Dynamic Pressure Enhancements Research Articles

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 storms on Mars

Based on data from the Mars Global Surveyor magnetometer we examine periods of significantly enhanced magnetic disturbances in the martian space environment. Using almost seven years of observations during the maximum and early declining phase of the previous solar cycle the occurrence pattern and typical time profile of such periods is investigated and compared to solar wind measurements at Earth. Typical durations of the events are 20–40 h, and there is a tendency for large events to last longer, but a large spread in duration and intensity are found. The large and medium intensity events at Mars are found to occur predominantly in association with interplanetary sector boundaries, with solar wind dynamic pressure enhancements being the most likely interplanetary driver. In addition it is found that, on time scales of months to several years, the dominant cause of global variability of the magnetic field disturbance at Mars is solar wind dynamic pressure variations associated with the eccentricity of the martian orbit around the Sun.

Simulations of observed auroral brightening caused by solar wind dynamic pressure enhancements under different interplanetary magnetic field conditions

Solar wind dynamic pressure (P(dyn)) enhancements have been observed to cause large-scale auroral brightening. The mechanism for this kind of auroral brightening is still a topic of current space research. Using the global piecewise parabolic method Lagrangian remap (PPMLR)-MHD simulation model, we investigate three auroral brightening events caused by dynamic pressure enhancement under different interplanetary magnetic field (IMF) conditions: (1) B(z) 0 on 11 August 2000, (2) B(z) = 0 on 21 January 2005. We show that the auroral location depends on the IMF conditions. Under southward IMF conditions, when B(y) is negative, the duskside aurora is located more equatorward at around 70 degrees magnetic latitude (MLAT) for all magnetic local times; when B(y) is positive, the duskside aurora can even reach beyond 80 degrees MLAT. A smaller and more localized response is seen when the IMF B(z) is nearly zero or northward, as shown in previous studies. Our simulation results are consistent with these observations, indicating that the observed aurora activities could be caused by solar wind dynamic pressure enhancements. The simulation results suggest that the enhancement of P(dyn) can increase the ionospheric transpolar potential and the corresponding field-aligned currents, leading to the observed auroral brightening.

Estimation of radial gradients of phase space density from POLAR observations during a quiet period prior to a sudden solar wind dynamic pressure enhancement

We have analyzed POLAR electron flux data to estimate the radial gradients of electron phase space density (PSD) immediately prior to a sudden solar wind dynamic pressure enhancement at ∼1135 UT on 12 August 2001. In this event, the instantaneous flux changes from the magnetospheric compression at L ∼ 7.6 in the postmidnight magnetic local time sector are observed to be both energy and pitch angle (PA) dependent: a substantial flux increase for lower energies (<∼200 keV) particularly at smaller local PA (<∼50°) and a large flux decrease for relativistic energies that is more pronounced for a larger PA. For this event, because of the extremely steady and quiet solar wind conditions prior to the pressure impact, we can reasonably assume that the temporal variation is negligible and so deduce the spatial and pitch angle distribution of initial electron fluxes to construct the event‐specific electron flux model for a localized region of the nightside outer radiation belt. PSD radial profiles are then estimated for 6.5 < L < 8.5 using the constructed electron flux model and Tsyganenko magnetospheric model for quiet times. The estimates show a positive PSD gradient for low energies (<∼a few 100 s keV at the POLAR location right after the pressure impact) and a negative gradient for relativistic energies. In addition, a rather gradual transition of the radial gradient from highly positive to more negative is found with increasing energy. The expected immediate flux responses using these estimated PSD radial gradients are qualitatively consistent with those observed, partially validating the veracity of the estimated profiles.

