Solar Wind Conditions Research Articles

Characteristics of GPS TEC variations in the polar cap ionosphere

AbstractThis paper presents statistical characteristics (occurrence rate, amplitude, and frequency) of low‐frequency (<100 mHz) variations in total electron content (TEC) observed in the polar cap ionosphere. TEC variations were primarily associated with mesoscale (tens to hundreds of kilometers) ionization structures and were observed by five Global Positioning System (GPS) receivers over a 6 year period (2009–2014). The altitude of ionization structures was estimated by using colocated ionosonde radars. High data rate receivers combined with broad spatial coverage of multisatellite TEC measurements provided high‐resolution magnetic local time/latitude maps of TEC variation characteristics, which were examined as a function of solar cycle and season. These high‐resolution maps improve upon the current observational picture of mesoscale structuring in the polar cap and provide accurate links to particular magnetospheric source regions. Occurrence of TEC variations was consistently highest in dayside regions mapping to low latitude and plasma mantle boundary layers, while largest‐amplitude TEC variations were observed in dayside regions close to the polar cusp, and lower latitudes around midnight. Occurrence and amplitude of TEC variations increased significantly during the ascending phase of the solar cycle, independent of solar wind conditions, while seasonal statistics showed highest dayside occurrence and amplitude in winter months, lowest in summer, and highest nightside occurrence and amplitude around equinox. A surprising result in the frequency distributions of TEC variations was discrete frequencies of about 2 and 4 mHz, which appeared to originate from regions corresponding to the plasma mantle, immediately poleward of the polar cusp.

Subsolar magnetopause observation and kinetic simulation of a tripolar guide magnetic field perturbation consistent with a magnetic island

AbstractThe Polar satellite recorded two reconnection exhausts within 6 min on 1 April 2001 across a subsolar magnetopause that displayed a symmetric plasma density, but different out‐of‐plane magnetic field signatures for similar solar wind conditions. The first magnetopause crossing displayed a bipolar guide field variation in a weak external guide field consistent with a symmetric Hall field from a single X line. The subsequent crossing represents the first observation of a tripolar guide field perturbation at Earth's magnetopause in a strong guide field. This perturbation consists of a significant guide field enhancement between two narrow guide field depressions. A particle‐in‐cell simulation for the prevailing conditions across this second event resulted in a magnetic island between two simulated X lines across which a tripolar guide field developed consistent with the observation. The simulated island supports a scenario whereby Polar encountered the asymmetric quadrupole Hall magnetic fields between two X lines for symmetric conditions across the magnetopause.

Cassini observations of Saturn's southern polar cusp

AbstractThe magnetospheric cusps are important sites of the coupling of a magnetosphere with the solar wind. The combination of both ground‐ and space‐based observations at Earth has enabled considerable progress to be made in understanding the terrestrial cusp and its role in the coupling of the magnetosphere to the solar wind via the polar magnetosphere. Voyager 2 fully explored Neptune's cusp in 1989, but highly inclined orbits of the Cassini spacecraft at Saturn present the most recent opportunity to repeatedly study the polar magnetosphere of a rapidly rotating planet. In this paper we discuss observations made by Cassini during two passes through Saturn's southern polar magnetosphere. Our main findings are that (i) Cassini directly encounters the southern polar cusp with evidence for the entry of magnetosheath plasma into the cusp via magnetopause reconnection, (ii) magnetopause reconnection and entry of plasma into the cusp can occur over a range of solar wind conditions, and (iii) double cusp morphologies are consistent with the position of the cusp oscillating in phase with Saturn's global magnetospheric periodicities.

