Regions Of Magnetosphere Research Articles

Electric field variability and classifications of Titan's magnetoplasma environment

Abstract. The atmosphere of Saturn's largest moon Titan is driven by photochemistry, charged particle precipitation from Saturn's upstream magnetosphere, and presumably by the diffusion of the magnetospheric field into the outer ionosphere, amongst other processes. Ion pickup, controlled by the upstream convection electric field, plays a role in the loss of this atmosphere. The interaction of Titan with Saturn's magnetosphere results in the formation of a flow-induced magnetosphere. The upstream magnetoplasma environment of Titan is a complex and highly variable system and significant quasi-periodic modulations of the plasma in this region of Saturn's magnetosphere have been reported. In this paper we quantitatively investigate the effect of these quasi-periodic modulations on the convection electric field at Titan. We show that the electric field can be significantly perturbed away from the nominal radial orientation inferred from Voyager 1 observations, and demonstrate that upstream categorisation schemes must be used with care when undertaking quantitative studies of Titan's magnetospheric interaction, particularly where assumptions regarding the orientation of the convection electric field are made.

Climate changes associated with high-amplitude Sq geomagnetic variations

When the solar irradiance propagates between the outer magnetospheric regions and the ionosphere, dynamic processes of the magnetosphere-ionosphere-thermosphere system are affected at the lower end of their paths by the interaction of radiation with the neutral troposphere. The main target of this work is to investigate the relationship between the diurnal magnetic field variations resulting from solar activities and the variation in the troposphere temperature. Meteorological and geomagnetic data acquired from different observatories located in Egypt, Portugal and Slovakia in a long-term and daily-term scales were analyzed.

Statistical evidence for O<sup>+</sup> energization and outflow caused by wave-particle interaction in the high altitude cusp and mantle

Abstract. We present a statistical study of the low (<1 Hz) frequency electric and magnetic field spectral densities observed by Cluster spacecraft in the high altitude cusp and mantle region. At the O+ gyrofrequency (0.02–0.5 Hz) for this region the electric field spectral density is on average 0.2–2.2 (mV m−1)2 Hz−1, implying that resonant heating at the gyrofrequency can be intense enough to explain the observed O+ energies of 20–1400 eV. The relation between the electric and magnetic field spectral densities results in a large span of phase velocities, from a few hundred km s−1 up to a few thousand km s−1. In spite of the large span of phase velocity, the ratio between the calculated local Alfvén velocity and the estimated phase velocity is close to unity. We provide average values of a coefficient describing diffusion in ion velocity space at different altitudes, which can be used in studies of ion energization and outflow. The observed average waves can explain the average O+ energies measured in the high altitude (8–15 RE) cusp/mantle region of the terrestrial magnetosphere according to our test particle calculations.

3-D mesoscale MHD simulations of a cusp-like magnetic configuration: method and first results

Abstract. We present a local mesoscale model of the magnetospheric cusp region with high resolution (up to 300 km). We discuss the construction and implementation of the initial configuration and give a detailed description of the numerical simulation. An overview of simulation results for the case of strongly northward interplanetary magnetic field (IMF) is then presented and compared with data from Cluster 2 spacecraft from 14 February 2003. Results show a cusp diamagnetic cavity (CDC) with depth normal to the magnetospheric boundary on the order of 1–2 RE and a much larger extent of ~5–9 RE tangential to the boundary, bounded by a gradual inner boundary with the magnetospheric lobe and a more distinct exterior boundary with the magnetosheath. These results are qualitatively consistent with observational data.

