Solar Wind Number Density Research Articles

Ion precipitation in the cusp for northward IMF and its relationship with magnetosheath flow

If the interplanetary magnetic field (IMF) has a northward component, a portion of the solar wind ions flowing in the high-latitude magnetosheath drastically alters the flow direction owing to lobe reconnection and follows the Earth's magnetic field lines toward the low-altitude cusp. Partly because of this folded path, the relationship between the speed of ion flow in the magnetosheath and the energy of the precipitating ions in the low-altitude cusp remains unclarified. Herein, we examined particle data from 11 years of observations by Defense Meteorological Satellite Program satellites to obtain 1990 cases through an automated event identification of the cusp. This corresponds to the largest number of cusp ion events reported to date for a stable northward IMF. The results revealed a positive correlation between the energy of the precipitating ions and the estimated flow speed in the magnetosheath, which answers the question that has remained unexplained for decades: why is the brightness of the cusp proton aurora more strongly correlated with the solar wind dynamic pressure than with the solar wind number density? We suggest additional energizations of the exhausted ions from lobe reconnection in the outflow region because of the turbulent electric field formed by the opposite flow of two ion populations—one in the tailward-directed magnetosheath flow and the other in the earthward-directed outflow jet.Graphical

Auroral Electrojet Activity for Long‐Duration Radial Interplanetary Magnetic Field Events

AbstractIn this paper, we provide statistical evidence that the level of solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling is weaker under radial (Sun‐Earth component dominant) interplanetary magnetic field (IMF) conditions than non‐radial IMF conditions. This is performed by analyzing auroral electrojet activity (using SuperMAG auroral electrojet indices) in the sunlit and dark ionospheres for long‐duration (at least 4 hr) radial IMF events and comparing against the same for long‐duration azimuthal (dusk‐dawn component dominant) IMF events. We show that the north‐south IMF component (IMF Bz) plays a crucial role in controlling the level of auroral electrojet activity as a negative half‐wave rectifier even for both IMF orientation categories. However, it is found that the magnitudes of the auroral electrojet indices are generally lower for radial IMF than for azimuthal IMF under similar sets of solar wind (radial bulk velocity and number density) and IMF Bz conditions, regardless of whether these indices are derived in the sunlit or dark regions. Moreover, the efficiency of coupling functions is lower for radial IMF than for azimuthal IMF, implying that increased coupling strength due to the azimuthal IMF component alone cannot well explain weaker auroral electrojets during radial IMF periods. Lastly, the contribution of the radial IMF component itself to auroral electrojet activity is also lower compared to the azimuthal IMF component. Our results suggest that the level of SW‐M‐I coupling characterized by auroral electrojet activity can be modulated by the radial IMF component, although the effect of this component is weaker than the other two IMF components.

Solar wind H+ fluxes at 1 AU for solar cycles 23 and 24

The solar wind consists of electrons and ion species having kinetic energy ≲10 keVs. Ion stream is mainly composed of protons. Their flux magnitude is especially important for radiation hardness assurance scientists who qualify materials for future space missions. Materials irreversibly degrade as soon as are exposed to solar radiation. Hence, proton differential and integral flux spectra play a crucial role in properly estimating radiation loads deposited within satellite functional components. Up to now, the wind fluxes were read out, e.g., from figures made for specific periods of a solar cycle. This method is not effective and is a source of a flux, and further on, a fluence uncertainty. The objective of this work was to calculate and tabulate the solar wind differential and integral proton fluxes, which can be used further on to evaluatespace mission proton fluence spectra. Solar wind proton bulk velocity and number density from four space solar observatories (SOHO, ACE, WIND, and DSCOVR) were numerically processed to achieve that goal. Fluxes were tabulated and represented in calendar year periods.Now, the radiation hardness assurance community can quickly access, compute and compare SW proton fluxes from different energy ranges, calendar years, and satellite data sources. Proton fluxes presented here indicate magnitude variations along with the solar cycle period. A correlation between the number of sunspots and the proton average flux was found, i.e., the larger the number of spots, the larger the flux magnitude. More importantly, it was discovered that proton fluxes indicate huge variations in magnitude for a selected year based on different satellite data sources. For some years, the difference reaches 240%. This value is reported for the first time and shows that to have a complete picture of the SW proton stream population, one must compare data from all four satellites. Also, it is reported that satellites, for a given calendar year, record different ranges of proton velocity and number density values. This finding, even stronger, suggests a necessity of comparing SW data from as many solar observatories as possible. The spread in the proton flux magnitude (240%) has a direct implication for planning and executing satellite material on-ground radiation test campaigns. Not carefully choosing a proton flux may result in a false impression of how examined material would degrade under an interplanetary space radiation environment. In this article, recommendations for selecting proper proton spectra are given.

