Magnetosphere-ionosphere System Research Articles

Substorms and polar cap convection: the 10 January 2004 interplanetary CME case

Abstract. The expansion-contraction model of Dungey cell plasma convection has two different convection sources, i.e. reconnections at the magnetopause and in the magnetotail. The spatial-temporal structure of the nightside source is not yet well understood. In this study we shall identify temporal variations in the winter polar cap convection structure during substorm activity under steady interplanetary conditions. Substorm activity (electrojets and particle precipitations) is monitored by excellent ground-satellite DMSP F15 conjunctions in the dusk-premidnight sector. We take advantage of the wide latitudinal coverage of the IMAGE chain of ground magnetometers in Svalbard – Scandinavia – Russia for the purpose of monitoring magnetic deflections associated with polar cap convection and substorm electrojets. These are augmented by direct observations of polar cap convection derived from SuperDARN radars and cross-track ion drift observations during traversals of polar cap along the dusk-dawn meridian by spacecraft DMSP F13. The interval we study is characterized by moderate, stable forcing of the magnetosphere-ionosphere system (EKL = 4.0–4.5 mV m−1; cross polar cap potential (CPCP), Φ (Boyle) = 115 kV) during Earth passage of an interplanetary CME (ICME), choosing an 4-h interval where the magnetic field pointed continuously south-west (Bz < 0; By < 0). The combination of continuous monitoring of ground magnetic deflections and the F13 cross-track ion drift observations in the polar cap allows us to infer the temporal CPCP structure on time scales less than the ~10 min duration of F13 polar cap transits. We arrived at the following estimates of the dayside and nightside contributions to the CPCP (CPCP = CPCP/day + CPCP/night) under two intervals of substorm activity: CPCP/day ~110 kV; CPCP/night ~50 kV (45% CPCP increase during substorms). The temporal CPCP structure during one of the substorm cases resulted in a dawn-dusk convection asymmetry measured by DMSP F13 which is opposite to that expected from the prevailing negative By polarity of the ICME magnetic field, a clear indication of a nightside source.

Characteristics of the terrestrial field-aligned current system

Abstract. We present the first ever comprehensive statistical study of the spatiotemporal characteristics of field-aligned currents in the terrestrial magnetosphere-ionosphere system using multi point measurements. We determine how the FAC density, variability and scale size are coupled. The three ST 5 satellites were in a pearls-on-a-string formation making measurements of the magnetic field with variable inter-spacecraft separations ranging from a few seconds to about 10 min. More than 4700 sets of satellite passes are analyzed using a robust correlation analysis aimed at determining the variability of the FAC system as a function of scale size and satellite spacing. We find significant differences between the FAC characteristics on the dayside and on the nightside in terms of dynamics of the current systems. On the dayside the FAC characteristics are found to be independent of IMF Bz and geomagnetic activity while the nightside indicates increased variability during disturbed conditions. The boundary separating highly and poorly correlated FACs can be fitted by a linear line for satellite separations shorter than 60 s (dayside) and 160 s (nightside). We interpret this as the dayside and nightside magnetospheric reconfiguration times respectively. For times exceeding this the FAC characteristics are suggested to be controlled by the solar wind (dayside) and plasma sheet (nightside) dynamics. Finally, the characteristics of FAC system with scale sizes larger than ~200 km (at ionospheric altitude) appear to be stable and repeatable on time scales of the order of a minute (i.e. comparable to the low-altitude orbiting satellite's traverse time across the auroral belt). In this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on average the large-scale currents with scale sizes of the Region1 and Region2 are quasi-persistently significant in the transport of energy and momentum between the magnetosphere and the ionosphere.

SuperDARN ionospheric space weather

The Super Dual Auroral Radar Network (SuperDARN) of high frequency radars monitors ionospheric space weather at middle- to high-latitudes in both hemispheres. SuperDARN is an international collaboration involving scientists and engineers from over a dozen countries. The backscatter targets of interest are irregularities in the ionospheric plasma density aligned along the geomagnetic field. The Doppler motion of the irregularities can be used to infer the strength and direction of the ionospheric electric field. These measurements, obtained continuously, provide valuable information about the electrodynamics of the coupled magnetosphere-ionosphere system over extended spatial scales and with high time resolution. Herein, the history of SuperDARN is briefly reviewed with a particular emphasis on the recent expansion of the network to middle- and higher-latitudes. A technique for assimilating multi-radar data to produce space weather maps of the hemispheric state of ionospheric plasma motion is also described.

