R. D. Belian

October 1995 magnetic cloud and accompanying storm activity: Ring current evolution

The passage at Earth of the October 1995 magnetic cloud and the high-speed corotating stream overtaking it, monitored by the Global Geospace Science (GGS) spacecraft Wind, caused two consecutive geomagnetic storms: a major one during the strong Bz < 0 nT phase of cloud passage and a moderate one during the intermittent Bz < 0 activity in the fast corotating stream. Large dynamic pressure changes were observed in the sheath region ahead of the cloud and in the cloud-stream interface region at its rear, resulting in substantial corrections to the measured Dst index. A burst of superdense plasma sheet extending over ∼2 hours in local time was observed at geostationary orbit during the second storm. We simulate the ring current development during this storm period using our kinetic model and calculate the magnetic field perturbation caused by the ring current. The plasma inflow on the nightside is modeled throughout the investigated period using data measured at geosynchronous orbit. The modeled Dst index is compared with the observed Dst values corrected for magnetopause and telluric currents. The temporal evolution of the ring current H+ and O+ distribution functions is computed, considering losses due to charge exchange, Coulomb collisions, and ion precipitation. We find that (1) the storm time enhancement of the plasma sheet ion population contributed significantly to the ring current buildup; (2) an additional ∼12 nT decrease in Dst is achieved when the symmetry line of the plasma convection paths is rotated eastward from the dawn-dusk direction with 3 hours during the first storm; (3) the major loss process is charge exchange, followed by Coulomb collisions and ion precipitation; (4) however, the energy losses due to ion precipitation increase monotonically during the more active periods, reaching the level of Coulomb losses at peak storm intensity. We argue that the losses due to ion precipitation considered in this study are closely related to the enhanced convection electric field, which in our model is parameterized with the planetary Kp index. Correspondingly, we find that (5) there is a very good correlation between the variations in time of this index and the magnitude of the ion precipitation losses.

The relativistic electron response at geosynchronous orbit during the January 1997 magnetic storm

The first geomagnetic storm of 1997 began on January 10. It is of particular interest because it was exceptionally well observed by the full complement of International Solar Terrestrial Physics (ISTP) satellites and because of its possible association with the catastrophic failure of the Telstar 401 telecommunications satellite. Here we report on the energetic electron environment observed by five geosynchronous satellites. In part one of this paper we examine the magnetospheric response to the magnetic cloud. The interval of southward IMF drove strong substorm activity while the interval of northward IMF and high solar wind density strongly compressed the magnetosphere. At energies above a few hundred keV, two distinct electron enhancements were observed at geosynchronous orbit. The first enhancement began and ended suddenly, lasted for approximately 1 day, and is associated with the strong compression of the magnetosphere. The second enhancement showed a more characteristic time delay, peaking on January 15. Both enhancements may be due to transport of electrons from the same initial acceleration event at a location inside geosynchronous orbit but the first enhancement was due to a temporary, quasi-adiabatic transport associated with the compression of the magnetosphere while the second enhancement was due to slower diffusive processes. In the second part of the paper we compare the relativistic electron fluxes measured simultaneously at different local times. We find that the >2-MeV electron fluxes increased first at noon followed by dusk and then dawn and that there can be difference of two orders of magnitude in the fluxes observed at different local times. Finally, we discuss the development of data-driven models of the relativistic electron belts for space weather applications. By interpolating fluxes between satellites we produced a model that gives the >2-MeV electron fluxes at all local times as a function of universal time. In a first application of this model we show that, at least in this case, magnetopause shadowing does not contribute noticeably to relativistic electron dropouts.

Substorm injection of relativistic electrons to geosynchronous orbit during the great magnetic storm of March 24, 1991

The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50–300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron e × B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm.

