Showing papers on "Substorm published in 2013"
TL;DR: The authors studied the three-dimensional structure of the dipolarization front current sheet (DFCS), which demarcates the magnetic boundary of a dipolarizing flux bundle (DFB) in Earth's magnetotail.
Abstract: [1] Using Time History of Events and Macroscale Interactions during Substorms observations from four tail seasons, we study the three-dimensional structure of the dipolarization front current sheet (DFCS), which demarcates the magnetic boundary of a dipolarizing flux bundle (DFB, the strong magnetic field region led by a dipolarization front) in Earth's magnetotail. An equatorial cross section of the DFCS is convex; a meridional cross section is consistent with a dipolarized field line. The equatorial flow pattern in the ambient plasma ahead of the DFCS exhibits diversions of opposite sense on its evening and morning sides. The magnetic field perturbations are consistent with local field-aligned current generation of region-2 sense ahead of the front and region-1 sense at the front. The median thickness of the DFCS increases from 800 to 2000 km with increasing distance from the neutral sheet, indicating bundle compression near the neutral sheet. On a meridional cross section, DFCS's linear current density (1.2–1.8 nA/m) peaks ~±0.55 l from the neutral sheet (where l is the ambient cross-tail current sheet half-thickness, l ~1.5 RE in our database). This peak, reminiscent of active-time cross-tail current sheet bifurcation noted in past studies, suggests that the intense but thin DFCS (10 to 20 nA/m2) may be produced by redistribution (diversion) of the extended but weaker cross-tail current (~1 nA/m2). Near the neutral sheet, the average DFCS current over the dipolarization front (DF) thickness is perpendicular to both the magnetic field interior to the DFB and the average field direction over the DF thickness. Away from the neutral sheet, the average current becomes progressively parallel to the internal field and the average field direction. The average current directions are indicative of region-1-sense field-aligned current on the DF. As few as approximately three DFBs can carry sufficient total current that, if redirected into the auroral ionosphere, can account for the substorm current wedge's peak current for a sizable substorm (~1 MA). A collapsing DFB could thus be an elemental substorm current wedge, or “wedgelet,” that can divert a sizable portion of the cross-tail current into the auroral ionosphere.
293 citations
TL;DR: In this paper, the Dessler-Parker-Sckopke relation was used to predict the time history of Dst as a function of solar wind coupling and the rate of loss by charge exchange.
Abstract: A typical magnetic storm is characterized by an initial phase, a main phase and a recovery phase. The initial phase is caused by increased solar wind dynamic pressure, the main phase by injection into and energization of particles in the radiation belts, and the recovery phase by charge exchange loss of these particles. The effect of the Radiation belt particles is roughly equivalent to that of a geocentric ring of current of radius 3-4 Re which decreases the horizontal component of the earth's surface field by as much as 500 nT. The magnitude of this effect is generally indexed by the Dst index. The Dessler-Parker-Sckopke relation shows that in the absence of other effects Dst is directly proportional to the total energy of the radiation belt particles. Thus if Dst is corrected for other effects its time rate of change is a direct measure of the rate at which energy is flowing into or out of the ring current. Various models have been developed to explain the rate of energy input via solar wind coupling and the rate of loss by charge exchange. Statistical studies have parameterized these models and the resulting differential equation solved to predict the time history of Dst as a function of solar wind coupling. It is noteworthy that these models are remarkably accurate, and that they do not involve any measure of substorm activity such as the AL index. In fact, when the rectified solar wind electric field is used to predict both Dst and AL, the prediction residuals for these two indices are completely uncorrelated. This result suggests that the effect of particles injected and energized by the expansion phase of substorms is undetectable in the pressure corrected Dst index. It thus seems more likely that it is the global convection electric field that injects and energizes ring current particles. Thus, despite the fact that substorms occur throughout the main phase development of the ring current, the inductive electric field of the expansion phase is not likely to be the primary source of ring current particles or energy. However, it almost certainly plays a role in trapping the particles transported by the global convection field.
135 citations
TL;DR: The authors' observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a “tongue” of ionization (TOI).
Abstract: Patches of ionization are common in the polar ionosphere, where their motion and associated density gradients give variable disturbances to high-frequency (HF) radio communications, over-the-horizon radar location errors, and disruption and errors to satellite navigation and communication. Their formation and evolution are poorly understood, particularly under disturbed space weather conditions. We report direct observations of the full evolution of patches during a geomagnetic storm, including formation, polar cap entry, transpolar evolution, polar cap exit, and sunward return flow. Our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a “tongue” of ionization (TOI).