Ducted propagation of chorus waves: Cluster observations

Abstract. Ducted propagation of whistler waves in the terrestrial magnetosphere-ionosphere system was discussed and studied long before the first in-situ spacecraft measurements. While a number of implicit examples of the existence of ducted propagation have been found, direct observation of ducts has been hampered by the low sampling rates of measurements of the plasma density. The present paper is based on Cluster observations of chorus waves. The ability to use measurements of the spacecraft potential as a proxy for high time resolution electron density measurements is exploited to identify a number of cases when increased chorus wave power, observed within the radiation belts, is observed simultaneously with density enchantments. It is argued that the observation of ducted propagation of chorus implies modification of numerical models for plasma-wave interactions within the radiation belts.

The Earth–atmosphere–geospace system: main properties and processes

On the basis of the author's long-term experimental and theoretical studies, the concept that the Earth–atmosphere–ionosphere–magnetosphere system (EAIMS) is a complex, open, dissipative, nonlinear dynamical system whose most important property is a trigger mechanism for energy release has been validated, and the basic aspects of the system paradigm are described. Highly energetic phenomena in the system are shown to give rise to a complex cluster of processes and to reconstruction in the subsystem coupling. Active experiments in the EAIMS have established the limitations of the linear description of the subsystem response to large energy inputs, determined the possibility of the onset of large-scale and global-scale perturbations from local and localized energy releases, and also revealed and identified the types of waves transferring these disturbances. Most of the highly variable processes in the EAIMS have been found to be accompanied by energetic particle precipitation from the magnetosphere at middle latitudes. A few remote sensing techniques have been used to study the dynamic processes operating in the EAIMS and to confirm the basic aspects of the system paradigm.

Observations of an auroral streamer in a double oval configuration

Abstract. During the late evening and night of 14 September 2004, the nightside auroral oval shows a distinct double oval configuration for several hours after a substorm onset at ~18:45 UT. This structure is observed both by the IMAGE satellite optical instruments focusing on the Southern Hemisphere, and by the MIRACLE ground-based instrument network in Scandinavia. At ~21:17 UT during the recovery phase of the substorm, an auroral streamer is detected by these instruments and the EISCAT radar, while simultaneously the Cluster satellites observe a bursty bulk flow in the conjugate portion of the plasma sheet in the magnetotail. Our combined data analysis reveals significant differences between the ionospheric equivalent current signature of this streamer within a double oval configuration, as compared to previously studied streamer events without such a configuration. We attribute these differences to the presence of an additional poleward polarization electric field between the poleward and the equatorward portions of the double oval, and show with a simple model that such an assumption can conceptually explain the observations. Further, we estimate the total current transferred in meridional direction by this recovery phase streamer to ~80 kA, significantly less than for previously analysed expansion phase streamer events. Both results indicate that the development of auroral streamers is dependent on the ambient background conditions in the magnetosphere-ionosphere system. The auroral streamer event studied was simultaneously observed in the conjugate Northern and Southern Hemisphere ionosphere.

Understanding the response of the ionosphere-magnetosphere system to sudden solar wind density increases

[1] The different responses of the magnetosphere to sudden solar wind density enhancements are investigated. The majority of the responses are observed to have two phases: the first is due to an inductive E field propagating through the system, while the second is due to a vortex formation in the magnetosphere. However, during some events, the system displays a single-phase response. In order to understand the controlling parameters in the system response, statistical analyses of several solar wind external characteristics as well as the internal index Dst are conducted. Results of these analyses show that none of those characteristics seem to be associated with the way the geospace system responds to the abrupt solar wind density increase. Owing to the lack of statistics in the analysis, simulations investigating the effect of the solar wind Alfven Mach number on the magnetospheric response have been conducted. The simulation results indicate that the two-phase response is always present but that a low solar wind Mach number causes an indistinct two-phase response, which is difficult to identify as having two phases. Compression signals following the sudden compression at the magnetopause are carried by a fast mode wave in the magnetosphere, which propagates toward the Earth and is reflected back toward the magnetopause by the plasmapause or ionosphere. The wave is subsequently reflected back again, leading to a secondary Earthward propagation. These multiple bounces of the fast mode wave result in two groups of the two-phase responses in the ionosphere, although the second group is extremely difficult to identify in ionospheric data.