Magnetic cloud boundary layer of 9 November 2004 and its associated space weather effects

On 9 November 2004, the WIND spacecraft detected a magnetic cloud boundary layer (MCBL) during the interval from 19:07 UT to 20:30 UT. Within the MCBL, there is intense southward magnetic field and the dynamic pressure is rather high, which makes it much geoeffective. Twenty‐three minutes later, the MCBL arrived on the magnetopause. An intense geomagnetic storm main phase was driven by the sustaining strong southward magnetic field within the MCBL. During the passage of the MCBL, a typical magnetospheric substorm was triggered. The substorm onset was synthetically identified by the aurora breakup, magnetic dipolarization, dispersionless particle injection, Pi2 pulsation, and the polar bay onset. The substorm triggering is related to the special magnetic and plasma structure within the MCBL. The MCBL accompanying adjacent sheath region formed a dynamic pressure enhancement region, which strongly compressed the magnetosphere and even pushed the magnetopause into the geosynchronous orbit so that two dayside spacecraft GOES‐10 and GOES‐12 were directly exposed in the magnetosheath for a long interval during the passage of the MCBL. In terms of Shue et al. (1998) model, the closest subsolar standoff distance even reached 5.1 RE during the passage of the MCBL. It can be inferred that the strong dynamic pressure and the strong discontinuities within the MCBL determine the intense compression effect. In addition, a very intense geomagnetically induced current (GIC) event was directly caused by the MCBL. Similar to this case, majority of MCBLs are dynamic pressure enhancement regions, and there are strong southward magnetic field and several strong discontinuities inside these regions, which can potentially drive large‐scale magnetospheric activities. In this paper, we take a case study to discuss the magnetospheric activities and the space weather effects caused by MCBLs.

Response of the Poleward Boundary of the Nightside Auroral Oval to Impacts of Solar Wind Dynamic Pressure Enhancement

In this paper we have investigated latitudinal variations of the poleward boundary of the nightside auroral oval when the magnetosphere is hit by an enhanced solar wind dynamic pressure front. We used precipitating particle data obtained from Defense Meteorological Satellite Program satellites to identify the locations of the boundary before and after enhanced pressure impacts. The boundary locations are represented by a parameter called "b5e". After performing the analysis for a number of events, we found that the basic effect of the solar wind pressure increase impact is often (but not always) to move the poleward boundary of the nightside auroral oval poleward. However, this effect can be often modified by other factors, such as simultaneous variations of the interplanetary magnetic field with a pressure increase, and thus the boundary response is not necessarily a poleward shift in many cases. We demonstrate this with specific examples, and discuss other possible complicating factors.

High‐latitude reconnection effect observed at the dayside dip equator as a precursor of a sudden impulse

Geomagnetic sudden impulses (SI) observed at the dayside dip equator normally show a decrease and then an increase in the H magnetic field component, i.e., a preliminary reverse impulse (PRI) followed by a main impulse. Using global geomagnetic field measurements, we examine an unusual SI event observed at the dayside dip equator, which shows a clear precursor ∼1 min before the PRI onset. The precursor was observed simultaneously at both the dayside dip equator and in the southern polar region but was not observed at all in the northern polar region. The global ground variations after the PRI onset were, however, consistent with a conventional SI model of the magnetospheric response to a sudden enhancement of the solar wind dynamic pressure. Considering that the interplanetary magnetic field Bx, By, and Bz components were positive with the By component dominant for this event, we suggest that the SI precursor was caused by high‐latitude magnetic reconnection that occurred only (or first) in the dawn quadrant of the southern hemisphere, the effect of which has for the first time been clearly identified at the dayside dip equator. This implies that electric field effects occurring in the polar ionosphere due to magnetopause reconnection may be rapidly monitored at the dayside dip equator. In addition, we argue that the quasi‐simultaneous (time difference is less than 10 s) appearance of the disturbance fields both in the polar region and at the dip equator confirmed in this study is extremely important for theoretically explaining the transmission of the polar electric field to the dayside dip equator, because different theories give different travel times for a disturbance field propagating from polar region to the dip equator. Our observations lend strong evidence for validity of the waveguide model.

A split in the outer radiation belt by magnetopause shadowing: Test particle simulations

We developed a three‐dimensional relativistic test particle code and used it to calculate the trajectories of relativistic electrons in the outer radiation belt. By applying time‐varying magnetic field data calculated from the Tsyganenko model and using observed solar wind data and the Dst index, we examined the drift loss of relativistic electrons by magnetopause shadowing (MPS). Since other loss processes such as wave‐particle interactions are not included in this simulation, the pure MPS effect can be discussed. A split was found in the outer radiation belt after the enhancement of the solar wind dynamic pressure. Isolated electrons outside of the split have a narrow pitch angle distribution around 90° and are confined to a narrow range of the L shell. We found that the existence of the isolated electrons depends on the large geomagnetic tilt angle. These findings indicate that the split can be seen during summer and winter after MPS occurs. We suggest that this split in the outer radiation belt during summer and winter is evidence that MPS actually causes the loss of the outer radiation belt.

