Steady Solar Wind Conditions Research Articles

Model‐Based Assessments of Magnetic Reconnection and Kelvin‐Helmholtz Instability at Jupiter's Magnetopause

AbstractThe interaction between the solar wind and Jupiter's magnetic field confines the planetary field to the largest magnetosphere in the Solar System. However, the full picture of when and where key processes operate at the magnetopause boundary of the system remains unclear. This is essential for testing understanding with observations and for determining the relative importance of different drivers of Jovian magnetospheric dynamics. Here we present a global analytical model of Jovian magnetopause conditions under steady state, which forms the basis of boundary process assessments. Sites of magnetic reconnection at Jupiter's magnetopause are expected to be in regions of sufficiently high magnetic shear across the boundary, controlled by the orientation of the interplanetary magnetic field. Reconnection rates are also most sensitive to changes in the highly variable IMF, followed by changes in the solar wind plasma mass density. The largest plasma flow shear across the boundary is in the equatorial dawn region, producing a region that is typically unstable to growth of the Kelvin‐Helmholtz (K‐H) instability. Compared to magnetopause reconnection site locations, this K‐H‐unstable region at dawn is less sensitive to changing conditions. Motion of K‐H boundary perturbations typically includes dawn‐to‐dusk motion across the subsolar region. Model‐predicted reconnection voltages are typically hundreds of kV but rely on steady solar wind conditions on a time scale that is longer than typical at Jupiter's orbit. How the reconnection voltage compares to the voltage applied due to the “viscous‐like” interaction involving K‐H instability remains unclear.

Influence of ion outflow in coupled geospace simulations: 2. Sawtooth oscillations driven by physics‐based ion outflow

AbstractWe present the first simulations of magnetospheric sawtooth oscillations under steady solar wind conditions that are driven internally by heavy ion outflow from a physics‐based model. The simulations presented use the multifluid Lyon‐Fedder‐Mobarry magnetohydrodynamics model two‐way coupled to the ionosphere/polar wind model (IPWM). Depending on the type of wave‐particle interactions utilized within IPWM, the coupled simulations exhibit either sawtooth oscillations or steady magnetospheric convection. Contrasting the simulations that do and do not develop sawtooth oscillations yields insights into the relationship between outflow and sawtooth oscillations. The total outflow rate is not an adequate predictor of the convection mode that will emerge. The simulations that develop sawtooth oscillations are characterized by intense outflow concentrated in the midnight auroral region. This outflow distribution mass loads the tail reconnection region without excessively mass loading the dayside reconnection region and leads to an imbalance between the dayside and nightside reconnection rates.

Martian ionospheric responses to dynamic pressure enhancements in the solar wind

AbstractAs a weakly magnetized planet, Mars ionosphere/atmosphere interacts directly with the shocked solar wind plasma flow. Even though many numerical studies have been successful in reproducing numerous features of the interaction process, these earlier studies focused mainly on interaction under steady solar wind conditions. Recent observations suggest that plasma escape fluxes are significantly enhanced in response to solar wind dynamic pressure pulses. In this study, we focus on the response of the ionosphere to pressure enhancements in the solar wind. Through modeling of two idealized events using a magnetohydrodynamics model, we find that the upper ionosphere of Mars responds almost instantaneously to solar wind pressure enhancements, while the collision dominated lower ionosphere (below ~150 km) does not have noticeable changes in density. We also find that ionospheric perturbations in density, magnetic field, and velocity can last more than an hour after the solar wind returns to the quiet conditions. The topside ionosphere forms complicated transient shapes in response, which may explain unexpected ionospheric behaviors in recent observations. We also find that ionospheric escape fluxes do not correlate directly with simultaneous solar wind dynamic pressure. Rather, their intensities also depend on the earlier solar wind conditions. It takes a few hours for the ionospheric/atmospheric system to reach a new quasi‐equilibrium state.