Determining the Timing of Driver Influences on 1.8–3.5 MeV Electron Flux at Geosynchronous Orbit Using ARMAX Methodology and Stepwise Regression

AbstractAlthough lagged correlations have suggested influences of solar wind velocity (V) and number density (N), Bz, ultralow frequency (ULF) wave power, and substorms (as measured by the auroral electrojet (AE) index) on MeV electron flux at geosynchronous orbit over an impressive number of hours and days, a satellite's diurnal cycle can inflate correlations, associations between drivers may produce spurious effects, and correlations between all previous time steps may create an appearance of additive influence over many hours. Autoregressive‐moving average transfer function (ARMAX) multiple regressions incorporating previous hours simultaneously can eliminate cycles and assess the impact of parameters, at each hour, while others are controlled. ARMAX influences are an order of magnitude lower than correlations uncorrected for time behavior. Most influence occurs within a few hours, not the many hours suggested by correlation. A log transformation accounts for nonlinearities. Over all hours, solar wind velocity (V) and number density (N) show an initial negative impact, with longer term positive influences over the 9 (V) or 27 (N) hr. Bz is initially a positive influence, with a longer term (6 hr) negative effect. ULF waves impact flux in the first (positive) and second (negative) hour before the flux measurement, with further negative influences in the 12–24 hr before. AE (representing electron injection by substorms) shows only a short term (1 hr) positive influence. However, when only recovery and after‐recovery storm periods are considered (using stepwise regression), there are positive influences of ULF waves, AE, and V, with negative influences of N and Bz.

Operational inner magnetosphere particle transport and acceleration model (IMPTAM) for 1–300 keV electrons

Surface charging, the process of charge deposition on covering insulating surfaces of satellites is directly linked to the space environment at a time scale of a few tens of seconds. Accurate specification of the space environment at different orbits is of key importance. We introduce the model operating online in near-real time for low energy (< 200 keV) electrons in the inner magnetosphere, called Inner Magnetosphere Particle Transport and Acceleration model (IMPTAM). This model in its various versions has been operating online since March 2013 (latest version at imptam.engin.umich.edu) and it is driven by the real time solar wind (solar wind number density, dynamic pressure and velocity) and Interplanetary Magnetic Field (IMF), Y and Z components and total magnitude, parameters and by the real time Dst and Kp indices. The model provides the low energy electron (and proton) flux at all Magnetic Local Times (MLTs) and at L-shells from 3 to 10 and at all satellite orbits inside the modeling region, when necessary. IMPTAM output can serve as an input to assess surface charging levels of spacecraft immersed in severe environments and inside radiation belts models as distributions of electron seed population for further acceleration to MeV energies.

Effect of solar wind density and velocity on the subsolar standoff distance of the Martian magnetic pileup boundary

Using a 3D multispecies magnetohydrodynamic model, we investigated the effect of the solar wind dynamic pressure (Pd) with different densities and velocities on the subsolar standoff distance (r0) of the Martian magnetic pileup boundary (MPB). We fixed the solar maximum condition, the strongest crustal field located in the dayside region, and the Parker spiral interplanetary magnetic field at Mars. We simulated 35 cases with a Pd range of 0.1494 to 7.323 nPa (solar wind number density n ∈ [1, 9] cm−3, and solar wind velocity V ∈ [−258, −1344] km s−1). The main results are as follows. (1) r0 decreases with increasing Pd according to the power-law relations. For the same Pd, a higher solar wind velocity (lower density) results in a larger r0 of the Martian MPB. (2) A higher solar wind density leads to a lower ratio of the compressed magnetic field strength to the crustal field strength and a larger plasma β under the same Pd. This indicates that the thermal pressure at the Martian MPB plays a significant role for the compressed magnetic field. Because the magnetic pileup process is stronger for a higher solar wind velocity, the magnetic pressure at the Martian MPB is increased. As a result, the thermal pressure decreases and r0 of the Martian MPB increases. (3) We present a new formula of r0 with the parameters of the solar wind dynamic pressure, number density, and velocity.