Simulation of electromagnetic ion cyclotron triggered emissions in the Earth's inner magnetosphere

[1] In a recent observation by the Cluster spacecraft, emissions triggered by electromagnetic ion cyclotron (EMIC) waves were discovered in the inner magnetosphere. We perform hybrid simulations to reproduce the EMIC triggered emissions. We develop a self-consistent one-dimensional hybrid code with a cylindrical geometry of the background magnetic field. We assume a parabolic magnetic field to model the dipole magnetic field in the equatorial region of the inner magnetosphere. Triggering EMIC waves are driven by a left-handed polarized external current assumed at the magnetic equator in the simulation model. Cold proton, helium, and oxygen ions, which form branches of the dispersion relation of the EMIC waves, are uniformly distributed in the simulation space. Energetic protons with a loss cone distribution function are also assumed as resonant particles. We reproduce rising tone emissions in the simulation space, finding a good agreement with the nonlinear wave growth theory. In the energetic proton velocity distribution we find formation of a proton hole, which is assumed in the nonlinear wave growth theory. A substantial amount of the energetic protons are scattered into the loss cone, while some of the resonant protons are accelerated to higher pitch angles, forming a pancake velocity distribution.

Numerical simulations to study whistler turbulence by kinetic Alfvén wave

AbstractThis work presents the model equations governing the excitation of weak whistler by a stronger Kinetic Alfvén wave (KAW) in the plasma having β value (β ≫ me/mi, where beta is the ratio of the ion sound speed to the Alfvén speed), applicable to magnetotail in Earth's magnetosphere, when the ponderomotive nonlinearity is incorporated in the KAW dynamics. Numerical solution of the model equations has been obtained when the incident pump KAW is having a small perturbation. Energy exchange between main KAW and perturbation and the resulting localized structures of the KAW have been studied. A weak whistler signal propagating in these localized structures is amplified and leads to the development of envelope solitons. Our result reveals that the amplified (excited) whistler has an electric field power spectrum that is steeper than k−8/3. This result is consistent with recent observations by the Cluster spacecraft Eastwood et al. (Phys. Rev. Lett., vol. 102, 2009, 035001) in the magnetotail region of the Earth's magnetosphere.

Statistical characteristics of field-aligned currents in Saturn's nightside magnetosphere

During 2008, the Cassini spacecraft traversed Saturn's high-latitude field-aligned current systems on sequential north-south periapsis passes in the nightside magnetosphere. Two types of current systems have previously been identified, associated with antisymmetric azimuthal field signatures in the northern and southern hemispheres. The first exhibits exclusively lagging field morphology, while the second also includes an equatorward interval of leading field. Here we report the statistical characteristics of these currents, their strength, ionospheric location, and relationship to plasma boundaries. From high to low latitude, the first type comprises a downward current followed by an upward current, whose strengths are similar to 0.5-3.5 MA per radian of azimuth. The downward current maps to ionospheric colatitudes of similar to 13.5 degrees and similar to 16 degrees in the north and south, respectively, usually centered in the outer magnetosphere, while the upward current maps to similar to 16.5 degrees and similar to 19 degrees in the north and south, located within the ring current region. The second type comprises a distributed downward current of similar to 1-2 MA rad(-1) flowing in the open field and outer magnetosphere regions, followed by an upward current of similar to 2.5-5 MA rad(-1) mapping to similar to 15.5 degrees and similar to 18 degrees in the north and south, corresponding to the outer magnetosphere and outer ring current, and a further downward current of similar to 1-2.5 MA rad(-1) mapping to similar to 17.5 degrees and similar to 20 degrees in the north and south, corresponding to the inner ring current.

VARIABLE TeV EMISSION AS A MANIFESTATION OF JET FORMATION IN M87?

It is proposed that the variable TeV emission observed in M87 may be produced in a starved magnetospheric region, above which the outflow associated with the VLBA jet is established. It is shown that annihilation of MeV photons emitted by the radiative inefficient flow in the vicinity of the black hole, can lead to injection of seed charges on open magnetic field lines, with a density that depends sensitively on accretion rate, $n_\pm\propto\dot{m}^{4}$. For an accretion rate that corresponds to the inferred jet power, and to a fit of the observed SED by an ADAF model, the density of injected pairs is found to be smaller than the Goldreich-Julian density by a factor of a few. It is also shown that inverse Compton scattering of ambient photons by electrons (positrons) accelerating in the gap can lead to a large multiplicity, $\sim 10^3$, while still allowing photons at energies of up to a few TeV to freely escape the system. The estimated gap width is not smaller than $0.01 r_s$ if the density of seed charges is below the Goldreich-Julian value. The VHE power radiated by the gap can easily account for the luminosity of the TeV source detected by H.E.S.S. The strong dependence of injected pair density on accretion rate should render the gap emission highly intermittent. We also discuss briefly the application of this mechanism to Sgr A$^\star$.

