AE Index Research Articles

Studying ionospheric responses to magnetic storms depending on the local time of the storm main phase onset

A morphological analysis of vertical sounding data obtained in Irkutsk from 2003 to 2008 has been performed. The AE index was used to determine the geomagnetic activity level, and the storm main phase onset was registered based on the D st index. The ionospheric response to a magnetic storm was estimated based on the relative deviation of the critical frequency and altitude of the ionospheric F2 region from the median values. Superstrong magnetic storms and storms without positive initial phases were not considered when the data were selected. We found that positive ionospheric disturbances, which were accompanied by an increase in the F2 region maximum altitude, predominated between the storm initial phase and main phases during all considered magnetic storms. Between these storm phases, negative disturbances were only registered at night. Predominance of positive ionospheric disturbances over negative ones can be related to the selection of storms for studies.

High-latitude geomagnetic disturbances during the initial phase of a recurrent magnetic storm (from February 27 to March 2, 2008)

A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a prolonged period of negative IMF Bz values, and the most intense wave-like disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with Np reaching 30 cm−3) in the solar wind under the southward IMF in the sheath region of the high-speed solar wind stream responsible for the discussed storm caused a great (the AE index is ∼1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ∼ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF Bz resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere.

Response of low latitude ionosphere to the geomagnetic storm of May 30

Response of low latitude ionosphere to the geomagnetic storm of May 30, 2005 in the Indian longitude sector has been investigated by using the GPS data recorded at three stations namely, Udaipur, Hyderabad and Bengaluru. The event is noteworthy due to the fact that the Z component of interplanetary magnetic field (IMF Bz) remained southward for about 10 hours, coincident with the local day time for the Indian longitude sector, along with significantly higher values of AE and ASY-H indices. However, we neither found any evidence for the presence of long lasting storm time electric fields nor could we infer episodes of eastward-westward penetration of electric fields under steady southward IMF Bz and unsteady ring current conditions. On the storm day, the maximum enhancement in the total electron content has been found to be about 60%. The ionosonde observations also showed increased critical frequency (foF2) and the height (hPF2) of the F layer. The foF2 was enhanced by ∼60% which is consistent with the enhancement in total electron content. The slow rise and long duration enhancement of hPF2 and foF2 have been attributed to the upwelling by the meridional neutral winds, caused by continuous energy inputs at higher latitudes. The poleward expansion of the equatorial ionization anomaly has also been observed on May 30. On May 31, the following day of the storm, significantly suppressed anomaly with near absence of its northern crest in the Indian longitude sector, revealed the effect of storm induced disturbance dynamo electric fields.

EMIC waves observed at geosynchronous orbit during solar minimum: Statistics and excitation

[1] We identified 1875 wave events in magnetic field data from geosynchronous orbit. Most of these events were transverse with respect to the background magnetic field, left-hand polarized, and were observed in the post-noon magnetic local time sector at frequencies just below the helium gyrofrequency. Combined, these observations strongly suggest that most of these events are Electromagnetic Ion Cyclotron (EMIC) waves. Average wave amplitudes are presented, binned by frequency, geomagnetic activity and magnetic local time. The amplitude increases with increasing geomagnetic activity; increased activity also narrows the local time sector in which the waves are observed. A superposed epoch analysis of solar wind parameters and geomagnetic activity indices shows that 12 hours before wave onset the AE and Kp index increased, indicating storm and substorm activity that injects hot ion populations needed to drive the EMIC instability and providing ample time for those populations to drift into the post-noon local time sector. Just before wave onset a sudden enhancement in the AE index and the solar wind dynamic pressure are observed, indicating that a final perturbation of the magnetosphere is needed to excite EMIC wave growth. EMIC waves are thought to cause loss of relativistic particles in the radiation belt. Large solar wind densities have been associated with low flux of relativistic particles during the recovery phase of geomagnetic storms. We show that EMIC waves are preferentially generated during intervals of large solar wind density, indicating that such conditions drive EMIC waves which in turn cause enhanced loss of relativistic particles.

