Interplanetary Magnetic Field Configuration Research Articles

Asymmetric deformation of the Earth’s magnetosphere under low Alfvén Mach number solar wind: Observations and MHD simulation

AbstractThe density of the solar wind plasma near the Earth’s magnetosphere sometimes decreases to only several per cent of the usual value, and such density extrema result in a significant reduction of the dynamic pressure and Alfvén Mach number ($$M_A$$ M A ) of the solar wind flow. While a symmetric expansion of the Earth’s magnetosphere by the low dynamic pressure was assumed in previous studies, a global magnetohydrodynamic (MHD) simulation study predicted a remarkable dawn-dusk asymmetry of the magnetospheric shape under low-density solar wind and Parker-spiral interplanetary magnetic field (IMF) configuration. Here, we present observations consistent with the asymmetric deformation of the magnetosphere under low-$$M_A$$ M A solar wind and Parker-spiral IMF conditions, focusing on the significant expansion of the dawn-flank magnetosphere detected by the Geotail spacecraft. A global MHD simulation reproduced the dawnward expansion of the near-Earth magnetosphere, which was consistent with the observation by Geotail. The solar wind flow had a non-negligible dusk-to-dawn component and partly affected the dawnward expansion of the magnetosphere. Local, roughly Alfvénic sunward acceleration of magnetosheath ions at the dawn flank magnetopause suggests magnetosheath plasma entry into the magnetosphere through open field lines generated by magnetic reconnection at the dayside magnetopause. At the same time, Cluster 1 and 3, located near the southern polar cusp, also detected continuous antisunward ion jets and occasional sunward jets, which are consistent with the occurrence of magnetic reconnection near the southern cusp. These observations suggest that enhanced plasma acceleration at the dayside magnetopause operates under the low-$$M_A$$ M A solar wind and Parker spiral IMF conditions and that plasma influx across the dawnside magnetopause is at work under such a low-$$M_A$$ M A condition. These results can be helpful in understanding interactions between low-$$M_A$$ M A solar/stellar winds and celestial objects, such as inner planets and exoplanets. Graphic Abstract

Solar Wind Impact on ENAs From Earth's Subsolar Magnetosheath

AbstractAs the solar wind (SW) crosses Earth's bow shock, it is heated and decelerated, resulting in a hot and dense magnetosheath ion population. This increases the probability of charge exchange between a magnetosheath proton and a geocoronal Hydrogen atom, which produces energetic neutral atoms (ENAs). These ENAs propagate away from the magnetosheath carrying information about the inherent properties of the progenitor plasma. In this work, ENA fluxes from the subsolar magnetosheath, observed by the Interstellar Boundary Explorer, are compared to SW conditions. These comparisons reveal that the passband‐weighted average differential flux of ENAs is strongly influenced by the SW density, and that ENA spectra flatten with increasing SW speed and temperature, and steepen with increasing SW density. In addition, there is evidence from ENA spectra for different interplanetary magnetic field configurations that most of the ENAs observed by Interstellar Boundary Explorer come from the specularly reflected proton population in the magnetosheath.

Ion acceleration in Mars’ twisted magnetotail

ABSTRACT Mars lacks a global magnetic field and instead has a unique, ‘hybrid’ magnetosphere composed of crustal magnetic fields, which are stronger in the Southern hemisphere than the Northern hemisphere. These crustal fields are believed to drive a twist in the magnetotail at Mars. This is important because heavy ions escape from this induced magnetotail, which is a major source of atmospheric erosion. We present analysis of heavy ions produced in Mars’ upper atmosphere that are transported downtail along the twisted current sheet and observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We identified O+, O$_2\, ^+$, and CO$_2\, ^+$ being accelerated to different energies at a range of distances downtail and found that heavy ions in the 10–50 eV energy population have the highest flux. We present tail slices of ion fluxes for different interplanetary magnetic field (IMF) configurations based on the BY component of the IMF and find that the different configurations affect the degree of tail twist and the transport of heavy ions in the induced magnetotail lobes. We find that the total escape rate for O+, O$_2\, ^+$, and CO$_2\, ^+$ is 5.5e24 s-1, with 50% escaping between 2–200 eV.

