S. Wing

Central plasma sheet ion properties as inferred from ionospheric observations

A method of inferring central plasma sheet (CPS) temperature, density, and pressure from ionospheric observations is developed. The advantage of this method over in situ measurements is that the CPS can be studied in its entirety, rather than only in fragments. As a result, for the first time, comprehensive two-dimensional equatorial maps of CPS pressure, density, and temperature within the isotropic plasma sheet are produced. These particle properties are calculated from data taken by the Special Sensor for Precipitating Particles, version 4 (SSJ4) particle instruments onboard DMSP F8, F9, F10, and F11 satellites during the entire year of 1992. Ion spectra occurring in conjunction with electron acceleration events are specifically excluded. Because of the variability of magnetotail stretching, the mapping to the plasma sheet is done using a modified Tsyganenko [1989] magnetic field model (T89) adjusted to agree with the actual magnetotail stretch at observation time. The latter is inferred with a high degree of accuracy (correlation coefficient ∼0.9) from the latitude of the DMSP b2i boundary (equivalent to the ion isotropy boundary). The results show that temperature, pressure, and density all exhibit dawn-dusk asymmetries unresolved with previous measurements. The ion temperature peaks near the midnight meridian. This peak, which has been associated with bursty bulk flow events, widens in the Y direction with increased activity. The temperature is higher at dusk than at dawn, and this asymmetry increases with decreasing distance from the Earth. In contrast, the density is higher at dawn than at dusk, and there appears to be a density enhancement in the low-latitude boundary layer regions which increases with decreasing magnetic activity. In the near-Earth regions, the pressure is higher at dusk than at dawn, but this asymmetry weakens with increasing distance from the Earth and may even reverse so that at distances X < ∼−10 to −12 RE depending on magnetic activity, the dawn sector has slightly higher pressure. The temperature and density asymmetries in the near-Earth region are consistent with the ion westward gradient/curvature drift as the ions E×B convect earthward. When the solar wind dynamic pressure increases, CPS density and pressure appear to increase, but the temperature remains relatively constant. Comparison with previously published work indicates good agreement between the inferred pressure, temperature, and density and those obtained from in situ data. This new method should provide a continuous mechanism to monitor the pressure, temperature, and density in the magnetotail with unprecedented comprehensiveness.

The auroral oval position, structure, and intensity of precipitation from 1984 onward : an automated on-line data base

An on-line data base consisting of the auroral oval boundaries, structure, and particle fluxes from the DMSP F7 and F9 satellites from December 1983 through the present (with about an 7-month lag) is announced. The data are divided into distinct regions (e.g., diffuse aurora, discrete aurora, cusp proper, polar rain, etc.). The magnetospheric sources of the various particle precipitation regions are identified using a sophisticated pattern recognition technique (a neural network). For each region the boundaries are specified in both geographic and geomagnetic coordinates, and the average and peak values of the particle fluxes are given, along with the average energies. The DMSP satellites are continuously monitored, so that data gaps are comparatively rare. It is anticipated that the location of the auroral oval (and precipitation intensities) will be of particular interest to ground-based observers; although anyone interested in the state of the auroral oval may find it of value. SPAN data requests are processed on a completely automated basis, with up to 48 hours of auroral oval parameters provided in response to a single request.

Characterizing the state of the magnetosphere : Testing the ion precipitation maxima latitude (b2i) and the ion isotropy boundary

Recently, efforts to characterize and monitor the state of the magnetosphere have intensified, along with the rising interest in space weather. The latitude of the ion energy flux precipitation maxima (“b2i”), which almost invariably occurs near the equatorward edge of the nightside main auroral oval, has been suggested as one such parameterization. It has been suggested that b2i corresponds to the ion isotropy boundary (IB), which has been independently researched as a measure of the extent to which the magnetotail is stretched. By comparing simultaneous observations by the Defense Meteorological Satellite Program (DMSP) and NOAA spacecraft, we confirm a close association between b2i and the isotropy boundary of 30 keV protons. Using 2.5 years of simultaneous data from DMSP and GOES spacecraft, we verified that magnetic field inclination (the extent to which the magnetotail is stretched) strongly controls the b2i/IB latitude. Based on use of the b2i latitude, corrected for local time variation, as an index of magnetic stretching in the tail to show a considerable dawn-dusk asymmetry, we find that the magnetic field is more depressed and stretched at dusk than at dawn, and asymmetry increases with increasing magnetotail stretching. This asymmetry is consistent with the rotation of the symmetry line of the b2i(MLT) curve toward premidnight hours and suggests the growth of a so-called “partial ring current” system with increasing activity. Finally, the utility of the b2i/IB boundary as a characterization of the state of the magnetosphere is shown by demonstrating that the average pressure in the magnetotail is better specified by b2i than by Kp.

