A simulation study of the thermosphere mass density response to substorms using GITM

The temporal and spatial variations of the thermospheric mass density during a series of idealized substorms were investigated using the Global Ionosphere Thermosphere Model (GITM). The maximum mass density perturbation of an idealized substorm with a peak variation of hemispheric power (HP) of 50 GW and interplanetary magnetic field (IMF) Bz of −2 nT was ~14% about 50 min after the substorm onset in the nightside sector of the auroral zone. The mass density response to different types of energy input has a strong local time dependence, with the mass density perturbation due to only an IMF Bz variation peaking in the dusk sector and the density perturbation due to only HP variation peaks in the nightside sector. Simulations with IMF Bz changes only and HP changes only showed that the system behaves slightly nonlinearly when both IMF and HP variations are included (a maximum of 6% of the nonlinearity) and that the nonlinearity grows with energy input. The neutral gas heating rate due to Joule heating was of same magnitude as the heating rate due to precipitation, but the majority of the temperature enhancement due to the heating due to precipitation occurs at lower altitude as compared to the auroral heating. About 110 min after onset, a negative mass density perturbation (~−5%) occurred in the night sector, which was consistent with the mass density measurement of the CHAMP satellite.Key PointsMass density response to different types of energy input has strong local time dependenceNegative mass density perturbation (~−5%) occurred in the night sector during storm recover phaseMass density responses to different types of energy source are nearly a linear systemPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115970/1/jgra52067_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115970/2/jgra52067.pd

More about arc-polarized structures in the solar wind

We report results from a Cluster-based study of the properties of 28 arc-polarized magnetic structures (also called rotational discontinuities) in the solar wind. These Alfve ́nic events were selected from the database created and analyzed by Knetter (2005) by use of criteria chosen to elim- inate ambiguous cases. His studies showed that standard, four-spacecraft timing analysis in most cases lacks sufficient accuracy to identify the small normal magnetic field compo- nents expected to accompany such structures, leaving unan- swered the question of their existence. Our study aims to break this impasse. By careful application of minimum vari- ance analysis of the magnetic field (MVAB) from each indi- vidual spacecraft, we show that, in most cases, a small but significantly non-zero magnetic field component was present in the direction perpendicular to the discontinuity. In the very few cases where this component was found to be large, ex- amination revealed that MVAB had produced an unusual and unexplained orientation of the normal vector. On the whole, MVAB shows that many verifiable rotational discontinuities (Bn ̸= 0) exist in the solar wind and that their eigenvalue ratio (EVR=intermediate/minimum variance) can be extremely large (up to EVR = 400). Each of our events comprises four individual spacecraft crossings. The events include 17 ion- polarized cases and 11 electron-polarized ones. Fifteen of the ion events have widths ranging from 9 to 21 ion iner- tial lengths, with two outliers at 46 and 54. The electron- polarized events are generally thicker: nine cases fall in the range 20–71 ion inertial lengths, with two outliers at 9 and 13. In agreement with theoretical predictions from a one- dimensional, ideal, Hall-MHD description (Sonnerup et al., 2010), the ion-polarized events show a small depression in field magnitude, while the electron-polarized ones tend to show a small enhancement

Accuracy of multi-point boundary crossing time analysis

Recent multi-spacecraft studies of solar wind discontinuity crossings using the timing (boundary plane triangulation) method gave boundary parameter estimates that are significantly different from those of the well-established single-spacecraft minimum variance analysis (MVA) technique. A large survey of directional discontinuities in Cluster data turned out to be particularly inconsistent in the sense that multi-point timing analyses did not identify any rotational discontinuities (RDs) whereas the MVA results of the individual spacecraft suggested that RDs form the majority of events. To make multi-spacecraft studies of discontinuity crossings more conclusive, the present report addresses the accuracy of the timing approach to boundary parameter estimation. Our error analysis is based on the reciprocal vector formalism and takes into account uncertainties both in crossing times and in the spacecraft positions. A rigorous error estimation scheme is presented for the general case of correlated crossing time errors and arbitrary spacecraft configurations. Crossing time error covariances are determined through cross correlation analyses of the residuals. The principal influence of the spacecraft array geometry on the accuracy of the timing method is illustrated using error formulas for the simplified case of mutually uncorrelated and identical errors at different spacecraft. The full error analysis procedure is demonstrated for a solar wind discontinuity as observed by the Cluster FGM instrument

