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

Observations of electrons of energy 125 ev to ∼2 kev with the OGO 1 satellite and of electrons of energy 40 ev to ∼2 kev with OGO 3 (by means of modulated Faraday cup detectors) are used to investigate the low-energy electron population in the magnetosphere within the local-time range ∼17 to ∼22 hours. Intense fluxes of these electrons are confined to a spatial region, termed the plasma sheet, which is an extension of the magnetotail plasma sheet discovered by the Vela satellites and is identified with the soft electron band first detected by Gringauz. The plasma sheet extends over the entire local-time range studied in this investigation, from the magnetospheric tail past the dusk meridian toward the dayside magnetosphere. In latitude it is confined to within 4–6 RE of the geomagnetic and/or solar magnetospheric equatorial plane, in agreement with observations already reported from the Vela satellites; no electron fluxes are detected high above the equator, not even very near the magnetopause. In radial distance the plasma sheet is terminated by the magnetopause on the outside and by a well-defined sharp inner boundary on the inside. The inner boundary has been traced from the equatorial region to the highest latitudes investigated, ∼40°; during geomagnetically quiet periods, it is observed at an equatorial distance of 11 ± 1 RE and appears to extend to higher latitudes along magnetic field lines. Weak or no electron fluxes are found between the inner boundary of the plasma sheet and the outer boundary of the plasmasphere. Detection (by an indirect process) of the very high ion densities within the plasmasphere gives positions for its boundary in good agreement with other determinations. During periods of magnetic bay activity, the plasma sheet extends closer to the earth; the inner boundary of the plasma sheet is then found at equatorial distances of 6–8 RE. This is most simply interpreted as the result of an actual inward motion of the plasma during a bay. In one case, it was possible to associate the beginning of this motion with the onset of the bay and to estimate an average radial speed of ∼12 km/sec, from which an electric field corresponding to ∼48 kilovolts across the magnetospheric tail was inferred. Within the plasma sheet, the electron population is characterized by number densities from 0.3 to 30 cm−3 and mean energies from 50 to 1600 ev and higher, with a strong anticorrelation between density and mean energy, so that the electron energy density (∼1 kev cm−3) and energy flux (∼3 ergs cm−2 sec−1) show relatively little variation. The lower energies and higher densities tend to occur during periods of geomagnetic disturbance. The nonobservation of electrons in regions above the plasma sheet implies an upper limit on the electron number density of 5 × 10−2 cm−3 if their mean energy is assumed to be ∼50 ev (typical of the magnetosheath) and 10−2 cm−3 if the energy is ∼1 kev (typical of the plasma sheet). At the inner boundary of the plasma sheet there is a sharp softening of the electron spectrum with decreasing radial distance but apparently little change in the electron number density. The electron energy density decreases across the inner boundary roughly as ∼exp (distance/0.4 RE) during quiet periods; during times of magnetic bay activity the energy density decreases as ∼exp (distance/0.6 RE), and there is a more complicated spatial structure of density and mean energy.

A survey of low-energy electrons in the evening sector of the magnetosphere with OGO 1 and OGO 3.

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is the fifth NASA Medium-class Explorer (MIDEX), launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. The mission employs five identical micro-satellites (hereafter termed “probes”) which line up along the Earth’s magnetotail to track the motion of particles, plasma and waves from one point to another and for the first time resolve space–time ambiguities in key regions of the magnetosphere on a global scale. The probes are equipped with comprehensive in-situ particles and fields instruments that measure the thermal and super-thermal ions and electrons, and electromagnetic fields from DC to beyond the electron cyclotron frequency in the regions of interest. The primary goal of THEMIS, which drove the mission design, is to elucidate which magnetotail process is responsible for substorm onset at the region where substorm auroras map (∼10 RE): (i) a local disruption of the plasma sheet current (current disruption) or (ii) the interaction of the current sheet with the rapid influx of plasma emanating from reconnection at ∼25 RE. However, the probes also traverse the radiation belts and the dayside magnetosphere, allowing THEMIS to address additional baseline objectives, namely: how the radiation belts are energized on time scales of 2–4 hours during the recovery phase of storms, and how the pristine solar wind’s interaction with upstream beams, waves and the bow shock affects Sun–Earth coupling. THEMIS’s open data policy, platform-independent dataset, open-source analysis software, automated plotting and dissemination of data within hours of receipt, dedicated ground-based observatory network and strong links to ancillary space-based and ground-based programs. promote a grass-roots integration of relevant NASA, NSF and international assets in the context of an international Heliophysics Observatory over the next decade. The mission has demonstrated spacecraft and mission design strategies ideal for Constellation-class missions and its science is complementary to Cluster and MMS. THEMIS, the first NASA micro-satellite constellation, is a technological pathfinder for future Sun-Earth Connections missions and a stepping stone towards understanding Space Weather.