132 citations
TL;DR: In this paper, two Van Allen Probes were used to observe plasmaspheric hiss and chorus waves in association with substorm-injected energetic electrons, and the upper energy of injected electrons agrees well with the minimum cyclotron resonant energy calculated for the lower cutoff frequency of the observed hiss, and computed convective linear growth rates indicate instability at the observed low frequencies.
Abstract: [1] Both plasmaspheric hiss and chorus waves were observed simultaneously by the two Van Allen Probes in association with substorm-injected energetic electrons. Probe A, located inside the plasmasphere in the postdawn sector, observed intense plasmaspheric hiss, whereas Probe B observed chorus waves outside the plasmasphere just before dawn. Dispersed injections of energetic electrons were observed in the dayside outer plasmasphere associated with significant intensification of plasmaspheric hiss at frequencies down to ~20 Hz, much lower than typical hiss wave frequencies of 100–2000 Hz. In the outer plasmasphere, the upper energy of injected electrons agrees well with the minimum cyclotron resonant energy calculated for the lower cutoff frequency of the observed hiss, and computed convective linear growth rates indicate instability at the observed low frequencies. This suggests that the unusual low-frequency plasmaspheric hiss is likely to be amplified in the outer plasmasphere due to the injected energetic electrons.
130 citations
19 Mar 2013
TL;DR: In this paper, it was shown that plasmoid release is a fundamental process whereby the magnetosphere gives up excess stored energy and plasma, much like comets are seen to do, and that the phenomena of the substorm seen at earth are a byproduct of that fundamental process.
Abstract: Auroral or magnetic substorms are periods of enhanced auroral and geomagnetic activity lasting one to a few hours that signify increased dissipation of energy from the magnetosphere to the earth Data acquired during the past decade from satellites in the near-earth sector of the magnetotail have suggested that during a substorm part of the plasma sheet is severed from earth by magnetic reconnection, forming a plasmoid, ie, a body of plasma and closed magnetic loops, that flows out of the tail into the solar wind, thus returning plasma and energy that have earlier been accumulated from the solar wind Very recently this picture has been dramatically confirmed by observations, with the ISEE 3 spacecraft in the magnetotail 220 R/sub E/ from earth, of plasmoids passing that location in clear delayed response to substorms It now appears that plasmoid release is a fundamental process whereby the magnetosphere gives up excess stored energy and plasma, much like comets are seen to do, and that the phenomena of the substorm seen at earth are a by-product of that fundamental process
112 citations
TL;DR: In this article, the build-up and evolution of the substorm current wedge (SCW) and its association with plasma flows from the tail were investigated using MHD simulations of magnetotail dynamics.
Abstract: [1] Using magnetohydrodynamic (MHD) simulations of magnetotail dynamics, we investigate the build-up and evolution of the substorm current wedge (SCW) and its association with plasma flows from the tail. Three different scenarios are considered: the propagation of magnetic flux ropes of artificially reduced entropy (bubbles), and the formation and propagation of bubbles resulting from magnetic reconnection in the near and far tail. The simulations confirm the important role of the entropy reduction in the earthward penetration of bubbles, as well as in the build-up of field-aligned current signatures attributed to the SCW. Low-entropy flow channels can indeed propagate close to the Earth from the distant tail, as suggested recently. However, this requires substantial entropy reduction, presumably from progression of reconnection into the lobes. The major SCW and pressure build-up occurred when the low-entropy flow channels were braked and the flow diverted azimuthally in the near-Earth region. The flows commonly exhibit multiple narrow channels, separated in space and time, whereas the associated increases in Bz (dipolarization) accumulate over a wider spatial range, spreading both azimuthally and radially. This suggests a picture of the SCW as being composed of multiple smaller “wedgelets,” rather than one big wedge.
102 citations
TL;DR: In this article, a review of previous spacecraft observations and proposed mechanisms/processes that generate O+-rich energetic plasma population, and outlines possible scenarios of O+ pressure abundance in the Earth's inner magnetosphere.
