Solar Wind Discontinuities Research Articles

Multipoint observations of the structure and evolution of foreshock bubbles and their relation to hot flow anomalies

AbstractWhen backstreaming foreshock ions pass through approaching solar wind discontinuities, they may get concentrated upstream of the discontinuities and form foreshock bubbles (FBs). Because FBs result in very intense global pressure variations upstream of and inside Earth's magnetosphere, they are important for solar wind‐magnetosphere coupling. Information about these recently discovered phenomena is limited, however. To elucidate FB spatial structure, evolution, expansion, and formation conditions, we use multipoint Time History of Events and Macroscale Interactions during Substorms observations in which three or more spacecraft observed the same events. From single‐case studies of two foreshock bubbles and one hot flow anomaly (HFA), we demonstrate how we determine FB spatial structure, evolution, and expansion and propose a model to explain these properties. We discuss the different conditions leading to formation of FBs and HFAs. Multiple case studies of six FBs, five HFAs, and one spontaneous HFA show that FBs typically expand faster than HFAs and their expansion speed is likely determined by the solar wind speed.

Planar magnetic structures in coronal mass ejection-driven sheath regions

Abstract. Planar magnetic structures (PMSs) are periods in the solar wind during which interplanetary magnetic field vectors are nearly parallel to a single plane. One of the specific regions where PMSs have been reported are coronal mass ejection (CME)-driven sheaths. We use here an automated method to identify PMSs in 95 CME sheath regions observed in situ by the Wind and ACE spacecraft between 1997 and 2015. The occurrence and location of the PMSs are related to various shock, sheath, and CME properties. We find that PMSs are ubiquitous in CME sheaths; 85 % of the studied sheath regions had PMSs with the mean duration of 6 h. In about one-third of the cases the magnetic field vectors followed a single PMS plane that covered a significant part (at least 67 %) of the sheath region. Our analysis gives strong support for two suggested PMS formation mechanisms: the amplification and alignment of solar wind discontinuities near the CME-driven shock and the draping of the magnetic field lines around the CME ejecta. For example, we found that the shock and PMS plane normals generally coincided for the events where the PMSs occurred near the shock (68 % of the PMS plane normals near the shock were separated by less than 20° from the shock normal), while deviations were clearly larger when PMSs occurred close to the ejecta leading edge. In addition, PMSs near the shock were generally associated with lower upstream plasma beta than the cases where PMSs occurred near the leading edge of the CME. We also demonstrate that the planar parts of the sheath contain a higher amount of strong southward magnetic field than the non-planar parts, suggesting that planar sheaths are more likely to drive magnetospheric activity.

Validation of an operational product to determine L1 to Earth propagation time delays

AbstractWe describe the development and validation of an operational space weather tool to forecast propagation delay times between L1 and Earth using the Weimer and King (2008) tilted phase front technique. A simple flat plane convection delay method is currently used by the NOAA Space Weather Prediction Center (SWPC) to propagate the solar wind from a monitoring satellite located at L1 to a point upstream of the magnetosphere. This technique assumes that all observed solar wind discontinuities, such as interplanetary shocks and interplanetary coronal mass ejection boundaries, are in a flat plane perpendicular to the Sun‐Earth line traveling in the GSE X direction at the observed solar wind velocity. In reality, these phase plane fronts can have significantly tilted orientations, and by relying on a ballistic propagation method, delay time errors of ±15 min are common. In principle, the propagation time delay product presented here should more accurately predict L1 to Earth transit times by taking these tilted phase plane fronts into account. This algorithm, which is based on the work of Weimer and King (2008), is currently running in real time in test mode at SWPC as part of the SWPC test bed. We discuss the current algorithm performance, and via our detailed validation study, show that there is no significant difference between the two propagation methods when run in a real‐time operational environment.

THEMIS observations of tangential discontinuity‐driven foreshock bubbles

AbstractForeshock bubbles (FBs), transient ion foreshock phenomena formed from highly concentrated suprathermal foreshock ions upstream of solar wind discontinuities, produce intense disturbances in the magnetosphere‐ionosphere system and can accelerate particles to even higher energies. Rotational discontinuities are known to drive FBs. Tangential discontinuities (TDs), however, have not previously been considered as drivers of FBs because they have no normal magnetic field component, preventing access of field‐aligned particles upstream. However, given that suprathermal foreshock ions have gyroradii larger than the width of TDs, they may pass upstream of TDs and generate FBs. Using multipoint observations from Advanced Composition Explorer, WIND, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), we report on two cases of TD‐driven FBs. The FBs were identified using classical FB selection criteria, and the driving TDs were identified using ideal MHD criteria, to within the limits of observational error. Our results add another potential solar wind driver of FBs and imply that FBs may be even more common than previously thought.

