C. M. Ho

Monitoring of global ionospheric irregularities using the Worldwide GPS Network

A prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Globa Positioning System (GPS) receivers. Case studies in this pape indicate that GPS receiver loss of lock of signal tracking may be associated with strong phase fluctuations. It is shown that a network-based GPS monitoring system will enable us to study the generation and evolution of ionospheric irregularities continuously around the globe under various solar and geophysical conditions, which is particularly suitable for studies of ionospheric storms, and for space weather research and applications.

Global ionosphere perturbations monitored by the Worldwide GPS Network

For the first time, measurements from the Global Positioning System (GPS) worldwide network are employed to study the global ionospheric total electron content (TEC) changes during a magnetic storm (November 26, 1994). These measurements are obtained from more than 60 world-wide GPS stations which continuously receive dual-frequency signals. Based on the delays of the signals, we have generated high resolution global ionospheric maps (GIM) of TEC at 15 minute intervals. Using a differential method comparing storm time maps with quiet time maps, we find that significant TEC increases (the positive effect) are the major feature in the winter hemisphere during this storm (the maximum percent change relative to quiet times is about 150%). During this particular storm, there is almost no negative phase. A traveling ionospheric disturbance (TID) event is identified that propagates from the northern subauroral region to lower latitudes (down to about 30°N) at a speed of ∼460 m/s. This TID is coincident with significant increases in the TEC around the noon sector. We also find that another strong TEC enhancement occurs in the pre-dawn sector in the northern hemispheric subauroral latitudes, in the beginning of the storm main phase. This enhancement then spreads into almost the entire nightside. The nighttime TEC increase in the subauroral region is also noted in the southern hemisphere, but is less significant. These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a powerful tool for studying the global pattern and evolution process of the entire ionospheric perturbation.

The relationship between interplanetary discontinuities and Alfven waves: Ulysses observations

The rate of occurrence of interplanetary discontinuities (ROID) is examined using Ulysses magnetic field and plasma data from 1 to 5 AU radial distance from the Sun and at high heliographic latitudes. We find two regions where the ROID is high: in stream-stream interaction regions and in Alfven wave trains. This latter feature is particularly obvious at high latitudes when Ulysses enters a high speed stream associated with a polar coronal hole. These streams are characterized by the presence of continuous, large-amplitude (ΔB→/|B|∼1−2)Alfven waves and an extraordinarily high ROID value (∼150 discontinuities/day). In a number of intervals examined, it is found that (rotational) discontinuities are an integral part of the Alfven waves. The nonlinear Alfven waves are spherically polarized, i.e., the tip of the perturbation vector resides on the surface of a sphere (a consequence of constant |B|). The slowly rotating part of the wave rotates ∼270° in phase. There is a slight arc in the B1-B2 hodogram, suggesting an almost linear polarization. The phase rotation associated with the discontinuity is ∼90°, lies in the same plane as the slowly rotating part of the Alfven wave, and therefore completes the 360° phase rotation. The best description of the overall Alfven wave plus discontinuity is a spherical, arc-polarized, phase-steepened wave.

A REVIEW OF DISCONTINUITIES AND ALFVEN WAVES IN INTERPLANETARY SPACE : ULYSSES RESULTS

The Ulysses mission is the first to explore our heliosphere at all latitudes up to ±80° and therefore is an ideal mission to study potential gradients in heliolatitude (and radial distance) of discontinuity occurrence rates and types. Directional discontinuities (DDs) are shown to be dependent on the type of solar wind streams that they are embedded in. The occurrence rate of DDs is 5–10 times higher in high-speed streams than in slow streams. The explanation is that nonlinear Alfven waves dominate the high-speed streams and rotational discontinuities are the phase-steepened edges of the Alfven waves. Dissipation at these phase-steepened Alfven waves have been sought but not found. An e−(R−1)/5 decrease in discontinuity rate with increasing radial distance (R in units of AU) is partially an artifact of the selection criteria (discontinuity thickening), but dissipation at a relatively slow rate cannot be ruled out at this time. There is no obvious latitudinal gradients in discontinuity types or occurrence rates. Somewhat surprisingly, tangential discontinuities are detected at high latitudes. These have been associated with the edges of local small-scale magnetic decreases. A pair of slow shocks were detected at 5.3 AU. The speeds are similar to fast mode shock speeds. When Alfven waves in high-speed streams impinge upon the Earths magnetosphere, near-continuous substorms (called HILDCAAs) occur, leading to the pumping of an extraordinary amount of energy into the nightside ionosphere. Current discontinuity and Alfven wave research problems are discussed.

