Energetic Particle Data Research Articles

Ground‐satellite correlation of low‐latitude Pi 2 pulsations: A quasi‐periodic field line oscillation in the magnetosphere

Magnetometer data at dip‐equator and at geosynchronous altitudes (GOES 5 and 6 satellites) and energetic particle data from geosynchronous spacecraft (1982‐019, 1984‐037, 1984‐129) are studied to investigate field line and particle signatures associated with ground Pi 2 onset. Pi 2 pulsations observed at the ground station are discussed as field disturbances that axe launched in the midnight sector of the magnetosphere in association with field reconfiguration at substorm onset. In the geosynchronous orbit, both quasi‐periodic field line oscillations and particle injections are often observed in the nightside sector in association with the onset of ground Pi 2 pulsations. Of 216 Pi 2 events selected in this report, 16% showed associated quasi‐periodic field line oscillations. The dominant periods of the oscillations in space are similar to but not equal to those of the corresponding ground Pi 2 pulsations. The oscillations in space are polarized primarily in the azimuthal direction with a dc offset that develops to form a field‐aligned current signature, namely, eastward (westward) directed in the premidnight (postmidnight) sector. Simulation of forced field line oscillations has been carried out by adding the sudden increase in plasma pressure in the equatorial midnight sector region that is expected to come from particle injection at substorm expansion onset. We discuss the relevance of the quasi‐periodic oscillations to plasma injections at geosynchronous altitudes with reference to the result of the model calculations.

Observations of magnetospheric substorms occurring with no apparent solar wind/IMF trigger

An outstanding topic in magnetospheric physics is whether substorms are always externally triggered by disturbances in either the interplanetary magnetic field (IMF) or solar wind, or whether they can also occur solely as the result of an internal magnetospheric instability. Over the past decade, arguments have been made on both sides of this issue. Horwitz [1985] and McPherron et al. [1986] have shown examples of substorm onsets which they claimed were not externally triggered. However, as pointed out by Lyons [1995, 1996], there are several problems associated with these studies that make their results somewhat inconclusive. In particular, in the McPherron et al. study, fluctuations in the By component were not considered as possible triggers. Furthermore, Lyons suggests that the sharp decreases in the AL index during intervals of steady IMF/solar wind are not substorms at all but rather that they are just enhancements of the convection driven DP 2 current system that are often observed to occur during steady magnetospheric convection events. In the present study, we utilize a much more comprehensive data set (consisting of particle data from the Los Alamos energetic particle detectors at geosynchronous orbit, IMP 8 magnetometer and plasma data, Viking UV auroral imager data, midlatitude Pi 2 pulsation data, ground magnetometer data, and ISEE 1 magnetic field and energetic particle data) to show as unambiguously as possible that typical substorms can indeed occur in the absence of an identifiable trigger in the solar wind/IMF.

ULYSSES observations of energetic particle acceleration and the superposed CME and CIR events of November 1992

Abstract. During November 1992, a series of forward and reverse shocks passed the ULYSSES spacecraft. Spectral and anisotropy measurements are reported for protons and alpha particles between 0.28 and 6 MeV observed by the Energetic Particle Composition Experiment, data recorded by the Magnetometer Experiment and the high-energy (2.7–300 MeV) proton data from the Kiel Electron Telescope. An analysis of energetic particle, plasma and magnetometer data from ULYSSES has allowed a unique study of the corresponding arrival of fare particles, particles within a corotating interaction region and particles transported with a coronal mass ejection. We present an analysis of these data in terms of possible diffusive shock acceleration but conclude that this is likely to be incompatible with the short transit time of the particles. Shock drift acceleration of particles with energies 0.3 MeV/nucleon or solar acceleration followed by particle trapping behind the shock front are alternative possibilities.