The THEMIS Mission

The magnetic field experiment on WIND will provide data for studies of a broad range of scales of structures and fluctuation characteristics of the interplanetary magnetic field throughout the mission, and, where appropriate, relate them to the statics and dynamics of the magnetosphere. The basic instrument of the Magnetic Field Investigation (MFI) is a boom-mounted dual triaxial fluxgate magnetometer and associated electronics. The dual configuration provides redundancy and also permits accurate removal of the dipolar portion of the spacecraft magnetic field. The instrument provides (1) near real-time data at nominally one vector per 92 s as key parameter data for broad dissemination, (2) rapid data at 10.9 vectors s−1 for standard analysis, and (3) occasionally, snapshot (SS) memory data and Fast Fourier Transform data (FFT), both based on 44 vectors s−1. These measurements will be precise (0.025%), accurate, ultra-sensitive (0.008 nT/step quantization), and where the sensor noise level is <0.006 nT r.m.s. for 0–10 Hz. The digital processing unit utilizes a 12-bit microprocessor controlled analogue-to-digital converter. The instrument features a very wide dynamic range of measurement capability, from ±4 nT up to ±65 536 nT per axis in eight discrete ranges. (The upper range permits complete testing in the Earth's field.) In the FTT mode power spectral density elements are transmitted to the ground as fast as once every 23 s (high rate), and 2.7 min of SS memory time series data, triggered automatically by pre-set command, requires typically about 5.1 hours for transmission. Standard data products are expected to be the following vector field averages: 0.0227-s (detail data from SS), 0.092 s (‘detail’ in standard mode), 3 s, 1 min, and 1 hour, in both GSE and GSM coordinates, as well as the FFT spectral elements. As has been our team's tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the solar wind: (1) as a collisionless plasma laboratory, at all time scales, macro, meso and micro, but concentrating on the kinetic scale, the highest time resolution of the instrument (=0.022 s), (2) as a consequence of solar energy and mass output, (3) as an external source of plasma that can couple mass, momentum, and energy to the Earth's magnetosphere, and (4) as it is modified as a consequence of its imbedded field interacting with the moon. Since the GEOTAIL Inboard Magnetometer (GIM), which is similar to the MFI instrument, was developed by members of our team, we provide a brief discussion of GIM related science objectives, along with MFI related science goals.

https://wind.nasa.gov/docs/MFI_Lepping_SSR1995.pdf

The WIND magnetic field investigation

In the eight preceding papers, two magnetospheric substorms on August 15, 1968, were studied with data derived from many sources. In this, the concluding paper, we attempt a synthesis of these observations, presenting a phenomenological model of the magnetospheric substorm. On the basis of our results for August 15, together with previous reports, we believe that the substorm sequence can be divided into three main phases: the growth phase, the expansion phase, and the recovery phase. Observations for each of the first three substorms on this day are organized according to this scheme. We present these observations as three distinct chronologies, which we then summarize as a phenomenological model. This model is consistent with most of our observations on August 15, as well as with most previous reports. In our interpretation we expand our phenomenological model, briefly described in several preceding papers. This model follows closely the theoretical ideas presented more quantitatively in recent papers by Coroniti and Kennel (1972a, b; 1973). A southward turning of the interplanetary magnetic field is accompanied by erosion of the dayside magnetosphere, flux transport to the geomagnetic tail, and thinning and inward motion of the plasma sheet. Our observations indicate, furthermore, that the expansionmore » phase of substorms can originate near the inner edge of thc plasm sheet as a consequence of rapid plasma sheet thinnig. At this time a portion of the inner edge of the tail current is short circuited' through the ionosphere. This process is consistent with the formation of a neutral point in the near-tail region and its subsequent propagation tailward. However, the onset of the expansion phase of substorms is found to be far from a simple process. Expansion phases can be centered at local times far from midnight, can apparently be localized to one meridian, and can have multiple onsets centered at different local times. Such behavior indicates that, in comparing observations occurring in different substorms, careful note should be made of the localization and central meridian of cach substorm. (auth)« less

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Satellite studies of magnetospheric substorms on August 15, 1968: 9. Phenomenological model for substorms

Observations with the Los Alamos Scientific Laboratory (LASL) plasma probe and the Goddard Space Flight Center (GSFC) magnetometer on the IMP 6 satellite show that the magnetospheric boundary layer, first identified along the flanks of the magnetosphere, is also present at the magnetosphere's sunward surface. The magnetic field lines in this sunward sector of the boundary layer are closed, and the plasma flow has a component transverse to the field. These observations suggest that the boundary layer is a site of continual transfer of plasma, momentum and energy from the magnetosheath to the magnetosphere. These transfer processes supply plasma and magnetic field to the magnetotail. Also, they produce, indirectly, the dawn-to-dusk electric field across the polar cap, the field-aligned currents that border the dayside polar cap, and the soft particle fluxes that characterize the cleft precipitation, including recently reported dawn-dusk asymmetries of these fluxes. Magnetosheath plasma directly enters the outer few hundred to few thousand kilometers of the magnetosphere's surface to form the boundary layer. There it is enabled to flow across the magnetic field (and approximately parallel to the magnetosphere's surface) by becoming electrically polarized. Leakage of the polarization charge along magnetic field lines to the earth produces themore » dayside high latitude effects mentioned above. The polarizing current flowing across the boundary layer interacts with the magnetic field to oppose the boundary layer plasma flow, taking up its momentum. In this way the magnetic field lines are pulled downstream. The process described here is independent of the interplanetary magnetic field (IMF) and thus may constitute the principal transfer mechanism during prolonged periods of northward IMF when the magnetosphere is very quiet. It is not clear how the effects of southward IMF are superposed on this process. (AIP)« less

