The most remarkable and unexpected feature of Saturn's magnetosphere is that it pulses but the period of the pulses slowly varies with time. The pulsations were originally seen remotely in kilometric radio wave emission but then identified in a rotating magnetic signal. The source of the enigma remains open and it remains easier to undermine proposed explanations than to substantiate them as will be shown.
Outer planet auroras have been imaged for more than a decade, yet understanding their origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were obtained in January 2007, when the Hubble Space Telescope imaged the ultraviolet aurora, while the Cassini spacecraft crossed field lines connected to the auroral oval in the high-latitude magnetosphere near noon. The Cassini data show that the oval lies in the boundary between open and closed field lines, where a layer of upward-directed field-aligned current flows whose density requires downward acceleration of magnetospheric electrons sufficient to produce the aurora. These observations indicate that Saturn's auroras are produced by the magnetosphere-solar wind interaction, through the shear in rotational flow across the open-closed field line boundary.
We present a new model of Saturn's bow shock surface. Shock crossings are identified in Cassini magnetic field and plasma data and these are added to the crossings made by Pioneer 11, Voyager 1 and Voyager 2. Using electron densities determined from Langmuir wave observations by the Radio and Plasma Wave System, the solar wind dynamic pressure for the Cassini crossings is estimated. Corrections for the planet's orbital motion and solar wind dynamic pressure variation are made and a conic section is fitted to the crossings using a least squares technique. We examine and discuss the role played by different parameters in determining the size and shape of the kronian bow shock.
We present strong evidence that on 23 February 2007 between 2230 and 2330 UT the SPEAR system successfully modified the ionospheric electron density inside the northern polar cap. The modulated modification led to the generation of a small scale Ultra-Low Frequency (ULF) wave on open field lines which was detected by ground based magnetometers in the vicinity of the radar. It was not registered by magnetometer stations elsewhere inside the northern polar cap or at lower latitudes. The solar wind conditions are investigated and although wave activity at the modulation frequency is present, it is shown that the local ULF wave is unlikely to originate from that activity.
Gravity waves, tides and planetary waves transport energy and momentum between the troposphere, stratosphere and mesosphere. The waves and tides strongly interact with the mean flow of the mesosphere and largely drive its planetary-scale circulation. This talk will give an overview of these processes and present two particular studies. In the first study, we explore the origins of a "two-day" planetary wave observed in the polar mesosphere. Ground-based radar observations show that this wave reaches large amplitudes in summer, but - unexpectedly - it is also present in winter. Observations with the Microwave Limb Sounder on the EOS Aura satellite reveal that the summer and winter waves are actually entirely different phenomena. The summertime wave appears to be the well-known mesospheric two-day wave, but the winter wave appears to be an Eastward-travelling planetary wave launched from the underlying polar stratosphere. The second study develops new techniques by which a meteor radar can be used to detect gravity waves. Measurements made over Esrange (68N) and Rothera (68S) reveal a seasonal variation in gravity-wave activity. Estimates of momentum flux show strong driving of the mean flow and the short-term variability suggests that ducting and/or other processes may be very important at these heights.
A new optical instrument, the Scanning Doppler Imager (SCANDI) was installed in the Auroral Station at Longyearbyen, Svalbard (78N 16E) in December 2006. All-sky thermospheric neutral wind and temperature fields are produced over a 1000km-diameter projected area, in time scales of minutes through measurements of the Doppler shift and Doppler broadening of the 630nm oxygen red line. The data presented are compared against data from an existing standard Fabry-Perot Interferometer (FPI) which detects the same emission line; and ionospheric parameters measured by the EISCAT Svalbard radar (ESR) and CUTLASS radar. The collocated field-of-views of these instruments and high spatial and temporal resolutions from SCANDI allow further studies of the coupled ionosphere-thermosphere system at meso-scale resolution, i.e. resolutions of 10.s kilometres and minutes.