Abstract: [1] In situ observations and modeling work have confirmed that singly charged oxygen ions, O+, which are of Earth's ionospheric origin, are heated/accelerated up to >100 keV in the magnetosphere. The energetic O+ population makes a significant contribution to the plasma pressure in the Earth's inner magnetosphere during magnetic storms, although under quiet conditions, H+ dominates the plasma pressure. The pressure enhancements, which we term energization, are caused by adiabatic heating through earthward transport of source population in the plasma sheet, local acceleration in the inner magnetosphere and near-Earth plasma sheet, and enhanced ion supply from the topside ionosphere. The key issues regarding stronger O+ energization than H+ are nonadiabatic local acceleration, responsible for increase in O+ temperature, and more significant O+ supply than H+, responsible for the increase in O+ density. Although several acceleration mechanisms and O+ supply processes have been proposed, it remains an open question what mechanism(s)/process(es) play the dominant role in stronger O+ energization. This review paper summarizes important previous spacecraft observations, introduces the proposed mechanisms/processes that generate O+-rich energetic plasma population, and outlines possible scenarios of O+ pressure abundance in the Earth's inner magnetosphere.
100 citations
TL;DR: In this article, the energy transport of an earthward bursty bulk flow observed by Cluster C1 on 30 July 2002 was studied using the kinetic approach, i.e., ion velocity distribution function.
Abstract: [1] The energy transport of bursty bulk flows (BBFs) is very important to the understanding of substorm energy transport. Previous studies all use the MHD bulk parameters to calculate the energy flux density of BBFs. In this paper, we use the kinetic approach, i.e., ion velocity distribution function, to study the energy transport of an earthward bursty bulk flow observed by Cluster C1 on 30 July 2002. The earthward energy flux density calculated using kinetic approach QKx is obviously larger than that calculated using MHD bulk parameters QMHDx. The mean ratio QKx/QMHDx in the flow velocity range 200–800 km/s is 2.7, implying that the previous energy transport of BBF estimated using MHD approach is much underestimated. The underestimation results from the deviation of ion velocity distribution from ideal Maxwellian distribution. The energy transport of BBF is mainly provided by ions above 10 keV although their number density Nf is much smaller than the total ion number density N. The ratio QKx/QMHDx is basically proportional to the ratio N/Nf. The flow velocity v(E) increases with increasing energy. The ratio Nf/N is perfectly proportional to flow velocity Vx. A double ion component model is proposed to explain the above results. The increase of energy transport capability of BBF is important to understanding substorm energy transport. It is inferred that for a typical substorm, the ratio of the energy transport of BBF to the substorm energy consumption may increase from the previously estimated 5% to 34% or more.
87 citations
TL;DR: In this article, the effect of the pressure pulse on the ionospheric current using a global network of more than 100 ground magnetometers, images from the POLAR spacecraft, and solar wind measurements from the WIND and Geotail spacecraft was studied.
Abstract: On January 10, 1997, a strong pressure pulse, observed by the WIND spacecraft between 1030 and 1055 UT, hit the magnetosphere after about a one-half hour delay, causing the strengthening and widening of the auroral electrojet at all local times. The duration of the electrojet perturbation was the same as the duration of the solar wind pressure pulse. The pulse occurred during the well-studied January 10-11, 1997, magnetic storm and during strong geomagnetic activity. We study the effect of the pressure pulse on the ionospheric current using a global network of more than 100 ground magnetometers, images from the POLAR spacecraft, and solar wind measurements from the WIND and Geotail spacecraft. We find that the magnetospheric and ionospheric response is directly driven by the solar wind conditions and clearly related to the onset, duration and end of the pressure pulse. In addition, it appears that the enhancement of the Region 1 currents opposed the effect of the enhancement of the magnetopause current for locations near noon. These responses are not characteristics of a typical substorm.
80 citations
TL;DR: A series of computer experiments has been carried out with the Magnetospheric Specification and Forecast Model, in an attempt to illuminate this key question, as well as several other cause-and-effect relationships as discussed by the authors.
Abstract: Over the last twenty years, many quantitative models have been developed to represent the idea that the storm-time ring current consists primarily of particles from the plasma sheet that are injected into the inner magnetosphere by strong westward electric fields. These models, which have gradually become more sophisticated and realistic, have achieved rough quantitative agreement with observed ring-current fluxes, but the tests are still imprecise. The models are still not sophisticated enough to provide accurate and reliable theoretical estimates of Dst. Controversy still surrounds the question of whether the injection of the storm-time ring current mainly results from induction electric fields associated with magnetospheric substorms or from potential electric fields associated with periods of strong convection. A series of computer experiments has been carried out with the Magnetospheric Specification and Forecast Model, in an attempt to illuminate this key question, as well as several other cause-and-effect relationships. The MSFM runs indicate that potential convection electric fields play a far more important role in ring current injection than do substorm-associated induction fields. The computer experiments also demonstrate the importance of fluctuations in the convection field: periods of very strong convection, separated by periods of weak flow, inject particles deeper into the magnetosphere than a long period of moderately strong convection. A run carried out with strong convection limited to one hour - an imitation of a very strong isolated substorm - shows the injection of only a weak ring current. Run results also show reduced populations of ∼ 50 keV ions in regions that are in the shadow of the magnetopause; these depleted regions are particularly prominent when the magnetosphere is highly compressed.
