N. E. Turner

Evaluation of the Tail Current Contribution to Dst

The Dst index is produced using low-latitude ground magnetic field measurements and frequently is used as an estimate of the energy density of the ring current carried mainly by energetic (∼ 10 – 200 keV) ions relatively close to the Earth. However, other magnetospheric current systems can cause field perturbations at the Earths surface: for example, dayside magnetopause currents are known to contribute to the Dst index. It has also been suggested that the nightside tail current sheet can significantly affect the Dst index during high magnetic activity periods when the currents are intense and flow relatively close to the Earth. In this study, several disturbed periods are input into Tsyganenko magnetic field models. From the time series of the external and internal fields an artificial Dst index is computed using the same procedure followed in the actual Dst calculation. A tail region in the magnetosphere is explicitly defined and the T96 and T89 models are used to calculate the effect of current within this tail region on ground measurements and therefore on Dst. The results are then compared with the measured Dst to determine the tail current contribution to Dst. It is found that for a geomagnetic storm and a storm-time substorm with Dst of ∼ 80 nT the tail current contribution is between 22 and 26 nT. The same analysis is also applied to several isolated non-storm-time substorms, yielding a nearly linear relationship between Dst and the tail current contribution. This contribution is approximately one quarter of Dst.

Correlation of changes in the outer‐zone relativistic‐electron population with upstream solar wind and magnetic field measurements

A study has been made of the correlation of the population of relativistic electrons in the outer-zone magnetosphere with the properties of the solar wind (speed, density, magnetic field) during a solar minimum period. The study is based upon observations made in the Spring of 1995 with sensors aboard 1994-026 and WIND. It is found that a large relativistic electron enhancement depends upon a substantial solar-wind speed increase associated with precursor solar-wind density enhancement, and, in particular, upon a southward turning of the interplanetary magnetic field.

Recurrent geomagnetic storms and relativistic electron enhancements in the outer magnetosphere: ISTP coordinated measurements

New, coordinated measurements from the International Solar-Terrestrial Physics (ISTP) constellation of spacecraft are presented to show the causes and effects of recurrent geomagnetic activity during recent solar minimum conditions. It is found using WIND and POLAR data that even for modest geomagnetic storms, relativistic electron fluxes are strongly and rapidly enhanced within the outer radiation zone of the Earths magnetosphere. Solar wind data are utilized to identify the drivers of magnetospheric acceleration processes. Yohkoh solar soft X-ray data are also used to identify the solar coronal holes that produce the high-speed solar wind streams which, in turn, cause the recurrent geomagnetic activity. It is concluded that even during extremely quiet solar conditions (sunspot minimum) there are discernible coronal holes and resultant solar wind streams which can produce intense magnetospheric particle acceleration. As a practical consequence of this Sun-Earth connection, it is noted that a long-lasting E>1MeV electron event in late March 1996 appears to have contributed significantly to a major spacecraft (Anik E1) operational failure.

Energetics of Magnetic Storms Driven by Corotating Interaction Regions: A Study of Geoeffectiveness

We investigate the energetics of magnetic storms associated with corotating interaction regions (CIRs). We analyze 24 storms driven by CIRs and compare to 18 driven by ejecta-related events to determine how they differ in overall properties and in particular in their distribution of energy. To compare these different types of events, we look at events with comparable input parameters such as the epsilon parameter and note the properties of the resulting storms. We estimate the energy output by looking at the ring current energy along with ionospheric Joule heating derived from the PC and Dst indices. We also include the energy of auroral precipitation, estimated from NOAA/TIROS and DMSP observations. In general, ejecta-driven storms produce more intense events, as parameterized by Dst*, but they are usually not as long lasting, and in most cases deposit less energy. This is observed even for events that have similar input quantities, such as epsilon. This may be related to the high speed of the solar wind, in that an increased magnetosonic Mach number may influence the reconnection rate and therefore the coupling. Additionally, we find the efficiency of the coupling varies greatly from CIR-driven to ejecta-driven storms, with the CIR-driven storms coupling substantially more efficiently, particularly in the recovery phase. The efficiency of coupling (output energy divided by input energy) for CIR-driven storms in recovery phase was double that of ejecta-driven storms.

Ring current ion composition during solar minimum and rising solar activity: Polar/CAMMICE/MICS results

Abstract : This report shows statistical results of the ring current ion composition and its variability as a function of solar cycle and magnetospheric activity. Spin-averaged energetic particle (1-200 keV) measurements from the POLAR/CAMMICE/MICS instrument are combined with geomagnetic indices as well as solar wind and IMF observations from the WIND spacecraft during a period from September 1996 to March 1999. The statistics are performed both for time-averaged values for all periods as well as for peak flux values during geomagnetic storms (defined as Dst < -50 nT) that occurred during this period. The average O(+) energy density increases by about a factor of 5 during the rising phase of the solar cycle from the minimum values in 1996, while the average values of H(+) and He show variability but no consistently increasing trend. The O(+) flux is small (below 10%) compared to the hydrogen flux, and the average energy density ranges from a few percent at solar minimum to about 10% at high solar activity time in early 1999. The O(+) flux is typically smaller than the He(+) flux, reaching comparable values only during the latter part of the period when the solar activity increased. Analogously, the energy densities of O(+) and He(+) are about equal during 1996 and 1997, whereas the O(+) energy density is about twice the He(+) energy density during the higher solar activity period in 1998 and early 1999.

