Solar Wind Input Research Articles

Field-aligned currents observed by CHAMP during the intense 2003 geomagnetic storm events

Abstract. This study concentrates on the characteristics of field-aligned currents (FACs) in both hemispheres during the extreme storms in October and November 2003. High-resolution CHAMP magnetic data reflect the dynamics of FACs during these geomagnetic storms, which are different from normal periods. The peak intensity and most equatorward location of FACs in response to the storm phases are examined separately for both hemispheres, as well as for the dayside and nightside. The corresponding large-scale FAC peak densities are, on average, enhanced by about a factor of 5 compared to the quiet-time FACs' strengths. And the FAC densities on the dayside are, on average, 2.5 times larger in the Southern (summer) than in the Northern (winter) Hemisphere, while the observed intensities on the nightside are comparable between the two hemispheres. Solar wind dynamic pressure is correlated with the FACs strength on the dayside. However, the latitudinal variations of the FACs are compared with the variations in Dst and the interplanetary magnetic field component Bz, in order to determine how these parameters control the large-scale FACs' configuration in the polar region. We have determined that (1) the equatorward shift of FACs on the dayside is directly controlled by the southward IMF Bz and there is a saturation of the latitudinal displacement for large value of negative Bz. In the winter hemisphere this saturation occurs at higher latitudes than in the summer hemisphere. (2) The equatorward expansion of the nightside FACs is delayed with respect to the solar wind input. The poleward recovery of FACs on the nightside is slower than on the dayside. The latitudinal variations on the nightside are better described by the variations of the Dst index. (3) The latitudinal width of the FAC region on the nightside spreads over a wide range of about 25° in latitude.

Reply to comment by Haaland et al. on “A new interpretation of Weimer et al.'s solar wind propagation delay technique”

[1] We welcome the comments offered by Haaland et al. [2006] on Bargatze et al. [2005]. Our paper examines a technique developed by Weimer et al. [2003] that can be used to estimate solar wind propagation delay times by applying minimum variance analysis (MVA) to study interplanetary magnetic field (IMF) time series. Solar wind propagation delay is the term used to denote the time duration between the instants that related conditions are measured at a pair of monitors spatially separated in the interplanetary medium or equivalently the time duration between the moment that interplanetary conditions are measured at an upstream monitor and the time when the same conditions would putatively reach the nose of the magnetosphere. Correction for propagation delay is important because one must apply time shifting adjustments to observed solar wind time series to properly correlate changes in upstream interplanetary conditions with subsequent response variations of magnetospheric processes. [2] Tests show that use of the Weimer et al. [2003] technique in place of the convection or corotation delay correction techniques leads to a modest increase in the ability to correlate geomagnetic activity with measures of solar wind input to the magnetosphere [Bargatze and McPherron, 2004]. However, in the course of our effort to validate the Weimer et al. [2003] technique, an error was discovered in the computer algorithm they employ to perform MVA on the magnetic field variance matrix [Bargatze et al., 2005]. A correction paper has been published that addresses the error in the algorithm [Weimer, 2004]. Still, the error turned out to be serendipitous as use of the Weimer et al. [2003] algorithm yields propagation delay time estimates that agree well with delay times obtained using an independent technique based on multispacecraft observations [Weimer et al., 2002] when both techniques are applied to study the same data interval. [3] Now, in the work of Haaland et al. [2006] the authors state that their comment paper is motivated by a sentence that appears at the end of the abstract in the work of Bargatze et al. [2005]. The sentence in question states: ‘‘Given that the new technique appears to improve the accuracy of estimating solar wind propagation delays and that it requires IMF data from only one interplanetary monitor, testing it as a space weather forecasting tool is clearly motivated.’’ Unfortunately, the statement’s use of the pronouns is ambiguous and did not convey our recommendations clearly. Here we wish to clarify this statement. [4] It is clear that solar wind propagation delay techniques that are based on MVA provide the best means yet available to account for the propagation of solar wind phase fronts from an upstream monitor to the nose of the magnetosphere. Also, we would recommend using any MVA technique that produces results that mimic those produced by the Weimer et al. method that does not have the flaw present in their analysis algorithm. The MVAB-0 method suggested by Haaland et al. [2006] is one such option. MVAB-0 is the name that Sonnerup et al. [1998] assign to the form of MVA that one obtains under the condition that the average IMF unit vector, jBj, lies in the plane of the discontinuity. In fact, a technique that produces results that are identical to those found using the MVAB-0 method is presented by Bargatze et al. [2005]. It is based on applying MVA to analyze the magnetic field variance matrix calculated using B ! ?, the magnetic field fluctuations that are perpendicular to the average background IMF. The correspondence between MVAB-0 and our analysis technique can be confirmed by comparing the results presented in Table 4 of Bargatze et al. [2005] with the second set of results listed in Table 1 of the comment paper. The eigenvector values and the eigenvalues listed in the two tables agree to four significant figures. This agreement is expected given that both of these mathematical techniques are based on the same underlying assumption. That is, both assume that the phase plane normal points in a direction that is perpendicular to the average IMF direction. [5] Still though, there are many styles of MVA and it is still not clear whether other MVA variants could be used to obtain more accurate estimates of propagation delay times. It is also important to consider the fact that the solar wind contains many types of structures including rotational discontinuities, tangential discontinuities, and interplanetary shocks. Tangential discontinuities are those that have no normal magnetic field crossing the plane that defines the discontinuity surface. These discontinuities convect passively with the solar wind. The orientations of tangential JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A06103, doi:10.1029/2005JA011557, 2006