Magnetospheric ULF waves with increasing amplitude related to solar wind dynamic pressure changes: The Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations

AbstractUltralow frequency (ULF) waves play an important role in transferring energy by buffeting the magnetosphere with solar wind pressure impulses. The amplitudes of magnetospheric ULF waves, which are induced by solar wind dynamic pressure enhancements or shocks, are thought to damp in one half a wave cycle or an entire wave cycle. We report in situ observations of solar wind dynamic pressure impulse‐induced magnetospheric ULF waves with increasing amplitudes. We found six ULF wave events induced by solar wind dynamic pressure enhancements with slow but clear wave amplitude increase. During three or four wave cycles, the amplitudes of ion velocities and electric field of these waves increased continuously by 1.3–4.4 times. Two significant events were selected to further study the characteristics of these ULF waves. We found that the wave amplitude growth is mainly contributed by the toroidal mode wave. Three possible mechanisms of causing the wave amplitude increase are discussed. First, solar wind dynamic pressure perturbations, which are observed in a duration of 20–30 min, might transfer energy to the magnetospheric ULF waves continually. Second, the wave amplitude increase in the radial electric field may be caused by superposition of two wave modes, a standing wave excited by the solar wind dynamic impulse and a propagating compressional wave directly induced by solar wind oscillations. When superposed, the two wave modes fit observations as does a calculation that superposes electric fields from two wave sources. Third, the normal of the solar wind discontinuity is at an angle to the Sun‐Earth line. Thus, the discontinuity will affect the dayside magnetopause continuously for a long time.

The magnetic hole as plasma inhomogeneity in the solar wind and related interplanetary medium perturbations

We considered magnetic hole-type plasma structures with a constant total pressure, which are often observed in the flux of the solar wind. The interaction between a linear magnetic hole and the front of the primary or bow shock wave before the Earth’s magnetosphere was studied, and the appearance of a fast shock wave in the magnetosheath and displacement of the bow shock front in the direction of the Earth’s magnetosphere is described. The magnetic hole in the scope of the MHD theory is considered a plasma inhomogeneity bounded by two tangential discontinuities: the front and rear boundaries. Based on the MHD theory of nonlinear interactions of solar wind discontinuity structures with a magnetic hole, the appearance of new automodel and MHD shock waves inside the magnetic hole is shown. The obtained results, which provide evidence of a change in the configuration of the magnetic hole and a displacement of the bow shock front due to the perturbation from the solar wind, are qualitatively verified in many aspects by observations performed earlier by the Cluster and ACE spacecrafts.

Multipoint observation of the response of the magnetosphere and ionosphere related to the sudden impulse event on 19 November 2007

The aim of this study is to provide a complete scope of a magnetic sudden impulse (SI) event along its way through interplanetary space and the magnetosphere until its arrival to the ground. In our case, we chose the event of 19th November 2007 because of the availability of enough well-located spacecraft at that moment for our purpose. We have used a 16 spacecraft data set. We calculated the mass flux variation and the change in magnetic field components across the discontinuity. Thus, we identified the solar wind discontinuity as a shock. We also calculated the orientation of the solar wind shock front. Then, we examined the effects of the shock front propagation in detail. With this large data set, we obtained a global view of the travelling wave front and identified the effects of the compressional wave front. Thus, we determined in detail the shock front passing through the different parts of the magnetosphere. We described the compressional effects in the bow shock, the magnetosheath, and the magnetopause and we depicted the propagation inside the inner magnetosphere. Moreover, we used an extensive data set from magnetic observatories on the ground and so we studied the global distribution of the SI waveform. Finally, the comparison of the observational facts with those derived from the theoretical model showed a good consistency. On the basis of the waveforms and polarizations of this SI, we determined the location in latitude where ionospheric currents (ICs) changed their sense. And also, we related polarization at ground to polarization measured by GOES spacecraft.