Ionospheric VTEC and thermospheric infrared emission dynamics during corotating interaction region and high-speed stream intervals at solar minimum: 25 March to 26 April 2008

[1] We analyze a portion of the Whole Heliospheric Interval from 25 March to 26 April 2008 to identify the ionospheric and thermospheric responses to high-speed solar wind streams. This period during solar sunspot minimum is of moderate geomagnetic activity (with the minimum Dst ∼ −50 nT) with enhanced auroral activity seen in High-Intensity Long-Duration Continuous Auroral Activity events. The solar wind data show several corotating interaction regions (CIRs) and recurrent high-speed streams (HSSs). Using the infrared emission data obtained with Sounding of the Atmosphere using Broadband Emission Radiometry on Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics, we identify a distinct relationship between the infrared emission irradiated from the thermosphere and CIR-HSS intervals. Specifically, zonal flux of NO infrared radiation correlates well with AE indices. The most pronounced effects are found at high latitudes. We used Jet Propulsion Laboratory Global Ionospheric Maps (GIMs) software and the GPS total electron content (TEC) database to calculate the vertical total electron content (VTEC) and construct GIMs. It is shown that VTEC intensifies during HSSs periods. To illustrate the point, dynamics of daytime VTEC averaged over different latitude bands are presented. These results are compared to quiet time observations to contrast variations associated with geomagnetic activity. Data analyses show fast, global, and continuous ionospheric responses to external solar wind forcing. The largest variations are found in low-latitude (between −30° and 30°) VTEC. In conclusion, we suggest that CIRs/HSSs are external drivers for both thermospheric and ionospheric phenomena during the solar sunspot minimum. We discuss both prompt penetration electric fields and disturbance dynamos as possible mechanisms responsible for the observed VTEC effects. It is clear that efficient heliospheric-magnetospheric-ionospheric-thermospheric coupling occurs during CIR-HSS intervals even during solar sunspot minimum in 2008.

Solar wind high-speed streams and related geomagnetic activity in the declining phase of solar cycle 23

Context. Coronal holes (CHs) are the source of high-speed streams (HSSs) in the solar wind, whose interaction with the slow solar wind creates corotating interaction regions (CIRs) in the heliosphere. Aims. We investigate the magnetospheric activity caused by CIR/HSS structures, focusing on the declining phase of the solar cycle 23 (years 2005 and 2006), when the occurrence rate of coronal mass ejections (CMEs) was low. We aim to (i) perform a systematic analysis of the relationship between the CH characteristics, basic parameters of HSS/CIRs, and the geomagnetic indices Dst, Ap and AE; (ii) study how the magnetospheric/ionospheric current systems behave when influenced by HSS/CIR; (iii) investigate if and how the evolution of the background solar wind from 2005 to 2006 affected the correlations between CH, CIR, and geomagnetic parameters. Methods. The cross-correlation analysis was applied to the fractional CH area (CH) measured in the central meridian distance interval ±10 ◦ , the solar wind velocity (V), the interplanetary magnetic field (B), and the geomagnetic indices Dst, Ap ,a ndAE. Results. The performed analysis shows that Ap and AE are better correlated with CH and solar wind parameters than Dst ,a nd quantitatively demonstrates that the combination of solar wind parameters BV 2 and BV plays the central role in the process of energy transfer from the solar wind to the magnetosphere. Conclusions. We provide reliable relationships between CH properties, HSS/CIR parameters, and geomagnetic indices, which can be used in forecasting the geomagnetic activity in periods of low CME activity.

GPS TEC observations of dynamics of the mid-latitude trough during substorms

[1] In this study, we combine GPS vertical total electron content (VTEC) and other complementary instruments, such as the Poker Flat incoherent scatter radar and all-sky imagers, to investigate the dynamics of the mid-latitude trough during non-storm time substorms for solar minimum condition and focus on Alaska region. We find that the poleward wall of the mid-latitude trough shifts equatorward rapidly after substorm onset with a maximum speed reaching 4°–5° of geomagnetic latitude per hour. This equatorward motion results in narrowing and even disappearance of the mid-latitude trough and is due to enhanced energetic electron precipitation. The mid-latitude trough can reappear during the substorm recovery phase as auroral activity retreats poleward. This phenomenon has not been reported before probably because of limited field-of-view of previous instruments used in trough studies. Comparisons of the trough minimum location predicted by models that are based on global Kp and AE indices show good agreement before substorm activity reaches the peak and relatively poor agreement during the recovery phase. The observations suggest that a local index, such as the local AL index, may be a better index to use to parameterize the trough location at a given meridian than a global index.

