The equatorial ring-current energy during magnetic storms as related to Dst activity indices

On the basis of geomagnetic activity indices Dst and solar wind parameters (velocity and density), we calculate the kinetic energy of the equatorial ring-current during the development of a magnetic storm, following the hypothesis of Sckopke's theorem.Supposing that the relaxation mechanisms of the plasmaspheric drift current which lead to the pre-storm state are always present and depend solely on the kinetic energy, it is found that the major part of the energy gain is lost, except during the main phase, when the interplanetary magnetic field permits it to be stored.     

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.     

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.     

Effects of geomagnetic storms in the lower ionosphere,middle atmosphere and troposphere

Geomagnetic storm effects at heights of about 0–100 km are briefly (not comprehensively) reviewed, with emphasis being paid to middle latitudes, particularly to Europe. Effects of galactic cosmic rays, solar particle events, relativistic and highly relativistic electrons, and IMF sector boundary crossings are briefly mentioned as well. Geomagnetic storms disturb the lower ionosphere heavily at high latitudes and very significantly also at middle latitudes. The effect is almost simultaneous at high latitudes, while an after-effect dominates at middle latitudes. The lower thermosphere is disturbed significantly. In the mesosphere and stratosphere, the effects become weaker and eventually non-detectable. There is an effect in total ozone but only under special conditions. Surprisingly enough, correlations with geomagnetic storms seem to reappear in the troposphere, particularly in the Northern Hemisphere. Atmospheric electricity is affected by geomagnetic storms, as well. We essentially understand the effects of geomagnetic storms in the lower ionosphere, but there is a lack of mechanisms to explain correlations found deeper in the atmosphere, particularly in the troposphere. There seem to be two different groups of effects with possibly different mechanisms—those observed in the lower ionosphere, lower thermosphere and mesosphere, and those observed in the troposphere.     

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.     

Model calculation and analysis of magnetosphere-ionosphere-thermosphere coupling

Model calculations are used to analyse the function of the magnetosphere-ionosphere electrodynamic coupling and that of the ionospheric tide-dynamo. The phenomena concerned are discussed.It is shown that for geomagnetic and ionospheric research in low-latitude and equatorial regions, the penetration and shielding of magnetospheric disturbances should be taken into account. On the other hand, the effect of the tidal dynamo on the ionosphere of the auroral region should not be ignored, especially on geomagnetically disturbed days when the conductivity is very high. The features and the relative importance of these two effects are different at different stages of a magnetic storm, and depend on latitude. There is an evident dawn-dusk asymmetry in these effects. The influence of weak geomagnetic disturbances on the low-latitude ionosphere should also be taken into account.     

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.     

Dependence of ionospheric response on the local time of sudden commencement and the intensity of geomagnetic storms

A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations.     

Modelling of transequatorial propagation of HF radio waves

In the geometrical optics approximation, a synthesis oblique ionogram of ionospheric and magnetospheric HF radio wave signals propagating between magnetic conjugate points has been carried out. The magnetospheric HF propagation is considered for a model of the waveguide formed by field-aligned irregularities with depleted electron density. The characteristic peculiarities of the magnetospheric mode have been determined: (i) strong disperion of the group delay with a frequency at 14–18 MHz, from − 1.4 to 0.6 ms/MHz for magnetically conjugate points at geomagnetic latitudes φ = 30°, 40° and 50°, respectively, (ii) spreading ∼ 1–2 ms, and (iii) a possibility of propagation between magnetic conjugates points at moderately low geomagnetic latitudes φ₀ ∼ 30–40° at frequencies exceeding 1.5 times the maximum usable frequency (MUF) of multi-hop ionospheric propagation.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.     

Low latitude signatures of substorm activity

Although magnetospheric substorms have their most easily detectable observable consequences in the high latitude auroral oval, most of the early studies of the phenomenon took place using data from low latitude observing sites. Many of these early studies concentrated on magnetic storms, yielding observations and interpretations of the ring current and sudden storm commencements which are valid to this day. Over the years, ground-based observations have been concentrated in the high latitude regions under and immediately adjacent to the auroral oval and studies of storm and substorm phenomena using low latitude data have been few in number. Despite the intensity of research activity in recent years, the physics of the substorm process still remains a matter of controversy. The STEP period represents an excellent opportunity for substorm physicists to solve some of the outstanding problems they still face. Space scientists monitoring the geospace environment at observing sites well equatorward of the auroral oval have an excellent opportunity to contribute to the ultimate solution of the substorm problem during the STEP interval, however they will have to focus on specific observational manifestations of substorms if they are to optimize their probability of success. In this paper I shall try to point out some areas where successful observations and interpretation of substorm phenomena at low latitudes would be extremely useful in helping the substorm community resolve their outstanding scientific problems.     