Ultra‐low Frequency Foreshock Waves and Ion Dynamics at Mars

AbstractWe study the solar wind interaction with Mars in a global three‐dimensional hybrid model. A well‐developed, vast ion foreshock forms under a strongly flow‐aligned interplanetary magnetic field (IMF) configuration but otherwise nominal solar wind and solar minimum photon flux conditions. Large‐scale ultra‐low frequency (ULF) waves are excited in the foreshock by backstreaming ions. The foreshock ULF waves constitute two distinct regions in the analyzed solar wind and IMF situation: the near region where the wave period is 71–83 s and the far region where the wave period is 25–28 s. The near foreshock region waves transmit downstream through the bow shock and affect dynamics of the solar wind and planetary ion populations. Especially, ion precipitation rate into the exobase and planetary ion escape rates fluctuate at the ULF wave period corresponding to the near foreshock region. The peak‐to‐peak amplitude of the modulation is few percent or less. Interestingly, ionospheric oxygen ion escape fluxes show more than two orders of magnitude local modulations in the heavy plume at the same period. Finally, the escape rates of the ionospheric oxygen ion populations are enhanced by 60–70% under flow‐aligned IMF compared to nominal upstream conditions.

Remote sensing of cometary bow shocks: modelled asymmetric outgassing and pickup ion observations

ABSTRACT Despite the long escort by the ESA Rosetta mission, direct observations of a fully developed bow shock around 67P/Churyumov–Gerasimenko have not been reported. Expanding on our previous work on indirect observations of a shock, we model the large-scale features in cometary pickup ions, and compare the results with the ESA Rosetta Plasma Consortium Ion Composition Analyser ion spectrometer measurements over the pre-perihelion portion of the escort phase. Using our hybrid plasma simulation, an empirical, asymmetric outgassing model for 67P, and varied interplanetary magnetic field (IMF) clock angles, we model the evolution of the large-scale plasma environment. We find that the subsolar bow shock standoff distance is enhanced by asymmetric outgassing with a factor of 2 to 3, reaching up to $18\,000\, \rm {km}$ approaching perihelion. We find that distinct spectral features in simulated pickup ion distributions are present for simulations with shock-like structures, with the details of the spectral features depending on shock standoff distance, heliocentric distance, and IMF configuration. Asymmetric outgassing along with IMF clock angle is found to have a strong effect on the location of the spectral features, while the IMF clock angle causes no significant effect on the bow shock standoff distance. These dependences further complicate the interpretation of the ion observations made by Rosetta. Our data-model comparison shows that the large-scale cometary plasma environment can be probed by remote sensing the pickup ions, at least when the comet’s activity is comparable to that of 67P, and the solar wind parameters are known.

Solar Cycle, Maunder Minimum and Pandemic Influenza

We explore the idea that influenza pandemics may arise from the transference of new virions (new sub-types of the influenza virus) of cosmic origin in general accord with the theory of cometary panspermia. Such a transfer process will be modulated by the sunspot cycle and through its role in affecting the interplanetary magnetic field configurations in the Earth’s vicinity. Transfers of virus could take place directly from comets or indirectly from a transient repository represented for instance by the Kordylewski if dust clouds at the L4 and L5 Lagrange libration points of the Earth-Moon system. In either case an active sun appears to be a perequisite for effective transfers. The long remission of influenza pandemics throughout the period 1645-1715, during the Maunder sunspot minimum, might be understood on the basis of our model.

Solar Cycle, Maunder Minimum and Pandemic Influenza

We explore the idea that influenza pandemics may arise from the transference of new virions (new sub-types of the influenza virus) of cosmic origin in general accord with the theory of cometary panspermia. Such a transfer process will be modulated by the sunspot cycle and through its role in affecting the interplanetary magnetic field configurations in the Earth’s vicinity. Transfers of virus could take place directly from comets or indirectly from a transient repository represented for instance by the Kordylewski if dust clouds at the L4 and L5 Lagrange libration points of the Earth-Moon system. In either case an active sun appears to be a perequisite for effective transfers. The long remission of influenza pandemics throughout the period 1645-1715, during the Maunder sunspot minimum, might be understood on the basis of our model.