Wide field‐of‐view soft X‐ray imaging for solar wind‐magnetosphere interactions

AbstractSoft X‐ray imagers can be used to study the mesoscale and macroscale density structures that occur whenever and wherever the solar wind encounters neutral atoms at comets, the Moon, and both magnetized and unmagnetized planets. Charge exchange between high charge state solar wind ions and exospheric neutrals results in the isotropic emission of soft X‐ray photons with energies from 0.1 to 2.0 keV. At Earth, this process occurs primarily within the magnetosheath and cusps. Through providing a global view, wide field‐of‐view imaging can determine the significance of the various proposed solar wind‐magnetosphere interaction mechanisms by evaluating their global extent and occurrence patterns. A summary of wide field‐of‐view (several to tens of degrees) soft X‐ray imaging is provided including slumped micropore microchannel reflectors, simulated images, and recent flight results.

The atmosphere of comet 67P/Churyumov-Gerasimenko diagnosed by charge-exchanged solar wind alpha particles

Context. The ESA/Rosetta mission has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, measuring its dayside plasma environment. The ion spectrometer onboard Rosetta has detected two ion populations, one energetic with a solar wind origin (H+, He2+, He+), the other at lower energies with a cometary origin (water group ions such as H2O+). He+ ions arise mainly from charge-exchange between solar wind alpha particles and cometary neutrals such as H2O. Aims. The He+ and He2+ ion fluxes measured by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA) give insight into the composition of the dayside neutral coma, into the importance of charge-exchange processes between the solar wind and cometary neutrals, and into the way these evolve when the comet draws closer to the Sun. Methods. We combine observations by the ion spectrometer RPC-ICA onboard Rosetta with calculations from an analytical model based on a collisionless neutral Haser atmosphere and nearly undisturbed solar wind conditions. Results. Equivalent neutral outgassing rates Q can be derived using the observed RPC-ICA He+/He2+ particle flux ratios as input into the analytical model in inverse mode. A revised dependence of Q on heliocentric distance Rh in AU is found to be Rh -7.06Rh-7.06 between 1.8 and 3.3 AU, suggesting that the activity in 2015 differed from that of the 2008 perihelion passage. Conversely, using an outgassing rate determined from optical remote sensing measurements from Earth, the forward analytical model results are in relatively good agreement with the measured RPC-ICA flux ratios. Modelled ratios in a 2D spherically-symmetric plane are also presented, showing that charge exchange is most efficient with solar wind protons. Detailed cometocentric profiles of these ratios are also presented. Conclusions. In conclusion, we show that, with the help of a simple analytical model of charge-exchange processes, a mass-capable ion spectrometer such as RPC-ICA can be used as a remote-sensing instrument for the neutral cometary atmosphere. © 2016 ESO.

Average field‐aligned current configuration parameterized by solar wind conditions

AbstractWe present the first large‐scale comparison of the spatial distribution of field‐aligned currents as measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment, with the location and brightness of the average auroral oval, determined from the Imager for Magnetopause‐to‐Aurora Global Exploration far ultraviolet instrument. These distributions are compared under the same interplanetary magnetic field magnitude and clock angle conditions. The field‐aligned currents and auroral oval drop to lower latitudes, as the interplanetary magnetic field becomes both increasingly stronger in magnitude and increasingly southward. We find that the region 2 currents are more closely aligned with the distribution of auroral UV emission, whether that be in the discrete auroral zone about dusk or in the postmidnight diffuse aurora sector. The lack of coincidence between the region 1 field‐aligned currents with the auroral oval in the dusk sector is contrary to expectation.

