Gordon Rostoker

What is a geomagnetic storm

After a brief review of magnetospheric and interplanetary phenomena for intervals with enhanced solar wind-magnetosphere interaction, an attempt is made to define a geomagnetic storm as an interval of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial energization in the magnetosphere-ionosphere system, to an intensified ring current sufficiently strong to exceed some key threshold of the quantifying storm time Dst index. The associated storm/substorm relationship problem is also reviewed. Although the physics of this relationship does not seem to be fully understood at this time, basic and fairly well established mechanisms of this relationship are presented and discussed. Finally, toward the advancement of geomagnetic storm research, some recommendations are given concerning future improvements in monitoring existing geomagnetic indices as well as the solar wind near Earth.

On the origin of relativistic electrons in the magnetosphere associated with some geomagnetic storms

There has been considerable interest of late in the sudden appearance of large fluxes of relativistic electrons (E>1 MeV) at geostationary orbit due to the adverse effect these electrons can have on operational spacecraft. The question arises as to how these electrons are accelerated to relativistic energies in the relatively short time of a few hours. We shall show that large amplitude ULF pulsations are a unique feature of intervals of time in which the fluxes of relativistic electrons rise to high levels at radial distances beyond the normal L-shell range occupied by the outer radiation belt. We use SAMPEX polar orbiter data for the magnetic storm of November 1993 to show that the fluxes of E>400 keV electrons increase simultaneously over a broad range of L-shells and use this fact to suggest that large amplitude ULF pulsations have the potential to supply the energy necessary to create the relativistic electron fluxes.

Canopus — A ground-based instrument array for remote sensing the high latitude ionosphere during the ISTP/GGS program

Proper interpretation ofin situ satellite data requires a knowledge of the global state of the magnetosphere-ionosphere system. CANOPUS is a large-scale array of remote sensing equipment monitoring the high latitude ionosphere from the north-central to the north-west portion of North America. The array comprises thirteen magnetometers and riometers four meridian scanning photometers, a digital allsky imager and an auroral radar linked by geostationary satellite to a central receiving node in Ottawa, where the data are archived and made available in near real time to participating scientists. This paper provides a technical description of the various instruments in the CANOPUS array, and contains a summary of the key parameters which will be provided to the Central Data Handling Facility (CDHF) located at NASA/Goddard Space Flight Center, for use by the ISTP/GGS community.

Recurrent geomagnetic storms and relativistic electron enhancements in the outer magnetosphere: ISTP coordinated measurements

New, coordinated measurements from the International Solar-Terrestrial Physics (ISTP) constellation of spacecraft are presented to show the causes and effects of recurrent geomagnetic activity during recent solar minimum conditions. It is found using WIND and POLAR data that even for modest geomagnetic storms, relativistic electron fluxes are strongly and rapidly enhanced within the outer radiation zone of the Earths magnetosphere. Solar wind data are utilized to identify the drivers of magnetospheric acceleration processes. Yohkoh solar soft X-ray data are also used to identify the solar coronal holes that produce the high-speed solar wind streams which, in turn, cause the recurrent geomagnetic activity. It is concluded that even during extremely quiet solar conditions (sunspot minimum) there are discernible coronal holes and resultant solar wind streams which can produce intense magnetospheric particle acceleration. As a practical consequence of this Sun-Earth connection, it is noted that a long-lasting E>1MeV electron event in late March 1996 appears to have contributed significantly to a major spacecraft (Anik E1) operational failure.

The roles of direct input of energy from the solar wind and unloading of stored magnetotail energy in driving magnetospheric substorms

This paper presents the consensus arrived at by the authors with respect to the contributions to the substorm expansive phase of direct energy input from the solar wind and from energy stored in the magnetotail which is released in a sometimes unpredictable manner. Two physical processes, neither of which can be ignored, are considered to be of importance in the dispensation of the energy input from the solar wind. One of these is the ‘driven process’ in which energy, supplied from the solar wind, is directly dissipated in the ionosphere with the only clearly definable delay being due to the inductance of the magnetosphere-ionosphere system. The other is the ‘loading-unloading process’ in which energy from the solar wind is first stored in the magnetotail and then is suddenly released to be deposited in the ionosphere as a consequence of external changes in the interplanetary medium or internal triggering processes. Although the driven process appears to be more dominant on a statistical basis in terms of solar wind-geomagnetic activity relationships, one or the other of the two above processes may dominate for any individual cases. Moreover, the two processes may operate simultaneously during a given phase of the substorm, e.g., the magnetotail may experience loading as the driven system increases in strength. Thus, in our approach, substorms are described in terms of physical processes which we infer to be operative in the magnetosphere and the terminology of the past (e.g., phases) is related to those inferred physical processes. The pattern of substorm development in response to changes in the interplanetary medium is presented for a canonical isolated substorm.

