Joseph E. Borovsky

Differences between CME‐driven storms and CIR‐driven storms

Twenty one differences between CME-driven geomagnetic storms and CIR-driven geomagnetic storms are tabulated. (CME-driven includes driving by CME sheaths, by magnetic clouds, and by ejecta; CIR-driven includes driving by the associated recurring high-speed streams.) These differences involve the bow shock, the magnetosheath, the radiation belts, the ring current, the aurora, the Earths plasma sheet, magnetospheric convection, ULF pulsations, spacecraft charging in the magnetosphere, and the saturation of the polar cap potential. CME-driven storms are brief, have denser plasma sheets, have strong ring currents and Dst, have solar energetic particle events, and can produce great auroras and dangerous geomagnetically induced currents; CIR-driven storms are of longer duration, have hotter plasmas and stronger spacecraft charging, and produce high fluxes of relativistic electrons. Further, the magnetosphere is more likely to be preconditioned with dense plasmas prior to CIR-driven storms than it is prior to CME-driven storms. CME-driven storms pose more of a problem for Earth-based electrical systems; CIR-driven storms pose more of a problem for space-based assets.

Auroral arc thicknesses as predicted by various theories

Twenty-two theoretical mechanisms for auroral arcs are examined (12 electron-acceleration mechanisms and 10 generator mechanisms) and a characteristic auroral-arc thickness is worked out for each mechanism except one. The arc thicknesses are then mapped down to the ionosphere along the terrestrial magnetic-field lines: near the Earth a dipole magnetic-field model is used and farther from the Earth the mapping includes the effects of magnetic-field-line draping. The predicted thicknesses are compared with published ground-based measurements of the optical thicknesses of auroral arcs in the ionosphere, which typically find arcs to be 100 m wide. The 21 theoretical models all predict auroral-arc thicknesses that are at least an order of magnitude wider than the optically observed arcs. As an alternative explanation of the optical observations of narrow auroral arcs, the acceleration of ionospheric electrons to produce airglow in electrical-discharge mechanisms is explored: these electrical-discharge mechanisms are found to be improbable. Also explored is the possibility that the observed narrow auroral arcs are caused by interference effects when Alfven waves reflect off the ionosphere: this is found to be an unlikely explanation. Suggestions are made for future ground-based auroral-arc measurements.

The Earth's plasma sheet as a laboratory for flow turbulence in high-β MHD

The bulk flows and magnetic-field fluctuations of the plasma sheet are investigated using single-point measurements from the ISEE-2 Fast Plasma Experiment and fluxgate magnetometer. Ten several-hour-long intervals of continuous data (with 3 s and 12 s time resolution) are analysed. The plasma-sheet flow appears to be strongly ‘turbulent’ (i.e. the flow is dominated by fluctuations that are unpredictable, with rms velocities[Gt ]mean velocities and with field fluctuations≈mean fields). The flow velocities are typically sub-Alfvenic. The flow-velocity probability distribution P ( v ) is constructed, and is found to be well fitted by exponential functions. Autocorrelation functions [Ascr ](τ) are constructed, and the autocorrelation times τ corr for the flow velocities are found to be about 2 min. From the flow measurements, an estimate of the mixing length in the plasma sheet is produced, yielding L mix ≈2 Earth radii; correspondingly, the plasma-sheet material appears to be well mixed in density and temperature. An eddy viscosity for the plasma sheet is also estimated. Power spectra, which are constructed from the v ( t ) and B ( t ) time series, have portions that are power laws with spectral indices that are near the range of those expected for turbulence theories. The plasma sheet may provide a laboratory for the study of turbulence in parameter regimes different from that of solar-wind turbulence: the plasma sheet is a β[Gt ]1, hot-ion plasma, and the turbulence may be strongly driven rather than well developed. The turbulent nature of the flow and the disordered nature of the magnetic field have implications for the transport of plasma-sheet material, for the penetration of the solar-wind electric field into the plasma sheet, and for the calculation of particle orbits in the magnetotail.

The occurrence rate of magnetospheric-substorm onsets: Random and periodic substorms

Particle-injection events are monitored on three geosynchronous satellites to determine the occurrences of magnetospheric substorms: for every consecutive pair of substorms found, the time interval [Delta]t between substorm onsets is determined. In this manner, 1001 values of [Delta]t are obtained. A statistical analysis of the [Delta]t values finds that the most-probable time between substorm onsets is [Delta]t [approx] 2.75 hours; this is interpreted to be the period between substorms when substorms occur cyclically. The statistical analysis of the [Delta]t values also finds a random probability for the occurrence of substorms with a mean time between random substorms of about 5 hours: it is speculated that this random occurrence may be caused by a property of the solar wind that varies randomly with an approximately 5-hour time scale. About 1500 substorms occur per year: about half are periodic and about half occur randomly. 59 refs., 5 figs.