THE MAJOR GEOEFFECTIVE SOLAR ERUPTIONS OF 2012 MARCH 7: COMPREHENSIVE SUN-TO-EARTH ANALYSIS

ABSTRACT During the interval 2012 March 7–11 the geospace experienced a barrage of intense space weather phenomena including the second largest geomagnetic storm of solar cycle 24 so far. Significant ultra-low-frequency wave enhancements and relativistic-electron dropouts in the radiation belts, as well as strong energetic-electron injection events in the magnetosphere were observed. These phenomena were ultimately associated with two ultra-fast (>2000 km s−1) coronal mass ejections (CMEs), linked to two X-class flares launched on early 2012 March 7. Given that both powerful events originated from solar active region NOAA 11429 and their onsets were separated by less than an hour, the analysis of the two events and the determination of solar causes and geospace effects are rather challenging. Using satellite data from a flotilla of solar, heliospheric and magnetospheric missions a synergistic Sun-to-Earth study of diverse observational solar, interplanetary and magnetospheric data sets was performed. It was found that only the second CME was Earth-directed. Using a novel method, we estimated its near-Sun magnetic field at 13 R ⊙ to be in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun CME magnetic field are required to match the magnetic fields of the corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream solar-wind conditions, as resulting from the shock associated with the Earth-directed CME, offer a decent description of its kinematics. The magnetospheric compression caused by the arrival at 1 AU of the shock associated with the ICME was a key factor for radiation-belt dynamics.

The geomagnetic cutoff rigidities at high latitudes for different solar wind and geomagnetic conditions

Abstract. Studying the access of the cosmic rays (CRs) into the magnetosphere is important to understand the coupling between the magnetosphere and the solar wind. In this paper we numerically studied CRs' magnetospheric access with vertical geomagnetic cutoff rigidities using the method proposed by Smart and Shea (1999). By the study of CRs' vertical geomagnetic cutoff rigidities at high latitudes we obtain the CRs' window (CRW) whose boundary is determined when the vertical geomagnetic cutoff rigidities drop to a value lower than a threshold value. Furthermore, we studied the area of CRWs and found out they are sensitive to different parameters, such as the z component of interplanetary magnetic field (IMF), the solar wind dynamic pressure, AE index, and Dst index. It was found that both the AE index and Dst index have a strong correlation with the area of CRWs during strong geomagnetic storms. However, during the medium storms, only AE index has a strong correlation with the area of CRWs, while Dst index has a much weaker correlation with the area of CRWs. This result on the CRW can be used for forecasting the variation of the cosmic rays during the geomagnetic storms.

A Statistical Study of the Plasma Sheet in the Near and Middle Earth Magnetotail

AbstractMagnetotail plasma sheet is the most active area in Earth's magnetosphere. It has been found that the features of plasma sheet are controlled by the conditions of solar wind and Interplanetary Magnetic Field (IMF). Some previous statistical studies have found that some parameters of the plasma sheet are dawn‐dusk asymmetric. But the thickness of the magnetotail plasma sheet in the near and mid‐tail region and how they are influenced by the IMF is still unclear. In this paper, the probability of the Cluster‐C1 satellite encountering the plasma sheet is examined statistically by utilizing the proton flux and ß data from the CODIF and FGM equipment on board the Cluster‐C1. Using data from July to November of year 2001–2004, the distributions of the probability of satellite in the plasma sheet are mapped on the Y‐Dz plane (Dz denotes the distance between the satellite and the neutral sheet) during the southward and northward IMF periods, respectively. By comparison, we found that the plasma sheet is thinner during southward IMF periods than that during northward IMF periods. It is more obvious in the flank regions of the plasma sheet. We also found that the plasma sheet in the dusk side is thinner than that in the dawn side.

Global MHD modeling of resonant ULF waves: Simulations with and without a plasmasphere.

We investigate the plasmaspheric influence on the resonant mode coupling of magnetospheric ultralow frequency (ULF) waves using the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic (MHD) model. We present results from two different versions of the model, both driven by the same solar wind conditions: one version that contains a plasmasphere (the LFM coupled to the Rice Convection Model, where the Gallagher plasmasphere model is also included) and another that does not (the stand‐alone LFM). We find that the inclusion of a cold, dense plasmasphere has a significant impact on the nature of the simulated ULF waves. For example, the inclusion of a plasmasphere leads to a deeper (more earthward) penetration of the compressional (azimuthal) electric field fluctuations, due to a shift in the location of the wave turning points. Consequently, the locations where the compressional electric field oscillations resonantly couple their energy into local toroidal mode field line resonances also shift earthward. We also find, in both simulations, that higher‐frequency compressional (azimuthal) electric field oscillations penetrate deeper than lower frequency oscillations. In addition, the compressional wave mode structure in the simulations is consistent with a radial standing wave oscillation pattern, characteristic of a resonant waveguide. The incorporation of a plasmasphere into the LFM global MHD model represents an advance in the state of the art in regard to ULF wave modeling with such simulations. We offer a brief discussion of the implications for radiation belt modeling techniques that use the electric and magnetic field outputs from global MHD simulations to drive particle dynamics.

