Strong Southward Interplanetary Magnetic Field Research Articles

Significant depletions of the ionospheric plasma density at middle latitudes: A possible signature of equatorial spread F bubbles near the plasmapause

Plasma density irregularities in the equatorial F region ionosphere are generally called equatorial spread F (ESF). Large‐scale plasma bubbles associated with ESF processes are generated in the bottomside F region and penetrate the F peak to the topside F region. It is still not fully understood how high ESF bubbles can reach. In this paper, we present the measurements of deep depletions of the ionospheric plasma density at 840‐km altitude with the Defense Meteorological Satellite Program (DMSP) satellites in the evening sector on 29 October 2003. The plasma depletions occurred after an interval of strong southward interplanetary magnetic field (IMF). A prominent feature of the observations is that the plasma depletions occurred at very high magnetic latitudes (MLAT), as well as at equatorial latitudes. The equatorial depletions were identified as plasma bubbles. The depletions at middle latitudes (20°–46° MLAT) were separated from the conventional ionospheric midlatitude trough and identified as the extension of ESF flux tubes into these latitudes. It implies that the plasma bubble, which caused the emptied flux tube at 46° MLAT, reached the altitude of 6800 km over the magnetic equator. The observations may represent the highest altitude/latitude of plasma bubbles that has ever been reported in the literature. For the plasma depletions at even higher latitudes (47°–49° MLAT) near the subauroral polarization stream (SAPS)/midlatitude trough, there are two candidate generation mechanisms. One is the extension of ESF flux tubes into these latitudes, implying that the plasma bubbles reached the altitude of 8400 km over the magnetic equator. Because the SAPS/midlatitude trough corresponds to the plasmapause at the plasmaspheric heights, the observations suggest that ESF plasma bubbles may have reached the plasmapause. Another mechanism is a field‐aligned, low‐density structure between the plasmasphere and ionosphere, and the plasma structure is generated through the strong electric field in the subauroral polarization stream region.

“Shoulders” on the high-latitude magnetopause: Polar/GOES observations

[1] MHD simulations predict that under strongly southward interplanetary magnetic field (IMF) conditions the polar cap potential saturates and attendant Region 1 currents affect the dayside magnetosphere by weakening subcusp magnetic fields and enhancing them in the high-latitude lobe. Consequently, “shoulder-like” structures appear as sunward projections of the high-latitude magnetopause relative to the location of the subsolar magnetopause that retreats earthward. We present observations from the Polar satellite, acquired during a period of strong southward IMF and saturated transpolar potential, which illustrate the predicted lobe field enhancement and resulting “shoulder” structure. At about the same time the GOES satellite observed weakening magnetic fields near the subsolar magnetopause. Similar types of observations, obtained under weakly and strongly driven magnetospheric conditions, are shown to be consistent with the MHD predictions. We demonstrate that including the Region 1 current generated magnetic field in the force balance equation at the magnetopause provides a quantitative basis for predicting magnetopause erosion during strongly southward IMF intervals.

Response of neutral atom emissions in the low‐latitude and high‐latitude magnetosheath direction to the magnetopause motion under extreme solar wind conditions

On 11 April 2001 the high velocity and density of the solar wind and the strong southward interplanetary magnetic field moved the dayside magnetopause inside of geosynchronous orbit. The Low Energy Neutral Atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft in the magnetosphere observed significant emission in the magnetosheath direction. The total neutral atom flux from the dayside region, ignoring the neutral solar wind flux directly from the Sun, shows a threefold enhancement, and each of the three increases is coincident with the occurrence of the magnetopause inside 6.6 RE. Observations by LENA also show that emission in the direction of the low‐latitude and high‐latitude magnetosheath is modulated in such a manner that the sources shift earthward/sunward and equatorward/poleward in the low‐latitude and high‐latitude sheath, respectively. A model based on the distributions of the sheath flux and of the number density of the hydrogen exosphere explains these characteristics as a result of the motion of the magnetopause having an indentation at the cusp, suggesting a means for monitoring the cusp motion using IMAGE/LENA.