Particle injections with auroral expansions

We compare the onset of dispersionless energetic particle injections, observed as a sudden increase of energetic (tens to hundreds of keV) electron and ion fluxes on a timescale of ∼1 min, with the start of auroral breakups. A total of 34 dispersionless injections observed by Los Alamos National Laboratory (LANL) satellites are analyzed, and their corresponding auroral breakups are determined with global auroral images acquired from the Polar ultraviolet imager. An important finding is that dispersionless injections can actually be associated with substorm intensification. The injection time at LANL relative to the start of auroral breakups varies from −2 to 8 min and can sometimes be more than 10 min. The average lag time for the injections compared to the auroral breakups is 1.8 min with a standard deviation of 2.5 min. It is suggested that particle energization must take place in the magnetotail ∼l min earlier than the start of the explosive auroral substorm onset, while the delay of the injections at LANL is due to a propagation effect. An implied average earthward injection boundary is estimated to be ∼ 6.9 – 9.2 RE. Further analysis of the delay time indicates that the transport of substorm injections is associated with the enhancement of convection electric field by a factor of ∼5, corresponding to an earthward convection flow speed of 5 – 120 km s−1. Dispersionless injections can take place in a fairly wide magnetic local time (MLT) region from 2000 to 0100 MLT with a peak at 2200 MLT, where auroral breakups occur most frequently. More importantly, dispersionless injections have ionospheric footprints clustered around the location of auroral breakup within ±1 hour of MLT, further supporting the concept of the close relationship between the substorm injections and the auroral breakups.

Model for the geostationary electron environment: POLE

Two and a half solar cycles of electron outer radiation belt measurements were analyzed. The data were acquired with the full range of Los Alamos National Laboratory, Los Alamos, NM, geostationary satellites, covering the period 1976-2001. Cross calibrations of the instruments were performed amongst themselves and referenced to CRRES observations. A model was derived [particle ONERA-LANL electron (POLE)] valid from 30 keV up to 2.5 MeV, which takes into account the solar cycle variation. In general, lower energy ( 500 keV) electrons show variations of over an order of magnitude. It is shown that the variability of these high energy electrons can change quite drastically from one solar cycle to the next.

The radial and longitudinal propagation characteristics of substorm injections

Abstract In this paper we present a statistical study of the radial and azimuthal propagation of substorm effects in the near-geosynchronous magnetotail. Data from five spacecraft (AMPTE/CCE, 1979-053, 1982-019, GOES-5, and GOES-6) have been used in our study. Since CCE has an apogee of 8.8 R E , those data allow for the study of both the radial and azimuthal propagation characteristics of substorm events. A list of ion injections was compiled from CCE energetic particle data obtained in 1985 and 1986. Those injections are dispersionless over an energy range of 25 keV to 285 keV on a 72-sec time scale. Dispersionless injections during which 1979-053 or 1982-019 were on the nightside in close longitudinal proximity to CCE were selected for our study. The most significant correlation in the data is between the local time separation between any two spacecraft and the time delay between the local onsets. On the other hand, in cases during which CCE was within 1 hour of local time from a geosynchronous satellite, there is little correlation between the radial separation between CCE and the geosynchronous satellite and the local onset time delay. Moreover, there is no evidence to suggest that local substorm onset propagates earthward. In fact, examination of data from several events during which there were unusually close alignments of spacecraft along the radial direction lends support to the view that local substorm onsets propagate radially outward.

Signatures of the substorm recovery phase at high‐altitude spacecraft

The substorm recovery phase typically commences ∼30 min after the substorm expansion phase onset and covers a period of roughly 1 hour. Several signatures have previously been associated with the recovery phase such as plasma sheet expansion in the midtail and magnetic field return toward the quiet time configuration. However, the detailed temporal sequence during the recovery phase is still not very well established. A total of 66 events in February-April 1979 have been investigated where ISEE 2 observed a plasma sheet expansion associated with fast earthward flows. Of these, 50 events were clearly associated with the substorm recovery phase as identified in ground magnetic records. For this data set, energetic electron (>30 keV) and proton (>145 keV) observations from two geostationary spacecraft were available in 41 cases. Of the 41 cases, 32 of the midtail plasma sheet recoveries were associated with distinctive ion or electron flux increases at geostationary orbit. These flux increases, often in both protons and electrons, were generally observed from the predusk to the postdawn sector. However, very few enhancements were found near local noon. The lack of large energy dispersion in the flux increases and the simultaneous occurrence of both electron and ion enhancements suggests that the particles do not drift from a more distant location (as in substorm expansion phase onsets) but are accelerated or redistributed locally. These events are suggested to be associated with a large-scale reconfiguration of the near-Earth tail as the neutral line retreats to large distances.