77 citations
TL;DR: The Geotail mission has added significantly to our understanding of magnetic reconnection in the near-Earth plasma sheet for substorm onset as mentioned in this paper, where tailward fast convection flows in association with onsets are found to occur mostly beyond 22 R E in the magnetotail, whereas earthward fast convolutions in association of onsets occur only inside 30 R E.
Abstract: The Geotail mission has added significantly to our understanding of magnetic reconnection in the near-Earth plasma sheet for substorm onset. Tailward fast convection flows in association with onsets are found to occur mostly beyond 22 R E in the magnetotail, whereas earthward fast convection flows in association with onsets are found to occur only inside 30 R E These convection flows are seen mainly in the premidnight plasma sheet. The observations locate the site of typical observation of magnetic reconnection for substorm onsets in the premidnight plasma sheet between X GSM = -22 R E and X GSM = -30 R E . Furthermore, the flows are found to precede the ground onset time in a number of cases near this site, which implies that magnetic reconnection is the cause of the substorm. In the vicinity of the magnetic reconnection site, ion and electron distribution functions show characteristics that depend on distance from the neutral sheet. Ions convect away from the X-type neutral line near the neutral sheet, whereas ions form field-aligned outflows off the neutral sheet. Heating and acceleration processes are observed for ions being convected toward the neutral sheet. High-energy (10-keV) electrons appear in this region. Off the neutral sheet, the highest-energy component of electrons escapes from the X-type neutral line region. Farther off the neutral sheet, high-energy (10-keV) electrons form outward stream, while medium-energy (3-keV) electrons form stream toward the X-type neutral line region. Thus, the micro structure and distribution functions within the magnetic reconnection site has been emerging with Geotail observations.
19 Mar 2013
TL;DR: In this paper, a survey of all the electric field data from the double probe experiment on ISEE-1 (apogee approximately 22 earth radii) during 1980 when the satellite was in the magnetotail was made.
Abstract: A survey has been made of all the electric field data from the University of California, Berkeley, double probe experiment on ISEE-1 (apogee approximately 22 earth radii) during 1980 when the satellite was in the magnetotail. This study was restricted to the 74 events where E cross B flows could be calculated and were equal to or greater than 100 km/s. Substorm times were determined by examining the Ae index for peaks equal to or greater than 250 gamma. In association with substorms, approximately 70 percent of the flows were earthward, and approximately 20 percent had a signature called 'near satellite reconnection' (first described by Nishida et al. (1983) of tailward flow followed by earthward flow which can be interpreted in terms of a model where the x-line forms earthward of the satellite and subsequently propagates tailward of X(GSM) = -21 earth radii and within the absolute value of Y(GSM) equal to or less than 4.5 earth radii. These data suggest that the near earth x-line usually forms tailward of X(GSM) approximately -20 earth radii.
TL;DR: In this article, the authors used current density data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to identify the location of maximum region 1 current at all magnetic local times (MLTs).
Abstract: [1] We use current density data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to identify the location of maximum region 1 current at all magnetic local times (MLTs). We term this location the R1 oval. Comparing the R1 oval location with particle precipitation boundaries identified in DMSP data, we find that the R1 oval is located on average within 1° of particle signatures associated with the open/closed field line boundary (OCB) across dayside and nightside MLTs. We hence conclude that the R1 oval can be used as a proxy for the location of the OCB. Studying the amount of magnetic flux enclosed by the R1 oval during the substorm cycle, we find that the R1 oval flux is well organized by it: during the growth phase the R1 oval location moves equatorward as the amount of magnetic flux increases whereas after substorm expansion phase onset significant flux closure occurs as the R1 current location retreats to higher latitudes. For about 15 min after expansion phase onset, the amount of open magnetic flux continues to increase indicating that dayside reconnection dominates over nightside reconnection. In the current density data, we find evidence of the substorm current wedge and also show that the dayside R1 currents are stronger than their nightside counterpart during the substorm growth phase, whereas after expansion phase onset, the nightside R1 currents dominate. Our observations of the current distribution and OCB movement during the substorm cycle are in excellent agreement with the expanding/contracting polar cap paradigm.
