Heliospheric Magnetic Field Research Articles

27-day variation of galactic cosmic ray intensity by PAMELA experiment. Relationship with heliospheric parameters

We analyzed the dynamics of the 27-day variation of galactic cosmic rays (GCR) using PAMELA data in the solar cycle 24 covering the period of 2006-2015. We studied the time line of the proton and helium spectra for different rigidities measured by PAMELA and compared with neutron monitor (NM) observations. We studied the relationship between the 27-day variations of GCR and heliospheric parameters: solar wind velocity and heliospheric magnetic field (HMF). We found that the 27-day variation observed by PAMELA was changing over the solar cycle 24. Near the solar minimum in the 2007-2008 it was observed the stationary 27-day variation with stable amplitude and time of maximum for almost ∼8 solar rotations. Solar wind velocity and Bx component of the HMF also showed the same clear 27-day periodicity. The recurrent changes of solar wind velocity were in strong antiphase with changes of GCR. In contrary in the rising phase of the solar cycle 24 the period of quasi-recurrent GCR variations was much more variable in time and in duration, in average 27-days, but also with the strong contamination of the higher harmonics. The antiphase relation between GCR and solar wind is not so clearly pronounced. Moreover, in the rising phase of the solar cycle 24 we established in all analyzed parameters some wave packages, periodically enhanced, taking place averagely each 1.5-2 years, suggesting the occurrence of the quasi biannual oscillations (QBOs).

Long‐Term and Solar Cycle Variation of Galactic Cosmic Rays: Evidence for Variable Heliospheric Turbulence

AbstractThe Sun modulates the flux of galactic cosmic rays (GCR) reaching the Earth's orbit. GCR flux has been measured by ground‐based neutron monitors (NMs) for several decades, which provides an interesting long‐term monitor of solar activity and the heliospheric magnetic field. Here we study the long‐term evolution of the power spectrum of GCR over the last six solar cycles, using the power law slope in the frequency range 5.56·10−6 to 2.14·10−6 Hz (between 50 and 130 hr). We use data from 31 neutron monitors during 1953–2016. We show that the power law slopes vary within the solar cycle, with a Kolmogorov‐type slope observed at solar minimum and a random‐walk‐type slope observed at solar maximum. This implies that the different conditions in the different phases of the solar cycle affect the scaling properties of heliospheric turbulence and, thereby, cosmic ray variability.

On Contribution of Poloidal Branch of Solar Activity to Heliosphere and GCR Modulation

The point of view is substantiated that during periods of low sunspot activity many heliospheric characteristics (the solar wind velocity, the form of the heliospheric current sheet, the strength of the heliospheric magnetic field) can be considered as the manifestations in the heliosphere of the poloidal branch of solar activity. As these characteristics strongly influence the process of the GCR transport through the heliosphere the contribution of the poloidal branch of solar activity to the GCR solar modulation should also be substantial. This conclusion is at odds with the wide-spread belief that the modulated GCR intensity is mainly determined by the sunspot cycle.

All-sky Measurement of the Anisotropy of Cosmic Rays at 10 TeV and Mapping of the Local Interstellar Magnetic Field

Abstract We present the first full-sky analysis of the cosmic ray arrival direction distribution with data collected by the High-Altitude Water Cherenkov and IceCube observatories in the northern and southern hemispheres at the same median primary particle energy of 10 TeV. The combined sky map and angular power spectrum largely eliminate biases that result from partial sky coverage and present a key to probe into the propagation properties of TeV cosmic rays through our local interstellar medium and the interaction between the interstellar and heliospheric magnetic fields. From the map, we determine the horizontal dipole components of the anisotropy δ 0h = 9.16 × 10−4 and δ 6h = 7.25 × 10−4 (±0.04 × 10−4). In addition, we infer the direction (229.°2 ± 3.°5 R.A., 11.°4 ± 3.°0 decl.) of the interstellar magnetic field from the boundary between large-scale excess and deficit regions from which we estimate the missing corresponding vertical dipole component of the large-scale anisotropy to be .