Titan's thermospheric response to various plasma environments

[1] TheCassini‐HuygensmissionhasbeenobservingTitansinceOctober2004,resultingin over 70 targeted flybys. Titan’s thermosphere is sampled by the Ion and Neutral Mass Spectrometer (INMS) during several of these flybys. The measured upper atmospheric density varies significantly from pass to pass. In order to quantify the processes controlling this variability, we calculate the nitrogen scale height for a variety of parameters related to the solar and plasma environments and, from these, we infer an effective upper atmospheric temperature. In particular, we investigate how these calculated scale heights and temperatures correlate with the plasma environment. Measured densities and inferred temperatures are found to be reduced when INMS samples Titan within Saturn’s magnetospheric lobe regions, while they are enhanced when INMS samples Titan in Saturn’s plasma sheet. Finally the data analysis is supplemented with Navier‐Stokes model calculations using the Titan Global Ionosphere Thermosphere Model. Our analysis indicates that, during the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may override the expected solar‐driven diurnal variation in temperatures in the upper atmosphere. Citation: Westlake, J. H., J. M. Bell, J. H. Waite Jr., R. E. Johnson, J. G. Luhmann, K. E. Mandt, B. A. Magee, and A. M. Rymer (2011), Titan’s thermospheric response to various plasma environments, J. Geophys. Res., 116, A03318,

Research Spotlight: Observing Jupiter's polar flares

Jupiter's aurora often emits dramatic flares of ultraviolet light lasting several tens of seconds. Bonfond et al. captured high‐time‐resolution image sequences of the flares using the Space Telescope Image Spectrograph on board the Hubble Space Telescope. The scientists found that these flares occur quasiperiodically, with a time scale of about 2–3 minutes. They also identified the magnetospheric region that corresponds to these emissions, and by analogy with similar flares on Earth, they determined that the flares are probably related to pulsed reconnections of the magnetic field at the planet's dayside magnetopause. (Geophysical Research Letters, doi:10.1029/2010GL045981, 2011)

First radar observations in the vicinity of the plasmapause of pulsed ionospheric flows generated by bursty bulk flows

[1] Recent expansion of the SuperDARN network to mid-latitudes and the addition of a new high-time resolution mode provides new opportunities to observe mid-latitude ultra-low frequency waves and other ionospheric sub-auroral features at high temporal resolution. On 22 February 2008, the Blackstone SuperDARN radar and THEMIS ground magnetometers simultaneously observed substorm Pi2 pulsations. Similarities in measurements from the Blackstone radar and a magnetometer at Remus suggest a common generating mechanism. Cross-phase analysis of magnetometer data places these measurements at the ionospheric projection of the plasmapause, while fine spatial and temporal details of the radar data show evidence of field line compressions. About 1 min prior to ground Pi2 observation, 2 Earthward-moving Bursty Bulk Flows (BBFs) were observed by THEMIS probes D and E in the near-Earth plasma sheet. We conclude that the first 2 pulses of the Pi2s observed at Blackstone and Remus result from compressional energy generated by BBFs braking against the magnetospheric dipolar region.

