ALBERT

Description: ABSTRACT With the objective to understand the generation, propagation and nonlinear evolution of ion cyclotron waves (ICWs) in the corona and solar wind, we use electromagnetic hybrid (kinetic ions, fluid electrons) simulations with a non-uniform magnetic field. ICWs are generated by the temperature anisotropy of O 5+ ions as minority species in a proton-electron plasma with uniform density. A number of magnetic field models are used including radial and spiral with field strength decreasing linearly or with the square of the radial distance. O 5+ ions with perpendicular temperature larger than parallel are initially placed in the high magnetic field regions. These ions are found to expand outward along the magnetic field. Associated with this expansion, ion cyclotron waves propagating along the magnetic field are also seen to expand outward. These waves are generated at frequencies below the local gyro-frequency of O 5+ ions propagating parallel and anti-parallel to the magnetic field. Through analysis of the simulation results we demonstrate that wave generation and absorption takes place at all radial distances. Comparing the simulation results to observations of ICWs in the solar wind shows some of the observed wave characteristics may be explained by the mechanism discussed in this paper.

Description: We use a new magnetohydrodynamic (MHD) model to study the effects of thermal-electron heating in Titan's ionosphere. This model improves the previously used multispecies MHD model by solving both the electron and ion pressure equations instead of a single plasma pressure equation. This improvement enables a more accurate evaluation of ion and electron temperatures inside Titan's ionosphere. The model is first applied to an idealized case, and the results are compared in detail with those of the single-pressure MHD model to illustrate the effects of the improvement. Simulation results show that the dayside ionosphere thermal pressure is larger than the upstream pressure during normal conditions, when Titan is located in the dusk region; thus Saturn's magnetic field is shielded by the highly conducting ionosphere, similar to the interaction of Venus during solar maximum conditions. This model is also applied to a special flyby of Titan, the T34 flyby, which occurred near the dusk region. It is shown that better agreement with the magnetometer data can be achieved using the two-fluid MHD model with the inclusion of the effects of thermal electron heating. The model results clearly demonstrate the importance of thermal-electron heating in Titan's ionosphere.

Description: [1]  Rockets and satellites have previously observed small-scale Alfvén waves inside large-scale downward field-aligned currents and numerical simulations have associated their formation with self-consistent magnetosphere-ionosphere coupling. The origin of these waves was previously attributed to ionospheric feedback instability, however we show that they arise in numerical experiments in which the instability is excluded. A new interpretation is proposed in which strong ionospheric depletion and associated current broadening (a nonlinear steepening/wavebreaking process) form magnetosphere-ionosphere waves inside a downward current region and these oscillations drive upgoing inertial Alfvén waves in the overlying plasma. The resulting waves are governed by characteristic periods, which are a good match to previously observed periods for reasonable assumed conditions. Meanwhile, wavelengths perpendicular to the magnetic field initially map to an ionospheric scale comparable to the electron inertial length for the low-altitude magnetosphere, but become shorter with time due to frequency-based phase mixing of boundary waves (a new manifestation of phase mixing). Under suitable conditions, these could act as seeds for the ionospheric feedback instability.

Description: [1]  This paper reports a new global multispecies single-fluid MHD model that was recently developed for Venus. This model is similar to the numerical model that has been successfully applied to Mars. Mass densities of proton and three important ionospheric ion species (O + , O 2 + , and CO 2 + ) are self-consistently calculated in the model by including related chemical reactions and ion-neutral collision processes. The simulation domain covers the region from 100 km altitude above the surface up to 24 R V in the tail. An adaptive spherical grid structure is constructed with radial resolution of about 5 km in the lower ionosphere. Bow shock locations are well reproduced for both solar-maximum and solar-minimum conditions using appropriate solar wind parameters for each case. It is shown that the shock locations are farther from the planet during the solar maximum condition, because of both the enhanced solar radiation strength and the relatively small Mach number. The simulation results also agree well with Venus Express observations, as shown by comparisons between model results with magnetic fields observed by the spacecraft.