Cyclic reformation of a quasi‐parallel bow shock at Mercury: MESSENGER observations

AbstractWe here document with magnetic field observations a passage of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft through Mercury's magnetosphere under conditions of a quasi‐parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25‐min‐long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasiperiodicity allows us to study all aspects of the shock reconfiguration, including ultra‐low‐frequency waves in the upstream region and large‐amplitude magnetic structures observed in the vicinity of the magnetosheath‐solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak‐to‐peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large‐amplitude magnetic structures, as is generally the case at the terrestrial bow shock under quasi‐parallel conditions.

Spontaneous hot flow anomalies at quasi‐parallel shocks: 2. Hybrid simulations

AbstractMotivated by recent THEMIS observations, this paper uses 2.5‐D electromagnetic hybrid simulations to investigate the formation of Spontaneous Hot Flow Anomalies (SHFAs) upstream of quasi‐parallel bow shocks during steady solar wind conditions and in the absence of discontinuities. The results show the formation of a large number of structures along and upstream of the quasi‐parallel bow shock. Their outer edges exhibit density and magnetic field enhancements, while their cores exhibit drops in density, magnetic field, solar wind velocity, and enhancements in ion temperature. Using virtual spacecraft in the simulation, we show that the signatures of these structures in the time series data are very similar to those of SHFAs seen in THEMIS data and conclude that they correspond to SHFAs. Examination of the simulation data shows that SHFAs form as the result of foreshock cavitons interacting with the bow shock. Foreshock cavitons in turn form due to the nonlinear evolution of ULF waves generated by the interaction of the solar wind with the backstreaming ions. Because foreshock cavitons are an inherent part of the shock dissipation process, the formation of SHFAs is also an inherent part of the dissipation process leading to a highly nonuniform plasma in the quasi‐parallel magnetosheath including large‐scale density and magnetic field cavities.

PENGUIn/AGO and THEMIS conjugate observations of whistler mode chorus waves in the dayside uniform zone under steady solar wind and quiet geomagnetic conditions

We perform a case study of conjugate observations of whistler mode chorus waves on the dayside made on 26 July 2008 by three THEMIS spacecraft and ground‐based ELF/VLF receivers at the Automatic Geophysical Observatories (AGO) in Antarctica supported by the U.S. Polar Experiment Network for Geospace Upper‐atmosphere Investigations (PENGUIn) project. The dayside chorus waves were excited during a period of no substorm activity with geomagnetic indices indicating quiet conditions (Dst ∼ −10 nT; AE < 200 nT). The solar wind dynamic pressure was almost constant during the chorus wave intensification. Conjugate observations in the outer magnetosphere confirm that the chorus intensification was localized within the radial distance R = 7–10 RE near noon (12.5 < MLT < 13.5 h). The waves persisted for at least 1.5 h in the same location, where field lines are not accompanied by off‐equatorial minimum‐B pockets but rather exhibit nearly zero dB/ds, the field‐aligned gradient in B‐magnitude, over a wide range of magnetic latitudes (∼±20°). The location did not seem to corotate with the Earth or drift with the energetic electrons. The chorus waves consisted of discrete, rising tone elements, propagating away from the magnetic equator, quasi‐parallel to the ambient magnetic field (wave‐normal angles < 20°). We conclude that the long‐lasting, localized, quiet time dayside chorus amplification was due to the nearly zero dB/ds conditions that occur naturally in the dayside uniform zone (DUZ), the transition region between the near‐Earth dipole and the compressed, off‐equatorial double‐minimum field configuration found closer to the magnetopause. We thus suggest that the magnetic field configuration in the dayside outer magnetosphere plays a key role in the generation of dayside chorus waves under quiet geomagnetic conditions.