Non-storm erosion of MeV electron outer radiation belt down to &lt;em&gt;L&lt;/em&gt;* &amp;lt; 4.0 associated with successive enhancements of solar wind density

We report an unusual non-storm erosion event of outer zone MeV electron distribution during three successive solar wind number density enhancements (SWDEs) on November 27−30, 2015. Loss of MeV electrons and energy-dependent narrowing of electron pitch angle distributions (PAD) first developed at <italic>L</italic>* = 5.5 and then moved down to <italic>L</italic>* &lt; 4. According to the evolution of the electron phase space density (PSD) profile, losses of electrons with small pitch angles at <italic>L</italic>* &gt; 4 during SWDE1 are mainly due to outward radial diffusion. However during SWDE2&amp;3, scattering loss due to EMIC waves is dominant at 4 &lt; <italic>L</italic>* &lt; 5. As for electrons with large pitch angles, outward radial diffusion is the primary loss mechanism throughout all SWDEs which is consistent with the incursion of the Last Closed Drift Shell (LCDS). The inner edge of EMIC wave activity moved from <italic>L</italic>* ~5 to <italic>L</italic>* ~4 and from <italic>L</italic> ~6.4 to <italic>L</italic> ~4.2 from SWDE1 to SWDE2&amp;3, respectively, observed by Van Allen Probes and by ground stations. This is consistent with the inward penetration of anisotropic energetic protons from <italic>L</italic>* = 4.5 to <italic>L</italic>* = 3.5, suggesting that the inward extension of EMIC waves may be driven by the inward injection of anisotropic energetic protons from the dense plasma sheet.

A 3D Parametric Martian Bow Shock Model with the Effects of Mach Number, Dynamic Pressure, and the Interplanetary Magnetic Field

Abstract Using global MHD simulations, we construct a 3D parametric Martian bow shock model that employs a generalized conic section function defined by seven parameters. Effects of the solar wind dynamic pressure (P d ), the magnetosonic Mach number (M MS), and the intensity and the orientation of the interplanetary magnetic field (IMF) on the seven parameters are examined based on 250 simulation cases. These 250 cases have a P d range of 0.36–9.0 nPa (the solar wind number density varying from 3.5 to 12 cm−3 and the solar wind speed varying from 250 to 670 km s−1), an M MS range of 2.8–7.9, and an IMF strength range from zero to 10 nT. The results from our parametric model show several things. (1) The size of the Martian bow shock is dominated by P d . When P d increases, the bow shock moves closer to Mars, and the flaring of the bow shock decreases. (2) The M MS has a similar effect on the bow shock as P d but with different coefficients. (3) The effects of IMF components on the bow shock position are associated with the draping and pileup of the IMF around the Martian ionosphere; hence, we find that both the subsolar standoff distance and the flaring angle of the bow shock increase with the field strength of the IMF components that are perpendicular to the solar wind flow direction (B Y and B Z in the MSO coordinate system). The parallel IMF component (B X ) has little effect on the subsolar standoff distance but affects the flaring angle. (4) The cross section of the bow shock is elongated in the direction perpendicular to the IMF on the Y–Z plane, and the elongation degree is enhanced with increasing intensities of B Y and B Z . The north–south (dawn–dusk) asymmetry is controlled by the cone angle when the IMF is on the X–Z (X–Y) plane. These results show a good agreement with the previous empirical and theoretical models. The current parametric model is obtained under solar maximum conditions with the strongest Martian crustal magnetic field located at the subsolar point. In fact, the bow shock shape can also be affected by both the solar extreme ultraviolet radiation and the orientation of crustal magnetic anomalies to the Sun, which should be considered in future models.

