scholarly journals Evidence of the Nonstationarity of the Terrestrial Bow Shock from Multi-Spacecraft Observations: Methodology, Results and Quantitative Comparison with PIC Simulations

2020 ◽  
Author(s):  
Christian Mazelle ◽  
Bertrand Lembege
Keyword(s):  
Magnetic Field ◽  
Lower Bound ◽  
Data Processing ◽  
Shock Front ◽  
Upper Bound ◽  
Spatial Scales ◽  
Large Fraction ◽  
Field Measurements ◽  
Bow Shock ◽  
Time Variability

Abstract. The nonstationarity of the terrestrial bow shock is analyzed in detail from in situ magnetic field measurements issued from the FGM experiment on board of Cluster mission. Attention is focused on statistical analysis of quasiperpendicular supercritical shock crossings. The present analysis stresses for the first time the importance of a careful and accurate methodology in the data processing which can be a source of confusion/misunderstanding if not treated properly. The analysis performed using 96 shock front crossings shows evidence of a strong variability of the microstructures of the shock front (foot and ramp) which are analyzed in deep details. Main results are: (i) most statistics clearly evidence that the ramp thickness is very narrow and can be as low as a few c/ωpe (electron inertia length), (ii) the width is narrower when the angle θBn (between the shock normal and the upstream magnetic field) approaches 90°, (iii) the foot thickness strongly varies but its variation has an upper limit provided by theoretical estimates given in previous studies (e.g., Schwartz et al., 1983; Gosling and Thomsen, 1985; Gosling and Robson, 1985); (iv) the presence of foot and overshoot, as shown in all front profiles confirms the importance of dissipative effects. Present results indicate that these features can be signatures of the shock front self-reformation among a few mechanisms of nonstationarity identified from numerical simulation/theoretical works. A comparison 2D PIC simulation for a perpendicular supercritical shock (used as reference), has been performed and it shows that: (a) the ramp thickness varies only slightly in time over a large fraction of the reformation cycle and reaches a lower bound value of the order of a few electron inertial length, (ii) in contrast, the foot width strongly varies during a self-reformation cycle but always stays lower than an upper bound value in agreement with the value given by Woods (1971), and (iii) as a consequence, the time variability of the whole shock front is depending on both ramp and foot variations. Moreover, a detailed comparative analysis shows that much elements of analysis were missing in previous reported works concerning both (i) the important criteria used in the data selection and (ii) the different and careful steps of the methodology used in the data processing itself. This absence of these precise elements of analysis makes the comparison with present work difficult, worse, it makes some final results and conclusive statements quite questionable at present time. A least, looking for a precise estimate of the shock transition thickness presents nowadays a restricted interest, since recent results show that the terrestrial shock is rather nonstationary and one unique typical spatial scaling of the microstructures of the front (ramp, foot) must be replaced by some variation ranges (with lower bound/upper bound values) within which the spatial scales of the fine structures can extend.

scholarly journals Evidence of the nonstationarity of the terrestrial bow shock from multi-spacecraft observations: methodology, results, and quantitative comparison with particle-in-cell (PIC) simulations

2021 ◽  
Vol 39 (4) ◽  
pp. 571-598
Author(s):  
Christian Mazelle ◽  
Bertrand Lembège
Keyword(s):  
Magnetic Field ◽  
Lower Bound ◽  
Data Processing ◽  
Shock Front ◽  
Upper Bound ◽  
Spatial Scales ◽  
Bow Shock ◽  
Time Variability ◽  
Fluxgate Magnetometer ◽  
Particle In Cell

