Solar Wind Pressure Enhancements Research Articles

Simultaneous Global Ionospheric Disturbances Associated With Penetration Electric Fields During Intense and Minor Solar and Geomagnetic Disturbances

AbstractA new observational phenomenon, named Simultaneous Global Ionospheric Density Disturbance (SGD), is identified in GNSS total electron content (TEC) data during periods of three typical geospace disturbances: a Coronal Mass Ejection‐driven severe disturbance event, a high‐speed stream event, and a minor disturbance day with a maximum Kp of 4. SGDs occur frequently on dayside and dawn sectors, with a ∼1% TEC increase. Notably, SGDs can occur under minor solar‐geomagnetic disturbances. SGDs are likely caused by penetration electric fields (PEFs) of solar‐geomagnetic origin, as they are associated with Bz southward, increased auroral AL/AU, and solar wind pressure enhancements. These findings offer new insights into the nature of PEFs and their ionospheric impact while confirming some key earlier results obtained through alternative methods. Importantly, the accessibility of extensive GNSS networks, with at least 6,000 globally distributed receivers for ionospheric research, means that rich PEF information can be acquired, offering researchers numerous opportunities to investigate geospace electrodynamics.

Magnetosphere‐Ionosphere‐Thermosphere (M‐I‐T) Coupling Leading to Equatorial Upward and Westward Drifting Supersonic Plasma Bubble Development and Amplified Subauroral Polarization Streams (SAPS) During the January 21, 2005 Moderate Storm

AbstractBy utilizing multipoint observations, we investigate the January 21, 2005 moderate storm's prompt penetration electric field (PPEF) effects in the American sector, Rayleigh‐Taylor (R‐T) instability driven polarization electric (E) field effects in the African sector, and disturbance dynamo electric field (DDEF) effects in the Northern Hemisphere. We study how these E field effects impacted the equatorial ionization anomaly (EIA) and underlying forward fountain, the midlatitude trough, and the subauroral polarization streams (SAPS). In the daytime sector during the initial phase, the PPEF development was produced by solar wind pressure enhancements. In the dusk sector, the undershielding PEFs triggered R‐T instability mechanisms, which impacted the EIA during a prereversal enhancement (PRE) like scenario and afterwards. These led to the development of supersonic plasma bubbles drifting upward and westward. Supersonic bubbles are characteristics of superstorms, and their detections further verify the January 21, 2005 moderate storm's turbulent and superstorm nature. While the disturbance dynamo‐driven anti‐Sq current system operated, the net subauroral westward currents became enhanced and the net zonal (E‐ and F‐region) drift created a deep O+ trough that deepened the midlatitude trough and led to amplified SAPS flows via positive feedback mechanisms. We conclude that strong magnetosphere‐ionosphere‐thermosphere coupling was present at equatorial latitudes where forward fountain plasma drift‐signatures were seen in the thermospheric neutral wind speed data and where the supersonic bubbles developed, and at subauroral latitudes where the midlatitude trough became deepened during the anti‐Sq current system's operation amplifying the SAPS flows by positive feedback mechanisms.

Simultaneous Observations of Electromagnetic Ion Cyclotron (EMIC) Waves and Pitch Angle Scattering During a Van Allen Probes Conjunction

AbstractOn 22 December 2015, the two Van Allen Probes observed two sets of electromagnetic ion cyclotron (EMIC) wave bursts during a close conjunction when both Probe A and Probe B were separated by 0.57 to 0.68 RE. The EMIC waves occurred during an active period in the recovery phase of a coronal mass ejection‐driven geomagnetic storm. Both spacecraft observed EMIC wave bursts that had similar spatial structure within a 1–2 min time delay. The EMIC waves occurred outside the plasmasphere, within ΔL ≈ 1–2 of the plasmapause and within a few degrees in magnetic latitude of the equatorial plane. The spatial structure of the EMIC wave bursts may have been related to the proton drift paths outside the plasmasphere and influenced by total magnetic field strength variations associated with solar wind pressure enhancements. The EMIC waves were observed in a narrow L shell region from L ≈ 4.55–5.32 between 10 and 11 magnetic local time (MLT) on the outbound halves of the spacecraft orbits and from L ≈ 4.82–5.51 between 13 and 14 MLT on the inbound halves of the spacecraft orbits. However, Pc1 pulsations were observed on the ground over a broad range of local times. The anisotropy of the proton pitch angle distributions was enhanced when the EMIC waves were observed. Although the overall radiation belt response during this storm was dominated by acceleration and transport processes, the EMIC waves produced local pitch angle scattering of 13–15 keV protons and 2.1–2.6 MeV electrons, consistent with calculations of the expected resonant energies.