Analysis of the October 3-7 2000 GEM storm with the WINDMI model

[1] The 8 dimensional physics model WINDMI is used to analyze the October 3–7, 2000 geomagnetic storm using solar wind input data from the ACE satellite. This period was chosen because it contains an extended interval of well-defined and quasi-periodic auroral activations called sawtooth oscillations, a phenomena whose relationship to substorm processes and to upstream solar wind drivers is still under debate. The question of whether multiple sawtooth oscillations are triggered by periodic upstream solar wind features or by internal magnetospheric processes is addressed. The model predicts both the occurrence of 8 auroral activations identified as sawtooth events during the 24 hour period on the 4th of October, in agreement with the measured AL index, and also an earlier multiple sawtooth interval on the 3rd of October, in agreement with the measured AL index. These intervals occur during steady but moderate solar wind IMF Bz values and the periodicity of the sawtooth events was not directly related to any periodic features in the upstream solar wind. The model also predicts the geomagnetic Dst index through the main and recovery phase of the storm.

Statistical study of the substorm onset: its dependence on solar wind parameters and solar illumination

Abstract. Based on 1829 well-defined substorm onsets in the Northern Hemisphere, observed during a 2-year period by the FUV Imager on board the IMAGE spacecraft, a statistical study is performed. From the combination of solar wind parameter observations by ACE and magnetic field observations by the low altitude satellite CHAMP, the location of auroral breakups in response to solar illumination and solar coupling parameters are studied. Furthermore, the correspondence of the onset location with prominent large-scale field-aligned currents and electrojets are investigated. Solar illumination and the related ionospheric conductivity have significant effects on the most probable substorm onset latitude and local time. In sunlight, substorm onsets tend to occur 1h earlier in local time and 1.5° more poleward than in darkness. The solar wind input, represented by the merging electric field, integrated over 1h prior to the substorm, correlates well with the latitude of the breakup. Most poleward latitudes of the onsets are found to range around 73° magnetic latitude during very quiet times. Field-aligned and Hall currents observed concurrently with the onset are consistent with the signature of a westward travelling surge evolving out of the Harang discontinuity. The observations suggest that the ionospheric conductivity has an influence on the location of the precipitating energetic electron which causes the auroral break-up signature. Keywords. Ionosphere (Auroral ionosphere) – Magnetospheric Physics (Current systems; Magnetosphereionosphere interactions)

