar data as input, predictions one-three days or even a month in advance are possible, while using solar wind data as input predictions about an hour in advance are possible. The latter predictions have been very successful. Finally, the effects of geomagnetic storms on power and satellite systems are reviewed.     

Aircraft measurements of the geomagnetic latitude effect on air-earth current density

Recent investigations of the electrode effect and the phenomenon of bubble electrification processes at the air-sea interface throw doubt on the applicability of using surface atmospheric electric observations made at sea by the Carnegie for proving the latitude effect in the columnar resistance of the atmosphere. Conduction current measurements were taken on flights during a period of decades by two instrumented aircraft in oceanic areas remote from sources of pollution. A composite of these measurements is given and confirms the notion that there is a latitude variation in air-Earth current. With the reasonable assumptions of an equal ionospheric potential and either low concentrations of Aitken nuclei or at least suitably small variations in their density with respect to latitude, the observed variation is apparently the integrated result of the Earth's magnetic field acting on cosmic ray activity throughout the troposphere.