Ionospheric total electron content perturbations monitored by the GPS global network during two northern hemisphere winter storms

The global evolution of two major ionospheric storms, occurring on November 4, 1993, and November 26, 1994, respectively, is studied using measurements of total electron content (TEC) obtained from a worldwide network of ground-based GPS receivers. The time-dependent features of ionospheric storms are identified using TEC difference maps based on the percent change of TEC during storm time relative to quiet time. The onset of each ionospheric storm is indicated by the appearance of auroral/subauroral TEC enhancements which occur within 1 hour of the beginning of the geomagnetic storm main phase. Significant TEC enhancements (> 100%) are observed in the winter northern hemisphere. The rate at which TEC enhancements appear is found to correlate with gradients in the Dst index. The large scale ionospheric structures identified during the storms are (1) nightside auroral/subauroral enhancements which surround the auroral oval, (2) dayside (around noon) high-latitude and middle-latitude enhancements associated with traveling ionospheric disturbances, and (3) conjugate latitudinal enhancements. For the November 1993 storm, a short positive phase (about 15 hours) is followed by a long negative phase (∼60 hours). In the November 1994 storm, we have identified the clear signature of a traveling ionospheric disturbance (TID) which propagated at a speed of ∼460 m/s from ∼60° N to ∼40° N. The motion of this disturbance appears to conserve angular momentum.

Magnetic cloud field intensities and solar wind velocities

For the sets of magnetic clouds studied in this work we have shown the existence of a relationship between their peak magnetic field strength and peak velocity values, with a clear tendency that clouds which move at higher speeds also possess higher core magnetic field strengths. This result suggests a possible intrinsic property of magnetic clouds and also implies a geophysical consequence. The relatively low field strengths at low velocities is presumably the cause of the lack of intense storms during low speed ejecta. There is also an indication that this type of behavior is peculiar for magnetic clouds, whereas other types of non cloud-driver gas events do not seem to show a similar relationship, at least for the data studied in this paper. We suggest that a field/speed relationship for magnetic clouds, as that obtained in our present study, could he associated with the cloud release and acceleration mechanism at the sun. Since for magnetic clouds the total field tyically has a substantial southward component, B s , our results imply that the interplanetary dawn-dusk electric field, given by v X B s (where v is the clouds velocity), is enhanced by both factors. Therefore, the consequent magnetospheric energization (that is governed by this electric field) becomes more efficient for the occurrence, of magnetic storms.

Large Amplitude IMF Fluctuations in Corotating Interaction Regions: Ulysses at Midlatitudes

Corotating Interaction Regions (CIRs), formed by high-speed corotating streams interacting with slow speed streams, have been examined from −20° to −36° heliolatitudes. The high-speed streams emanate from a polar coronal hole that Ulysses eventually becomes fully embedded in as it travels towards the south pole. We find that the trailing portion of the CIR, from the interface surface (IF) to the reverse shock (RS), contains both large amplitude transverse fluctuations and magnitude fluctuations. Similar fluctuations have been previously noted to exist within CIRs detected in the ecliptic plane, but their existence has not been explained. The normalized magnetic field component variances within this portion of the CIR and in the trailing high-speed stream are approximately the same, indicating that the fluctuations in the CIR are compressed Alfven waves. Mirror mode structures with lower intensities are also observed in the trailing portion of the CIR, presumably generated from a local instability driven by free energy associated with compression of the high-speed solar wind plasma. The mixture of these two modes (compressed Alfven waves and mirror modes) plus other modes generated by three wave processes (wave-shock interactions) lead to a lower Alfvenicity within the trailing portion of the CIR than in the high-speed stream proper. The results presented in this paper suggest a mechanism for generation of large amplitude Bz fluctuations within CIRs. Such phenomena have been noted to be responsible for the generation of moderate geomagnetic storms during the declining phase of the solar cycle.

A comparative study of ionospheric total electron content measurements using global ionospheric maps of GPS, TOPEX radar, and the Bent model

