Solar Wind Dynamic Pressure Variations Research Articles

Quasi-elastodynamic Processes Involved in the Interaction between Solar Wind and Magnetosphere

The interaction between the solar wind and the magnetosphere is one of the most important research subjects in the fields of astrophysics and space physics. For more than half a century, based on the pressure balance assumption between the solar wind and the magnetosphere and considering other important factors, such as the interplanetary magnetic field and magnetic reconnection process, the dynamic processes at the magnetopause have been extensively analyzed. However, the responses of magnetopause to the solar wind dynamic pressure variations are still complicated to understand. Here, we show that the interaction between the solar wind and the magnetosphere can be regarded as a quasi-elastodynamic process. The driving frequency of the solar wind is determined as a crucial reason for the phase difference between solar wind dynamic pressure variations and magnetopause standoff distance. The low-pass filter effect and oscillation properties of the magnetopause can also be well explained by the forced damped vibrations. Moreover, the quasi-elastodynamic processes predict deformations at the magnetopause, which resemble the magnetopause surface wave. Finally, a three-dimensional time-dependent magnetopause model is constructed and verified by observation. Based on 12,242 magnetopause crossing events, it is found that the new model reveals ∼9.7% better prediction accuracy than the widely used time-independent model. These results can also shed light on our understanding of the solar-wind–magnetopause interaction for other planets.

On Differentiating Multiple Types of ULF Magnetospheric Waves in Response to Solar Wind Periodic Density Structures

Identifying the nature and source of ultra‐low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in‐ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang‐Sheeley‐Arge model, we mapped the source of this solar wind stream to an active region and a mid‐latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz.

Jovian UV Aurora's Response to the Solar Wind: Hisaki EXCEED and Juno Observations

AbstractWe summarize Jupiter's ultraviolet (UV) auroral response to solar wind dynamic pressure variations during Juno's approach to Jupiter in 2016. The response time of Jupiter's aurora to external drivers has thus far been unknown owing to a sparsity of upstream in situ solar wind measurements. Combining the Juno solar wind observations with continuous UV aurora data obtained by Hisaki EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) and Juno UV spectrograph, the UV aurora brightenings in response to three major shock arrivals showed time lags of 10–15 hr. These time lags are longer than the time required for ballistic propagation of the shocks by the solar wind. In addition to that puzzle, while an enhancement in the UV auroral power was observed with an increase in dynamic pressure to ~0.03 nPa, no associated brightening was observed with a dynamic pressure elevation of >0.1 nPa. These imply that internal magnetospheric aspects need to be taken into consideration to fully resolve the issue.

Small‐Scale Aurora Associated With Magnetospheric Flow Vortices After a Solar Wind Dynamic Pressure Decrease

AbstractSudden changes of the solar wind dynamic pressure have significant impacts on the dynamics of the magnetosphere‐ionosphere system. On 18 February 2011, a sudden decrease in solar wind dynamic pressure was observed by the Wind satellite, which drove the entire magnetosphere‐ionosphere system as recorded in many ground‐based and space‐based measurements. In the magnetosphere, Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observed a counterclockwise plasma flow vortex propagating tailward. Near the magnetic footprints of the THEMIS spacecraft, the ground magnetometers observed magnetic field variations that corresponded to a counterclockwise vortex in the equivalent ionospheric currents, which in turn indicated the presence of upward field‐aligned currents. The all‐sky imager at RANK station near the THEMIS footprints also observed a simultaneous enhancement of the auroral brightness. Therefore, this comprehensive case study demonstrates a causal chain that links the solar wind dynamic pressure variations to magnetospheric, ionospheric, and auroral activities.

Prompt Disappearance and Emergence of Radiation Belt Magnetosonic Waves Induced by Solar Wind Dynamic Pressure Variations

AbstractMagnetosonic waves are highly oblique whistler mode emissions transferring energy from the ring current protons to the radiation belt electrons in the inner magnetosphere. Here we present the first report of prompt disappearance and emergence of magnetosonic waves induced by the solar wind dynamic pressure variations. The solar wind dynamic pressure reduction caused the magnetosphere expansion, adiabatically decelerated the ring current protons for the Bernstein mode instability, and produced the prompt disappearance of magnetosonic waves. On the contrary, because of the adiabatic acceleration of the ring current protons by the solar wind dynamic pressure enhancement, magnetosonic waves emerged suddenly. In the absence of impulsive injections of hot protons, magnetosonic waves were observable even only during the time period with the enhanced solar wind dynamic pressure. Our results demonstrate that the solar wind dynamic pressure is an essential parameter for modeling of magnetosonic waves and their effect on the radiation belt electrons.

