Interplanetary Magnetic Field Polarity Research Articles

Fast and accurate evaluation of deep-space galactic cosmic ray fluxes with HelMod-4/CUDA

The accurate knowledge of cosmic ion fluxes is essential for fundamental physics, deep space missions, and exploration activities in the solar system. In the HelMod-4 model the Parker transport equation is solved using a Monte Carlo approach to evaluate the solar modulation effect on local interstellar spectra of Galactic Cosmic Rays (GCRs). This work presents the latest updates to the HelMod-4 model parameters, focusing on the descending phase of solar cycle 24. The updates are motivated by the latest high-precision measurements from the AMS-02 detector which revealed for the first time with high accuracy the features of GCR fluxes’ evolution during a period of positive interplanetary magnetic field polarity. Furthermore, we present HelMod-4/CUDA, a GPU-accelerated approach for solving the Parker equation in the heliosphere using a stochastic differential equation method. The code is an evolution of the HelMod-4 code, porting the algorithm to GPU architecture using the CUDA programming language. This approach achieves significant speedup compared to a CPU implementation. The HelMod-4/CUDA code has been validated by comparing its results with the most precise and updated experimental GCR spectra observed during high and low solar activity periods, both in the inner and outer heliosphere, at the Earth location, and outside the ecliptic plane. The comparison shows that HelMod-4 and HelMod-4/CUDA can be equivalently used to provide solar-modulated spectra with a similar degree of accuracy in reproducing observed data.

Determining the Influence of the IMF and Planetary Magnetic Field Models on Mercury's Magnetosphere Along Spacecraft Trajectories of MESSENGER, BepiColombo and MPO

AbstractMercury's planetary magnetic field models (PMFMs) agree on a majorly dipolar field structure with a northward shift of the magnetic equator. However, due to the northerly biased orbit coverage of past spacecraft missions and different data analyzing methods, the available PMFMs differ in the determined multipole magnitudes for the dipole, quadrupole and octupole moments. While the PMFMs agree well with northern observations, we find that the predicted magnetic field values differ in the unexplored equatorial and southern regions. In the forward modeling approach of this study, we apply three different PMFM representatives, differing in their values of the dipole, quadrupole and octupole moments, and model the resulting solar wind interaction via a global hybrid model with sets of the 4 most common interplanetary magnetic field (IMF) directions under otherwise average solar wind conditions. Extracting our modeled fields along the flybys of MESSENGER and BepiColombo, as well as along four representative orbits of the Mercury Planetary Orbiter (MPO), allows us to estimate local field variations due to IMF and PMFM influence. Our modeled magnetic field components separate significantly by up to 60 nT between the PMFMs in low‐altitude southern regions, leading to displacements of magnetopause, cusps and current sheet locations. We find that MESSENGER flyby observations agree best with modeled field ranges resulting from a quadrupolar PMFM and that certain nightside regions are useful to estimate upstream IMF polarity. We also demonstrate how comparing BepiColombo swingby and MPO orbit phase observations with modeled results can help determine the correct PMFM for Mercury.

Anomalous Response of Mercury’s Magnetosphere to Solar Wind Compression: Comparison to Earth

Magnetic field intensity increases when solar wind compresses a planet’s magnetosphere. The compression can be measured using the ratio of compressed magnetic fields to purely dipolar magnetic fields just inside the magnetopause. For Earth, the ratio is proportional to the subsolar standoff distance of the magnetopause. Data from in-orbit observations by the MESSENGER spacecraft indicate an opposite ratio for Mercury; the compression ratio is inversely proportional to the subsolar standoff distance. The additional magnetic fields induced by currents at the top of Mercury’s core enhance the total magnetic field strength. We also evaluated differences in the subsolar standoff of Mercury’s magnetopause according to the north–south polarity of the interplanetary magnetic field (IMF). Previous studies have not identified meaningful differences in subsolar standoff distance between those in northward versus southward IMF polarities for Mercury; however, we found that the difference is statistically significant at a large IMF B Z (15–20 nT). The magnetic reconnection that occurs behind the cusp for a large northward IMF transfers the magnetic flux to the dayside and increases the subsolar standoff distance. The eroded magnetic flux for a large southward IMF is compensated by the induced magnetic fields.