Energetic particles in the environment of the Earth's magnetosphere

This review summarizes the work in the field of magnetospheric energetic particles during the years 1987–1989. Out of a wealth of contributions it concentrates on a few topics. First it follows the path of ions extracted out of the polar ionosphere and their acceleration parallel and perpendicular to the magnetic field, as well as their subsequent transport into the equatorial magnetosphere and tail region. Then it focuses on acceleration of ions in the magnetotail and the related characteristics in the boundary layers including consequences for current substorm modeling. In the ring current region, the AMPTE and VIKING missions have made possible detailed studies of charge state and pitch-angle distributions as well as their variations during magnetospheric storms and substorms, from which conclusions on the transport and loss processes can be drawn. Recently, observations of energetic particles from orbiting nuclear reactors in the magnetosphere, which can be used as tracers for these satellites, have been made public. However, this may also constitute a serious background problem for future γ-astronomy missions with an increased sensitivity of the instrumentation. Finally, leading beyond the boundaries of the magnetosphere, attention is drawn to the still ongoing debate on the source of energetic particles upstream of the Earth's bow-shock and the respective importance of particle leakage and/or acceleration at the magnetospheric boundaries.     

Observations and modelling of ionospheric and thermospheric disturbances during major geomagnetic storms: A review

The thermosphere is primarily energised by the combination of three sources of energy and momentum. Solar UV and EUV energy is absorbed globally on the dayside within the middle and upper thermosphere. There is a persistent, but highly variable, inflow of energy and momentum from the magnetosphere. These magnetospheric inputs are usually confined to high latitudes, except at times of very large geomagnetic disturbances. Tides and gravity waves upwell from their sources in the troposphere and stratosphere to deposit energy and momentum at levels from the middle mesosphere to the upper thermosphere. Solar EUV radiation between 120 ran and 250 nm photo-dissociates the molecules which dominate the composition of the lower thermosphere, in particular producing atomic oxygen which dominates the composition of the upper thermosphere. The combination of solar EUV radiation at wavelengths shorter than 120 nm, plus energetic (mainly) charged particles from the magnetosphere, also ionise the neutral constituents of the thermosphere, creating the ionosphere. Particularly at high latitudes, within the geomagnetic polar caps and auroral ovals, the energetic, dynamical and chemical coupling and interactions between the thermosphere and ionosphere dominate the structural and dynamical response of both the thermosphere and ionosphere to solar and geomagnetic inputs of energy and momentum.Comparisons between predictions using global thermosphere-ionosphere coupled models and comparable observational sets have shown encouraging agreement during periods of relatively quiet geomagnetic activity. This indicates that the major energetic, ionisation, chemical and dynamical processes and interactions can be described in models with reasonable accuracy. During periods of high geomagnetic activity, and particularly during major geomagnetic storms, large rapid disturbances of the thermosphere occur with extremely rapid variations. These disturbances are observed as large increases of temperature, density, major changes of neutral composition, and with the development of high speed wind flows and large amplitude waves which may propagate to affect the entire globe. Since the ionosphere is formed from thermospheric constituents and affected by thermospherc dynamics, the gross disturbances of the ionosphere during highly disturbed periods are related to contemporary changes of density, composition and flows of the thermosphere, as well as changes of ionisation sources and electric fields. Observations which describe the nature and scale of disturbances of the thermosphere during geomagnetic storms will be used, in combination with appropriate global numerical simulations, to aid interpretation of storm-time ionospheric phenomena. The role of energetic, dynamical and chemical coupling between the thermosphere and ionosphere is emphasised.     