Longitudinal study of the ionospheric response to the geomagnetic storm of 15 May 2005 and manifestation of TADs

Abstract. Response of low latitude ionosphere to the geomagnetic storm of 15 May 2005 has been studied using total electron content (TEC) data, obtained from three GPS stations namely, Yibal, Udaipur and Kunming situated near the northern crest of equatorial ionization anomaly at different longitudes. Solar wind parameters, north-south component of the interplanetary magnetic field (IMF Bz) and AE index data have been used to infer the strength of the geomagnetic storm. A large value of eastward interplanetary electric field at 06:15 UT, during the time of maximum southward IMF Bz has been used to infer the transmission of an eastward prompt penetration electric field (PPEF) which resulted in a peak in TEC at 07:45 UT due to the local uplift of plasma in the low latitudes near the anomaly crest over a wide range of longitudes. Wave-like modulations superposed over the second enhancement in TEC between 09:15 UT to 10:30 UT have been observed at all the three stations. The second enhancement in TEC along with the modulations of up to 5 TECU have been attributed to the combined effect of super plasma fountain and traveling atmospheric disturbances (TAD). Observed large enhancements in TEC are a cause of concern for satellite based navigation and ground positioning. Increased [O/N2] ratio between 09:15 UT to 10:15 UT when modulations in TEC have been also observed, confirms the presence of TADs over a wide range of longitudes.

High Speed Stream Properties and Related Geomagnetic Activity During the Whole Heliosphere Interval (WHI): 20 March to 16 April 2008

We study the interplanetary features and concomitant geomagnetic activity of the two high-speed streams (HSSs) selected by the Whole Heliosphere Interval (WHI) campaign participants: 20 March to 16 April 2008 in Carrington rotation (CR) 2068. This interval was chosen to perform a comprehensive study of HSSs and their geoeffectiveness during this “deep” solar minimum. The two HSSs within the interval were characterized by fast solar-wind speeds (peak values > 600 km s−1) containing large-amplitude Alfvenic fluctuations, as is typical of HSSs during normal solar minima. However, the interplanetary magnetic field (IMF) magnitude [Bo] was exceptionally low (≈3 – 5 nT) during these HSSs, leading to lower than usual IMF Bz values. The first HSS (HSS1) had favorable IMF polarity for geomagnetic activity (negative during northern Spring). The average AE and Dst for the HSS1 proper (HSS1P) were + 258 nT and − 21 nT, respectively. The second HSS (HSS2) had a positive sector IMF polarity, one that is less favorable for geomagnetic activity. The AE and Dst index averages were + 188 nT and − 7 nT, both lower than corresponding numbers for the first event, as expected. The HSS1P geomagnetic activity is comparable to, and the HSS2P geomagnetic activity lower than, corresponding observations for the previous minimum (1996). Both events’ geomagnetic activities are lower than HSS events previously studied in the declining phase (in 2003). In general, Vsw was faster for the HSSs in 2008 compared to 1996. The southward IMF Bz was lower in the former. The product of these two parameters [Vsw and IMF Bz] comprises the solar-wind electric field, which is most directly associated with the energy input into the magnetosphere during the HSS intervals. Thus the combined effects led to the solar wind energy input in 2008 being slightly less than that in 1996. A detailed analysis of magnetic-field variances and Alfvenicity is performed to explore the characteristics of Alfven waves (a central element in the geoeffectiveness of HSSs) during the WHI. The Bz variances in the proto-CIR (PCIR) were ≈ 30 nT2 and < 10 nT2 in the high speed streams proper.

Classification of cracking mode in concrete by acoustic emission parameters

The study occupies with acoustic emission monitoring of several types of concrete during bending. The signals emitted at the different fracturing stages exhibit distinct signatures. Specifically, frequency and shape parameters of the acquired waveforms shift during the experiment, closely following the sequence of fracture mechanisms from tensile micro-cracking to brittle macro-cracking and fiber pull out. A number of AE indices are proposed, the use of which will enable classification of the cracks according to their mode. The study sheds light to the fracture process of cementitious materials, and enables a warning against the final failure. The simplicity of the scheme renders it applicable in situ.