Statistical study of non-adiabatic energization and transport in Kelvin-Helmholtz vortices at mercury

Non-adiabatic energization and transport of planetary ions in the vicinity of Kelvin-Helmholtz (KH) vortices at Mercury are statistically investigated using a magnetohydrodynamic model combined with a test particle tracing technique. The measurements from the MErcury Surface, Space ENvironment, Geochemistry, and Ranging spacecraft show signatures of KH waves in the magnetospheric flanks, especially on the duskside. We examine here the energization and transport of heavy ions of planetary origin such as Na+ associated with the growth of the KH instability on both the dawn and dusk sides of the magnetosphere, and for both northward and southward interplanetary magnetic field configurations. We find that in all cases those planetary ions in the magnetosphere can be accelerated non-adiabatically with a level of acceleration larger on the dawnside. The transport of those ions is controlled by the convection electric field in the magnetosheath. Those planetary ions when picked up in the duskside magnetosphere where the KH waves have been mainly observed at Mercury stagnate inside the magnetosphere and, hence, are not expected to escape. The present study indicates that these ions may likely stay inside the magnetosphere and sputter the planetary surface on the duskside preferentially, being the source of secondary neutrals from the planetary surface.

Global MHD simulations of the Response of Jupiter's Magnetosphere and Ionosphere to Changes in the Solar Wind and IMF

AbstractWe have developed a new global magnetohydrodynamic (MHD) model for Jupiter's magnetosphere based on the BATSRUS code and an ionospheric electrodynamics solver. Our model includes the Io plasma torus at its appropriate location and couples the global magnetosphere with the planetary ionosphere through field‐aligned currents. Through comparisons with available particle and field observations as well as empirical models, we show that the model captures the overall configuration of the magnetosphere reasonably well. In order to understand how the magnetosphere responds to different solar wind drivers, we have carried out time‐dependent simulations using various kinds of upstream conditions, such as a forward shock and a rotation in the interplanetary magnetic field (IMF). Our model predicts that compression of the magnetosphere by a forward shock of typical strength generally weakens the corotation enforcement currents on the dayside and produces an enhancement on the nightside. However, the global response varies depending on the IMF orientation. A forward shock with a typical Parker‐spiral IMF configuration has a larger impact on the magnetospheric configuration and large‐scale current systems than with a parallel IMF configuration. Plasmoids are found to form in the simulation due to tail reconnection and have complex magnetic topology, as they evolve and propagate down tail. For a fixed mass input rate in the Io plasma torus, the frequency of plasmoid occurrence in our simulation is found to vary depending on the upstream solar wind driving.

Localized Ionization Hypothesis for Transient Ionospheric Layers

AbstractThe persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies, respectively, determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring nonglobally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bow shock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers' wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN Radio Occultation Science Experiment in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed.

Modeling Solar Energetic Particle Transport near a Wavy Heliospheric Current Sheet

Abstract Understanding the transport of solar energetic particles (SEPs) from acceleration sites at the Sun into interplanetary space and to the Earth is an important question for forecasting space weather. The interplanetary magnetic field (IMF), with two distinct polarities and a complex structure, governs energetic particle transport and drifts. We analyze for the first time the effect of a wavy heliospheric current sheet (HCS) on the propagation of SEPs. We inject protons close to the Sun and propagate them by integrating fully 3D trajectories within the inner heliosphere in the presence of weak scattering. We model the HCS position using fits based on neutral lines of magnetic field source surface maps (SSMs). We map 1 au proton crossings, which show efficient transport in longitude via HCS, depending on the location of the injection region with respect to the HCS. For HCS tilt angles around 30°–40°, we find significant qualitative differences between A+ and A− configurations of the IMF, with stronger fluences along the HCS in the former case but with a distribution of particles across a wider range of longitudes and latitudes in the latter. We show how a wavy current sheet leads to longitudinally periodic enhancements in particle fluence. We show that for an A+ IMF configuration, a wavy HCS allows for more proton deceleration than a flat HCS. We find that A− IMF configurations result in larger average fluences than A+ IMF configurations, due to a radial drift component at the current sheet.