Proton isotropy boundaries as measured on mid- and low-altitude satellites

Abstract. Polar CAMMICE MICS proton pitch angle distributions with energies of 31-80 keV were analyzed to determine the locations where anisotropic pitch angle distributions (perpendicular flux dominating) change to isotropic distributions. We compared the positions of these mid-altitude isotropic distribution boundaries (IDB) for different activity conditions with low-altitude isotropic boundaries (IB) observed by NOAA 12. Although the obtained statistical properties of IDBs were quite similar to those of IBs, a small difference in latitudes, most pronounced on the nightside and dayside, was found. We selected several events during which simultaneous observations in the same local time sector were available from Polar at mid-altitudes, and NOAA or DMSP at low-altitudes. Magnetic field mapping using the Tsyganenko T01 model with the observed solar wind input parameters showed that the low- and mid-altitude isotropization boundaries were closely located, which leads us to suggest that the Polar IDB and low-altitude IBs are related. Furthermore, we introduced a procedure to control the difference between the observed and model magnetic field to reduce the large scatter in the mapping. We showed that the isotropic distribution boundary (IDB) lies in the region where Rc/ρ~6, that is at the boundary of the region where the non-adiabatic pitch angle scattering is strong enough. We therefore conclude that the scattering in the large field line curvature regions in the nightside current sheet is the main mechanism producing isotropization for the main portion of proton population in the tail current sheet. This mechanism controls the observed positions of both IB and IDB boundaries. Thus, this tail region can be probed, in its turn, with observations of these isotropy boundaries. Keywords. Magnetospheric physics (Energetic particles, Precipitating; Magnetospheric configuration and dynamics; Magnetotail)

Polar cap size during 14–16 July 2000 (Bastille Day) solar coronal mass ejection event: MHD modeling and satellite imager observations

This study investigates the development of the polar cap area as simulated by global magnetohydrodynamic models of the Earth's magnetosphere during the 14–16 July 2000 (Bastille Day) event. Around 1440 UT on 15 July, a magnetic cloud hit the magnetosphere and in the following hours high levels of activity in the magnetosphere and ionosphere were driven by the frequent changes in solar wind conditions. We compare the size of the polar cap (region of open magnetic field lines) as computed with two MHD models (UCLA‐GGCM and BATSRUS) with observation data obtained from the IMAGE and Polar satellites. The two models in general reproduce the changes of the polar cap size that are seen by the satellite imagers. The range of modeled polar cap sizes, however, is limited to about 50–80% of the size range seen by the imagers and the shapes of the polar caps sometimes differ considerably among the models and compared to the observations. We found that a smoothing of solar wind parameters occurs, suggesting that solar wind inputs are stored in the magnetospheric system over a certain “memory” timescale. Cross‐correlations are computed between smoothed solar wind input and the time history of the resulting polar cap size. Modeled magnetospheric “memory” timescales are estimated to be less than 12 min, whereas the satellite image data suggest a timescale of more than 20 min. The driver of magnetospheric activity in the models was found to be the Bz (north‐south) component of the solar wind magnetic field and to a lesser degree, the Akasofu ε parameter which is closely related to Bz but is also influenced by the solar wind velocity Vx and magnetic field By. N, Vx, and Pdyn ∝ NVx2 show some degree of anticorrelation with observed polar cap sizes, but anticorrelations are barely significant for polar caps computed from either model. Memory timescales and reaction time delays could be derived from some of the parameters within the limits of statistical significance of the correlation coefficients.

Relationship between the ionospheric conductance, field aligned current, and magnetopause geometry: Global MHD simulations

The role of the ionospheric conductance in the solar wind–magnetosphere coupling is studied using global MHD simulations. The simulations with varying conductance and a constant solar wind input show that the field-aligned currents, whose magnitude depends on the ionospheric conductance, affect the size of the magnetopause at the flanks by increasing the local magnetic pressure and thus altering the surface equilibrium at the magnetopause. A current system that generates the magnetic stresses required to account for the location and geometrical structure of the magnetosphere observed in the simulations is proposed.