Circulation of ionospheric and solar wind particle populations during extended southward interplanetary magnetic field

Single particles trajectories using time‐dependent electric and magnetic fields from multifluid simulations are used to examine the energy and mass transport due to entry of solar wind particles and from ionospheric outflows incorporating both light and heavy ions. It is shown the particles move with the convection cells derived from the multifluid simulations, and that strong particle acceleration occurs at both the dayside and nightside tips of the flow reversal regions. However, different particle populations have limited access to various parts of the convection pattern, and this leads to substantial dawn‐dusk asymmetries in the particle populations within the magnetosphere. For the strong southward interplanetary magnetic field (IMF) conditions for the event studied, fast solar wind particles are able to enter from the dawnside and contribute to the asymmetric ring current. The rest of the fast particles flow down the mantle and do not enter the near‐Earth magnetosphere. The remaining slow component is convected into the magnetosphere over a few tens of minutes to form cold populations in the mantle and LLBL. Solar wind He++ and O6+ are shown to penetrate deeper than the solar wind protons. Light ionospheric ions from the cusp and dawnside auroral oval have similar access problems and flow primarily down the tail and make little contribution to the plasma content to the near‐Earth plasma sheet. Nightside auroral protons are shown to form a cold component in the plasma sheet boundary layer. When they are eventually convected in to the near‐Earth plasma sheet they are accelerated preferentially to the duskside and experience further energy gains as they are convected sunward. These energetic protons contribute to the asymmetric ring current, with leakage from the magnetosphere occurring as they approach the dayside reconnection region. Heavy ionospheric ions enter the convection pattern differently from their light ions counterparts. O+ ions from the cusp can enter the near‐Earth plasma sheet to provide some mass loading, but the biggest contributor is the dawnside auroral oval. The dawn‐dusk electric field accelerates these ions toward the dusk side, but on sufficient low L shells that these ions can dominate the symmetric ring current composition. The O+ ions that originate from the midnight and dusk sectors go to forming hot populations in the dusk LLBL.

Tail‐dominated storm main phase: 31 March 2001

On 31 March 2001 a fast solar wind transient with strong southward interplanetary magnetic field Bz produced a large geomagnetic storm at Earth, with a drop in the Dst index to −350 nT between 0400 and 0900 UT. The Earth's magnetosphere was very compressed during this interval, with the bow shock crossing geosynchronous orbit on at least two occasions. Here we present space‐based and ground‐based observations demonstrating that tail currents, rather than ring currents, were the dominant contributor to the Dst index during the main phase of this storm. The plasma sheet during this interval was exceptionally dense and penetrated very deeply towards the Earth, leading to extremely strong tail currents flowing quite close to the Earth. These tail currents produced a very distorted magnetosphere, with strong stretching of the magnetic field lines in the nightside plasma sheet. Energetic neutral atom (ENA) images from the MENA and HENA instruments on IMAGE show a very narrow spatial distribution, with ENAs confined to the nightside until a magnetic field dipolarization at ∼0630 UT when Dst was −250 nT. Ground magnetometer measurements confirm that the disturbance was localized on the nightside and dominated by tail currents up until the field dipolarization. Following the dipolarization, higher energy ENAs began to drift toward dusk, forming a partial ring current. Even at that time, low‐energy ENAs were not observed on the dayside, either due to inhibited access or to strong charge exchange losses.

