Heliospheric Magnetic Field Polarity Research Articles

Solar modulation of cosmic proton and helium with AMS-02

We investigate the solar modulation effect with the long-time cosmic ray (CR) proton and helium spectra measured by AMS-02 on the timescale of a Bartels rotation (27 days) between May 2011 and May 2017. The time covers the negative heliospheric magnetic field polarity cycle, the polarity reversal period, and the positive polarity cycle. The precise AMS-02 data provide an excellent opportunity to improve the understanding of the time-dependent solar modulation effect. In this work, a two-dimensional solar modulation model is used to compute the propagation of cosmic rays in the heliosphere. The CR propagation equation is numerically solved by the public code solarprop. We find that the drift effect is suppressed during the high solar activity period for a long time but nearly recovered in the first half of 2017. The time-dependent rigidity dependence of the mean free path is critical to reproduce the observations during the polarity reversal period. We also confirm that protons and helium have the same diffusive mean free path. The future monthly AMS-02 and PAMELA data will further confirm the vital assumption about the universal mean free path for all species. The monthly antiproton data will be crucial to break the degeneracy between the diffusion and drift effect and determine the level of drift effect during different epochs.

A detailed comparison of techniques used to model drift in numerical cosmic ray modulation models

Drifts have long been known to play a significant role in the transport and modulation of galactic cosmic rays (GCRs), occurring due to gradients in, as well as the curvature of, the heliospheric magnetic field (HMF). GCRs also drift along the wavy current sheet, a structure separating regions of opposite HMF polarity. Furthermore, the influence of drift effects is solar cycle dependent, as can, for example, be seen in the 22-year cycle observed in neutron monitor observations, corresponding to the 22-year HMF polarity cycle. As such, over the previous decades several techniques have been proposed to incorporate these effects into numerical cosmic ray modulation models. The present study compares the effects of employing these techniques, using a 3D, stochastic, ab initio GCR modulation model with a simulated solar cycle dependence. It is found that, for solar minimum conditions, all methods considered yield similar GCR intensities, but that the choice of method becomes significant when heliospheric conditions approaching solar maximum are to be modelled.

Lightning as a space‐weather hazard: UK thunderstorm activity modulated by the passage of the heliospheric current sheet

AbstractLightning flash rates, RL, are modulated by corotating interaction regions (CIRs) and the polarity of the heliospheric magnetic field (HMF) in near‐Earth space. As the HMF polarity reverses at the heliospheric current sheet (HCS), typically within a CIR, these phenomena are likely related. In this study, RL is found to be significantly enhanced at the HCS and at 27 days prior/after. The strength of the enhancement depends on the polarity of the HMF reversal at the HCS. Near‐Earth solar and galactic energetic particle fluxes are also ordered by HMF polarity, though the variations qualitatively differ from RL, with the main increase occurring prior to the HCS crossing. Thus, the CIR effect on lightning is either the result of compression/amplification of the HMF (and its subsequent interaction with the terrestrial system) or that energetic particle preconditioning of the Earth system prior to the HMF polarity change is central to solar wind lightning coupling mechanism.

Modulation of UK lightning by heliospheric magnetic field polarity

Observational studies have reported solar magnetic modulation of terrestrial lightning on a range of time scales, from days to decades. The proposed mechanism is two-step: lightning rates vary with galactic cosmic ray (GCR) flux incident on Earth, either via changes in atmospheric conductivity and/or direct triggering of lightning. GCR flux is, in turn, primarily controlled by the heliospheric magnetic field (HMF) intensity. Consequently, global changes in lightning rates are expected. This study instead considers HMF polarity, which doesnʼt greatly affect total GCR flux. Opposing HMF polarities are, however, associated with a 40–60% difference in observed UK lightning and thunder rates. As HMF polarity skews the terrestrial magnetosphere from its nominal position, this perturbs local ionospheric potential at high latitudes and local exposure to energetic charged particles from the magnetosphere. We speculate as to the mechanism(s) by which this may, in turn, redistribute the global location and/or intensity of thunderstorm activity.