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.

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

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

Responses of the magnetotail plasma sheet to two interplanetary shocks: TC-1 observations

Two interplanetary shocks are examined to determine the responses of the magnetic field and plasma in the plasma sheet upon the shock impacts by using TC-1 observational data. The two shocks are observed by WIND on November 7, 2004. Prior to and after the shock, the IMF is either weakly southward or northward. The responses of the plasma sheet to the two shocks are intense and much similar. When the shock interacts with the magnetosphere, the magnetic field impulsively increases 1–2 min after the geomagnetic field sudden impulse (SI) judged from the Sym-H index change, and the magnetic field line is stretched. On the other hand, all of the ion density, the ion temperature, and the velocity of ion flow in the plasma sheet increase. Interestingly, quasi-periodical oscillations of the ion flow are suddenly enhanced, and the plasma flow is basically perpendicular to the local magnetic field. The responses of the magnetic field and the plasma are nearly simultaneous. The responses in the plasma sheet are probably caused by the lateral compression due to the dynamic pressure enhancement downstream the shock when the shock propagates antisunward in the magnetosheath.

Influence of solar wind parameters on ground magnetic field near equator during intense storms

Using a simple time-lagged correlation technique, present study aims to identify the solar wind (SW) parameter, which is better associated with the ground magnetic field variations of shorter time duration near equator, during intense geomagnetic storms. It is found that out of all SW parameters, successively occurring enhancements in the SW dynamic pressure have substantial influence on the horizontal component of magnetic field at ground. Present analysis reveals a time lag of ~30–45 min between the SW pressure changes seen at L1 location and ground magnetic field variations, and hence providing a good approximation of an averaged propagation time during entire storm interval; the time lag varies with solar wind velocity. Separate study during day and nighttime suggests that the SW dynamic pressure enhancements recorded by the dayside outer magnetospheric satellite have impact on the ground horizontal magnetic field measurements near equator, irrespective of day or nighttime.

Features of energetic particle radial profiles inferred from geosynchronous responses to solar wind dynamic pressure enhancements

Abstract. Determination of the radial profile of phase space density of relativistic electrons at constant adiabatic invariants is crucial for identifying the source for them within the outer radiation belt. The commonly used method is to convert flux observed at fixed energy to phase space density at constant first, second and third adiabatic invariants, which requires an empirical global magnetic field model and thus might produce some uncertainties in the final results. From a different perspective, in this paper we indirectly infer the shape of the radial profile of phase space density of relativistic electrons near the geosynchronous region by statistically examining the geosynchronous energetic flux response to 128 solar wind dynamic pressure enhancements during the years 2000 to 2003. We thus avoid the disadvantage of using empirical magnetic field models. Our results show that the flux response is species and energy dependent. For protons and low-energy electrons, the primary response to magnetospheric compression is an increase in flux at geosynchronous orbit. For relativistic electrons, the dominant response is a decrease in flux, which implies that the phase space density decreases toward increasing radial distance at geosynchronous orbit and leads to a local peak inside of geosynchronous orbit. The flux response of protons and non-relativistic electrons could result from a phase density that increases toward increasing radial distance, but this cannot be determined for sure due to the particle energization associated with pressure enhancements. Our results for relativistic electrons are consistent with previous results obtained using magnetic field models, thus providing additional confirmation that these results are correct and indicating that they are not the result of errors in their selected magnetic field model.

