Bernd Heber

Rigidity dependence of cosmic ray proton latitudinal gradients measured by the Ulysses spacecraft: Implications for the diffusion tensor

The latitudinal gradient of cosmic ray protons observed by Ulysses during September 1994 to July 1995 is small, and it increases as function of rigidity up to ∼ 2 GV and then decreases. Although previous drift models could reproduce the observed small positive gradient for an A > 0 solar polarity cycle, they produced a maximum at a rigidity well below 1 GV, in contrast to the observations. After exploring various options, it turns out that changing the rigidity dependence of the perpendicular diffusion coefficient (DC) in the polar direction so that it differs from that of the parallel DC is the most effective way to obtain good agreement with data. Specifically, we find that this DC must have a flatter rigidity dependence than the parallel DC in order to reproduce the observed rigidity dependence of the latitudinal gradient of protons during an A > 0 solar polarity cycle. We argue that the present study, combined with studies by other authors, suggests that the perpendicular mean-free path of particles with rigidity R ≳ 0.1 GV has at least two distinct components. One is independent of particle rigidity, and one is proportional to the square of the particle rigidity at low rigidity and flattens to become almost independent of it at higher rigidity. We also show an example of the rigidity dependence of this gradient that Ulysses might observe during an A 0 cycle and could impose stricter constraints on the diffusion tensor.

Cosmic Ray and Solar Particle Investigations Over the South Polar Regions of the Sun

Observations of galactic cosmic radiation and anomalous component nuclei with charged particle sensors on the Ulysses spacecraft showed that heliospheric magnetic field structure over the south solar pole does not permit substantially more direct access to the local interstellar cosmic ray spectrum than is possible in the equatorial zone. Fluxes of galactic cosmic rays and the anomalous component increased as a result of latitude gradients by less than 50% from the equator to -80�. Thus, the modulated cosmic ray nucleon, electron, and anomalous component fluxes are nearly spherically symmetric in the inner solar system. The cosmic rays and the anomalous nuclear component underwent a continuous, -26 day recurrent modulation to -80.2�, whereas all recurring magnetic field compressions and recurring streams in the solar wind disappeared above ∼55�S latitude.

The modelling of the latitude dependence of cosmic ray protons and electrons in the inner heliosphere

Abstract Observations during the fast latitude scan of Ulysses show that the latitude dependence of cosmic ray protons is significantly less than predicted by drift models which were based on the Parker geometry for the heliospheric magnetic field (HMF) and diffusion coefficients ∞ 1/Bp. Recent modeling with a modified HMF and enhanced turbulence in the polar zones of the heliosphere can reproduce the observed latitude dependence for protons reasonably well at high rigidities. It is shown here that incorporating enhanced perpendicular diffusion in the polar direction in a drift model also gives a latitude dependence for protons, over a wide range of rigidities, and a latitude dependence for electrons and therefore a charge-dependence that are compatible with observations.

Statistical survey of widely spread out solar electron events observed with STEREO and ACE with special attention to anisotropies

Context. In February 2011, the two STEREO spacecrafts reached a separation of 180 degrees in longitude, offering a complete view of the Sun for the first time ever. When the full Sun surface is visible, source active regions of solar energetic particle (SEP) events can be identified unambiguously. STEREO, in combination with near-Earth observatories such as ACE or SOHO, provides three well separated viewpoints, which build an unprecedented platform from which to investigate the longitudinal variations of SEP events. Aims. We show an ensemble of SEP events that were observed between 2009 and mid-2013 by at least two spacecrafts and show a remarkably wide particle spread in longitude (wide-spread events). The main selection criterion for these events was a longitudinal separation of at least 80 degrees between active region and spacecraft magnetic footpoint for the widest separated spacecraft. We investigate the events statistically in terms of peak intensities, onset delays, and rise times, and determine the spread of the longitudinal events, which is the range filled by SEPs during the events. Energetic electron anisotropies are investigated to distinguish the source and transport mechanisms that lead to the observed wide particle spreads. Methods. According to the anisotropy distributions, we divided the events into three classes depending on different source and transport scenarios. One potential mechanism for wide-spread events is efficient perpendicular transport in the interplanetary medium that competes with another scenario, which is a wide particle spread that occurs close to the Sun. In the latter case, the observations at 1 AU during the early phase of the events are expected to show significant anisotropies because of the wide injection range at the Sun and particle-focusing during the outward propagation, while in the first case only low anisotropies are anticipated. Results. We find events for both of these scenarios in our sample that match the expected observations and even different events that do not agree with the scenarios. We conclude that probably both an extended source region at the Sun and perpendicular transport in the interplanetary medium are involved for most of these wide-spread events.

