Global Magnetic Field Configuration Research Articles

A New Method Linking the Solar Wind Speed to the Coronal Magnetic Field

The release and acceleration of the solar wind is still an outstanding question. There are several aspects related to this phenomenon that still need to be investigated, and one of these is the identification of the region within the inner corona where the larger fraction of acceleration occurs. To address this matter, it is necessary to have reliable measurements of the solar wind speed between 1 and 10 solar radii (R ⊙). Moreover, in order to describe the coronal plasma behavior, it is important to consider its interaction with the magnetic field. Within this context, our intent is to investigate a method to combine measurements of the solar wind with the extrapolated magnetic field in the corona to derive how the solar wind velocity evolves along the magnetic field lines, aiming at better understanding the sources, origins, and acceleration of the solar wind. To this purpose, we used outflow speed measurements of the coronal plasma derived by applying the Doppler dimming technique, as well as the global magnetic field configuration derived from the measured photospheric magnetic field by using the Wang–Sheeley–Arge model. These two sets of data are then combined for heliocentric distances between 2.6 and 5 R ⊙. This paper presents the proposed method and the results obtained over two different Carrington rotations (CR 1923 and CR 1924), demonstrating the applicability of the method and the capability to link measured solar wind velocity to the extrapolated coronal magnetic field in order to derive the velocity profile.

Radiation belt data assimilation of a moderate storm event using a magnetic field configuration from the physics-based RAM-SCB model

Abstract. Data assimilation using Kalman filters provides an effective way of understanding both spatial and temporal variations in the outer electron radiation belt. Data assimilation is the combination of in situ observations and physical models, using appropriate error statistics to approximate the uncertainties in both the data and the model. The global magnetic field configuration is one essential element in determining the adiabatic invariants for the phase space density (PSD) data used for the radiation belt data assimilation. The lack of a suitable global magnetic field model with high accuracy is still a long-lasting problem. This paper employs a physics-based magnetic field configuration for the first time in a radiation belt data assimilation study for a moderate storm event on 19 December 2002. The magnetic field used in our study is the magnetically self-consistent inner magnetosphere model RAM-SCB, developed at Los Alamos National Laboratory (LANL). Furthermore, we apply a cubic spline interpolation method in converting the differential flux measurements within the energy spectrum, to obtain a more accurate PSD input for the data assimilation than the commonly used linear interpolation approach. Finally, the assimilation is done using an ensemble Kalman filter (EnKF), with a localized adaptive inflation (LAI) technique to appropriately account for model errors in the assimilation and improve the performance of the Kalman filter. The assimilative results are compared with results from another assimilation experiment using the Tsyganenko 2001S (T01S) magnetic field model, to examine the dependence on a magnetic field model. Results indicate that the data assimilations using different magnetic field models capture similar features in the radiation belt dynamics, including the temporal evolution of the electron PSD during a storm and the location of the PSD peak. The assimilated solution predicts the energy differential flux to a relatively good degree when compared with independent LANL-GEO in situ observations. A closer examination suggests that for the chosen storm event, the assimilation using the RAM-SCB predicts a better flux at most energy levels during storm recovery phase but is slightly worse in the storm main phase than the assimilation using the T01S model.

Rotation of an erupting filament observed by the STEREO EUVI and COR1 instruments

On August 31, 2007, a prominence eruption was observed by the Solar TErrestrial RElations Observatory (STEREO) in the Extreme-UltraViolet Imager (EUVI) 304 images and later on, as the core of a three-part coronal mass ejection (CME) in images acquired by the inner STEREO coronagraph (COR1). Because they were covered by both STEREO spacecraft from right vantage points, these observations provide an excellent opportunity to perform a three-dimensional (3D) prominence reconstruction and study its evolution. We employed the tie-pointing technique to reconstruct the 3D shape and trajectory of the prominence, which has been followed from an heliocentric distance of ~1.3 up to ~2.4 R ⊙ during the first 1.3 h of eruption. Data show evidence for a progressive clockwise prominence rotation by ~90° occurring not only in the early phase of the eruption sampled by EUVI, but also at larger heliocentric distances as seen by COR1. Interestingly, a counter-clockwise rotation of the filament was observed in Hα images in the week before the eruption; the filament does not show a twisted shape. In the same period, the potential field extrapolated at different times shows a clockwise rotation of closed lines overlying the filament. This suggests that a magnetic helicity storage occurred not in the filament itself, but in the global magnetic field configuration of the surrounding corona. Moreover, close inspection to the high-resolution EUVI images revealed a small scale helical feature along the erupting prominence. The sense of rotation of this helix agrees with the observed prominence rotation, providing evidence for the conversion of twist into writhe. The observed rotation of an erupting prominence, if representative of the flux rope rotation, may have a strong impact on the definition of geo-effectiveness of CMEs for space weather forecasting purposes.