EMIC waves observed at geosynchronous orbit under quiet geomagnetic conditions (Kp ≤ 1)

AbstractWe statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500–1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100–1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM‐H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM‐H for quiet time EMIC wave occurrence.

Fast damping of ultralow frequency waves excited by interplanetary shocks in the magnetosphere

AbstractAnalysis of Cluster spacecraft data shows that intense ultralow frequency (ULF) waves in the inner magnetosphere can be excited by the impact of interplanetary shocks and solar wind dynamic pressure variations. The observations reveal that such waves can be damped away rapidly in a few tens of minutes. Here we examine mechanisms of ULF wave damping for two interplanetary shocks observed by Cluster on 7 November 2004 and 30 August 2001. The mechanisms considered are ionospheric joule heating, Landau damping, and waveguide energy propagation. It is shown that Landau damping provides the dominant ULF wave damping for the shock events of interest. It is further demonstrated that damping is caused by drift‐bounce resonance with ions in the energy range of a few keV. Landau damping is shown to be more effective in the plasmasphere boundary layer due to the higher proportion of Landau resonant ions that exist in that region.

Mars ionospheric response to solar wind variability

AbstractAt planets with induced magnetospheres, the coupling between the ionosphere, the weak draped magnetosphere, and the solar wind is very direct in comparison to Earth. The weak induced magnetosphere itself is created by the prevailing Solar wind conditions and therefore in its shape and strength dynamically depending on it. In early 2010, Mars was located behind Earth in the Solar wind; thus, we can use coordinated data from multiple near‐Earth spacecraft (Stereo, Wind) to evaluate what kind of Solar wind disturbances have passed by Earth and might consecutively hit Mars, and when. We employ plasma data from the ESA Mars‐Express mission, the ASPERA‐3 particle instrument, and the MARSIS Active Ionospheric Sounder (AIS) to investigate, for a number of isolated events in March and April 2010, how the ionosphere and the induced magnetosphere at Mars develop and decay in response to Solar wind variability in the magnetic field, density, and velocity. In a dedicated campaign mode, we use frequent long‐duration MARSIS AIS operations for several consecutive orbits, to monitor for the first time the long‐term development of the Martian plasma environment during solar wind disturbances. We find that the magnetosphere and ionosphere of Mars can become considerably compressed by solar wind dynamic pressure variations, which usually are also associated with changes in the magnetic draping of the interplanetary magnetic field around the planet. These are typically associated with corotating interaction regions and coronal mass ejections, and can last for several days. During such episodes of compression, we see signatures of increased plasma transport over the terminator and enhanced ion outflow from the upper atmosphere.

Responses of Earth's radiation belts to solar wind dynamic pressure variations in 2002 analyzed using multisatellite data and Kalman filtering

It has been difficult to establish a definitive causal link between radiation belt electron dropouts and solar wind dynamic pressure, due to adiabatic effects and observations limited to a single location in space at one time. Here we reconstruct the radial profiles of radiation belt electron phase space density (PSD) for the entire year of 2002 (solar maximum), based on a combination of the VERB radial diffusion model and data assimilation of the sparse observations from six satellites including GEO1989, GEO1990, LANL‐97A, LANL‐01A, GPSns33, and POLAR, to perform a statistical analysis of the potential relationship between electron PSD dropout and solar wind dynamic pressure variation. We capture 59 electron PSD dropout events, 81% associated with solar wind dynamic pressure sudden jumps (i.e., pulses) or modest increase, consistent with the results of Shprits et al. (2012) for the period of 1990–1991. With the continuous availability of high quality solar wind data in 2002, we further identify 41 pressure pulses and determine that 68% of them are associated with electron PSD dropouts. We also identify 49 pressure enhancements, 41% of which are associated with electron PSD dropouts. Our results support that gradual or sharp enhancements in solar wind dynamic pressure can play an important role in producing electron PSD dropouts, owing to inward intrusion of the magnetopause that enhances the loss to the magnetopause and outward radial diffusion. But we conclusively demonstrate that solar wind dynamic pressure pulses and enhancements are neither a necessary nor a sufficient condition for the formation of electron PSD dropouts, which suggests that some other mechanism(s), which remains mysterious, is required to explain electron PSD dropout occurrences without pressure pulse or modest pressure increase. For the first time, we also perform a quantitative comparison of conjunctions between electron PSD reanalysis results and satellite PSD data, which indicates a tolerable and reasonable error in assimilated PSD within a factor of 5. Differences in assimilated PSD and satellite PSD have a potential for use to evaluate the ignored processes in the physical model and to estimate the errors associated with satellite measurements.

