Interplanetary Magnetic Field Cone Angle Research Articles

Solar Wind Parameters Influencing Magnetosheath Jet Formation: Low and High IMF Cone Angle Regimes

AbstractMagnetosheath jets are localized flows of enhanced dynamic pressure that are frequently observed downstream of the Earth's bow shock. They are significantly more likely to occur downstream of the quasi‐parallel shock than the quasi‐perpendicular shock. However, as the quasi‐perpendicular geometry is a more common configuration at the Earth's subsolar bow shock, quasi‐perpendicular jets comprise a significant fraction of the observed jets. We study the influence of solar wind conditions on jet formation by looking separately at jets during low and high interplanetary magnetic field (IMF) cone angles. According to our results, jet formation commences when Alfvén Mach number MA ≳ 5. We find that during low IMF cone angles (downstream of the quasi‐parallel shock) other solar wind parameters do not influence jet occurrence. However, during high IMF cone angles (downstream of the quasi‐perpendicular shock) jet occurrence is higher during low IMF magnitude, low density, high plasma beta (β), and high MA conditions. The distribution of quasi‐parallel (quasi‐perpendicular) jet sizes parallel to flow peaks at ∼0.3 RE (∼0.1 RE). Some quasi‐perpendicular jets formed during high β and MA are particularly small. We show two examples of high β and MA quasi‐perpendicular shock crossings. Jets were observed in the transition region, but not deeper in the magnetosheath. A more detailed look into one jet revealed signatures of gyrating ions, indicating that gyrobunched ions near the shock may produce jet‐like enhancements. Our results suggest that jets form as part of the quasi‐perpendicular shock dynamics amplified by high solar wind MA and β.

The Dependence of the Location and Pressure Variations of the Martian Magnetic Pileup Boundary on the Interplanetary Magnetic Field: An MHD Simulation Study

Abstract In this paper, using a three-dimensional multispecies MHD model, we study the effect of the interplanetary magnetic field (IMF) intensity and orientation on the subsolar standoff distance of the Martian magnetic pileup boundary (r 0) and the pressure balance across it. The results show that: (1) with the increasing magnitude of the Y- or Z-component of the IMF, B Y or B Z , r 0 increases, while the radial IMF component, B X , has little effect. With the increasing magnitude of B Y or B Z , the compression degree of the magnetic field (f) increases, while the solar wind pressure coefficient (k) remains unchanged, resulting in the enhancement of r 0. (2) Under the same IMF intensity, B t , the impact of the IMF cone angle on r 0 and f is controlled by the ratio of the IMF Y- and Z-components to B t , B Y 2 + B Z 2 / B t . When the ratio is enhanced, both r 0 and f increase, while k generally remains unchanged. Compared with the IMF cone angle, the influence of the IMF clock angle is relatively less. We suggest that the stronger magnetic pileup process controlled by the perpendicular IMFs (B Y or B Z ) causes the larger r 0, while the weaker magnetic pileup under the radial IMF leads to the smaller r 0. The difference in the IMF effect on the size of the Martian magnetic pileup boundary and the terrestrial magnetopause reveals different solar wind interactions with a magnetized and unmagnetized planet. Last, the location of the intense crustal field can also affect r 0 and the pressure balance condition, and the specific impact needs to be further studied.

A magnetohydrodynamic simulation of the dayside magnetic reconnection between the solar wind and the Martian crustal field