Active current sheets and candidate hot flow anomalies upstream of Mercury's bow shock

Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities interacting with collisionless bow shocks. They are typically formed when the normal component of the motional (convective) electric field points toward the embedded current sheet on at least one of its sides. The core region of an HFA contains hot and highly deflected ion flows and rather low and turbulent magnetic field. In this paper, we report observations of possible HFA‐like events at Mercury identified over a course of two planetary years. Using data from the orbital phase of the MESSENGER mission, we identify a representative ensemble of active current sheets magnetically connected to Mercury's bow shock. We show that some of these events exhibit magnetic and particle signatures of HFAs similar to those observed at other planets, and present their key physical characteristics. Our analysis suggests that Mercury's bow shock does not only mediate the flow of supersonic solar wind plasma but also provides conditions for local particle acceleration and heating as predicted by previous numerical simulations. Together with earlier observations of HFA activity at Earth, Venus, Mars, and Saturn, our results confirm that hot flow anomalies could be a common property of planetary bow shocks and show that the characteristic size of these events is controlled by the bow shock standoff distance and/or local solar wind conditions.

Collision of a solar wind shock wave with the Earth’s bow shock. Wave flow pattern

The wave pattern of the flow developed when a solar wind shock wave propagates along the surface of the Earth's bow shock is studied. The investigation is carried out in the three-dimensional non-plane-polarized formulation within the framework of the ideal magneto- hydrodynamic model in which the medium is assumed to be inviscid and non-heat-conducting and to have the infinite conductivity. The global three-dimensional pattern of the interaction which is a function of the latitude and longitude of elements on the surface of the bow shock is constructed as a mosaic of solutions to the problem of breakdown of a discontinuity developed between the states behind the impinging and bow shocks on the moving curve of intersection of their fronts. The investigation is carried out for typical solar wind parameters and interplanetary magnetic field strength in the Earth's orbit and for several Mach numbers of the interplanetary shock wave, which makes it possible to trace the evolution of the flow developed as a function of the intensity of the shock perturbation of the solar wind. The solution obtained is necessary for interpreting measurements carried out by spacecraft located in the neighborhood of the Lagrange point and the Earth's magnetosphere. At present, spacecraft located in the solar wind in the neighborhood of the Lagrange point L1 at distances of approximately 250 Earth's radii RE from the Earth (Wind, SOHO, and ACE) and groups of spacecraft in the neighborhood of the Earth's bow shock, in the magnetosheath, and in the outer magnetosphere (THEMIS, Cluster, and Double Star) are measuring the state of the interplanetary medium and magnetic field and transferring the data to the Earth. The measurements are used to identify sharp jumpwise changes occurring in the solar wind and related to shock waves, rotational and tangential discontinuities, and their manifestations recorded on spacecraft in the neighborhood of the Earth with the aim to forecast the cosmic weather in the form of sudden storm commencements, magnetic substorms, and sudden impulses in the Earth's magnetosphere (1-4). The results of numerical MHD simulations carried out by means of various methods (2-6) are used in analyzing the events. However, the simulations have insufficient spatial resolution, and due to this fact several MHD waves merge with one another and can hardly be identified; for example, slow or Alfven waves are unified with the contact discontinuity (5). For the correct interpretation of measurements it is necessary to use the exact solutions to the problem of interaction between a solar wind discontinuity and the Earth's bow shock Sb (7-11). The quasi-steady-state method of finding them within the framework of magnetohydrodynamics of an ideally conducting medium was first proposed in (7, 8) as the solution to the problem of breakdown of a discontinuity between the states behind the interacting waves on the moving curve of intersection of their fronts. The wave flow pattern and the dependences of the physical parameters of the medium and the magnetic field were first obtained as functions of the angle of inclination of Sb to the solar wind velocity Vsw in the

Propagation delay of solar wind discontinuities: Comparing different methods and evaluating the effect of wavelet denoising

We present a statistical study of the performance of three methods used to predict the propagation delay of solar wind structures. These methods are based on boundary normal estimations between the Advanced Composition Explorer (ACE) spacecraft orbiting the L1 libration point and the Cluster spacecraft near the Earth's magnetopause. The boundary normal estimation methods tested are the cross product method (CP), the minimum variance analysis of the magnetic field (MVAB), and the constrained minimum variance analysis (MVAB0). The estimated delay times are compared with the observed ones to obtain a quantitative measure of each method's accuracy. Boundary normal estimations of magnetic field structures embedded in the solar wind are known to be sensitive to small‐scale fluctuations. Our study uses wavelet denoising to reduce the effect of these fluctuations. The influence of wavelet denoising on the performance of the three methods is also analyzed. We find that the free parameters of the three methods have to be adapted to each event in order to obtain accurate propagation delays. We also find that by using denoising parameters optimized to each event, 88% of our database of 356 events are estimated to arrive within ±2 min from the observed time delay with MVAB, 74% with CP, and 69% with the MVAB0 method. Our results show that wavelet denoising significantly improves the predictions of the propagation time delay of solar wind discontinuities.