The distribution of polar and auroral ionospheric plasma on horizontal spatial scales of ~100 km is largely dependent on the high-latitude convection flow pattern, which in turn is governed by the interplanetary magnetic field (IMF). The Coupled Thermosphere Ionosphere Plasmasphere (CTIP) model can be used to simulate ion densities in these high-latitude regions. However, the electric potential patterns available to drive the plasma flow has been restrictive, taken for example from a library of Millstone Hill incoherent scatter radar electric potential patterns and generally incorporating a two-cell convection pattern. In a recent development to the model, electric potential patterns provided by the SuperDARN radar network are used as the convection input, allowing more flexibility and covering both IMF Bz positive and IMF Bz negative cases.
Results are presented from a model run under IMF Bz negative. The output is compared with a previous model run that used a library electric potential pattern as input and with observations by the Aberystwyth University radio tomography experiment. It is concluded that the model with SuperDARN input yields better agreement with the observations. Results are also presented from a case study under IMF Bz positive, where reasonable agreement is again obtained between the model output and tomography observations.
The global pattern of the ionospheric plasma convection can be deduced from characteristics of GPS signals acquired by ground network of dual-frequency GPS receivers. The tomographic inversion of these GPS data in a three-dimensional time-dependent inversion algorithm can reveal the spatial and temporal distribution of ionospheric plasma density. This algorithm has been applied to reconstruct the 4D dynamics of ionospheric plasma content (TEC) and plasma density during major magnetic storms of the recent solar maximum including Oct. 28-30, 2003 ("Halloween") and Nov.20, 2003 super-storms. Comparison between the results of GPS tomography and in-situ measurements of plasma bulk motion by LEO satellites allows conclusions to be made about the degree at which the ionospheric convection flow expands during the major storms and the efficiency of electromagnetic magnetosphere-ionosphere coupling at sub-auroral latitudes.
Non-Thermal Continuum (NTC) radiation is believed to be emitted from sources located in the vicinity of the plasmapause region. Cluster orbit is well adapted to study the beaming properties of those emissions. The two main purposes of the WHISPER experiment are to record the natural waves in the bandwidth 2-83 kHz and to make a diagnostic of the electron density using the sounding technique. The various working modes and the Fourier transforms calculated on board provide a good time and frequency resolution and allow us to detect the fine structure of NTC emissions as well as their spectral characteristics. On September 26, 2003, the Cluster satellites crossed two Continuum radiation beams localized on each side of the geomagnetic equator. According to Jones theory (D. Jones, Planet. Space Sci., 1982), these two beams are the manifestation of a same source which emits beams symmetrical to the geomagnetic equator and at an angle which depends on plasma characteristics at the source localisation. Spin modulation properties, i.e. directivity and modulation factor, give access to the beam direction projected in the spin plane and to an estimation of the angle between the propagation vector and its projection in the spin plane. We use this approach to establish the beam direction observed by each satellite. We discuss these results in the context of Jones theory.
Using recent measurements of energetic electrons by the ACE/EPAM experiment the dependence of the characteristic decay time of impulsive electron events on the large-scale structure of the Interplanetary Magnetic Field (IMF) within which the electrons are emitted is examined. The observations shed light on the physical mechanisms that contribute to the establishment and maintenance of particle reservoirs in the heliosphere in terms of magnetic 'barriers' formed in space beyond 1 AU. Furthermore, Solar Energetic Particle (SEP) fluxes and their anisotropy characteristics observed by the HI-SCALE instrument onboard the ULYSSES spacecraft and the EPAM experiment onboard ACE are utilized as diagnostic tracers of the large-scale structure and topology of the (IMF) embedded within well-identified Interplanetary Coronal Mass Ejections (ICMEs) observed in the heliosphere in and out of the ecliptic plane. The still controversial issue of whether ICMEs have been detached from the solar corona or are still magnetically anchored to it when they arrive at the spacecraft is tackled. Such knowledge of the near-Sun interplanetary field structure and connectivity will be very important for the interpretation of data obtained in the inner heliosphere by Solar Orbiter (SOLO launch 2015) and the Inner Heliospheric Sentinels (IHS launch 2017) recently christened as the HELEX joint ESA/NASA mission. Magnetic connectivity will be determined at multiple points in the inner heliosphere by measuring energetic electrons and the associated X-ray and radio emissions and these measurements will be used to trace the magnetic field lines directly to the solar source regions.