Geomagnetic storms driven by ICME- and CIR-dominated solar wind

The interaction of the solar wind and the Earths magnetosphere is complex and the phenomenology of the interaction is very different for solar wind dominated by interplanetary coronal mass ejections (ICMEs) compared to solar wind dominated by corotating interaction regions (CIRs). We perform a superposed epoch study of the effects of ICME- and CIR-dominated solar wind upon the storm-time plasma at geosynchronous orbit using data from the magnetospheric plasma analyzer (MPA) instruments on board seven Los Alamos National Laboratory (LANL) satellites. Using 78 ICME events and 32 CIR events, we examine the electron and ion plasma sheets that are formed during each type of solar wind driver, at energy-per-charge between ∼0.1 and 45 keV/q. The results demonstrate that CIR events produce a more significant modulation in the plasma sheet temperature than ICME events, whilst ICME events produce a more significant modulation in the plasma sheet density than CIR events. We attribute these differences to the average speed in the solar wind and a combination of the density of the solar wind and the ionospheric component of the plasma sheet, respectively. We also show that for CIR events, the magnitude of the spacecraft potential is, on average, significantly greater than during ICME-events, with consequent effects upon the performance of instrumentation within this environment.

The superdense plasma sheet: Plasmaspheric origin, solar wind origin, or ionospheric origin?

A few times per month, the density of the plasma sheet is several times higher than its usual density. Such superdense plasma sheet intervals are observed both in the midtail region and at geosynchronous orbit. Typically at geosynchronous orbit, a superdense plasma sheet occurs on the first day of a geomagnetic storm and lasts about 12–18 hours. The occurrences of superdense plasma sheets are found to be related to a distinct pattern of Kp: Kp rising after it has been low for an extended period. The occurrences are also associated with high-density solar wind. Three sources for the material of the superdense plasma sheet are explored: (1) the outer plasmasphere, which is stripped away and drawn into the dayside neutral line when Kp increases, wherein it joins the lobe and eventually joins the plasma sheet; (2) high-density solar-wind, which may have its entry into the plasma sheet controlled by the solar-wind magnetic field; and (3) ionospheric outflow, which is known to be Kp dependent. The occurrence of a superdense plasma sheet has several consequences: it adds to the intensity of the ring current, it may alter the dynamics of the magnetotail and the nature of substorms, and it may provide an enhanced source population for storm-time energetic particles.

The transport of plasma sheet material from the distant tail to geosynchronous orbit

Several aspects of mass transport in the Earths plasma sheet are examined. The evolution of plasma sheet material as it moves earthward is examined by statistically comparing plasma sheet properties at three different downtail distances: near-Earth plasma sheet properties obtained from measurements by 1989-046 near the geomagnetic equator near midnight at 6.6 RE, midtail plasma sheet properties obtained from ISEE 2 measurements during 333 encounters with the neutral sheet, and distant-plasma sheet properties obtained from ISEE 2 measurements during 53 encounters with the interface between the plasma sheet and the plasma sheet boundary layer. Examination of the evolution of the plasma sheet through pressure-density space shows that the transport is nearly adiabatic (γ = 1.52), with a loss of entropy observed in the near-Earth region. The estimated pressure loss from the plasma-sheet associated with the aurora is able to account for the observed decrease in entropy. The near-Earth plasma sheet plasma is also found to be compressed much less than would be expected from magnetic field models. Examination of the evolution of the plasma sheet through density-flux tube-volume space (with the aid of the T89c magnetic field model) indicates that there is a substantial loss of mass from plasma sheet flux tubes. Global magnetic reconnection during substorms and patchy reconnection at other times is invoked to account (1) for the required mass loss, (2) for the related lack of compression, and (3) for an observed disconnection between ionospheric convection and plasma sheet convection. This reconnection must occur closer than 20 RE downtail. Selective transport is examined by statistically analyzing the ISEE 2 neutral sheet crossing data set: strong transport is found to be associated with low densities, with weak Bz, and with large flux tube volume. A correlation between the direction of the flow in the plasma sheet and the solar wind velocity indicates that earth-ward transport is stronger when the solar wind velocity is lower. An examination of near-Earth and of midtail plasma sheet densities, temperatures, and entropies shows that the plasma sheet is usually spatially homogeneous, contrary to a “bubbles and blobs” picture of transport. Several new points of view about plasma sheet transport are discussed, including the dominant role of near-Earth reconnection, the importance of auroral zone pressure loss, the control of the plasma sheet properties by the density and speed of the solar wind, and the disconnection of the ionospheric and plasma sheet flow patterns.

