Changes In Solar Wind Dynamic Pressure Research Articles

Automated Technique for the Detection of Step‐Like Solar Wind Dynamic Pressure Changes: Application to the Response of the Transpolar Potential to Solar Wind Dynamic Pressure Fronts

AbstractThe input from the solar wind (SW) and the accompanying Interplanetary Magnetic Field (IMF) is the defining feature of the solar wind‐magnetosphere interactions. One of the most important SW drivers is the solar wind dynamic pressure, and most significantly its sharp changes known as pressure fronts. Sudden enhancements in solar wind dynamic pressure, , have been shown to lead to intensification of the auroral electrojets (and Region 1 field‐aligned currents), increase in auroral emissions, and significant enhancement in ionospheric convection and the transpolar potential. Case studies of the interaction between pressure fronts and the terrestrial space environment shed great light on their importance to the structure and dynamics of the system. However, thorough understanding of the interaction requires fully statistical investigations. These statistical studies require a large number of suitable pressure front events. In the past these have been obtained through laborious manual inspection of solar wind data. We present an automated procedure that identifies sudden step enhancements in that satisfy a desired set of limiting step‐like characteristics, such as magnitude of the change, sign of the change (positive/negative), and rate of change. The procedure yields a list of suitable events, along with their step features within minutes. The technique is tested for missed events and false positives by direct comparison to manual inspection of the data. Its utility is illustrated by application to the statistical effect of pressure fronts to the transpolar potential as measured by the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique.

Asymmetric Structure of the Solar Wind and Heliosphere from IBEX Observations

Abstract Our heliosphere is formed by the interaction between the dynamic solar wind (SW) and partially ionized, local interstellar medium. “Pickup ions” (PUIs) are produced by charge exchange between SW ions and interstellar neutral atoms and propagate with the SW into the outer heliosphere. Neutralization of these PUIs in the heliosheath creates energetic neutral atoms (ENAs) that are observed near Earth by the Interstellar Boundary Explorer (IBEX). In this study, we analyze the evolution of ENAs observed by IBEX between ∼0.5 and 6 keV from 2009 to 2018, presenting the evolution of ENA spectral indices as a function of latitude in different regions of the sky. Our results show that IBEX ENA spectra reflect unique differences in the northern and southern polar coronal holes (PCHs) as a function of latitude and time. ENA spectra reflect a delayed closure of the southern PCH compared to the north by ∼1–2 yr in solar cycle 24, but then a rapid opening of the southern PCH beginning ∼1–2 yr later, in agreement with evolving SW speeds emanating from the PCHs. While ENA spectra from the front and flanks of the heliosphere exhibit similar behavior, ENA spectra from the back behave differently, suggesting that the heliotail and ENA source structures are significantly different. The mean ENA spectral index observed by IBEX evolves gradually and quasi-periodically over the ∼11 yr solar cycle, reflecting large-scale changes in the PCHs and SW speed. The standard deviation of the spectral index exhibits bumps at times when the SW dynamic pressure changes abruptly.

The Effects of Solar Wind Dynamic Pressure Changes on the Substorm Auroras and Energetic Electron Injections on 24 August 2005

AbstractAfter the passage of an interplanetary (IP) shock at 06:13 UT on 24 August 2005, the enhancement (>6 nPa) of solar wind dynamic pressure and the southward turning of interplanetary magnetic field (IMF) cause the earthward movement of dayside magnetopause and the drift loss of energetic particles near geosynchronous orbit. The persistent electron drift loss makes the geosynchronous satellites cannot observe the substorm electron injection phenomenon during the two substorm expansion phases (06:57–07:39 UT) on that day. Behind the IP shock, the fluctuations (~0.5–3 nPa) of solar wind dynamic pressure not only alter the dayside auroral brightness but also cause the entire auroral oval to swing in the day‐night direction. However, there is no Pi2 pulsation in the nightside auroral oval during the substorm growth phase from 06:13 to 06:57 UT. During the subsequent two substorm expansion phases, the substorm expansion activities cause the nightside aurora oval brightening from substorm onset site to higher latitudes, and meanwhile, the enhancement (decline) of solar wind dynamic pressure makes the nightside auroral oval move toward the magnetic equator (the magnetic pole). These observations demonstrate that solar wind dynamic pressure changes and substorm expansion activities can jointly control the luminosity and location of the nightside auroral oval when the internal and external disturbances occur simultaneously. During the impact of a strong IP shock, the earthward movement of dayside magnetopause probably causes the disappearance of the substorm electron injections near geosynchronous orbit.