Using a three-dimensional multispecies magnetohydrodynamic model, we study the effects of the orientation of the interplanetary magnetic field (IMF), solar wind dynamic pressure (Pd), and the location of the intense crustal field, on the dayside magnetic reconnection between the solar wind and the Martian crustal field. Our main results are as follows: (1) Different IMF orientations result in different magnetic field configurations and reconnection conditions on the Martian dayside. When the intense crustal field is located on the dayside, the dayside magnetic reconnection tends to occur in the region with solar zenith angles (SZA) ≈45° in the southern hemisphere for the IMF with a southward component. When the IMF has a northward component, the magnetic field lines are piled up in the same place and the Martian magnetic pileup boundary (MPB) appears as a local bulged “mini-magnetopause”. Under the pure radial IMF, the magnetic reconnection is absent, which might be due to the presence of additional outward magnetic tension and kinetic effects. (2) Dayside reconnection can change the shape of the Martian MPB, while the bow shock is weakly affected. When the IMF has a southward component, the dayside magnetic reconnection happens and the MPB is located closer to Mars with a “cusp” shape. When the IMF has a northward component, the Martian MPB expands with a local bulged “mini-magnetopause”. For the pure radial IMF condition, the subsolar region of the MPB is located closer to Mars than that under other IMF directions. The influence of the IMF cone angles on the Martian bow shock is much less than that on the MPB, and the bow shock locations are very close to the model results of another author found in the literature. (3) With increasing Pd, the size of the crustal field region decreases and the draped fields correspondingly move to lower altitudes where the IMF and crustal field have the same direction. When the IMF has a southward component and the magnetic reconnection occurs at SZA ≈ 45°, the reconnection site, the region of the closed topology of the crustal field, and the draped IMF, do not change much with increasing Pd. We suggest that the multipolar crustal magnetic fields can protect the solar wind IMF from further reconnecting with the crustal field to a lower altitude when Pd is enhanced.

A global view of Pc3 wave activity in near-Earth space: Results from hybrid-Vlasov simulations

Ultra-low frequency (ULF) waves in the Pc3 range, with periods between 10–45 s, are routinely observed in Earth’s dayside magnetosphere. They are thought to originate in the foreshock, which extends upstream of the quasi-parallel bow shock and is populated with shock-reflected particles. The foreshock is permeated with ULF waves generated by ion beam instabilities, most notably the “30-s” waves whose periods match those of the Pc3 waves and which are carried earthward by the solar wind flow. However, the global picture of Pc3 wave activity from the foreshock to the magnetosphere and its response to changing solar wind conditions is still poorly understood. In this study, we investigate the global distribution and properties of Pc3 waves across near-Earth space using global simulations performed with the hybrid-Vlasov model Vlasiator. The simulations enable us to study the waves in their global context, and compare their properties in the foreshock, magnetosheath and dayside magnetosphere, for different sets of upstream solar wind conditions. We find that in all three regions the Pc3 wave power peaks at higher frequencies when the interplanetary magnetic field (IMF) strength is larger, consistent with previous studies. The Pc3 wave power is significantly enhanced in all three regions for higher solar wind Alfvén Mach number. As this parameter is known to affect the shock properties but has little impact inside the magnetosphere, this brings further support to the magnetospheric waves originating in the foreshock. Other parameters that are found to influence the foreshock wave power are the solar wind density and the IMF cone angle. Inside the magnetosphere, the wave power distribution depends strongly on the IMF orientation, which controls the foreshock position upstream of the bow shock. The wave power is largest when the angle between the IMF and the Sun-Earth line is smallest, suggesting that wave generation and transmission are most efficient in these conditions.

Variations of IMF Cone Angle and IMF Clock Angle to Solar Cycles and Geomagnetic Storms

Interplanetary magnetic field (IMF) carried by solar wind and rooted at solar corona. Variations of IMF depend on solar surface activities, e.g. sunspots. Moreover, solar magnetic storms are connected by the IMF. IMF parameters, sunspot number, and geomagnetic index for more than 55 years were downloaded from OMNIWeb, which the data were obtained from several spacecraft in geostasionary and L1 Lagrange orbits. Solar cycles are measured in terms of variations in the number of sunspots and their surface areas. Both magnetic events of IMF and sunspots indicate the relation between solar cycles and IMF orientation which is clock angle and cone angle. IMF also interacts with the Earth’s magnetosphere, and it can cause geomagnetic storms. Cone angle and clock angle are derived from IMF components on X-axis, Y-axis, and Z-axis. Their frequency distribution charts on each solar cycle of 20 to 24 and their maxima phase and minima phase are the key points for the analysis. The plot of clock angle to Dst index as a geomagnetic index for each solar cycle was also derived. Based on the analysis, solar cycles are indicated to give an impact on the magnitude of each IMF component. It makes the peak of the charts shifted from the minima phase to the maxima phase. Other than that, the reconnection in magnetopause mostly occurs when the clock angle is between 1400–2520 or when the clock angle directions are southward. But the 23rd solar cycle shows the northward clock angle can also cause extreme geomagnetic storms.

