Ioannis A. Daglis

Current understanding of magnetic storms : Storm-substorm relationships

This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high-intensity, long-duration, continuous auroral electrojet activity) and the so-called Russell-McPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood. (2) A serious controversy exists as to whether the successive occurrence of intense substorms plays a direct role in the energization of ring current particles or whether the enhanced electric field associated withmorexa0» southward IMF enhances the effect of substorm expansions. While most of the {ital Dst} variance during magnetic storms can be solely reproduced by changes in the large-scale electric field in the solar wind and the residuals are uncorrelated with substorms, recent satellite observations of the ring current constituents during the main phase of magnetic storms show the importance of ionospheric ions. This implies that ionospheric ions, which are associated with the frequent occurrence of intense substorms, are accelerated upward along magnetic field lines, contributing to the energy density of the storm-time ring current. An apparently new controversy regarding the relative importance of the two processes is thus created. It is important to identify the role of substorm occurrence in the large-scale enhancement of magnetospheric convection driven by solar wind electric fields. (3) Numerical schemes for predicting geomagnetic activity indices on the basis of solar/solar wind/interplanetary magnetic field parameters continue to be upgraded, ensuring reliable techniques for forecasting magnetic storms under real-time conditions. There is a need to evaluate the prediction capability of geomagnetic indices on the basis of physical processes that occur during storm time substorms. (4) It is crucial to differentiate between storms and nonstorm time substorms in terms of energy transfer/conversion processes, i.e., mechanical energy from the solar wind, electromagnetic energy in the magnetotail, and again, mechanical energy of particles in the plasma sheet, ring current, and aurora. To help answer the question of the role of substorms in energizing ring current particles, it is crucial to find efficient magnetospheric processes that heat ions up to some minimal energies so that they can have an effect on the strength of the storm time ring current. (5) The question of whether the {ital Dst} index is an accurate and effective measure of the storm time ring-current is also controversial. In particular, it is demonstrated that the dipolarization effect associated with substorm expansion acts to reduce the {ital Dst} magnitude, even though the ring current may still be growing. {copyright} 1998 American Geophysical Union«xa0less

Fast ionospheric response to enhanced activity in geospace: Ion feeding of the inner magnetotail

The role of the ionospheric particle source in the dynamic processes of the geospace was neglected, underestimated, or questioned for a long period. Recent studies have indicated a fast and effective feeding of the magnetosphere with energetic ionospheric-origin ions during periods of enhanced auroral electrojets, i.e., during the expansion and the late growth phase of substorms. Such a feeding with ionospheric ions can alter the plasma composition in the geospace system and thus influence its dynamics and evolution during substorms and magnetic storms. This paper addresses the issue of short-timescale (rather than long-term) feeding of the magnetosphere with ionospheric ions. We consider this feeding to be a result of the ionospheric response to increased auroral current dissipation due to increased solar wind - magnetosphere coupling or increased magnetospheric activity. We review and combine old and recent observations of both the ionospheric outflow and the ionospheric ions in the magnetosphere, along with transport and energization mechanisms for the relatively cold ionospheric outflowing ions and the potential implications of their enhanced presence in the magnetosphere for the dynamics of geospace processes (substorms/storms). We suggest that a fast feeding of the near magnetotail with ionospheric ions can lead to a transient localized dominance of heavy ionospheric ions (namely O + ) and consequently to an ionospheric regulation of the evolution of dynamic geospace processes. Accordingly, we suggest that the role of the ionosphere within the dynamic geospace coupling be given more emphasis : the ionosphere should be regarded not only as a material source for the magnetosphere but also as an active element in dynamic geospace processes.