Geoeffective interplanetary magnetic field (IMF) from in situ data: realistic versus idealized spiral IMF

The geoeffective, southward IMF (B_text {s}) given in the GSM reference frame as nature presents is compared with that based on idealized, spiral IMF. We obtained B_text {s} and B_text {s} sorted by the IMF polarity (B_text {s} fields) from in situ data at a high 16-second resolution. Idealized IMF is derived by omitting the fluctuation of the IMF in the GSEQ Z-direction. Results are: the absolute value of realistic B_text {s} is larger than the one from idealized IMF; realistic B_text {s} polarity fields exist in all seasons, while those from idealized IMF exist only around spring/fall when the IMF points toward/away from the Sun; idealized B_text {s} fields match the predictions of the Russell–McPherron (RM) model almost ideally. The present study has resolved the issue related to the patterns and absolute values of the observed B_text {s} fields and those from the RM model that assumes an idealized IMF. It confirms that B_{z,text {GSEQ}} plays a crucial role for B_text {s}. Finally, it paves a way to properly link the variations seen in geomagnetic activity with the pattern of the measured B_text {s} fields.

The Effect of the Fluctuating Interplanetary Magnetic Field on the Cosmic Ray Intensity Profile of the Ground-level Enhancement (GLE) Events

We numerically integrate the equations of motion of a large number of GeV protons, released impulsively near the Sun, in order to study their time–intensity behavior at the location of an observer at 1 au. This is relevant to the interpretation of Ground Level Enhancements (GLEs) detected by neutron monitors on Earth. Generally, the observed time–intensity profiles reveal a single sharp rise, followed by slow decay. However, in the 1989 October 22 GLE event, there was an initial sharp spike followed by a secondary smaller spike in the particle intensity. We consider whether the propagation of the high-energy protons in a large-scale turbulent interplanetary magnetic field (IMF) can lead to this unusual time–intensity profile. The IMF model includes large-scale magnetic turbulence and a heliospheric current sheet. Ad-hoc scattering is used to mimic the effect of smaller-scale fluctuations resulting in pitch-angle scattering. Proton fluxes as a function of time and location for an observer are determined for various turbulence parameters, IMF polarities, and the size of the particle source near the Sun. We find that the fluctuating IMF leads to considerable variation in the arrival location of the particles crossing 1 au, and the time–intensity profile depends significantly on the observer's location and can have multiple peaks. An alternate explanation for the unusual structure in the 1989 October 22 GLE event is provided. Our findings show that the large-scale turbulent IMF enhances the access of the high-energy protons to the HCS at the early time of the event, which leads to efficient cross-field transport.

Solar Wind Model Supported by Parker Solar Probe Observations During Faint Venusian Auroral Emission

Abstract The encounter of the Parker Solar Probe (PSP) with Venus during the Venus Gravity Assist 3 on 2020 July 11 provided a unique opportunity to gather in situ solar wind data in the Venusian environment while also being able to observe Venus from ground-based facilities on Earth. The Wang–Sheeley–Arge (WSA) model was used to make accurate predictions of solar wind velocity and interplanetary magnetic field polarity at Earth and STEREO-A, as compared to in situ data at each spacecraft. The same model was then used to predict solar wind conditions at Venus. The predictions were in good agreement with in situ PSP data, as they match the overall magnitude and structure of the solar wind velocity and magnetic polarity at multiple spacecraft. This demonstrates that WSA can be used to make reliable predictions at locations in the heliosphere when in situ data is not available. Venusian aurorae were detected via emission in the oxygen green line 5577 Å OI(1 S − 1 D) at the same time that PSP captured a heliospheric current sheet crossing, and shortly thereafter, detected an increase in proton count rate. This is the first observation of oxygen green line aurora on Venus that is not the direct result of a coronal mass ejection, a solar flare, or corotating interaction regions.