On consecutive bursts of low-latitude Pi2 pulsations

Consecutive bursts of low-latitude Pi2 pulsations are examined with magnetic field data obtained by the Sino Magnetic Array at Low Latitudes (SMALL) in 1999. To ascertain the relationship with the substorm onset, 33 events consisting of two consecutive Pi2 bursts are selected from the observations at the Wuhan and Beijing stations, with reference to H-component magnetic bays in high-latitude magnetograms. The dominant frequencies of these consecutive Pi2 bursts are mostly in the range 10– 20 mHz . The frequencies of the two Pi2 bursts are less correlated with each other at the lower latitude station, Wuhan, than at Beijing. The dominant frequency increases as the Kp index increases. Observations with ACE and Wind satellite show that the first Pi2 burst occurs significantly after the southward turning of the interplanetary magnetic field (IMF) and the second Pi2 often occurs shortly after a northward turning of the IMF. Thus, the first Pi2 burst is not a signature of the beginning of the flux transfer to the tail but perhaps signals the initial onset of reconnection in the tail. The interplanetary data can be used to estimate the amount of reconnected magnetic flux transported to the tail. The delay time of two consecutive bursts of low-latitude Pi2 is correlated with this estimated flux pileup. If the second burst of Pi2 signals the onset of reconnection of the open field lines in the tail lobes, then this observation implies that the rate of reconnection of the closed field lines in the plasma sheet is more rapid the greater is the rate of reconnection at the nose. Thus, we would expect plasmoids to be created more quickly for strong southward IMF than for weakly southward IMF.

Polar observations and model predictions during May 4, 1998, magnetopause, magnetosheath, and bow shock crossings

During the rise to the maximum phase of solar cycle 23, several periods of extreme solar wind conditions have occurred. During such an example on May 4, 1998, the solar wind monitors observed a period of strong southward interplanetary magnetic field (IMF) accompanied by a solar wind dynamic pressure that was 30 times higher than average. During this period the Polar spacecraft crossed the magnetopause and bow shock and experienced its first solar wind encounter. This case provides a rare opportunity to study the magnetopause, low‐latitude boundary layer, magnetosheath, and bow shock and to test our ability to model the dynamic behavior of these boundary regions under extreme and highly variable solar wind conditions. In this study we use the gas dynamic convected field model to predict the time‐dependent magnetic field and plasma properties upstream from the magnetopause and the location of the Polar spacecraft relative to the magnetopause and bow shock during the event. To test the accuracy of the prediction, model magnetic field characteristics are compared to the fields observed along the satellite track by the magnetometer on Polar. The predicted model plasma characteristics (density, velocity, and temperature) are compared to moments derived from TIDE observations, extrapolated to account for the higher energy portion of the magnetosheath distributions. Where ambiguities occur in identifying the satellite location, plasma distribution functions from two additional ion detectors (TIMAS and HYDRA) are used to resolve the observed location of Polar relative to the boundaries. With this procedure, carried out separately for ACE and Wind and for two different magnetopause models, observed features at Polar can be traced back to drivers in the solar wind, providing a unique opportunity to assess the evolution of the solar wind and its predictability from the solar wind monitors to the magnetopause. When the Polar apogee drifts to low latitudes in the future, it will provide more and more observations in this region. Therefore what we learn from this case can be indicative of what we will see in the future in Polar operations. The high‐level correlation between the predictions and in situ measurements indicates that the solar wind monitors often provide adequate and useful solar wind conditions near the Earth. The tests indicate that in this event the vacuum dipole field magnetosphere model predicts a significantly larger magnetosphere than observed. While the empirical magnetopause model predictions are more consistent with the observations, they overpredict the response of the magnetosphere to transient southward turnings of the IMF, indicating that the magnetosphere does not respond to the southward IMF turnings on small timescales. Sometimes, there are significant differences at relatively small time scales between the measurements from the two solar wind monitors. It is possible for the solar wind conditions near the Earth to be different from the measurements made by at least one of the solar wind monitors. Cautions should be taken when interpreting near‐Earth observations if the observations are inconsistent at small timescales with the predictions based on one solar wind monitor.