Discrete Aurora at Mars: Dependence on Upstream Solar Wind Conditions.

Discrete aurora at Mars, characterized by their small spatial scale and tendency to form near strong crustal magnetic fields, are emissions produced by particle precipitation into the Martian upper atmosphere. Since 2014, Mars Atmosphere and Volatile EvolutioN's (MAVEN's) Imaging Ultraviolet Spectrograph (IUVS) has obtained a large collection of UV discrete aurora observations during its routine periapsis nightside limb scans. Initial analysis of these observations has shown that, near the strongest crustal magnetic fields in the southern hemisphere, the IUVS discrete aurora detection frequency is highly sensitive to the interplanetary magnetic field (IMF) clock angle. However, the role of other solar wind properties in controlling the discrete aurora detection frequency has not yet been determined. In this work, we use the IUVS discrete aurora observations, along with MAVEN observations of the upstream solar wind, to determine how the discrete aurora detection frequency varies with solar wind dynamic pressure, IMF strength, and IMF cone angle. We find that, outside of the strong crustal field region (SCFR) in the southern hemisphere, the aurora detection frequency is relatively insensitive to the IMF orientation, but significantly increases with solar wind dynamic pressure, and moderately increases with IMF strength. Interestingly however, although high solar wind dynamic pressures cause more aurora to form, they have little impact on the brightness of the auroral emissions. Alternatively, inside the SCFR, the detection frequency is only moderately dependent on the solar wind dynamic pressure, and is much more sensitive to the IMF clock and cone angles. In the SCFR, aurora are unlikely to occur when the IMF points near the radial or anti‐radial directions when the cone angle (arccos(B x /|B|)) is less than 30° or between 120° and 150°. Together, these results provide the first comprehensive characterization of how upstream solar wind conditions affect the formation of discrete aurora at Mars.

On the Interplay Between Solar Wind Parameters and ULF Wave Power as a Function of Geomagnetic Activity at High‐ and Mid‐latitudes

AbstractUsing ground magnetometer measurements from the IMAGE array over the timespan of one solar cycle, we apply wavelets to statistically assess the power of ultra‐low frequency (ULF) waves from L = 3.34 to L = 13.6 as a function of Kp, solar wind speed (), solar wind dynamic pressure (), Dst, and . We find that although geomagnetic storms account for the overwhelming majority of ULF wave power at mid latitudes, this effect decreases for higher L‐shells. We present analysis of the effect of interplanetary magnetic field (IMF) on ULF wave power for different values of Kp and , revealing a strong inter‐dependency especially in relation to . A parameterization additionally incorporating appears to be more important at large‐L. Further, analysis for positive reveals a perhaps unexpected additional dependence on which points to effects of the interplanetary magnetic field cone angle and Kelvin‐Helmholtz instabilities. Finally, we derive ULF wave radial diffusion coefficients from the measured ULF wave power spectral densities and briefly discuss their importance for radiation belt dynamics. Interestingly, in part because of the correlation between Dst‐defined storm times and high Kp, we find that the low‐Kp radial diffusion coefficients during storms are higher than previously assumed almost certainly because of the dominance of nonstorm epochs in the statistics of median ULF wave power under conditions of low Kp. These inter‐dependencies should be taken into account in future specification models for ULF wave radial diffusion coefficients when applied in storm time simulations.