Description: [1]  Geomagnetic storms are frequently associated with the formation of multiple bands of energetic electrons inside the inner radiation belt at L = 1.1-1.9 and with prominent energy structures of protons inside the slot region at L = 2.2–3.5. These structures typically from 100 keV up to the MeV range result from coherent interactions of energetic particles with quasi- monochromatic Ultra Low Frequency waves (ULF). These waves are induced by magnetospheric changes due to the arrival of dense solar material and related nightside injections of particles from the outer magnetosphere that destabilize field lines in the inner magnetosphere down to L = 1.1. Using low-altitude data from the polar orbiting Demeter spacecraft, we perform case and statistical studies of these structures. We show that with such a spacecraft, these structures are best seen near the South Atlantic Anomaly because of lowering of the belt particle mirror point. As evidenced from ground measurements, energy bands are associated with quasi-sinusoidal ULF Pc5 and Pc4 waves with periods in the 1000 second range for L = 1.1–1.9 and in the 60 second range for L = 2.2–3.5. Numerical simulations of the coherent drift resonance of energetic particles with Ultra Low Frequency waves show how the particles are accelerated and how the observed structures build up.

Description: This work considers the global correlation of geomagnetic activity as a way to evaluate magnetotail disturbances, such as the substorm. Impulsive magnetotail disturbances are generally associated with geomagnetic pulsations, which can be coherent over wide ranges of latitude and longitude, and which display distinctive phase reversals collocated with power maxima. Analyzing a disturbance period chosen for its breadth in local time, we find that pulsations can be detected from the coherence that they generate within a magnetometer array, and identify an extended line of nodes across which the phase reversals occur. Phase reversals consistent with the same line of nodes persist for five hours, beginning clearly in a 0.7 mHz pulsation one and a half hours before the disturbance, and persisting in a 5.8 mHz pulsation three and one half hours after the initial disturbance. Under the hypothesis that the line of nodes maps to a source in the central plasma sheet (CPS), we note that the persistence of this extended source of disturbances suggests memory in the CPS. We define a quantitative “coherence index” that characterizes geomagnetic activity according to the degree of global coherence that it generates, and observe that a narrowly peaked coherence signal leads a much broader peak in power. We relate these results to models of the magnetosphere based in critical phenomena.

Description: In this work we study the waves in regions adjacent to ten interplanetary (IP) shocks formed by the interactions between interplanetary coronal mass ejections and the solar wind. We analyze the STEREO data for the years 2007–2010. Shocks in our sample have low magnetosonic Mach numbers (Mms ≤ 2.3), their criticality ratios range between 0.8 and 2.3 and θBn are between 38° and 85°. We find ultra-low frequency (ULF, 0.01 Hz–0.05 Hz) waves and higher-frequency (HF, ≥ 1 Hz) whistler precursors upstream of these shocks. Downstream of them we observe irregular ULF fluctuations and regular HF waves with similar frequencies as in the upstream case. We find that IP shocks with relatively small Mms can excite waves in large regions in front of them (2.2 × 10−3 AU–4.6 × 10−3 AU), thereby forming large ULF wave foreshocks. We do not find any evidence for the steepening of these waves. We do observe suprathermal (E ≤ 30 keV) proton foreshocks upstream of some of the shocks in the sample. The extensions of suprathermal proton foreshocks range between 0.02 AU and 0.1 AU. However, not all foreshocks with suprathermal ions show ULF waves or vice versa. The extensions of ULF and proton foreshocks can be very different. Enhanced ULF waves and suprathermal protons can be observed upstream of local quasi-perpendicular shocks. We propose that the observed discordance between the shock geometries and the presence of the foreshock phenomena may be explained in terms of temporal and spatial variations of the local geometry of the IP shocks.