Abstract. The nonstationarity of the terrestrial bow shock is analyzed in detail from in situ magnetic field measurements issued from the fluxgate magnetometer (FGM) experiment of the Cluster mission. Attention is focused on statistical analysis of quasi-perpendicular supercritical shock crossings. The present analysis stresses for the first time the importance of a careful and accurate methodology in the data processing, which can be a source of confusion and misunderstanding if not treated properly. The analysis performed using 96 shock front crossings shows evidence of a strong variability of the microstructures of the shock front (foot and ramp), which are analyzed in great detail. The main results are that (i) most statistics clearly show that the ramp thickness is very narrow and can be as low as a few c/ωpe (electron inertia length); (ii) the width is narrower when the angle θBn (between the shock normal and the upstream magnetic field) approaches 90∘; (iii) the foot thickness strongly varies, but its variation has an upper limit provided by theoretical estimates given in previous studies (e.g., Schwartz et al., 1983; Gosling and Thomsen, 1985; Gosling and Robson, 1985); and (iv) the presence of foot and overshoot, as shown in all front profiles, confirms the importance of dissipative effects. Present results indicate that these features can be signatures of the shock front self-reformation among a few mechanisms of nonstationarity identified from numerical simulation and theoretical studies. A comparison with 2D particle-in-cell (PIC) simulation for a perpendicular supercritical shock (used as reference) has been performed and shows the following: (a) the ramp thickness varies only slightly in time over a large fraction of the reformation cycle and reaches a lower-bound value on the order of a few electron inertial length; (b) in contrast, the foot width strongly varies during a self-reformation cycle but always stays lower than an upper-bound value in agreement with the value given by Woods (1971); and (c) as a consequence, the time variability of the whole shock front is depending on both ramp and foot variations. Moreover, a detailed comparative analysis shows that many elements of analysis were missing in previous reported studies concerning both (i) the important criteria used in the data selection and (ii) the different and careful steps of the methodology used in the data processing itself. The absence of these precise elements of analysis makes the comparison with the present work difficult; worse, it makes some final results and conclusive statements quite questionable at the present time. At least, looking for a precise estimate of the shock transition thickness presents nowadays a restricted interest, since recent results show that the terrestrial shock is rather nonstationary, and one unique typical spatial scaling of the microstructures of the front (ramp, foot) must be replaced by some “variation ranges” (with lower-bound and upper-bound values) within which the spatial scales of the fine structures can extend.

scholarly journals Scattering of field-aligned beam ions upstream of Earth's bow shock

2007 ◽  
Vol 25 (3) ◽  
pp. 785-799 ◽  
Author(s):  
A. Kis ◽  
M. Scholer ◽  
B. Klecker ◽  
H. Kucharek ◽  
E. A. Lucek ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Field Measurements ◽  
Bow Shock ◽  
Ion Distribution ◽  
Parallel Side ◽  
Earth’S Bow Shock ◽  
High Mach ◽  
The Magnetic Field

Abstract. Field-aligned beams are known to originate from the quasi-perpendicular side of the Earth's bow shock, while the diffuse ion population consists of accelerated ions at the quasi-parallel side of the bow shock. The two distinct ion populations show typical characteristics in their velocity space distributions. By using particle and magnetic field measurements from one Cluster spacecraft we present a case study when the two ion populations are observed simultaneously in the foreshock region during a high Mach number, high solar wind velocity event. We present the spatial-temporal evolution of the field-aligned beam ion distribution in front of the Earth's bow shock, focusing on the processes in the deep foreshock region, i.e. on the quasi-parallel side. Our analysis demonstrates that the scattering of field-aligned beam (FAB) ions combined with convection by the solar wind results in the presence of lower-energy, toroidal gyrating ions at positions deeper in the foreshock region which are magnetically connected to the quasi-parallel bow shock. The gyrating ions are superposed onto a higher energy diffuse ion population. It is suggested that the toroidal gyrating ion population observed deep in the foreshock region has its origins in the FAB and that its characteristics are correlated with its distance from the FAB, but is independent on distance to the bow shock along the magnetic field.

scholarly journals Low-frequency magnetic variations at the high-<i>β</i> Earth bow shock

2019 ◽  
Vol 37 (5) ◽  
pp. 877-889
Author(s):  
Anatoli A. Petrukovich ◽  
Olga M. Chugunova ◽  
Pavel I. Shustov
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Spatial Scales ◽  
Low Frequency ◽  
Background Field ◽  
Bow Shock ◽  
Stable Phase ◽  
Main Magnetic Field ◽  
Astrophysical Plasmas ◽  
Earth’S Bow Shock

Abstract. Observations of Earth's bow shock during high-β (ratio of thermal to magnetic pressure) solar wind streams are rare. However, such shocks are ubiquitous in astrophysical plasmas. Typical solar wind parameters related to high β (here β>10) are as follows: low speed, high density, and a very low interplanetary magnetic field of 1–2 nT. These conditions are usually quite transient and need to be verified immediately upstream of the observed shock crossings. In this report, three characteristic crossings by the Cluster project (from the 22 found) are studied using multipoint analysis, allowing us to determine spatial scales. The main magnetic field and density spatial scale of about a couple of hundred of kilometers generally corresponds to the increased proton convective gyroradius. Observed magnetic variations are different from those for supercritical shocks, with β∼1. Dominant magnetic variations in the shock transition have amplitudes much larger than the background field and have a frequency of ∼ 0.3–0.5 Hz (in some events – 1–2 Hz). The wave polarization has no stable phase and is closer to linear, which complicates the determination of the wave propagation direction. Spatial scales (wavelengths) of variations are within several tens to a couple of hundred of kilometers.