Temporal and spatial evolution of the solar energetic particle event on 20 January 2005 and resulting radiation doses in aviation

The solar energetic particle event on 20 January 2005 was one of the largest ground level events ever observed. Neutron monitor stations in the Antarctic recorded count rate increases of several thousand percent caused by secondary energetic particles, and it took more than 36 h to return to background level. Such huge increases in high energetic solar cosmic radiation on the ground are obviously accompanied by considerable changes in the radiation environment at aviation altitudes. Measurements of 28 neutron monitor stations were used in this work to numerically approximate the primary solar proton spectra during the first 12 h of the event by minimizing the differences between measurements and the results of Monte-Carlo calculated count rate increases. The primary spectrum of solar energetic protons was approximated by a power law in rigidity and a linear angular distribution. The incoming direction of the solar energetic particles was determined and compared to the interplanetary magnetic field direction during the event. The effects on the radiation exposure at altitudes of about 12 km during that time were estimated to range from none at low latitudes up to almost 2 mSv/h for a very short time in the Antarctic region and about 0.1 mSv/h at high latitudes on the Northern Hemisphere. After 12 h, dose rates were still increased by 50% at latitudes above 60 degrees whereas no increases at all occurred at latitudes below 40 degrees during the whole event.

Spatial variation of >106 Mev proton fluxes observed during the Ulysses rapid latitude scan: Ulysses COSPIN/KET results

The basic physical processes that lead to the long-term modulation of cosmic rays in the heliosphere have been known for many years. However, our knowledge of the relative importance of the various processes is still incomplete. Observations of cosmic rays at high latitudes can be used to improve our understanding of modulation processes. In this paper we present measurements of galactic proton fluxes with energies above 106 MeV made by the Kiel Electron Telescope on board the Ulysses spacecraft during the fast scan from the South polar passage in September 1994 to the North pole in August 1995, under solar minimum conditions. Comparison of proton fluxes at high latitudes and in the ecliptic shows a 20% higher flux in polar regions. The flux increase is not symmetric with respect to the heliographic equator but rather with respect to a surface shifted by 7° South. In such a coordinate system the latitudinal gradient in both hemispheres has a value of (0.33±0.02)%/deg.

A comparative study of cosmic ray radial and latitudinal gradients in the inner and outer heliosphere

The radial and latitudinal intensity gradients of 145–255 MeV/nucleon He, 34–50 MeV/nucleon He and 30–69 MeV H are studied over an extensive range of heliocentric distances and latitudes for the 1993.0–1996.0 time period using data from cosmic ray experiments on the Ulysses, IMP 8, Voyager 1 and 2, and Pioneer 10 spacecraft. The radial gradients are found to decrease rapidly with increasing heliocentric distance and agree with those measured 20 years earlier at a similar phase of the heliomagnetic cycle. The latitudinal gradients measured in the inner and outer heliosphere are in reasonable agreement and positive albeit exceedingly small. In agreement with other Ulysses energetic particle experiments it is found that a shift of heliolatitude by −7° to −10° is necessary to get reasonable symmetry in the measurements at midlatitudes. From the Ulysses data it appears there is a significantly reduced latitudinal variation in the intensity of the three energetic particle components at (magnetic) heliolatitudes above about 50° at this phase of the modulation cycle. Such a reduced entry of cosmic rays over such an extensive area above the solar poles implies a strong modification of the previously assumed cosmic ray transport processes at high latitudes, most probably a considerably increased rate of scattering combined with reduced particle gradient and curvature drifts. A significant higher intensity is observed over the north solar pole than over the south pole for the low-energy components after the corrections have been applied for the temporal changes at the 1-AU baseline.