On the generation and topology of flux transfer events

We use resistive magnetohydrodynamics (MHD) simulations to investigate the generation and topology of flux transfer events (FTEs) at Earth's dayside magnetopause under generic southward interplanetary magnetic field (IMF) conditions. We focus on three questions: (1) Can FTEs form spontaneously under steady solar wind conditions? (2) What is the role of magnetic reconnection in the generation of FTEs? (3) What effect do FTEs have on the magnetopause magnetic field topology? We find that in the simulations, FTEs form spontaneously under steady solar wind conditions. In contrast to the multiple X line model of Raeder (2006), dipole tilt is not required to produce FTEs in resistive MHD simulations. Further, FTEs are generated by flow vortices which begin to grow well before magnetic reconnection changes the magnetopause magnetic field topology. FTEs form under steady southward IMF conditions but not under steady northward IMF conditions, suggesting that the magnetopause becomes unstable when the IMF turns southward. In our simulations, this instability appears to be triggered by a movement of the flow stagnation point away from the magnetic separator, which modifies the subsolar stagnation point flow, thereby rendering it unstable. Finally, examination of the internal magnetic structure of the simulated FTEs reveals a complex topology resulting from multiple separator bifurcations driven by plasma flows within the FTEs.

Integration of the radiation belt environment model into the space weather modeling framework

We have integrated the Fok radiation belt environment (RBE) model into the space weather modeling framework (SWMF). RBE is coupled to the global magnetohydrodynamics component (represented by the Block-Adaptive-Tree Solar-wind Roe-type Upwind Scheme, BATS-R-US, code) and the Ionosphere Electrodynamics component of the SWMF, following initial results using the Weimer empirical model for the ionospheric potential. The radiation belt (RB) model solves the convection–diffusion equation of the plasma in the energy range of 10 keV to a few MeV. In stand-alone mode RBE uses Tsyganenko's empirical models for the magnetic field, and Weimer's empirical model for the ionospheric potential. In the SWMF the BATS-R-US model provides the time dependent magnetic field by efficiently tracing the closed magnetic field-lines and passing the geometrical and field strength information to RBE at a regular cadence. The ionosphere electrodynamics component uses a two-dimensional vertical potential solver to provide new potential maps to the RBE model at regular intervals. We discuss the coupling algorithm and show some preliminary results with the coupled code. We run our newly coupled model for periods of steady solar wind conditions and compare our results to the RB model using an empirical magnetic field and potential model. We also simulate the RB for an active time period and find that there are substantial differences in the RB model results when changing either the magnetic field or the electric field, including the creation of an outer belt enhancement via rapid inward transport on the time scale of tens of minutes.

Plasma flow across the cusp-magnetosheath boundary under northward IMF

In this paper, we have used two closely spaced (INTERBALL-1 and MAGION-4) satellites orbiting in the region adjacent to the magnetopause with motivation to determine the structure of the cusp-magnetosheath boundary under steady solar wind conditions and under the northward interplanetary magnetic field (IMF). Two-point observations reveal that a steady reconnection takes place tailward of the cusp and the position of the reconnection site moves in accordance with the actual IMF direction. Reconnection produces a latitudinally dispersed cusp precipitation with highest energies in the cusp poleward edge. The reconnection site is widely spread in local time and thus changes of the IMF By component only slightly modulate observed patterns. The change of the plasma flow direction suggests the presence of the vortex-like structure in the exterior cusp.

Spatial features observed in the cusp under steady solar wind conditions

Multispacecraft studies using Polar, Fast Auroral Snapshot (FAST), and Interball observations revealed that many cusp structures observed during stable solar wind conditions are spatial features. While individual cusp events appear to be very different from each other, multiple observations of the same cusp event during periods of stable solar wind conditions are remarkably similar. These spatial cusp structures have been interpreted as a result of the independent evolution of ion distributions from multiple reconnection sites (X‐lines) in both hemispheres. The location of reconnection sites depends critically on the direction of the interplanetary magnetic field. Thus individual cusp events should have similar cusp structures if they occur in the same magnetic local time sector during comparable solar wind conditions, especially during similar interplanetary magnetic field orientations. Data presented in this paper show just such an effect. For three individual cusp events observed by Polar and FAST, similar stepped ion structures are seen by both satellites passing though these large‐scale spatial structures.