RAPID: The imaging energetic particle spectrometer on Cluster

The RAPID spectrometer (Research with Adaptive Particle Imaging Detectors) for the Cluster mission is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20–400 keV for electrons, 40 keV–1500 keV (4000 keV) for hydrogen, and 10 keV nucl-1–1500 keV (4000 keV) for heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for either species. Identification of the ionic component (particle mass A) is based on a two-dimensional analysis of the particles velocity and energy. Electrons are identified by the well-known energy-range relationship. Details of the detection techniques and in-orbit operations are described. Scientific objectives of this investigation are highlighted by the discussion of selected critical issues in geospace.

Energy density of ionospheric and solar wind origin ions in the near-Earth magnetotail during substorms

Comprehensive energy density studies provide an important measure of the participation of various sources in energization processes and have been relatively rare in the literature. We present a statistical study of the energy density of the near-Earth magnetotail major ions (H+, O+, He++, He+) during substorm expansion phase and discuss its implications for the solar wind/magnetosphere/ionosphere coupling. Our aim is to examine the relation between auroral activity and the particle energization during substorms through the correlation between the AE indices and the energy density of the major magnetospheric ions. The data we used here were collected by the charge-energy-mass (CHEM) spectrometer on board the AMPTE/CCE satellite in the near-equatorial nightside magnetosphere, at geocentric distances ∼7-9 RE. CHEM provided the opportunity to conduct the first statistical study of energy density in the near-Earth magnetotail with multispecies particle data extending into the higher energy range (≥ 20 keV/e). The use of 1-min AE indices in this study should be emphasized, as the use (in previous statistical studies) of the (3-hour) Kp index or of long-time averages of AE indices essentially smoothed out all the information on substorms. Most distinct feature of our study is the excellent correlation of O+ energy density with the AE index, in contrast with the remarkably poor He++ energy density - AE index correlation. Furthermore, we examined the relation of the ion energy density to the electrojet activity during substorm growth phase. The O+ energy density is strongly correlated with the pre-onset AU index, that is the eastward electrojet intensity, which represents the growth phase current system. Our investigation shows that the near-Earth magnetotail is increasingly fed with energetic ionospheric ions during periods of enhanced dissipation of auroral currents. The participation of the ionosphere in the substorm energization processes seems to be closely, although not solely, associated with the solar wind/magnetosphere coupling. That is, the ionosphere influences actively the substorm energization processes by responding to the increased solar wind/magnetosphere coupling as well as to the unloading dissipation of stored energy, with the increased feeding of new material into the magnetosphere.

Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in the course of an isolated substorm event

On January 15, 1994, the ion spectrometer high energy particle - low energy particle detector (HEP-LD) on the Japanese spacecraft Geotail observed five quasi-periodic energetic ion bursts in the deep tail (X=−96 RE). These bursts were associated with plasmoid-like structures in the magnetic field components. In. addition, three multiple TCR groups were identified in the interval. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP-LD observations show that Bz bipolar plasmoid-like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are predominantly associated with bipolar signatures in By. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and Bz bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP-LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam-like structure which occupied only 6 ∼ 7% of the full solid angle (4π). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid-plasma sheet which trails the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger quantities of energy during this very intense substorm event by ejecting multiple relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.

Energetic ion distributions on both sides of the Earth's magnetopause

The AMPTE/CCE spacecraft, with an apogee of ∼8.8RE and an inclination of ∼4.3°, sampled the outer dayside equatorial magnetosphere for extended time periods and often crossed into the magnetosheath whenever the solar wind pressure was sufficiently high to compress the magnetopause to 10 keV and an earthward gradient in the subsolar magnetosheath. In addition to the steady state magnetosheath population there exists a burst-type component indicative of a magnetospheric source, and most of the time this is recognized as a flux transfer event. Overall, the results about the origin of the ≥50 keV magnetosheath ions are consistent with the continuous leakage of magnetospheric particles across a tangential discontinuity magnetopause, locally distributed according to magnetospheric drift paths. Magnetic reconnection, although present, should not be a dominant source on average, because it is not continuous in time. Fermi acceleration should not be dominant because it predicts the opposite local time asymmetry, and shock drift acceleration should be a minor contributor at E≥50 keV because of upper-energy cutoff limitations. Our observations also indicate a significant magnetospheric contribution to energies as low as ∼10 keV, where the magnetosphere-magnetosheath intensity gradient reverses. However, in order to examine the relative strength and local time distribution of all possible sources at these energies, a detailed analysis is required.