Equatorial electrojet plasma irregularities observed during late afternoon by the 30 MHz coherent scatter radar in São Luís, Brazil

Coherent scatter radar observations conducted right before twilight by the 30 MHz radar in São Luís (Brazil) during both quiet (December 10, 2001) and disturbed (December 12, 2001) conditions detected type I echoes on the shoulders of broad type II spectra (near zero Doppler velocity). Zonal electric fields inferred from the São Luís ionosonde have a larger magnitude during the period where these spectral observations were made than on December 18, 2001 when type I echoes were not observed during late afternoon. The IMF B z and AE index indicate that the extended geomagnetic disturbance in the afternoon of December 12, 2001 has allowed the interaction between disturbance dynamo and prompt penetration electric fields. The latter electric field component could have exceeded the former to give rise to the observed electric fields, and make possible observation of type I echoes.

Multiple magnetic dipolarizations observed by THEMIS during a substorm

Abstract. The magnetic field dipolarization in the vicinity of substorm onset and during substorm expansion phase during the period of 06:00–06:40 UT on 15 February 2008 is investigated with observations from multiple probes of THEMIS. It is found that the magnetic dipolarization at the substorm onset (the onset time was about 06:14 UT) was not accompanied by obvious magnetic disturbance and ion bulk speed variation. The magnetic dipolarizations taking place during the substorm expansion phase observed by P4~(−10.97, 2.04, −3.03) RE and P3~(−11.32, 1.15, −3.10) RE were mostly accompanied by high speed earthward ion bulk flow, but the magnetic dipolarizations occurring during the substorm expansion phase observed by P5~(−9.45, 1.07, −2.85) RE were not accompanied by high speed earthward ion bulk flow. Before substorm onset THEMIS P3, P4, P5 all observed the Bx component fluctuation with a period of about 300 s. After substorm onset earthward high speed ion bulk flow and significant magnetic disturbances both occurred at P3 and P4 locations. These results indicate that there is no one-to-one relationship between the near-Earth magnetic dipolarization and the earthward ion bulk flow. In particular, the magnetic dipolarization occurring on the earthward side of the inner near-Earth plasma sheet is not accompanied by high speed earthward ion bulk flow. The dipolarization at substorm onset is a local and small scale phenomenon. There are multiple magnetic dipolarizations occurring during the substorm expansion phase. The dipolarization process is very complex and is not simply an MHD process. It is accompanied by some kinds of plasma instabilities, the plasma sheet azimuthal expansion not only by earthward ion bulk flow during substorm. A sharp increase of the AE index does not always give an accurate substorm onset time for substorm analysis.

Relationship between interplanetary field/plasma parameters with geomagnetic indices and their behavior during intense geomagnetic storms

This paper presents a correlative study between the peak values of geomagnetic activity indices (Dst, Kp, ap and AE) and the peak values of various interplanetary field (Bt, Bz, E and σ B ) and plasma (T, D, V, P and β) parameters along with their various products (BV, BzV and B 2V) during intense geomagnetic storms (GMSs) for rising, maximum and decay phases as well as for complete solar cycle 23. The study leads to the conclusion that the peak values of different geomagnetic activity indices are in good correlation with Bt, Bz, σ B , V, E, BV, BzV and B 2V, therefore these parameters are most useful for predicting GMSs and substorms. These parameters are also reliable indicators of the strength of GMSs. We have also presented the lag/lead time analysis between the maximum of Dst and peak values of geomagnetic activity indices, various interplanetary field/plasma parameters for all GMSs. We have found that the average of peak values of geomagnetic activity indices and various field/plasma parameters are larger in decay phase compare to rising and maximum phases of cycle 23. Our analyses show that average values of lag/lead time lie in the ≈−4.00 h interval for Kp, ap and AE indices as well as for Bt, Bz, σ B , E, D and P. For a more meaningful analysis we have also presented the above study for two different groups G1 (CME-driven GMSs) and G2 (CIR-driven GMSs) separately. Correlation coefficients between various interplanetary field/plasma parameters, their various products and geomagnetic activity indices for G1 and G2 groups show different nature. Three GMSs and associated solar sources observed during three different phases of this solar cycle have also been studied and it is found that GMSs are associated with large flares, halo CMEs and their active regions are close to the solar equator.