The Detached Auroras Induced by the Solar Wind Pressure Enhancement in Both Hemispheres From Imaging and In Situ Particle Observations

AbstractThis paper presents simultaneous detached proton auroras that appeared in both hemispheres at 11:06 UT, 08 March 2012, just 2 min after a sudden solar wind pressure enhancement (~11:04 UT) hit the Earth. They were observed under northward interplanetary magnetic field Bz condition and during the recovery phase of a moderate geomagnetic storm. In the Northern Hemisphere, Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observed that the detached arc occurred within 60°–65° magnetic latitude and covered a few magnetic local time (MLT) hours ranging from 0530 to 0830 MLT with a possible extension toward noon. At the same time (11:06 UT), Polar Orbiting Environment Satellites 19 detected a detached proton aurora around 1300 MLT in the Southern Hemisphere, centering ~62° magnetic latitude, which was at the same latitudes as the northern detached arc. This southern aurora was most probably a part of a dayside detached arc that was conjugate to the northern one. In situ particle observations indicated that the detached auroras were dominated by protons/ions with energies ranging from around 20 keV to several hundreds of keV, without obvious electron precipitations. These detached arcs persisted for less than 6 min, consistent with the impact from pressure enhancement and the observed electromagnetic ion cyclotron (EMIC) waves. It is suggested that the increasing solar wind pressure pushed the hot ions in the ring current closer to Earth where the steep gradient of cold plasma favored EMIC wave growth. By losing energy to EMIC waves the energetic protons (>20 keV) were scattered into the loss cone and produced the observed detached proton auroras.

Effects of sudden commencement on the ionosphere: PFISR observations and global MHD simulation

AbstractSudden commencement (SC) induced by solar wind pressure enhancement can produce significant global impact on the coupled magnetosphere‐ionosphere (MI) system, and its effects have been studied extensively using ground magnetometers and coherent scatter radars. However, very limited observations have been reported about the effects of SC on the ionospheric plasma. Here we report detailed Poker Flat Incoherent Scatter Radar (PFISR) observations of the ionospheric response to SC during the 17 March 2015 storm. PFISR observed lifting of the F region ionosphere, transient field‐aligned ion upflow, prompt but short‐lived ion temperature increase, subsequent F region density decrease, and persistent electron temperature increase. A global magnetohydrodynamic (MHD) simulation has been carried out to characterize the SC‐induced current, convection, and magnetic perturbations. Simulated magnetic perturbations at Poker Flat show a satisfactory agreement with observations. The simulation provides a global context for linking localized PFISR observations to large‐scale dynamic processes in the MI system.

Dynamic responses of the Earth's radiation belts during periods of solar wind dynamic pressure pulse based on normalized superposed epoch analysis

AbstractUsing the electron flux measurements obtained from five satellites (GOES 15 and POES 15, 16, 18, and 19), we investigate the flux variations of radiation belt electrons during forty solar wind dynamic pressure pulses identified between September 2012 and December 2014. By utilizing the mean duration of the pressure pulses as the epoch timeline and stretching or compressing the time phases of individual events to normalize the duration by means of linear interpolation, we have performed normalized superposed epoch analysis to evaluate the dynamic responses of radiation belt energetic electrons corresponding to various groups of solar wind and magnetospheric conditions in association with solar wind dynamic pressure pulses. Our results indicate that by adopting the timeline normalization we can reproduce the typical response of the electron radiation belts to pressure pulses. Radiation belt electron fluxes exhibit large depletions right after the Pdyn peak during the periods of northward interplanetary magnetic field (IMF) Bz and are more likely to occur during the Pdyn pulse under southward IMF Bz conditions. For the pulse events with large negative values of (Dst)min, radiation belt electrons respond in a manner similar to those with southward IMF Bz, and the corresponding postpulse recovery can extend to L ~ 3 and exceed the prepulse flux levels. Triggered by the solar wind pressure enhancements, deeper earthward magnetopause erosion provides favorable conditions for the prompt electron flux dropouts that extend down to L ~ 5, and the pressure pulses with longer duration tend to produce quicker and stronger electron flux decay. In addition, the events with high electron fluxes before the Pdyn pulse tend to experience more severe electron flux dropouts during the course of the pulse, while the largest rate of electron flux increase before and after the pulse occurs under the preconditioned low electron fluxes. These new results help us understand how electron fluxes respond to solar wind dynamic pressure pulses and how these responses depend on the solar wind and geomagnetic conditions and on the preconditions in the electron radiation belts.