The Venus Express observation of Venus’ induced magnetosphere boundary at solar maximum

The Venusian plasma environment is divided into two distinct regions by the induced magnetosphere boundary (IMB): the domain of solar wind protons and the domain of local planetary ions. Previous studies on the identification of the IMB gave various IMB definitions. Here we study the well-structured Venusian IMB with a sudden magnetic field rotation and a sharp magnetosheath plasma decrease using Venus Express observations. We statistically investigate the location of such well-structured IMB and give an average location of the IMB at solar maximum. The solar activity and solar wind controls of the Venusian IMB location is also studied in this work. Our results show that the dayside Venusian IMB distance increases with solar activity, but it decreases with increasing solar wind dynamic pressure and interplanetary magnetic field cone angle. The behaviors of the IMB under these conditions are similar to those of the ionopause, indicating that the distance of the Venusian IMB is much correlated with the scale of the ionosphere. We suggest that the variation of the IMB is partially contributed to by the variation of the ionopause, whose altitude is determined by the pressure balance between ionospheric thermal pressure and external magnetic pressure.

Comparison of MMS Observations of Foreshock Bubbles With a Global Hybrid Simulation

AbstractWe present multi‐point observations of foreshock bubbles (FBs) for comparison with the predictions of hybrid simulations. The four Magnetospheric Multiscale (MMS) spacecraft observed a series of discontinuities in the region upstream from the bow shock on December 18, 2017. Two solar wind discontinuities were associated with fully developed FBs with core regions exhibiting greatly decelerated and deflected antisunward flows, significant increases in temperature, and depressed plasma densities and magnetic field strengths. A single shock lies upstream (sunward) from each of the two bubbles. The connection to the foreshock before and/or after the discontinuities can play an important role in the foreshock transient formation. We present the predictions of a 2.5‐D global hybrid simulation for the presence of the FBs for two different interplanetary magnetic field (IMF) cone angles (66° and 25°). The topology of the simulated FB is nearly hemispherical for small angles, but becomes linear for large angles. We show that the characteristics of the two simulated FBs for the two different discontinuity normals and IMF cone angles are in good agreement with the observational results.

Analysis of IMF Orientation and Pc5 Power During St. Patrick Geomagnetic Storm on 17 March 2015

Solar storms have a hazardous effect on the Earth, one of the most powerful is Coronal Mass Ejection (CME). St. Patrick geomagnetic storm is a severe geomagnetic storm which is caused by partial halo CME. Propagation of CME at interplanetary space is detected by solar wind parameters by speed and magnetic field. Interplanetary Magnetic Field (IMF) has orientation to determine the shift of solar wind particle. IMF orientation can be classified by IMF cone angle and IMF clock angle. Both can be used as an indicator of Kelvin-Helmholtz instability. This study aims to determine the flow of solar wind to the magnetosphere. The solar wind will enter the magnetosphere can be detected by THEMIS satellite orbits at ∼100 km to ∼12 RE at magnetotail and ground-based Fluxgate Magnetometer at LAPAN Pasuruan. Meanwhile, the geomagnetic storm is observed by Dst index. We combine data from interplanetary space, magnetosphere, and ground-based magnetometer and processed to Pc5 power. The results are IMF clock angle was closer to zero and IMF cone angle got wider on 15-18 March 2015, FM and FGM Pc5 power surged since March 16, 2015. Finally, a severe geomagnetic storm observed on March 17, 2015 with Dst index was -222 nT.

Helium in the Earth's foreshock: a global Vlasiator survey

Abstract. The foreshock is a region of space upstream of the Earth's bow shock extending along the interplanetary magnetic field (IMF). It is permeated by shock-reflected ions and electrons, low-frequency waves, and various plasma transients. We investigate the extent of the He2+ foreshock using Vlasiator, a global hybrid-Vlasov simulation. We perform the first numerical global survey of the helium foreshock and interpret some historical foreshock observations in a global context. The foreshock edge is populated by both proton and helium field-aligned beams, with the proton foreshock extending slightly further into the solar wind than the helium foreshock and both extending well beyond the ultra-low frequency (ULF) wave foreshock. We compare our simulation results with Magnetosphere Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA) measurements, showing how the gradient of suprathermal ion densities at the foreshock crossing can vary between events. Our analysis suggests that the IMF cone angle and the associated shock obliquity gradient can play a role in explaining this differing behaviour. We also investigate wave–ion interactions with wavelet analysis and show that the dynamics and heating of He2+ must result from proton-driven ULF waves. Enhancements in ion agyrotropy are found in relation to, for example, the ion foreshock boundary, the ULF foreshock boundary, and specular reflection of ions at the bow shock. We show that specular reflection can describe many of the foreshock ion velocity distribution function (VDF) enhancements. Wave–wave interactions deep in the foreshock cause de-coherence of wavefronts, allowing He2+ to be scattered less than protons.