Solar wind fluctuations typically are suggestive both of intermittent turbulence (for example, a robust power law region of the power spectrum with ~ -5/3 exponent) and at lower frequencies, of fluctuations of coronal origin (with scaling close to ~Ò1/fÓ). The respective roles of the coronal driver, and the evolving turbulence, in generating the observed scaling signature are yet to be unambiguously determined. In order to eliminate the effect of the large scale complex magnetic topology of the corona, which can also show scaling, we examine ULYSSES magnetic field data during intervals when the spacecraft spent many months in the quiet fast solar wind above the Sun's polar coronal holes. We quantify the scaling properties of fluctuations in a statistical sense, on different time-scales τ using generalised structure functions (GSF) to test for a power law scaling ~ τζ(p). We recover approximate power law scaling for the Ò1/fÓ range using GSF and test the scaling exponents for secular trend with latitude and radial distance from the sun. At higher frequencies, were we would expect an inertial range of turbulence, the structure functions do not show power law scaling with ; however power law scaling is recovered under Extended Self Similarity (ESS), that is, in the ratios of structure functions. Thus the inertial range scaling is of the form g(τ)ζ(p). We show that a single function g(τ) captures all the time periods examined, i.e. no latitudinal or radial dependency was found. This is highly suggestive that the higher frequency range is indeed generated by local phenomenology, that is, evolving turbulence, whereas at lower frequencies (the "1/f" range) the relevant phenomenology may be coronal, with implications for our understanding of coronal heating of the solar wind.
Using multi-instrument and multi-wavelength observations (SOHO/EIT, SOHO/MDI, Hα, Kanzelhöhe Observatory, SOHO/LASCO) combined with ACE in situ data, we describe and characterise an interplanetary magnetic cloud (MC) observed near Earth on 13 April 2006. We attempt to determine the solar source of the MC, and provide evidence that the eruption could have originated from this region. Active region (AR) 10869, which was observed slightly south west of the disk centre was the only active region on the disk on 10 April 2006; the time at which the MC observed near Earth was likely to have been ejected from the Sun. The ACE spacecraft detected, 77 hours later, the arrival of the MC.
The link between this eruption from AR10869 and the interplanetary MC is supported by several pieces of evidence including the propagation time of the ejecta and the location of the solar source near disk centre. However, there is evidence that suggests that the magnetic helicity sign in the MC and the AR may not agree, presenting the possibility that despite the strong links that can be made between AR10869 and the MC, this may not in fact be the source region for this interplanetary MC.
A preliminary survey of other possible solar sources of the MC indicates that active regions located very near to the east and west solar limbs may be candidate regions, since both areas show evidence of eruptive activity on and around the estimated time of the CME eruption.
EISCAT Interplanetary Scintillation observation and modelling techniques are used to investigate the effects on the solar wind of a series of solar eruptive events that occured between the 3rd. May and 7th. May 2007. Extrapolations are made from transient solar wind features seen in these observations to attempt to identify the location and timing of the eruptions. Coronal configuration at that time suggests interaction between CME and CIR material maybe possible which makes this quite a complex series of events. Results thus far are presented and direction of further research into these events are discussed.
The response of the magnetosphere and ionosphere to the CME events of 07-12 November 2004 is presented using Cluster and ground-based (EISCAT Svalbard radar or ESR, EISCAT VHF radar, Jicamarca IS radar, GPS-TEC, ionosondes and magnetometer) observations. The response of the ionosphere is also modelled. The observations show (i) strong magnetospheric compression at the impact of the CME and dawnward-duskward oscillation of the magnetosphere due to changes in IMF By, (ii) a rare super double geomagnetic storm with three positive initial phases, (iii) strongest ever recorded prompt penetration electric field (PPEF), (iv) direct response of the high latitude ionosphere to the CMEs, and (v) delayed positive ionospheric storms at low-mid latitudes in longitudes where the main phase onset of the geomagnetic storm occurred in the morning sector. The model results suggest that penetration electric field is unlikely to contribute to the positive ionospehric storms, and it is the direct effect of the equatorward neutral wind that is the main driver of positive ionospheric storms.
