Joachim Raeder

Modeling the magnetosphere for northward interplanetary magnetic field: Effects of electrical resistivity

We develop a simple analytic model and use global simulations of Earths magnetosphere to investigate the effects of electrical resistivity on the topology of the magnetosphere for northward interplanetary magnetic field (IMF). We find that for low resistivity values (≲104 Ω m) the magnetosphere remains open after 6 hours of northward IMF. For larger values (≳2×105 Ω m) the magnetic flux of the tail lobes decreases rapidly on the timescale of ∼1 hour. In this case the tail becomes closed, tadpole-shaped, steady state, and of finite length. The tail length decreases with increasing resistivity and becomes as short as about 50 RE for a resistivity value of 106 Ω m. Reconnection between IMF and lobe field lines occurs in all cases and is not significantly affected by the resistivity. However, large values of the resistivity annihilate lobe flux and break the frozen-in condition for closed tail flux tubes, leading to a decoupling of the flux tube motion from plasma convection. These effects make the development of a steady, closed tail of finite length possible. Because resistivity values larger than 102 Ω m are unrealistic for the quiet time tail, we conclude that the magnetosphere is unlikely to ever close and that models which predict the rapid closure and a steady, finite length tail are possibly in error due to numerical resistivity.

Geospace Environment Modeling 2008–2009 Challenge: Ground magnetic field perturbations

helps the users of the modeling products to better understand the capabilities of the models and to choose the approach that best suits their specific needs. Further, metrics!based analyses are important for addressing the differences between various modeling approaches and for measuring and guiding the progress in the field. In this paper, the metrics!based results of the ground magnetic field perturbation part of the Geospace Environment Modeling 2008‐2009 Challenge are reported. Predictions made by 14 different models, including an ensemble model, are compared to geomagnetic observatory recordings from 12 different northern hemispheric locations. Five different metrics are used to quantify the model performances for four storm events. It is shown that the ranking of the models is strongly dependent on the type of metric used to evaluate the model performance. None of the models rank near or at the top systematically for all used metrics. Consequently, one cannot pick the absolute“winner”: the choice for the best model depends on the characteristics of the signal one is interested in. Model performances vary also from event to event. This is particularly clear for root!mean!square difference and utility metric!based analyses. Further, analyses indicate that for some of the models, increasing the global magnetohydrodynamic model spatial resolution and the inclusion of the ring current dynamics improve the models’capability to generate more realistic ground magnetic field fluctuations.

Initial results of high‐latitude magnetopause and low‐latitude flank flux transfer events from 3 years of Cluster observations

We present initial results from a statistical study of Cluster multispacecraft flux transfer event (FTE) observations at the high-latitude magnetopause and low-latitude flanks from February 2001 to June 2003. Cluster FTEs are observed at both the high-latitude magnetopause and low-latitude flanks for both southward and northward IMF. Among the 1222 FTEs, 36%, 20%, 14%, and 30% are seen by one, two, three, and four Cluster satellites, respectively. There are 73% (27%) of the FTEs observed outside ( inside) the magnetopause, which might be caused by the motion of FTEs toward the magnetosheath when they propagate from subsolar magnetopause to the midlatitude and high-latitude magnetopause and low-latitude flanks. We obtain an average FTE separation time of 7.09 min, which is at the lower end of the previous results. The mean B-N peak-peak magnitude of Cluster FTEs is significantly larger than that from low-latitude FTE studies. FTE B-N peak-peak magnitude clearly increases with increasing absolute magnetic latitude (MLAT), it has a weaker dependence on magnetic local time (MLT) with a peak near the magnetic local noon, and it has a complex dependence on Earth dipole tilt with a peak at around zero. FTE periodic behavior is found to be controlled by MLT, with a general increase of FTE separation time with increasing MLT, and by Earth dipole tilt, with a peak FTE separation time at around zero Earth dipole tilt. There is no clear dependence of FTE separation time on MLAT. There is a weak increase of FTE BN peak-peak magnitude with increasing FTE separation time, and we see no clear dependence of it on FTE B-N peak-peak time. When no FTE identification thresholds are used, more accurate calculations of some FTE statistical parameters, including the mean B-N peak-peak time, can be obtained. Further, comparing results with different thresholds can help obtain useful information about FTEs.