Dayside auroral hiss observed at South Pole Station

We performed a statistical study of low frequency (LF) auroral hiss recorded at South Pole Station in 2004, 2005, and 2007, and very low frequency (VLF) hiss recorded in 2000–2008. As expected, most auroral hiss occurs in the pre‐midnight sector. However, there is a secondary peak in occurrence in the pre‐noon sector (1000–1530 UT; ∼ 0630–1200 magnetic local time (MLT)) and somewhat more events occur in the post‐noon sector (1530–2100 UT; ∼ 1200–1730 MLT), with a null in occurrence around noon MLT. Individual dayside events appear similar to nightside hiss, but statistically they do not extend to as high frequencies. Solar wind discontinuities or impulses on the magnetopause are not correlated with these events. All‐sky camera, photometer, magnetometer, riometer, and VLF receiver data show that dayside LF hiss almost always extends to the VLF range and is often associated with active aurora. Examination of interplanetary magnetic field (IMF), substorm conditions, and Kp/AE/QI indices at times of dayside hiss suggests differences between the pre‐noon and post‐noon events: pre‐noon events are associated with IMF By < 0, whereas post‐noon events favor Bz < 0 and show a weaker correlation with By > 0. The correlation between pre‐noon events and By < 0 may arise because under those conditions, the pattern of field‐aligned currents (FACs) shifts to later magnetic local times, causing upward FACs to be dominant during pre‐noon hours at 74°, the invariant latitude of the South Pole. Unlike pre‐noon events, post‐noon events are more often associated with substorm activity on the nightside and favor elevated Kp indices, suggesting a connection of post‐noon events to nightside activity.

Magnetosheath dynamic pressure enhancements: occurrence and typical properties

Abstract. The first comprehensive statistical study of large-amplitude (> 100%) transient enhancements of the magnetosheath dynamic pressure reveals events of up to ~ 15 times the ambient dynamic pressure with durations up to 3 min and an average duration of around 30 s, predominantly downstream of the quasi-parallel shock. The dynamic pressure transients are most often dominated by velocity increases along with a small fractional increase in the density, though the velocity is generally only deflected by a few degrees. Superposed wavelet transforms of the magnetic field show that, whilst most enhancements exhibit changes in the magnetosheath magnetic field, the majority are not associated with changes in the Interplanetary Magnetic Field (IMF). However, there is a minority of enhancements that do appear to be associated with solar wind discontinuities which cannot be explained simply by random events. In general, it is found that during periods of magnetosheath dynamic pressure enhancements the IMF is steadier than usual. This suggests that a stable foreshock and hence foreshock structures or processes may be important in the generation of the majority of magnetosheath dynamic pressure enhancements.

Dynamics of the foreshock compressional boundary and its connection to foreshock cavities

AbstractWe use several global hybrid (kinetic ions, fluid electrons) simulation runs for steady and time‐varying interplanetary magnetic field (IMF) conditions to examine the dynamics of the foreshock compressional boundary (FCB) and its connection to foreshock cavities. The results demonstrate that for steady IMF conditions, the FCB forms and evolves over a long period of time due to the dynamics of the bow shock and ion foreshock. Formation of the FCB is tied to the generation and nonlinear evolution of ULF waves associated with large‐amplitude fluctuations in magnetic field and density within the foreshock. As a result, even during steady IMF conditions, the transitions in the magnetic field strength and direction across an FCB evolve. Although the FCB itself is associated with increases in the magnetic field strength and density, these quantities are reduced on the turbulent side of the FCB as compared to the pristine solar wind. Hybrid simulations with time‐varying IMF have been performed to examine the relationship between the FCB and foreshock cavities generated under two possible scenarios. In the first scenario, a bundle of field lines connects to an otherwise quasi‐perpendicular bow shock and results in the formation of a finite‐sized foreshock region that travels with this bundle of field lines as it connects to different parts of the bow shock surface. Two FCBs bound the traveling foreshock region. In the second scenario, solar wind discontinuities cause the IMF cone angle (angle between the IMF and the solar wind flow direction) to vary and thereby modify the foreshock geometry and the position of the FCB. We demonstrate that structures similar to foreshock cavities bounded by FCBs form in both scenarios. We show that the two scenarios cannot be distinguished based on convecting or nonconvecting FCBs. We also demonstrate that depending on spacecraft location and the nature of the solar wind discontinuities, foreshock cavities may be bounded by an FCB on one side and a foreshock bubble on the other.