Global MHD simulations of Mercury's magnetosphere with coupled planetary interior: Induction effect of the planetary conducting core on the global interaction

AbstractMercury's comparatively weak intrinsic magnetic field and its close proximity to the Sun lead to a magnetosphere that undergoes more direct space‐weathering interactions than other planets. A unique aspect of Mercury's interaction system arises from the large ratio of the scale of the planet to the scale of the magnetosphere and the presence of a large‐size core composed of highly conducting material. Consequently, there is strong feedback between the planetary interior and the magnetosphere, especially under conditions of strong external forcing. Understanding the coupled solar wind‐magnetosphere‐interior interaction at Mercury requires not only analysis of observations but also a modeling framework that is both comprehensive and inclusive. We have developed a new global MHD model for Mercury in which the planetary interior is modeled as layers of different electrical conductivities that electromagnetically couple to the surrounding plasma environment. This new modeling capability allows us to characterize the dynamical response of Mercury to time‐varying external conditions in a self‐consistent manner. Comparison of our model results with observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft shows that the model provides a reasonably good representation of the global magnetosphere. To demonstrate the capability to model induction effects, we have performed idealized simulations in which Mercury's magnetosphere is impacted by a solar wind pressure enhancement. Our results show that due to the induction effect, Mercury's core exerts strong global influences on the way Mercury responds to changes in the external environment, including modifying the global magnetospheric structure and affecting the extent to which the solar wind directly impacts the surface. The global MHD model presented here represents a crucial step toward establishing a modeling framework that enables self‐consistent characterization of Mercury's tightly coupled planetary interior‐magnetosphere system.

The relation between transpolar potential and reconnection rates during sudden enhancement of solar wind dynamic pressure: OpenGGCM‐CTIM results

AbstractThis study investigates how solar wind energy is deposited into the magnetosphere‐ionosphere system during sudden enhancements of solar wind dynamic pressure (Psw), using the coupled Open Geospace General Circulation Model–Coupled Ionosphere Thermosphere Model (OpenGGCM‐CTIM) 3‐D global magnetosphere‐ionosphere‐thermosphere model. We simulate three unique events of solar wind pressure enhancements that occurred during negative, near‐zero, and positive interplanetary magnetic field (IMF) Bz. Then, we examine the behavior of the dayside and nightside reconnection rates and quantify their respective contributions to cross polar cap potential (CPCP), a proxy of ionospheric plasma convection strength. The modeled CPCP increases after a Psw enhancement in all three cases, which agrees well with observations from the Defense Meteorological Satellite Program spacecraft and predictions from the assimilative mapping of ionospheric electrodynamics technique. In the OpenGGCM‐CTIM model, dayside reconnection increases within 9–13 min of the pressure impact, while nightside reconnection intensifies about 13–25 min after the pressure increase. As the strong Psw compresses the dayside magnetosheath and, subsequently, the magnetotail, their magnetic fields intensify and activate stronger antiparallel reconnection on the dayside magnetopause first and near the central plasma sheet second. For southward IMF, dayside reconnection contributes to the CPCP enhancement 2–4 times more than nightside reconnection. For northward IMF, the dayside contribution weakens, and nightside reconnection contributes more to the CPCP enhancement. We find that high‐latitude magnetopause reconnection during northward IMF produces sunward ionospheric plasma convection, which decreases the typical dawn‐to‐dusk ionosphere electric field. This results in a weaker dayside reconnection contribution to the CPCP during northward IMF.