Asymmetrical response of dayside ion precipitation to a large rotation of the IMF

AbstractWe have carried out global magnetohydrodynamics (MHD) simulations together with large‐scale kinetic simulations to investigate the response of the dayside magnetospheric ion precipitation to a large rotation (135°) of the interplanetary magnetic field (IMF). The study uses simplified global MHD model (no dipole tilt and constant ionospheric conductance) and idealized solar wind conditions where the IMF rotates smoothly from a southward toward a northward direction (BX = 0) to clearly identify the effects of the impact of the discontinuity on the magnetopause. Results of the global simulations reveal that a strong north‐south asymmetry develops in the pattern of precipitating ions during the interaction of the IMF rotation with the magnetopause. For a counterclockwise IMF rotation from its original southward direction (BY < 0), a spot of high‐energy particle injections occurs in the Northern Hemisphere but not in the Southern Hemisphere. The spot moves poleward and dawnward as the interacting field rotates. In that case, reconnection is found close to the poleward edge of the northern cusp, while it occurs farther tailward in the Southern Hemisphere. Tracing magnetic field lines shows an asymmetry in the tilt of the cusps and indicates that the draping and subsequent double reconnection of newly opened field lines from the Southern Hemisphere over the dayside magnetosphere cause the symmetry breaking. The reverse north‐south asymmetry is found for a clockwise IMF rotation from its original southward direction (BY > 0). Trends observed in the ion dispersions predicted from the simulations are in good agreement with Cluster observations of the midaltitude northern cusp, which motivated the study.

Ion-driven instabilities in the solar wind: Wind observations of 19 March 2005.

Intervals of enhanced magnetic fluctuations have been frequently observed in the solar wind. But it remains an open question as to whether these waves are generated at the Sun and then transported outward by the solar wind or generated locally in the interplanetary medium. Magnetic field and plasma measurements from the Wind spacecraft under slow solar wind conditions on 19 March 2005 demonstrate seven events of enhanced magnetic fluctuations at spacecraft‐frame frequencies somewhat above the proton cyclotron frequency and propagation approximately parallel or antiparallel to the background magnetic field B o. The proton velocity distributions during these events are characterized by two components: a more dense, slower core and a less dense, faster beam. Observed plasma parameters are used in a kinetic linear dispersion equation analysis for electromagnetic fluctuations at k x B o = 0; for two events the most unstable mode is the Alfvén‐cyclotron instability driven by a proton component temperature anisotropy T⊥/T|| > 1 (where the subscripts denote directions relative to B o), and for three events the most unstable mode is the right‐hand polarized magnetosonic instability driven primarily by ion component relative flows. Thus, both types of ion anisotropies and both types of instabilities are likely to be local sources of these enhanced fluctuation events in the solar wind.

IPS observations at 140 MHz to study solar wind speeds and density fluctuations by MEXART

The interplanetary scintillation (IPS) technique is a remote-sensing method for monitoring the inner heliosphere. These observations supply information on solar wind conditions covering heliocentric ranges that no other technique can provide. The Mexican Array Radio Telescope (MEXART) is a single-station instrument operating at 140MHz, fully dedicated to performing solar-wind studies employing the IPS technique. We present solar-wind parameters (scintillation indices and solar-wind speeds), using the initial measurements from this array of four IPS sources (3C273, 3C283, 3C286, 3C298) detected during October–December 2014. We report the transit of an IPS radio source (3C298), observed at 140MHz, from weak- to strong-scattering regions at around 0.36 AU, and sky projection maps of solar wind conditions associated presumably with the passing of one or more Coronal Mass Ejections (CMEs). These results show the progress to operate the full array in the near future. The MEXART observations will complement the solar wind IPS studies using other frequencies, and the tracking of solar wind disturbances by other stations located at different longitudes. These solar wind measurements, provided in real time, can have space-weather forecasting applications.