After 35 years manned space exploration of the Moon and Mars is back on the agenda. With it comes the far greater concern for the health risks to astronauts due to Space Weather events causing acute radiation sickness and/or death. The probability is very high that a significant Solar Proton Event will envelope a space craft on these longer missions.
Several innovative "Active Shield" mitigation solutions, reminiscent of Star Trek "deflector shields", have being proposed along with material shielding and biochemical protection[1].The work presented here concerns recent modelling and laboratory experiments done on the artificial or mini-magnetosphere concept.
The idea, first seriously considered in the 1960's, is to put a portable artificial magnetosphere around the space craft to create a magnetically confined plasma barrier, not unlike an in-side out tokamak. This solution imitates the terrestrial magnetosphere, it deflects or buffers much of the worst effects of SEPs and prevents otherwise lethal radiation levels on the Earth's surface.
However the question is does the magnetosphere have to be as big as the Earth's in order to work? Some recent computer simulations will be shown which demonstrate that in principle the small scale shield does work. This requires the use of a 3D Particle-In-a-Cell (PIC) code, dHybrid[2], which includes the particle kinetics. Although computationally demanding, it can simulate aspects like the Bow Shock because, unlike an MHD code, the code differentiates between particle species and thus allows for the generation of electric fields and currents which is fundamental to understanding how the magnetosphere "transport barrier" or shield will work.
Now we have the first experimental results, movies, photos and data, showing a flowing "solar wind" plasma, with an equivalent embedded IMF, impacting a magnetised object representing the inhabited space craft. The narrowness of the mini-magnetopause which is of the order of the electron skin depth, demonstrates the importance of including the particle kinetics in the modelling.
[1] Adams, J.H. et al., "Revolutionary Concepts of Radiation Shielding for Human Exploration of Space", NASA/TM-2005-213688, March 2005.
[2] Gargate, L.; Bingham, R.; Fonseca, R. A.; Silva, L. O., "dHybrid: A massively parallel code for hybrid simulations of space plasmas", Computer Physics Communications, Volume 176, Issue 6, Pages 419-425, 15 March 2007, doi:10.1016/j.cpc.2006.11.013
Reconnection is an intrinsic requirement to the description of magnetospheric dynamics. The CLUSTER multi-spacecraft mission has, with its wide range of instruments, provided in-situ data of higher resolution than ever before. Analytical simulation models based on a Petschek-type reconnection layer, using the Rankine-Hugoniot jump conditions, have been constructed using information from dayside magnetopause crossings. In the past the jump conditions have been used as verifiers of MHD wave mode presence rather than comprising, as in this case, a complete analytic framework. Preliminary comparison between the models and observed data suggest that the method may be useful for, among other things, acting as a tool providing a description of the event structure, or allowing statistical classification of MHD wave modes involved in events in terms of crossing location or local conditions. Preliminary case studies have rendered promising correlations between models and observed data.
Between 1920 and 1950 UT on 27th January 2006, the Cluster spacecraft straddled the dayside magnetopause slightly equatorward of the cusp in the post-noon sector. The spacecraft formed a tetrahedron with a scale length of 10,000 km. The lagged IMF was largely directed sunward, but with weak and approximately equal duskward and southward components. A series of flux transfer events (FTEs) were observed by differing subsets of the Cluster spacecraft, three of which were observed by all four spacecraft. Multi-spacecraft timing analysis indicates that these FTEs moved predominantly latitudinally, but their observation at all four spacecraft indicates that these three FTEs extend longitudinally for at least 10,000 km. Simultaneously, pulsed ionospheric flows (PIFs) were observed, although there is no clear one-to-one correlation between the PIFs and the FTEs. We present in situ observations, along with attempts to quantify the flux reconnected during this interval using ground-based observations.
On July 20th 2005 a structured Pc1 pulsation was observed simultaneously across more than 18 degrees of corrected geomagnetic latitude using the Finnish Pulsation Magnetometer Chain. The pulsation, consisting of a series of band limited, falling tones displays anomalous polarization features and a periodicity that is latitudinally invariant.
