George L. Siscoe

The Magnetospheric Sash and the Cross‐Tail S

As revealed in MHD simulation, the magnetospheric sash is a band of weak magnetic field that, for the usual case in which the IMF is approximately perpendicular to the geomagnetic dipole, runs tailward along the high-latitude magnetopause flanks from one dayside cusp to the other, closing via the cross-tail neutral sheet. On the magnetopause flanks, it contains the magnetic separator line, at which all three topological types of field lines meet. Seen in a cross-sectional plane through the near-Earth tail, the magnetospheric sash takes the form of the cross-tail S, a weak-field feature comprised of the tail neutral sheet with diagonally symmetric extensions along the magnetopause flanks connecting it to the separator line. The cross-tail S is evident in the MHD results and in cross-sectional maps based on IMP 8 data. The magnetopause expression of the sash is latent in prior works that described the geometry of antiparallel fields across the magnetopause and the consequent cancellation of the fields within the magnetopause layer. The sash picture bears a strong resemblance to antiparallel merging geometry.

A dawn‐dusk density asymmetry in Earth's magnetosheath

Magnetosheath data from IMP 8 and solar wind data from ISEE 1, ISEE 3, and WIND are used to investigate Earths magnetosheath structure. Magnetosheath data are transformed into a coordinate system which allows data from regions with similar physical histories to be binned together. The results show a significant dawn-dusk asymmetry in Earths magnetosheath near solar maximum but not near solar minimum, with larger densities on the dawnside than on the duskside. We use a magnetohydrodynamic (MHD) simulation to model the Parker spiral interplanetary magnetic field (IMF) case; an asymmetry does develop in the same sense as observed in the data but with a somewhat smaller magnitude. Near solar minimum the observed density asymmetry is dependent on the IMF orientation, but this is not true near solar maximum, apparently ruling out both foreshock effects and different compressions/deflections by parallel and perpendicular shocks as causes. Earths magnetosphere thus could be a contributor to this effect.

Energetic ions, large diamagnetic cavities, and Chapman‐Ferraro cusp

[1]xa0Extremely large diamagnetic cavities with a size of as large as 6 RE have been observed in the dayside high-altitude cusp regions. These diamagnetic cavities were associated with strong magnetic field turbulence. Associated with these cavities are >40 keV ions that are more typical of the trapped ring current and radiation belt populations than the solar wind. The charge state distribution of these cusp cavity ions was indicative of their seed populations being a mixture of ionospheric and solar wind particles. In April 1999, the cusp diamagnetic cavities were observed by the Polar spacecraft almost in every orbit, indicating that such cavities are always there day by day. Some of the diamagnetic cavities were independent of the interplanetary magnetic field directions, suggesting that the cusp diamagnetic cavities are different from the magnetospheric sash predicted by MHD simulations. During a high solar wind pressure period on 21 April 1999, the Polar spacecraft observed lower energetic (>20 keV/e) ion fluxes in the dayside high-latitude magnetosheath than that in the neighboring cusp cavities. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers. These energetic particles in the cusp diamagnetic cavity together with the cusps connectivity probably have significant global impacts on the geospace environment.

Properties of the mantle‐like magnetotail boundary layer: GEOTAIL data compared with a mantle model

Near the tail boundary beyond about 100 Re, GEOTAIL often encounters a plasma mantle-like boundary layer in which the plasma flowing tailward transitions smoothly from magnetosheath values of speed and density to much smaller values, more characteristic of the tail lobe. This boundary layer had earlier been recognized on the basis of ISEE 3 measurements. GEOTAIL confirms the existence of this layer and extends the documentation on its behavior. Boundary oscillations sweep the boundary layer over the spacecraft enabling GEOTAIL to “sound” the layers profile of plasma parameters. The density-versus-speed correlogram of the mantle-like part—a useful diagnostic for comparisons—is reasonably well simulated by a 1-D, MHD slow-mode expansion fan model of the plasma mantle. Flow directions are consistent with an open plasma mantle on the northern, duskside flank of the tail, as expected for the standard magnetic merging model of mantle formation.

Probabilistic forecasting of geomagnetic indices using solar wind air mass analysis

[1]xa0Measurements of the Suns photospheric magnetic field can in principle be used to predict geomagnetic activity 1–3 days in advance. The accuracy of such predictions is low, however, because they do not include the north-south component of the interplanetary magnetic field (IMF Bz), which is the most geomagnetically relevant parameter. Aside from that carried by large-scale transients, IMF Bz is mainly a product of small-scale, in-transit turbulence and so is inherently unpredictable from solar measurements. Routine 1- to-3-day forecasts of geomagnetic activity based on deterministic algorithms are, therefore, not possible. Probabilistic forecasts offer the next best thing to deterministic forecasts, and air mass climatology offers a way to develop the advantages inherent in probabilistic forecasts for space weather applications. Here we address the IMF Bz indeterminacy problem (or better, get around it) by applying the concept of air mass climatology to the solar wind. We give criteria and statistics for solar wind air masses, which provide poof of concept for routine, midrange (1- to 3-day) probabilistic air mass forecasts of daily levels of geomagnetic activity.