Solar wind reconstruction from magnetosheath data using an adjoint approach

Abstract. We present a new method to reconstruct solar wind conditions from spacecraft data taken during magnetosheath passages, which can be used to support, e.g., magnetospheric models. The unknown parameters of the solar wind are used as boundary conditions of an MHD (magnetohydrodynamics) magnetosheath model. The boundary conditions are varied until the spacecraft data matches the model predictions. The matching process is performed using a gradient-based minimization of the misfit between data and model. To achieve this time-consuming procedure, we introduce the adjoint of the magnetosheath model, which allows efficient calculation of the gradients. An automatic differentiation tool is used to generate the adjoint source code of the model. The reconstruction method is applied to THEMIS (Time History of Events and Macroscale Interactions during Substorms) data to calculate the solar wind conditions during spacecraft magnetosheath transitions. The results are compared to actual solar wind data. This allows validation of our reconstruction method and indicates the limitations of the MHD magnetosheath model used.

Assessing the performance of community‐available global MHD models using key system parameters and empirical relationships

AbstractGlobal magnetohydrodynamic (MHD) modeling is a powerful tool in space weather research and predictions. There are several advanced and still developing global MHD (GMHD) models that are publicly available via Community Coordinated Modeling Center's (CCMC) Run on Request system, which allows the users to simulate the magnetospheric response to different solar wind conditions including extraordinary events, like geomagnetic storms. Systematic validation of GMHD models against observations still continues to be a challenge, as well as comparative benchmarking of different models against each other. In this paper we describe and test a new approach in which (i) a set of critical large‐scale system parameters is explored/tested, which are produced by (ii) specially designed set of computer runs to simulate realistic statistical distributions of critical solar wind parameters and are compared to (iii) observation‐based empirical relationships for these parameters. Being tested in approximately similar conditions (similar inputs, comparable grid resolution, etc.), the four models publicly available at the CCMC predict rather well the absolute values and variations of those key parameters (magnetospheric size, magnetic field, and pressure) which are directly related to the large‐scale magnetospheric equilibrium in the outer magnetosphere, for which the MHD is supposed to be a valid approach. At the same time, the models have systematic differences in other parameters, being especially different in predicting the global convection rate, total field‐aligned current, and magnetic flux loading into the magnetotail after the north‐south interplanetary magnetic field turning. According to validation results, none of the models emerges as an absolute leader. The new approach suggested for the evaluation of the models performance against reality may be used by model users while planning their investigations, as well as by model developers and those interesting to quantitatively evaluate progress in magnetospheric modeling.

Statistical mapping of ULF Pc3 velocity fluctuations in the Earth’s dayside magnetosheath as a function of solar wind conditions

In this paper, we present the results of a statistical study of Pc3 velocity fluctuations in the Earth’s dayside magnetosheath. There exists a notable dawn–dusk asymmetry, such that velocity fluctuations generally exhibit enhanced spectral power in the magnetosheath downstream of the quasi-parallel shock. The fluctuations in the central magnetosheath and close to bow shock tend to dampen with increasing tail-ward distance while the opposite trend is observed close to the magnetopause. This strongly suggests that velocity shear driven processes such as the Kelvin–Helmholtz instability drive Pc3 flow variations close to the magnetopause as the velocity shear increases with increasing tail-ward distance. We also show strong evidence that Pc3 velocity fluctuations are significantly enhanced during intervals of faster solar wind speeds. We see negligible differences between data collected during northward and southward IMF orientations, but in general, a dawn-favoured asymmetry persists.