Magnetosheath pressure pulses: Generation downstream of the bow shock from solar wind discontinuities

We present multipoint Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of transient dynamic pressure pulses in the magnetosheath 3–10 times the background in amplitude, due to enhancements in both the ion density and velocity. Their spatial dimensions are of the order of ∼1 RE parallel to the flow and ∼0.2–0.5 RE perpendicular to it, inferred from the difference in the amplitudes observed by the different spacecraft. For the first time, simultaneous observations of the solar wind and foreshock are also shown, proving no similar dynamic pressure enhancements exist upstream of the bow shock and that the majority of pulses are downstream of the quasi‐parallel shock. By considering previously suggested mechanisms for their generation, we show that the pressure pulses cannot be caused by reconnection, hot flow anomalies, or short, large‐amplitude magnetic structures and that at least some of the pressure pulses appear to be consistent with previous simulations of solar wind discontinuities interacting with the bow shock. These simulations predict large‐amplitude pulses when the local geometry of the shock changes from quasi‐perpendicular to quasi‐parallel, while the opposite case should also produce notable pulses but typically of lower amplitude. Therefore, in a given region of the magnetosheath, some of the discontinuities in the solar wind should generate pressure pulses, whereas others are expected not to. There is also evidence that the pulses can impinge upon the magnetopause, causing its motion.

Accuracy of multi-point boundary crossing time analysis

Abstract. Recent multi-spacecraft studies of solar wind discontinuity crossings using the timing (boundary plane triangulation) method gave boundary parameter estimates that are significantly different from those of the well-established single-spacecraft minimum variance analysis (MVA) technique. A large survey of directional discontinuities in Cluster data turned out to be particularly inconsistent in the sense that multi-point timing analyses did not identify any rotational discontinuities (RDs) whereas the MVA results of the individual spacecraft suggested that RDs form the majority of events. To make multi-spacecraft studies of discontinuity crossings more conclusive, the present report addresses the accuracy of the timing approach to boundary parameter estimation. Our error analysis is based on the reciprocal vector formalism and takes into account uncertainties both in crossing times and in the spacecraft positions. A rigorous error estimation scheme is presented for the general case of correlated crossing time errors and arbitrary spacecraft configurations. Crossing time error covariances are determined through cross correlation analyses of the residuals. The principal influence of the spacecraft array geometry on the accuracy of the timing method is illustrated using error formulas for the simplified case of mutually uncorrelated and identical errors at different spacecraft. The full error analysis procedure is demonstrated for a solar wind discontinuity as observed by the Cluster FGM instrument.

Multiple responses of magnetotail to the enhancement and fluctuation of solar wind dynamic pressure and the southward turning of interplanetary magnetic field

During the interval from 06:15 to 07:30 UT on 24 August 2005, the Chinese Tan-Ce 1 (TC1) satellite observed the multiple responses of the near-Earth magnetotail to the combined changes in solar wind dynamic pressure and interplanetary magnetic field (IMF). The magnetotail was highly compressed by a strong interplanetary shock because of the dynamic pressure enhancement (similar to 15 nPa), and the large shrinkage of magnetotail made a northern lobe and plasma mantle move inward to the position of the inbound TC1 that was initially in the plasma sheet. Meanwhile, the dynamic pressure fluctuations (similar to 0.5-3 nPa) behind the shock drove the quasi-periodic oscillations of the magnetopause, lobe-mantle boundary, and geomagnetic field at the same frequencies: one dominant frequency was around 3 mHz and the other was around 5 mHz. The quasi-periodic oscillations of the lobe-mantle boundary caused the alternate entries of TC1 into the northern lobe and the plasma mantle. In contrast to a single squeezed or deformed magnetotail by a solar wind discontinuity moving tailward, the compressed and oscillating magnetotail can better indicate the dynamic evolution of magnetotail when solar wind dynamic pressure increases and fluctuates remarkably, and the near-Earth magnetotail is quite sensitive even to some small changes in the solar wind dynamic pressure when it is highly compressed. Furthermore, it is found that a considerable amount of oxygen ions (O(+)) appeared in the lobe after the southward turning of IMF.