Improved understanding of the Earth's radiation belts from the CRRES satellite

Energetic particle data gathered on the CRRES spacecraft have been used to produce new and more accurate models of high-energy electron and proton fluxes as well as total dose models out to geosynchronous altitude. In addition to providing the information necessary to improve designs and operations of near-Earth space systems, the models also give insight into the dynamic behavior of the radiation belts not considered in previous models. Sample orbit runs are compared to the earlier NASA models to elucidate their weaknesses. Areas of improved understanding in the radiation environment, gained from CRRES, and how they impact systems are summarized.

ULYSSES observations of energetic particle acceleration and the superposed CME and CIR events of November 1992

During November 1992, a series of forward and reverse shocks passed the ULYSSES spacecraft. Spectral and anisotropy measurements are reported for protons and alpha particles between 0.28 and 6 MeV observed by the Energetic Particle Composition Experiment, data recorded by the Magnetometer Experiment and the high-energy (2.7–300 MeV) proton data from the Kiel Electron Telescope. An analysis of energetic particle, plasma and magnetometer data from ULYSSES has allowed a unique study of the corresponding arrival of fare particles, particles within a corotating interaction region and particles transported with a coronal mass ejection. We present an analysis of these data in terms of possible diffusive shock acceleration but conclude that this is likely to be incompatible with the short transit time of the particles. Shock drift acceleration of particles with energies 0.3 MeV/nucleon or solar acceleration followed by particle trapping behind the shock front are alternative possibilities.

Solar Atmospheric Abundances and Energy Content in Flare-accelerated Ions from Gamma-Ray Spectroscopy

We used SMM gamma-ray data from 19 solar flares to study ambient elemental abundances in the solar atmosphere. We found that the abundance ratios of low FIP (first ionization potential) to high FIP elements are enhanced relative to photospheric abundances, but that the variability of these ratios from flare to flare is limited to a narrower range than that inferred from EUV and X-ray observations. The mean of the gamma-ray derived Mg/O (a low FIP to high FIP element abundance ratio) is coronal and the individual values are always higher than the photospheric Mg/O. The value of Ne/O (~0.25) is higher than the coronal value of 0.15 obtained from solar energetic particle data, but not inconsistent with some EUV and X-ray determinations. To avoid Ne/O higher than 0.3 a steep accelerated particle energy spectrum extending down to about 1 MeV per nucleon is needed. This implies that a large fraction of the available flare energy is contained in accelerated ions.

In situ observations of magnetotail reconnection prior to the onset of a small substorm

On April 15, 1979 at 0645 UT, several minutes prior to the expansion phase onset of a weak substorm, the ISEE 1 and 2 spacecraft were located 16 RE downtail and close to local midnight. The two spacecraft were on opposite sides of the current sheet, allowing us to better constrain the possible interpretations of the available plasma, magnetic field, and energetic particle data. Plasma sheet acceleration occurring earthward of the spacecraft was evident in the energetic particle data 2–3 min before the first ground‐based signatures of substorm onset. The ensuing magnetic field and particle changes are consistent with an X‐type neutral line moving tailward of ISEE 1 and 2 with an apparent speed of ∼30 km/s, while the plasma sheet thickness decreased to a minimum of less than 0.2 RE. Although the substorm effects at geosynchronous altitude and on the ground were weak, the reconnection rate was locally quite intense (>1 mV/m) and produced earthward bursty bulk flows of peak velocity >600 km/s. Ion velocity‐space distributions confirm that the flows are indeed convective flows of a single‐ion population and not counterstreaming beams, as is often the case at the plasma sheet boundary. The fast tailward flows observed at the initial stage of reconnection were localized to a thin layer surrounding the neutral sheet. The magnitude of the current disruption and the region 1 sense field‐aligned currents associated with reconnection were sufficiently large to explain the intensity of the substorm current wedge, as inferred from ground magnetometers.