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.

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

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

Response of magnetic fields at geosynchronous orbit and on the ground to the sudden changes of IMF B Z

The rapid change in the Earth’s magnetosphere caused by solar wind disturbances has been an important part of the solar wind-magnetosphere interaction. However most of the previous studies focused on the perturbation of the Earth’s magnetic field caused by solar wind dynamic pressure changes. In this paper, we studied the response of geosynchronous magnetic field and the magnetic field to the rapid southward turning of interplanetary magnetic field during the interval 1350–1420 UT on 7 May 2007. During this event, BZ component of the interplanetary magnetic field decreased from 15 nT to −10 nT within 3 min (1403–1406 UT). The geosynchronous magnetic field measured by three geosynchronous satellites (GOES 10–12) first increased and then decreased. The variations of magnetic field strength in the morning sector (9–10 LT) were much larger than those in the dawn sector (5 LT). Meanwhile, the H components of geomagnetic field on the ground have similar response features but exhibit latitude and LT dependent variations. Compared with H components, the D components do not have regular variations. Although the solar wind dynamical pressure encounters small variations, the magnetic field both in space and on the ground does not display similar variations. Therefore, the increase of geomagnetic field in the dawn sector is caused by the southward turning of IMF (interplanetary magnetic field) BZ. These results will help to better understand the coupling process of geomagnetic filed and interplanetary magnetic field.

Geostationary magnetic field response to solar wind pressure variations: Time delay and local time variation

The relationship between solar wind dynamic pressure changes and geosynchronous magnetic field response is studied using 15 years of OMNI2 and GOES data at 1‐minute resolution. Significant magnetospheric response to solar wind‐forcing is found to be most frequent near noon (30% of all intervals), and virtually absent on the night‐side. The strongest response occurs when IMF Bz is strongly northward and the effect of reducing IMF Bz is most pronounced in the dusk sector. Approximately 25% of dayside Bz variance for related intervals can be attributed to direct response from solar wind dynamic pressure forcing. Time lag between changes in the solar wind at the bow shock nose and similar fluctuations in the magnetosphere at geosynchronous orbit (6.6 Re) is typically 2 to 4 minutes, with responses occurring first in the post‐noon sector and approximately 2 minutes later near dawn. The OMNI2 HRO time‐shifting algorithm appears to be quite effective, with a slight (2 minute) systematic increase in lag and no increased scatter for the most distant upstream solar wind satellite location.

Polar cap convection and PC index during sudden changes in solar wind dynamic pressure

The polar cap (PC) index derived from polar magnetic variations is primarily a measure of the intensity of the transpolar ionospheric currents generated by the solar wind interaction with the Earth's magnetosphere. We have examined the influence of the solar wind dynamic pressure on the PC index in cases of global sudden impulses (SI) or storm sudden commencement (SSC) magnetic impulses. Such events usually give rise to a sequence of a negative preliminary reverse impulse (PRI) followed by a positive main impulse (MI) in the PC index. Sudden decreases in the solar wind dynamic pressure generate the corresponding sequence of pulses with inverted polarity. These pulses have amplitudes of typically 0.5 in the PC index and a total duration of 10–30 min. Using magnetic recordings from a network of observatories in the polar cap we have examined in detail the development in the polar ionospheric convection following sudden solar wind dynamic pressure changes. The PRI impulses are related to the occurrence of a pair of transient dayside reverse convection vortices at cusp latitudes. We conclude that the short negative PRI pulses in the PC index following sudden changes in solar wind dynamic pressure are caused by the diversion of magnetopause currents along cusp field lines to the polar cap ionosphere as the pressure front sweeps over the magnetosphere. The more extended positive MI main impulses are related to the formation of a pair of forward convection vortices in the dayside auroral oval supported by enhanced Region 1 (R1) field‐aligned currents flowing to and from the auroral ionosphere.

Geosynchronous magnetic field response to the large and fast solar wind dynamic pressure change

We present a comparison of large and sharp solar wind dynamic pressure changes, observed by several spacecraft, with fast disturbances in the magnetospheric magnetic field measured by the GOES-8, 9 and 10 geosynchronous satellites. Almost 400 solar wind pressure changes in the period 1996–2003 were selected for this study. Using the large statistics we confirmed that increases (decreases) in the dynamic pressure always results in increases (decreases) in the magnitude of geosynchronous B z component. The amplitude of the geosynchronous B z response strongly depends on the location of observer relative to the noon meridian, from the value of solar wind pressure before the disturbance arriving and firstly – from the amplitude of the pressure change. It is shown that 3D MHD numerical simulation of geosynchronous B z response performed for different pressure changes inside 9–15 LT range is consistent well with the experimental results.