Statistical characteristics of geomagnetic storm activity during solar cycle 24, 2009–2020

Urgency. The atmosphere and geospace are widely used as a radio channel in solving problems of radar, radio navigation, direction finding, radio communication, radio astronomy, and the remote sensing of the Earth from space or the near-earth environment from the surface of the planet. The parameters of the atmospheric-space radio channel are determined by the state of tropospheric and space weather, which is formed mainly by non-stationary processes on the Sun (solar storms) and partly by high-energy processes on the Earth and in the atmosphere. Geospace storms give rise to the strongest disturbances of the atmospheric-space radio channel, and it is important to note that these storms are diverse, so that no two storms are alike. At the same time, storms have both similar and individual features. Currently, there is insufficient knowledge about both of these features, and their study remains an urgent task of space geophysics and space radio physics. In particular, the identification of general patterns is advisable by performing a statistical analysis of a large number of storms. The aim of this work is to statistically analyze the parameters of the solar wind and geomagnetic field during the Solar Cycle 24 activity (2009–2020). Methods and Methodology. The parameters of the disturbed solar wind (number density nsw, velocity Vsw, and temperature Tsw), the disturbed values of the By- and Bz-components of the interplanetary magnetic field, which is the cause of magnetic storms on Earth, as well as the indices of geomagnetic activity (AE, Dst and Kp) are selected as source input to the study. In this paper, geomagnetic storms with Kр ≥ 5 or G1, G2, G3, and G4 geomagnetic storms are considered. In total, there were 153 storms with Kp ≥ 5. The time series of the nsw, Vsw, Tsw maximum values, of the By- and Bz-components, and of the AE, Dst and Kp indices, as well as of the Bz-component and the Dst index minimum values have been analyzed. Results. The main statistical characteristics of the parameters of the solar wind, interplanetary magnetic field, and of the geomagnetic field have been determined for 153 events that took place during Solar Cycle 24. Conclusions. The geomagnetic situation during Solar Cycle 24 was calmer than during Solar Cycle 23.

Comparison of Multiple and Logistic Regression Analyses of Relativistic Electron Flux Enhancement at Geosynchronous Orbit Following Storms

AbstractMany factors influence relativistic outer radiation belt electron fluxes, such as waves in the ultralow frequency (ULF) Pc5, very low frequency (VLF), and electromagnetic ion cyclotron (EMIC) frequency bands, seed electron flux, Dst disturbance levels, substorm occurrence, and solar wind inputs. In this work we compared relativistic electron flux poststorm versus prestorm using three methods of analysis: (1) multiple regression to predict flux values following storms, (2) multiple regression to predict the size and direction of the change in electron flux, and (3) multiple logistic regression to predict only the probability of the flux rising or falling. We determined which is the most predictive model and which factors are most influential. We found that a linear regression predicting the difference in prestorm and poststorm flux (Model 2) results in the highest validation correlations. The logistic regression used in Model 3 had slightly weaker predictive abilities than the other two models but had the most value in providing a prediction of the probability of the electron flux increasing after a storm. Of the variables used (ULF Pc5 and VLF, seed electrons, substorm activity, and EMIC waves), the most influential in the final model were ULF Pc5 waves and the seed electrons. IMF Bz, Dst, and solar wind number density, velocity, and pressure did not improve any of the models, and were deemed unnecessary for effective predictions.

The Source, Significance, and Magnetospheric Impact of Periodic Density Structures Within Stream Interaction Regions

AbstractWe present several examples of magnetospheric ultralow frequency pulsations associated with stream interaction regions and demonstrate that the observed magnetospheric pulsations were also present in the solar wind number density. The distance of the solar wind monitor ranged from just upstream of Earth's bow shock to 261 RE, with a propagation time delay of up to 90 min. The number density oscillations far upstream of Earth are offset from similar oscillations observed within the magnetosphere by the advection timescale, suggesting that the periodic dynamic pressure enhancements were time stationary structures, passively advecting with the ambient solar wind. The density structures are larger than Earth's magnetosphere and slowly altered the dynamic pressure enveloping Earth, leading to a quasi‐static and globally coherent “forced‐breathing” of Earth's dayside magnetospheric cavity. The impact of these periodic solar wind density structures was observed in both magnetospheric magnetic field and energetic particle data. We further show that the structures were initially smaller‐amplitude, spatially larger, structures in the upstream slow solar wind and that the higher‐speed wind compressed and amplified these preexisting structures leading to a series of quasiperiodic density structures with periods typically near 20 min. Similar periodic density structures have been observed previously at L1 and in remote images, but never in the context of solar wind shocks and discontinuities. The existence of periodic density structures within stream interaction regions may play an important role in magnetospheric particle acceleration, loss, and transport, particularly for outer zone electrons that are highly responsive to ultralow frequency wave activity.