Dependence of flux transfer events on solar wind conditions from 3 years of Cluster observations

We investigate the dependence of Cluster high-latitude magnetopause and low-latitude flank flux transfer events (FTEs) on solar wind conditions using measurements from Cluster FGM and CIS and ACE MFI and SWEPAM between February 2001 and June 2003. We find that there are strong dependences of Cluster FTE occurrence on the IMF B-xgsm, B-ygsm, and B-zgsm components but not on the IMF magnitude. There are strong dependences of Cluster FTE occurrence on the IMF clock, tilt, spiral, and cone angles. However, some patterns are significantly different from previous results. The solar wind density, speed, Beta, VxBz, dynamic pressure, and magnetosonic Mach number have different degrees of control on FTE occurrence. FTE separation time is found to be clearly controlled by IMF B-ygsm, B-zgsm, and magnitude, and the IMF clock, tilt, spiral, and cone angles, and weakly controlled by the solar wind VxBz and magnetosonic Mach number. There is no obvious control of it by other IMF and solar wind parameters. FTE peak-peak magnitude is found to be controlled by IMF B-ygsm, B-zgsm, and magnitude and by the solar wind density and dynamic pressure but not by other IMF and solar wind parameters. The FTE dawn-dusk asymmetry is not likely caused by the Parker spiral IMF. Some FTE statistical patterns are strongly dependent on FTE locations. Finally, we see similar to 4% of the FTEs corresponding to a single change in IMF B-zgsm from positive to negative, similar to 4% corresponding to a single change from negative to positive, and similar to 43% corresponding to multiple changes in the sign of IMF B-zgsm, all within the 10-min interval preceding the FTEs. There is still no evidence for a direct correlation between IMF B-zgsm changing sign and FTEs.

The Io masshloading diskc Model calculations

The observations of ion cyclotron waves up to 0.5 RJ beyond the orbit of Io are best explained by the presence of a thin disk of fast neutrals whose ionization and pickup provide the free energy for the waves. We extend the model of Wilson and Schneider l1999r in order to explain the observed properties of this masshloading region, especially the most recent Galileo observations near Io. In the extended model, some of the molecules of sulfur compounds in Ios exobase are first ionized by photoionization, impact ionization, and charge exchange. These charged particles are accelerated in the corotation electric field associated with the motion of the magnetized Io torus plasma that is moving through its exosphere. After a period of acceleration the heavy ions are neutralized by charge exchange with other exospheric neutral particles or combined with local electrons. These newly neutralized particles continue with high velocities similar to those of their former charged state, moving only under the influence of the gravity fields of Jupiter and Io, not affected by the electric and magnetic field. If they do not impact Io or its atmosphere, these neutral particles can propagate large distances across the magnetic field before they are reionized. Eventually, the reionized particles are lost by dissociation. Characteristic Io masshloading particle distributions, such as high torus plasma density outside Ios orbit and the lower density inside, and the directional feature of the masshloading neutral cloud, are qualitatively reproduced in the model. Meanwhile, the configuration of the masshloading region, in which the ion cyclotron waves are observed, is obtained, and the results are consistent with Galileo wave observations. Three parameters are found to control the structure of the neutral and ion loading disks: the characteristic lifetimes of the initially created ions, of the neutral molecules, and of the ions generated by neutral particle reionization. In addition, particle source geometry, gravity, and the near Io field configurations play important roles in the Io masshloading process.

Solar wind pressure pulse‐driven magnetospheric vortices and their global consequences

We report the in situ observation of a plasma vortex induced by a solar wind dynamic pressure enhancement in the nightside plasma sheet using multipoint measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. The vortex has a scale of 5–10 Re and propagates several Re downtail, expanding while propagating. The features of the vortex are consistent with the prediction of the Sibeck (1990) model, and the vortex can penetrate deep (~8 Re) in the dawn-dusk direction and couple to field line oscillations. Global magnetohydrodynamics simulations are carried out, and it is found that the simulation and observations are consistent with each other. Data from THEMIS ground magnetometer stations indicate a poleward propagating vortex in the ionosphere, with a rotational sense consistent with the existence of the vortex observed in the magnetotail.