Identifying the evolution of Southern Hemisphere poleward moving auroral forms (PMAFs) in the context of plasma convection and magnetic reconnection

AbstractIn this study we examine the dynamic evolution of Southern Hemisphere auroral precipitation patterns, specifically poleward moving auroral forms (PMAFs). The spatial, structural, and temporal characteristics of these events have been linked to underlying magnetopause‐boundary layer flux transfer events which are responsible for the generation of these ionospheric transient forms. We present five case studies of Southern Hemisphere PMAFs using optical data for 630.0 nm and 427.8 nm wavelengths from the US Automatic Geophysical Observatory Network in Antarctica. Emphasis is put on the unique capabilities of two‐dimensional optical data when it comes to studying PMAFs in the context of pulsed magnetic reconnection. In a detailed examination of three PMAFs recorded in 2007, we have found that their motion closely follows the underlying plasma convection patterns as determined from backscatter radar data, which in turn are governed by the interplanetary magnetic field (IMF) configuration. Of particular interest are periods when the positive IMF By component causes an asymmetric convection pattern which is biased toward the dawn region. These cases are consistent with the expected antisymmetric nature of the northern and southern hemisphere convection. In addition, we present two “pseudo” PMAFs which are actually independent auroral arcs executing poleward motions and occurring when the IMF Bz was positive. Using two‐dimensional images instead of one‐dimensional keograms or meridian scanning photometer recordings, we were able to distinguish these arcs from PMAFs. The misidentification of these arcs as PMAFs would cause inconsistencies with the theories of cusp aurora dynamics and magnetic reconnection, and some previous studies may have included such non‐PMAF events.

Solar Energetic Particle Transport Near a Heliospheric Current Sheet

Abstract Solar energetic particles (SEPs), a major component of space weather, propagate through the interplanetary medium strongly guided by the interplanetary magnetic field (IMF). In this work, we analyze the implications that a flat Heliospheric Current Sheet (HCS) has on proton propagation from SEP release sites to the Earth. We simulate proton propagation by integrating fully 3D trajectories near an analytically defined flat current sheet, collecting comprehensive statistics into histograms, fluence maps, and virtual observer time profiles within an energy range of 1–800 MeV. We show that protons experience significant current sheet drift to distant longitudes, causing time profiles to exhibit multiple components, which are a potential source of confusing interpretations of observations. We find that variation of the current sheet thickness within a realistic parameter range has little effect on particle propagation. We show that the IMF configuration strongly affects the deceleration of protons. We show that in our model, the presence of a flat equatorial HCS in the inner heliosphere limits the crossing of protons into the opposite hemisphere.

Solar energetic particle interactions with the Venusian atmosphere

Abstract. In the context of planetary space weather, we estimate the ion production rates in the Venusian atmosphere due to the interactions of solar energetic particles (SEPs) with gas. The assumed concept for our estimations is based on two cases of SEP events, previously observed in near-Earth space: the event in October 1989 and the event in May 2012. For both cases, we assume that the directional properties of the flux and the interplanetary magnetic field configuration would have allowed the SEPs' arrival at Venus and their penetration to the planet's atmosphere. For the event in May 2012, we consider the solar particle properties (integrated flux and rigidity spectrum) obtained by the Neutron Monitor Based Anisotropic GLE Pure Power Law (NMBANGLE PPOLA) model (Plainaki et al., 2010, 2014) applied previously for the Earth case and scaled to the distance of Venus from the Sun. For the simulation of the actual cascade in the Venusian atmosphere initiated by the incoming particle fluxes, we apply the DYASTIMA code, a Monte Carlo (MC) application based on the Geant4 software (Paschalis et al., 2014). Our predictions are afterwards compared to other estimations derived from previous studies and discussed. Finally, we discuss the differences between the nominal ionization profile due to galactic cosmic-ray–atmosphere interactions and the profile during periods of intense solar activity, and we show the importance of understanding space weather conditions on Venus in the context of future mission preparation and data interpretation.