Update on scales and energetics of auroral field-aligned currents as observed by Swarm

2020 ◽  
Author(s):  
Leonie Pick ◽  
Joachim Vogt ◽  
Adrian Blagau ◽  
Nele Stachlys
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Model Comparison ◽  
Energy Transport ◽  
Spatial Scales ◽  
Electromagnetic Coupling ◽  
Geomagnetic Latitude ◽  
Field Measurements ◽  
Boundary Crossing ◽  
Poynting Flux

&lt;p&gt;Auroral field-aligned currents (FACs) are of key importance for the electromagnetic coupling and the energy transport in the magnetosphere-ionosphere system. We use Swarm multi-spacecraft magnetic and electric field measurements from a selection of auroral oval crossing events to advance our understanding of the spatial scales and the electromagnetic energy flux (Poynting flux) associated with sheets of auroral FACs. Our study comprises the derivation of a scale-dependent correlation function based on dual-satellite vectorial magnetic field perturbation time series, in order to identify and analyze planar current structures. Applying concepts from multi-point boundary crossing analysis to data from Swarm-A and Swarm-C, a correlation measure is constructed using the mean square deviation of the observed magnetic perturbations and an empirical pattern function. Peak correlations indicate the positions and the scales of auroral FAC sheets, which we contextualize with the magnetic local time, the geomagnetic latitude, and geomagnetic activity indices (e.g., AL). In a parallel strand of work, we estimate the associated Poynting flux from the combination of the magnetic field perturbations and those of the electric field as deduced from the observed cross-track ion drift velocity. We assess the quality of our Swarm-based estimate by a comparison to the Poynting flux given by the &amp;#8220;Cosgrove-PF&amp;#8221; empirical model, which is based on FAST data from 1996 to 2001 and available from NASA&amp;#8217;s Community Coordinated Modeling Center. Connecting both strands of work, we check to what degree this data-model comparison depends on the current sheets&amp;#8217; spatial scale. Throughout the study, we adopt a framework for describing planar magnetic structures that facilitates error analysis and accommodates not only boundary analysis, but also single-spacecraft polarization techniques.&lt;/p&gt;

scholarly journals Corotation-driven magnetosphere-ionosphere coupling currents in Saturn’s magnetosphere and their relation to the auroras

2003 ◽  
Vol 21 (8) ◽  
pp. 1691-1707 ◽  
Author(s):  
S. W. H. Cowley ◽  
E. J. Bunce
Keyword(s):  
Magnetic Field ◽  
Angular Velocity ◽  
Equatorial Plane ◽  
Large Scale ◽  
Spatial Scales ◽  
Outer Boundary ◽  
Time Variability ◽  
Small Scale ◽  
Velocity Models ◽  
Field Aligned Current

Abstract. We calculate the latitude profile of the equatorward-directed ionospheric Pedersen currents that are driven in Saturn’s ionosphere by partial corotation of the magnetospheric plasma. The calculation incorporates the flattened figure of the planet, a model of Saturn’s magnetic field derived from spacecraft flyby data, and angular velocity models derived from Voyager plasma data. We also employ an effective height-integrated ionospheric Pedersen conductivity of 1 mho, suggested by a related analysis of Voyager magnetic field data. The Voyager plasma data suggest that on the largest spatial scales, the plasma angular velocity declines from near-rigid corotation with the planet in the inner magnetosphere, to values of about half of rigid corotation at the outer boundary of the region considered. The latter extends to ~ 15–20 Saturn radii (RS) in the equatorial plane, mapping along magnetic field lines to ~ 15° co-latitude in the ionosphere. We find in this case that the ionospheric Pedersen current peaks near the poleward (outer) boundary of this region, and falls toward zero over ~ 5°–10° equator-ward of the boundary as the plasma approaches rigid corotation. The peak current near the poleward boundary, integrated in azimuth, is ~ 6 MA. The field-aligned current required for continuity is directed out of the ionosphere into the magnetosphere essentially throughout the region, with the current density peaking at ~ 10 nA m-2 at ~ 20° co-latitude. We estimate that such current densities are well below the limit requiring field-aligned acceleration of magnetospheric electrons in Saturn’s environment ( ~ 70 nAm-2), so that no significant auroral features associated with this ring of upward current is anticipated. The observed ultraviolet auroras at Saturn are also found to occur significantly closer to the pole (at ~ 10°–15° co-latitude), and show considerable temporal and local time variability, contrary to expectations for corotation-related currents. We thus conclude that Saturn’s ‘main oval’ auroras are not associated with corotation-enforcing currents as they are at Jupiter, but instead are most probably associated with coupling to the solar wind as at Earth. At the same time, the Voyager flow observations also suggest the presence of radially localized ‘dips’ in the plasma angular velocity associated with the moons Dione and Rhea, which are ~ 1–2 RS in radial extent in the equatorial plane. The presence of such small-scale flow features, assumed to be azimuthally extended, results in localized several-MA enhancements in the ionospheric Pedersen current, and narrow bi-polar signatures in the field-aligned currents which peak at values an order of magnitude larger than those associated with the large-scale currents. Narrow auroral rings (or partial rings) ~ 0.25° co-latitude wide with intensities ~ 1 kiloRayleigh may be formed in the regions of upward field-aligned current under favourable circumstances, located at co-latitudes between ~ 17° and ~ 20° in the north, and ~ 19° and ~22° in the south.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; planetary magnetospheres)