Location of high‐altitude cusp during steady solar wind conditions

We have investigated the high‐altitude cusp location and its dependence on the interplanetary magnetic field (IMF) Bz, solar wind Ey, and solar wind dynamic pressure during steady solar wind conditions. We have adopted strict event selection criteria, where we have studied only events during stationary IMF Bz, when the cusp latitude is supposed to be stationary. Prior to this study, the statistical studies have shown a great deal of scatter between the cusp latitude and the IMF Bz; however, with the strict event selection criteria, we have been able to reduce the scatter. The strongest source of scatter comes from large and largely fluctuating IMF By. When the events are selected during small IMF By, the scattering between the cusp latitude and the IMF Bz is reduced. When IMF is southward, also the fluctuation of IMF By has strong influence on the cusp latitude, whereas for northward IMF, the IMF By fluctuation has less influence on the cusp latitude. For larger southward IMF the cusp appears further equatorward than with smaller southward IMF. When the scatter is reduced by introducing the IMF By control, the larger northward IMF brings the cusp to lower latitudes than the smaller northward IMF, i.e., the maximum cusp latitude is obtained for small positive IMF Bz. For increasing solar wind Ez the cusp moves equatorward. Furthermore, with increasing solar wind dynamic pressure, the cusp appears at lower latitudes. This behavior depends on the orientation of the IMF Bz, with southward IMF the cusp moves faster equatorward than with northward IMF.

Response of ionospheric convection to changes in the interplanetary magnetic field: Lessons from a MHD simulation

Characteristics of magnetospheric and high‐latitude ionospheric convection pattern responses to abrupt changes in the interplanetary magnetic field (IMF) orientation have been investigated using an MHD model with a step function reversal of IMF polarity (positive to negative BY) in otherwise steady solar wind conditions. By examining model outputs at 1 min intervals, we have tracked the evolution of the IMF polarity reversal through the magnetosphere, with particular attention to changes in the ionosphere and at the magnetopause. For discussion, times are referenced relative to the time of first contact (t = 0) of the IMF reversal with the subsolar nose of the magnetopause at ∼ 10.5 RE. The linear change in large‐scale ionospheric convection pattern begins at t = 8 min, reproducing the difference pattern results of Ridley et al. [1997, 1998]. Field‐aligned current difference patterns, similarly derived, show an initial two‐cell pattern earlier, at t = 4 min. The current difference two‐cell pattern grows slowly at first, then faster as the potential pattern begins to change. The first magnetic response to the impact of the abrupt IMF transition at the magnetopause nose is to reverse the tilt of the last‐closed field lines and of the “first”‐open field lines. This change in tilt occurs within the boundary layer before merging of IMF with closed magnetospheric field lines starts. In the case of steady state IMF BY, IMF field lines undergo merging or “changing partners” with other IMF field lines, as they approach the nose and tilt in response to currents. When the BY reversal approaches the magnetopause nose, IMF field lines from behind the reversal overtake and merge with those in front of the reversal, thus puncturing the reversal front and uncoupling the layer of solar wind plasma in the reversal zone from the magnetosphere. The uncoupled layer propagates tail ward entirely within the magnetosheath. Merging of closed magnetospheric field lines with the new polarity IMF begins at t = 3 min and starts to affect local currents near the cusp 1 min later. While merging starts early and controls the addition of open flux to the polar cap, large‐scale convection pattern changes are tied to the currents, which are controlled in the boundary layers. The resulting convection pattern is an amalgamation of these diverse responses. These results support the conclusion of Maynard et al. [2001b], that the small convection cell is driven from the opposite hemisphere in BY‐dominated situations.