Tailward flowing energetic oxygen ion bursts associated with multiple flux ropes in the distant magnetotail: GEOTAil observations

An event of tailward flowing energetic (144–7959 keV) oxygen ion bursts was observed in the distant magnetotail (X=−63, Y=+7, Z=−3.8 RE) on February 13, 1994. The observations were made with the HEP-LD spectrometer on board the GEOTAIL spacecraft. The event was associated with magnetic field signatures characteristic of multiple flux ropes. During the event, which lasted from 1847 to 1907 UT, strong impulsive increases in the oxygen flux were observed. From 1846 to 1900 UT the proton counting rate also exhibited an increase, followed by a decrease until the end of the oxygen event. The oxygen flux was confined to a rather narrow range in polar and azimuthal angle (only 7–10% of 4π was occupied). This implies a streaming distribution or beam-like structure. Comparison of the particle flow angles with the polar and azimuthal angles of the magnetic field indicates that the ion beam may have been embedded in flux ropes, which may be connecting the polar ionosphere and the distant magnetotail. During the observed oxygen event the ratio is significantly higher than the ratios usually found in the center of the distant magnetotail. There is some evidence that the observed oxygen ions were more efficiently accelerated in this event than hydrogen and helium ions.

Model calculation of energetic neutral atoms precipitation at low altitudes

It has become apparent during recent years that global magnetospheric imaging through the detection of energetic neutral atoms (ENA) will be a crucial tool for the advancement of magnetospheric research via remote sensing technique. This report presents model calculations of ENA precipitation as detectable from a satellite orbiting at low altitudes (<1500 km). The ENA generation via charge exchange between energetic ions and geocoronal neutrals is simulated in the inner magnetosphere, between 1.5RE and 10RE, where both neutral densities and ion fluxes are significantly high. AMPTE/CCE energetic ion measurements and the geocoronal neutral H altitude profile are used to simulate the charge exchange process leading to ENA production. We discuss the relative contribution of ring current and radiation belts, as well as the relative contribution of H+ and O+ to the ENA generation.

Ionospheric contribution to the cross-tail current enhancement during the substorm growth phase

Abstract We present a growth phase case study in the near-Earth nightside magnetosphere with AMPTE/CCE data. The observations indicate an increase of the difference between parallel and transverse pressures of the particle distribution, which can lead to or favour the enhancement of the near-Earth cross tail current, necessary for the magnetic field configuration changes during the growth phase. The highest relative pressure enhancement is observed for the ionospheric origin O + ions, indicating an active participation of the ionosphere in the substorm growth phase.

Energetic neutral atoms in the outer magnetosphere: An upper flux limit obtained with the HEP-LD spectrometer on board GEOTAIL

The energetic particle spectrometer HEP-LD on board the Japanese mission GEOTAIL is the first instrument in the energy range above 77 keV which can discriminate between energetic neutral atoms (ENA) and charged particles. The principle for particle identification is based on a time-of-flight (T) and energy (E) measurement which uniquely determines the particle species. A rather compact and highly anisotropic active collimation system in front of the (E,T)-detector head acts as an effective charged particle rejector (CPR) in the ENA mode. The resulting high-pass filter function for charged particles allows the detection of ENAs with energies below 200 keV. This report concentrates on ENA observations in the outer magnetosphere on the dayside (GSE x=−3, y=+12, z=+1 RE) and establishes an upper limit for the ENA flux in quiet conditions. The measurements agree reasonable well with predictions, however, the statistical uncertainties are considerable.