TL;DR: Zhou et al. as discussed by the authors presented cluster observations of a plasma-depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near Earth tail around X = -18 R-E.
Abstract: Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasma-depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near-Earth tail around X = -18 R-E. The bubble is bounded by both sharp leading (partial derivative b(z)/partial derivative x 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field-aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super-Alfvenic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E x B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of B-z, or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field-aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed. Citation: Zhou, M., X. Deng, M. Ashour-Abdalla, R. Walker, Y. Pang, C. Tang, S. Huang, M. El-Alaoui, Z. Yuan and H. Li (2013), Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble, J. Geophys. Res. Space Physics, 118, 674-684, doi:10.1029/2012JA018323.
TL;DR: In this article, magnetically conjugate satellite observations of substorm SAID near the magnetic equator and in the topside ionosphere confirming and expanding on previous results are explored.
Abstract: [1] Subauroral ion drifts (SAID) are the prominent feature of the active subauroral geospace, just earthward of the electron plasma sheet (PS) boundary in the premidnight sector. We explore magnetically conjugate satellite observations of substorm SAID near the magnetic equator and in the topside ionosphere confirming and expanding on our previous results. The SAID channels reside between the hot electron (PS/auroral) boundary at the plasmapause and the drop in the hot ion flux inside the plasmasphere. The overall features are inconsistent with the paradigm of voltage and current generators. Rather, they are explained in terms of a short circuiting of substorm‒injected hot plasma jets over the plasmapause and formation of a turbulent plasmaspheric boundary layer. The short circuiting occurs when the cold plasma density exceeds a critical value of 5–10 cm−3. As the polarization field at the front of the hot plasma jet is shorted out, the hot electrons are arrested, while the hot ions yet move inward. This provides a natural explanation of the long‒known dispersionless auroral precipitation boundary. Enhanced plasma turbulence within the SAID channel provides anomalous circuit resistivity and magnetic diffusion, as with the well‒documented plasmoid‒magnetic barrier problem.
TL;DR: In this article, a two-dimensional view of the field-aligned currents (FACs) during the late growth and expansion phases for three isolated substorms utilizing in situ observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and from ground-based magnetometer and optical instrumentation from the Canadian Array for Realtime Investigations of Magnetic Activity and Time History of Events and Macroscale Interactions during Substorms groundbased arrays is presented.
Abstract: [1] We present a comprehensive two-dimensional view of the field-aligned currents (FACs) during the late growth and expansion phases for three isolated substorms utilizing in situ observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and from ground-based magnetometer and optical instrumentation from the Canadian Array for Realtime Investigations of Magnetic Activity and Time History of Events and Macroscale Interactions during Substorms ground-based arrays. We demonstrate that the structure of FACs formed during the expansion phase and associated with the substorm current wedge is significantly more complex than a simple equivalent line current model comprising a downward FAC in the east and upward FAC in the west. This two-dimensional view demonstrates that azimuthal bands of upward and downward FACs with periodic structuring in latitude form across midnight and can span up to 8 h of magnetic local time. However, when averaged over latitude, the overall longitudinal structure of the net FACs resembles the simpler equivalent line current description of the substorm current wedge (SCW). In addition, we demonstrate that the upward FAC elements of the structured SCW are spatially very well correlated with discrete aurora during the substorm expansion phase and that discrete changes in the FAC topology are observed in the late growth phase prior to auroral substorm expansion phase onset. These observations have important implications for determining how the magnetosphere and ionosphere couple during the late growth phase and expansion phase, as well as providing important constraints on the magnetospheric generator of the FACs comprising the SCW.
TL;DR: In this paper, the authors examined the magnetic disturbances on March 24, 1940, February 11, 1958, August 4, 1972, and March 13, 1989 that were responsible for the most significant power system effects.
Abstract: Space weather disturbances cause geomagnetic field variations that induce electric currents into power transmission systems on the ground. These geomagnetically induced currents (GIC) flow to ground through the windings of power transformers where they produce extra magnetic flux that can saturate the transformer core. This leads to transformer heating, increased power demand, and ac harmonic generation, which can interfere with power system operation. This paper examines the magnetic disturbances on March 24, 1940, February 11, 1958, August 4, 1972, and March 13, 1989 that were responsible for the most significant power system effects. The blackout of the Hydro-Quebec system on March 13, 1989 was due to an enhancement of a westward substorm electrojet resulting from loading and unloading of energy in the magnetosphere. Power system effects, including transformer overheating, later on March 13 can be attributed to an eastward convection electrojet caused by the 'directly-driven' flow of energy from the solar wind. Power system problems during the earlier disturbances are also shown to be caused by rapid changes of the convection electrojets. This shows that the convection current systems, as well as substorm currents, need to be included when predicting space weather effects on power systems.