The role of magnetic field in the transition to streaming ablation in wire arrays

In wire array Z-pinches, the magnetic field configuration and the global field penetration of individual wires play a key role in the ablation plasma dynamics. Knowledge of the magnetic field configuration is necessary to understand the ablation plasma acceleration process near the wires. Two-dimensional resistive magnetohydrodynamics simulations show that a change in the global magnetic field configuration is critical to initiating inward flow of the ablation plasma. Analysis of these simulations show that the initially compressive J×B force around a wire in its vacuum field configuration undergoes a transition to a configuration in which the Lorentz force can accelerate plasma toward the array axis. This transition is achieved through a low magnetic Reynolds number diffusive flow in which the plasma and the magnetic field are decoupled. The plasma current follows the expanding plasma toward the array axis and, after traveling a critical distance scaling with the array radius divided by the wire number, the global magnetic field threads the wire core, thereby allowing J×B coronal acceleration into ablation streams.

Recovery of the global magnetic field configuration of 78 Virginis from StokesIQUVline profiles

The surface magnetic field configuration of the Ap star HD 118022 (78 Vir) has been reconstructed in the framework of the magnetic charge distribution (MCD) method from the analysis of Stokes IQUV spectra obtained using the MuSiCoS spectropolarimeter at Pic du Midi Observatory. Magnetically-sensitive Fe ii lines were primarily employed in the analysis, supposing that iron is evenly distributed over the stellar surface. We show that the Stokes IQUV profile shapes and variations of 78 Vir can be approximately fit assuming a global magnetic field configuration described by a slightly decentered, inclined magnetic dipole of polar surface intensity approximately 3.3 kG. The derived inclinations of the stellar rotational axis to the line of sight i = 24 ± 5 ◦ as well as to the magnetic dipole axis β = 124 ± 5 ◦ are in good agreement with previous .

Effect of the latitudinal distribution of temperature at the coronal base on the interplanetary magnetic field configuration and the solar wind flow

Using a two‐dimensional MHD model of the corona and solar wind, we investigate the role of the temperature distribution with latitude at the coronal base on the global magnetic field configuration and solar wind properties at 1 AU. The latitudinal distribution of temperature is aimed at modeling the transition in electron temperature at the Sun from a polar coronal hole to the quiet Sun to active regions. The results of the model calculations illustrate how the variation of temperature with latitude impacts the coronal magnetic field configuration and the distribution of wave energy flux in the solar wind and consequently its thermodynamic properties. The sharp temperature changes at the coronal base lead to the formation of current sheets in the corona. They also modify the location of the streamer cusp and the neutral line originating there. Two different approaches in treating electron heat flux are also compared, one assumes a Spitzer expression throughout the computational domain and the other assumes a collisionless expression beyond some radial distance. Model results thus derived differ little in terms of proton flux and terminal speed.

Two examples of magnetic clouds with double rotations observed by the Ulysses spacecraft

A survey of Ulysses magnetometer data has been carried out between Oct 1990 (launch) and Apr 2003 (just after the second northern polar pass) cataloguing magnetic cloud (MC) signatures, i.e., a smooth magnetic field rotation and enhanced magnetic field magnitude. After confirming that each of these events was a true MC by checking for a low proton temperature signature, it was possible to fit a constant α, force‐free flux rope model to each event. This allows parameters describing the global magnetic field configuration of the event in the region of the intersection with the spacecraft's trajectory to be determined. The survey has shown that associated with many MC field rotations there is a second, less well‐defined, rotation. Usually this rotation is so poorly defined that it is impossible to fit the flux rope model to it. However, in two specific cases, presented here, it has been possible to produce good model fits to both rotations. This gives insights into the chirality and relative orientations of the two flux ropes, assuming that is what they are. The results suggest that the two rotations are part of the same, deformed, rope through which the spacecraft has passed at two different points.