Global ionospheric mapping (GIM) is a new and emerging technique for determining global ionospheric TEC (total electron content) based on measurements from a worldwide network of Global Positioning System (GPS) receivers. In this study, GIM accuracy in specifying TEC is investigated by comparison with direct ionospheric measurements from the TOPEX altimeter. A climatological model (Bent model) is also used to compare with the TOPEX altimeter data. We find that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model. The difference between GIM and TOPEX in TEC measurements is very small (less than 1.5 TEC units (TECU)) within a 1500-km range from a reference GPS station. The RMS gradually increases with increasing distance from the station, while the Bent model shows a constant large RMS, unrelated to any station location. Within a 1000-km distance of a GPS site (elevation angle > 25°), GIM has a good correlation (R > 0.93) to TOPEX with respect to TEC measurements. The slope of the linear fitting line to the data set from two TOPEX cycles is 44.5° (near the ideal 45°). In the northern hemispheric regions, ionospheric specification by GIM appears to be accurate to within 3-10 TECU up to 2000+ km away from nearest GPS station (corresponding to ∼1° elevation angle cutoff). Beyond 2000 km, GIM accuracy, on average, is reduced to the Bent model levels. In the equatorial region, the Bent model predictions are systematically lower (∼5.0 TECU) than TOPEX values and often show a saturation at large TEC values. During ionospheric disturbed periods, GIM sometimes shows differences from TOPEX values due to transient variations of the ionosphere. Such problems may be improved by the continuous addition of new GPS stations in data-sparse regions. Thus, over a GPS stations measurement realm (up to 2000 km in radius), GIM can produce generally accurate TEC values. Through a spatial and temporal extrapolation of GPS-derived TEC measurements, the GIM technique provides a powerful tool for monitoring global ionospheric features in near real time.

Interplanetary discontinuities and Alfvén waves at high heliographic latitudes : Ulysses

This paper presents the results of the first statistical study of interplanetary directional discontinuities at both low and high heliographic latitudes measured by the Ulysses magnetometer. There is a gradual decrease in the rate of occurrence of interplanetary discontinuities (ROIDs) with increasing radial distance. From 1 to 5 AU, an e -(r-1)/5 dependence is derived. Much of this decrease is believed to be an artifact due to the discontinuity thickening with decreasing IBI, falling outside the detection criteria. As Ulysses goes from the ecliptic plane to high (-80°) heliographic latitudes, the ROID value increases dramatically. The increase is about a factor of 5 as Ulysses moves from Jupiter at 5 AU to 2.5 AU over the south pole. There is a one-to-one correspondence between high ROID values and high-speed streams. This is particularly dramatic just after the Jovian encounter when there are ∼25.4-day period corotating streams present. Thus the increase with latitude is primarily due to Ulysses spending an increasing percentage of time within a high-speed stream emanating from the solar coronal hole. High-speed streams are characterized by the presence of nonlinear Alfven waves with peak-to-peak transverse fluctuations as large as |ΔB|/| B| of 1 to 2. Over the south pole, the normalized transverse wave power can be characterized by P = 2.5 × 10 -4 f -1.6 Hz -1 and the compressional power 1.8 × 10 -4 f -1.2 Hz -1 for frequencies between 10 -5 and 10 -2 Hz. The normalized wave power spectra in different regions of the polar coronal hole streams, from midlatitudes to high heliographic latitudes, appear to be quite similar. The wave power in the ecliptic plane is somewhat lower, perhaps due to contamination from low-speed streams. The Alfven waves in the high-speed stream are found to be propagating outward from the Sum, even at these large heliocentric distances (2.5-5.0 AU). The waves typically have arclike polarizations and conserve field magnitude to first order. Directional (rotational) discontinuities often form the edges of the phase-steepened Alfven waves, thus offering a natural explanation for the high ROID rates within high-speed streams.

Global Ionospheric TEC Variations During January 10, 1997 Storm

The ionospheric storm evolution process was monitored during the January 10, 1997 magnetic cloud event, through measurements of the ionospheric total electron content (TEC) from 150 GPS stations. The first significant response of the ionospheric TEC to the geomagnetic storm was at 0300 UT as an auroral/subauroral enhancement around the Alaskan evening sector. This enhancement then extended to both noon and midnight. Around 0900 UT, the enhancement at noon broke from the subauroral band and moved to lower latitudes. This day side northern hemisphere enhancement also corresponded to a conjugate geomagnetic latitude enhancement in the southern hemisphere and lasted about 5 hours. At 1500 UT a large middle latitude enhancement appeared over Mexico and the southern US, and persisted until 2200 UT. The enhancement was probably caused by the equatorward neutral wind which pushed the plasma up. On the basis of this assumption, the kinetic energy of the neutral wind which caused the middle latitude enhancement is estimated as ∼4.1×l09 Joules. This is about 0.03% of solar wind energy impinging on the magnetosphere and about 3% of the energy deposited on polar cap ionosphere. After 2000 UT, a negative phase gradually became stronger (especially in the southern hemisphere), although the northern subauroral enhancement persisted one more day. The entire ionosphere gradually recovered to normal on January 12. Thus, large middle latitude enhancement, equatorward motion of the dayside enhancement (probably related to a TID), the persistence of the subauroral enhancement, and the conjugate features at both hemispheres are the main characteristics of this storm.