Long‐Lasting Ground‐Satellite High Coherence of Compressional Dayside Pc3–Pc4 Pulsations

AbstractWe present observations of broadband (∼10–50 mHz) Pc3–Pc4 waves on 4 January 2014. The waves were detected on the dayside simultaneously in a compressional component (δBz) at the Radiation Belt Storm Probes A (RBSP‐A) in the inner magnetosphere and in the north‐south component (δH) on the ground at a low‐latitude Bohyun (BOH) station (L = 1.3) during an interval of small interplanetary magnetic field cone angle, suggesting that upstream ultralow frequency (ULF) waves were the source of the magnetospheric Pc3–Pc4 waves. We observed the ground‐satellite high coherence between RBSP‐A δBz and BOH δH for a prolonged time interval lasting 6.5 hr, which has not been reported previously, during which RBSP‐A was on the inbound and outbound legs moving from L = ∼3.3 to ∼6.3. In order to understand the spatial mode structure of the compressional waves, we examined the ground‐satellite cross phase for the high‐coherence interval and found that the waves observed in and out of the plasmasphere propagated earthward at the average fast‐mode speed of ∼700–1,000 km/s. We also observed cross‐phase values smoothly changing with the radial distance of the spacecraft across the plasmapause. This indicates that the presence of the plasmapause has little effect on our fast‐mode waves propagating into the inner magnetosphere and to the ground low‐latitude station.

Influence of the Interplanetary Magnetic Field Cone Angle on the Geometry of Bow Shocks

Abstract The frozen-in interplanetary magnetic field (IMF) in the solar wind is one of the most important parameters affecting the Earth’s space weather. In the early studies of the IMF’s influence on space weather, significant effects of the north–south component of the IMF B Z were emphasized, while the radial component of the IMF B X was largely ignored. However, the IMF near the Earth is not always dominated by the north–south component of the IMF B Z , and the radial component of the IMF B X also plays an important role. However, while the effects of the IMF B X (cone angle) on the magnetopause have been studied in recent years, there has been much less effort to quantify the B X (cone angle) effects on the bow shock. In this paper, using the bow shock crossing data from multiple satellites, we investigate the IMF cone angle effect on the dayside and nightside of the bow shock. Our results show that under the radial IMF condition, the dayside of the bow shock is located closer to the Earth than the average. At the same time, on the nightside, the bow shock is farther away from the Earth than the average. The mechanism explaining the bow shock location under the radial IMF is not completely understood. We believe that the magnetosonic Mach number and unusual conditions of the magnetosheath, especially for low dynamic pressure, play an important role. In the future, more work is needed to describe the reactions of the Earth’s magnetosphere to different IMF orientations, especially to the radial IMF.

Effects of Orbital Eccentricity and IMF Cone Angle on the Dimensions of Mercury’s Magnetosphere

Abstract We investigate the influence of orbital eccentricity and interplanetary magnetic field (IMF) direction on the dimensions of Mercury’s magnetosphere using four Earth years of data collected by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. The variations in magnetopause subsolar distance, flank distance, and tail radius are compared to determine the global dimensions and compressibility of the magnetosphere. As Mercury moves from perihelion (aphelion) to aphelion (perihelion) its magnetosphere expands (contracts) globally by ∼15% on average. After mapping all of the magnetopause crossings to a fixed radial distance from the Sun, we find that IMF cone angle changes lead to reconfigurations of the magnetosphere of comparable magnitude to radial solar distance effects. Furthermore, the magnetosphere is found to expand globally under the quasi-radial IMF, while contracting under the quasi-perpendicular conditions. A new Mercury magnetopause model parameterized by the heliocentric distance and IMF cone angle was constructed. Unlike Earth, the model gives a closed magnetopause for the nightside in most cases, and its flaring decreases with the contraction of the magnetosphere. The results demonstrate the highly variable reconfigurations of Mercury’s magnetosphere under its extreme environmental conditions.