The generation mechanism for this category of pulsation is known to be ion-cyclotron resonant interactions; and polarisation and dispersion features will be discussed in terms of that interaction between highly energetic particle populations and local wave activity. The latitudinal invariance of intensity maxima separation will be discussed and its modulation by a Field Line Resonance will be supported by observations made with the IMAGE magnetometer network.
This study investigates the relationship between the trapped and precipitated components of the >30 keV and >100 keV auroral electron flux measured by the polar orbiting environmental satellites (POES) which are in sun-synchronous orbits at an altitude of about 850 km. The data are restricted to selected over-passes of the Kilpisjärvi imaging riometer by the POES NOAA-12 and NOAA-14. The analysis of the data addresses the short-term variation of the fluxes and the relation between particle flux and the ionospheric radiowave absorption (A dB) measured by the imaging riometer. The precipitated flux during each over-pass was found to fluctuate to a much greater extent than the trapped, particularly in the night sector, though this variation decreased with local time through the morning sector into the day-side. The continuity equation appropriate to the D-region (q=αeNe2) indicates that ; A∝ flux2 it was found that though this relation does indeed hold for the trapped flux, for the precipitated it is only applicable on the day-side, and less so than for the trapped. By considering the theory of pitch angle diffusion applied to the detector geometry and its orientation with respect to the geomagnetic field, it is shown that the reason for this discrepancy might be that not all of the particles reaching auroral altitudes are registered by the precipitated particle detector. Prediction formulae are provided which relate ground-based measurements of absorption to spacecraft measurements of particle fluxes.
Using Cluster observations of the Earth's quasi-perpendicular bowshock we find that, contrary to previous findings, there can be a significant disagreement between the timing method estimate of the shock normal and the Peredo model estimate. We propose that in most cases, this disagreement may be due to shock variability.
Using cross-correlation of magnetic field magnitude profiles from each spacecraft, we investigate the spatial and temporal scales of this variability, aiming to discover a characteristic value for each. We find that the variability correlation decreases as the spacecraft spatial and temporal separation increases.
The results are discussed in the light of elements which are central to this analysis: the transition to the NIF frame and the angle at which the spacecraft crosses the shock.
The interplanetary medium upstream of Saturn can have a very strong impact on magnetospheric dynamics. During the declining phase of the solar cycle, the heliosphere is highly structured by Corotating Interaction Regions (CIRs), while this pattern breaks down somewhat closer to solar minimum.
We compare Cassini magnetometer data from the cruise to Saturn, and from a period after Saturn Orbit Insertion (SOI) where Cassini had a prolonged excursion into the solar wind. We present the predicted and observed values for the Parker Spiral angle over this long time interval upstream of Saturn and discuss the implications of this.
The pre-SOI data encompass an interval where the solar wind was highly disturbed by the "Halloween Storms", a period of extreme solar activity during October/November 2003. The effects of these storms have been widely studied in the vicinity of the Earth, where some of the highest solar wind speeds ever were recorded. We examine how this unusual solar wind evolved by the time it reached Cassini, and discuss the expected magnetospheric response. This is then in turn contrasted with the more typical solar wind conditions, and solar cycle effects taken into account.
The solar wind flow has a magnetic Reynolds number estimated to be ~105; and fluctuations in solar wind bulk plasma parameters typically show a clear region of power law scaling in the power spectrum with an exponent close to the Kolmogorov prediction of -5/3. Quantitative analysis of solar wind fluctuations are thus often performed in the context of intermittent turbulence and center around methods to quantify statistical scaling, such as generalized structure functions which assume a weakly stationary process. The solar wind exhibits large scale secular changes and so the question arises as to whether the timeseries of the fluctuations is non-stationary. One approach is to seek a local stationarity by restricting the time interval over which statistical analysis is performed. However multifractality implies that the scaling exponents are local in time. Computing scaling exponents over different intervals of a stationary multifractal process can thus potentially yield anomalously time varying values for the scaling exponents, suggestive of non-stationarity. We investigate this using synthetic datasets generated from a (self affine) Levy flight and from a (multifractal) p-model. We are able to estimate the minimum interval (number of datapoints) needed to quantify the scaling exponents in a stationary multifractal process. With fewer datapoints the stationary timeseries becomes indistinguishable from a nonstationary process and we illustrate this with nonstationary synthetic datasets. Finally we apply these ideas to in-situ solar wind observations.