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.

Tracing ions in the cusp and low‐latitude boundary layer using multispacecraft observations and a global MHD simulation

High‐latitude observations from the Polar spacecraft on 21 January 1998 show a region of closed magnetic field lines containing several distinct solar wind ion populations in the energy range from <10 eV to >200 keV e−1. Precipitating ion fluxes in this region are consistent with the low‐latitude boundary layer (LLBL). A global MHD simulation of this event (using input from the Wind spacecraft upstream from the Earth's bow shock) reproduces Geotail and Interball/Tail spacecraft observations in the outer magnetosphere and magnetosheath. These results demonstrate that the simulation faithfully reproduces the global magnetic field configuration of the magnetosheath and magnetosphere and provides confidence for the interpretation of the LLBL observations from Polar. Results from the simulation show that a single evolutionary process of plasma entry into the cusp and evolution to the LLBL can account for the development of a multi‐energy ion population of solar wind origin on closed magnetic field lines in the magnetosphere. Sources with direct access to the ultimately closed LLBL field lines observed by Polar are the dayside magnetosheath (low‐energy ions below 3 keV e−1), the quasi‐parallel bow shock (higher‐energy ions between 3 and 100 keV e−1), and possibly the duskside magnetopause (highest‐energy ions above 100 keV e−1).

Bulk flows of hot plasma in the Jovian magnetosphere: A model of anisotropic fluxes of energetic ions

Since the Ulysses flyby of Jupiter in February 1992, a consistent picture of the bulk flows of hot plasma has yet to be formed. The present study reports a comprehensive analysis of energetic particle anisotropies from the Ulysses mission. We present an empirical model which separates flow from gradient anisotropies. The model employs a multidimensional minimization routine to fit simultaneously 13 free parameters using 98 measurements. The large number of independent measurements is possible because the Ulysses Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energy (HI‐SCALE) has nearly full angular coverage at high time resolution and moderately high angular resolution. The model uses the full energy range (∼50–3000 keV) of the low‐energy magnetic spectrometers and also the pure proton channels of the composition aperture on the instrument. We present flow results during the inbound pass of Ulysses in which the radial velocity component shows a persistent radial outflow. The azimuthal component is in the sense of planetary rotation but generally below rigid corotation, and the azimuthal velocity is greater near the plasma current sheet than at higher magnetic latitudes. Outbound, the flows in the high‐latitude duskside magnetosphere inferred from the particle anisotropies indicate, surprisingly, sunward flows opposite to planetary rotation. We find evidence of gradient anisotropies especially near the magnetospheric current sheet on the dayside. We present a picture of the global magnetic field configuration and flows of hot plasma, consistent with our model results, dominated by subcorotation at low latitudes but by sunward flow at high latitudes.

A study of weak anisotropy in electron pressure in the tail current sheet

We adopt a magnetotail model with stretched field lines where ion motions are generally nonadiabatic and where it is assumed that the pressure anisotropy resides only in the electron pressure tensor. We show that the magnetic field lines with p⊥ > p∥ are less stretched than the corresponding field lines in the isotropic model. For p∥ > p⊥, the magnetic field lines become more and more stretched as the anisotropy approaches the marginal firehose limit, p∥ = p⊥+B²/μ0. We also show that the tail current density is highly enhanced at the firehose limit, a situation that might be subject to a microscopic instability. However, we emphasize that the enhancement in the current density is notable only near the center of the tail current sheet (z = 0). Thus it remains unclear whether any microscopic instability can significantly alter the global magnetic field configuration of the tail. By comparing the radius of the field‐line curvature at z = 0 with the particle's gyroradius, we suspect that even the conventional adiabatic description of electrons may become questionable very close to the marginal firehose limit.