Polar cap potential during magnetic storms: MHD simulations

In this paper, we will examine the variation of the polar cap potential during magnetic storms using global magnetohydrodynamic (MHD) simulations of the interaction of the solar wind with the magnetosphere. We present the time evolution of the polar cap potential and correlate changes in the potential with changes in the solar wind input during three events. We show that the simulation potential behaves very similar to predictions based on the solar wind electric field, but that there is also a noticeable dependence on solar wind density, with higher values of polar cap potential for larger values of solar wind density.

Solar wind-driven electron radiation belt response functions at 100-min time scales

We present a simple yet numerically robust technique, using autoregressive linear filters, to remove unwanted “colored noise” from solar wind and radiation belt electron data at sub-daily resolution. The remaining signal is then studied using finite impulse response linear prediction filters to represent the driven portion of the linear dynamics that describe the coupling between solar wind speed and electron flux. Sub-daily resolution response profiles covering magnetic L-shells between 1.1 and 8.0 R E are presented which are consistent with daily resolution response functions. Namely, while there is strong global coherence governing electron flux dynamics, there are at least two distinct responses. The first response is an immediate dropout of electrons between L = 4 and L = 7 that is at least a partly adiabatic effect associated with enhancements in the ring current. This is followed by a 1–2 day delayed enhancement across the same L-shells that is likely a result of increased radial diffusion. The second response is an immediate enhancement seen between L = 3 and L = 4 with a typical duration of less than one day. Plausible explanations for this second response are briefly discussed, but neither empirical nor theoretical evidence can establish conclusively a definite physical cause. Finally, the response profiles show significant solar cycle and seasonal dependencies, indicating that better model output might be achieved with: (1) additional simultaneous solar wind inputs; (2) more sophisticated dynamical model structures capable of incorporating non-linear feedback; and/or (3) time-adaptive linear filters that can track non-stationary dynamics in time.

3D modeling of shock-induced trapping of solar energetic particles in the Earth's magnetosphere

The prompt trapping of solar energetic particles (SEPs) in the inner magnetosphere around L = 2 –2.5, including protons and heavier ions, has been observed at both the Cycle 22 and 23 solar maxima, in association with high-speed interplanetary shocks and storm sudden commencements (SSCs). Recent observations include the Bastille Day 2000 CME-driven storm as well as two in November 2001, which produced a long-lived new proton belt, as well as trapping of heavy ions up to Fe in all three cases. A survey of such events around the most recent solar maximum, including high altitude measurements from Polar and HEO satellites along with low altitude measurements from `the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)', indicates similarities to the well-studied March 24, 1991 SSC event. In this event, electrons and protons in drift resonance with a magnetosonic impulse were transported radially inward. A requirement for such shock-induced acceleration is a high-speed CME-shock at 1 AU, which launches a perturbation with comparable velocity inside the magnetosphere. Secondly, there must be a source population which is drift-resonant with the impulse. The CME-shock itself is a source of solar energetic particles, both protons and heavy ions, with higher fluxes and harder spectra associated with faster moving CMEs. Arrival of the interplanetary shock compresses and changes the magnetosphere topology, leading to a reduction of the geomagnetic cutoff, initially around L = 4 for SEP protons. This effect is modeled using a 3D Lorentz integration of SEP trajectories in electric and magnetic fields taken from the Lyon–Fedder–Mobarry (LFM) global MHD code, with solar wind input parameters taken from spacecraft measurements upstream from the bow shock, carried out for the November 24, 2001 SEP event. The results indicate that an enhancement in solar wind dynamic pressure for this event plays a role in the observed injection of ions to low L-values, to form a new proton belt which has lasted for more than 2 years.