The 11.08.1999 solar eclipse and the ionosphere: a search for the distant bow-wave

The advantage of studying eclipse disturbances is the perfect predictability of their 4D source geometry, which allows for preparation of adapted systems and schedules. The total solar eclipse period of August 11, 1999 across Europe was notable for exceptionally uniform solar disk, steady solar wind and quiet magnetospheric conditions. Large-scale gravity wave activity prior to the eclipse however disturbed the initial 0900 LT thermosphere weather. This rapid letter is an advance summary about one particular aspect of the West European ionosonde and radar results of the eclipse experiment. It focusses on the possible emergence of a distant eclipse frontal bow-wave. This was expected as a consequence of the supersonic shock of stratospheric Ozone cooling. First-look data of Vertical Incidence Digisonde records are greatly improved by their Real-Time acquisition of inverted true-height profiles. The EBRE (Tortosa, Spain) foF1 and foF2 simultaneous oscillations observed from the second to the fourth hour following maximum solar occultation appear as convincing indicators of the bow-wave signature. Large fluctuations in foF1 and foF2 during some of our control days, of usual gravity wave character, emphasize the importance of meteorologic disturbances on mid-latitude ionosphere variability.

Disturbances of the western European ionosphere during the total solar eclipse of 11 August 1999 measured by a wide ionosonde and radar network

The 11 August 1999 Solar eclipse totality path ran across western Europe at near-constant latitudes of about 49°N. It occurred at mid-time of a sequence of three days with steady solar wind and quiet magnetospheric conditions. Its response was observed by a score of ionospheric facilities, which will provide high-resolution probing of the various disturbances. First results allow us to compare the time fluctuations at various distances from totality on the eclipse and adjacent days, inside a 5° West to 5° East longitude area. In this preliminary work the foF1 and foF2 time changes are presented in contour maps on a 50 km size grid. They show the expected longitude transit of eclipse perturbation. We venture brief comments on the eclipse-own signatures as separate from the various wave oscillations detected prior to eclipse time by 12.4 MHz panoramic azimuth scans of the Losquet radar near Lannion (Brittanny).

Dynamical properties of self‐consistent magnetotail configurations

This study of the magnetotail employs a large‐scale kinetic ion model specifically developed to consider the region's approach to equilibrium and its dynamics under the influence of an imposed electric field. Results from the self‐consistent, two‐dimensional model indicate that the magnetotail achieves equilibrium by maintaining a delicate balance between the influx of plasma from a mantle source and the accelerated loss of these particles resulting from nonadiabatic particle dynamics. The quasi‐steady oscillations that result from instantaneous imbalances between sources and losses occur independent of solar wind variations. This variability occurs even for steady solar wind conditions and is caused by the rapid nonadiabatic energization of ions incident on the current sheet and their loss through the flanks of the magnetotail faster than they can be replenished. To determine the influence of external parameters on the dynamical state of the magnetotail, we performed a parameter search in which we varied the magnitude of the convection electric field and the influx of plasma mantle ions. This parameter search showed that the magnetotail configuration can adjust itself to compensate for the various combinations of solar wind parameters. As a rule, this self‐adjustment occurs in the form of quasi‐steady magnetotail states, where only the average characteristics of the configuration are steady but pronounced cyclic variations occur in the magnetotail topology. For cases in which the magnetotail is underpopulated, the current sheet is depleted via the flanks. In overpopulation cases a flaring of magnetotail field lines caused by the additional current in the neural sheet limits access of additional mantle particles to the region. The magnetotail system is flexible enough to self‐consistently regulate its own population. However, the resulting configuration becomes dynamic and only a quasi‐steady state is achieved.

A global view of the role of acceleration processes in solar-terrestrial coupling as provided by the ISTP theory and ground-based experiments