Залежність величини варіацій геомагнітного поля від початкового стану магнітосферно-іоносферної системи

Залежність величини варіацій геомагнітного поля від початкового стану магнітосферно-іоносферної системи

Spontaneous and trigger-associated substorms compared: Electrodynamic parameters in the polar ionosphere

[1] An attempt is made to study the difference, if any, between the response of the polar ionosphere to spontaneous substorms and that to trigger-associated substorms in terms of electrodynamic parameters including ionospheric current vectors, the electric potential, and the current function. The results show that, in the first approximation, the ionospheric parameters for the two types of substorms are quite similar. It is therefore conceived that spontaneous substorms are not very different from trigger-associated substorms in the development of substorm processes in the magnetosphere-ionosphere system. We demonstrate, however, that spontaneous substorms seem to have a more clearly identifiable growth phase, whereas trigger-associated substorms have a more powerful unloading process. Changes in the current intensity and the electric potential drop across the polar cap in the recovery phase are also quite different from each other. Both the current intensity and the cross-polar cap potential drop show a larger decrease in the recovery phase of trigger-associated substorms, but the potential drop decreases only slightly and the currents in the late morning sector are still strong for spontaneous substorms. We interpret these findings as an indication of the relative importance of the unloading process and the directly driven process in conjunction with the north-south polarity of the interplanetary magnetic field. There still exists a strong directly driven process in the recovery phase of spontaneous substorms. For trigger-associated substorms, however, both the directly driven process and the unloading process become weak after the peak time.

Electric circuits and their generators in the earth’s magnetosphere: The concept of electric circuits as applied to the first phase of the April 6, 2000, superstorm

{1} The first phase of the superstorm on April 6, 2000 was studied based on the analogy between systems of magnetospheric currents and wire electric currents. The conventional dataset supplemented with maps of ionospheric equivalent currents (ECs) and field-aligned currents (FACs) was also used. The application of this analogy made it possible to introduce spatial R.N inhomogeneities into FAC distributions in the two-dimensional ionosphere and three types of meridional current systems, MCS-0, MCS-1, and MCS-2, providing electric coupling of three Iijima and Potemra FAC Regions. This basis was used to describe the formation and observed dynamics of ionospheric auroral electrojets and three-dimensional current systems in a disturbed magnetosphere-ionosphere system. The results the modify known paradigms of the substorm current wedge (SCW). A new important fact was noted: simultaneously with the beginning of the disturbance expansion phase due to the stepwise growth in the dynamic pressure of the solar wind (SW), the stepwise growth in the area of polar cap and in the electromagnetic energy flux coming to the magnetosphere from the SW were observed.

Electric circuits of the disturbed magnetosphere-ionosphere system and their generators

The simplest theory of electric circuits is applied to analysis of the observed large-scale electric field and currents in a disturbed magnetosphere-ionosphere system. Maps of distribution of field-aligned currents (FACs) obtained from ground-based magnetic measurements using the original magnetogram inversion method (MIT) and measurements by satellites were used. A method for circuit determination according to the data of such maps based on the detection of spatial R.N inhomogeneities in each of three Iijima and Potemra FAC zones is proposed. The results of the new method are used to describe some electric field and current generators not known before, new types of current systems in tail lobes and plasma sheet, and the formation and dynamics of new types of three-dimensional systems with auroral electrojets and meridional ionospheric Pedersen current, which have not been paid due attention in the literature.

Optimization of a Magnetosphere Model for Real-Time Space Weather Prediction Using a Modified Genetic Algorithm

A low-dimensional plasma physics based on the nonlinear dynamical model of the magnetosphere-ionosphere system called WINDMI is used as the basis for a real-time space weather prediction system. The input into the model is a driving voltage derived from solar wind parameters and the interplanetary magnetic field measured by the Advanced Composition Explorer satellite. The output is a field-aligned current proportional to the westward auroral electrojet AL index and the energy stored in the Earth's ring current which is proportional to the Dst index. In order to use the model for the real-time prediction of geomagnetic activity, the model parameters are required to update periodically. We developed a modified genetic algorithm (GA) with micromovement (MGAM) to train the parameters of the model in order to achieve the lowest mse against the measured AL and Dst indexes. The MGAM implements a particle-swarm-optimization-inspired movement phase that helps to improve the convergence rate while employing the efficient GA mechanism for maintaining the population diversity. The performance of the MGAM is compared to a basic real-valued GA (RGA) on five standard test functions and historical geomagnetic storm data sets. While the MGAM performs substantially better than the RGA when evaluating the standard test functions, the improvement is about 6%-12% when used on the 20-D nonlinear dynamical WINDMI model.

