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.

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What is a geomagnetic storm

The near-Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere-tail coupling, large-scale mapping, onset trigger- ing, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near-tail, midtail, and deep- tail features of substorms. Nonetheless, the NENL model still seems to provide the best avail- able framework for ordering the complex, global manifestations of substorms.

/pdf/neutral-line-model-of-substorms-past-results-and-present-alhlk5bw71.pdf

Neutral line model of substorms: Past results and present view

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide measurements of the E × B drift of ionospheric plasma over extended regions of the high-latitude ionosphere. With the recent augmentation of the northern hemisphere component to six radars, a sizable fraction of the entire convection zone (approximately one-third) can be imaged nearly instantaneously (∼2 min). To date, the two-dimensional convection velocity has been mapped by combining line-of-sight velocity measurements obtained from pairs of radars within common-volume areas. We describe a new method of deriving large-scale convection maps based on all the available velocity data. The measurements are used to determine a solution for the distribution of electrostatic potential, Φ, expressed as a series expansion in spherical harmonics. The addition of data from a statistical model constrains the solution in regions of no data coverage. For low-order expansions the results provide a gross characterization of the global convection. We discuss the processing of the radar velocity data, the factors that condition the fitting, and the reliability of the results. We present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field (IMF). In the case of a sudden polarity change from northward to southward IMF, the convection is seen to reconfigure globally on very short (<6 min) timescales.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JA01288

Large-scale imaging of high-latitude convection with Super Dual Auroral Radar Network HF radar observations

[1] Electron acceleration inside the Earth's magnetosphere is required to explain increases in the ∼MeV radiation belt electron flux during magnetically disturbed periods. Recent studies show that electron acceleration by whistler mode chorus waves becomes most efficient just outside the plasmapause, near L = 4.5, where peaks in the electron phase space density are observed. We present CRRES data on the spatial distribution of chorus emissions during active conditions. The wave data are used to calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and to obtain a timescale for acceleration. We show that chorus emissions in the prenoon sector accelerate electrons most efficiently at latitudes above 15° for equatorial pitch angles between 20° and 60°. As electrons drift around the Earth, they are scattered to large pitch angles and further accelerated by chorus on the nightside in the equatorial region. The timescale to accelerate electrons by whistler mode chorus and increase the flux at 1 MeV by an order of magnitude is approximately 1 day, in agreement with satellite observations during the recovery phase of storms. During wave acceleration the electrons undergo many drift orbits and the resulting pitch angle distributions are energy-dependent. Chorus scattering should produce pitch angle distributions that are either flat-topped or butterfly-shaped. The results provide strong support for the wave acceleration theory.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2004JA010811

Timescale for radiation belt electron acceleration by whistler mode chorus waves

The response of outer radiation belt relativistic electrons to the October 9, 1990, magnetic storm is analyzed in detail using a radial diffusion model and data from the Combined Release and Radiation Effects Satellite (CRRES) and the Los Alamos National Laboratory (LANL) geosynchronous satellite 1989-046 Electron measurements are expressed in terms of phase space density as a function of the three adiabatic invariants determined from CRRES magnetic field data and the Tsyganenko 1989 Kp-dependent magnetic field model The radial diffusion model is implemented with a time-dependent radial diffusion coefficient parameterized by Kp, and a time-dependent outer boundary condition scaled by geosynchronous electron data The results show that radial diffusion propagates outer boundary variations into the heart of the outer radiation belt, accounting for both significant decreases and increases in the 1 MeV electrons is inconsistent with the radial diffusion model given the parameter regime chosen for this study Greatly enhanced whistler chorus waves observed by CRRES throughout the recovery phase suggest that a possible explanation for the inconsistency may be electron acceleration via wave-particle interaction

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JA900344

Radial diffusion analysis of outer radiation belt electrons during the October 9, 1990, magnetic storm

The temporal relationship between the solar wind and magnetospheric activity has been studied using 34 intervals of high time resolution IMP 8 solar wind data and the corresponding AL auroral activity index. The median values of the AL index for each interval were utilized to rank the intervals according to geomagnetic activity level. The linear prediction filtering technique was then applied to model magnetospheric response as measured by the AL index to the solar wind input function VB(s). The linear prediction filtering routine produces a filter of time-lagged response coefficients which estimates the most general linear relationship between the chosen input and output parameters of the magnetospheric system. It is found that the filters are composed of two response pulses speaking at time lags of 20 and 60 min. The amplitude of the 60-min pulse is the larger for moderate activity levels, while the 20-min pulse is the larger for strong activity levels. A possible interpretation is that the 20-min pulse represents magnetospheric activity driven directly by solar wind coupling and that the 60-min pulse represents magnetospheric activity driven by the release of energy previously stored in the magnetotail. If this interpretation is correct, the linear filtering results suggest that both the driven and the unloading models of magnetospheric response are important facets of a more comprehensive response model.

/pdf/magnetospheric-impulse-response-for-many-levels-of-bogeyouw8q.pdf

Magnetospheric impulse response for many levels of geomagnetic activity

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 Earth's 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.

/pdf/recurrent-geomagnetic-storms-and-relativistic-electron-3o8q30smma.pdf

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

We review the evidence for the control of magnetospheric substorms by the interplanetary magnetic field (IMF). Early statistical results, as well as recent correlation studies, are cited to show the strong degree of control exerted by the IMF over substorms as measured by auroral electrojet (AE, AL) parameters. Recent case studies and linear prediction filtering methods reveal quite clearly that substorms have both a “driven” and an “unloading” aspect. The driven component has a response time of 10-20min, while the unloading time scale is -1hr. ISEE-3 deepmagnetotail results and recent Dynamics Explorer polar imaging data are related in this paper to these two separate facets of the substorm sequence. Dimensional analysis studies have delineated which solar wind-magnetosphere coupling formalisms are physically tenable and which give the best representation of the empirical data. The review concludes with a discussion of the total magnetospheric energy output parameter, UT, and some of the problems inherent in its calculation. It is shown that as presently formulated, UT cannot be meaningfully compared to solar wind input parameters by the usual correlation methods.

Magnetospheric response to the IMF - Substorms

The technique of linear prediction filtering has been used to generate empirical response functions relating the solar wind electric field to the most frequently used magnetic indices, AL, AU, Dst and ASYM. Two datasets, one from 1967-1968 and one from 1973-1974, provided the information needed to calculate the empirical response functions. These functions have been convolved with solar wind observations obtained during the IMS to predict the indices. These predictions are compared with the observed indices during two, three-day intervals studied extensively by participants in the CDAW-6 workshop. Differences between the observed and predicted indices are discussed in terms of the linear assumption and in terms of physical processes other than direct solar wind-magnetosphere interaction.

L. F. Bargatze

Papers

Magnetospheric impulse response for many levels of geomagnetic activity

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

Magnetospheric response to the IMF - Substorms

Coupling of the solar wind to measures of magnetic activity