TL;DR: The role of substorms in the geomagnetic storm of 2000 is examined in this article, where the authors show that the entire day of the storm was characterized by a sequence of sub-storms and a gradual build-up of the ring current.
Abstract: The geomagnetic storm of October 4-6, 2000 provides an exceptionally good opportunity to examine the role of substorms during the storm because of the relatively moderate solar wind driving and because of the excellent set of satellite observations. We show that the entire day of October 4 was characterized a sequence of substorms and a gradual build-up of the storm-time ring current. We examined one of those substorms in some detail showing that it had the expected signatures of a magnetospheric substorm. ENA observations and in situ measurements show that the substorms clearly do provide a mechanism for transporting energetic ions from the magnetotail to the inner magnetosphere. Substorm injections do not, however, provide the only mechanism. As other studies have suggested, the evidence from this storm shows that the presence of a quasi-steady convection electric field plays an additional important role. This is seen through the comparison of Dst which decreases slowly and smoothly over nearly 10 hours with geosynchronous particle injections which occur impulsively at roughly 2-hour intervals producing a sawtooth injection profile. Further evidence comes from ENA observations that show both impulsive injection during substorms and continued intensification and eastward expansion in response to convection electric fields. We also investigate the transport and symmetry of the ring current particles. We find that, even more than 10-hours into the storm when Dst had decreased below -100 nT and at least five clear substorm injections had occurred, the ring current remained highly asymmetric with essentially no ENA emissions from the dawn-to-noon sector. We also find that Dst, SYM-H, and ASY-H all respond approximately equally in spite of the fact that there is apparently no symmetric component to the ring current until later in the storm.
TL;DR: In this paper, the authors studied the current system ahead and within the dipolarization fronts (DFs) with high-resolution magnetic field measurements from Cluster constellation in 2003 and classified the DFs into two types: DFs with magnetic dips immediate ahead of the fronts (type I) and DFs without magnetic dips (type II).
Abstract: [1] Recently, observational results on currents around the dipolarization fronts (DFs) of earthward flow bursts have attracted much research attention. These currents are found to have close association with substorm intensifications. This paper devotes to further study of the current system ahead and within the DFs with high-resolution magnetic field measurements from Cluster constellation in 2003. The separation of four spacecraft is much smaller than the scales of spatial structures ahead and within the DF layer so that the currents can be reliably obtained. Based on features of the magnetic field variations prior to the fronts, we categorized the DFs into two types: DFs with magnetic dips immediate ahead of the fronts (type I) and DFs without magnetic dips (type II). For type I DFs, it is found that dawnward currents along the DFs exist in the dip region; duskward currents exist within the fronts. Furthermore, the dawnward currents in the dip region are found to be mainly parallel to the local magnetic field with a spatial scale of ~1000 km, whereas the duskward currents within the fronts have both significant parallel and perpendicular components. On the other hand, for type II DFs, only significant duskward and mainly perpendicular currents show up within the fronts; no dawnward currents exist ahead of DFs. The dawnward and mainly parallel current in the type I DFs is important in the current coupling process between magnetosphere and ionosphere and may lead to local current disruptions for substorm initiations.
TL;DR: In this paper, the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions was analyzed with 11 years of DMSP SSJ/4/5 data.
Abstract: [1] Substorms release a large amount of energy, some of which is used to energize the precipitating particles in the polar region. Superposed epoch analysis was performed with 11 years of DMSP SSJ/4/5 data to characterize the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions. Although substorms only increase the ion pressure by 30%, they increase the power of the diffuse, monoenergetic, and wave electron aurora by 310%, 71%, and 170%, respectively. Substorms energize the ion aurora mainly in the 21:00–05:00 magnetic local time (MLT) sector. The dynamics of the diffuse electron aurora are different from those of the other two electron aurorae. The expansion phase duration is approximately 15 min for the monoenergetic and wave electron aurorae, whereas it is 1 h for the diffuse electron aurora. The monoenergetic and wave electron aurorae appear to complete the substorm cycle within a 5 h interval, whereas the diffuse electron aurora takes more than 5 h. The diffuse electron aurora power and energy flux start increasing at 15 min before the substorm onset, whereas those for the monoenergetic and wave electron aurorae start increasing at 1 h and 15 min before the onset. The increase in the monoenergetic electron aurora power and energy flux may result from the increase in the magnetotail stretching and region-1 field-aligned current during the growth phase. The monoenergetic electrons may also be associated with fast flows, which have been previously observed more frequently in the dusk-midnight sector.