A new semiempirical model of Saturn's bow shock based on propagated solar wind parameters

[1] A new semiempirical model of Saturn’s dayside bow shock is presented. The model uses observations made during the Pioneer 11, Voyager 1, and Voyager 2 flybys as well as data from the first 6 years of the Cassini mission (2004–2010) to derive the average shape of the shock surface and the variation of shock subsolar distance with solar wind dynamic pressure. The 574 bow shock crossings used to construct the model provide good local time coverage of the dayside shock surface up to latitudes of roughly 45°, allowing the three‐dimensional shape of the shock surface to be investigated for the first time. Narrowband Langmuir waves observed by the Radio and Plasma Wave Science instrument are combined with propagated solar wind velocities in order to estimate the solar wind dynamic pressure associated with each of the Cassini crossings. An axisymmetric second‐order surface is then fit to the resulting crossing distribution, self‐consistently accounting for solar wind dynamic pressure variations. The new semiempirical model is compared with existing models of Saturn’s bow shock and magnetopause, and the physical implications of the model are discussed. On the basis of these comparisons, it is proposed that the new semiempirical model is the most accurate representation of Saturn’s bow shock surface to date.

Magnetic storms on Mars

Based on data from the Mars Global Surveyor magnetometer we examine periods of significantly enhanced magnetic disturbances in the martian space environment. Using almost seven years of observations during the maximum and early declining phase of the previous solar cycle the occurrence pattern and typical time profile of such periods is investigated and compared to solar wind measurements at Earth. Typical durations of the events are 20–40 h, and there is a tendency for large events to last longer, but a large spread in duration and intensity are found. The large and medium intensity events at Mars are found to occur predominantly in association with interplanetary sector boundaries, with solar wind dynamic pressure enhancements being the most likely interplanetary driver. In addition it is found that, on time scales of months to several years, the dominant cause of global variability of the magnetic field disturbance at Mars is solar wind dynamic pressure variations associated with the eccentricity of the martian orbit around the Sun.

Timing analysis of the relationship between solar wind parameters and geosynchronous Pc5 amplitude

Solar wind‐driven magnetospheric ULF waves in the Pc5 band (1.7–6.7 mHz) have been suggested to mediate the radial transport of electrons in the outer radiation belt. To identify the wave generation mechanism and the impact of the waves on the electrons, we performed correlation and superposed epoch analyses of the solar wind control parameters measured by the ACE spacecraft and the amplitude of Pc5 waves and the flux of radiation belt electrons measured at the GOES 12 geosynchronous satellite. The analyses were carried out for solar minimum year 2006, when the solar wind exhibited recurrent co‐rotating interaction regions. We used hourly averages of solar wind velocity Vsw, the root‐mean‐square amplitudes of the solar wind dynamic pressure variations in the Pc5 band σPsw, the azimuthal (σBy) and compressional (σBz) magnetic field components of geosynchronous Pc5 waves, as well as relativistic (>2 MeV) electron flux Je at geosynchronous orbit. We found that σPsw has a higher correlation with σBy and σBz than Vsw and that the correlation between σPsw and the wave amplitudes peaks at or very near zero time shift. We conclude that the major driver of geosynchronous Pc5 waves is solar wind pressure variations rather than the Kelvin–Helmholtz instability on the magnetopause. Solar wind pressure pulses propagate into the inner magnetosphere at the MHD wave speed and it takes less than 1 hour for the signal to reach GEOS 12 from the bow shock nose. The pressure pulses also excite standing Alfvén waves that contribute to the σPsw–σBy correlation. There is a positive correlation between Vsw and Pc5 amplitude, but the correlation peaks at a time shift of about −1 day (the Pc5 peak occurs earlier than the Vsw peak). We attribute this shift to an intrinsic time shift between σPsw and Vsw. Je exhibits a minimum at the Dst minimum, coincident with the maximum of σPsw, σBy, and σBz, and increases by 2 to 3 orders of magnitude over a period of about one day during which σBy and σBz decrease by one order of magnitude and Vsw reaches a peak value. However there are storms that do not exhibit electron flux enhancement despite the presence of strong Pc5 activity. Therefore the correlation between Pc5 amplitude and electron flux at geosynchronous orbit is not unique. The difference in electron response implies either the existence of other competing acceleration/loss mechanisms or that electron response to the Pc5 driver is nonlinear: similar driving conditions produce different response.

