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Register a new userSpatial Variation in the Responses of the Surface External and Induced Magnetic Field to the Solar Wind
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We analyze the spatial variation in the response of the surface geomagnetic field (or the equivalent ionospheric current) to variations in the solar wind. Specifically, we regress a reanalysis of surface external and induced magnetic field (SEIMF) variations onto measurements of the solar wind. The regression is performed in monthly sets, independently for 559 regularly spaced locations covering the entire northern polar region above 50 magnetic latitude. At each location, we find the lag applied to the solar wind data that maximizes the correlation with the SEIMF. The resulting spatial maps of these independent lags and regression coefficients provide a model of the localized SEIMF response to variations in the solar wind, which we call {guillemotleft}Spatial Information from Distributed Exogenous Regression.{guillemotright} We find that the lag and regression coefficients vary systematically with ionospheric region, season, and solar wind driver. In the polar cap region the SEIMF is best described by the (B$_{y}$) component of the interplanetary magnetic field (50-75% of total variance explained) at a lag the SEIMF is best described by the solar wind that varies with season and magnetic local time (MLT), from $sim$15-20 min for dayside and afternoon MLT (except in Oct-Dec) to typically 30-40 min for nightside and morning MLT and even longer (60-65 min) around midnight MLT.
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We investigate the spatial correlation properties of the solar wind using simultaneous observations by the ACE and WIND spacecraft. We use mutual information as a nonlinear measure of correlation and compare this to linear correlation. We find that the correlation lengthscales of fluctuations in density and magnetic field magnitude vary strongly with the solar cycle, whereas correlation lengths of fluctuations in B field components do not. We find the correlation length of |B| ~ 120 Re at solar minimum and ~ 270 Re at maximum and the correlation length of density ~ 75 Re at minimum and ~ 170 Re at minimum. The components of the B field have correlation lengths ~ correlation length |B| at minimum.
A statistical relationship between magnetic reconnection, current sheets and intermittent turbulence in the solar wind is reported for the first time using in-situ measurements from the Wind spacecraft at 1 AU. We identify intermittency as non-Gaussian fluctuations in increments of the magnetic field vector, $mathbf{B}$, that are spatially and temporally non-uniform. The reconnection events and current sheets are found to be concentrated in intervals of intermittent turbulence, identified using the partial variance of increments method: within the most non-Gaussian 1% of fluctuations in $mathbf{B}$, we find 87%-92% of reconnection exhausts and $sim$9% of current sheets. Also, the likelihood that an identified current sheet will also correspond to a reconnection exhaust increases dramatically as the least intermittent fluctuations are removed from the dataset. Hence, the turbulent solar wind contains a hierarchy of intermittent magnetic field structures that are increasingly linked to current sheets, which in turn are progressively more likely to correspond to sites of magnetic reconnection. These results could have far reaching implications for laboratory and astrophysical plasmas where turbulence and magnetic reconnection are ubiquitous.
We investigate the scattering of strahl electrons by microinstabilities as a mechanism for creating the electron halo in the solar wind. We develop a mathematical framework for the description of electron-driven microinstabilities and discuss the associated physical mechanisms. We find that an instability of the oblique fast-magnetosonic/whistler (FM/W) mode is the best candidate for a microinstability that scatters strahl electrons into the halo. We derive approximate analytic expressions for the FM/W instability threshold in two different $beta_{mathrm c}$ regimes, where $beta_{mathrm c}$ is the ratio of the core electrons thermal pressure to the magnetic pressure, and confirm the accuracy of these thresholds through comparison with numerical solutions to the hot-plasma dispersion relation. We find that the strahl-driven oblique FM/W instability creates copious FM/W waves under low-$beta_{mathrm c}$ conditions when $U_{0mathrm s}gtrsim 3w_{mathrm c}$, where $U_{0mathrm s}$ is the strahl speed and $w_{mathrm c}$ is the thermal speed of the core electrons. These waves have a frequency of about half the local electron gyrofrequency. We also derive an analytic expression for the oblique FM/W instability for $beta_{mathrm c}sim 1$. The comparison of our theoretical results with data from the emph{Wind} spacecraft confirms the relevance of the oblique FM/W instability for the solar wind. The whistler heat-flux, ion-acoustic heat-flux, kinetic-Alfven-wave heat-flux, and electrostatic electron-beam instabilities cannot fulfill the requirements for self-induced scattering of strahl electrons into the halo. We make predictions for the electron strahl close to the Sun, which will be tested by measurements from emph{Parker Solar Probe} and emph{Solar Orbiter}.
One of the discoveries made by Parker Solar Probe during first encounters with the Sun is the ubiquitous presence of relatively small-scale structures standing out as sudden deflections of the magnetic field. They were called switchbacks as some of them show up the full reversal of the radial component of the magnetic field and then return to regular conditions. Analyzing the magnetic field and plasma perturbations associated with switchbacks we identify three types of structures with slightly different characteristics: 1. Alfvenic structures, where the variations of the magnetic field components take place while the magnitude of the field remains constant; 2. Compressional, the field magnitude varies together with changes of the components; 3. Structures manifesting full reversal of the magnetic field (extremal class of Alfvenic structures). Processing of structures boundaries and plasma bulk velocity perturbations lead to the conclusion that they represent localized magnetic field tubes with enhanced parallel plasma velocity and ion beta moving together with respect to surrounding plasma. The magnetic field deflections before and after the switchbacks reveal the existence of total axial current. The electric currents are concentrated on the relatively narrow boundary layers on the surface of the tubes and determine the magnetic field perturbations inside the tube. These currents are closed on the structure surface, and typically have comparable azimuthal and the axial components. The surface of the structure may also accommodate an electromagnetic wave, that assists to particles in carrying currents. We suggest that the two types of structures we analyzed here may represent the local manifestations of the tube deformations corresponding to a saturated stage of the Firehose instability development.
The twisted local magnetic field at the front or rear regions of the magnetic clouds (MCs) associated with interplanetary coronal mass ejections (ICMEs) is often nearly opposite to the direction of the ambient interplanetary magnetic field (IMF). There is also observational evidence for magnetic reconnection (MR) outflows occurring within the boundary layers of MCs. In this paper a MR event located at the western flank of the MC occurring on 2000-10-03 is studied in detail. Both the large-scale geometry of the helical MC and the MR outflow structure are scrutinized in a detailed multi-point study. The ICME sheath is of hybrid propagation-expansion type. Here the freshly reconnected open field lines are expected to slip slowly over the MC resulting in plasma mixing at the same time. As for MR, the current sheet geometry and the vertical motion of the outflow channel between ACE-Geotail-WIND spacecraft was carefully studied and tested. The main findings on MR include: (1) First-time observation of non-Petschek-type slow-shock-like discontinuities in the inflow regions; (2) Observation of turbulent Hall magnetic field associated with a Lorentz force deflected electron jet; (3) Acceleration of protons by reconnection electric field and their back-scatter from the slow shock-like discontinuity; (4) Observation of relativistic electron near the MC inflow boundary/separatrix; these electron populations can presumably appear as a result of non-adiabatic acceleration, gradient B drift and via acceleration in the electrostatic potential well associated with the Hall current system; (5) Observation of Doppler shifted ion-acoustic and Langmuir waves in the MC inflow region.
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