Dependence of magnetopause erosion on southward interplanetary magnetic field

On May 4, 1998, the Wind and ACE satellites observed a period of extremely strong southward interplanetary magnetic field (IMF) followed by a period of highly variable IMF. The magnetopause moved inside the geosynchronous orbit, exposing Los Alamos National Laboratory (LANL) geosynchronous satellites and Polar to the magnetosheath. In a simple model the magnetopause erosion may be linearly proportional to the magnitude of southward IMF. A careful comparison of the predictions derived from various magnetopause location models to the in situ observations indicates a nonlinear dependence of the magnetopause erosion on IMF Bz. The observations from the Wind and ACE satellites are significantly different in the period of highly variable IMF. The predictions based on the ACE solar wind data are more consistent with the LANL observations than those based on the Wind solar wind data. The positions of the satellites show that ACE and LANL were mainly on the same side of the Sun‐Earth meridian plane. However, for the Polar observations the predictions based on the Wind data are better than those based on the ACE data. Wind and Polar were on the same side of the equatorial plane. These results suggest that we choose a solar wind monitor that is on the same side of the meridian or equatorial plane as the magnetospheric satellite for better predictions.

Global simulation of magnetospheric space weather effects of the Bastille Day storm

We present results from a global simulation of the interaction of the solar wind with Earth's magnetosphere, ionosphere, and thermosphere for the Bastille Day geomagnetic storm and compare the results with data. We find that during this event the magnetosphere becomes extremely compressed and eroded, causing 3 geosynchronous GOES satellites to enter the magnetosheath for an extended time period. At its extreme, the magnetopause moves at local noon as close as 4.9 R E to Earth which is interpreted as the consequence of the combined action of enhanced dynamic pressure and strong dayside reconnection due to the strong southward interplanetary magnetic field component B z, which at one time reaches a value of −60 nT. The lobes bulge sunward and shield the dayside reconnection region, thereby limiting the reconnection rate and thus the cross polar cap potential. Modeled ground magnetic perturbations are compared with data from 37 sub-auroral, auroral, and polar cap magnetometer stations. While the model can not yet predict the perturbations and fluctuations at individual ground stations, its predictions of the fluctuation spectrum in the 0–3 mHz range for the sub-auroral and high-latitude regions are remarkably good. However, at auroral latitudes (63° to 70° magnetic latitude) the predicted fluctuations are slightly too high.

Solar wind control of the penetration of electric fields in the inner magnetosphere

Particle and field measurements from the DMSP F8 and CRRES satellites were used to study interplanetary influences on the electrodynamics of the duskside inner magnetosphere during the magnetic storm of June 1991. The storm was triggered by an increase in the solar wind dynamic pressure accompanied by a southward turning of the interplanetary magnetic field (IMF). Satellite data show that: (1) Particle and field boundaries initially moved equatorward/earthward, probably in response to the strong southward IMF turning. (2) Electric field boundaries were close to the inner edge of ring current ions throughout the main and early recovery phases, except during rapid increases in the polar cap potential when shielding was ineffective. (3) Potentials at subauroral latitudes were large fractions of the total potentials of the afternoon cell, twice exceeding 60 kV. This source of ring current energization depends on the polar cap potential to magnetopause standoff distance ratio. (4) Auroral electron boundaries mapped to lower L shells than where CRRES detected plasma sheet electrons reflecting magnetic inflation by the ring current.

Electrodynamics of the inner magnetosphere observed in the dusk sector by CRRES and DMSP during the magnetic storm of June 4–6, 1991

We compare equatorward/earthward boundaries of convection electric fields and auroral/plasma sheet electrons detected by the DMSP F8 and CRRES satellites during the June 1991 magnetic storm. Measurements come from the dusk magnetic local time sector where the ring current penetrates closest to the Earth. The storm was triggered by a rapid increase in the solar wind dynamic pressure accompanied by a southward turning of the interplanetary magnetic field (IMF). Satellite data show the following: (1) All particle and field boundaries moved equatorward/earthward during the initial phase, probably in response to the strong southward IMF turning. (2) Electric field boundaries were either at lower magnetic L shells or close to the inner edge of ring current ions throughout the main and early recovery phases. Penetration earthward of the ring current occurred twice as the polar cap potential increased rapidly. (3) Electric potentials at subauroral latitudes were large fractions of the total potentials in the afternoon cell, twice exceeding 60 kV. (4) The boundaries of auroral electron precipitation were more variable than those of electric fields and mapped to lower L shells than where CRRES encountered plasma sheet electrons. Observations qualitatvely agree with predictions of empirical models for auroral electron and electric field boundaries.