Thin current sheets in the magnetotail during substorms: CDAW 6 revisited

The global magnetic field configuration during the growth phase of the CDAW 6 substorm (March 22, 1979, 1054 UT) is modeled using data from two suitably located spacecraft and temporally evolving variations of the Tsyganenko magnetic field model. These results are compared with a local calculation of the current sheet location and thickness carried out by McPherron et al. (1987) and Sanny et al. (this issue). Both models suggest that during the growth phase the current sheet rotated away from its nominal location, and simultaneously thinned strongly. The locations and thicknesses obtained from the two models are in good agreement. The global model suggests that the peak current density is ∼120 nA/m2, and that the cross‐tail current almost doubled its intensity during this very strong growth phase. The global model predicts a field configuration that is sufficiently stretched to scatter thermal electrons, which may be conducive to the onset of ion tearing in the tail. The electron plasma data further support this scenario, as the anisotropy present in the low‐energy electrons disappears close to the substorm onset. The electron contribution to the intensifying current in this case is of the order of 10% before the isotropization of the distribution.

Global and local estimates of the current sheet thickness: CDAW-6

The growth phase development of the near-Earth magnetotail configuration during the CDAW-6 substorm (22 March 1979, 1054 UT) is studied using two complementary methods. The global magnetic field configuration is modeled using data from two suitably located spacecraft and temporally evolving variations of the Tsyganenko magnetic field model. These results are compared with a local calculation of the current sheet location and thickness previously carried out by McPherron et al. /1/. The good agreement between the two methods provides a positive test for the accuracy of the global model. Both models indicate the formation of an extremely thin current sheet during the substorm growth phase. The plasma electron data together with the rate of pitch-angle scattering estimated from the global field model suggest that the onset may be initiated by the chaotization of electrons and consequent growth of the tearing instability.

Quasi-static MHD processes in Earth's magnetosphere

The purpose of this paper is to demonstrate how the MHD equilibrium theory can be used to describe the global magnetic field configuration of Earth's magnetosphere and its time evolution under the influence of magnetospheric convection. The MHD equilibrium theory represents magneto-hydrodynamics in the slow-flow approximation. In this approximation time scales are long compared to typical Alfvén wave travel times, and plasma flow velocities are small compared to the Alfvén speed. Under those conditions, the inertial term ρ(dv/dt) in the MHD equation of motion is a small second order term which can be neglected. The MHD equilibrium theory is not a static theory, though, because time derivatives and flow velocities remain first order quantities in the continuity equation, in the thermodynamic equation of state, and in the induction equation. Therefore one can compute slowly time-dependent processes, such as magnetospheric convection, in terms of series of static equilibrium states. However, those series are not arbitrary; they are constrained by thermodynamic conditions according to which the magnetosphere evolves in time.It is an interesting question, whether or not the magnetosphere, driven by slow, lossless, adiabatic, earthward convection of magnetotail flux tubes, can reach a steady state. There exist magnetospheric equilibria in which magnetotail flux tubes satisfy the steady-state condition d/dt (Pρ−γ) = 0. Those configurations exhibit a deep magnetic field minimum in the equatorial plane, near the inner edge of the tail plasma sheet. The magnetosphere becomes tearing-mode unstable in the neighborhood of such a minimum, thus leading to periodic onsets of substorms in the inner plasma sheet. This explains why distinct magnetic field minima have not been observed in this region. Magnetic substorms seem to be an inevitable element of the global convection cycle which inhibit the establishment of an ultimate steady state.MHD equilibria discussed in this paper result from linear and non-linear solutions to the two-dimensional Grad-Shafranov equation for isotropic thermal plasma pressure.

Studies of the configuration of the Venus ionospheric magnetic field

The dayside ionospheric magnetic field of Venus has been modelled from two different points of view. The Cloutier et al. electrodynamic model makes specific predictions about the behavior of the global magnetic field configuration that can be compared with those expected from the alternate diffusion/convection model. Although the diffusion/convection model is currently only one-dimensional, it is found that it is consistent with the observations in several areas where the 3-dimensional electrodynamic model is not.