Spectroscopic measurements of element abundances in the solar corona: Variations on the FIP theme

Solar wind and solar energetic particle (SEP) data yield systematic differences between elemental abundances in the corona and in the photosphere related to the first ionization potential (FIP) of the elements: low-FIP elements are preferentially enhanced relative to high-FIP elements by about a factor of four. Spectroscopic studies of the inner corona show that such a pattern may apply on average but not in detail for coronal loops: substantial abundance differences occur between different types of coronal structures, and variations have been found from flare to flare, from one active region to another, and over time in the same region; further, in some flares, anomalies such as enhanced Ne:O ratios, distinctly at odds with the FIP pattern, show that a competing element selection mechanism sometimes operates. Details of the observed abundance variability — such as the magnitude of the variations, the relevant temporal and spatial scales, and correlations with other properties of the given coronal structure — may give important clues to the processes which supply and heat the corona, or they may reflect the changing physical conditions or locations where those processes take place. However, many such details remain to be established definitively. At present, abundance variability is primarily a major complication to data analysis and interpretation. However, once it is better understood, it may provide a new diagnostic tool for probing the lower layers of the solar atmosphere.

Initial GEOTAIL survey of magnetic substorm signatures in the magnetotail

Substorm signatures in the magnetotail at radial distances from −30 to −210 Re are studied using magnetic field data acquired with the GEOTAIL spacecraft. The current data set consists of GEOTAIL magnetic field data transmitted in real time to Japan from October 1992 to September 1993, and the corresponding GMS‐4 (140°E geographic, geostationary) energetic particle data and 1‐s Kakioka (L=1.2, 140°E) magnetic field record. We first identify substorms from major increases in the particle flux at the geostationary satellite, then determine accurate onset times from Pi 2 pulsations at the Kakioka ground station, and finally examine the corresponding magnetic field signatures at GEOTAIL. Eighty‐nine substorm events occurred when GEOTAIL was either in the plasma sheet or in the tail lobe, and these events were analyzed with a special emphasis on the temporal development of the north‐south magnetic field component Bz. A clear bipolar signature (northward perturbation followed by southward) is observed in Bz for 53 events. The bipolar events have characteristics consistent with the plasmoid or the traveling compression region reported in earlier studies. In the middle and distant tail (X<−60 Re), most of events seem to indicate the passage of a plasmoid. The present results provide evidence for near‐Earth reconnection and the formation and release of plasmoids in association with substorm onsets. A simple model yields the following plasmoid parameters: near‐Earth reconnection around X=−30 Re, the center of the newly‐created plasmoid around X=−60 Re, and the expansion of the tailward‐moving plasmoid to a dimension of 100 Re in the X direction and almost fully across the distant tail (X=−200 Re).

Tailward energetic ion streams observed at ∼100 RE by GEOTAIL‐EPIC associated with geomagnetic activity intensification

We report energetic particle (EPIC instrument) and magnetic field (MGF instrument) measurements collected onboard GEOTAIL located at ∼100 RE downstream in the magnetotail during a 13‐hour long active period on January 20–21, 1993. During this period, a series of 6 geomagnetic activity intensifications (breakup or pseudo‐breakup) have been studied with ground magnetograms and energetic particle data at geosynchronous orbit. We show that: (1) as a general trend, the level of the energetic ion fluxes was related to the intensity of the westward electrojet; (2) a systematic temporal (within a few minutes) correlation exists between energetic ion flux enhancements observed by GEOTAIL/EPIC in the distant tail and near‐Earth geomagnetic activity intensification onsets; and (3) for 5 of 6 cases, these energetic ion flux enhancements corresponded to strong tailward streams which included particles of both solar wind and ionospheric origin. Their amplitude seemed also to be related to the electrojet intensity change.

Prognoz 10 energetic particle data: Leakage from the magnetosphere versus bow shock acceleration

We present a case study and a statistical survey of Prognoz 10 energetic particle and magnetic field observations from the solar wind to the magnetosphere. The results provide evidence for both Fermi acceleration and leakage from the magnetosphere. Energetic ion (E > 15 keV) flux levels upstream of the bow shock and in the outer magnetosphere generally exceed those in the magnetosheath. Energetic electrons rarely accompany ion fluxes in the outer magnetosheath and upstream of the bow shock, but commonly accompany those in the inner magnetosheath. Upstream flux levels increase as the interplanetary magnetic field (IMF) rotates toward the bow shock normal, consistent with Fermi acceleration. Although the north/south magnetosheath magnetic field orientation has no effect upon energetic particle flux levels in the outer magnetosheath, flux levels in the inner magnetosheath increase during periods of southward IMF. We interpret the results as indicating that Fermi acceleration supplies most of the ions observed in the outer dayside magnetosheath but that leakage from the magnetosphere supplies most of the ions observed in the inner magnetosheath.