Permutation Entropy Analysis of Magnetic Field Turbulence at 1AU Revisited

AbstractPermutation entropy analysis is a relatively recent addition to the palette of techniques used to study spacecraft observations of heliospheric magnetic field fluctuations at Earth, allowing one to characterize the underlying processes driving the observed fluctuations. This study investigates the effects of data averaging, data gaps, and underlying periodicities on the results of such an analysis, utilizing synthetic data sets and thereby developing efficient treatments for each of these effects. Furthermore, WIND spacecraft observations are employed to validate the results of an earlier permutation entropy analysis by Weck et al. (2015, https://doi.org/10.1103/PhysRevE.91.023101), confirming the results of that study and showing how dat averaging effects can significantly affect the results so acquired. Lastly, as a novel application of this technique, Advanced Composition Explorer spacecraft data taken from 1998 to 2008 are analyzed to investigate whether the permutation entropy so calculated displays a solar cycle dependence. It is shown that although solar cycle dependencies have been reported for observed turbulence quantities such as the magnetic variance, there is no significant dependence discernible in the permutation entropy, and therefore in the underlying processes driving the turbulence.

Generation of Inverted Heliospheric Magnetic Flux by Coronal Loop Opening and Slow Solar Wind Release

Abstract In situ spacecraft observations provide much-needed constraints on theories of solar wind formation and release, particularly the highly variable slow solar wind, which dominates near-Earth space. Previous studies have shown an association between local inversions in the heliospheric magnetic field (HMF) and solar wind released from the vicinity of magnetically closed coronal structures. We here show that in situ properties of inverted HMF are consistent with the same hot coronal source regions as the slow solar wind. We propose that inverted HMF is produced by solar wind speed shear, which results from interchange reconnection between a coronal loop and open flux tube, and introduces a pattern of fast–slow–fast wind along a given HMF flux tube. This same loop-opening process is thought to be central to slow solar wind formation. The upcoming Parker Solar Probe and Solar Orbiter missions provide a unique opportunity to directly observe these processes and thus determine the origin of the slow solar wind.

The association of the Hale sector boundary with RHESSI solar flares and active longitudes

Context. The heliospheric magnetic field (HMF) is structured into large sectors of positive and negative polarity. The parts of the boundary between these sectors where the change in polarity matches that of the leading-to-following sunspot polarity in that solar hemisphere, are called Hale sector boundaries (HSB). Aims. We investigate the flare occurrence rate near HSBs and the association between HSBs and active longitudes. Methods. Previous work determined the times HSBs were at solar central meridian, using the detection of the HMF sector boundary crossing at the Earth. In addition to this, we use a new approach which finds the HSB locations at all times by determining them from potential field source surface (PFSS) extrapolations of photospheric magnetograms. We use the RHESSI X-ray flare list for comparison to the HSB as it provides accurate flare locations over 14 years, from February 2002 to February 2016, covering both Cycles 23 and 24. For the active longitude positions we use previously published work based on sunspot observations. Results. We find that the two methods of determining the HSB generally agree and that 41% (Cycle 23) and 47% (Cycle 24) of RHESSI flares occur within 30° of the PFSS determined-HSB. The behaviour of the HSBs varies over the two Cycles studied, and as expected they swap in hemisphere as the Cycles change. The HSBs and active longitudes do overlap but not consistently. They often move at different rates relative to each other (and the Carrington solar rotation rate) and these vary over each Cycle. The HSBs provide a useful additional activity indicator, particularly during periods when active longitudes are difficult to determine.

Southward shift of the coronal neutral line and the heliospheric current sheet: Evidence for radial evolution of hemispheric asymmetry