The Effects of Geomagnetic Storms and Solar Wind Conditions on the Ultrarelativistic Electron Flux Enhancements

AbstractUsing data from the Relativistic Electron Proton Telescope on the Van Allen Probes, the effects of geomagnetic storms and solar wind conditions on the ultrarelativistic electron (E &gt; ~3 MeV) flux enhancements in the outer radiation belt, especially regarding their energy dependence, are investigated. It is showed that, statistically, more intense geomagnetic storms are indeed more likely to cause flux enhancements of ~1.8‐ to 7.7‐MeV electrons, though large variations exist. As the electron energy gets higher, the probability of flux enhancement gets lower. To shed light on which conditions of the storms are preferred to cause ultrarelativistic electron flux enhancement, detailed superposed epoch analyses of solar wind parameters and geomagnetic indices during moderate and intense storms with/without flux enhancements of different energy electrons are conducted. The results suggest that the storms with higher solar wind speed, sustained southward interplanetary magnetic field Bz, lower solar wind number density, higher solar wind Ey, and elevated and sustained substorm activity are more likely to cause ultrarelativistic electron flux enhancements in the outer belt. Comparing results of different energy electrons, the solar wind speed and AE index are the two parameters mostly correlated with the energy‐dependent acceleration of ultrarelativistic electrons: Storms with higher solar wind speed and elevated and sustained substorm activity are more likely to cause flux enhancement of ultrarelativistic electrons with higher energies. This suggests the important roles of inward radial diffusion as well as the source and seed populations provided by substorms on the energy‐dependent acceleration of ultrarelativistic electrons.

The Development of a Space Climatology: 1. Solar Wind Magnetosphere Coupling as a Function of Timescale and the Effect of Data Gaps

AbstractDifferent terrestrial space weather indicators (such as geomagnetic indices, transpolar voltage, and ring current particle content) depend on different coupling functions (combinations of near‐Earth solar wind parameters), and previous studies also reported a dependence on the averaging timescale, τ. We study the relationships of the am and SME geomagnetic indices to the power input into the magnetosphere Pα, estimated using the optimum coupling exponent α, for a range of τ between 1 min and 1 year. The effect of missing data is investigated by introducing synthetic gaps into near‐continuous data, and the best method for dealing with them when deriving the coupling function is formally defined. Using Pα, we show that gaps in data recorded before 1995 have introduced considerable errors into coupling functions. From the near‐continuous solar wind data for 1996–2016, we find that α = 0.44 ± 0.02 and no significant evidence that α depends on τ, yielding Pα∝B0.88Vsw1.90(mswNsw)0.23sin4(θ/2), where B is the interplanetary magnetic field, Nsw the solar wind number density, msw its mean ion mass, Vsw its velocity, and θ the interplanetary magnetic field clock angle in the geocentric solar magnetospheric reference frame. Values of Pα that are accurate to within ±5% for 1996–2016 have an availability of 83.8%, and the correlation between Pα and am for these data is shown to be 0.990 (between 0.972 and 0.997 at the 2σ uncertainty level), 0.897 ± 0.004, and 0.790 ± 0.03, for τ of 1 year, 1 day, and 3 hr, respectively, and that between Pα and SME at τ of 1 min is 0.7046 ± 0.0004.

On the Relation Between Soft Electron Precipitations in the Cusp Region and Solar Wind Coupling Functions