Mechanisms and time-scales of the magnetospheric response to the interplanetary magnetic field changes during the 8 May 1986 substorm

On 8 May 1986, between 1113 and 1600 UT, an isolated magnetospheric substorm was observed, during which the AE-index exceeded 700 nT (CDAW 9E event). Three available sets of measurements (a) of the solar-wind parameters (IMP-8 satellite), (b) of the magnetotail energy flux (ISEE-1 spacecraft), and (c) of ground magnetic observatories, allowed us to make a detailed study of the overall magnetospheric response to changes of the interplanetary magnetic field (IMF) direction, during this event of weak solar-wind coupling.In order to study the mechanisms and time-delays of the magnetospheric response to the abrupt increase of the solar-wind energy input, we have evaluated the total magnetospheric energy output U_T following two different methods: (a) Akasofu's method, taking the ring current decay time τ_R constant, and (b) Vasyliunas' method where the values of u_t are independent of the solar-wind energy input as determined from the epsilon parameter. Both methods suggest that the driven system has been considerably developed during this substorm, while an unloading event has been superposed at the expansion onset.     

A comparison of northern and southern hemisphere TEC storm behaviour

Changes in total electron content during magnetic storms are compared at stations with similar geographic and geomagnetic latitudes and eastward declinations in the northern and southern hemispheres.Mean patterns are obtained from 58 storms at ±35° and 28 storms at ± 20° latitude. The positive storm phase is generally larger (and earlier) in the southern hemisphere, while negative storm effects are larger in the north. These changes reduce the normal asymmetry in TEC between the two hemispheres. Composition changes calculated from the MSIS86 atmospheric model agree well with the maximum decreases in TEC in both seasons (when changes in the F-layer height are ignored). Recovery occurs with a time constant of about 35 h; this is 50% longer than in the MSIS86 model. There is a marked diurnal variation at 35°S, with a rapid overnight decay and enhanced values of TEC in the afternoon. This pattern is inverted (and weaker) at 35°N, where night-time decay is consistently slower than on undisturbed nights. These results require a diurnal change in composition of opposite sign in the two hemispheres, or enhanced westward winds at night changing to eastward near sunrise. There is some evidence for both these mechanisms. Following a night-time sudden commencement there is a large annual effect with daytime TEC increasing for storms near the June solstice and decreasing near December. Storms occurring between November and April tend to give large, irregular increases in TEC for several days, particularly at low latitudes. In summer and winter at both stations, the mean size of the negative phase does not increase for storms with K_p> 6. The size of the positive phase is proportional to the size of the change in a_p in winter, while in summer a positive phase is seen only for the larger storms.     

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.     

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.     

Response of the outer ionosphere to the magnetic storm of 20–22 March 1990

Measurements of the thermal plasma parameters in the interval 17–23 March 1990, made within the scope of the ACTIVE Project, were analysed to study the response of the outer ionosphere to the storm with SSC at 22.45 UT on 20 March. These measurements in the morning sector at altitudes around 2000 km clearly reflect the enhanced geomagnetic activity. They allow us to estimate the radial depth to which the plasmasphere has been affected by increased magnetospheric convection and at which a new equatorial plasmapause has formed. They also provide the possibility of monitoring the initial phase of recovery.The measurements in the dusk altitudes of 500–1000 km indicate a distincttrough in electron concentration and also a trough of light ions under relatively quiet, as well as under disturbed, conditions. The position of the equatorial edge of these troughs and the position of electron temperature peaks enable us to judge whether a plasmaspheric bulge has formed, and whether an inner plasmapause exists.     

Ionosphere-thermosphere space weather issues

Weather disturbances in the ionosphere-thermosphere system can have a detrimental effect on both ground-based and space-based systems. Because of this impact and because our field has matured, it is now appropriate to develop specification and forecast models, with the aim of eventually predicting the occurrence, duration, and intensity of weather effects. As part of the new National Space Weather Program, the CEDAR community will focus on science issues concerning space weather, and this tutorial/review is an expanded version of a tutorial presentation given at the recent CEDAR annual meeting. The tutorial/review provides a brief discussion of weather disturbances and features, the causes of weather, and the status of weather modeling. The features and disturbances discussed include plasma patches, boundary and auroral blobs, sun-aligned polar cap arcs, the effects of traveling convection vortices and SAID events, the lifetime of density structures, sporadic E and intermediate layers, spread F and equatorial plasma bubbles, geomagnetic storms and substorms, traveling ionospheric disturbances (TID's), and the effects of tides and gravity waves propagating from the lower atmosphere. The tutorial/review is only intended to provide an overview of some of the important scientific issues concerning ionospheric-thermospheric weather, with the emphasis on the ionosphere. Tutorials on thermospheric and magnetospheric weather issues are given in companion papers.