Anomalous transport effects on the parallelEfield in downward auroral current regions of the Earth's magnetosphere

[1] The physical processes that determine the fluid quantities and the self-consistent electric field (E∥) parallel to the magnetic field have been an unresolved problem in magnetospheric physics for over 40 years. We review a recently developed kinetic and multimoment fluid theory for inhomogeneous, nonuniformly magnetized plasma with temperature anisotropy in the guiding-center and gyrotropic approximation and apply the theory to solve for the quasi steady state in the long-range potential region of a downward Birkeland current sheet when electrostatic ion cyclotron turbulence is dominant. We find that an electron, bump-on-tail-driven ion cyclotron instability produces the turbulence and that a large enhancement in ∣E∥∣ by nearly a factor of 40 occurs when the turbulence is present compared to the case when it is absent. Anomalous momentum transfer (anomalous resistivity) by itself has a very small effect on E∥; however, the presence of the turbulence and the anomalous energy transfers (anomalous heating and cooling) that result have a very large effect on the entire solution. In the electron and ion momentum balance equations for E∥, the turbulence enhances the magnitude of E∥ by reducing the effect of the generalized parallel pressure gradients and thereby enhancing the effect of the mirror forces. A new, nonlinear formula for the current-voltage relationship in downward Birkeland current regions is also given.

Nonlinear disturbance of the dipole field by an axisymmetric plasma distribution

The results of numerical modeling of magnetic field variations with increasing pressure in the inner magnetosphere regions in the axisymmetric case with pressure isotropy are presented. The radial dependence of magnetic field disturbance for the given plasma pressure distribution is determined. The dependence of magnetic field disturbance near the Earth on the total pressure with allowance for the influence of magnetic field of the currents in the plasma on the field value and distribution is obtained. The obtained solutions are applied in an analysis of the large magnetic storm of February, 1986, during which the minimum value of the Dst variation was 307 nT. The plasma pressure values experimentally measured near the equatorial plane were used. It is shown that, with allowance for nonlinear effects, the value of observed Dst variation corresponds to the disturbance caused by the axisymmetric part of a ring current.

Whistler propagation and modulation in the presence of nonlinear Alfvén waves

This paper presents the modulation of whistler waves due to linear interactions between a weak whistler signal and a pump kinetic Alfvén wave (KAW) in intermediate‐β(me/mi ≪ β ≪ 1) plasmas (where β is the ratio of the ion sound speed to the Alfvén speed). As a consequence of ponderomotive nonlinearity, the pump KAW becomes filamented when its power exceeds the threshold for the filamentation instability. The periodic enhancements of the whistler amplitude occur in the locations where the beam width is narrower and are thus produced by the focusing of the whistler beam. Our results reveal that KAW has a (magnetic) power spectrum with an approximate scaling of k−5/3 (where k is ∣k∣). The amplified whistler has electric field power spectra that are steeper than k−8/3 (where k is ∣k∣). These results are consistent with recent observations by the Cluster spacecraft in the magnetotail region of the Earth's magnetosphere.

On the possibility of coupling satellite and ground-based optical measurements in the region of pulsating auroras

The possibility of comparing in detail CLUSTER satellite data with data on pulsating spots registered with a TV camera at the Lovozero Observatory when the satellite crossed the magnetospheric region related to the camera’s field of view is discussed. The satellite ionospheric projections were calculated using the T89, T96, and T01 models. It was shown that the projection allows us to judge with confidence whether or not a satellite will find itself in a region of pulsating auroras when only the level of geomagnetic activity and the characteristics of the interplanetary medium are a priori known. When different models are used in the projection, the spread is not less than the characteristic dimensions of the pulsating spots and can be as high as 100 km. The corresponding satellite flight time is ∼4 min. Such a large spatial and time uncertainty does not allow us to compare in detail the satellite data with ground-based optical measurements without a priori information on, e.g., the character of precipitation above a spot, as has been done by other researchers in the case of auroral arcs. The situation becomes even more complex if a satellite is in the region of greatly stretched magnetic field lines.