Plasma sheet climatology: Geotail observations and LFM model comparisons

We present a statistical comparison of plasma sheet properties from Geotail measurements and from the Lyon–Fedder–Mobarry (LFM) global magnetohydrodynamic (MHD) model. We compare more than three years of Geotail data with 10 consecutive days of simulation results driven by real solar wind data. Both studies have statistically similar solar wind inputs and a plasma sheet definition that follows from criteria that are widely used in the literature. We map the average plasma pressures, fields, and flows as functions of XY ( GSM ) position within the equatorial plasma sheet. The comparisons show a persistent dawn–dusk asymmetry in the statistical patterns of plasma flows and electric fields in both the Geotail and the simulation results. Geotail observes a less rapid fall-off of thermal pressure with downtail distance than is seen in the simulation. The Geotail dataset suffers from an orbital bias, but we show the dawn–dusk asymmetry persists after accounting for the bias. Overall, our study finds similar statistical plasma pressures, magnetic pressures, and perpendicular flows in Geotail measurements, and the LFM MHD regions of the plasma sheet.

Magnetohydrodynamic simulation of an equatorial dipolar paleomagnetosphere

During polarity reversals of the Earth's internal geomagnetic field, the paleomagnetosphere must have undergone dramatic changes because the core field was essentially different from the present‐day case, where the dipole moment is approximately aligned with the Earth's rotation axis (axial dipole) and perpendicular to the solar wind flow. According to one of the possible transition scenarios, the internal dipole moment, having much reduced in magnitude, could slowly turn around, staying close to the equatorial plane for a significant time. In this paper we present magnetohydrodynamic (MHD) simulations of a so‐called equatorial dipolar paleomagnetosphere, where the internal dipole moment is perpendicular to the rotation axis, i.e., the dipole axis is in the equatorial plane. The magnitude of the dipole moment was chosen as small as one‐tenth of the present value. With such a dipole field orientation, the dipole tilt in GSM coordinates changes, due to the Earth's rotation, between zero and 360 degrees in the course of the day, resulting in an extremely dynamic magnetosphere on the diurnal timescale. As a first approximation, we calculated steady‐state solutions with different dipole tilts, or in other words with different angles between the dipole axis and the Sun‐Earth line, to represent the paleomagnetosphere at different times of the day. We describe the regular diurnal variation of the geomagnetic field‐line configuration and the topology of large‐scale current systems, like the magnetopause currents and the tail current sheet. We investigate the so‐called pole‐on magnetosphere, where the dipole axis is aligned with the Sun‐Earth line, in more details using different solar wind input parameters.

Data‐derived forecasting model for relativistic electron intensity at geosynchronous orbit

In this paper we present a data‐derived model of relativistic electron flux at geosynchronous orbit. The model is driven by multiple solar wind and magnetospheric inputs and combines the deterministic approach of nonlinear dynamics with conditional probability consideration. The model is used for one‐day predictions of daily flux maxima for the years 1995–2000. The deterministic part of the model yields average prediction efficiency of 0.77 and linear correlation coefficient of 0.89. It identifies solar wind velocity and SymH index as most relevant input parameters. The probabilistic part of the model quantifies the risks of deviations from deterministic predictions as a function of average solar wind and magnetosphere conditions.

Case study of solar wind pressure variations and neutral atom emissions observed by IMAGE/LENA

The Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft was designed to observe neutral atom emissions at low energy (10 eV–∼4 keV) from the high‐latitude ionosphere. In this study we present data from an ion outflow event on 24 June 2000 showing increases in neutral flux that appear to be associated with dynamic pressure variations in the solar wind. Results from a correlation analysis show the neutral flux to be well correlated (∼0.64) with the solar wind dynamic pressure. The neutral atom data are most consistent with a source region at a radial distance of 2.2–2.3 RE, interpreted as the region over which the bulk ion outflow escapes the ionosphere and is partly converted to neutral atoms. Very short time delays (∼2 min) were calculated between the solar wind pressure increase and neutral atom response, indicating a prompt ionospheric response to the solar wind input.