Martian ionospheric responses to dynamic pressure enhancements in the solar wind

AbstractAs a weakly magnetized planet, Mars ionosphere/atmosphere interacts directly with the shocked solar wind plasma flow. Even though many numerical studies have been successful in reproducing numerous features of the interaction process, these earlier studies focused mainly on interaction under steady solar wind conditions. Recent observations suggest that plasma escape fluxes are significantly enhanced in response to solar wind dynamic pressure pulses. In this study, we focus on the response of the ionosphere to pressure enhancements in the solar wind. Through modeling of two idealized events using a magnetohydrodynamics model, we find that the upper ionosphere of Mars responds almost instantaneously to solar wind pressure enhancements, while the collision dominated lower ionosphere (below ~150 km) does not have noticeable changes in density. We also find that ionospheric perturbations in density, magnetic field, and velocity can last more than an hour after the solar wind returns to the quiet conditions. The topside ionosphere forms complicated transient shapes in response, which may explain unexpected ionospheric behaviors in recent observations. We also find that ionospheric escape fluxes do not correlate directly with simultaneous solar wind dynamic pressure. Rather, their intensities also depend on the earlier solar wind conditions. It takes a few hours for the ionospheric/atmospheric system to reach a new quasi‐equilibrium state.

SuperMAG‐based partial ring current indices

Using the extensive set of stations in the SuperMAG collaboration, we introduce partial ring current indices, which provide new insights into ring current development. The indices are labeled SMR‐00, SMR‐06, SMR‐12 and SMR‐18 for their center local time range. These indices incorporate data from 98 mid and low latitude stations. The behavior of these local time indices during storms and substorms, on both an individual and superposed epoch basis, produces consistent patterns. The initial positive spike before a storm, which results from solar wind pressure enhancements, is seen simultaneously at all local times. Once the main phase of the storm begins, however, SMR‐18 nearly always drops fastest and furthest in magnitude, while SMR‐06 drops more slowly (i.e., has weaker ring current signatures), and never as far. Symmetry is, in fact, not reached until storm recovery is well underway, with a typical symmetry point of about 20–25 h after onset. If the main phase continues to new depths (larger magnitude negative SMR) over a longer time period, the SMR‐18 sector will continue to lead, and SMR‐06 to lag. There has been controversy over the extent to which substorm auroral and cross‐tail currents perturb Dst and SYM‐H signatures. The signature of substorms can be seen very clearly as a positive spike of roughly 10 nT magnitude in SMR‐00, and only to a much lesser extent elsewhere. The SMR global index, and thus also SYM‐H, experiences only a small immediate perturbation from a substorm onset, ending with a net drop of a few nT. Since there are typically only 1–2 substorms in a main phase, substorms are a minor factor in the development of the storm time ring current. Indeed, because even the peak perturbation of substorms currents in the most affected sector (which is midnight) is nearly an order of magnitude smaller than the storm perturbation (about 10 nT versus 80 nT), fluctuations in the cross‐tail and field‐aligned currents in general are not a major influence over SMR. The pattern of LT substorm responses, with a strong positive SMR‐00 effect, weak positive SMR‐06 effect, and negative SMR‐18 effect implies it is field‐aligned currents and not the cross‐tail currents which create modest perturbations in the putatively ring current indices.

Analysis of radiation belt energetic electron phase space density using THEMIS SST measurements: Cross-satellite calibration and a case study

[1] In this study we perform an energy channel dependent cross-satellite calibration of Time History of Events and Macroscale Interactions during Substorms (THEMIS) solid state telescope (SST) flux measurements based on electron phase space density (PSD) conjunctions at fixed phase space coordinates. By comparing PSD around L* = 6 between THEMIS SST and Los Alamos National Laboratory (LANL) satellite LANL-01A synchronous orbit particle analyzer (SOPA) for a half year period starting from 1 July 2007, we evaluate systematic errors in the THEMIS SST measurements and quantify the cross-instrument calibration factors for the 11 SST energy channels from 40 to 2159 keV. Good consistency in electron PSD conjunctions between the five THEMIS probes indicates that the SST instrument aboard each spacecraft responds quite similarly to the ambient electron radiation environment. Compared to the LANL-01A SOPA instrument, the THEMIS SST underestimates the electron fluxes within a factor of 2 for the 40–140 keV energy channels and overestimates the electron fluxes within a factor of 3 for the 204–2159 keV energy channels. Using the cross-satellite calibrated SST flux data for the five THEMIS spacecraft and the SOPA measurements from the LANL-01A and 1989–048 satellites, we analyze the response of radiation belt electrons to a sudden solar wind pressure enhancement event. The electron PSD conjunctions between the THEMIS probes and the two geostationary satellites show good agreement, suggesting a reasonable cross-satellite calibration of the SST measurements. Our results also indicate a clear correlation between electron PSD dropouts and the solar wind pressure pulse, which is likely due to the combination of magnetopause shadowing and outward radial diffusion. The gradual buildup of electron PSD after the abrupt pressure enhancement most likely results from a combined effect of local acceleration and inward and outward radial diffusion that refills the main phase PSD dropout when the magnetopause moves outward. A longer-term quantitative analysis of the temporal evolution and radial profile of electron PSD based on multiple satellite measurements, including the cross-satellite calibrated THEMIS SST data, will be required to improve our understanding of the dynamic responses of radiation belt electrons to solar activity.