The effect of a brief northward turning in IMF Bz on solar wind‐magnetosphere coupling in a global MHD simulation

AbstractIn this paper we examine the response of the magnetosphere‐ionopshere (M‐I) system to a transient northward excursion in the interplanetary magnetic field (IMF) using the Lyon‐Fedder‐Mobarry (LFM) global MHD simulation. The simulated IMF transitions hold from a steady southward IMF to a steady northward IMF before suddenly transitioning back to southward IMF after 20 min. Once the IMF returns southward, the M‐I system is in a state of reduced energy dissipation for approximately an hour as it reconfigures back into a standard southward IMF configuration. We find that the northward IMF excursion affects both the viscous and reconnection interactions with the solar wind. The flow of plasma in the magnetosphere is significantly disrupted by the reconnection cycle under northward IMF. This reduce the transfer of mechanical energy from the solar wind due to the viscous interaction, and the magnetosphere‐ionosphere system is in a mixed topological configuration containing elements produced by both of southward IMF reconnection and the Dungey cycle, as well as northward IMF reconnection and the presence of reverse cell convection at high latitudes. The effects of the transient northward IMF must be completely cleared out before the system can return to an optimal state of energy transfer characteristic of steady southward IMF. As a result, a simple 20 min excursion of northward IMF can put the magnetosphere‐ionosphere system into a reduced state of coupling to the solar wind for some time following the return to steady southward IMF; for LFM we saw a reduced state lasting an hour

Hemispheric asymmetries of the Venus plasma environment

We study the Venus‐solar wind interaction and the hemispheric asymmetries of the Venus plasma environment in the global HYB‐Venus hybrid simulation. We concentrate especially on the role of the flow‐aligned interplanetary magnetic field (IMF) component (i.e., the Parker spiral angle or the IMF cone angle) and analyze the dawn‐dusk and Esw asymmetries between four magnetic quadrants around Venus. Using the simulation model, we study two upstream condition cases in detail: the perpendicular IMF to the solar wind flow case and the nominal Parker spiral case (dominant flow‐aligned IMF component). Several differences and similarities were found in these two simulation runs. Common features of the Venus plasma environment between the two cases include asymmetric magnetic barrier and tail lobes and asymmetric planetary ion escape in the direction of the solar wind convection electric field. Further, protons of planetary origin and of solar wind origin were found to follow similar velocity patterns in the Venus plasma wake in both cases. The differences when the IMF flow‐aligned component is dominating compared to the perpendicular IMF case, the so‐called (magnetic) dawn‐dusk asymmetries, include the parallel bow shock and the foreshock region, the asymmetric magnetic barrier, the asymmetric tail current system, and the asymmetric central tail current sheet. Further, the escaping planetary H+ and O+ ion fluxes are concentrated more on the hemisphere of the parallel bow shock. When interpreting in situ plasma and magnetic observations from Venus, the features of at least these two basic IMF configurations should be considered.

Interstellar He+ ring‐beam distributions: Observations and implications

We report systematic measurements of the distribution of the incident angle of interstellar pickup He+ ions as observed by STEREO/Plasma and Supra‐Thermal Ion Composition (PLASTIC). We have organized these observations according to the angle spanned by the Interplanetary Magnetic Field (IMF), , and the bulk solar wind velocity, . Our measurements show clear evidence of a relatively local injection of He+ pickup ions into the solar wind, which are then seen as a ring distribution perpendicular to . Changes of the spectral shape and a reduced flux of interstellar He+ during radial IMF configuration, as observed by, e.g., Ulysses/Solar Wind Ion Composition Spectrometer (SWICS), SOHO/Charge Time‐Of‐Flight (CTOF), Active Magnetospheric Particle Tracer Explorers/SUprathermaL Energy Ionic Charge Analyzer (SULEICA), have generally been attributed to inefficient scattering across 90° pitch‐angle. Our observations of the pitch‐angle distribution of interstellar He+ suggest that these changes are instead a result of locally injected pickup ions that escape detection for IMF configuration in which the Solar Wind Sector of PLASTIC, as well as SWICS, CTOF, and SULEICA, are not sensitive to the measurement of the locally injected pickup ion ring.