Long-term North–South Asymmetry of the Heliospheric Current Sheet

Abstract In this paper, we evaluate the heliospheric current sheet (HCS) north–south asymmetry using the ecliptical sector structure of the interplanetary magnetic field (IMF) reconstructed since the second half of the 19th century. During the last five solar cycles, the inferred IMF polarities fairly reproduce the observed dominance of the sectors with the polarity of the northern solar hemisphere, i.e., the prolonged southward shift of the HCS. For the presatellite era, we found that the northward shift of the HCS was more common in cycles 10, 15, and 17–19, and the southward HCS shift was more common in cycles 9, 11–14, and 16. We also analyzed the north–south asymmetry in sunspot group numbers since 1749 and found that the northern hemisphere dominated in cycles 2–3, 7–9, and 15–20, and the southern hemisphere activity was stronger in cycles 4, 9–14, and 21–24. Moreover, other solar phenomena bear similar long-term asymmetry variations. The regularity of these variations is not clear. According to the available proxies of the solar data, the dominance of the northern hemisphere is found in the ascending phase of the secular solar cycle, and the dominance of the southern hemisphere coincides with the descending phase.

Discrete Aurora on the Nightside of Mars: Occurrence Location and Probability

AbstractThis paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons.

Average Ring Current Response to Solar Wind Drivers: Statistical Analysis of 61 Days of ENA Images

AbstractThis paper presents the first determination of the average global and regional response of ring current (RC) flux intensity versus four key solar wind (SW) and interplanetary magnetic field (IMF) parameters: SW electric field (ESW), IMF polarity (BZ,IMF), SW pressure (PSW), and SW speed (VSW). We analyze energetic neutral atom (ENA) images from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) to measure the RC response, and use 5‐min downstream‐propagated data from the Advanced Composition Explorer (ACE) and Wind to measure SW driving. Our imaging data comprises 61 events, including 1,838 global 2D equatorial maps of line‐of‐sight (LOS) averaged ion flux derived from background‐subtracted, 15 min TWINS ENA images spanning 1–30 keV. Average spatial and spectral distributions were organized into four driver strength groups for each SW parameter. ESW has the strongest correlation with RC ion flux (coefficients are [0.95, 0.94, 0.88] at [1, 16, 30] keV, respectively), confirming that magnetospheric convection is a primary driver. In case studies, the global RC responds rapidly (≤15 min) to changes in ESW ranging from strong to weak. PSW is correlated with RC intensity, though not as strongly as ESW, indicating the secondary importance of PSW‐driven compression in RC injections. Strong ESW increases lower‐energy ions farther eastward and higher‐energy ions closer to midnight, consistent with an energy‐dependent interplay between magnetic drift and convection. The strongest SW driving preferentially increases the least energetic ions, suggesting that quasi‐steady convection can load the nightside RC with cooler ions.

Galactic cosmic ray transport in inner heliosphere: Study with ion chamber and neutron monitor data for 1936–2006