Modeling magnetospheric current response to solar wind dynamic pressure enhancements during magnetic storms: 2. Application to different storm phases

Two patterns of ground H perturbation as response to pressure enhancements during different phases of storms are found. One is polarity asymmetry under southward IMF Bz, e.g., 25 September 1998, 29 May 2003, and 10 January 1997 events; the other is positive H perturbation everywhere under northward IMF Bz, e.g., 7 November 2000 event. The polarity asymmetry can be further divided into two categories: dawn‐dusk asymmetry with negative H perturbation at dusk extending through noon to late morning (25 September 1998 event) and positive H perturbation elsewhere, and day‐night asymmetry with a smaller region of negative H perturbation extending less far toward dusk (29 May 2003 and 10 January 1997 events). The net field‐aligned current (FAC) system from the R1, R2, and the partial ring current (PRC) closure FAC, the equatorial portion of the PRC, and Chapman‐Ferraro (CF) current are responsible for the polarity asymmetry. The stronger effect of the R1 is primarily responsible for the negative H perturbation at noon and positive at midnight. The PRC strength at the onset of pressure enhancements determines how far the negative H perturbation at noon extends toward dusk and which pattern is most likely. A stronger (weaker) PRC extends negative H perturbation more (less) toward dusk and thus increases the likelihood of dawn‐dusk (day‐night) asymmetry. Positive H perturbation everywhere results from the dominant contribution of the CF and lack of significant preexisting PRC and R1 and R2.

Modeling magnetospheric current response to solar wind dynamic pressure enhancements during magnetic storms: 1. Methodology and results of the 25 September 1998 peak main phase case

We present a methodology for using the modular Tsyganenko storm magnetic field model (TS04) as a tool to investigate the response of magnetospheric currents to solar wind dynamic pressure enhancements during magnetic storms. We demonstrate the technique by examining the contribution of each model current to the observed dawn‐dusk asymmetric ground H perturbation during a peak storm main phase event. We add present pressure terms to the parameterizations of several individual model currents and fit them to the observed low‐latitude and midlatitude H perturbations. We find that the asymmetric H perturbation for this case is primarily due to a net field‐aligned current (FAC) system, with upward FACs peaking in the evening and downward FACs peaking in the morning, and the equatorial portion of the partial ring current (PRC). This net FAC system includes the region 1 and region 2 FACs and the closure FAC of the PRC, with the largest contribution from the PRC closure FACs. The model results show that in this main phase case, the PRC played a more important role in causing the dawn‐dusk asymmetric H perturbation than did the other currents owing to its strength and asymmetry. The model results do not show significant contributions from the symmetric ring current and tail current, but the magnetopause current gives a significant positive perturbation at all MLTs. In the companion paper, our methodology is applied to obtain a more complete picture of the storm time current response to pressure enhancements by examining pressure increases occurring during different storm phases.

Dawnside particle injection caused by sudden enhancement of solar wind dynamic pressure

Here we present the first evidence for particle injection in the dawn-side equatorial plane caused by a sudden enhancement of the solar wind dynamic pressure (SEP dyn). The observational results demonstrate that the SEP dyn caused two effects: (1) instantaneous particle energization in the dayside, and (2) particle injection in the dawn side at approximately 7 min later. The former effect has been suggested to be energization of the preexisting particles by the sudden compression of the magnetosphere. We argue that the latter effect was due to acceleration of relatively hot ions in the dawn-side plasma sheet by a bipolar electric field caused by the SEP dyn.

Polar Ultraviolet Imager observations of solar wind‐driven ULF auroral pulsations

Analysis of a sequence of auroral images acquired by the Polar Ultraviolet Imager (UVI) during a long‐duration (∼1.5 hr) solar wind dynamic pressure enhancement reveals auroral intensity variations in the Pc5 band. Power spectral analysis indicates that the “auroral pulsations” appeared in multiple frequency bands predominantly in the day sector and the dawn‐dusk flank of the oval, with the maximum wave power skewing toward the dawn sector. Ground‐based magnetometer recordings showed Pc5 micropulsations modulated with the auroral pulsations. The lower frequency (1–3 mHz) auroral pulsations were modulated with solar wind dynamic pressure pulses moving tailward, whereas the higher frequency (6–8 mHz) ones were not. We concluded that the lower frequency auroral pulsations were driven directly by the solar wind, whereas the higher frequency ones were associated with global compressional cavity mode. This study shows for the first time the snapshot of global Pc5 ultralow‐frequency (ULF) waves and demonstrates that global far‐ultraviolet (FUV) imaging such as Polar UVI provides a powerful ancillary tool for studying ULF waves on a global scale.