An empirical model of Saturn's bow shock: Cassini observations of shock location and shape

We present a new empirical model of Saturn's bow shock that utilizes observations from the Cassini spacecraft. Shock crossings are identified in magnetic field and plasma observations made by Cassini between June 2004 and August 2005. The Cassini crossings are then combined with the crossings made during the Saturn flybys of Pioneer 11, Voyager 1, and Voyager 2. Solar wind dynamic pressures for the Cassini crossings are estimated using upstream electron densities determined from Langmuir wave observations made by the Radio and Plasma Wave System. The crossing positions are rotated into aberrated coordinates to correct for the effect of the planet's orbital motion. In the case of Saturn this rotation is by ∼1°. To correct for solar wind dynamic pressure variations, the crossing positions are normalized to the average pressure 〈PSW〉 = 0.048 nPa. The model is then obtained by fitting a conic section to the crossings using a nonlinear least squares technique. To validate the assumptions made in constructing the model, we treat the parameters previously assumed to be constants as variables and fit their values using an optimization routine; this leads to a conic section that is within the positional uncertainty of the model. The spacecraft trajectories are considered, and we conclude that they do not significantly bias the model. The new model is compared to the existing models, and the similarities and differences are discussed. We suggest that the new model gives the most accurate empirical representation of the shape and location of Saturn's bow shock.

Temporal evolution of the transpolar potential after a sharp enhancement in solar wind dynamic pressure

Recent studies of ionospheric convection have shown that sudden enhancements in solar wind dynamic pressure have significant effect on the transpolar potential and the coupling efficiency between the solar wind and the terrestrial magnetosphere. Super Dual Auroral Radar Network observations of the dayside convection have demonstrated that the strength of convection correlates well with solar wind dynamic pressure variations, implying an enhancement of dayside reconnection induced by changes in solar wind pressure. At the same time, dynamic pressure increases have been shown to lead to closing of the polar cap, particularly on the nightside, and thus directly drive enhanced tail reconnection. The enhanced dayside and nightside reconnection potentials can both lead to changes in the transpolar potential, but their individual contributions and the balance between the two is not known. We present a case study of the transpolar potential evolution after a long‐lasting solar wind pressure step increase. We show that the potential first rises in response to the increase in pressure, then gradually subsides a few hours later despite the solar wind pressure remaining high. We interpret this behavior in terms of pressure‐driven changes in dayside and nightside reconnection.

High‐time resolution dayside convection monitoring by incoherent scatter radar and a sample application

Convection and its changes, driven by the solar wind, are related to geomagnetic disturbances such as substorms, dynamic pressure disturbances and convection bays. Furthermore, convection is theoretically related to the evolution of the Harang electric field reversal (or “discontinuity”), which has been related observationally and theoretically to these disturbances. Interplanetary measurements generally used for studying and making associations of convection with disturbances are acceptable for statistical studies, but spatial structure and propagation timing errors due to the highly variable orientation of interplanetary structures create significant ambiguity for case studies and have prevented definitive understanding. Moreover, small but geoeffective structure in the solar wind may be missed by even multispacecraft observations. Direct observation of dayside convection with high time resolution offers a potential solution to this problem. In this paper, we demonstrate that Sondrestrom incoherent scatter radar (ISR) observations have the ability to accurately monitor the dayside convection with ∼2.5 min temporal resolution. The observations clearly show the dayside convection responses to IMF Bz, By and solar wind dynamic pressure variations and thus shed new light on how the magnetosphere and ionosphere respond to different external driving. Then, we illustrate an application of this technique by showing how it can be applied to study substorm triggering. We plan to use this technique in the future to study the relationships between convection, other geomagnetic disturbances, and the Harang reversal, as well as to evaluate the preceding convection strength and duration and the convection changes that lead to substorm onset.

Response of the magnetic field in the geosynchronous orbit to solar wind dynamic pressure pulses

We do a statistical survey of solar wind dynamic pressure (Pd) pulses and geosynchronous magnetic fields observed between 1998 and 2005. In geomagnetic quiet times with Dst > −50 nT, we find 111 solar wind dynamic pressure pulses which produce geosynchronous magnetic field responses. These responses are often observed by two or three GOES spacecraft at different local times in geosynchronous orbit. The magnitudes of the geosynchronous magnetic field changes (dBz) have a peak near the noon meridian, similar to the results obtained in the study of the response of the geosynchronous field to the large and sharp solar wind dynamic pressure variations. However, the relative change of the geosynchronous magnetic field dBz/AV‐Bz (where AV‐Bz is the average of the geosynchronous magnetic field Bz observed during the response to the pressure pulse) depends weakly on the local time; thus the change of Bz(dBz) is proportional to the average field (AV‐Bz). As the magnitude of the relative change of solar wind dynamic pressure (dPd/Pd) increases, the rate of geosynchronous magnetic field variation increases correspondingly. These results imply that the magnitude of the geosynchronous magnetic field response could be determined by AV‐Bz. In addition, the interplanetary field orientation does not affect the response significantly. Using an MHD code which models the global behavior of the solar wind‐magnetosphere‐ionosphere system, we reproduce the main characteristics of the observations.