Galactic cosmic ray (GCR) solar diurnal variation data recorded by shielded ion chambers at Cheltenham/Fredericksburg (CH-IC) and Yakutsk (Yak-IC) are corrected for diurnal temperature variation in upper atmosphere and used to calculate the ratio α of GCR free paths normal (λ⊥) and parallel (λ||) to mean interplanetary magnetic field (IMF) B for 1936–2006; α is an important parameter in theories for GCR modulations in heliosphere including anisotropies. Also, neutron monitor (NM) data at low latitudes (Huancayo / Haleakala) are analyzed for 1953–2006; they are not affected by temperature variation in upper atmosphere. Calculations assume negligible symmetric latitude GCR density gradient (Gθs) based on observational evidence. Results support Ahluwalia and Modzelewska (2020) inference for global network of NMs and Nagoya vertical muon telescope (NagV-MT) that α depends on polarity of B at Earth orbit. GCR modulation theories quote α values much smaller than those computed by us. Flat heliosphere current sheet (HCS) and Master equations derived by Ahluwalia and Dorman (1997) are used for computing α and study its correlation with solar activity for positive (p) and negative (n) polarity intervals of B at 1 AU; B points outward/ inward from the Sun during p/n interval in northern hemisphere, IMF polarity changes sign near sunspot number (SSN) maximum. The results challenge validity of the conceptual understanding of GCR modulation processes in the heliosphere, pointing to a need for an acceptable theory to explain empirical results reported in this paper. We propose two Ulysses-like spacecrafts be put in a fast scan mode, making complimentary observations in solar north and south hemispheres for at least a complete solar cycle to obtain more precise information about temporal variations of GCR latitude gradients with solar activity and its dependence on GCR rigidity, facilitating a physically meaningful computation of α.

Galactic cosmic ray transport in the heliosphere: Forbush ion chamber data analysis

The study of ratio α (=λ⊥/λ||) of galactic cosmic ray (GCR) free paths normal (λ⊥) and parallel (λ||) to the mean interplanetary magnetic field (IMF) B is continued with annual mean GCR solar diurnal anisotropy (SDA) data of the Cheltenham/Fredericksburg shielded ion chamber (CH-IC) for 1936–1977 previously analyzed by Bieber and Chen (1991) but not corrected for atmospheric temperature effect. This study uses CH-IC data corrected for temperature effect to calculate α for the first time assuming negligible latitudinal density gradient. Solar activity dependence of α shown by Bieber and Chen (BC) remains qualitatively unchanged. Present GCR modulation theories do not provide guidance for α, its rigidity or solar cycle dependence. Flat heliosphere current sheet (HCS) and Master equations derived by Ahluwalia and Dorman (1997) are used for computing α and study its rigidity dependence and correlation with solar activity for positive (p) and negative (n) polarity intervals of B at 1 AU; B points outward/ inward from the Sun during p/n interval in Northern hemisphere, IMF polarity changes sign near sunspot number (SSN) maximum. CH-IC median rigidity of response (Rm) for GCR differential spectrum is 67 GV. We show that computed α is high for p- and low for n-intervals, also α is low near SSN maximum and high near SSN minimum when effective limiting GCR rigidity (Rc) for SDA is known to be <100 GV. The results are consistent with those reported earlier by Ahluwalia and Modzelewska (2020) for NagV-MT (Rm = 60 GV) for 1971–2017; they challenge validity of present conceptual understanding of GCR modulation processes in the heliosphere, pointing to a great need to develop an acceptable modulation theory to explain empirical results reported in this paper.

The interconnection and phase asynchrony between the geomagnetic indices’ periodicities: A study based on the interplanetary magnetic field polarities 1967–2018 utilizing a cross wavelet analysis

In this article, we investigate the interconnection and phase asynchrony between the periodicities of geomagnetic activity indices Kp, Ap, aa, and Dst according to the interplanetary magnetic field (IMF) polarities, toward (T) or away (A), during the time interval 1967–2018. For this purpose, the daily data of Kp, Ap, aa, and Dst indices during the considered period have been sorted into two groups (T and A) according to the IMF polarities. The wavelet transform (WT), the cross-wavelet transform (XWT), and the wavelet coherence (WTC) have been applied on the monthly averages of T and A groups for each geomagnetic index. Moreover, the correlation analyses (linear and running correlations) between the annual averages of T and A groups of each geomagnetic index have been investigated. The results of XWT and WTC revealed the existence of a long-term periodicity in the frequency range 8–16 yr for the monthly averages of T and A groups for aa, Kp, and Ap indices existed during the entire period. In contrast, the Dst index revealed this periodicity during the period 1980–2018. In addition, during this frequency band, both T and A groups of each geomagnetic index revealed a highly positive correlation and nearly in-phase relationship behavior. Furthermore, during the frequency band 4–8 yr, the XWT displayed a prominent periodicity that occurred for the monthly averages of T and A groups of each geomagnetic index, revealing a phase change during some time intervals. Periodicity in the range 2–4 yr for the monthly averages of T and A groups of aa, Kp,Ap, and Dst occurred in the intervals; 1967–2009 for aa and Kp, 1967–2007 for Ap, and 1976–1995 for Dst. The annual periodicity is also identified for all considered indices. Both T and A groups of Dst shared a semiannual periodicity (~187 days) during different intervals in the entire period. On the other hand, a periodicity around 0.25-yr (~90 days) appeared only near the times of solar activity maxima in the case of the T and A groups of aa, Kp, and Ap indices.