Statistical Characteristics of Solar Wind Dynamic Pressure Enhancements During Geomagnetic Storms

Solar wind dynamic pressure enhancements are known to cause various types of disturbances to the magnetosphere. In particular, dynamic pressure enhancements may affect the evolution of magnetic storms when they occur during storm times. In this paper, we have investigated the statistical significance and features of dynamic pressure enhancements during magnetic storm times. For the investigation, we have used a total of 91 geomagnetic storms for 2001-2003, for which the Dst minimum ( ) is below –50 nT. Also, we have imposed a set of selection criteria for a pressure enhancement to be considered an event: The main selection criterion is that the pressure increases by %o r nPa within 30 min and remains to be elevated for 10 min or longer. For our statistical analysis, we define the storm time to be the interval from the main Dst decrease, through , to the point where the Dst index recovers by 50%. Our main results are summarized as follows. (i) 81% of the studied storms indicate at least one event of pressure enhancements. When averaged over all the 91 storms, the occurrence rate is 4.5 pressure enhancement events per storm and 0.15 pressure enhancement events per hour. (ii) The occurrence rate of the pressure enhancements is about three times higher for CME-driven storm times than for CIR-driven storm times. (iii) Only 21.1% of the pressure enhancements show a clear association with an interplanetary shock. (iv) A large number of the pressure enhancement events are accompanied with a simultaneous change of IMF and/or : For example, 73.5% of the pressure enhancement events are associated with an IMF change of either nT or nT. This last finding suggests that one should consider possible interplay effects between the simultaneous pressure and IMF changes in many situations.

The storm‐time access of solar wind ions to the nightside ring current and plasma sheet

We investigated the storm‐time entry of solar wind ions into the magnetosphere by carrying out a large‐scale kinetic particle tracing calculation for the 24–25 September 1998 and the 17 April 2002 geomagnetic storms. For each storm, we simulated the magnetosphere by using a global magnetohydrodynamic (MHD) code that was driven by solar wind and interplanetary magnetic field (IMF) data from spacecraft upstream of Earth. We then launched ions in the solar wind in the global time‐dependent fields obtained from the MHD simulation, beginning prior to storm onset and extending into the main phase. We collected those ions that successfully reached the nightside plasma sheet and ring current, and then determined the mechanisms of entry and transport responsible for the injection of these ions into the magnetotail. We found that, in agreement with quiet time entry results, the IMF By component strongly influenced whether ions entered through the dawn or dusk flanks, and changes in IMF By were immediately reflected on the ion entry pattern. Also, in addition to the usual dayside merging source, ion entry into the magnetosphere was facilitated by dynamic pressure enhancements, during which entry occurred over a wide swath of latitudes and extended from the dayside to locations far downtail. More significantly, because of the warmer storm‐time ion population in the magnetosheath, ions were more readily affected by inhomogeneities and rapid changes in the IMF, which caused solar wind ions to gradient drift onto open field lines and reach the inner magnetosphere.

Temporal evolution of the transpolar potential after a sharp enhancement in solar wind dynamic pressure

Recent studies of ionospheric convection have shown that sudden enhancements in solar wind dynamic pressure have significant effect on the transpolar potential and the coupling efficiency between the solar wind and the terrestrial magnetosphere. Super Dual Auroral Radar Network observations of the dayside convection have demonstrated that the strength of convection correlates well with solar wind dynamic pressure variations, implying an enhancement of dayside reconnection induced by changes in solar wind pressure. At the same time, dynamic pressure increases have been shown to lead to closing of the polar cap, particularly on the nightside, and thus directly drive enhanced tail reconnection. The enhanced dayside and nightside reconnection potentials can both lead to changes in the transpolar potential, but their individual contributions and the balance between the two is not known. We present a case study of the transpolar potential evolution after a long‐lasting solar wind pressure step increase. We show that the potential first rises in response to the increase in pressure, then gradually subsides a few hours later despite the solar wind pressure remaining high. We interpret this behavior in terms of pressure‐driven changes in dayside and nightside reconnection.