M. F. Thomsen

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.

Plasma sheet access to geosynchronous orbit

One years worth of magnetospheric plasma analyzer data from three Los Alamos geosynchronous satellites are used for a statistical study of proton and electron fluxes at geosynchronous orbit and their dependence on local time (LT) and geomagnetic activity level as measured by Kp. When displayed as a function of LT and Kp, the fluxes exhibit distinct boundaries, which are shown to be consistent with a combination of a global pattern of particle drift through the magnetosphere and loss processes mainly due to charge exchange of the ions and auroral precipitation of the electrons. A Hamiltonian energy conservation approach combined with the (U, B, K) coordinate transformation introduced by Whipple [1978] is used to calculate the theoretical position of the separatrix between open and closed drift trajectories (Alfven layer) as a function of particle species, energy, local time, and geomagnetic activity level. The comparison of the theoretical boundaries with the observations confirms the predictions of plasma sheet access to the geosynchronous region. The analysis also provides independent statistical support for previously derived relationships between Kp and the strength of the global convection electric field.

Bulk plasma properties at geosynchronous orbit

We present a comprehensive study of plasma properties at geosynchronous orbit for electron and ion energies between ∼1 eV and ∼45 keV, between 1990 and 2001. The variations of temperature and density are analyzed as functions of local time, magnetospheric convection strength, and the strength of the ring current. Various parameters derived from temperature and density are calculated to elucidate the temporal and spatial location of delivery of plasma sheet material into the inner magnetosphere. We find that the electron and proton densities are greatest in the dawn region for the periods of highest convection and ring current strength. We perform a superposed epoch analysis of 283 geomagnetic storms which occurred between 1991 and 2001 and examine the temporal variation of the plasma at geosynchronous orbit as a function of storm phase. This analysis demonstrates the local time variability of delivery from the plasma sheet into the inner magnetosphere and the concurrent changes in temperature and pressure. We demonstrate that the density of electrons in the plasma sheet is strongly dependent upon the magnetospheric convection strength and, for the first time, upon solar activity. Electron density at geosynchronous orbit is strongly correlated with solar activity. The average plasma sheet electron density at solar maximum can be a factor of two or more higher than that at solar minimum. We also outline a method to estimate the composition of the plasma sheet from MPA measurements and calculate the O+ and H+ density variations with solar cycle as a function of Kp and local time. We show that the O+ and H+ plasma sheet densities increase with increasing solar activity, as does the O+/H+ density ratio. During times of high solar activity and strong convection, the O+ and H+ densities may be comparable.

Variability of the ring current source population

The relationship between the strength of the storm-time ring current and the available density in the presumed source region, the plasma sheet, is examined for 23 storms with Dst * (minimum Dst corrected for magnetopause currents) ranging from -50 to -164 nT. A good correlation is found between the plasma-sheet density at geosynchronous orbit and the minimum Dst * , The minimum Dst * is also well correlated with the eastward interplanetary electric field measured upstream by the Wind spacecraft, in agreement with previous studies, but the dependence of Dst * on plasma-sheet density is unrelated to its dependence on the electric field. The best correlation is between the minimum Dst * and the product of the plasma-sheet density and the eastward interplanetary electric field. These results are consistent with a scenario in which the intensity of the storm-time ring current is determined by a combination of source strength (plasma-sheet density) and injection strength (interplanetary electric field).

Plasma sheet access to the inner magnetosphere

We present here plasma data from the Polar HYDRA instrument giving comprehensive coverage of the inner magnetospheric region from L ∼ 2 outward. Data is projected to an equatorial reference plane yielding a global view of the inner extend of the plasma sheet. We determine the inner boundary for plasma sheet electrons and ions in the μ range 0.05 - 50 eV nT -1 and we compare these to the predicted Alfven boundaries as a function of the geomagnetic activity index Kp. In general, the simple conventional drift paradigm is shown to be globally consistent with the averaged data in the inner magnetosphere, with electrons adhering better to the predicted boundaries than ions. The data are further compared to the geosynchronous slice as measured by the Los Alamos Magnetospheric Plasma Analyzer (MPA) which measures the crossing point of the Alfven boundaries at geosynchronous altitudes with much better statistical resolution than Polar. Integral to the drift model used is an assumption about the form of the global electric field. The agreement with data validates the simple corotation and convection electric field used and shows that this model describes well the average transport for a wide range of geomagnetic activity and over a large part of the inner magnetosphere.

Understanding storm‐time ring current development through data‐model comparisons of a moderate storm

[1] With three components, global magnetosphere (GM), inner magnetosphere (IM), and ionospheric electrodynamics (IE), in the Space Weather Modeling Framework (SWMF), the moderate storm on 19 May 2002 is globally simulated over a 24-hour period that includes the sudden storm commencement (SSC), initial phase, and main phase of the storm. Simulation results are validated by comparison with in situ observations from Geotail, GOES 8, GOES 10, Polar, LANL MPA, and the Sym-H and Dst indices. It is shown that the SWMF is reaching a sophistication level for allowing quantitative comparison with the observations. Major storm characteristics at the SSC, in the initial phase, and in the main phase are successfully reproduced. The simulated plasma parameters exhibit obvious dawn-dusk asymmetries or symmetries in the ring current region: higher density near the dawn and higher temperature in the afternoon and premidnight sectors; the pressure is highest on the nightside and exhibits a near dawn-dusk symmetry. In addition, it is found in this global modeling that the upstream solar wind/ IMF conditions control the storm activity and an important plasma source of the ring current is in the solar wind. However, the ionospheric outflow can also affect the ring current development, especially in the main phase. Activity in the high-latitude ionosphere is also produced reasonably well. However, the modeled cross polar cap potential drop (CPCP) in the Southern Hemisphere is almost always significantly larger than that in the Northern Hemisphere during the May storm.