Precision measurements of cosmic ray electron and positron spectra above 50 MeV with the PAMELA magnetic spectrometer

The PAMELA experiment with magnetic spectrometer operated almost ten years on board of the Resurs DK1 satellite. The satellite was launched on 15 June 2006 on polar orbit with an inclination of 70° and an altitude of 350–610 km. The spectrometer continuously measured charged cosmic ray particles in wide energy range from about 50 MeV up to several TeV. In this work the spectra of electrons and positrons averaged over one year were obtained from July of 2006 until January 2016, i.e. from the end of 23th and at the beginning of 24th solar cycle including the period of interplanetary magnetic field polarity reversal. These precise long duration time-dependent measurements of the electron and positron spectra are important to estimate possible contributions of exotic cosmic ray sources such as dark matter annihilation or decay.

A Comparison Study of Extrapolation Models and Empirical Relations in Forecasting Solar Wind

Coronal mass ejections and high speed solar streams serve as perturbations to the background solar wind that have major implications in space weather dynamics. Therefore, a robust framework for accurate predictions of the background wind properties is a fundamental step toward the development of any space weather prediction toolbox. In this pilot study, we focus on the implementation and comparison of various models that are critical for a steady state, solar wind forecasting framework. Specifically, we perform case studies on Carrington rotations 2,053, 2,082, and 2,104, and compare the performance of magnetic field extrapolation models in conjunction with velocity empirical formulations to predict solar wind properties at Lagrangian point L1. Two different models to extrapolate the solar wind from the coronal domain to the inner-heliospheric domain are presented, namely, a) Kinematics based [Heliospheric Upwind eXtrapolation (HUX)] model, and b) Physics based model. The physics based model solves a set of conservative equations of hydrodynamics using the PLUTO code and can additionally predict the thermal properties of solar wind. The assessment in predicting solar wind parameters of the different models is quantified through statistical measures. We further extend this developed framework to also assess the polarity of inter-planetary magnetic field at L1. Our best models for the case of CR2053 gives a very high correlation coefficient (∼0.73–0.81) and has an root mean square error of (∼75–90 km s−1). Additionally, the physics based model has a standard deviation comparable with that obtained from the hourly OMNI solar wind data and also produces a considerable match with observed solar wind proton temperatures measured at L1 from the same database.

North‐South Asymmetric Nightside Distorted Transpolar Arcs Within A Framework of Deformed Magnetosphere‐Ionosphere Coupling: IMF‐By Dependence, Ionospheric Currents, and Magnetotail Reconnection