20 Mar 2013
19 Mar 2013
TL;DR: The near-earth magnetotail of the terrestrial magnetosphere is very likely the region in which magnetospheric substorms are initiated and it is in this location that substorm-related magnetic reconnection appears to occur as mentioned in this paper.
Abstract: The near-earth magnetotail (10 less than or equal to r less than or equal to 20 R/sub E/) portion of the terrestrial magnetosphere is very likely the region in which magnetospheric substorms are initiated and it is in this location that substorm-related magnetic reconnection appears to occur. An observational advantage compared to other astrophysical regions is that the near magnetotail can be nearly continuously monitored by spacecraft that are relatively fixed in location. Observations of magnetic fields and plasma distribution functions in the neartail reveal a very regular and predictable sequence of variation in association with substorms. These data, considered in a global context, provide very strong evidence for the neutral line substorm model and, thus, for the regular occurrence of magnetic reconnection in the near-earth magnetotail during substorms.
21 Mar 2013
TL;DR: In this paper, the interaction of the solar wind with the magnetosphere is examined from the standpoint of fundamental processes in physics and, whenever possible, observational data, such as open magnetic field lines in the polar cap and a boundary layer just inside the magnetopause, and the problems associated with reversal of the electric field within the magnetotail are examined.
Abstract: The interaction of the solar wind with the magnetosphere is examined from the standpoint of fundamental processes in physics and, whenever possible, observational data. A conceptual model is developed which incorporates several features of the magnetosphere that are still open to debate, such as open magnetic field lines in the polar cap and a boundary layer just inside the magnetopause. It is shown that the length of the magnetotail, the existence of the polar rain, the bursts of particles at the edge of the plasma sheet, and the poleward motion of auroral forms during the recovery phase of a magnetospheric substorm may all have a natural explanation with the proposed model. The one exception, however, is the topology of the electric field. The problems associated with reversal of the electric field within the magnetotail are examined.
28 Mar 2013
TL;DR: In this paper, a population of 83 similar ULF wave events also associated with substorm-injected particles were identified using multiple Super Dual Auroral Radar Network radars in Europe and North America between June 2000 and September 2005.
Abstract: [1] A previous case study observed a ULF wave with an eastward and equatorward phase propagation (an azimuthal wave number m, of ∼13) generated during the expansion phase of a substorm. The eastward phase propagation of the wave suggested that eastward drifting energetic electrons injected during the substorm were responsible for driving that particular wave. In this study, a population of 83 similar ULF wave events also associated with substorm-injected particles have been identified using multiple Super Dual Auroral Radar Network radars in Europe and North America between June 2000 and September 2005. The wave events identified in this study exhibit azimuthal wave numbers ranging in magnitude from 2 to 92, where the direction of propagation depends on the relative positions of the substorm onsets and the wave observations. We suggest that azimuthally drifting energetic particles associated with the substorms are responsible for driving the waves. Both westward drifting ions and eastward drifting electrons are implicated with energies ranging from ∼1 to 70 keV. A clear dependence of the particle energy on the azimuthal separation of the wave observations and the substorm onset is seen, with higher energy particles (leading to lower m-number waves) being involved at smaller azimuthal separations.
TL;DR: In this paper, it was shown that the ionospheric component shows the largest increase at L < 4 during the main phase of magnetic storms, indicating that the frequent occurrence of intense substorms is important.
Abstract: There is much evidence in recent Yohkoh and Ulysses observations that the most intense magnetic storms are a manifestation of fast magnetic clouds, perhaps originating in Coronal Mass Ejections (CMEs). However, uncertainties exist as to what magnetic configuration is formed for CMEs and how it changes over time, how CMEs interact with the interplanetary medium, and how geoeffectiveness depends on the different size of ejections. The constituents of the ring current in the magnetosphere as a function of storm time are now observationally identified. In particular, the ionospheric component shows the largest increase at L < 4 during the main phase of magnetic storms, indicating that the frequent occurrence of intense substorms is important. There seems to be a consensus that, of the two processes playing essential roles in enhancing the storm-time ring current, the enhanced electric field driven by southward interplanetary magnetic fields dominates the effects of the induced electric field resulting from substorm expansion onsets. This creates a new controversy regarding the relative importance of the two processes, although they are not mutually exclusive. There is a need to evaluate observations and models in kinetic, chemical, and electrodynamic coupling between the ionospheric and thermospheric magnetic storms in a more quantitative manner. For example, global, not regional, observations of the electron density and the neutral wind are required in order to understand the ionospheric/thermospheric storms within the framework of the chain of processes from the Sun to the Earth. In view of the important effects of magnetic storms on a wide variety of human-societal systems, prediction schemes continue to be upgraded.