Aims. The heliospheric current sheet (HCS) has been observed to be southward shifted in the late declining to minimum phase of the solar cycle. Here we study the existence of a simultaneous shift in the heliosphere and in the corona using a robust new method. Methods. We use the synoptic maps of the photospheric field of the Wilcox Solar Observatory (WSO) and the Mount Wilson Observatory (MWO) together with the potential field source surface (PFSS) model to calculate the coronal magnetic field and compare it with the simultaneous heliospheric magnetic field of the NASA/NSSDC OMNI 2 dataset. We divide the magnetic field into the two sectors, towards (T) and away (A) from the Sun, and calculate how often the sector polarities at 1 AU and in the corona match each other. We divide the sectors both at 1 AU and in the corona. We also calculate the annual (T − A)/(T + A) ratios of sector occurrence both at 1 AU and in the corona. Results. We verify that the HCS/neutral line is southward shifted both in the corona and heliosphere. We find that the coronal shift is systematically larger than the simultaneous heliospheric shift. Conclusions. The fact that the southward shift of the coronal neutral line is larger than the simultaneous shift of the heliospheric current sheet at 1 AU implies that the radial evolution of the magnetic field between the two sites is different between the northern and southern hemispheres.

Global structure of the heliosphere: 3D kinetic-MHD model and the interpretation of spacecraft data

This paper is a brief overview of research into the interaction between the solar wind and local interstellar medium. This interaction determines the global structure of the heliosphere (the region occupied by the solar wind) and has a complex multicomponent character. We describe the three-dimensional kinetic-magnetohydrodynamic model of the interaction, which includes plasma and neutral components of the interstellar medium and solar wind, the heliospheric and interstellar magnetic fields, and the latitudinal and temporal variations of solar wind parameters. In describing the results, magnetic-field-related effects are given special attention, in particular, the magnetic-field-driven plasma depletion in the vicinity of the heliopause. The model explains a sufficiently large body of data from Voyager-1 and Voyager-2, the Interstellar Boundary Explorer (IBEX), the Solar and Heliospheric Observatory (SOHO), the Hubble Space Telescope (HST), and other spacecraft. Based on the experimental data and using the model, the parameters of the interstellar medium and of the interstellar magnetic field are determined. It is shown, however, that a single model using a single set of boundary conditions cannot self-consistently explain all the data available. The Voyager-1 data on the scattered solar Lyman-alpha radiation is taken as an example to illustrate the difficulties that arise.

Modulation of Galactic Cosmic Rays in the Inner Heliosphere over Solar Cycles

Abstract The 11- and 22-year modulation of galactic cosmic rays (GCRs) in the inner heliosphere is studied using a numerical model developed by Qin and Shen in 2017. Based on the numerical solutions of Parker’s transport equations, the model incorporates a modified Parker heliospheric magnetic field, a locally static time-delayed heliosphere, and a time-dependent diffusion coefficients model in which an analytical expression of the variation of magnetic turbulence magnitude throughout the inner heliosphere is applied. Furthermore, during solar maximum, the solar magnetic polarity is determined randomly with the possibility of A > 0 decided by the percentage of the solar north polar magnetic field being outward and the solar south polar magnetic field being inward. The computed results are compared at various energies with several GCR observations, e.g., the Interplanetary Monitoring Platform 8 (IMP 8), EPHIN on board the Solar and Heliospheric Observatory (SOHO), Ulysses, and Voyager 1 and 2, and they show good agreement. We show that our model has successfully reproduced the 11- and 22-year modulation cycles.

Long-term variations in the heliosphere

AbstractReconstructions of long-term solar variability underpin our understanding of the solar dynamo, potential tropospheric climate implications and future space weather scenarios. Prior to direct spacecraft measurements of the heliospheric magnetic field (HMF) and solar wind, accurate annual reconstructions are possible using geomagnetic and sunspot records. On longer timescales, information about the HMF can be extracted from cosmogenic radionuclide records, particularly 14C in ancient trees and 10Be in ice sheets. These proxies, and what they reveal about the HMF and solar wind, are briefly reviewed here.

Long-term evolution of the heliospheric magnetic field inferred from cosmogenic 44Ti activity in meteorites