Impact angle control of interplanetary shock geoeffectiveness

We use Open Geospace General Circulation Model global MHD simulations to study the nightside magnetospheric, magnetotail, and ionospheric responses to interplanetary (IP) fast forward shocks. Three cases are presented in this study: two inclined oblique shocks, hereafter IOS-1 and IOS-2, where the latter has a Mach number twice stronger than the former. Both shocks have impact angles of 30° in relation to the Sun-Earth line. Lastly, we choose a frontal perpendicular shock, FPS, whose shock normal is along the Sun-Earth line, with the same Mach number as IOS-1. We find that, in the IOS-1 case, due to the north-south asymmetry, the magnetotail is deflected southward, leading to a mild compression. The geomagnetic activity observed in the nightside ionosphere is then weak. On the other hand, in the head-on case, the FPS compresses the magnetotail from both sides symmetrically. This compression triggers a substorm allowing a larger amount of stored energy in the magnetotail to be released to the nightside ionosphere, resulting in stronger geomagnetic activity. By comparing IOS-2 and FPS, we find that, despite the IOS-2 having a larger Mach number, the FPS leads to a larger geomagnetic response in the nightside ionosphere. As a result, we conclude that IP shocks with similar upstream conditions, such as magnetic field, speed, density, and Mach number, can have different geoeffectiveness, depending on their shock normal orientation.

Impact Angle Control of Interplanetary Shock Geoeffectiveness: A Statistical Study

We present a survey of interplanetary (IP) shocks using Wind and ACE satellite data from January 1995 to December 2013 to study how IP shock geoeffectiveness is controlled by IP shock impact angles. A shock list covering one and a half solar cycle is compiled. The yearly number of IP shocks is found to correlate well with the monthly sunspot number. We use data from SuperMAG, a large chain with more than 300 geomagnetic stations, to study geoeffectiveness triggered by IP shocks. The SuperMAG SML index, an enhanced version of the familiar AL index, is used in our statistical analysis. The jumps of the SML index triggered by IP shock impacts on the Earths magnetosphere are investigated in terms of IP shock orientation and speed. We find that, in general, strong (high speed) and almost frontal (small impact angle) shocks are more geoeffective than inclined shocks with low speed. The strongest correlation (correlation coefficient R = 0.78) occurs for fixed IP shock speed and for varied IP shock impact angle. We attribute this result, predicted previously with simulations, to the fact that frontal shocks compress the magnetosphere symmetrically from all sides, which is a favorable condition for the release of magnetic energy stored in the magnetotail, which in turn can produce moderate to strong auroral substorms, which are then observed by ground-based magnetometers.

Feasibility of a multisatellite investigation of the Earth's magnetosphere with radio tomography

We describe the scientific motivation, basic principles, and feasibility of a relatively new measurement technique, radio tomography, and show how it can be used to investigate the Earths magnetosphere. We demonstrate that a multisatellite radio tomography experiment can produce two-dimensional images of plasma density in the Earths magnetosphere at sufficient spatial (1/2 RE) and temporal (∼10 s) resolution to address key problems of magnetospheric physics. The imaging technique incorporates well-established radio science methods and computed tomography. Several coplanar satellites are required in orbits that encompass the imaged area. We suggest that the large-scale images are more valuable when combined with in situ observations, supporting an unambiguous interpretation of the in situ data and an investigation of the interdependence of small- and large-scale plasma processes.

Comparison of predictive estimates of high‐latitude electrodynamics with observations of global‐scale Birkeland currents

Two of the geomagnetic storms for the Space Weather Prediction Center (SWPC) Geospace Environment Modeling (GEM) challenge [cf. Pulkkinen et al., 2013] occurred after data were first acquired by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We compare Birkeland currents from AMPERE with predictions from four models for the 4-5 April 2010 and 5-6 August 2011 storms. The four models are: the Weimer [2005b] field-aligned current statistical model; the Lyon-Fedder-Mobarry magnetohydrodynamic (MHD) simulation; the Open Global Geospace Circulation Model MHD simulation; and the Space Weather Modeling Framework MHD simulation. The MHD simulations were run as described in Pulkkinen et al. [2013] and the results obtained from the Community Coordinated Modeling Center (CCMC). The total radial Birkeland current, ITotal, and the distribution of radial current density, Jr, for all models are compared with AMPERE results. While the total currents are well correlated, the quantitative agreement varies considerably. The Jr distributions reveal discrepancies between the models and observations related to the latitude distribution, morphologies, and lack of nightside current systems in the models. The results motivate enhancing the simulations first by increasing the simulation resolution, and then by examining the relative merits of implementing more sophisticated ionospheric conductance models, including ionospheric outflows or other omitted physical processes. Some aspects of the system, including substorm timing and location, may remain challenging to simulate, implying a continuing need for real-time specification.