The Interplanetary Causes of the Anormalous SSC Events

磁暴急始(SSC)是强烈太阳风动压或行星际激波与磁层相互作用的结果.通常SSC事件的上升时间在4～10 min,我们把上升时间超过15 min的SSC事件称为异常SSC事件.本文利用地磁SYM-H指数鉴别出了5个有地磁观测历史以来发生的上升时间大于15 min的异常SSC事件,并利用Wind,ACE,IMP 8,Goes,Geotail多点卫星太阳风观测数据和地磁观测数据,分析了异常SSC事件的行星际原因.结果表明,异常SSC事件通常都是强烈行星际扰动引起的,5个异常SSC事件有4个对应于行星际激波,有3个对应于多步太阳风动压跃变,有1个对应于行星际电场大幅度变化;由行星际激波产生的异常SSC事件,其上升时间依赖于行星际激波的方向,方向相对于日地连线越偏,上升时间越长;异常SSC事件上升时间与行星际磁场方向关系不明显.

Large and sharp solar wind dynamic pressure variations as a source of geomagnetic field disturbances at the geosynchronous orbit

We present a comparison of large and fast solar wind dynamic pressure changes, observed by INTERBALL-1 satellite, with disturbances in the magnetospheric magnetic field measured by the GOES-8, 9 and 10 geosynchronous satellites. More than 300 solar wind pressure changes in the period from 1996 to 1998 were selected for this study. These fast and large solar wind pressure changes cause fast variations of the geomagnetic field magnitude in the outer magnetosphere. We found that increases (decreases) in the solar wind dynamic pressure always result in increases (decreases) in the geosynchronous magnetic field magnitude. The time delay between the solar wind pressure change and the geomagnetic response is significantly affected by the inclination of the pressure front to the Sun–Earth line. The amplitude of the geomagnetic field variation strongly depends on the location of the GOES satellite relative to the noon meridian, with the maximum amplitude near noon.

Magnetospheric reconnection driven by solar wind pressure fronts

Abstract. Recent work has shown that solar wind dynamic pressure changes can have a dramatic effect on the particle precipitation in the high-latitude ionosphere. It has also been noted that the preexisting interplanetary magnetic field (IMF) orientation can significantly affect the resulting changes in the size, location, and intensity of the auroral oval. Here we focus on the effect of pressure pulses on the size of the auroral oval. We use particle precipitation data from up to four Defense Meteorological Satellite Program (DMSP) spacecraft and simultaneous POLAR Ultra-Violet Imager (UVI) images to examine three events of solar wind pressure fronts impacting the magnetosphere under two IMF orientations, IMF strongly southward and IMF Bz nearly zero before the pressure jump. We show that the amount of change in the oval and polar cap sizes and the local time extent of the change depends strongly on IMF conditions prior to the pressure enhancement. Under steady southward IMF, a remarkable poleward widening of the oval at all magnetic local times and shrinking of the polar cap are observed after the increase in solar wind pressure. When the IMF Bz is nearly zero before the pressure pulse, a poleward widening of the oval is observed mostly on the nightside while the dayside remains unchanged. We interpret these differences in terms of enhanced magnetospheric reconnection and convection induced by the pressure change. When the IMF is southward for a long time before the pressure jump, open magnetic flux is accumulated in the tail and strong convection exists in the magnetosphere. The compression results in a great enhancement of reconnection across the tail which, coupled with an increase of magnetospheric convection, leads to a dramatic poleward expansion of the oval at all MLTs (dayside and nightside). For near-zero IMF Bz before the pulse the open flux in the tail, available for closing through reconnection, is smaller. This, in combination with the weaker magnetospheric convection, leads to a more limited poleward expansion of the oval, mostly on the nightside. Key words. Magnetospheric physics (solar windmagnetosphere interactions; magnetospheric configuration and dynamics; auroral phenomena)

Effect of solar wind pressure pulses on the size and strength of the auroral oval