Internal acceleration of relativistic electrons by large‐amplitude ULF pulsations

The flux of relativistic electrons in geostationary orbit exhibits a variability closely regulated by the solar wind, but the acceleration mechanism of relativistic electrons remains poorly understood. Recent observational evidence has shown that the intensification of relativistic electrons often takes place in a matter of several hours. The rapidity is difficult to reconcile with traditional diffusion-based models which often take days to produce appropriate high-energy electron fluxes and motivates us to search for alternative mechanisms of internal acceleration. Elaborating on the observation in an earlier paper of Rostoker et al. [1998], we propose that global oscillation of magnetosphere in the Pc4–5 range is capable of accelerating electrons under the catalysis of random pitch angle scattering. This view is developed theoretically and computationally in this paper. The most noteworthy result of the investigation is the demonstration that magnetic pumping by ULF waves can lead to the observed high relativistic electron flux in a time as short as a few hours under parameters appropriate for major magnetic storms. Further development and test of this theory are discussed.

A strong CME‐related magnetic cloud interaction with the Earth's Magnetosphere: ISTP observations of rapid relativistic electron acceleration on May 15, 1997

A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (BZ∼−25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst ∼−170 nT). SAMPEX data showed that relativistic electrons (E ≳ 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4. These electrons were not directly “injected” from higher altitudes (i.e., from the magnetotail), nor did they come from an interplanetary source. The electron increase was preceded (for ∼2 hrs) by remarkably strong low-frequency wave activity as seen by CANOPUS ground stations and by the GOES-8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high-energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi-point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi-MeV energies deep in the radiation belts on timescales of tens of minutes.

Phenomenology and physics of magnetospheric substorms

This paper provides a phenomenological and theoretical framework for understanding magnetospheric substorms based on the boundary layer dynamics model developed by Rostoker and Eastman [1987] updated to be consistent with modern observations. The model is designed to account for both the directly driven and storage-release aspects of substorm activity. The essence of the model is that enhanced frontside reconnection leads to both growth of the directly driven electrojets and storage of energy in the magnetic field and particle drift in the tail. The growth of particularly intense cross-tail current near the inner edge of the plasma sheet is terminated by an abrupt collapse marking the onset of the expansive phase of the substorm. This collapse normally takes place in the midnight sector inside ∼ −12 RE. The collapse of this cross-tail current is consistent with the breakdown of shielding associated with region 2 field-aligned currents. The expansive phase proceeds through antiearthward propagation in the tail of a wave triggered by the collapse. When the wave reaches the neutral line in the tail, it enhances the reconnection rate leading to stronger velocity shear along the interface between the low latitude boundary layer and central plasma sheet. This velocity shear is responsible for activity at the high latitude edge of the expanded auroral oval. The physics of the processes characterizing the model requires a reexamination of the Harang discontinuity as it would appear that there are actually two regions of electric field reversal in the nightside magnetosphere both of which play important roles in the substorm process.

Substorm onset timing: The December 31, 1995, event

The objective of the present study is to examine the timing of various onset-associated signatures and address the cause-and-effect relationship between the formation of a near-Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = −30.3 RE in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950-km/s) tailward convection flow accompanied with southward Bz (< −10 nT) indicates that the near-Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (≈1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near-Earth reconnection process as a major role in the substorm dynamics.

Satellite anomalies linked to electron increase in the magnetosphere

On January 20, 1994, at 1443 UT, the momentum wheel control circuitry of the Intelsat K spacecraft at geostationary orbit suffered an operational anomaly causing a loss of attitude control. The system was switched to the backup circuitry, and control was reestablished. The same day, at 1735 UT, the Anik E-l spacecraft, also at geostationary orbit, suffered the same kind of operational anomaly in the momentum wheel control circuitry [Rostoker, 1994]. According to newspaper accounts the next day, Telesat Canada operators struggled for 8 hours to regain control of the Anik E-1 satellite. They were able to finally switch to the backup momentum wheel controller and resume reasonably normal operations.