Modeling anomalous cosmic ray oxygen gradients over successive solar cycles

Cosmic ray drift directions depend both on the charge of the particle population under consideration, as well as the heliospheric magnetic field polarity which oscillates within a ∼22 year cycle. The differences in the cosmic ray drift patterns between successive solar cycles manifest themselves in the sign of the latitudinal cosmic ray gradient. For positively charged particles, a positive latitudinal gradient is expected in the qA > 0 polarity cycle, while a negative value is expected in the qA < 0 cycle. For the first time observed radial and latitudinal values for anomalous cosmic ray oxygen are available in both of these cycles. In the present qA < 0 cycle, the observations seem, however, inconsistent with the drift‐dominated considerations discussed above, as the latitudinal gradient is smaller than expected and perhaps even positive. This issue is addressed by using a comprehensive numerical model for anomalous cosmic ray acceleration and transport, based on new diffusion coefficients, to simulate these gradients over the previous two magnetic polarity cycles. Computed gradients and energy spectra (at Earth and at Ulysses) are compared to observations, with good agreement achieved between observations and model predictions. For the qA > 0 cycle, the model predicts a positive latitudinal gradient but for the qA < 0 cycle a latitudinal gradient that is smaller and that can be either positive or negative. It is concluded that drifts and poleward diffusion set up competing latitudinal gradients in the qA < 0 cycle, with the resulting sign of the gradient determined by the most effective of the two processes.

Galactic cosmic ray intensity in the inner and outer heliosphere in the epoch of the solar cycle 24 minimum

The behavior of the observed and expected Galactic cosmic ray (GRC) intensity both near the Earth and in the outer heliosphere is considered. The hypothesis is discussed of the strong dependence of the GCR intensity in the interaction region of the solar and interstellar winds on the heliospheric magnetic field polarity in the epoch of a solar activity minimum.

Observations of recurrent cosmic ray decreases during solar cycles 22 and 23

Abstract. During solar cycle 22, the modulation of several hundred MeV galactic cosmic rays (GCRs) by recurrent and transient cosmic ray decreases was observed by the Ulysses spacecraft on its descent towards the solar south pole. In solar cycle 23, Ulysses repeated this trajectory segment during a similar phase of the solar cycle, but with opposite heliospheric magnetic field polarity. Since cosmic ray propagation in the heliosphere should depend on drift effects, we determine in this study the latitudinal distribution of the amplitude of recurrent cosmic ray decreases in solar cycles 22 and 23. As long as we measure the recurrent plasma structures in situ, we find that these decreases behave nearly the same in both cycles. Measurements in the fast solar wind, however, show differences: in cycle 22 (A>0) the recurrent cosmic ray decreases show a clear maximum near 25° and are still present beyond 40°, whereas we see in cycle 23 (A<0) neither such a pronounced maximum nor significant decreases above 40°. In other words: the periodicity in the cosmic ray intensity, which can be clearly seen in the slow solar wind, appears to vanish there. Theoretical models for drift effects, however, predict quite the opposite behaviour for the two solar cycles. To closer investigate this apparent contradiction, we first put the visual inspection of the data onto a more solid basis by performing a detailed Lomb (spectral) analysis. The next step consists of an analysis of the resulting periodicities at 1 AU in order to distinguish between spatial and temporal variations, so that we can obtain statements about the question in how far there is a correlation between the in-situ data at 1 AU and those measured by Ulysses at larger latitudes. We find a good correlation being present during cycle 22, but not for cycle 23. As one potential explanation for this behaviour, we suggest the difference in the coronal hole structures between the cycles 22 and 23 due to a large, stable coronal hole structure, which is present during cycle 22, but not in cycle 23. We support this possibility by comparing Yohkoh SXT and SOHO EIT maps.

Success rate of predicting the heliospheric magnetic field polarity with Michelson Doppler Imager (MDI) synoptic charts

The photospheric and heliospheric magnetic fields have been continuously observed at the L1 point by the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) and Wind and the Advanced Composition Explorer (ACE) spacecraft since 1996. The combined observations of the photospheric and heliospheric magnetic fields make it possible to more reliably predict the heliospheric magnetic field and more accurately estimate the success rate of the prediction. On the basis of the latitude dependence of the zonal polar field inferred from high‐resolution MDI synoptic charts between 1996 and 2003 we fill in the polar data gaps in the MDI synoptic charts. To assess the influence of synoptic charts from different data sources on the prediction of the coronal and heliospheric magnetic fields, we compare the success rate of the heliospheric magnetic field (HMF) polarity predicted over 107 Carrington rotations from June 1996 to June 2004. We used the potential field source surface model and the synoptic charts from the Kitt Peak National Observatory, MDI, and the Wilcox Solar Observatory. The mean success rate of the HMF polarity predicted using MDI synoptic charts over 8 years is 0.862 ± 0.101, the best among the three photospheric data sources, although the difference among the three sources is small. This result validates the MDI synoptic charts in modeling coronal and heliospheric magnetic fields.