Double cusp: Model prediction and observational verification

Recent modeling of the entry of solar wind plasma into the magnetosphere and ionosphere has adequately simulated the large-scale particle precipitation features in the observed cusp, mantle, polar rain, and open-field line low-latitude boundary layer regions. The assumption of a simple dawn-dusk electric field limited the models to the near-noon region and southward interplanetary magnetic field (IMF) case. Here, we present an improved model that incorporates the electric field obtained from statistical convection patterns. When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower-latitude cusp ions originate from low-latitude magnetosheath, whereas the higher-latitude ions originate from the high-latitude magnetosheath. The lower-latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp, as would be observed from a typical meridional trajectory of a polar-orbiting satellite. The higher-latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher-latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. This prediction was subsequently confirmed in a large case study with Defense Meteorological Satellite Program (DMSP) data. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz ,30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double-cusp result is consistent with the following new statistical results: (1) the cusp latitudinal width increases with |IMF By| and (2) the cusp equatorward boundary moves to lower latitude with increasing |IMF By|.

Effects of interplanetary magnetic field z component and the solar wind dynamic pressure on the geosynchronous magnetic field

A study of the correlation of the geosynchronous magnetic field with interplanetary magnetic field (IMF) Bz and the solar wind dynamic pressure (Pd) is presented. Hourly averages of 5 years of GOES 6 and 6 years of GOES 7 observations are correlated with IMF Bz and Pd. As previously reported, increases in Pd enhance geosynchronous Bz on the dayside, most strongly around noon, but depress it on the nightside, most strongly around midnight. This has been interpreted in terms of increases in the cross-tail and magnetopause currents. Our study shows that the dayside geosynchronous magnetic field decreases with IMF Bz, particularly during periods of southward IMF. During periods of northward IMF, this trend continues but at much slower rate. The results of a multiple regression analysis of GOES Bz as a function of IMF Bz and Pd during periods of northward IMF show that roughly 30% of the IMF Bz uniformly “penetrates” the geosynchronous Bz. In contrast, during periods of southward IMF, the effect of IMF Bz on geosynchronous Bz is nonuniform and much larger at all local times, especially near dawn and dusk, apparently because of enhanced cross-tail current. During periods of southward IMF, geosynchronous Bx depends most strongly on IMF Bz near dawn, dusk, and midnight. The dawn and dusk correlations can be attributed to the enhanced region 1 Birkeland currents, and the midnight correlation can be attributed to enhanced cross-tail current. Geosynchronous By has the best correlations with IMF Bz near 2000 LT and 0400 LT, which can be attributed to enhanced region l Birkeland and/or cross-tail currents. Also, we show that these magnetic field perturbations can be interpreted in terms of fast rarefaction waves and merging at equatorial regions.

A large statistical study of the entry of interplanetary magnetic field Y‐component into the magnetosphere

We used 5 years of GOES-6 and 6 years of GOES-7 data to correlate the y-component of the geosynchronous magnetic field with the y-component of IMF. One motivation is that the subsolar merging model and the antiparallel model predict distinctly different patterns for By in the magnetosphere as a function of IMF By. Although both models predict that nightside magnetospheric magnetic field will tilt in the direction of the IMF By, the antiparallel merging model predicts that magnetospheric magnetic field lines in the vicinity of local noon will tilt in the direction of the IMF, where as the subsolar model does not. The correlation coefficients between the geosynchronous data and the IMF By peak at both noon (0.61) and midnight (0.50), favoring the antiparallel model. These results were obtained with 1789 (noon) and 1312 (midnight) data points. The slopes of the regression lines indicate 29% IMF By entry at noon and 79% at midnight. The local-time distribution of the slopes varies smoothly from midnight peak to a broad minimum near noon.

Relation to solar activity of intense aurorae in sunlight and darkness

The oldest documented, relationship between the number of sunspots (the solar cycle) and terrestrial effects is the increased frequency of aurorae in the period immediately after the solar maximum (the peak of the number of sunspots). This correlation is, however, based only on observations of the relatively rare events of ‘great aurorae’, which are those that reach mid-latitudes or lower. The overwhelming majority of intense aurorae, and therefore most of the energy put into the ionosphere, occurs at high latitudes, where aurorae appear nightly. Here we report the global frequency of aurorae as a function of solar cycle, determined by data from the US Air Force Defense Meteorological Satellite Program. We find that, contrary to expectations, the total number of intense aurorae is uncorrelated with solar activity in darkness, and is negatively correlated with solar activity in sunlit conditions. These findings imply a causal relationship between aurorae and ionospheric conductivity (the latter is maximal at solar maximum) and therefore indicate that the occurrence of intense aurorae is a discharge phenomenon, similar to lightning.