Description: Flux transfer events observed at Mercury, Earth, and Jupiter are attributed to spatially and temporally limited events in which the magnetosheath and magnetospheric magnetic field become interconnected and magnetic flux is transported from the dayside to the lobes of the magnetotail. Examination of the Saturnian magnetopause at local times from 1000 to 1400 shows no evidence for this phenomenon. Nevertheless, we do find brief intervals during which the normal component of the magnetic field across the magnetopause becomes significantly enhanced for typically one to ten minutes. Magnetosheath electrons appear during these episodes of enhanced magnetic field normal components indicating that indeed the magnetosphere is connected to the magnetosheath by these magnetic bridges. To determine if this magnetic connection leads to a measurable transfer of magnetic flux from the dayside, we check the location of the magnetopause standoff distance for both northward and southward magnetosheath fields. In 71 crossings, we find no obvious dependence of the distance on the direction of the magnetosheath field, indicating that the direction of the interplanetary magnetic field is not a major factor in the determination of the location of the Saturnian magnetopause. This is unlike the position of the terrestrial magnetosphere that undergoes significant motion through reconnection with the interplanetary magnetic field.

Description: Giant pulsations (Pgs; frequency ∼10 mHz) were detected with ground magnetometers on the North American continent on 19 October 2008, when the GOES-10, -11, -12, and -13 geostationary satellites and the THEMIS-A probe were magnetically connected to the region of the ground pulsation activity. This unique configuration allowed us to determine the properties of magnetospheric ultra-low-frequency (ULF) waves that caused the Pgs on the ground. All spacecraft detected monochromatic ULF waves at ∼10 mHz, and the coherence between the Pg at the Gillam ground station and the ULF wave at THEMIS-A was high when the magnetic field foot point of the spacecraft came close to the ground station. The ULF waves observed by the five spacecraft had perturbations in the radial and compressional components of the magnetic field and in the azimuthal component of the electric field, which are attributed to poloidal mode standing Alfvén waves. The poloidal waves were accompanied by multiharmonic toroidal waves, and from the frequency relationship among these, it is concluded that the ∼10 mHz oscillations correspond to the fundamental (odd, or symmetric) mode. The standing wave mode also explains the amplitude variation with latitude and the phase delay between the magnetic and electric fields. Numerical models of poloidal waves incorporating finite height integrated ionospheric conductivity indicate that the fundamental mode interpretation is valid even when the damping of the standing waves is strong. Our observations are the most comprehensive to date in terms of spacecraft data, and we believe that theoretical work on the Pg generation mechanism should focus on mechanisms specific to odd mode standing waves, such as drift resonance of ring current ions.

Description: Saturn's magnetosphere is replete with magnetospheric periodicities; magnetic fields, plasma parameters, energetic particle fluxes, and radio emissions have all been observed to vary at a period close to that of Saturn's assumed sidereal rotation rate. In particular, periodicities in Saturn's magnetotail can be interpreted in terms of periodic vertical motion of Saturn's outer magnetospheric plasma sheet. The phase relationships between periodicities in different measurable quantities are a key piece of information in validating the various published models that attempt to relate periodicities in different quantities at different locations. It is important to empirically extract these phase relationships from the data in order to distinguish between these models, and to provide further data on which to base new conceptual models. In this paper a simple structural model of the flapping of Saturn's plasma sheet is developed and fitted to plasma densities in the outer magnetosphere, measured by the Cassini electron spectrometer. This model is used to establish the phase relationships between magnetic field periodicities in the cam region of the magnetosphere and the flapping of the plasma sheet. We find that the plasma sheet flaps in phase with Br and B$\theta$ and in quadrature with the B$\varphi$ component in the core/cam region. The plasma sheet phase also has a strong local time asymmetry. These results support some conceptual periodicity models but are in apparent contradiction with others, suggesting that future work is required to either modify the models or study additional phase relationships that are important for these models.