Typical reconstructions of historic heliospheric magnetic field (HMF) BHMF are based on the analysis of the sunspot activity, geomagnetic data or on measurement of cosmogenic isotopes stored in terrestrial reservoirs like trees (14C) and ice cores (10Be). The various reconstructions of BHMF are however discordant both in strength and trend. Cosmogenic isotopes, which are produced by galactic cosmic rays impacting on meteoroids and whose production rate is modulated by the varying HMF convected outward by the solar wind, may offer an alternative tool for the investigation of the HMF in the past centuries. In this work, we aim to evaluate the long-term evolution of BHMF over a period covering the past twenty-two solar cycles by using measurements of the cosmogenic 44Ti activity (τ1∕2 = 59.2 ± 0.6 yr) measured in 20 meteorites which fell between 1766 and 2001. Within the given uncertainties, our result is compatible with a HMF increase from 4.87-0.30+0.24 nT in 1766 to 6.83-0.11+0.13 nT in 2001, thus implying an overall average increment of 1.96-0.35+0.43 nT over 235 years since 1766 reflecting the modern Grand maximum. The BHMF trend thus obtained is then compared with the most recent reconstructions of the near-Earth HMF strength based on geomagnetic, sunspot number, and cosmogenic isotope data.

What can the annual 10Be solar activity reconstructions tell us about historic space weather?

Context: Cosmogenic isotopes provide useful estimates of past solar magnetic activity, constraining past space climate with reasonable uncertainty. Much less is known about past space weather conditions. Recent advances in the analysis of 10Be by McCracken & Beer (2015, Sol Phys 290: 305–3069) (MB15) suggest that annually resolved 10Be can be significantly affected by solar energetic particle (SEP) fluxes. This poses a problem, and presents an opportunity, as the accurate quantification of past solar magnetic activity requires the SEP effects to be determined and isolated, whilst doing so might provide a valuable record of past SEP fluxes. Aims: We compare the MB15 reconstruction of the heliospheric magnetic field (HMF), with two independent estimates of the HMF derived from sunspot records and geomagnetic variability. We aim to quantify the differences between the HMF reconstructions, and speculate on the origin of these differences. We test whether the differences between the reconstructions appear to depend on known significant space weather events. Methods: We analyse the distributions of the differences between the HMF reconstructions. We consider how the differences vary as a function of solar cycle phase, and, using a Kolmogorov-Smirnov test, we compare the distributions under the two conditions of whether or not large space weather events were known to have occurred. Results: We find that the MB15 reconstructions are generally marginally smaller in magnitude than the sunspot and geomagnetic HMF reconstructions. This bias varies as a function of solar cycle phase, and is largest in the declining phase of the solar cycle. We find that MB15's excision of the years with very large ground level enhancement (GLE) improves the agreement of the 10Be HMF estimate with the sunspot and geomagnetic reconstructions. We find no statistical evidence that GLEs, in general, affect the MB15 reconstruction, but this analysis is limited by having too few samples. We do find evidence that the MB15 reconstructions appear statistically different in years with great geomagnetic storms.

Interplanetary Magnetic Field Turbulence and Rigidity Spectrum of the Galactic Cosmic Rays Intensity Variation (1969–2011)

AbstractThe present study investigates the relation between the rigidity R spectrum exponent γ of the galactic cosmic rays (GCR) flux changes (δD(R)/D(R) ∝ R−γ) and the exponents νy and νz of the power spectral density of the By and Bz components of the heliospheric magnetic field (HMF) fluctuations. Two parameters have been introduced: (1) the exponent γ demonstrating the rigidity dependence of long‐term changes of GCR flux on solar activity (SA) and (2) the exponents νy and νz signifying the state of turbulence which is divided of the By and Bz components. This study considers the period of time between 1969 and 2011 divided into four subperiods, according to the solar global magnetic field polarities: (I) 1969–1979, (A > 0) (positive polarity magnetic field lines are directed away from the Sun's northern hemisphere); (II) 1980–1988, (A < 0) (negative polarity magnetic field lines are directed toward the Sun's northern hemisphere); (III) 1991–2001, (A > 0); and (IV) 2002–2011, (A < 0). The paper finds that temporal changes of the resonant frequency (fres) of GCR protons in heliospheric magnetic field turbulence in relation to SA are mainly caused by variations of the induction of B components from year to year, while the contribution of the solar wind speed remains insignificant. The analyses of the four subperiods show that γ increases and νy and νz decrease in the periods of increased SA, and conversely, the γ decreases and νy and νz increase in the periods of reduced SA. The study concludes that the exponents γ, and νy and νz can be considered as important indices (proxies) for research on GCR transport in the heliosphere.