Central axial field direction in magnetic clouds and its relation to southward interplanetary magnetic field events and dependence on disappearing solar filaments

Extended periods of strong southward interplanetary magnetic field Bs are the primary cause of intense geomagnetic storms. Most such intervals are associated with magnetic clouds in the solar wind. We define a magnetic cloud Bs event as the interval of southward interplanetary magnetic field observed as a magnetic cloud passes by the Earth. For the 26 well‐characterized events studied here, we find that (1) magnetic cloud central axial field directions are almost evenly distributed between −90° and 90° ecliptic latitude, the longitudinal distribution is slightly peaked around the east and west, (2) the duration and intensity of magnetic cloud Bs events correlate linearly with the direction of the cloud's central axial field, and (3) cloud central axial field directions are correlated with the central axial field directions of the associated disappearing filament on the Sun. These findings are useful in predicting from solar observations the duration and intensity of those magnetic cloud Bs events that hit Earth.

Signatures of flux erosion from the dayside magnetosphere

The rate of merging and the strength of the region 1 Birkeland currents increase during periods of southward interplanetary magnetic field (IMF). Fringe fields of the Birkeland currents depress dayside magnetospheric magnetic field strengths and remove magnetic flux from the dayside magnetosphere, thereby allowing the dayside magnetopause to move inward and the cusp equatorward. We use previously derived fits to the magnetopause location as a function of IMF Bz, the condition of pressure balance at the magnetopause, and an idealized model of region 1 Birkeland currents to estimate that strong southward IMF turnings will produce ∼13‐ to 26‐nT depressions in the geosynchronous magnetic field strength over periods of 30‐60 min. We then present three case studies of geosynchronous magnetic field strength variations during periods of nearly constant solar wind dynamic pressure and southward IMF. The dayside magnetospheric magnetic field strength was depressed ∼10 nT during a period of strongly southward IMF (Bz = −6 nT), but only ∼5 nT during two more typical periods of slightly southward IMF (Bz = −2 to −3 nT). The depressions correspond to periods of enhanced AL index, which we interpret as evidence for directly driven solar wind‐magnetosphere interaction rather than the unloading of energy stored within the magnetotail. Dayside geosynchronous magnetic field strengths are weakly correlated with IMF Bz.

Observation of electromagnetic ion cyclotron waves and hot plasma in the polar cusp

On July 24, 1986 narrow band magnetic field fluctuations were observed by VIKING near the apogee altitude of 13,500 km in the polar cusp region during a period of strong southward interplanetary magnetic field (IMF). The VIKING magnetic field experiment obtained vector magnetic field measurements up to nearly the proton cyclotron frequency (fH=28 Hz). Simultaneously, magnetosheath‐like electrons and ions as well as an upward‐flowing field‐aligned current were observed. The most intense flux of electrons and ions were colocated with a 35 nT depression in the total magnetic field strength of 1860 nT, interpreted as a diamagnetic effect. The density of magnetosheath‐like plasma, estimated from the diamagnetic depression, reached ∼500 cm−3. This extremely high density was also consistent with total plasma density estimates using high frequency waves. Magnetic field fluctuations were observed from 18 to 27 Hz and occurred when the magnetic field strength depression was maximum. These waves were left‐hand elliptically polarized with ellipticity between 0.2 and 0.3 and may propagate in the Alfvén‐ion cyclotron branch of the cold electromagnetic dispersion relation. Possible free energy sources included anisotropies in the electron and ion distributions.