Direct measurements of the Poynting flux associated with convection electric fields in the magnetosphere

Observations of Poynting fluxes associated with onset of convection electric fields are essential for understanding of electromagnetic energy transport from the solar wind toward the magnetosphere leading to changes in the convection electric field, which is one of the most fundamental parameters in the magnetosphere‐ionosphere coupled system. We present Cluster multispacecraft observations of Poynting fluxes associated with abrupt changes in large‐scale electric fields during sudden commencements and southward turning of the interplanetary magnetic field (IMF). The Cluster spacecraft detected Poynting fluxes dominated by the field‐aligned upward component during the preliminary impulse of sudden commencements and in the initial period after southward turning of the IMF. The upward Poynting flux indicates existence of Alfvén waves transporting electromagnetic energy from the ionosphere toward the magnetosphere leading to magnetospheric convection changes. The waveguide model and global magnetohydrodynamic (MHD) simulation calculating evolution of the Poynting flux following solar wind pressure enhancements also show upward Poynting fluxes propagating from the ionosphere toward the magnetosphere faster than the propagation of compressional waves. We conclude that the ionosphere acts as a channel to transmit electromagnetic energy supplied as field‐aligned currents toward a wide region in the magnetosphere‐ionosphere system instantaneously, leading to changes in magnetospheric convection electric fields.

A series of plasma flow vortices in the tail plasma sheet associated with solar wind pressure enhancement

A series of earthward‐moving (∼140 km/s) plasma flow vortices with anticlockwise (when viewed from above the ecliptic plane) rotation was detected in the dawnside tail plasma sheet between 1255 and 1400 UT on 6 July 2003. These flow vortices were observed under the condition of northward interplanetary magnetic field with an enhanced solar wind dynamic pressure. Analysing the plasma and magnetic field data from the Cluster spacecraft and using the Grad‐Shafranov streamline reconstruction technique, we show that the vortex‐like plasma structures have a very similar shape: a Vx component dominant in the dawnside, while a distinct Vy component appears in the duskside, and each structure has a size of about 1.8 × 0.68 RE, approximately in the xy plane of GSM coordinates. It is found that the vortices contain both magnetosphere‐originated hot (N ∼ 0.1 cm−3, E > 3 keV) and magnetosheath‐originated denser and colder (N > 0.2 cm−3, E < 1 keV) populations on the closed field lines. The vortices involve fast earthward flows (Vx > 200 km/s) of mainly sheath‐originated plasmas. We suggest that these observed plasma flow vortices are generated inside the magnetotail during the prolonged and intensified compression of the magnetosphere by the enhanced solar wind dynamic pressure.

On phase space density radial gradients of Earth's outer‐belt electrons prior to sudden solar wind pressure enhancements: Results from distinctive events and a superposed epoch analysis

Here, we present the results of a study of phase space density radial gradients for outer‐belt electrons at and beyond geosynchronous orbit prior to 86 sudden solar wind pressure enhancements from 1993 through 2007. All of the events are classified and analyzed based on the results for equatorial electrons with first adiabatic invariants of 50, 200, 750, and 2000 MeV/G. Examples of three distinctive events are compared, and the results from a superposed epoch analysis are presented. We find that the radial gradients are dependent on the first adiabatic invariant (i.e., energy), and that for the majority of cases, the gradient is negative for electrons with energies above a couple of hundred keV, while it is either positive or relatively flat for electrons with energies lower than this, which is evidence of two distinct populations. In the cases where a positive gradient is observed for 2000 MeV/G electrons, the solar wind and geomagnetic conditions are very quiet for at least two days prior to the event, but for the events when the gradient for the same electrons is negative, there is a consistent evidence of enhanced substorm activity and/or convection in the days leading up to the events. Overall, this study puts previous observations of phase space density (PSD) gradients into a broader context of solar wind and geomagnetic conditions, while encompassing a broad range of energies, from the source population of tens to hundreds of keV electrons to relativistic electrons with energies exceeding 1 MeV. We discuss how 41 of the 86 events are consistent with and can be explained by local heating by wave‐particle interactions, and we provide evidence of the solar wind and geomagnetic conditions that are important to different types of sources of outer‐belt electron PSD.