AbstractIn this study, the correlations between the fluxes of precipitating soft electrons in the cusp region and solar wind coupling functions are investigated utilizing the Lyon‐Fedder‐Mobarry global magnetosphere model simulations. We conduct two simulation runs during periods from 20 March 2008 to 16 April 2008 and from 15 to 24 December 2014, which are referred as “Equinox Case” and “Solstice Case,” respectively. The simulation results of Equinox Case show that the plasma number density in the high‐latitude cusp region scales well with the solar wind number density (ncusp/nsw=0.78), which agrees well with the statistical results from the Polar spacecraft measurements. For the Solstice Case, the plasma number density of high‐latitude cusp in both hemispheres increases approximately linearly with upstream solar wind number density with prominent hemispheric asymmetry. Due to the dipole tilt effect, the average number density ratio ncusp/nsw in the Southern (summer) Hemisphere is nearly 3 times that in the Northern (winter) Hemisphere. In addition to the solar wind number density, 20 solar wind coupling functions are tested for the linear correlation with the fluxes of precipitating cusp soft electrons. The statistical results indicate that the solar wind dynamic pressure p exhibits the highest linear correlation with the cusp electron fluxes for both equinox and solstice conditions, with correlation coefficients greater than 0.75. The linear regression relations for equinox and solstice cases may provide an empirical calculation for the fluxes of cusp soft electron precipitation based on the upstream solar wind driving conditions.

Dependence of the amplitude of magnetosonic waves on the solar wind and AE index using Van Allen Probes

AbstractWe present the dependence of the magnetosonic wave amplitudes both outside and inside the plasmapause on the solar wind and AE index using Van Allen Probe‐A spacecraft during the time period of 1 October 2012 to 31 December 2015, based on a correlation and regression analysis. Solar wind parameters considered are the southward interplanetary magnetic field (IMF BS), solar wind number density (NSW), and bulk speed (VSW). We find that the wave amplitudes outside (inside) the plasmapause are well correlated with the preceding AE, IMF BS, and NSW with time delays, each corresponding to 2–3 h (3–4 h), 4–5 h (3–4 h), and 2–3 h (8–9 h), while the correlation with VSW is ambiguous both inside and outside the plasmapause. As measured by the correlation coefficient, the IMF BS is the most influential solar wind parameter that affects the dayside wave amplitudes both outside and inside the plasmapause, while NSW contributes to enhancing the duskside waves outside the plasmapause. The AE effect on wave amplitudes is comparable to that of IMF BS. More interestingly, regression with time histories of the solar wind parameters and the AE index preceding the wave measurements outside the plasmapause shows significant dependence on the IMF BS, NSW, and AE: the region of peak coefficients is changed with time delay for IMF BS and AE, while isolated peaks around duskside remain gradually decrease with time for NSW. In addition, the regression with magnetosonic waves inside the plasmapause shows high coefficients around prenoon sector with preceding IMF BS and VSW.

Space climate and space weather over the past 400 years: 1. The power input to the magnetosphere

Using information on geomagnetic activity, sunspot numbers and cosmogenic isotopes, supported by historic eclipse images and in conjunction with models, it has been possible to reconstruct annual means of solar wind speed and number density and heliospheric magnetic field (HMF) intensity since 1611, when telescopic observations of sunspots began. These models are developed and tuned using data recorded by near-Earth interplanetary spacecraft and by solar magnetograms over the past 53 years. In this paper, we use these reconstructions to quantify power input into the magnetosphere over the past 400 years. For each year, both the annual mean power input is computed and its distribution in daily means. This is possible because the distribution of daily values divided by the annual mean is shown to maintain the same lognormal form with a constant variance. This study is another important step towards the development of a physics-based, long-term climatology of space weather conditions.

Coronal magnetic field profiles from shock‐CME standoff distances

AbstractCoronagraphs observe coronal mass ejections (CMEs) and driven shocks in white light images. From these observations the shock's speed and the shock's standoff distance from the CME's leading edge can be derived. Using these quantities, theoretical relationships between the shock's Alfvénic Mach number MA and standoff distance, and empirical radial profiles for the solar wind velocity and number density, the radial magnetic field profile upstream of the shock can be calculated. These profiles cannot be measured directly. We test the accuracy of this method for estimating the radial magnetic field profile upstream of the shock by simulating a sample CME that occurred on 29 November 2013 using the three‐dimensional (3‐D) magnetohydrodynamic Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme code, retrieving shock‐CME standoff distances from the simulation, and comparing the estimated and simulated radial magnetic field profiles. We find good agreement between the two profiles (within ±30%) between 1.8 and 10 R⊙. Our simulations confirm that a linear relationship exists between the standoff distance and the inverse compression ratio at the shock. We also find very good agreement between the empirical and simulated radial profiles of the number density and speed of the solar wind and inner corona.