Present global simulations of the Earth's upper atmosphere and magnetosphere, which have been developed under the auspices of the International Solar Terrestrial Physics (ISTP) Program, can provide an effective, near-quantitative, global view of the role of acceleration processes in the transport of energy, momentum, and mass from the solar wind, through the magnetosphere, and to their final deposition in the upper atmosphere. In addition, ground-based experiments can provide not only more realistic inner boundary-conditions for the global MHD magnetosphere simulations, but give improved estimates of the roles of magnetospheric acceleration processes in the upper atmosphere. An overview of a detailed case study of a series of events which occurred during 19–20 May 1996 will be used to illustrate the power of theory and ground-based observations to complement spacecraft observations in providing a global view of acceleration processes.The period from 0100 UT 19 May to 0300 UT 20 May 1996 was chosen as an illustrative case since it contains times of steady solar wind conditions, and periods of slow variations in the IMF followed by sudden changes in the IMF These conditions are near ideal for global simulation-observation comparisons. It also has two distinct substorms at 2020 UT 19 May and 0035 UT 20 May. The slow changes in the IMF followed by the rapid changes provided excellent conditions for comparing theoretical predictions of the global effects of acceleration processes and observations from ground-based and spacecraft experiments.

Simultaneous observations of solar wind plasma entry from FAST and POLAR

On November 15, 1996, NASA FAST and POLAR satellites obtained data in the cusp/cleft region for extended intervals. POLAR sampled a narrow range of local times. FAST observed cusp/cleft plasma for more than 5 hours in local time. At 07:51 UT the satellites were in near magnetic conjunction, and the interplanetary magnetic field was steady for an extended interval before this time. We confirm prior observations that show solar wind plasma often enters the magnetosphere over extended regions of local time. For the one hour interval of steady solar wind conditions when POLAR encountered cusp/cleft plasma, irregularly spaced ion injections interspersed with intervals of nearly constant cusp/cleft ion fluxes were observed. The data are consistent with temporal variations in the reconnection rate. At the time of the near conjugate observations the data suggest that solar wind plasma entered the magnetosphere over an extended region of the magnetopause, i.e. not in small discontinuous patches.

A large southward magnetic field of −23.5 nT in the January 10, 1995, plasmoid

This paper reports a large southward magnetic field of −23.5 nT in the Earth's magnetotail at a radial distance of 46 RE, observed with Geotail on January 10, 1995. The large southward field was part of a bipolar Bz signature of a plasmoid in association with a well‐isolated substorm under steady solar wind conditions. Magnetic field and plasma observations are analyzed and compared with results from a hybrid simulation for magnetic reconnection. The large southward field is produced by the piling up of field lines due to colliding fast tailward flowing plasmas. This study implies that the piling‐up mechanism plays an important role in the evolution of plasmoids traveling in the magnetotail.

Plasma sheet pressure changes during the substorm growth phase

This paper reports clear evidence for increases in plasma pressure in the plasma sheet at 20–50 Re during the growth phase of substorms. The spacecraft WIND has provided the IMF Bz and other solar wind conditions, while the spacecraft GEOTAIL has observed plasma and magnetic field in the plasma sheet. We have examined 19 isolated substorms after clear southward turnings of the IMF Bz in steady solar wind conditions. Pressure increases start immediately after the southward turnings of the IMF Bz. Increases in plasma pressure are caused mainly by increases in plasma number density. Temperature does not show any significant changes. These observations suggest that the near‐Earth plasma sheet is compressed by enhanced magnetic field intensity in the tail lobes during the substorm growth phase.

Galileo flybys of earth: The nature of the distant shock

We report on observations of the Earth's bow shock at unprecedentedly large downtail distances, some as remote as 360 R(sub E). Suprisingly, we find that even at these large distances, the bow shock signature remains clear. The cases we report are among the weakest shocks ever clearly identified. These shocks reveal patterns of field changes remarkably similar to those observed for stronger shocks. Indeed, several of the shocks could serve as textbook examples because they occur in unusually quiet and steady solar wind conditions. The quasi- perpendicular shocks are, in some cases preceded by whistler wave trains. Several of the shocks which have normal vectors in the transitional region between quasiperpendicular and quasiparallel are associated with large amplitude wave disturbances in the downstream plasma. Although low Mach number, these shocks are the source of large amplitude turbulence. We show a range of fits to the locations of the shock crossings, and argue that, for steady solar wind conditions, there is no sign of tail flapping. The displacement of the shock from the tail axis appears to be governed by the interplanetary magnetic field orientation as previously reported for Venus.