Solar Wind Deflection in the Foreshock: Model‐Data Comparison

AbstractWe present the results of a 2.5‐D (2‐D in space and 3‐D in currents and electromagnetic fields) electromagnetic hybrid simulation run for foreshock wave activity during an interval of a radial interplanetary magnetic field (IMF). The simulation predicts nonlinear waves and foreshock expansion in connection with the development of cavitons and the formation of the foreshock compressional boundary. Fluctuations of the ion velocity component are associated with wave fronts corresponding to small wave normal angles and oscillate around 0 in the far upstream region. At distances closer to the bow shock, these fluctuations become asymmetric with duskward flow perturbations dominating at dusk and dawnward flow perturbations at dawn. We compare this simulation with THEMIS observations of large‐amplitude quasi‐monochromatic variations of ion velocity in the foreshock under radial IMF. A case study of simultaneous dual observations of unipolar fluctuations is followed by a statistical study of similar solar wind deflections in the foreshock. The observations that confirm the simulation predictions indicate that, in addition to ion heating, ultralow‐frequency waves participate in the magnetosheath‐like plasma flow deflection in the foreshock during periods of small IMF cone angles.

Upstream proton cyclotron waves: occurrence and amplitude dependence on IMF cone angle at Mars — from MAVEN observations

Proton cyclotron waves (PCWs) can be generated by ion pickup of Martian exospheric particles in the solar wind. The solar wind ion pickup process is highly dependent on the “IMF cone angle” — the angle between the solar wind velocity and the interplanetary magnetic field (IMF), which also plays an important role in the generation of PCWs. Using data from 2.15 Martian years of magnetic field measurements collected by the Mars Atmosphere and Volatile Evolution (MAVEN) mission, we have identified 3307 upstream PCW events. Their event number distribution decreases exponentially with their duration. A statistical investigation of the effects of IMF cone angle on the amplitudes and occurrence rates of PCWs reveals a slight tendency of PCWs’ amplitudes to decrease with increasing IMF cone angle. The relationship between the amplitude and IMF cone angle is weak, with a correlation coefficient r = –0.3. We also investigated the influence of IMF cone angle on the occurrence rate of PCWs and found that their occurrence rate is particularly high for intermediate IMF cone angles (~18°–42°) even though highly oblique IMF orientation occurs most frequently in the upstream region of the Martian bow shock. We also conclude that these variabilities are not artefacts of temporal coverage biases in MAVEN sampling. Our results demonstrate that whereas IMF cone angle strongly influences the occurrence of PCWs, IMF cone angle may also weakly modulate their amplitudes in the upstream region of Mars.

The Difference Between Isolated Flux Transfer Events and Flux Transfer Event Cascades

AbstractThis flux transfer event (FTE) study is based on 984 FTEs originally identified by Wang et al. (2005, https://doi:10.1029/2005JA011150) in Cluster data. Due to Cluster's orbit, the FTE list exclusively contains events detected at the high‐latitude dayside magnetopause and low‐latitude flanks. The focus of this study is on FTE separation time. The results show that FTEs appearing in cascades are mainly located at the northern dusk and southern dawn magnetopause, while isolated FTEs are equally spread over the region covered by Cluster. This difference may be explained by the different interplanetary magnetic field (IMF) conditions during which the subsets occur. For isolated FTEs, average IMF By is close to zero. During such conditions, FTEs are expected to form at arbitrary longitudes along an equatorial merging line. After formation, they propagate northward and southward, causing an equal distribution at higher latitudes. In contrast, FTE cascades typically occur during weakly southward IMF with a negative By component. Their asymmetric distribution at higher latitudes is consistent with both the component and the antiparallel merging model for nonzero By. In both scenarios, newly formed FTEs are expected to move to the northern dusk and southern dawn regions, as observed. Many FTE cascades appearing during northward IMF are located close to the low‐latitude flanks, confirming previous reports. We discovered that such FTEs appear during large IMF values. Another new result is that 16% of all isolated FTEs appear during small IMF cone angles, suggesting that these may form as a result of magnetosheath jets impacting on the magnetopause.