An evidence for prompt electric field disturbance driven by changes in the solar wind density under northward IMF Bz condition

AbstractBefore the onset of a geomagnetic storm on 22 January 2012 (Ap = 24), an enhancement in solar wind number density from 10/cm3 to 22/cm3 during 0440–0510 UT under northward interplanetary magnetic field (IMF Bz) condition is shown to have enhanced the high‐latitude ionospheric convection and also caused variations in the geomagnetic field globally. Conspicuous changes in ΔX are observed not only at longitudinally separated low‐latitude stations over Indian (prenoon), South American (midnight), Japanese (afternoon), Pacific (afternoon) and African (morning) sectors but also at latitudinally separated stations located over high and middle latitudes. The latitudinal variation of the amplitude of the ΔX during 0440–0510 UT is shown to be consistent with the characteristics of prompt penetration electric field disturbances. Most importantly, the density pulse event caused enhancements in the equatorial electrojet strength and the peak height of the F layer (hmF2) over the Indian dip equatorial sector. Further, the concomitant enhancements in electrojet current and F layer movement over the dip equator observed during this space weather event suggest a common driver of prompt electric field disturbance at this time. Such simultaneous variations are found to be absent during magnetically quiet days. In absence of significant change in solar wind velocity and magnetospheric substorm activity, these observations point toward perceptible prompt electric field disturbance over the dip equator driven by the overcompression of the magnetosphere by solar wind density enhancement.

Compressional perturbations of the dayside magnetosphere during high‐speed‐stream‐driven geomagnetic storms

AbstractThe quasi‐DC compressions of the Earth's dayside magnetic field by ram‐pressure fluctuations in the solar wind are characterized by using multiple GOES spacecraft in geosynchronous orbit, multiple Los Alamos spacecraft in geosynchronous orbit, global MHD simulations, and ACE and Wind solar wind measurements. Owing to the inward‐outward advection of plasma as the dayside magnetic field is compressed, magnetic field compressions experienced by the plasma in the dayside magnetosphere are greater than the magnetic field compressions measured by a spacecraft. Theoretical calculations indicate that the plasma compression can be a factor of 2 higher than the observed magnetic field compression. The solar wind ram‐pressure changes causing the quasi‐DC magnetospheric compressions are mostly owed to rapid changes in the solar wind number density associated with the crossing of plasma boundaries; an Earth crossing of a plasma boundary produces a sudden change in the dayside magnetic field strength accompanied by a sudden inward or outward motion of the plasma in the dayside magnetosphere. Superposed epoch analysis of high‐speed‐stream‐driven storms was used to explore solar wind compressions and storm time geosynchronous magnetic field compressions, which are of particular interest for the possible contribution to the energization of the outer electron radiation belt. The occurrence distributions of dayside magnetic field compressions, solar wind ram‐pressure changes, and dayside radial plasma flow velocities were investigated: all three quantities approximately obey power law statistics for large values. The approximate power law indices for the distributions of magnetic compressions and ram‐pressure changes were both −3.

Empirical predictive models of daily relativistic electron flux at geostationary orbit: Multiple regression analysis

AbstractThe daily maximum relativistic electron flux at geostationary orbit can be predicted well with a set of daily averaged predictor variables including previous day's flux, seed electron flux, solar wind velocity and number density, AE index, IMF Bz, Dst, and ULF and VLF wave power. As predictor variables are intercorrelated, we used multiple regression analyses to determine which are the most predictive of flux when other variables are controlled. Empirical models produced from regressions of flux on measured predictors from 1 day previous were reasonably effective at predicting novel observations. Adding previous flux to the parameter set improves the prediction of the peak of the increases but delays its anticipation of an event. Previous day's solar wind number density and velocity, AE index, and ULF wave activity are the most significant explanatory variables; however, the AE index, measuring substorm processes, shows a negative correlation with flux when other parameters are controlled. This may be due to the triggering of electromagnetic ion cyclotron waves by substorms that cause electron precipitation. VLF waves show lower, but significant, influence. The combined effect of ULF and VLF waves shows a synergistic interaction, where each increases the influence of the other on flux enhancement. Correlations between observations and predictions for this 1 day lag model ranged from 0.71 to 0.89 (average: 0.78). A path analysis of correlations between predictors suggests that solar wind and IMF parameters affect flux through intermediate processes such as ring current (Dst), AE, and wave activity.