Magnetic reconnection as an element of turbulence

Abstract. In this work, recent advances on the study of reconnection in turbulence are reviewed. Using direct numerical simulations of decaying incompressible two-dimensional magnetohydrodynamics (MHD), it was found that in fully developed turbulence complex processes of reconnection locally occur (Servidio et al., 2009, 2010a). In this complex scenario, reconnection is spontaneous but locally driven by the fields, with the boundary conditions provided by the turbulence. Matching classical turbulence analysis with a generalized Sweet-Parker theory, the statistical features of these multiple-reconnection events have been identified. A discussion on the accuracy of our algorithms is provided, highlighting the necessity of adequate spatial resolution. Applications to the study of solar wind discontinuities are reviewed, comparing simulations to spacecraft observations. New results are shown, studying the time evolution of these local reconnection events. A preliminary study on the comparison between MHD and Hall MHD is reported. Our new approach to the study of reconnection as an element of turbulence has broad applications to space plasmas, shedding a new light on the study of magnetic reconnection in nature.

The geometric parameters of solar wind discontinuities based on STEREO, ACE and WIND measurements

A case study of solar wind discontinuity geometry analysis, based on magnetosphere satellites Solar TErrestrial RElations Observatory (STEREO) A, STEREO B, ACE (Advanced Composition Explorer) and Wind measurements in 2007, is presented. The normal wind direction obtained by different spacecraft, depending on the cross spacecraft distance, has been analysed. The value of the curvature radius of a discontinuity's front appeared to be independent from the spatial scale within the range of STEREO A and STEREO B observation. The curvature radii of processed events were estimated to be of the order of a thousand R E (where R E is the radius of the earth).

Foreshock bubbles and their global magnetospheric impacts

We employ 2.5‐D electromagnetic, hybrid simulations that treat ions kinetically via particle‐in‐cell methods and electrons as a massless fluid to study the formation and properties of a new structure named the foreshock bubble upstream from the bow shock. This structure forms due to changes in the interplanetary magnetic field (IMF) associated with solar wind discontinuities and their interaction with the backstreaming ions in the foreshock prior to these discontinuities encountering the bow shock. The leading edge of the foreshock bubble consists of a fast magnetosonic shock and the compressed and heated plasma downstream of the shock. The leading edge surrounds the core which consists of a less‐dense and hotter plasma and lower magnetic field strength. Ultra low frequency turbulence is present in both the outer and core regions of the foreshock bubbles. The size of the foreshock bubble transverse to the flow direction scales with the width of the ion foreshock and at Earth corresponds to tens of RE. The size along the flow depends on the age of the bubble and grows with time. Although they expand sunward, foreshock bubbles are carried antisunward by the solar wind, and for small IMF cone angles (angle between IMF and solar wind flow) when the foreshock lies upstream of the dayside magnetosphere they collide with the bow shock. This collision is shown to have significant magnetospheric impacts. Upon encountering the bow shock, the low pressures within the core of the bubble result in the reversal of the magnetosheath flow from antisunward to sunward direction. This in turn results in the outward motion of the magnetopause and expansion of the dayside magnetosphere. The interaction is found to noticeably impact the density and energy of trapped radiation belt ions and plasma injection into the cusp. Foreshock bubbles are found to be highly effective sites for ion reflection and acceleration to high energies via first‐ and second‐order Fermi acceleration. The interaction of the foreshock bubble with the bow shock results in the release of energetic ions into the magnetosheath. Some of these ions are subsequently injected into the cusp.

Geoefficiency of solar wind discontinuities

We have investigated the solar wind–magnetosphere coupling efficiency in response to solar wind dynamic pressure enhancements. We investigate fast forward shocks, corotating interaction regions (CIRs), and pressure pulses separately, by conducting a statistical driver-response analysis. We complement the observations by results from the GUMICS-4 global magnetohydrodynamic (MHD) simulation, which in all cases is in good agreement with the observational results. Analysis of shock propagation inside the magnetosphere shows that the events with higher speed are associated with higher ionospheric activity. The coupling efficiency depends on the disturbance characteristics: it is higher for small solar wind E y and smaller for high E y . The coupling efficiency is highest for small values of IMF B z and increases with increasing solar wind speed. These results confirm that pressure changes drive dynamic changes in the ionosphere independent of the changes associated with the driving reconnection electric field.