Radial gradients of phase space density of the outer radiation belt electrons prior to sudden solar wind pressure enhancements

When Earth's magnetosphere is impacted by a sudden solar wind pressure enhancement, dayside trapped electrons are transported radially inwards, conserving their first and second adiabatic invariants (μ and K). Thus, with magnetic field and particle flux measurements at geosynchronous orbit (GEO) before and after the impact, the phase space density (PSD) radial gradients of the particles prior to the impact can be reconstructed. We show two examples, in which the PSD of low‐μ electrons, which correspond to energies less than ∼100 keV, increases slightly with increasing radial distance for one event and remains unchanged for the other, while that of high‐μ electrons decreases significantly with increasing radial distance from GEO for both events. These results suggest that the PSD radial gradients are μ dependent, and a significant heating, which violates μ and K, occurs inside GEO for the high energy electrons for the two cases examined.

Low‐latitude ionospheric electric and magnetic field disturbances in response to solar wind pressure enhancements

We study the characteristics of the low‐latitude ionospheric electric field and geomagnetic field in response to a sudden enhancement of the solar wind pressure. When the magnetosphere is compressed by an interplanetary shock, a significant enhancement in the dayside equatorial ionospheric ion vertical velocity occurs over 30–40 min and is measured by the Jicamarca incoherent scatter radar. A similar enhancement occurs in the high‐latitude ionospheric convection and is detected by the SuperDRAN HF radars. The simultaneous enhancements of the ionospheric ion velocity at high and low latitudes provide strong evidence of the occurrence of penetration electric fields produced by solar wind pressure enhancements. The geomagnetic field first increases rapidly over 2–3 min and then falls over 30–40 min to an asymptotic value. The enhanced magnetic field occurs from the subauroral region to the equator at all local times. The time scale of 30–40 min is much longer than the conventional preliminary and main impulses of geomagnetic response to interplanetary shocks. There are two possible mechanisms that may be responsible for the generation of the enhanced ionospheric electric and magnetic fields. One mechanism is that the solar wind shock causes an over‐compression of the magnetosphere, and the other is that the field‐aligned and ionospheric currents driven by the solar wind shock cause the enhancements of the ionospheric electric and magnetic fields. However, neither of the mechanisms appears to be able to provide a complete explanation of all observed features.

On the excitation of ULF waves by solar wind pressure enhancements

Abstract. We study the onset and development of an ultra low frequency (ULF) pulsation excited by a storm sudden commencement. On 30 August 2001, 14:10 UT, the Cluster spacecraft are located in the dayside magnetosphere and observe the excitation of a ULF pulsation by a threefold enhancement in the solar wind dynamic pressure. Two different harmonics are observed by Cluster, one at 6.8 mHz and another at 27 mHz. We observe a compressional wave and the development of a toroidal and poloidal standing wave mode. The toroidal mode is observed over a narrow range of L-shells whereas the poloidal mode is observed to have a much larger radial extent. By looking at the phase difference between the electric and magnetic fields we see that for the first two wave periods both the poloidal and toroidal mode are travelling waves and then suddenly change into standing waves. We estimate the azimuthal wave number for the 6.8 mHz to be m=10±3. For the 27 mHz wave, m seems to be several times larger and we discuss the implications of this. We conclude that the enhancement in solar wind pressure excites eigenmodes of the geomagnetic cavity/waveguide that propagate tailward and that these eigenmodes in turn couple to toroidal and poloidal mode waves. Thus our observations give firm support to the magnetospheric waveguide theory.