Particle acceleration consequences from fluctuating electric fields superposed on an X-type magnetic field in collisionless solar plasma are studied. Such a system is chosen to mimic generic features of dynamic reconnection, or the reconnective dissipation of a linear disturbance. We explore numerically the consequences for charged particle distributions of fluctuating electric fields superposed on an X-type magnetic field. Particle distributions are obtained by numerically integrating individual charged particle orbits when a time varying electric field is superimposed on a static X-type neutral point. This configuration represents the effects of the passage of a generic MHD disturbance through such a system. Different frequencies of the electric field are used, representing different possible types of wave. The electric field reduces with increasing distance from the X-type neutral point as in linear dynamic magnetic reconnection. The resulting particle distributions have properties that depend on the amplitude and frequency of the electric field. In many cases a bimodal form is found. Depending on the timescale for variation of the electric field, electrons and ions may be accelerated to different degrees and often have energy distributions of different forms. Protons are accelerated to gamma-ray producing energies and electrons to and above hard X-ray producing energies in timescales of 1 second. The acceleration mechanism is possibly important for solar flares and solar noise storms but is also applicable to all collisionless plasmas.
Plasma bubbles are one proposed explanation of Bursty Bulk Flows, which are thought to be a major contributor to the Earthward transport of magnetic flux and plasma through the magnetotail. They consist of depleted flux tubes with a lower PVγ content than the ambient plasma and convect Earthward through the action of the interchange instability. The multiscale and large-separation (up to 10,000km) configurations of the four Cluster spacecraft in 2005, 2006 and 2007 give us the opportunity to observe these structures, and the space local to them, simultaneously, allowing us to measure the effect a bubble's propagation has on its surroundings.
Here we present observations from 21st September 2005, where a plasma bubble passes over three of the Cluster spacecraft, missing the fourth. An enhanced electric field is observed within the bubble structure, consistent with the observed Earthward flow, while a tailward-directed, return flow was measured at the bubble edge. Evidence of field-aligned currents is seen at the edges of the plasma bubble, and enhanced wave activity in its wake. We compare these observations to theoretical predictions and the results of 3D MHD simulations.
We will investigate the position and energy distribution of ions in the Venusian upper atmosphere. These exhibit a complex pattern, with both ionospheric and pickup ions present. We will present an analysis of the populations of heavy ions ordered with particular respect to the boundary layers. The data is being taken from the ASPERA-4 IMA instrument onboard Venus Express and being analysed using the CL software.
The WIND and ACE spacecraft spend extended (from weeks to years) intervals in the solar wind, potentially providing observations of the spatial correlation properties of both coherent structures and evolving turbulence. We estimate the correlation length scale in the solar wind using a nonlinear measure- mutual information- and linear cross correlation, the latter to facilitate comparison with previous work. We consider quantities such as Elsasser variables expected to characterize Alfvenic turbulent fluctuations alongside 'non cascade' quantities such as the magnetic field magnitude and the proton density. We find that the magnetic field magnitude and the proton density have different correlation length scales to each other. We also consider variation with the solar cycle; this strongly modulates the correlation lengthscale seen in ρ and |B|, but not that seen in the Elsasser variables for example. Our quantitative estimates of correlation lengthscales relate directly to the phenomenology of the solar wind, either in the context of turbulence, or that of propagating structures of coronal origin.
Various energetic particle populations are observed in the vicinity of the Earth's bow shock. Amongst these are beams of ions propagating upstream along interplanetary magnetic field lines that intersect the quasi-perpendicular region of the shock. The origin of these field-aligned beams has defied a full explanation since their discovery forty years ago. In this paper we obtain the conditions that a particle in the incident solar wind distribution needs to satisfy in order to escape upstream after undergoing specular reflection at the shock. The number density and minimum energy of ion beams produced through specular reflection are calculated and shown to be in good agreement with past observations and computer simulations.
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