Plasmaspheric material at the reconnecting magnetopause

During geomagnetic storms, cold and dense plasmaspheric material is observed to drain toward the dayside magnetopause when the solar wind pressure is strong and the interplanetary magnetic field (IMF) is southward. What is the fate of draining plasmaspheric material at the magnetopause? Does the plasmaspheric material participate in the dayside reconnection and then convect on open field lines through the polar cap? Or does the material become captured into the low-latitude boundary layer and then convect on closed field lines around the flanks of the magnetosphere? In this paper, we present observations from the Los Alamos magnetospheric plasma analyzers (MPA) onboard five satellites at geosynchronous orbit during 86 plasmaspheric drainage events. For a set of events where cold plasmaspheric material is observed immediately adjacent to the magnetopause/low-latitude boundary layer, we examine the detailed ion distributions, from ∼1 eV to ∼40 keV, for evidence that the draining plasmaspheric ions and the entering magnetosheath ions are simultaneously present on the same flux tube. Ten cases out of 57 are found where magnetosheath ions and plasmaspheric ions were unambiguously present simultaneously in the same flux tube, which is a signature that the plasmaspheric flux tubes do experience dayside reconnection. An additional ten cases strongly, but not as definitively, support this conclusion. Further, six of seven events with available IMF information have velocity space signatures that are consistent with expectations based on the reconnection process.

A statistical comparison of hot‐ion properties at geosynchronous orbit during intense and moderate geomagnetic storms at solar maximum and minimum

Hot-ion measurements at geosynchronous orbit from the Los Alamos Magnetospheric Plasma Analyzer (MPA) instrument during geomagnetic storms at solar maximum (July 1999–June 2002) and at solar minimum (July 1994–June 1997) are collected, categorized, and analyzed through the superposed epoch technique. To investigate this source of the storm-time ring current, the local time (LT) and universal time (UT) dependence of the average variations of hot-ion fluxes (at the energies of ∼30, ∼17, ∼8, and ∼1 keV), density, temperature, entropy, and temperature anisotropy are examined and compared among four storm categories, i.e., 44 intense storms and 120 moderate storms, defined by the pressure corrected Dst (Dst*), at the two solar extrema. All the hot-ion parameters are highly disturbed around Dst*min; they show distinct peaks or minima and display obvious increase or decrease regions, whose locations do not change much with levels of geomagnetic activity and solar activity. It is also found that intense storms at solar minimum always have the highest (lowest) average peak value (minimum) in each hot-ion parameter. Around Dst*min in each storm category, hot ions are clearly denser near dawn than those near dusk. On the nightside and in the afternoon sector, temperature and entropy during solar minimum storms are usually higher than those during solar maximum storms; there is actually no clear temperature and entropy enhancement during solar maximum storms. During each type of storm, hot ions are isotropic on the nightside but anisotropic (T per /T par > 1) close to noon.

Quiet time densities of hot ions at geosynchronous orbit

Densities of hot (100 eV{endash}40 keV) ions vary with local time at geosynchronous orbit. Typically, the density is maximum near midnight, falls off toward both terminators, and is minimum near noon. This local time dependence is routinely observed in the Los Alamos geosynchronous plasma data. We use a month of these data (June 1996) and a simple model of particle drifts within the magnetosphere to explore its origins. We first show that the nightside density profile is shaped according to the drift path topology: trapped orbits are depleted, and only particles drifting on open trajectories contribute to this population, particularly for observations taken at times of low K{sub p}. Then, we highlight spectral absorption in dayside fluxes, which occurs when the particles pass through the geocorona. A model of charge exchange accounts for the shape and energy range of the absorption signatures observed at low K{sub p} for varying pitch angles. The density absorption computed versus local time matches the observations. Hence these basic principles of particle motion in the terrestrial magnetosphere readily explain the local time dependence of the hot ion density at geosynchronous orbit. {copyright} 1998 American Geophysical Union

Observations of the Earth’s Plasma Sheet at Geosynchronous Orbit

Geosynchronous orbit typically lies within the near‐Earth portion of the plasma sheet and its dayside extension. Los Alamos magnetospheric plasma analyzers (MPA) on three geosynchronous satellites routinely observe the plasma‐sheet ion and electron distributions over the energy range of ∼1 eV to ∼40 keV. Based on these observations, we describe the typical appearance of the plasma sheet at synchronous altitude under both fairly steady and fairly active conditions. We also present a statistical analysis of the bulk properties (density temperature, and anisotropy) of the plasma sheet ion and electron populations, and we illustrate the dependence of these average properties on local time.