The magnetospheric boundary layer: Site of plasma, momentum and energy transfer from the magnetosheath into the magnetosphere

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

Vela satellite observations of electrons and protons with E > 100 ev have shown that a sheet of plasma with enhanced energy density stretches across the earth's magnetotail from the dusk to the dawn boundaries of the magnetosphere. The plasma has been observed at geocentric distances between 15.5 and 20.5 earth radii; this plasma sheet probably extends from the night-side termination of the radiation belt to beyond 31 RE in the antisolar direction. Near the midnight meridian at ∼17 RE the sheet is often ∼4–6 RE thick, while toward the dusk and dawn boundaries the sheet flares out to about twice that thickness. The plane of symmetry of the sheet lies above or below the solar magnetospheric equatorial plane depending on whether the geomagnetic dipole axis tilts toward or away from the sun. The plasma is located in the vicinity of the ‘neutral sheet’ region of magnetic field reversal; inside the plasma sheet the measured kinetic energy densities of the electrons are comparable to the expected magnetic field energy densities, while outside the sheet the kinetic energy densities are much lower. The energy spectrums are quasi-thermal in character, having average energies extending from ∼200 ev to above 12 kev; omnidirectional fluxes extend to above 109 cm−2 sec−1. Both rapid (100–200 sec) and slower changes occur in the average energies of the plasma sheet electrons, resulting in large variations in the flux of energetic electrons (E > 45 kev). The plasma sheet boundaries, which are frequently in motion, are clearly defined by large changes in the electron flux. The average energy of the electron population is higher on the dawn side of the magnetotail than the dusk side, resulting in a more frequent appearance of energetic electrons on the dawn side.

Characteristics of the plasma sheet in the Earth's magnetotail

Various theories regarding the magnetotail are reviewed and discussed. These include the work of Dungey (1961) and Eastman et al., (1976) regarding the generation of the magnetotail, Frank et al., (1976) concerning the so-called magnetotail fireball and its characteristics, and Hones et al., (1976 and 1976a) on the formation of a neutral line across the near-earth plasma sheet near the substorm onset. A detailed discussion of a fireball encounter during 0900-1400 UT in April 1974 is presented, noting plasma and magnetic phenomena observed, and magnetic records from the earth. A critique is made by Hones of the interpretation of this fireball made by Frank et al. In an accompanying reply, Frank et al. comment on the observations made by Hones, with attention to the most evident discrepancy between the two theories, i.e., the generation of large closed magnetic loops in the plasma sheet during magnetic substorms.

Substorm processes in the magnetotail - Comments on 'On hot tenuous plasmas, fireballs, and boundary layers in the earth's magnetotail' by L. A. Frank, K. L. Ackerson, and R. P. Lepping

The flow of plasma in the earth's magnetotail has been measured with an electrostatic analyzer on Vela 4B at geocentric distances of ∼18 RE. The analyzer on the rotating (64-sec period) satellite measures proton energy spectra from 79 ev to 19 kev, and the plasma, flow is detected and measured by the substantial spin modulation that it often causes in the measured proton fluxes. The satellite's spin axis is kept directed radially outward along a radius vector from the earth, and so the analyzer, whose aperture is in the satellite's equatorial plane, most effectively senses flows in the direction perpendicular to the radius vector. Some results of the measurements are that (1) plasma flow speeds of several hundred km/sec are frequently measured in the plasma sheet, particularly during substorms, and these sometimes approach 1000 km/sec; however, evident flow in a given direction seldom persists for more than a few minutes; (2) these rapid substorm-related flows are usually directed generally sunward; (3) flow in the anti-sunward (tailward) direction is observed early in some substorms as the plasma sheet thins down; this may suggest the formation of a neutral line at geocentric distances <18 RE;, (4) the magnetotail is separated from the surrounding magnetosheath by a boundary layer a few thousand kilometers thick in which magnetosheath-like flow occurs but at reduced particle density and velocity; and (5) averaging of all flow measurements made in the plasma, sheet over many months does not reveal any distinct pattern of flow either sunward or anti-sunward; an average of the flows observed during periods of a few minutes of clearly evident flow, however, does reveal a flow in the general direction of the sun. It appears that the plasma sheet may often be in turbulent motion with turbulence-cell dimensions no greater than a few RE.

E. W. Hones

Papers

The magnetospheric boundary layer: Site of plasma, momentum and energy transfer from the magnetosheath into the magnetosphere

Magnetospheric impulse response for many levels of geomagnetic activity

Characteristics of the plasma sheet in the Earth's magnetotail

Substorm processes in the magnetotail - Comments on 'On hot tenuous plasmas, fireballs, and boundary layers in the earth's magnetotail' by L. A. Frank, K. L. Ackerson, and R. P. Lepping

Measurements of magnetotail plasma flow made with Vela 4B