Freja's contribution to the ISTP event of October 27, 1992. A distorted magnetosphere

A unique arrangement of satellites and ground‐based magnetometers has provided the opportunity to study the distortion of the magnetosphere by conditions of elevated solar wind dynamic pressure and high geomagnetic activity on October 27, 1992. Data were acquired by the near‐Earth satellites Freja, UARS, DMSP‐F8 and DMSP‐F11, and by IMP‐8. This study will focus on patterns of large‐scale Birkeland currents deduced from the APL magnetic field experiments on Freja and UARS. Because of the relatively low inclinations of their orbits (63° for Freja and 57° for UARS) these satellites skim the current systems over a wide range of local time and provide a kind of “global image” of the Birkeland currents. Energetic particle data from the DMSP satellites are used in an attempt to complete the image of the auroral region. The principal findings of this study include the following. The dayside Birkeland current system is displaced 6° to 9° equatorward of the statistical pattern determined by Iijima and Potemra [1978] for undisturbed conditions (|AL|< 100 nT) and 3° equatorward of their disturbed statistical pattern (|AL|≥ 100 nT). The densities of these daytime Birkeland currents are estimated to be 1 to 2 µA/m², comparable to, but not larger than the statistical values determined by Iijima and Potemra for disturbed conditions. The nightside Birkeland currents are located close to the Iijima and Potemra disturbed statistical pattern and they are less intense than the dayside currents.

Effects of the intense solar activity of March/June 1991 observed in the outer heliosphere

The properties of the large‐scale global merged interaction region (GMIR) generated by the intense solar events of March and June 1991 are studied using the available solar wind, interplanetary magnetic field, and energetic particle data from the observing network of Pioneer 10 and Voyagers 1 and 2 in the outer heliosphere. At heliocentric distances extending to 55 AU the delayed effects of this enhanced solar activity are observed in the form of large increases in the solar wind velocity and interplanetary magnetic field and significant decreases in the galactic cosmic ray intensity. For low‐energy ions (5‐MeV protons) there was a single long‐lived event extending over a period of some 6 months. Near the strongest interplanetary disturbances the H and He spectra are best represented by similar exponentials in momentum/nucleon (i.e., particle velocity at these energies). Over the rest of the event the characteristic momentum for He, (P0)He is generally ∼0.66 for hydrogen. These spectra and the consistently low H/He ratio (25.3) at 2 MeV/nucleon closely resemble that observed in corrotating interaction regions events. Despite the strong north/south asymmetry in the solar activity, the interplanetary disturbances produced the same net decrease in the galactic cosmic ray intensity of ions >70 MeV at the three widely separated spacecraft when the effects of the long‐term recovery are taken into account. A comparison of the relative intensity of MeV ions at these three spacecraft suggest that the most intense solar events occurred on the back side of the Sun in time periods adjacent to the March and June episodes of solar activity. It is argued that this GMIR as a system is responsible for the low‐frequency radio emission observed by the Voyager Plasma Wave experiment some 1.46 years after the onset of the March 1991 activity.