Solar Energetic Particle Events in the 23rd Solar Cycle: Interplanetary Magnetic Field Configuration and Statistical Relationship with Flares and CMEs

We study the influence of the large-scale interplanetary magnetic field configuration on the solar energetic particles (SEPs) as detected at different satellites near Earth and on the correlation of their peak intensities with the parent solar activity. We selected SEP events associated with X and M-class flares at western longitudes, in order to ensure good magnetic connection to Earth. These events were classified into two categories according to the global interplanetary magnetic field (IMF) configuration present during the SEP propagation to 1AU: standard solar wind or interplanetary coronal mass ejections (ICMEs). Our analysis shows that around 20% of all particle events are detected when the spacecraft is immersed in an ICME. The correlation of the peak particle intensity with the projected speed of the SEP-associated coronal mass ejection is similar in the two IMF categories of proton and electron events, $\approx 0.6$. The SEP events within ICMEs show stronger correlation between the peak proton intensity and the soft X-ray flux of the associated solar flare, with correlation coefficient $r=\,$0.67$\pm$0.13, compared to the SEP events propagating in the standard solar wind, $r=\,$0.36$\pm$0.13. The difference is more pronounced for near-relativistic electrons. The main reason for the different correlation behavior seems to be the larger spread of the flare longitude in the SEP sample detected in the solar wind as compared to SEP events within ICMEs. We discuss to which extent observational bias, different physical processes (particle injection, transport, etc.), and the IMF configuration can influence the relationship between SEPs and coronal activity.

The interplanetary magnetic structure that guides solar relativistic particles

Relating in situ measurements of relativistic solar particles to their parent activity in the corona requires understanding the magnetic structures that guide them from their acceleration site to the Earth. Relativistic particle events are observed at times of high solar activity, when transient magnetic structures such as Interplanetary Coronal Mass Ejections (ICMEs) often shape the interplanetary magnetic field (IMF). They may introduce interplanetary paths that are longer than nominal, and magnetic connections rooted far from the nominal Parker spiral. We present a detailed study of the IMF configurations during ten relativistic solar particle events of the 23rd activity cycle to elucidate the actual IMF configuration guiding the particles to Earth, where they are measured by neutron monitors. We use magnetic field (MAG) and plasma parameter measurements (SWEPAM) from ACE, and determine interplanetary path lengths of energetic particles through a modified version of the velocity dispersion analysis based on energetic particle measurements with SoHO/ERNE. We find that the majority (7/10) of the events is detected in the vicinity of an ICME. Their interplanetary path lengths are found to be longer (1.5-2.6 AU) than those of the two events propagating in the slow solar wind (1.3 AU). The largest apparent path length is found in an event within the fast solar wind, probably due to enhanced pitch angle scattering. The derived path lengths imply that the first energetic and relativistic protons are released at the Sun at the same time as electron beams emitting type III radio bursts. The timing of the first high-energy particle arrival at Earth is dominantly determined by the type of IMF in which the particles propagate. Initial arrival times are as expected from Parker's model in the slow solar wind, and significantly larger in or near transient structures such as ICMEs.

The Sun-Earth Connection of Energetic Particles

The origin of solar energetic particle (SEP) events is an issue for the understanding of particle acceleration and transport in astrophysics, as well as for space weather. We outline some recent studies addressing from the observational viewpoint the relationship between interacting and escaping particles, conditions for particle escape from the Sun, and the interplanetary magnetic field configurations where the particles propagate.