Disturbances of the ionospheric conductivity owing to the generation of magnetic pulsations in the Pc1 frequency range by means of the heating of electrons by a ground-based high-power high-frequency transmitter

Experiments on the generation of artificial electromagnetic pulsations constitute an important part of investigations of the magnetosphere-ionosphere system with the use of an active action. The investigation of the generation of magnetic pulsations in the Pc1 frequency range has shown that the response of the ionosphere to heating is detected only in a few experiments. Although the primary perturbed parameter is the electron temperature, the efficiency of the generation of pulsations is determined by the perturbations of the ionospheric conductivity. The magnitude of these hertz perturbations depends complexly on the electron density profile and the parameters of a pump wave. The numerical experiment demonstrates the determining effect of the electron density in the D region on the magnitude of perturbations of the ionospheric conductivity. Under conditions of a low electron density, it is impossible to create a large perturbation of the conductivity in the Pc1 frequency range, although perturbations of the electron temperature can be large in this case. In view of a large number of electrons at altitudes of 70–90 km, which absorb a considerable fraction of the energy of a high-frequency wave, the electron temperature in the E region of the ionosphere cannot be sharply increased, but the amplitude of the variations of the ionospheric conductivity in this case is larger than that for the profiles with a low electron density. In the presence of the developed D region, the efficiency of the modification of the conductivity in the indicated frequency range can be increased by choosing the optimal frequency and polarization of the pump wave. A low efficiency of the experiments on the generation of artificial magnetic pulsations in the Pc1 frequency range is apparently explained by the fact that they were performed in winter in the absence of a well-developed D region of the ionosphere.

The physics of near-earth space plasma and energetic particles in the magnetosphere

The magnetosphere-ionosphere plasma system, like the Earth’s other envelopes, is a complex nonlinear open dissipative system with self-organization. Moreover, it is embedded in the extremely nonstationary environment formed by the solar wind. Explorations that were initiated by the pioneer studies of S.N. Vernov and his colleagues resulted in deep insight into the processes occurring in this system.

Statistical analysis of the dependence of large-scale Birkeland currents on solar wind parameters

Abstract. The spatial distributions of large-scale field-aligned Birkeland currents have been derived using magnetic field data obtained from the Iridium constellation of satellites from February 1999 to December 2007. From this database, we selected intervals that had at least 45% overlap in the large-scale currents between successive hours. The consistency in the current distributions is taken to indicate stability of the large-scale magnetosphere–ionosphere system to within the spatial and temporal resolution of the Iridium observations. The resulting data set of about 1500 two-hour intervals (4% of the data) was sorted first by the interplanetary magnetic field (IMF) GSM clock angle (arctan(By/Bz)) since this governs the spatial morphology of the currents. The Birkeland current densities were then corrected for variations in EUV-produced ionospheric conductance by normalizing the current densities to those occurring for 0° dipole tilt. To determine the dependence of the currents on other solar wind variables for a given IMF clock angle, the data were then sorted sequentially by the following parameters: the solar wind electric field in the plane normal to the Earth–Sun line, Eyz; the solar wind ram pressure; and the solar wind Alfvén Mach number. The solar wind electric field is the dominant factor determining the Birkeland current intensities. The currents shift toward noon and expand equatorward with increasing solar wind electric field. The total current increases by 0.8 MA per mV m−1 increase in Eyz for southward IMF, while for northward IMF it is nearly independent of the electric field, increasing by only 0.1 MA per mV m−1 increase in Eyz. The dependence on solar wind pressure is comparatively modest. After correcting for the solar dynamo dependencies in intensity and distribution, the total current intensity increases with solar wind dynamic pressure by 0.4 MA/nPa for southward IMF. Normalizing the Birkeland current densities to both the median solar wind electric field and dynamic pressure effects, we find no significant dependence of the Birkeland currents on solar wind Alfvén Mach number.

Macro–micro interlocked simulation algorithm: an exemplification for aurora arc evolution

Using an innovative holistic simulation algorithm that can self-consistently treat a system that evolves as cooperation between macroscopic and microscopic processes, the evolution of a colorful aurora arc is beautifully reproduced as the result of cooperation between the global field-aligned feedback instability of the coupled magnetosphere–ionosphere system and the ensuing microscopic ion-acoustic instability that generates electric double layers and accelerates aurora electrons. These results are in agreement with rocket and satellite observations. This shows that the proposed holistic algorithm could be a reliable tool to reveal complex real dramatic events and become, in the near future, a viable scientifically secure prediction tool for natural disasters such as earthquakes, landslides and floods caused by typhoons.