TL;DR: In this paper, the authors show a new in situ example of a dipolarization identified in the Cassini magnetometer data and use this formalism to estimate the ionospheric current density that would arise based on in situ tail measurements of the magnetic field and the implications for corresponding auroral electron acceleration.
TL;DR: In this paper, the mechanisms by which ionospheric O+ stretches the plasma sheet causing variations in polar cap flux during magnetospheric sawtooth substorms are investigated using the multi-fluid Lyon-Fedder-Mobarry simulation code.
Abstract: [1] The mechanisms by which ionospheric O+ stretches the plasma sheet causing variations in polar cap flux during magnetospheric sawtooth substorms are investigated using the multi-fluid Lyon-Fedder-Mobarry simulation code. O+ outflow is induced in the simulation by Alfvenic Poynting flux flowing to low altitude during the sawtooth expansion and growth phases. The O+ fluid populates the plasma sheet and causes it to stretch tailward in response to the increased mass density. The ionospheric outflow thus functions as a feedback loop in the magnetosphere-ionosphere system wherein O+ ions released during the expansion phase alter the magnetospheric configuration and enable the development of the next substorm. We also find that the nightside reconnection rate is strongly dependent on the position of the tail merging line. As the x-line moves tailward, the inflow speed and magnetic field strength are reduced, creating an imbalance between dayside and nightside merging. This imbalance is responsible for the buildup and release of open magnetic flux in the magnetosphere during the sawtooth cycle.
21 Mar 2013
TL;DR: In the past decade, substantial progress has been made in the development of quantitative models of convection in the magnetosphere and of the electrodynamic processes that couple that magnetosphere/ionosphere and ionosphere as mentioned in this paper.
Abstract: During the past ten years, substantial progress has been made in the development of quantitative models of convection in the magnetosphere and of the electrodynamic processes that couple that magnetosphere and ionosphere. Using a computational scheme first proposed by Vasyliunas, the convection models under consideration separate the three-dimensional problem of convection in the inner magnetosphere/ionosphere into a pair of two-dimensional problems coupled by Birkeland currents flowing between the two regions. The logic, development, and major results of the inner magnetosphere convection model are reviewed with emphasis on ionospheric and magnetospheric currents. A major theoretical result of the models has been the clarification of the relationship between the region 1/region 2 picture of field-aligned currents and the older partial ring current/tail current interruption picture of substorm dynamics.
TL;DR: In this article, the authors studied the characteristics of particles and fields in the flow-bouncing region of the near-Earth tail during a substorm event around 12:50 UT on 7 September 2007 and showed that flow bouncing took place associated with a tailward-directed j × B force in a disturbed current sheet in addition to an enhanced tailward pressure gradient force.
Abstract: [1] Characteristics of particles and fields in the flow-bouncing region are studied based on multipoint observations from Cluster located at 13–15RE downtail during a substorm event around 12:50 UT on 7 September 2007. The Cluster spacecraft were separated by a distance of up to 10,000 km and allowed to determine the mesoscale evolution of the current sheet as well as the development of the dipolarization front. We show that the flow bouncing took place associated with a tailward-directed j × B force in a disturbed current sheet in addition to an enhanced tailward pressure gradient force. Multiple Earthward propagating dipolarization fronts accompanied by enhanced flux of energetic electrons were observed before the flow bouncing. The sequence of events started with a localized dipolarization front and ended with a large scale (>10RE) dipolarization front accompanied by a major increase in energetic electrons at all spacecraft and immediately followed by flow bouncing. Multiple dipolarization fronts result in the formation of compressed magnetic field with a plasma bulge bounded by thin ion-scale current layers, a favorable condition for flow bouncing. These observations suggest that to understand the flow bouncing and related acceleration of plasma in the near-Earth tail, both the large-scale MHD properties and the transient and small-scale effect of the plasma interaction with the Earth-dipole field need to be taken into account.