Transformation of high‐latitude ionospheric F region patches into blobs during the March 21, 1990, storm

Discrete F region electron density enhancements of a factor of 2 or more have been observed in the high-latitude ionosphere. These enhancements have been termed patches if they occur within the polar cap and blobs if they occur outside of the polar cap. It is important to understand the formation and evolution of these structures because they are associated with large phase and amplitude scintillation in transionospheric radio signals. Blobs are generally thought to result from the breakup of patches as they exit the polar cap; however, this process has not previously been observed. Detailed study of high-latitude ionospheric plasma transport is generally difficult because of the sparseness (spatial and temporal) of electron density and velocity observations. In this paper, we present electron density enhancements measured from the Qaanaaq Digisonde, the Millstone Hill incoherent scatter radar, and the DMSP F8 satellite during a 5-hour interval of the March 21, 1990, storm period and show definitively how a patch is transformed into a blob. We present a new trajectory analysis package that is capable of using ionospheric convection patterns to determine the motion of ionospheric plasma over a period of several hours. The new package uses convection patterns from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique to track the motion of observed patches from one site to another and thus determines where the measured electron density enhancements originated and where they went after being observed. The trajectory analysis also establishes that there is a direct connection between the enhancements observed by the different instruments at different locations. In this case, within ∼4 hours, plasma observed by a Digisonde near the pole is convected through 35° of latitude to the northeastern United States, where it is observed by the Millstone Hill radar, then roughly equal portions are transported westward to Alaska and eastward to Scandinavia where they are observed by the DMSP satellite. This study demonstrates that the changing convection pattern can significantly distort the patch shape and trajectory, and illustrates the high degree of mixing of ionospheric plasma by convection. The changing convection pattern leads to the simultaneous existence of a boundary blob and a subauroral blob which are both observed by the Millstone Hill radar. This work is very relevant to our future ability to specify and forecast ionospheric conditions at high latitudes. It represents a critical step from a merely qualitative ability to model the evolution of patches and blobs to a quantitative ability.

Quiet time plasma sheet ion pressure contribution to Birkeland currents

Birkeland currents transport magnetic tangential stress resulting from J × B forces, which, in the plasma sheet, are balanced by the pressure gradient, Vp. However, derivation of nightside Birkeland currents from Vp observationally has not been possible because pressure must be known everywhere in the plasma sheet at high resolution, which in situ satellites have been unable to provide. Recently, a method of inferring plasma sheet temperature, density, and pressure from low-altitude satellites was developed. The quiet time Birkeland currents (or J // ) are computed from the pressure profile derived from DMSP F8, F9, F10, and F11 data for the year 1992 and magnetic field from a modified Tsyganenko [1989] magnetic field model. Our results show that (1) the region 1 Birkeland currents exhibit a dawn-dusk asymmetry which can be explained by the dawn-dusk asymmetry in the plasma pressure arising if the near-Earth plasma sheet ions are supplied largely by the deep tail plasma sheet and LLBL ions undergoing E X B earthward and gradient/curvature duskward motions; (2) the average region 1 J // is -0.6 and 0.7 nA/m 2 near the neutral sheet (negative current density indicates the currents flow out of the ionosphere and positive means into the ionosphere); and (3) the current system tailward/poleward of the region 1 has the opposite polarities from those in region 1 and apparently is generated from the midnight pressure maximum. These results are fairly consistent with those previously obtained with in situ magnetometer measurements.

Modeling the entry of magnetosheath electrons into the dayside ionosphere

It has recently been shown that it is possible to quantitatively model the entry of magnetosheath ions and their access to the dayside ionosphere with surprisingly good results. In the same model, electrons had access to the region poleward of the cusp at unrealistically high intensities. We improve the previous model by imposing the constraints of charge quasi-neutrality and introducing more realistic electron magnetosheath populations. It turns out that no potential drop within the cusp proper is either needed or observed in order to enforce charge neutrality, since ions, as well as electrons, can enter freely, and they originally have the same density in the magnetosheath. Poleward of the magnetic cusp, ion entry is sharply curtailed because of the tailward magnetosheath flow, and the potential required and observed rises rapidly. This potential eliminates access of the “core” population of the magnetosheath electrons to the ionosphere. The typical polar rain signature observed at low altitudes fits best with the suprathermal solar wind electron population (either halo or both halo and strahl components). The model clearly shows that ions previously identified at low altitude as “mantle” do indeed cross the magnetopause tailward of the magnetic cusp, that is, the ionospheric mantle signature consists of ions originating in the high-altitude mantle. A single ion spectrum within the low-altitude cusp proves to consist of magnetosheath ions which have crossed the frontside magnetopause from a range of positions which commences with the merging site and extends to the magnetic cusp, but which is typically only 1–3 RE wide along the direction of the field line convection.