AbstractThe terrestrial magnetosphere is perpetually exposed to and significantly deformed by the interplanetary magnetic field (IMF) in the solar wind. This deformation is typically detected at discrete locations by space‐ and ground‐based observations. Earth's aurora, on the other hand, is a globally distributed phenomenon that may be used to elucidate magnetospheric deformations caused by IMF variations, as well as plasma supply from the deformed magnetotail to the high‐latitude atmosphere. We report the utilization of an auroral form known as the transpolar arc (TPA) to diagnose the plasma dynamics of the globally deformed magnetosphere. Nine TPAs examined in this study have two types of a newly identified morphology, which are designated as “J”‐ and “L”‐shaped TPAs from their shapes and are shown to have antisymmetric morphologies in the Northern Hemisphere and Southern Hemisphere, depending on the IMF polarity. The TPA‐associated ionospheric current profiles suggest that electric currents flowing along the magnetic field lines (field‐aligned currents [FACs]), connecting the magnetotail and the ionosphere, may be related to the “J”‐ and “L”‐shaped TPA formations. The FACs can be generated by velocity shear between fast plasma flows associated with nightside magnetic reconnection and slower background magnetotail plasma flows. Complex large‐scale TPA FAC structures, previously unraveled by an magnetohydrodynamic (MHD) simulation, cannot be elucidated by our observations. However, our interpretation of TPA features in a global context facilitates the usage of TPA as a diagnostic tool to effectively remote sense globally deformed terrestrial and planetary magnetospheric processes in response to the IMF and solar wind plasma conditions.

The Interconnection between the Periodicities of Solar Wind Parameters Based on the Interplanetary Magnetic Field Polarity (1967–2018): A Cross Wavelet Analysis

In this article, we have examined the possible interconnection and phase asynchrony between the periodicities of solar wind parameters (solar wind velocity $V$ , plasma density $N$ , and dynamic pressure $P$ ), and the interplanetary magnetic field (IMF) magnitude $B$ , taking into account the polarity state, toward ( $T$ ) and away ( $A$ ) from the Sun, of the IMF. For this purpose, the daily measurements of $V$ , $N$ , $P$ , and $B $ during the period 1967–2018, thus covering almost five solar cycles, were sorted into two groups according to the IMF sense. Days of mixed IMF polarity or having no data were eliminated from our analysis during the entire period. The monthly averages of $T$ and $A$ polarity groups have been calculated for each parameter. $B_{\mathrm{T}}$ ( $B_{\mathrm{A}}$ ), $V_{\mathrm{T}}$ ( $V_{\mathrm{A}}$ ), $N_{\mathrm{T}}$ ( $N_{\mathrm{A}}$ ), and $P_{\mathrm{T}}$ ( $P_{\mathrm{A}}$ ), refer to the monthly averages of $T$ ( $A$ ) polarity group for $B$ , $V$ , $N$ , and $P$ , respectively. In this study, three techniques were applied, including the running correlation analysis, cross-wavelet transform (XWT) and wavelet coherence (WTC) to the monthly means of each polarity group. Our results reveal that the running correlation coefficient for toward and away groups of each parameter reflect a positive correlation value during most of the entire period. The long-term periodicities within the 8–16 yr band are also detected for all toward and away components of the considered parameters with a significant common power during most of the entire period. Moreover, the WTC reflects an almost in-phase relationship behavior between the toward and away groups of each parameter during this range. On the other hand, within the 4–8 yr, 2–4 yr, and 1–2 yr bands, the toward and away groups of each solar wind parameter show mid-term periodicities at different periods, displaying sometimes significant powers.

Data Assimilative Optimization of WSA Source Surface and Interface Radii using Particle Filtering

AbstractThe Wang‐Sheeley‐Arge (WSA) model estimates solar wind speed and interplanetary magnetic field polarity in the inner heliosphere using global photospheric magnetic field maps. WSA employs the Potential Field Source Surface (PFSS) and Schatten Current Sheet (SCS) models to determine the Sun's global coronal magnetic field. The PFSS and SCS models are connected through two radial parameters, the source surface and interface radii, which specify the overlap region between the inner SCS and outer PFSS models. Though both radii values are adjustable, they have typically been fixed to 2.5 solar radii. Our work highlights how solar wind predictions improve when the radii are allowed to vary over time. Data assimilation using particle filtering (sequential Monte Carlo) is used to infer optimal values over a fixed time window. Solar wind model predictions and satellite observations are compared with a newly developed quality‐of‐agreement prediction metric. The agreement metric between the model and observations is assumed to correspond to the probability of the two key WSA model parameters, the source surface and interface radii, where the highest metric value implies the optimal radii. We find that the optimal particle filter values of solar radii can perform twice as well as standard values for an exploratory period during Carrington Rotation 1901, with these values also reducing nonphysical kinking effects seen in solar magnetic field lines. Data assimilation choices of input realization and time frame have implications for variation in the solar wind over time. We present this work's theoretical context and practical applications for prediction accuracy.