The response of the magnetosphere-ionosphere system to a sudden dynamic pressure enhancement under southward IMF conditions

Abstract. The magnetospheric response to step-like solar wind dynamic pressure increases under southward IMF conditions is studied using the University of Michigan MHD code. A two phased response in the ionosphere is observed, similar to what is observed when the IMF is northward by looking into the residual potential and field-aligned current (FAC) patterns in the ionosphere. The first phase response right after the high pressure enhancement hits the magnetopause is associated with a pair of FACs downward in the postnoon and upward in the prenoon region. These FACs are caused by dusk-to-dawn electric fields inside the dayside magnetopause launched by a fast mode compressional wave. The second phase response shows another pair of potential cells as well as FACs in opposite polarity, which originates from magnetospheric vortices on the equatorial plane. The vortices appear to be formed by the recovery of the system from the fast mode wave.

Controlled injection of high-power radio pulses into the ionosphere-magnetosphere system and appearance of microsubstorms on October 2, 2007

Insignificant geomagnetic disturbances, which originated during the experimental injection of high-power radio pulses into the magnetosphere-ionosphere system with the help of an HF transmitter of the Sura heating facility, are considered. The experiment was performed at 1840–1900 UT on October 2, 2007 (∼2100 MLT) at geomagnetic latitudes close to the zone of generation of the current wedge westward branch, responsible for geomagnetic substorms. The series of two magnetic microsubstorms, with a sudden initial pulse and an insignificant delay relative to the facility switching, was observed at 1840–2000 UT. A disturbance was registered at many stations in the Northern Hemisphere as a global event. The equivalent ionospheric current system of an initial pulse was similar to such a system of the westward auroral surge and had an intensity maximum at Karpogory magnetic observatory, which is the closest station to the Sura facility. Under the conditions of a quiet solar wind and low planetary geomagnetic activity, the AE auroral index correlated with the interplanetary medium parameters (the correlation coefficient reached 0.65) at 1710–2000 UT. It has been confirmed that an initial geomagnetic pulse is generated as a result of radiowave injection. The arguments for and against the generation of microsubstorms due to stimulated precipitation of magnetospheric electrons, as well as the assumption that the geoeffective impact of the interplanetary medium is intensified during the injection of high-power radiowaves near the zone where the westward branch of the current wedge of magnetospheric substorms is generated, are considered.

Electric field enhancement in an auroral arc according to the simultaneous radar (EISCAT) and optical (ALIS) observations

The character of a change in the ionospheric electric field when several auroral arcs crossed the region of radar measurements has been analyzed. In one case the plasma conductivity and electric field normal component in an arc increased as compared to their undisturbed values. In another case the field and conductivity changed traditionally (in antiphase). Arcs with an increased field were previously classified as correlating arcs, but their existence was subsequently open to question during optical observations. The usage of the ALIS system of digital cameras made it possible to decrease the errors introduced by optical equipment. The measurements in the vicinity of correlating arcs were performed when these arcs were generated, and a traditional arc was a completed formation. In an originating arc, the field value can depend not only on the ionospheric plasma conductivity but also on the processes in the magnetospheric-ionospheric system resulting in the field enhancement.

Contribution from the Earth's Bow Shock to Region 1 Current under Low Alfvén Mach Numbers

Using global MHD simulations of the solar wind–magnetosphere–ionosphere system, we investigate the dependence of the contribution from the Earth's bow shock (I1bs) to ionospheric region 1 field aligned current (FAC) (I1). It is found that I1bs increases with increasing southward interplanetary magnetic field (IMF) strength Bs, if the Alfvén Mach number MA of the solar wind exceeds 2, a similar result as obtained by previous authors. However, if MA becomes close to or falls below 2, I1bs will decrease with Bs in both magnitude and percentage (i.e., I1bs/I1) because of the resultant reduction of the bow shock strength. Both the surface current density Jbs at the nose of the bow shock and the total bow shock current Ibs share nearly the same relationship with MA, and vary non-monotonically with MA or Bs. The maximum point is found to be located at MA = 2.7. Three conclusions are then made as follows: (1) The surface current density at the nose, which is much easier to be evaluated, may be used to largely describe the behaviour of the bow shock instead of the total bow shock current. (2) The peak of the total bow shock current is reached at about MA = 2.7 when only Bs is adjusted. (3) The non-monotonic variation of the bow shock current with MA causes a similar variation of its contribution to region 1 FAC. The turning point for such contribution is found to be nearly MA = 2. The implication of these conclusions to the saturation of the ionospheric transpolar potential is briefly discussed.