Relativistic‐electron dropouts and recovery: A superposed epoch study of the magnetosphere and the solar wind

During 124 high-speed-stream-driven storms from two solar cycles, a multispacecraft average of the 1.1–1.5 MeV electron flux measured at geosynchronous orbit is examined to study global dropouts of the flux. Solar wind and magnetospheric measurements are analyzed with a superposed epoch technique, with the superpositions triggered by storm-convection onset, by onset of the relativistic-electron dropouts, and by recovery of the dropouts. It is found that the onset of dropout occurs after the passage of the IMF sector reversal prior to the passage of the corotating interaction region (CIR) stream interface. The recovery from dropout commences during the passage of the compressed fast wind. Relativistic-electron-dropout onset is temporally associated with the onset of the superdense ion and electron plasma sheet, with the onset of the extra-hot ion and electron plasma sheet and with the formation of the plasmaspheric drainage plume. Dropout recovery is associated with the termination of the superdense plasma sheet and with a decay of the plasmaspheric drainage plume. When there is appreciable spatial overlap of the superdense ion plasma sheet with the drainage plume, dropouts occur, and when that overlap ends, dropouts recover. This points to pitch-angle scattering by electromagnetic ion-cyclotron (EMIC) waves as the primary cause of the relativistic-electron dropouts, with the waves residing in the lumpy drainage plumes driven by the superdense ion plasma sheet. The drainage plume is caused by enhanced magnetospheric convection associated with southward (GSM) magnetic field after the IMF sector reversal. The superdense plasma sheet has its origin in the compressed slow wind of the CIR.

Energetic electron precipitation during high‐speed solar wind stream driven storms

Electron precipitation from the Earths inner magnetosphere transmits solar variability to the Earths upper atmosphere and may affect surface level climate. Here we conduct a superposed epoch analysis of energetic electrons observed by the NOAA POES spacecraft during 42 high-speed solar wind stream (HSS) driven geomagnetic storms to determine the temporal evolution and global distribution of the precipitating flux. The flux of trapped and precipitating E > 30 keV electrons increases immediately following storm onset and remains elevated during the passage of the HSS. In contrast, the trapped and precipitating relativistic electrons (E > 1 MeV) drop out following storm onset and subsequently increase during the recovery phase to levels which eventually exceed the prestorm levels. There is no evidence for enhanced precipitation of relativistic electrons during the MeV flux drop out, suggesting that flux drop outs during the main phase of HSS-driven storms are not due to precipitation to the atmosphere. On average, the flux of precipitating E > 30 keV electrons is enhanced by a factor of similar to 10 during the passage of the high-speed stream at all geographic longitudes. In contrast, the precipitating relativistic electron count rate is observed to peak in the region poleward of the South Atlantic Anomaly. During the passage of the high-speed stream, the flux of precipitating E > 30 keV electrons peaks in the region from 2100 to 1200 magnetic local time at low L (4 30 keV electrons in both regions.

Effect of plasmaspheric drainage plumes on solar-wind/magnetosphere coupling

Evidence is uncovered that plasmaspheric drainage plumes flowing into the dayside reconnection site mass load the reconnection rate and thereby reduce the coupling of the solar wind to the Earths magnetosphere. Solar-wind/magnetosphere coupling is statistically analyzed with the 1963–2003 OMNI2 data set matched up with the AE, AU, and PCI geomagnetic indices. Times when plasmaspheric drainage plumes are flowing are discerned using multiple spacecraft with plasma detectors in geosynchronous orbit. It is found that for a given value of −vBz of the solar wind, the geomagnetic indices AE, AU, and PCI are statistically lower when plasmaspheric drainage plumes are present than when plumes are not present. This is taken as a measure of a weakened coupling of the solar wind to the Earths magnetosphere caused by the plumes. An Addendum examines the effects of polar-cap saturation on the auroral electrojet index.