Dependence of geomagnetic activity during magnetic storms on the solar wind parameters for different types of streams

The dependence of the maximal values of the |Dst| and AE geomagnetic indices observed during magnetic storms on the value of the interplanetary electric field (Ey) was studied based on the catalog of the large-scale solar wind types created using the OMNI database for 1976–2000 [Yermolaev et al., 2009]. An analysis was performed for eight categories of magnetic storms caused by different types of solar wind streams: corotating interaction regions (CIR, 86 storms); magnetic clouds (MC, 43); Sheath before MCs (ShMC, 8); Ejecta (95); Sheath (ShE, 56); all ICME events (MC + Ejecta, 138); all compression regions Sheaths before MCs and Ejecta (ShMC + ShE, 64); and an indeterminate type of storm (IND, 75). It was shown that the |Dst| index value increases with increasing electric field Ey for all eight types of streams. When electric fields are strong (Ey > 11 mV m−1), the |Dst| index value becomes saturated within magnetic clouds MCs and possibly within all ICMEs (MC + Ejecta). The AE index value during magnetic storms is independent of the electric field value Ey for almost all streams except magnetic clouds MCs and possibly the compressed (Sheath) region before them (ShMC). The AE index linearly increases within MC at small values of the electric field (Ey 11 mV m−1). Since the dynamic pressure (Pd) and IMF fluctuations (σB) correlate with the Ey value in all solar wind types, both geomagnetic indices (|Dst| and AE) do not show an additional dependence on Pd and IMF δB. The nonlinear relationship between the intensities of the |Dst| and AE indices and the electric field Ey component, observed within MCs and possibly all ICMEs during strong electric fields Ey, agrees with modeling the magnetospheric-ionospheric current system of zone 1 under the conditions of the polar cap potential saturation.

On the origin of the energetic ion events measured upstream of the Earth's bow shock by STEREO, Cluster, and Geotail

[1] In 2007 during the declining phase of the solar cycle the energetic upstream ion events occurred mainly after a corotating interaction region passed the Earth’s magnetosphere. We study the relation between these upstream events observed from about 70 to 1750 RE away from the Earth and observations in the vicinity of the terrestrial bow shock (up to 30 RE). For this purpose, simultaneous measurements of energetic ions from STEREO A and STEREO B (far upstream region) and from Cluster and Geotail (near the bow shock) are used. In all cases the energetic ions far upstream are associated with the upstream ion events near the bow shock. The upstream events are observed simultaneously mainly when the magnetic field is pointing along the line joining those satellites in the far upstream region with those near the terrestrial bow shock. The upstream events near the bow shock often coincide with sunward directed electron bursts, increased AE index (>200 nT), nonexponential proton spectra, and most important the presence of O + ions, all of which imply at least partly a magnetospheric origin. In ∼57% of cases the upstream ion events near the bow shock are associated with electron bursts and/or with the presence of O + , and ∼40% of the latter events are associated with electron bursts at STEREO A. Although we present strong evidence that the events are partially of magnetospheric origin, we do not exclude the presence of the ions accelerated at the bow shock.

Response of the geomagnetic activity to solar wind turbulence during solar cycle 23

The solar wind–magnetosphere coupled system is characterized by dynamical processes. Recent works have shown that nonlinear couplings and turbulence might play a key role in the study of solar wind–magnetosphere interaction processes. Within this framework, this study presents a statistical analysis aimed to investigate the relationship between solar wind MHD turbulence and geomagnetic activity at high and low latitudes as measured by the AE and SYM-H indices, respectively. This analysis has been performed for different phases of solar cycle 23. The state of turbulence was characterized by means of 2-D histograms of the normalized cross-helicity and the normalized residual energy. The geomagnetic response was then studied in relation to those histograms. The results found clearly show that, from a statistical point of view, solar cycle 23 is somewhat peculiar. Indeed, good Alfvénic correlations are found unexpectedly even during solar activity maximum. This fact has implications on the geomagnetic response as well since a statistical relationship is found between Alfvénic fluctuations and auroral activity. Conversely, solar wind turbulence does not seem to play a relevant role in the geomagnetic response at low latitudes.

Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006

Experimental data from a constellation of five NOAA Polar Orbiting Environmental Satellites (POES), satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long‐lasting SEP events on 5–15 December 2006. We determined cutoff latitudes for electrons with energies &gt;100 keV and &gt;300 keV and for protons with energies from 240 keV to &gt;140 MeV. The large number of satellites allowed us to derive snapshots of the cutoff boundaries with 1‐hour time resolution. The boundaries were fitted well by ellipses. On the basis of the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R, of particles; MLT, geomagnetic indices Dst, Kp, and AE; and dipole tilt angle PS. The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross‐tail current, field‐aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of polar cap absorption (PCA) effects observed at high latitudes by the Canadian Advanced Digital Ionosonde network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of &gt;2.5 MeV protons and &gt;100 keV electrons, which contribute mostly to the ionization of ionospheric D‐layer at altitudes of ∼75 to 85 km. This finding was confirmed independently by FORMOSAT‐3/COSMIC observations of the SEP‐associated enhancements of electron content at altitudes of ∼80 km.

Occurrence and properties of substorms associated with pseudobreakups

We investigate how substorms with and without growth‐phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE index analysis shows that substorms with growth‐phase pseudobreakups are typically weak and appear as isolated events after hours of low geomagnetic activity. During the hours before onset the average solar wind merging field Em is weaker, and the length of time with enhanced values shorter than for regular substorms. Integrating Em over the last southward IMF period before onset shows an upper limit above which these substorms do not occur. To estimate how much Em reaches the ionosphere, polar cap potential drop and unified PC indices are examined. It is found that substorms with growth‐phase pseudobreakups have on average lower PC index values than regular substorms. The temporal evolution of the PC indices is similar for both substorm groups; the summer index correlates better with Em, the winter index with AE. Also the average polar cap potential drop curves for both types of substorms resemble one other; the dayside and nightside curves are mainly influenced by Em and AE, respectively. Comparing growth‐phase, isolated and recovery pseudobreakups shows that solar wind and ionospheric conditions around the first substorm after a pseudobreakup are similar, independent of whether the last pseudobreakup appeared hours (recovery and isolated pseudobreakups) or minutes before substorm onset (growth‐phase pseudobreakups). Isolated and recovery pseudobreakups are less often associated with a northward IMF rotation than growth‐phase pseudobreakups or substorms.

Modeling and simulation of the horizontal component of the geomagnetic field by fractional stochastic differential equations in conjunction with empirical mode decomposition

In this paper, we investigate the characteristics and develop a stochastic model for the horizontal component Bx of the magnetic field at 22 stations of the global near‐real‐time magnetic observatory network INTERMAGNET. The model is in the form of a fractional stochastic differential equation. A method to estimate the parameters on the basis of observed data and to simulate the data using the model is given. The degree of fractional differentiation and the alpha‐stability exponent of the process are employed to cluster the stations. The Bx time series possess pronounced local trends, which must be removed before modeling and simulation can be performed. This trend removal is carried out by an empirical mode decomposition. An outcome is an efficient method to simulate the Bx time series by empirical mode decomposition and fractional stochastic differential equation. The numerical results indicate the existence of two distinct clusters of the INTERMAGNET: one in the mid‐ and low latitudes consistent with the Dst index, and the other above geomagnetic latitude 60°N consistent with the AE index. This clustering corresponds to the inner magnetosphere and the outer magnetosphere, respectively.

Geomagnetic activity effects on plasma sheet energy conversion

Abstract. In this article we use three years (2001, 2002, and 2004) of Cluster plasma sheet data to investigate what happens to localized energy conversion regions (ECRs) in the plasma sheet during times of high magnetospheric activity. By examining variations in the power density, E·J, where E is the electric field and J is the current density obtained by Cluster, we have studied the influence on Concentrated Load Regions (CLRs) and Concentrated Generator Regions (CGRs) from variations in the geomagnetic disturbance level as expressed by the Kp, the AE, and the Dst indices. We find that the ECR occurrence frequency increases during higher magnetospheric activities, and that the ECRs become stronger. This is true both for CLRs and for CGRs, and the localized energy conversion therefore concerns energy conversion in both directions between the particles and the fields in the plasma sheet. A higher geomagnetic activity hence increases the general level of energy conversion in the plasma sheet. Moreover, we have shown that CLRs live longer during magnetically disturbed times, hence converting more electromagnetic energy. The CGR lifetime, on the other hand, seems to be unaffected by the geomagnetic activity level. The evidence for increased energy conversion during geomagnetically disturbed times is most clear for Kp and for AE, but there are also some indications that energy conversion increases during large negative Dst. This is consistent with the plasma sheet magnetically mapping to the auroral zone, and therefore being more tightly coupled to auroral activities and variations in the AE and Kp indices, than to variations in the ring current region as described by the Dst index.