Dayside reconnection enhancement resulting from a solar wind dynamic pressure increase

It is well known that the Interplanetary Magnetic Field (IMF) is the major contributor to geomagnetic activity on Earth. Recent studies, however, have shown that solar wind dynamic pressure variations also cause global effects when they encounter the terrestrial magnetosphere. In particular, it has been shown that solar wind dynamic pressure enhancements significantly increase particle precipitation and cause global intensification of the aurora. Further studies using Defense Meteorological Satellite Program (DMSP) measurements have demonstrated that solar wind pressure increases also significantly affect the size of the polar cap and the cross‐polar cap potential drop. This implies that the dynamic pressure has an important effect on the coupling efficiency between the solar wind and the Earth’s magnetosphere, which is in addition to that due to the IMF. It was previously suggested, on the basis of the DMSP data, that solar wind dynamic pressure enhancements induce enhanced magnetotail reconnection and magnetospheric convection. We now present Super Dual Auroral Radar Network (SuperDARN) observations for a number of events that demonstrate significantly enhanced ionospheric convection in the dayside ionosphere associated with the impact of solar wind pressure fronts. The enhanced convection extends to the vicinity of the expected location of the dayside separatrix, suggesting that the solar wind dynamic pressure strongly affects dayside reconnection as well as polar‐cap convection.

Large and sharp solar wind dynamic pressure variations as a source of geomagnetic field disturbances at the geosynchronous orbit

We present a comparison of large and fast solar wind dynamic pressure changes, observed by INTERBALL-1 satellite, with disturbances in the magnetospheric magnetic field measured by the GOES-8, 9 and 10 geosynchronous satellites. More than 300 solar wind pressure changes in the period from 1996 to 1998 were selected for this study. These fast and large solar wind pressure changes cause fast variations of the geomagnetic field magnitude in the outer magnetosphere. We found that increases (decreases) in the solar wind dynamic pressure always result in increases (decreases) in the geosynchronous magnetic field magnitude. The time delay between the solar wind pressure change and the geomagnetic response is significantly affected by the inclination of the pressure front to the Sun–Earth line. The amplitude of the geomagnetic field variation strongly depends on the location of the GOES satellite relative to the noon meridian, with the maximum amplitude near noon.

Long-term evolution of magnetospheric current systems during storms

Abstract. We present a method to model the storm-time magnetospheric magnetic field using representations of the magnetic field arising from the various magnetospheric current systems. We incorporate the effects of magnetotail changes during substorms by introducing an additional localized thin current sheet into the Tsyganenko T89 model. To represent the storm-time ring current the T89 ring current is replaced by a bean-shaped current system, which has a cross section that is close to the observed distribution of trapped particles in the inner magnetosphere and has an eastward flowing inner and westward flowing outer components. In addition to the symmetric ring current, an asymmetric partial ring current is taken into account with closing Region 2 sense field-aligned currents. Magnetopause currents are varied in accordance with solar wind dynamic pressure variations. Three moderate geomagnetic storms when Dst reached about –150 nT and one big storm with Dst about –250 nT are modelled. The model free parameters are specified for each time step separately using observations from GOES 8 and 9, Polar, Interball and Geotail satellites and Dst measurements. The model gives a high time-resolution field representation of the large-scale magnetic field, and a very good reproduction of the Dst index. It is shown that the ring current is most important during intense storms, whereas the near-Earth tail currents contribute more to the Dst index than the ring current during moderate storms. Key words. Magnetospheric physics (Current systems; Magnetospheric configuration and dynamics; Storms and substorms)

Solar cycle dependence of Pi2 pulsation activity in the equatorial region of India

Solar Cycle dependence of Pi2 pulsation activity from the equatorial station at Choutuppal (Gm. Coord.: 7.5°N, 149°14′E), India, has been studied using pulsation data spanning nearly two solar cycles from 1970 to 1989. The Pi2 activity, primarily a nighttime phenomenon maximizing around local midnight shows an unambiguous inverse relationship with sunspot cycle. Equatorial Pi2 pulsation activity also shows a direct relationship with solar wind dynamic pressure variations. The results are discussed in relation to the associated phenomena like bursty flows in the plasma sheet and the possible influence of solar wind dynamic pressure variations.