Role of the plasmapause in dictating the ground accessibility of ELF/VLF chorus

This study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of ELF/VLF chorus are able to propagate to and be received by midlatitude stations on the ground. First, we explore the effects of plasmapause extent on ground‐based observations of chorus via a 3 month study of ground‐based measurements of chorus at Palmer Station, Antarctica (L = 2.4, 50°S geomagnetic latitude), and data on the plasmapause extent from the IMAGE EUV instrument. It is found that chorus normalized occurrence peaks when the plasmapause is at L ∼ 2.6, somewhat higher than Palmer's L shell, and that this occurrence peak persists across a range of observed chorus frequencies. Next, reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region, distributed in radial distance and wave normal, from which chorus is able to reach Palmer Station via propagation in a nonducted mode. The results of ray tracing are similar to those of observations, with a peak of expected occurrence when the plasmapause is at L ∼ 3. The exact location of the peak is frequency dependent. This supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the L shell of the plasmapause is somewhat beyond that of the observing station. These results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode.

ROMA (Rank-Ordered Multifractal Analyses) of intermittency in space plasmas – a brief tutorial review

Abstract. Intermittent fluctuations are the consequence of the dynamic interactions of multiple coherent or pseudo-coherent structures of varied sizes in the stochastic media (Chang, 1999). We briefly review here a recently developed technique, the Rank-Ordered Multifractal Analysis (ROMA), which is both physically explicable and quantitatively accurate in deciphering the multifractal characteristics of such intermittent structures (Chang and Wu, 2008). The utility of the method is demonstrated using results obtained from large-scale 2-D MHD simulations as well as in-situ observations of magnetic field fluctuations from the interplanetary and magnetospheric cusp regions, and the broadband electric field oscillations from the auroral zone.

Magnetic helicity of a flux rope in the magnetotail: THEMIS results

Abstract. The magnetic field in many regions of magnetosphere has a complex topological structure. As a parameter to measure the topological complexity, the concept of magnetic helicity is a useful tool in magnetospheric physics. Here we present a case study of magnetic helicity in the flux rope (FR) in the near-Earth plasma sheet (PS) based on the in-situ observation from THEMIS for the first time. With the help of the Grad-Shafranov reconstruction technique, we determine the spatial distribution of magnetic field and evaluate the magnetic helicity in the flux rope. The conservation of magnetic helicity during multiple X-line reconnections and the transport of magnetic helicity between different magnetic field configurations are also discussed. The further application of helicity in magnetosphere will provide us more knowledge about the topologic property of the magnetic fields there and more attention should be paid to that.

Cold plasma wave analysis in magneto-rotational fluids

This paper is devoted to investigate the cold plasma wave properties. The analysis has been restricted to the neighborhood of the pair production region of the Kerr magnetosphere. The Fourier analyzed general relativistic magnetohydrodynamical equations are dealt under special circumstances and dispersion relations are obtained. We find the x-component of the complex wave vector numerically. The corresponding components of the propagation vector, attenuation vector, phase and group velocities are shown in graphs. The direction and dispersion of waves are investigated.

A coordinated study of field-aligned currents and Pc5 ULF waves during ejecta 1997

We show examples of long period Pc5 magnetic field pulsations near field-aligned current (FAC) regions in the high-latitude magnetosphere, observed by INTERBALL-Au, and coordinated with POLAR, GOES-9 and ground-based observations during 11 January and 11 April 1997. Identification of corresponding magnetosphere regions and subregions is provided by electrons and protons in the energy-range of 0.01–100 keV measured onboard the spacecraft. The ULF Pc5 wave occurrence is observed in both upward and downward FACs. A fairly good correlation is demonstrated between these ULF Pc5 waves and the consecutive injection of magnetosheath low energy protons. The constancy of the observed frequency peak at 1.8 mHz during quite unsteady solar wind pressure conditions could be reconciled with the surface wave mode model. The 3.1 mHz peak location area probably resembles field-line fluctuations with an interesting appearance of poloidal mode oscillation. It is suggested that the 1.3 mHz wave and its harmonic 2.6 mHz represent global compressional oscillations.