Discontinuities and Alfvenic Fluctuations in the Solar Wind

We examine the Alfvenicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walen test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80 % of the Alfven speed at an average angular deviation of 7.7◦; 33 cases had speeds exceeding 90 % of the Alfven speed at an average angle of 6.4◦. We show that the angular deviation between flow velocity (in the HT frame) and the Alfven velocity can be obtained from a reduced form of the Walen correlation coefficient

Structure of the low latitude boundary layer

Observations at high temporal resolution of the frontside magnetopause and plasma boundary layer, made with the LASL/MPE fast plasma analyzer onboard the ISEE 1 and 2 spacecraft, revealed a complex quasiperiodic structure of some of the observed boundary layers. A cool tailward streaming boundary layer plasma was seen intermittently, with intervening periods of hot tenuous plasma which has properties similar to the magnetospheric population. While individual encounters with the boundary layer plasma last only a few minutes, the total observation time may extend over one hour or more

Optimal reconstruction of magnetopause structures from Cluster data

The Grad-Shafranov (GS) reconstruction tech- nique, a single-spacecraft based data analysis method for recovering approximately two-dimensional (2-D) magneto- hydrostatic plasma/field structures in space, is improved to become a multi-spacecraft technique that produces a single field map by ingesting data from all four Cluster spacecraft into the calculation. The plasma pressure, required for the technique, is measured in high time resolution by only two of the spacecraft, C1 and C3, but, with the help of spacecraft po- tential measurements available from all four spacecraft, the pressure can be estimated at the other spacecraft as well via a relationship, established from C1 and C3 data, between the pressure and the electron density deduced from the poten- tials. Consequently, four independent field maps, one for each spacecraft, can be reconstructed and then merged into a single map. The resulting map appears more accurate than the individual single-spacecraft based ones, in the sense that agreement between magnetic field variations predicted from the map to occur at each of the four spacecraft and those actually measured is significantly better. Such a composite map does not satisfy the GS equation any more, but is op- timal under the constraints that the structures are 2-D and time-independent. Based on the reconstruction results, we show that, even on a scale of a few thousand km, the magne- topause surface is usually not planar, but has significant cur- vature, often with intriguing meso-scale structures embedded in the current layer, and that the thickness of both the current layer and the boundary layer attached to its earthward side can occasionally be larger than 3000 km

Four-spacecraft determination of magnetopause orientation, motion and thickness: comparison with results from single-spacecraft methods

In this paper, we use Cluster data from one magnetopause event on 5 July 2001 to compare predictions from various methods for determination of the velocity, orientation, and thickness of the magnetopause current layer. We employ established as well as new multi-spacecraft techniques, in which time differences between the crossings by the four spacecraft, along with the duration of each crossing, are used to calculate magnetopause speed, normal vector, and width. The timing is based on data from either the Cluster Magnetic Field Experiment (FGM) or the Electric Field Experiment (EFW) instruments. The multi-spacecraft results are compared with those derived from various single-spacecraft techniques, including minimum-variance analysis of the magnetic field and deHoffmann-Teller, as well as Minimum-Faraday-Residue analysis of plasma velocities and magnetic fields measured during the crossings. In order to improve the overall consistency between multi- and single-spacecraft results, we have also explored the use of hybrid techniques, in which timing information from the four spacecraft is combined with certain limited results from single-spacecraft methods, the remaining results being left for consistency checks. The results show good agreement between magnetopause orientations derived from appropriately chosen single-spacecraft techniques and those obtained from multi-spacecraft timing. The agreement between magnetopause speeds derived from single- and multi-spacecraft methods is quantitatively somewhat less good but it is evident that the speed can change substantially from one crossing to the next within an event. The magnetopause thickness varied substantially from one crossing to the next, within an event. It ranged from 5 to 10 ion gyroradii. The density profile was sharper than the magnetic profile: most of the density change occured in the earthward half of the magnetopause.<br><br> <b>Key words.</b> Magnetospheric physics (magnetopause, cusp and boundary layers; instruments and techniques) – Space plasma physics (discontinuities

20 Years of Cluster Observations: The Magnetopause,

The terrestrial magnetopause forms the boundary between the solar wind plasma with its embedded interplanetary magnetic field on one side, and the terrestrial magnetosphere, dominated by Earth's dipole field, on the other side. It is therefore a key region for the transfer of mass, momentum, and energy from the solar wind to the magnetosphere. The Cluster mission, comprising a constellation of four spacecraft flying in formation was launched more than 20 years ago to study boundaries in space. During its lifetime, Cluster has provided a wealth of new knowledge about the magnetopause. In this paper, we give an overview of Cluster-based studies of this boundary, and highlight a selection of interesting results.publishedVersio