Entry of plasma sheet particles into the inner magnetosphere as observed by Polar/CAMMICE

Statistical results are presented from Polar/CAMMICE measurements of events during which the plasma sheet ions have penetrated deeply into the inner magnetosphere. Owing to their characteristic structure in energy-time spectrograms, these events are called “intense nose events.” Almost 400 observations of such structures were made during 1997. Intense nose events are shown to be more frequent in the dusk than in the dawn sector. They typically penetrate well inside L = 4, the deepest penetration having occurred around midnight and noon. The intense nose events are associated with magnetic (substorm) activity. However, even moderate activity (AE = 150–250 nT) resulted in formation of these structures. In a case study of November 3, 1997, three sequential inner magnetosphere crossings of the Polar and Interball Auroral spacecraft are shown, each of which exhibited signatures of intense nose-like structures. Using the innermost boundary determinations from these observations, it is demonstrated that a large-scale convective electric field alone cannot account for the inward motion of the structure. It is suggested that the intense nose structures are caused by short-lived intense electric fields (in excess of ∼1 mV/m) in the inner tail at L=4–5.

Energy Transport and Dissipation in the Magnetosphere During Geomagnetic Storms

Abstract Major geomagnetic storms represent a significant dissipation of energy by the magnetosphere. The energy is derived from the solar wind flow and the subsequent powerful conversion of that energy takes several different forms. Ring current injection and decay, ionospheric Joule heating, particle precipitation into the atmosphere, and several related physical processes are exhibited clearly in large storm events. The modern-day constellation of operating spacecraft gives an unprecedented opportunity to study magnetic storm processes and energetics. This paper focuses on recent coronal mass ejection (CME) events that have been well-observed near the sun. These disturbances are followed from the sun to their arrival in near-Earth space. The reconfiguration of the magnetosphere under the driving influence of CME/magnetic cloud events is examined and the energetics of various forms of input and output are assessed. It is concluded that the present observations in the ISTP era, along with modern modeling techniques, have given us new insights into geomagnetic stormtime energy dissipation. Ionospheric Joule heating and auroral particle precipitation account for a substantial majority of energy dissipation during CME-driven storms (i.e., ≳70%). However, ring current energy injection due to moderate energy ions is also a key. Direct in situ observations of such ions are found to be crucially important because indices such as D st may be contaminated by many other effects.

Energy dissipation during a geomagnetic storm: May 1998

Abstract The energy input to the magnetosphere and its dissipation in the inner magnetosphere during a storm on May 2–9, 1998 is discussed. The methods to evaluate the energy input and the energy dissipation into the ionospheric Joule heat and the ring current are reviewed. It is shown that the energy dissipated in the ionosphere during individual substorms is proportional to the energy input during the same period. However, during very large input, the energy dissipation into the ionosphere seems to grow only slowly, and thus there seems to be a limit to how much energy can be dissipated via Joule heating. The ring current energy is evaluated using the Dst index and direct particle measurements. The reasons for the smaller values obtained by direct particle measurements are discussed. In total, it seems that during this very strong storm the ring current energy was only about half of the Joule heat, but the two together are less than a third of the total energy input. It is suggested that most of the energy during very strong driving is lost back to the solar wind.

Solar Wind-Magnetosphere Coupling During an Isolated Substorm Event: A Multispacecraft ISTP Study

Multispacecraft data from the upstream solar wind, polar cusp, and inner magnetotail are used to show that the polar ionosphere responds within a few minutes to a southward IMF turning, whereas the inner tail signatures are visible within ten min from the southward turning. Comparison of two subsequent substorm onsets, one during southward and the other during northward IMF, demonstrates the dependence of the expansion phase characteristics on the external driving conditions. Both onsets are shown to have initiated in the midtail, with signatures in the inner tail and auroral oval following a few minutes later.

High-altitude polar cap electric field responses to southward turnings of the interplanetary magnetic field

Interplanetary electric field coupling with the magnetosphere has been analyzed predominantly using data from the Wind magnetometer and the Polar electric field instrument. The coupling was investigated using the Polar Electric Field Instrument (EFI) to measure the electric field in the northern polar cap immediately following sharp southward turnings of the IMF as observed by Wind. Southward turnings were chosen which exhibited a sudden change of the IMF north–south component from BZ > 0 to BZ < 0 (GSM coordinates) after an hour or more of relatively stable conditions, and for which Polar was in the northern polar cap. These BZ changes correspond to EY changes in the interplanetary electric field. For each of the 30 identified events, a time was estimated for the arrival of the IMF change at the magnetopause using the solar wind speed observed by the Wind Solar Wind Experiment (SWE), and Polar electric field data were examined to identify responses. For many of the selected events (about one third), abrupt changes of state in the magnetospheric electric field were evident with timing that matched the expected solar wind arrival time at Earth. For events for which additional data were available, we conducted in-depth examination of the individual events using IMP 8, Geotail, and GOES 9. In one such event, GOES 9 data showed a substorm growth phase and onset which also corresponded to features in the solar wind observed by Wind, Geotail, and IMP 8. In addition to the individual event studies, a superposed epoch analysis of all available events revealed a consistent rise in the mean polar cap electric field about 15 min following sharp IMF southward turnings.