Magnetospheric field and current distributions during the substorm recovery phase

We have studied 11 substorm recovery phase events in which magnetic field and energetic particle data were available near the midnight sector from the GEOS 2 satellite. Comparison with the Tsyganenko magnetic field model shows that, after the expansion phase, BZ is large and decreases gradually toward the model value during the recovery phase, whereas deviations of BX and BY relative to the model values are small after the effects of the substorm current wedge have disappeared. We have modeled this sequence by using temporally evolving current systems implemented as additions to the Tsyganenko model. The tail current sheet thickness and the cross‐tail current intensity at different radial distances were varied using six free parameters in the model. The parameters were evaluated using a least squares fit for each of the 11 events separately. The results suggest that at the beginning of the recovery phase the current sheet was relatively thick close to the inner edge of the plasma sheet. Model fittings produced two different field configurations. In seven events the cross‐tail current was weak, and the field configuration was highly dipolar. In four events the near‐Earth current was weak, but stronger currents remained in the midtail region. In these latter events the field configuration at the beginning of the recovery phase included a region where BZ was negative. This negative BZ and the associated near‐Earth neutral line disappeared later as the current system developed toward the quiet time configuration. The magnetic field configuration, current distributions, and particle drift paths during the substorm recovery phase are examined and compared with those prevailing during the substorm growth phase.

Particle and field signatures during pseudobreakup and major expansion onset

The temporal and spatial scales of the onset of two types of substorm events are investigated. These substorms were cases where the expansion onset had precursor localized auroral activation without significant negative bay enhancement, that is, “pseudobreakup”. High‐resolution energetic particle and magnetic field data at synchronous orbit are used for the analysis together with auroral and magnetic field data simultaneously taken from ground‐based instrumentation. The auroral structure following the pseudobreakup significantly resembled the major expansion aurora, except in its spatial scale. Typical magnetospheric onset signatures such as tail current diversion, dipolarization, and injection were observed associated with some of the pseudobreakups. The major expansion, on the other hand, consisted of a number of rather localized injections and expansions, each of which had timescales of 2‐8 min, a comparable timescale to that of pseudobreakups. This study shows that there does not appear to be any phenomenological differences between pseudobreakups and major expansion onsets. The major difference between pseudobreakups and major expansion onsets would be the number of occurrences, as well as the intensity and the scale size of the magnetospheric source.

Energetic particle absorption by atmosphereless satellites and rings

In outer planet magnetospheres, theoretical estimates of the rate of energetic particle absorption by solid satellites and rings have two primary applications. First, in regions where satellite absorption is the dominant energetic particle loss process, model calculations combined with measured fluxes and derived phase space densities can be employed to constrain the amplitude and L-dependence of the magnetospheric radial diffusion process that is responsible for populating the radiation belts. Since only flyby data are available, this approach provides the only present means for probing the dynamics of outer planet radiation belts. Second, if the rate of radial diffusion is independently constrained, observed ring absorption signatures can be combined with model calculations to solve for certain undetermined parameters such as the mean particulate size in a planetary ring. Examples of applications to radial diffusion in the Uranian radiation belts and to the interpretation of the Jovian ring absorption signature in Pioneer 11 energetic particle data are discussed.

ISEE 3 observations of traveling compression regions in the Earth's magnetotail

A traveling compression region (TCR) is a several‐minute long compression of the lobe magnetic field produced by a plasmoid as it moves down the tail. They are generally followed by a longer interval of southward tilting magnetic fields. This study reports the first comprehensive survey of TCRs in the distant magnetotail. A total of 116 TCRs were identified in the ISEE 3 magnetic field observations. Of this population, 37 TCRs were observed to be separated by 30 min or more from any other TCR and are termed “isolated” events. “Paired” events are defined as two TCRs separated by less than 30 min. There were 36 such TCRs corresponding to 18 paired events. “Multiple” events were also observed in which more than two TCRs occurred in a series without a gap between TCRs of more than 30 min. The 11 multiple events identified in this study had an average of about four traveling compression regions each for a total of 43 TCRs. The mean amplitude, ΔB/B, and duration, ΔT, for all TCRs were found to be 7.6% and 158 s, respectively. TCRs occurring as isolated events were the largest (ΔB/B = 8.8% and ΔT = 218 s) and those associated with multiple events were the smallest (ΔB/B = 5.6% and ΔT = 84 s). The mean duration of the period of southward tilting Bz following isolated TCRs was 12.3 min. This time interval was found to be quite similar to the average spacing between TCRs in paired and multiple events, 11.2 and 10.2 min, respectively. TCR amplitude and duration were found to be independent of location within the tail lobes suggesting that the plasmoids which cause the TCRs maintain approximately constant volume and shape as they move down the tail. Mean plasmoid dimensions estimated from TCR duration and amplitude under the assumption of a quasi‐rigid magnetopause are 35 RE (length) × 15 RE (width) × 15 RE (height). Utilizing auroral kilometric radiation, the AL index, Pi 2 pulsations at two ground stations, and energetic particle data from three geosynchronous spacecraft, it is found that over 91% of the TCR events identified in this study followed substorm onsets or intensifications. The number of TCR events identified in this study are consistent with their release in association with a new substorm onset every 4‐6 hrs. The results of this study strongly suggest that the release of plasmoids down the tail near the time of expansion phase onset is an integral step in the substorm process and an important element in the substorm energy budget.