It has recently been found that solar wind dynamic pressure changes can dramatically affect the precipitation of magnetospheric particles on the high‐latitude ionosphere. We have examined the effect of large solar wind dynamic pressure increases on the location, size, and intensity of the auroral oval using particle precipitation data from Defense Meteorological Satellite Program (DMSP) spacecraft. Three events have been selected for study during the time period after 1997 when four DMSP spacecraft (F11–F14) were simultaneously operational. Interplanetary magnetic field (IMF) orientation is different from event to event. For each event, we determine equatorward and poleward boundaries of the auroral oval before and after an increase in solar wind pressure. Also, using measured integral fluxes, we construct precipitating particle energy input maps for the auroral oval. All cases studied show a significant change of the auroral oval location, size, and intensity in response to the solar wind pressure pulse. Most prominent are an increase of the auroral zone width and a decrease of the polar cap size when the solar wind dynamic pressure increases under steady southward IMF conditions. An increase in total precipitating particle energy flux is also observed. A smaller response is seen when the IMF Bz has a simultaneous northward turning and when it is nearly zero before the pressure enhancement. Our results also point to significant differences between the auroral precipitation response to solar wind pressure changes and its response to isolated substorms, the former inducing a global auroral reaction while the latter is related to more localized premidnight disturbances. Auroral UV observations from the Polar spacecraft during our events are found to give results consistent with the results we get from the precipitating particle observations.

A Survey of large, rapid solar wind dynamic pressure changes observed by Interball-1 and IMP 8

Abstract. The high time-resolution solar wind ion flux measurements from Interball-1 and IMP 8 show about one hundred large, rapid dynamic the pressure changes each year. We cataloged these events by the size and transition time of the pressure changes and present a statistical survey of these events. We find that the majority of the pressure changes of more than 1–2 nPa occur over a very short time period, on the order of a few minutes or less. Most of the large pressure changes not associated with shocks are due solely to density changes with speed remaining constant. We find that pressure balance between the thermal and magnetic pressures is not maintained across most of these events, so these events are still evolving.Key words. Interplanetary physics (solar wind plasma)

Large-scale fields and flows in the magnetosphere-ionosphere system

Advances in our understanding of the large-scale electric and magnetic fields in the coupled magnetosphere-ionosphere system are reviewed. The literature appearing in the period January 1991–June 1993 is sorted into 8 general areas of study. The phenomenon of substorms receives the most attention in this literature, with the location of onset being the single most discussed issue. However, if the magnetic topology in substorm phases was widely debated, less attention was paid to the relationship of convection to the substorm cycle. A significantly new consensus view of substorm expansion and recovery phases emerged, which was termed the ‘Kiruna Conjecture’ after the conference at which it gained widespread acceptance. The second largest area of interest was dayside transient events, both near the magnetopause and the ionosphere. It became apparent that these phenomena include at least two classes of events, probably due to transient reconnection bursts and sudden solar wind dynamic pressure changes. The contribution of both types of event to convection is controversial. The realisation that induction effects decouple electric fields in the magnetosphere and ionosphere, on time scales shorter than several substorm cycles, calls for broadening of the range of measurement techniques in both the ionosphere and at the magnetopause. Several new techniques were introduced including ionospheric observations which yield reconnection rate as a function of time. The magnetospheric and ionospheric behaviour due to various quasi-steady interplanetary conditions was studied using magnetic cloud events. For northward IMF conditions, reverse convection in the polar cap was found to be predominantly a summer hemisphere phenomenon and even for extremely rare prolonged southward IMF conditions, the magnetosphere was observed to oscillate through various substorm cycles rather than forming a steady-state convection bay.

Effect of sudden solar wind dynamic pressure changes at subauroral latitudes: Change in magnetic field

The observations obtained during the International Magnetospheric Study (IMS) from the magnetometers of the IGS network extending from Cambridge, England, to Tromso, Norway, are used to study the response of subauroral current systems to sudden changes in solar wind dynamic pressure. Observations show that the response is very strong at subauroral latitudes. The preliminary response in the H component is a brief, small increase in the dayside morning sector and a decrease in the afternoon and night sectors. The main response in the horizontal field (the H and D components) is toward the pole except in the dayside morning sector. The inferred ionospheric current is mainly a circulatory system flowing counterclockwise when viewed from the north pole everywhere at subauroral latitudes except the dayside morning sector.