Results from the Lyon–Fedder–Mobarry global magnetospheric model for the electrojet challenge

This paper presents the results from the Lyon–Fedder–Mobarry global magnetosphere model (LFM) for the Electrojet prediction challenge sponsored by the Space Environment Center (SEC). The SEC selected the March 19–20, 1999 interval, a period with relatively simple solar wind conditions, as an initial test bed for methods of predicting the strength and location the auroral electrojets. The challenge consisted of predicting responses of ground magnetometers using a near real-time model driven by ACE solar wind observations and/or input from real-time magnetometer stations geographically isolated from the target output stations. The LFM was run in a real time mode using input from ACE to drive the simulation for the entire 2 day period. The currents extracted from the ionospheric portion of the simulation were used with a Biot–Savart integration technique to determine the ground magnetic field perturbations for the target output stations. The results indicate the simulation predicts electrojets which are wider in latitudinal extent than the observations and that the current strength in the afternoon sector is larger than inferred from the observations. In general, the results show the simulation can qualitatively reproduce the observations but, the RMS errors indicate significant disparity between the observations and the predictions from the LFM.

Locations of proton isotropic boundaries as measured by conjugate high-altitude and low-altitude satellites

Polar CAMMICE MICS data of proton pitch angle distributions with energies of 31–80 keV for the period of 1997–1998 were analyzed to determine the locations where anisotropic pitch angle distributions (perpendicular flux dominating) change to isotropic distributions. We study this high-altitude isotropic distribution boundary (IDB) in terms of its location in L-shell and MLT. Statistical results showed that this boundary is located at lower L-shells on the nightside and at higher L-shells on the dayside with most distant location at dawn. With the increase of magnetic activity, the IDB positions shift towards lower L-shells at all MLT. The locations of IDBs were also determined for different storm phases. A superposed epoch analysis revealed that the L-shells of the IDBs move to lower L-shells with decreasing Dst and to higher L-shells during Dst recovery. We made a comparison between Polar measurements of IDB at high altitudes and DMSP measurements of b2i boundary at low altitudes selecting several events during which simultaneous observations in the same local time sector were available. The magnetic field mapping using the Tsyganenko T01 model with the observed solar wind input parameters showed that Polar and DMSP were placed on nearly the same field lines, which leads us to suggest that the Polar IDB and the b2i boundary at DMSP are related.

Cusp dynamics and ionospheric outflow

One of the IMAGE mission science goals is to understand the dayside auroral oval and its dynamic relationship to the magnetosphere. Two ways the auroral oval is dynamically coupled to the magnetosphere are through the injection of magnetosheath plasma into the magnetospheric cusps and through the ejection of ionospheric plasma into the magnetosphere. The ionospheric footpoints of the Earth's magnetospheric cusps are relatively narrow regions in invariant latitude that map magnetically to the magnetopause. Monitoring the cusp reveals two important aspects of magnetic reconnection at the magnetopause. Continuous cusp observations reveal the relative contributions of quasi-steady versus impulsive reconnection to the overall transfer of mass, energy, and momentum across the magnetopause. The location of the cusp is used to determine where magnetic reconnection is occurring on the magnetopause. Of particular interest is the distinction between anti-parallel reconnection, where the magnetosheath and magnetospheric field lines are strictly anti-parallel, and component merging, where the magnetosheath and magnetospheric field lines have one component that is anti-parallel. IMAGE observations suggest that quasi-steady, anti-parallel reconnection is occurring in regions at the dayside magnetopause. However, it is difficult to rule out additional component reconnection using these observations. The ionospheric footpoint of the cusp is also a region of relatively intense ionospheric outflow. Since outflow also occurs in other regions of the auroral oval, one of the long-standing problems has been to determine the relative contributions of the cusp/cleft and the rest of the auroral oval to the overall ionospheric ion content in the Earth's magnetosphere. While the nature of ionospheric outflow has made it difficult to resolve this long-standing problem, the new neutral atom images from IMAGE have provided important evidence that ionospheric outflow is strongly controlled by solar wind input, is `prompt' in response to changes in the solar wind, and may have very narrow and distinct pitch angle structures and charge exchange altitudes.