Long‐Term Variations in Solar Wind Parameters, Magnetopause Location, and Geomagnetic Activity Over the Last Five Solar Cycles

AbstractWe use both solar wind observations and empirical magnetopause models to reconstruct time series of the magnetopause standoff distance for nearly five solar cycles. Since the average annual interplanetary magnetic field (IMF) Bz is about zero, and the annual IMF cone angle varies between 54.0° and 61.2°, the magnetopause standoff distance on this timescale depends mostly on the solar wind dynamic pressure. The annual IMF magnitude well correlates with the sunspot number (SSN) with a zero time lag, while the annual solar wind dynamic pressure (Pdyn) correlates reasonably well with the SSN but with 3‐year time lag. At the same time, we find an anticorrelation between Pdyn and SSN in cycles 20–21 and a correlation in cycles 22–24 with 2‐year time lag. Both the annual solar wind density and velocity well correlate with the dynamic pressure, but the correlation coefficient is higher for density than for velocity. The 11‐year solar cycles in the dynamic pressure variations are superimposed by an increasing trend before 1991 and a decreasing trend between 1991 and 2009. The average annual solar wind dynamic pressure decreases by a factor of 3 from 1991 to 2009. Correspondingly, the predicted standoff distance in Lin et al.'s (2010, https://doi.org/10.1029/2009JA014235) magnetopause model increases from 9.7 RE in 1991 to 11.6 RE in 2009. The annual SSN, IMF magnitude, and magnetospheric geomagnetic activity indices display the same trends as the dynamic pressure. We calculate extreme solar wind parameters and magnetopause standoff distance in each year using daily values and find that both extremely small and large standoff distances during a solar cycle preferably occur at solar maximum rather than at solar minimum.

Distribution of solar energetic particles and magnetic field orientations related to strong geomagnetic storms in solar cycle 24

Based on hourly data of solar energetic particles (velocity and density), interplanetary magnetic field (IMF) Bz component, and derived IMF orientations, e.g. IMF cone angle and clock angle, we studied their relations within 155 intense geomagnetic storms (Dst index less than -100 nT) during years 2009 to 2016 of solar cycle 24. The importance of negative and southward Bz was confirmed. It is suggested that strong geomagnetic storm also depends on IMF orientation angles, especially perpendicular IMF Bz. However, strong storms is not correlated with fast proton speed, which is aligned with perpendicular or parallel IMF cone angle. In the future, we can use those derived equations for prediction of solar storm.

Influence of the IMF Cone Angle on Invariant Latitudes of Polar Region Footprints of FACs in the Magnetotail: Cluster Observation

AbstractThe influence of the interplanetary magnetic field (IMF) cone angle θ (the angle between the IMF direction and the Sun‐Earth line) on the invariant latitudes of the footprints of the field‐aligned currents (FACs) in the magnetotail has been investigated. We performed a statistical study of 542 FAC cases observed by the four Cluster spacecraft in the Northern Hemisphere. The results show that there are almost no FACs when the IMF cone angle is less than 10°, and there are indications of the FACs in the plasma sheet boundary layers being weak under the radial IMF conditions. The footprints of the large FAC (>10 nA/m2) cases are within invariant latitudes <71° and mainly within IMF cone angles θ > 60°, which implies that the footprints of the large FACs mainly expand equatorward with large IMF cone angle. The equatorward boundary of the FAC footprints in the polar region decreases with increasing IMF cone angle (and has a better correlation for northward IMF), which shows that the IMF cone angle plays an important controlling role in FAC distributions in the magnetosphere‐ionosphere coupling system. There is almost no correlation between the poleward boundary and the IMF cone angle for both northward and southward IMF. This is because the poleward boundary movement is limited by an enhanced lobe magnetic flux. This is the first time a correlation between FAC footprints in the polar region and IMF cone angles has been determined.