Effect of sudden solar wind dynamic pressure changes at subauroral latitudes: Time rate of change of magnetic field

The observations obtained during the IMS from the IGS magnetometer chain extending from Cambridge, England, to Tromso, Norway are used to study the time rate of change of the magnetic field at subauroral latitudes at the time of interplanetary shock passages. The time rate of change of the H component maximizes in the high latitude dayside sector. For these typical interplanetary shocks, the dayside value of time rate of change can be as high as ∼ 3 nT/sec at Tromso and ∼ 1 nT/sec at York. The time rate of change in the dayside roughly depends on the change of square root of solar wind dynamic pressure. The largest of these time rates of change are similar to but slightly smaller than those known to cause disruptive disturbances in power distribution and communication systems. Thus, the daytime effects of sudden impulses may be equal to or greater than the nighttime effects associated with substorms as measured by their impact on terrestrial systems.

The effect of solar wind dynamic pressure changes on low and mid‐latitude magnetic records

Magnetic records from low and mid latitude stations have been examined to determine their response to solar wind pressure changes. The best correlation between ground level changes and the change in the square root of the solar wind dynamic pressure occurs for stations at latitudes from 15° to 30° such as Tahiti, Honolulu, San Juan and Midway. The horizontal component of the field changes on average 16.5 nT for each change of 1(nPa)½ of the square root of dynamic pressure. This is 50% greater than the vacuum model of Tsyganenko would predict for a non‐conducting Earth and therefore what would be expected for a perfectly conducting interior. Thus, low and mid latitude ground level response to solar wind pressure changes is dominated by the variation of the strength and location of the magnetopause current system and the corresponding induced currents within the Earth rather than ionospheric current sources.

Variability of the interplanetary medium at 1 a.u. over 24 years: 1963–1986

A survey is presented of hourly averages of observations of the interplanetary medium, made by satellites close to the Earth (i.e. at 1 a.u.) in the years 1963–1986. This survey therefore covers two complete solar cycles (numbers 20 and 21). The distributions and solar-cycle variations of IMF field strength, B, and its northward component (in GSM coordinates), B z , and of the solar-wind density, n, speed, υ, and dynamic pressure, P, are discussed. Because of their importance to the terrestrial magnetosphere/ionosphere, particular attention is given to B z and P. The solar-cycle variation in the magnitude and variability of B z , previously reported for cycle 20, is also found for cycle 21. However, the solar-wind data show a number of differences between cycles 20 and 21. The average dynamic pressure is found to show a solar-cycle variation and a systematic increase over the period of the survey. The minimum of dynamic pressure at sunspot maximum is mainly due to reduced solar-wind densities in cycle 20, but lower solar-wind speed in cycle 21 is a more significant factor. The distribution of the duration of periods of stable polarity of the IMF B z component shows that the magnetosphere could achieve steady state for only a small fraction of the time and there is some evidence for a solar-cycle variation in this fraction. It is also found that the polarity changes in the IMF B z fall into two classes: one with an associated change in solar-wind dynamic pressure, the other without such a change. However, in only 20% of cases does the dynamic pressure change exceed 50%.

The ionospheric signatures of flux transfer events and solar wind dynamic pressure changes

The generation of flow and current vortices in the dayside auroral ionosphere has been predicted for two processes ocurring at the dayside magnetopause. The first of these mechanisms is time‐dependent magnetic reconnection, in “flux transfer events” (FTEs); the second is the action of solar wind dynamic pressure changes. The ionospheric flow signature of an FTE should be a twin vortex, with the mean flow velocity in the central region of the pattern equal to the velocity of the pattern as a whole. On the other hand, a pulse of enhanced or reduced dynamic pressure is also expected to produce a twin vortex, but with the central plasma flow being generally different in speed from, and almost orthogonal to, the motion of the whole pattern. In this paper, we make use of this distinction to discuss recent observations of vortical flow patterns in the dayside auroral ionosphere in terms of one or other of the proposed mechanisms. We conclude that some of the observations reported are consistent only with the predicted signature of FTEs. We then evaluate the dimensions of the open flux tubes required to explain some recent simultaneous radar and auroral observations and infer that they are typically 300 km in north–south extent but up to 2000 km in longitudinal extent (i.e., roughly 5 hours of MLT). Hence these observations suggest that recent theories of FTEs which invoke time‐varying reconnection at an elongated neutral line may be correct. We also present some simultaneous observations of the interplanetary magnetic field (IMF) and solar wind dynamic pressure (observed using the IMP8 satellite) and the ionospheric flow (observed using the EISCAT radar) which are also only consistent with the FTE model. We estimate that for continuously southward IMF (<Bz > ≈ 5 nT) these FTEs contribute about 30 kV to the mean total transpolar voltage (∼30%).