Galactic cosmic ray transport in the heliosphere: Study with muon data

We use the galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) data of Nagoya vertical muon telescope (NagV-MT) to compute the ratio α of GCR free paths normal and parallel to mean interplanetary magnetic field (IMF) B. The values are compared to those computed earlier for the neutron monitors (NMs) of the global network for three sunspot cycles (21–23) and parts of the other two (19, 24). Current GCR modulation theories do not provide any guidance for α, its rigidity and solar cycle dependence. We use a flat heliosphere current sheet and the Master equations derived by Ahluwalia and Dorman [1997] for computing α and study its rigidity dependence and correlation with solar activity for positive (p) and negative (n) polarity intervals of B at 1 AU; B points outward/ inward from the sun during p/n interval in north hemisphere; IMF polarity changes sign near SSN maximum. For NagV-MT (Rm = 60 GV), Rm being the median value of response to GCR differential rigidity spectrum, we find that α is high for p- and low for n-intervals, compared to being nearly flat for NMs; α for NM2 (Rm = 25 GV) is larger than that for NM1 (Rm = 15 GV), implying an increase in α with GCR rigidity. The α values for NagV-MT are low near SSN maximum as is the case for NMs and high near SSN minimum, when effective limiting GCR rigidity (Rc) for SDA is known to be lower than 100 GV. The results challenge the validity of current conceptual understanding of GCR modulation processes in the heliosphere, pointing to a great need to develop an acceptable theory to explain the empirical results reported in this paper.

Assessment of CESE-HLLD ambient solar wind model results using multipoint observation

For a three-dimensional magnetohydrodynamics solar wind model, it is necessary to carry out assessment studies to reveal its ability and limitation. In this paper, the ambient solar wind results of year 2008 generated by the CESE-HLLD 3D MHD model are compared with multipoint in-situ measurements during the late declining phase of solar cycle 23. The near-ecliptic results are assessed both quantitatively and qualitatively by comparing with in-situ data obtained at the L1 point and by the twin STEREO spacecraft. The assessment reveals the model’s ability in reproducing the time series and statistical characteristics of solar wind parameters, and in catching the change of interplanetary magnetic field polarity and the occurrence of the stream interaction regions. We find that the two-stream structure observed near the ecliptic plane is reproduced, but the differences among observations at L1 and the twin STEREO spacecraft are not caught by the model. The latitudinal variation of the results is assessed by comparing with the Ulysses observation. The characters of variation in different latitudinal ranges are duplicated by the model, but biases of the results are seen, and the boundary layers between fast and slow solar wind are sometimes thicker than observation.

High‐Speed Solar Wind Streams in 2007–2008: Turning on the Russell‐McPherron Effect

AbstractTwo sequences of five high‐speed solar wind stream/corotating interaction region (HSS/CIR) events were observed at 1 AU in December 2007–2008. These two HSS/CIR sequences had opposite magnetic polarities, and they originated from two persistent low‐latitude coronal holes with corresponding polarities. Each HSS/CIR event triggered a geomagnetic storm and strong high‐latitude activity. We follow the evolution of the properties and geomagnetic effects of the two sequences and find that the sequence with negative interplanetary magnetic field polarity (toward sector) develops systematically a more negative Bz(GSM) component and becomes relatively more geoeffective when moving from winter solstice in 2007 to spring equinox in 2008. On the other hand, the sequence with positive polarity (away sector) develops systematically a less negative Bz(GSM) component and becomes relatively less geoeffective. These changes allow the first detailed monitoring of the turning on of the Russell‐McPherron effect when moving from solstice to equinox and the development of the related changes in high‐latitude geomagnetic activity and geomagnetic storms.