The morphology of the topside ionosphere of Mars under different solar wind conditions: Results of a multi-instrument observing campaign by Mars Express in 2010

Since the internally-generated magnetic field of Mars is weak, strong coupling is expected between the solar wind, planetary magnetosphere, and planetary ionosphere. However, few previous observational studies of this coupling incorporated data that extended from the solar wind to deep into the ionosphere. Here we use solar wind, magnetosphere, and ionosphere data obtained by the Mars Express spacecraft during March/April 2010 to investigate this coupling. We focus on three case studies, each centered on a pair of ionospheric electron density profiles measured by radio occultations, where the two profiles in each pair were obtained from the same location at an interval of only a few days. We find that high dynamic pressures in the solar wind are associated with compression of the magnetosphere, heating of the magnetosheath, reduction in the vertical extent of the ionosphere, and abrupt changes in electron density at the top of the ionosphere. The first three of these associations are analogous to the behavior of the plasma environment of Venus, but the final one is not. These results reinforce the notion that changes in solar forcing influence the behaviors of all of the tightly coupled regions within the Martian plasma environment.

MHD model results of solar wind interaction with Mars and comparison with MAVEN plasma observations

AbstractThe Mars Atmosphere and Volatile EvolutioN mission (MAVEN), launched on 18 November 2013, is now in its primary science phase, orbiting Mars with a 4.5 h period. In this study, we use a time‐dependent MHD model to interpret plasma observations made by MAVEN particle and field instruments. Detailed comparisons between the model and the relevant plasma observations from MAVEN are presented for an entire Mars rotation under relatively quiet solar wind conditions. Through comparison along MAVEN orbits, we find that the time‐dependent multispecies single‐fluid MHD model is able to reproduce the main features of the plasma environment around Mars. Using the model results, we find that photoionization beyond the terminator is the dominant ion source as compared with day‐night transport in maintaining the nightside ionosphere. Model results also show that both the time‐varying solar wind conditions and the continuously rotating crustal field work together to control the ion escape variation with time.

Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind‐induced magnetospheric compressions and expansions

AbstractWe examine planetary period oscillations (PPOs) observed in Saturn's magnetospheric magnetic field data from the time of Saturn's equinox in 2009. In particular, we focus on the time period commencing February 2011, when the oscillations started to display sudden and unexpected changes in behavior at ~100–200 day intervals. These were characterized by large simultaneous changes in the amplitude of the northern and southern PPO systems, together with small changes in period and jumps in phase. Nine significant abrupt changes have been observed in the postequinox interval to date, commencing as the Sun started to emerge from a long extended solar minimum. We perform a statistical study to determine whether these modulations in PPO behavior were associated with changes in the solar and/or upstream solar wind conditions. We report that the upstream solar wind conditions show elevated values of solar wind dynamic pressure and density around the time of PPO behavioral transitions, as opposed to before and after these times. We suggest that abrupt changes in PPO behavior may be related to significant changes in the size of the Saturnian magnetosphere in response to varying solar wind conditions.

Numerical modeling of the polar F region ionosphere taking into account the solar wind conditions

The numerical model predicts the 3-D distribution of electron density over the high-latitude F region ionosphere in the altitudes between 130 and 600km. The distinctive feature of the model is an analytical representation of the electric potential distribution over the high-latitude ionospheric shell which continuously evolving with the solar wind parameters, season and universal time. In this approach the convection electric field is directly related to the field-aligned currents of magnetospheric origin which are controlled by the solar wind. The time-dependent ion continuity and momentum equations are solved as a function of altitude within a convecting and corotating plasma flux tube. Modeling results show that the polar ionosphere F region responds strongly to the change in the IMF polarity and the solar zenith angle. Large-scale ionospheric irregularities are reproduced in details.