MHD Rankine—Hugoniot equations applied to earth's bow shock

1969 ◽  
Vol 3 (3) ◽  
pp. 449-463 ◽  
Author(s):  
J. D. Mihalov ◽  
C. P. Sonett ◽  
J. H. Wolfe
Keyword(s):  
Magnetic Field ◽  
Flow Velocity ◽  
Field Measurements ◽  
Bow Shock ◽  
Anisotropic Pressure ◽  
Convective Velocity ◽  
Ion Density ◽  
Velocity Magnitude ◽  
Earth’S Bow Shock ◽  
Plasma Probe

This paper compares computed results with upstream and downstream thermal pressures, and the downstream ion density and vector flow velocity measured by the Ames plasma probe on the Pioneer 6 spacecraft as earth's bow shock was traversed. The upstream ion density and vector flow velocity measured on Pioneer 6 by this experiment are used as independent variables, together with Pioneer 6 magnetic field measurements. MRD Rankine—Hugoniot equations for an isotropic plasma are used for these computations. Reasonable agreement is obtained between the measured and computed downstream ion density, thermal pressure and convective velocity magnitude and orientation, only when a 1γ change is made to a downstream magnetic field value used in the calculation. There is disagreement between several determinations of shock orientation and that deduced from coplanarity of the reported magnetic fields, and the shock orientation provides the co-ordinate system for solving the Rankine—Hugoniot equations. If a shock orientation determined by velocity coplanarity is used in the calculations, all computed quantities agree well with the experimental results. Other results suggest that a disagreement between computed and measured upstream thermal pressures may not be negligible in comparison with experimental uncertainties in upstream velocity and density. If the computations included anisotropic pressure terms and/or other factors such as plasma turbulence, better agreement might be obtained between computed and measured upstream thermal pressures.

scholarly journals Rosetta swing-by at Mars – an analysis of the ROMAP measurements in comparison with results of 3-D multi-ion hybrid simulations and MEX/ASPERA-3 data

2009 ◽  
Vol 27 (6) ◽  
pp. 2383-2398 ◽  
Author(s):  
A. Boesswetter ◽  
U. Auster ◽  
I. Richter ◽  
M. Fränz ◽  
B. Langlais ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Measurements ◽  
Bow Shock ◽  
Gravity Assist ◽  
Pile Up ◽  
Magnetic Field Model ◽  
Order Of Magnitude ◽  
Upstream Waves ◽  
Insight Into ◽  
Hybrid Simulations

Abstract. The Rosetta spacecraft flew by Mars at a distance of 260 km on 25 February 2007 during a gravity assist manoeuvre. During the closest approach (CA) the lander magnetometer ROMAP was switched on. The dataset taken during this swingby provides insight into the plasma environment around Mars: in addition to a pronounced bow shock crossing Rosetta recorded the signature of the pile up region of draped magnetic field. Also the Rosetta measurements showed signatures of crustal magnetic field anomalies which can be verified by results of a crustal magnetic field model. In order to understand the measured field morphology, multi-ion hybrid simulations were performed. Some of the input parameters for the simulations were obtained from Mars Express (MEX) data which were contemporaneously collected during the Rosetta swingby. These simulations reproduces ROMAP magnetic field measurements and show that the interplanetary magnetic field pointed northward during the encounter. A spectral analysis shows upstream waves ahead of the bow shock and indicates the presence of the magnetic pile-up boundary (MPB). The multi-ion model reproduces the ion fluxes measured by MEX/ASPERA-3 and is in agreement with the measurements to within one order of magnitude.