Neural network prediction of geomagnetic activity: a method using local Hölder exponents

Abstract. Local scaling and singularity properties of solar wind and geomagnetic time series were analysed using Hölder exponents . It was shown that in analysed cases due to the multifractality of fluctuations, α changes from point to point. We argued there exists a peculiar interplay between regularity/irregularity and amplitude characteristics of fluctuations which could be exploited for the improvement of predictions of geomagnetic activity. To this end, a layered back-propagation artificial neural network model with feedback connection was used for the study of the solar wind magnetosphere coupling and prediction of the geomagnetic Dst index. The solar wind input was taken from the principal component analysis of the interplanetary magnetic field, proton density and bulk velocity. Superior network performance was achieved in cases when the information on local Hölder exponents was added to the input layer.

Long‐term‐average, solar cycle, and seasonal response of magnetospheric energetic electrons to the solar wind speed

Among the interplanetary activity parameters the solar wind speed is the one best correlated with the energetic electron fluxes in the inner magnetosphere. We examine the radial and temporal characteristics of the 2–6 MeV electron response, approximating it in this paper with linear filters. The filter response is parameterized by the time delay (τ), measured from the time of solar wind impact, and the L shell (L). We examine solar cycle and seasonal effects using an 8‐year‐long database of Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX)/ Proton Electron Telescope (PET) measurements at the radial range L = 1.1–10. The main peak P1 of the long‐term‐average response is at (τ, L) = (2, 5.3) and has a simultaneous response over a wide range of radial distances, ΔL = 5. The duration of the response after the peak is inversely proportional to the L shell. The central part of the inner magnetosphere (L = 3.7–5.75) has a much more prolonged response (>10 days) than other parts. Prior to the main response, P1, a brief response, P0, of typically lower amplitude appears at (τ, L) = (0, 3), probably as a quasi‐adiabatic response to the compression of the magnetosphere by the solar wind pressure. Over the solar cycle the variation in solar wind input results in a systematic change of the position, amplitude, radial extent, and duration of the two peaks: during solar wind minimum the quasi‐adiabatic peak disappears, and the radial size of the responding region decreases; both are responses to low‐density, high‐speed streams. During solar minimum, the duration is at least 3 days (30%) longer than average, probably due to the sustained solar wind input. Systematic variations appear also as a function of season due to several magnetic and fluid effects. During equinoxes the coupling is stronger, and the duration is longer (by at least 2 days) compared to solstices. Between the two equinoxes the fall response has a significantly higher amplitude and longer duration than the spring equinox response. This is at least partly due to the higher GSE By component during the observation time, which acts to increase the effective GSM Bz component according to the Russell‐McPherron effect. The seasonal modulation of the response is consistent with the variation in the fluxes themselves [Baker et al., 1999]. The modulation is discussed in terms of the equinoctial and axial hypotheses [Boller and Stolov, 1970; Russell and McPherron, 1973; Cliver et al., 2000].

Global and multiscale aspects of magnetospheric dynamics in local‐linear filters

The magnetospheric dynamics consists of global and multiscale components. The local‐linear filters (LLFs) relating the solar wind input and the magnetospheric output have been used earlier to predict the global dynamical behavior. In this paper, the relative role of global and multiscale processes in the prediction of magnetospheric dynamics is studied. The filters are derived from the reconstructed input–output magnetospheric phase space using time series of VBS as the input and AL index as the output. We show that the conventional formula for the LLF can be broken into two parts corresponding to the global and multiscale constituents. The first part is the zeroth‐order term, which is obtained by the phase space average of the model outputs. This is a feature similar to the mean‐field model in phase transition physics, which yields iterative predictions of the global coherent component. The second part consists of the higher‐order terms of the filter, which are highly irregular and thus cannot be used in dynamical prediction. This irregular behavior represents the departure from the low‐dimensional dynamics underlying earlier studies using LLFs. The earlier prediction studies mixed these two components. However, by separating these two components, the prediction procedure is highly simplified and longer period predictions are achieved. The multiscale nature arises from the perturbations over a wide range of scales and has a power spectrum similar to that of colored noise. When these perturbations are taken into account in the prediction process, the iterative predictions yield a factor of four improvement in the accuracy compared to the mean‐field model. However, the filter technique does not provide a prescription for correctly including the multiscale aspects in a dynamical model and further improvement in forecasting can be achieved by a statistical approach. These results have important implications for space weather forecasting.