Quantification and hemispheric asymmetry of low‐latitude geomagnetic disturbances caused by solar wind pressure enhancements

The solar wind dynamic pressure has significant influence on the magnetic field of the Earth. When the magnetosphere is compressed by a solar wind pressure enhancement, the dayside magnetopause current is enhanced, resulting in an increase in the magnetic field inside the magnetosphere and on the ground. In this paper we present the statistical results of low‐latitude geomagnetic disturbances caused by solar wind pressure enhancements under different interplanetary magnetic field (IMF) orientations, including northward, southward, and fluctuating IMF. One of the prominent features of the geomagnetic disturbances revealed in this study is the hemispheric asymmetry and its seasonal dependence. Geomagnetic disturbances in the Northern Hemisphere summer are in general larger than those found simultaneously in the Southern Hemisphere and are smaller in the Northern Hemisphere winter than in the corresponding southern stations. The ratio of the average geomagnetic disturbance at magnetic latitude 36°S to that at magnetic latitude 36°N in winter is 1.36 for northward IMF, 1.64 for fluctuating IMF, and 1.79 for southward IMF. The corresponding ratio at 24° magnetic latitude is 1.49, 1.64, and 2.0, respectively. We suggest that the hemispheric asymmetry is caused by the tilt of the Earth's magnetic axis. Our statistics includes a large data set (160 cases in total), and we derive an empirical formula between the solar wind pressure enhancement and geomagnetic disturbances: ΔH = k( − ), where ΔH is the change of the low‐latitude geomagnetic field and measured with nT, Psw is the solar wind pressure and measured with nPa, and k is a constant. The value of k depends on seasons and is 18.4 if all data are included.

Effect of solar wind pressure enhancements on storm time ring current asymmetry

The effect of solar wind pressure enhancements on storm time ring current asymmetry is investigated by examining the asymmetric variations in the north‐south (H) and east‐west (D) components of the geomagnetic field during four magnetic storms. For two strong storms, on 25 September 1998 and 29 May 2003, pressure enhancements occurred during the main phase with strong and steadily southward IMF Bz. It is found that the pressure enhancements significantly increase the asymmetry of the already strong and asymmetric ring current under these conditions. For the moderate magnetic storm on 10 January 1997, a pressure enhancement occurred during the early recovery phase when IMF Bz was turning northward while it was still southward. Our result shows that the pressure enhancement also slightly enhanced the asymmetry of the slightly asymmetric ring current that existed during the early recovery phase. On 6 November 2000, a pressure enhancement occurred during the late recovery phase when the IMF Bz was strongly northward. For this case, the pressure enhancement did not increase the asymmetry of the already symmetric ring current. The above results of ring current asymmetry increases can be explained by considering the local energization of the preexisting ring current particles by the azimuthal electric field induced by the pressure enhancement. Our results show that the effect of pressure enhancements on the ring current depends strongly on the asymmetry state of the ring current at the times of the onsets of pressure enhancements, which is in turn determined by the IMF Bz preconditioning. In addition, the size and relative strength of a pressure enhancement also play important roles in affecting the ground asymmetric H perturbation.

Are sawtooth oscillations of energetic plasma particle fluxes caused by periodic substorms or driven by solar wind pressure enhancements?

Energetic electron and proton fluxes measured by geosynchronous satellites often show sawtooth‐like variations during magnetic storms. We examine whether the sawtooth oscillations and relevant magnetospheric‐ionospheric disturbances are caused by periodic substorms or driven by a series of enhancements in the solar wind pressure. We show that there are significant differences between periodic substorms and solar wind‐induced variations. The energetic fluxes at geosynchronous orbit may increase by orders of magnitude after each onset of periodic substorms and by 10–50% in response to a large solar wind pressure impulse. The sudden increases of the energetic fluxes during periodic substorms show significant time delays of 30–50 min at different longitudes/local times, indicating that the fluxes are injected on the nightside and then drift to the dayside. In contrast, the small flux increases caused by solar wind pressure enhancements occur almost simultaneously at all local times. The periodic substorms always have a strong spectrum peak at 2–3 hours, no matter whether the solar wind pressure and/or IMF have similar spectrum peaks. The nightside magnetospheric magnetic elevation angle shows a large (30–60°) increase at each onset of periodic substorms through dipolarization and a small (<10°) decrease in response to a solar wind pressure impulse. Each cycle of periodic substorms can cause a deviation of 40–60 nT in the midlatitude geomagnetic field; the midlatitude geomagnetic deviations caused by solar wind pressure enhancements are proportional to the square root of the pressure change. The increase of the polar cap index caused by substorms is ∼4 times that caused by solar wind pressure enhancements. We conclude that the sawtooth‐like flux oscillations represent flux injections during periodic substorms and that the period of substorms is determined by the magnetosphere.