scholarly journals Regional background O<sub>3</sub> and NO<sub><i>x</i></sub> in the Houston–Galveston–Brazoria (TX) region: a decadal-scale perspective

2017 ◽  
Vol 17 (11) ◽  
pp. 6565-6581 ◽  
Author(s):  
Loredana G. Suciu ◽  
Robert J. Griffin ◽  
Caroline A. Masiello
Keyword(s):  
Lower Bound ◽  
Upper Bound ◽  
Spatial Scales ◽  
Lower Troposphere ◽  
Principal Component ◽  
Ground Level ◽  
Ambient Air ◽  
Mixing Ratios ◽  
Regional Background ◽  
Decadal Scale

Abstract. Ozone (O3) in the lower troposphere is harmful to people and plants, particularly during summer, when photochemistry is most active and higher temperatures favor local chemistry. Local precursor emissions, such as those of volatile organic compounds (VOCs) and nitrogen oxides (NOx), together with their chemistry contribute to the O3 and NOx mixing ratios in the Houston–Galveston–Brazoria (HGB) region. In addition to local emissions, chemistry and transport, larger-scale factors also contribute to local O3 and NOx. These additional contributions (often referred to as regional background) are not well quantified within the HGB region, impeding more efficient controls on precursor emissions to achieve compliance with the National Ambient Air Quality Standards for O3. In this study, we estimate ground-level regional background O3 and NOx in the HGB region and quantify their decadal-scale trends.We use four different approaches based on principal component analysis (PCA) to quantify background O3 and NOx. Three of these approaches consist of independent PCA on both O3 and NOx for both 1 and 8 h levels to compare our results with previous studies and to highlight the effect of both temporal and spatial scales. In the fourth approach, we co-varied O3, NOx and meteorology.Our results show that the estimation of regional background O3 has less inherent uncertainty when it was constrained by NOx and meteorology, yielding a statistically significant temporal trend of −0.68 ± 0.27 ppb yr−1. Likewise, the estimation of regional background NOx trend constrained by O3 and meteorology was −0.04 ± 0.02 ppb yr−1 (upper bound) and −0.03 ± 0.01 ppb yr−1 (lower bound). Our best estimates of the 17-year average of season-scale background O3 and NOx were 46.72 ± 2.08 ppb and 6.80 ± 0.13 ppb (upper bound) or 4.45 ± 0.08 ppb (lower bound), respectively. Average background O3 is consistent with previous studies and between the approaches used in this study, although the approaches based on 8 h averages likely overestimate background O3 compared to the hourly median approach by 7–9 ppb. Similarly, the upper bound of average background NOx is consistent between approaches in this study (A–C) but overestimated compared to the hourly approach by 1 ppb, on average. We likely overestimate the upper-bound background NOx due to instrument overdetection of NOx and the 8 h averaging of NOx and meteorology coinciding with MDA8 O3.Regional background O3 and NOx in the HGB region both have declined over the past 2 decades. This decline became steadier after 2007, overlapping with the effects of controlling precursor emissions and a prevailing southeasterly–southerly flow.

Effects of upstream conditions on ULF waves and SLAMS formation at Saturn

2021 ◽  
Author(s):  
Zsofia Bebesi ◽  
Antal Juhasz
Keyword(s):  
Magnetic Field ◽  
Simple Model ◽  
Shock Front ◽  
Large Amplitude ◽  
Plasma Pressure ◽  
Bow Shock ◽  
Wave Formation ◽  
Ulf Waves ◽  
Magnetic Structures ◽  
Ulf Wave

&lt;p&gt;In this study we present occurences of SLAMS (short large-amplitude magnetic structures) upstream of the quasi-parallel bow shock of Saturn. Five events are analyzed in more detail using the data of the CAPS and MAG instruments of Cassini. Directional and speed analysis of the backstreaming particles related to ULF wave formation (and subsequent SLAMS evolution) in the foreshock region is presented. We also correlate the measured the ULF wave frequencies with the variations of the upstream magnetic field.&lt;br&gt;With a simple model we estimate the distance of the observed SLAMS from the bow shock front based on the measured plasma pressure. We also&amp;#160;&lt;br&gt;discuss the spatial characteristics of SLAMS observed near Saturn.&lt;/p&gt;

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