Quasi-periodic changes in the 3D solar anisotropy of Galactic cosmic rays for 1965–2014

Aims. We study features of the 3D solar anisotropy of Galactic cosmic rays (GCR) for 1965−2014 (almost five solar cycles, cycles 20−24). We analyze the 27-day variations of the 2D GCR anisotropy in the ecliptic plane and the north-south anisotropy normal to the ecliptic plane. We study the dependence of the 27-day variation of the 3D GCR anisotropy on the solar cycle and solar magnetic cycle. We demonstrate that the 27-day variations of the GCR intensity and anisotropy can be used as an important tool to study solar wind, solar activity, and heliosphere. Methods. We used the components Ar, Aϕ and At of the 3D GCR anisotropy that were found based on hourly data of neutron monitors (NMs) and muon telescopes (MTs) using the harmonic analyses and spectrographic methods. We corrected the 2D diurnal (~24-h) variation of the GCR intensity for the influence of the Earth magnetic field. We derived the north-south component of the GCR anisotropy based on the GG index, which is calculated as the difference in GCR intensities of the Nagoya multidirectional MTs. Results. We show that the behavior of the 27-day variation of the 3D anisotropy verifies a stable long-lived active heliolongitude on the Sun. This illustrates the usefulness of the 27-day variation of the GCR anisotropy as a unique proxy to study solar wind, solar activity, and heliosphere. We distinguish a tendency of the 22-yr changes in amplitude of the 27-day variation of the 2D anisotropy that is connected with the solar magnetic cycle. We demonstrate that the amplitudes of the 27-day variation of the north-south component of the anisotropy vary with the 11-yr solar cycle, but a dependence of the solar magnetic polarity can hardly be recognized. We show that the 27-day recurrences of the GG index and the At component are highly positively correlated, and both are highly correlated with the By component of the heliospheric magnetic field.

Sunward Strahl: A Method to Unambiguously Determine Open Solar Flux from In Situ Spacecraft Measurements Using Suprathermal Electron Data

AbstractA fraction of the magnetic flux which threads the photosphere reaches sufficient coronal altitude to be dragged out by the solar wind and form the heliospheric magnetic field (HMF). Directly measuring this “open solar flux” (OSF) component, however, is difficult. While OSF can be extrapolated from photospheric magnetic field measurements, the most direct method is from in situ spacecraft measurements of the HMF. The difficultly is unambiguously distinguishing between HMF which connects directly back to the Sun (the OSF) and that which is locally distorted by waves, turbulence, and near‐Sun reconnection. Suitable temporal filtering of the data can remove such “noise,” but the level of filtering cannot be known a priori and varies with solar cycle, solar wind types, etc. Here we use the suprathermal electron beam, or “strahl,” to distinguish between different HMF topologies. As strahl moves antisunward on global scales, times when strahl is observed to be moving sunward indicate that the HMF is locally inverted. By subtracting the inverted HMF, we compute the OSF without need for arbitrary filtering of the data. We find that the OSF obtained in this manner is slightly larger than the proposed “kinematic correction” based on observed solar wind velocity structure, though in general agreement. Our new OSF estimate agrees with methods based wholly on HMF data, if the data are first used to compute approximately 1 day averages during solar minimum and approximately 3 day averages during solar maximum, stressing the point that the filter method is unreliable because the required characteristics vary.

Structure of the Photospheric Magnetic Field During Sector Crossings of the Heliospheric Magnetic Field