Interplanetary magnetic field control of magnetotail field: IMP 8 data and MHD model compared

Magnetic field patterns in the magnetotail`s cross-sectional plane at {approximately}30 R{sub E} derived from IMP 8 data are compared with corresponding patterns derived from the Fedder-Lyon MHD model of magnetosphere - solar wind interaction. The comparisons emphasize features attributable to the influence of the interplanetary magnetic field (IMF). They reveal considerable correspondences in field asymmetries, nonuniform field perturbations, and current sheet twisting. Both data-based and MHD-based patterns are qualitatively similar to patterns obtained by superposing a uniform field on a dipole field, but they show that the field perturbations are stronger at the equatorial flanks of the tail than in the high-latitude lobes. These quantitative details go beyond the superdisposition model. From the point of view of interpretation, the fact that the MHD-based patterns reproduce the distinctive nonsuperposition features seen in the databased patterns indicates that these features result from MHD motions within the magnetosphere distributing the field that is merged at the magnetopause (i.e., the {open_quotes}penetrating{close_quotes} field). From the point of view of modeling, the agreement indicates that MHD modeling captures essential aspects of the solar wind - magnetosphere coupling. 10 refs., 7 figs.

The space-weather enterprise: past, present, and future

Abstract Space-weather impacts society in diverse ways. Societies’ responses have been correspondingly diverse. Taken together these responses constitute a space weather “enterprise”, which has developed over time and continues to develop. Technological systems that space-weather affects have grown from isolated telegraph systems in the 1840s to ocean and continent-spanning cable communications systems, from a generator electrifying a few city blocks in the 1880s to continent-spanning networks of high-tension lines, from wireless telegraphy in the 1890s to globe-spanning communication by radio and satellites. To have a name for the global totality of technological systems that are vulnerable to space weather, I suggest calling it the cyberelectrosphere . When the cyberelectrosphere was young, scientists who study space weather, engineers who design systems that space weather affects, and operators of such systems — the personnel behind the space-weather enterprise — were relatively isolated. The space-weather enterprise was correspondingly incoherent. Now that the cyberelectrosphere has become pervasive and indispensable to most segments of society, the space weather enterprise has become systematic and coherent. At present it has achieved considerable momentum, but it has barely begun to realize the level of effectiveness to which it can aspire, as evidenced by achievements of a corresponding but more mature enterprise in meteorology, a field which provides useful lessons. The space-weather enterprise will enter a new phase after it matures roughly to where the tropospheric weather enterprise is now. Then it will become indispensable for humankinds further global networking through technology and for humankinds further utilization of and expansion into space.

Reconciling prediction algorithms for Dst

[1]xa0The Burton et al. equation and the Ring Current Atmosphere Interaction Model (RAM) code give about equally good fits to Dst profiles of strong magnetic storms, yet they are driven by different functions of the interplanetary electric field, one rising linearly and the other eventually saturating. We show that by reformulating the Burton et al. equation such that a quadratic form of the driving term replaces the original linear form, the new prediction algorithm driven by the saturation form of the driving electric field produces about equally good fits to Dst as the original Burton et al. equation and the RAM code. The form of the quadratic driving term is constructed by specializing a general form given by dimensional analysis.

Stream interfaces and energetic ions closer than expected: Analyses of Pioneers 10 and 11 observations

An empirical study of corotating interaction regions (CIRs) observed between 3.9 AU and 5.9 AU on Pioneers 10 and 11 shows that the main corotation energetic ion population (CEIP), which is associated with the trailing reverse shock, terminates within the CIR at a definite, structural boundary, which we show here is the stream interface. This new result has significant implications for solar wind and energetic particle modeling. In particular it implies either that the reverse shock forms closer to the stream interface than models suggest or that the theories that treat the generation and transport of these energetic ions, such as preshock Fermi acceleration and cross-field diffusion must be combined or extended. We test these scenarios by comparing the CEIP intensity profiles on the two sides of the stream interface. We find that while each automatically accounts for one or two aspects of the results none of them alone can account for all of our empirical results.

Interstellar pickup H+ Ions at 8.3 AU: Pioneer 10 plasma and magnetic field analyses

Analysis of Pioneer 10 plasma and magnetic field observations at 8.3 AU in 1975 provides new evidence for the presence of interstellar pickup hydrogen (H + ) ions. Use of plasma sensors that look far from the solar wind direction confirms the spherical shell distribution of the pickup ions in velocity space. Phase space density and flux estimates are closely consistent with those from Ulysses measured under similar conditions. Power spectral analyses of the magnetic field data show a distinct signal a little above the proton gyrofrequency, consistent with the presence of Doppler-shifted ion-cyclotron waves generated by H + pickup ions. These results show that the Pioneer data set has the potential for systematic studies of the global properties of interstellar pickup ions.