Midtail plasma flows and the relationship to near-Earth substorm activity : a case study

Recent simulations of magnetotail reconnection have pointed to a link between plasma flows, dipolarization, and the substorm current wedge. In particular, Hesse and Birn (1991) have proposed that earthward jetting of plasma from the reconnection region transports flux into the near-Earth region. At the inner edge of the plasma sheet this flux piles up, producing a dipolarization of the magnetic field. The vorticity produced by the east-west deflection of the flow at the inner edge of the plasma sheet gives rise to field-aligned currents that have region 1 polarity. Thus in this scenario the earthward flow from the reconnection region produces the dipolarization and the current wedge in a self-consistent fashion. In this study we examine observations made on April 8,1985 by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (IRM), the geosynchronous satellites 1979-053,1983-019, and 1984-037, and Syowa station, as well as AE. This event is unique because IRM was located near the neutral sheet in the midnight sector for an extended period of time. Ground data show that there was ongoing activity in the IRM local time sector for several hours, beginning at 1800 UT and reaching a crescendo at 2300 UT. This activity was also accompanied by energetic particle variations, including injections, at geosynchronous orbit in the nighttime sector. Significantly, there were no fast flows at the neutral sheet until the great intensification of activity at 2300 UT. At that time, IRM recorded fast earthward flow simultaneous with a dipolarization of the magnetic field. We conclude that while the aforementioned scenario for the creation of the current wedge encounters serious problems explaining the earlier activity, the observations at 2300 UT are consistent with the scenario of Hesse and Birn (1991). On that basis it is argued that the physics of substorms is not exclusively rooted in the development of a global tearing mode. Processes at the inner edge of the cross-tail current that cause a disruption of the current and a consequent dipolarization and current wedge may be unrelated to the formation of a macroscale reconnection region. Thus the global evolution of a substorm is probably a complicated superposition of such processes operating on a very localized scale and a global macroscale process that allows for such things as releasing the energy stored in lobe flux and the creation of plasmoids.

Premidnight plasmaspheric “plumes”

To explain observations of brief intervals of cold, dense plasma by geosynchronous satellites in the midnight sector prior to or during substorm onset, it has recently been proposed that dense plasmaspheric plasma is drawn out to geosynchronous orbit in the premidnight region by inductive electric fields during the growth phase of a geomagnetic substorm. We present here the results of a statistical study of such intervals observed with the Los Alamos magnetospheric plasma analyzer (MPA) on geosynchronous satellite 1989-046 between March and December 1993. We find that these premidnight cold plasma intervals occur only after extended periods of low magnetospheric activity identified by Kp and the midnight boundary index (MBI). We also find that the satellite typically enters the cold plasma region from the trough region and exits it into the plasma sheet. Finally, while measurements of the flow velocity of the cold plasma are rendered uncertain by the asymmetric spacecraft charge or photoelectron sheath, such measurements show no evidence of the outward flow that would be expected from the extrusion hypothesis. Rather, there are some indications that these cold plasma regions flow sunward in the (corotation) satellite frame. These results suggest an alternative explanation for the premidnight cold plasma: corotation-dominated transport of day side plasmaspheric structures into the premidnight sector. The implications of our observations for the extrusion hypothesis and for the alternative explanation are discussed.

The October 22, 1989, solar cosmic ray event measured at geosynchronous orbit

The authors compare proton measurements (5-150 MeV) made on two geosynchronous satellites during the October 22, 1989 solar cosmic ray event to those from ground-based neutron monitors. The satellite and ground instruments detected similar signatures in the solar protons for this event: an intense initial {open_quotes}spike{close_quotes} made up of two individual peaks followed by a longer, slower pulse. The height of the spike relative to the pulse was larger for higher measured energies. Most of the differences between the event`s characteristics as seen on the ground and in orbit may be attributed to the different energy regimes sampled by the different detectors: 450 MeV for the neutron monitors. The existence of the spike at low energies argues against an interpretation of the spike`s origin as primary solar neutrons. 12 refs., 5 figs., 1 tab.