Cosmic ray decreases and magnetic clouds

A study has been made of energetic particle data, obtained from IMP 8, in conjunction with solar wind field and plasma data at the times of reported magnetic clouds. It is shown that magnetic clouds can cause a depression of the cosmic ray flux but high fields are required. A depression of 3% in a neutron monitor requires a field of about 25 nT. Such high fields are found only in a subset of coronal ejecta. The principal cause for Forbush decreases associated with energetic shocks is probably turbulence in the postshock region, although some shocks will be followed by an ejecta with a high field. Each event is different. The lower‐energy particles can help in identifying the dominant processes in individual events.

Global characteristics of particle precipitation and field‐aligned electron acceleration during isolated substorms

Global features of particle precipitation and field‐aligned electron acceleration during isolated substorms were investigated using energetic particle data from the DMSP F6 and F7 satellites. The onset time and the recovery phase of each isolated substorm were determined using both the AU and AL indices and low‐latitude Pi 2 magnetic pulsations. It is found that during the recovery phase the poleward edge of the precipitation region of central plasma sheet (CPS) type electrons expands to latitudes higher than 72° around midnight, while the equatorward boundary of this region expands to latitudes lower than those before the substorm onset in the postmidnight sector of 00‐06 MLT. In the recovery phase, many accelerated electron precipitation events were embedded in the CPS‐type electron precipitation region for the nightside local times from 18 MLT to 09 MLT. Several different kinds of latitudinal dispersion of electron and ion energy were observed near the lowest latitude of the particle precipitation region. The features vary significantly depending on the magnetic local time. These energy dispersion features can be explained by injected particle motions in the magnetosphere. These energy dispersion features also show a possibility of some particle injections in the magnetotail even before the substorm onset. The field‐aligned potential differences, estimated from the peak energy of accelerated electron energy spectra, do not change on the morningside before and after the substorm onset, while they increase up to several kilovolts on the eveningside during the recovery phase. On the other hand, the field‐aligned potential differences were estimated for the accelerated electron precipitation events embedded in the CPS‐type electron precipitation region and were found to be slightly larger on the morningside than on the eveningside during the recovery phase. These features of field‐aligned potential differences can be explained by assuming that the field‐aligned electric field is formed to maintain the large‐scale region I and II currents. The electron densities above the field‐aligned potential difference were also estimated from an accelerated Maxwellian fitting procedure. They do not change clearly before the onset and in the recovery phase. This fact suggests that the supply and loss of electrons into the loss cone at magnetospheric altitudes are balanced during the substorms.

Magnetic field models from energetic particle data at Neptune

The locations of features in the Voyager 2 energetic particle data from Neptune are combined with uncertainties in the multipole expansion of the planetary magnetic field to derive new magnetic field models that are consistent both with various interpretations of the particle features and with the magnetic field data. While assumptions as to the origin of the features must be made, they do not provide sufficient constraints to obtain significant new information on any of the unknown multipole coefficients. However, the magnetic L shell positions of the particle features, which are interpreted primarily as absorption signatures of Neptune's satellites, can, in general, be brought into agreement with expected values.