Heliospheric magnetic field polarity inversions driven by radial velocity field structures

Magnetic field polarity inversions embedded in the predominantly unipolar fast solar wind have been observed by the Ulysses spacecraft at high latitudes. Such reversals have the nature of folded back field lines which we suggest are generated by the interaction of standard large amplitude, low frequency, Alfvénic turbulence with velocity shears in the fast solar wind. We present 2D magnetohydrodynamic simulations of a very low frequency and high amplitude Alfvén wave propagating away from the sun embedded in a velocity shear structure such as a microstream and show how reversals in the magnetic field lines are generated naturally on a time‐scale consistent with their observation at Ulysses. The generated magnetic field and plasma signals are similar to those observed. We discuss the role turbulence‐stream shear interactions might play in limiting differential velocities in the asymptotic high speed solar wind.

Modulation of anomalous protons: Effects of different solar wind speed profiles in the heliosheath

Two termination shock acceleration modulation models are used to study the modulation of anomalous protons, in particular the effects of different scenarios for global solar wind speed (V) variations in the heliosheath. The first numerical model simulates a symmetric heliosphere and the second simulates an asymmetric heliosphere with respect to the Sun. The modulation differences between these models are illustrated and discussed. The geometry of the heliosphere in the latter model is deduced from a time‐dependent three‐dimensional hydrodynamic model of the heliosphere which provides the different scenarios for the V‐profiles in the heliosheath. The modulation models include the solar wind termination shock, global drifts, adiabatic energy changes, diffusion, convection, and a heliosheath. The anomalous protons are kinetically described using the Parker transport equation. A solar wind speed decreasing stronger than the generally assumed V ∝ 1/r2 dependence, with r the radial distance from the Sun, is studied as well as an extreme scenario with V ∝ r2. The stronger decrease produces a compressive flow in the heliosheath which results in additional acceleration of anomalous protons in the heliosheath. The solutions are shown for solar minimum and moderate maximum modulation conditions for both heliospheric magnetic field polarity cycles. Significant modulation differences are found to occur between these different scenarios for V in the heliosheath. If the stronger than V ∝ 1/r2 scenarios in the heliosheath are real, the anomalous intensities should increase beyond the TS, which should be measurable by the Voyager 1 spacecraft in the near future.

Modulation of Galactic Protons in an Asymmetrical Heliosphere

A previously used two-dimensional model of the heliospheric modulation of cosmic rays including a solar wind termination shock is extended to include an outer modulation boundary that is asymmetrically shaped with respect to the Sun. The modulation process is described kinetically using the Parker transport equation. The model includes drifts, adiabatic energy changes, diffusion, convection, a solar wind termination shock, and a heliosheath and is used here to compute modulation differences between a symmetrical and an asymmetrical modeled heliosphere, as well as differences between an asymmetrical heliosphere including a termination shock and one without such a shock for Galactic cosmic-ray protons. The solutions are for solar minimum and moderate maximum conditions for both heliospheric magnetic field polarity cycles. It is found that the modulation produced for cosmic-ray protons with an asymmetrical heliospheric model differs from that produced with a symmetrical model, but significantly mostly for the A < 0 polarity cycle, especially in the tail region of the heliosphere. The results of the symmetrical model are, however, surprisingly reasonable for studies of the heliospheric nose region, measured against the asymmetrical model.

Modulation and acceleration of anomalous protons in the outer heliosphere

A comprehensive time-dependent modulation model including particle drifts, enhanced perpendicular diffusion, a solar wind termination shock (TS), and a heliosheath is used to study the acceleration and modulation of anomalous protons in the outer heliosphere. It is shown how the spectral forms of the accelerated anomalous protons at 90 AU change for the two heliospheric magnetic field polarity configurations during solar minimum conditions. Further, it is shown how these shocked spectra are modified and modulated in the outer heliosphere if the compression ratio of the TS was changed. The compression ratio is found to be by far the most dominant modulation factor.

The global heliospheric magnetic field polarity distribution as seen at Ulysses

Abstract. The Ulysses spacecraft is in a near-polar solar orbit with a period of 6.2 years. The heliospheric magnetic field polarity detected by Ulysses from its 1992 Jupiter encounter to the current time is presented, following ballistic mapping of the polarity information to the solar wind source surface, at approximately 2.5 solar radii. The spacecraft’s first foray to polar latitudes and first rapid heliolatitude scan occurred in 1994–1995, near a minimum in solar activity. The heliospheric current sheet during this period was confined to low heliolatitudes. In 2000–2001, Ulysses returned in situ data from the same region of its orbit as in 1994–1995, but near to the maximum in solar activity. Unlike at solar minimum, heliospheric current sheet crossings were detected at the spacecraft over a wide heliolatitude range, which is consistent with the reversal of the solar magnetic dipole occurring during solar maximum. Despite complexity in the solar wind parameters during the latest fast latitude scan (McComas et al., 2002), the underlying magnetic field structure appears consistent with a simple dipole inclined at a large angle to the solar rotational axis. The most recent data show the heliospheric current sheet returning to lower heliolatitudes, indicating that the dipole and rotational axes are realigning, with the Sun’s magnetic polarity having reversed.Key words. Interplanetary physics (interplanetary magnetic fields; sources of the solar wind) – Solar physics, astrophysics and astronomy (magnetic fields)

Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23

The Voyager 1 (V1) observations of the heliospheric magnetic field strength B agree with Parker's model of the global heliospheric magnetic field from 1 to 81.0 AU and from 1978 to 2001.34 when one considers the solar cycle variations in the source magnetic field strength and the latitude/time variation in the solar wind speed. In particular, Parker's model, without adjustable parameters, describes the general tendency for B to decrease with increasing distance R from the Sun, the three broad increases of B around 1980, 1990, and 2000, and the minima of B in 1987 and 1997. During 1987 and 1997, B appears to be lower than Parker's model predicts, but that can be attributed to the presence of a heliospheric vortex street at these times and/or uncertainty in the observations. There is no evidence for a significant flux deficit increasing monotonically from 1 to 81.0 AU. By extrapolating these results and considering the limitations of the observations, V1 should continue to make useful measurements during the next few years at least. The magnetic field polarity in the distant heliosphere at V1 and Voyager 2 (V2) changed during the ascending phase of solar cycle 23. In the Northern Hemisphere, V1 observed a decrease in the percentage of positive polarities from ≈100% during 1997 to ≈50% during 2000. In the Southern Hemisphere, V2 observed the opposite behavior, an increase in the percentage of positive polarities from ≈0% during 1997 to ≈50% during 2000. The variation of magnetic polarity observed by V1 and V2 was caused by the increasing latitudinal width of the sector zone with increasing solar activity, which in turn was related to the increasing maximum latitudinal extent and the decreasing minimum latitudinal extent of the footprints of the heliospheric current sheet (HCS). There was a tendency for the speed and proton temperature to decrease and the density to increase at V2 from 1997 (when it observed flows from polar coronal holes) to 2001 (when it observed more complex and dynamic flows).

Reversal of Heliospheric Magnetic Field Polarity: Theoretical Model

A simple analytical model of the reversal of the heliospheric magnetic field is suggested. The shape of the heliospheric current sheet is found for each instant of time using a kinematic approximation. Calculation results are illustratively presented in graphic and animated forms, showing a 3-D dynamic picture of the reversal of the heliospheric magnetic field throughout a 22-year solar cycle.

Correlation studies of the inclinations of the heliospheric current sheet with: II. 27 day and 13.5 day waves of the cosmic ray intensity

The comparison of the amplitudes of 27-day (Amp27) and 13.5-day (Amp13) cosmic ray waves with the tilts, α, of heliospheric current sheet was studied during the period 1971–1995. Large Amp27 or Amp13 or both were observed following a relatively large excursions in α and large dips in the tilts, near the times of the interplanetary magnetic field (IMF) polarity reversals and also, during the declining phase of the solar activity cycles (SAC). The correlation between Amp13 and α dose not depend on the heliospheric magnetic field polarity, while the correlation and sensitivity of Amp27 to tilt changes are dependent on the solar polar magnetic field. The time variations of the energy spectrum of 27-day and 13.5-day modulations varied with SAC and depend on the inversions of the dipole solar polar magnetic field. The sensitivities of the Amp13 waves to α for both IMF polarity states are rigidity (R) dependent, with a power form of R−1.47±0.2 for the qA<0 and R−0.7±0.15 for the qA>0 epoch (positive solar polarity state). The variations spectra of the 27-day modulations revealed a clear dependence on the IMF polarity state, the spectra are harder during the period of 1981–1989, when qA<0, than during the epoch with qA>0.

Heliospheric magnetic field polarity inversions at high heliographic latitudes

The heliospheric magnetic field over the poles of the sun has been examined to identify intervals when the polarity of the magnetic field deviates significantly from the calculated Parker spiral, using Ulysses observations in 1994 and 1995, near solar minimum. Intervals with deviation &gt; 90° have been identified, corresponding to magnetic field vectors pointing in the hemisphere opposite to that of the dominant polarity in the high speed solar wind originating in the polar coronal holes. The number of inversion periods decreases as the averaging period is increased from 1 to 12 hours. Their distribution was found to vary in time over the northern solar pole, where the average direction also deviated from the Parker direction. The propagation direction of waves during polarity inversions show that these are caused by large‐scale folds in the magnetic field, rather than by opposite polarity magnetic flux tubes originating near the sun.