The photospheric magnetic field is the source of the coronal and heliospheric magnetic fields (HMF), but their mutual correspondence is non-trivial and depends on the phase of the solar cycle. The photospheric field during the HMF sector crossings observed at 1 AU has been found to contain enhanced field intensities and definite polarity ordering, forming regions called Hale boundaries. Here we separately study the structure of the photospheric field during the HMF sector crossings during Solar Cycles 21 – 24 for the four phases of each solar cycle. We use a refined version of Svalgaard’s list of major HMF sector crossings, mapped to the Sun using the solar wind speed observed at Earth, and the daily level-3 magnetograms of the photospheric field measured at the Wilcox Solar Observatory in 1976 – 2016. We find that the structure of the photospheric field corresponding to the HMF sector crossings and the existence and properties of the corresponding Hale bipolar regions varies significantly with solar cycle, solar cycle phase, and hemisphere. The Hale boundaries in more than half of the ascending, maximum, and declining phases are clear and statistically significant. The clearest Hale boundaries are found during the (+,−) HMF crossings in the northern hemisphere of odd Cycles 21 and 23, but less systematical during the (+,−) crossings in the southern hemisphere of even Cycles 22 and 24. No similar difference between odd and even cycles is found for the (−,+) crossings. This shows that the northern hemisphere has a more organized Hale pattern overall. The photospheric field distribution also depicts a larger area for the field of the northern hemisphere during the declining and minimum phases, in a good agreement with the bashful ballerina phenomenon.

The Open Flux Problem

Abstract The heliospheric magnetic field is of pivotal importance in solar and space physics. The field is rooted in the Sun’s photosphere, where it has been observed for many years. Global maps of the solar magnetic field based on full-disk magnetograms are commonly used as boundary conditions for coronal and solar wind models. Two primary observational constraints on the models are (1) the open field regions in the model should approximately correspond to coronal holes (CHs) observed in emission and (2) the magnitude of the open magnetic flux in the model should match that inferred from in situ spacecraft measurements. In this study, we calculate both magnetohydrodynamic and potential field source surface solutions using 14 different magnetic maps produced from five different types of observatory magnetograms, for the time period surrounding 2010 July. We have found that for all of the model/map combinations, models that have CH areas close to observations underestimate the interplanetary magnetic flux, or, conversely, for models to match the interplanetary flux, the modeled open field regions are larger than CHs observed in EUV emission. In an alternative approach, we estimate the open magnetic flux entirely from solar observations by combining automatically detected CHs for Carrington rotation 2098 with observatory synoptic magnetic maps. This approach also underestimates the interplanetary magnetic flux. Our results imply that either typical observatory maps underestimate the Sun’s magnetic flux, or a significant portion of the open magnetic flux is not rooted in regions that are obviously dark in EUV and X-ray emission.

The induced surface electric response in Europe to 2015 St. Patrick's Day geomagnetic storm

The largest geomagnetic storm in solar cycle 24 is investigated from the dual perspective of (1) solar source – solar wind – geomagnetic storm chain and (2) the hazardous induced response as shown by the surface electric field. The study is based on heliospheric magnetic field and solar wind data, on one hand, and on recorded 1-min geomagnetic data from the INTERMAGNET network of European geomagnetic observatories, on the other. The lateral distribution of the induced electric response for the European continent during the considered storm is shown, and the timing of the maximum induced horizontal geoelectric vector is discussed in terms of specific moments of the storm development.

Modulation of galactic cosmic rays in solar cycles 22–24: Analysis and physical interpretation

This work represents a physical interpretation of cosmic ray modulation in the 22nd–24th solar cycles, including an interpretation of an unusual behavior of their intensity in the last minimum of the solar activity (2008–2010). In terms of the Parker modulation model, which deals with regularly measured heliospheric characteristics, it is shown that the determining factor of the increased intensity of the galactic cosmic rays in the minimum of the 24th solar cycle is an anomalous reduction of the heliospheric magnetic field strength during this time interval under the additional influence of the solar wind velocity and the tilt angle of the heliospheric current sheet. We have used in the calculations the dependence of the diffusion tensor on the rigidity in the form K ij ∝ R 2−μ with μ = 1.2 in the sector zones of the heliospheric magnetic field and with μ = 0.8 outside the sector zones, which leads to an additional amplification of the diffusion mechanism of cosmic ray modulation. The proposed approach allows us to describe quite satisfactorily the integral intensity of protons with an energy above 0.1 GeV and the energy spectra in the minima of the 22nd–24th solar cycles at the same value of the free parameter. The determining factor of the anomalously high level of the galactic cosmic ray intensity in the minimum of the 24th solar cycle is the significant reduction of the heliospheric magnetic field strength during this time interval. The forecast of the intensity level in the minimum of the 25th solar cycle is provided.