CIRCUMSOLAR ENERGETIC PARTICLE DISTRIBUTION ON 2011 NOVEMBER 3

Late on 2011 November 3, STEREO-A, STEREO-B, MESSENGER, and near-Earth spacecraft observed an energetic particle flux enhancement. Based on the analysis of in situ plasma and particle observations, their correlation with remote sensing observations, and an interplanetary transport model, we conclude that the particle increases observed at multiple locations had a common single-source active region and the energetic particles filled a very broad region around the Sun. The active region was located at the solar backside (as seen from Earth) and was the source of a large flare, a fast and wide coronal mass ejection, and an EIT wave, accompanied by type II and type III radio emission. In contrast to previous solar energetic particle events showing broad longitudinal spread, this event showed clear particle anisotropies at three widely separated observation points at 1 AU, suggesting direct particle injection close to the magnetic footpoint of each spacecraft, lasting for several hours. We discuss these observations and the possible scenarios explaining the extremely broad particle spread for this event.

Modulation effects of anisotropic perpendicular diffusion on cosmic ray electron intensities in the heliosphere

The modulation of cosmic ray electrons provides a useful tool to study the diffusion tensor applicable to heliospheric modulation. Electron modulation responds directly to the assumed energy dependence of the diffusion coefficients below ∼500 MeV in contrast to protons which experience large adiabatic energy losses below this energy. As a result of this and because drifts become unimportant for electrons at these low energies, conclusions can be made about the appropriate diffusion coefficients. Using a modulation model, we illustrate the role of anisotropic perpendicular diffusion on electron modulation. In general, we find that perpendicular diffusion dominates electron modulation below ∼100 MeV. Enhancing it in the polar direction typically produced an increase in modulation for both the A > O (e.g., ∼1990 to ∼2000) and A < 0 (e.g., ∼1980 to ∼1990) solar magnetic polarity cycles. It also causes the radial dependence of the intensity to become more uniform throughout the heliosphere, and causes a significant reduction in the latitude dependence of the intensities at all radial distances, with the largest effects in the inner heliosphere and at low energies. This agrees with studies of cosmic ray protons, which suggest that perpendicular diffusion enhanced in the polar direction of the heliosphere is required in conventional drift models to explain the small latitudinal gradients observed for protons on board the Ulysses spacecraft. The role of enhanced perpendicular diffusion was further investigated by examining electron modulation as a function of the “tilt angle” α of the wavy current sheet. In general, a reduction occurred between the modulation differences caused by drifts as a function of α for both polarity cycles. This work illustrates that anisotropic perpendicular diffusion has profound effects on the modulation of galactic cosmic ray electrons during both polarity cycles.

Corotating Interaction Regions at high latitudes

Ulysses observed a stable strong CIR from early 1992 through 1994 during its first journey into the southern hemisphere. After the rapid latitude scan in early 1995, Ulysses observed a weaker CIR from early 1996 to mid-1997 in the northern hemisphere as it traveled back to the ecliptic at the orbit of Jupiter. These two CIRs are the observational basis of the investigation into the latitudinal structure of CIRs. The first CIR was caused by an extension of the northern coronal hole into the southern hemisphere during declining solar activity, whereas the second CIR near solar minimum activity was caused by small warps in the streamer belt. The latitudinal structure is described through the presentation of three 26-day periods during the southern CIR. The first at ∼24°S shows the full plasma interaction region including fast and slow wind streams, the compressed shocked flows with embedded stream interface and heliospheric current sheet (HCS), and the forward and reverse shocks with associated accelerated ions and electrons. The second at 40°S exhibits only the reverse shock, accelerated particles, and the 26-day modulation of cosmic rays. The third at 60°S shows only the accelerated particles and modulated cosmic rays. The possible mechanisms for the access of the accelerated particles and the CIR-modulated cosmic rays to high latitudes above the plasma interaction region are presented. They include direct magnetic field connection across latitude due to stochastic field line weaving or to systematic weaving caused by solar differential rotation combined with non-radial expansion of the fast wind. Another possible mechanism is particle diffusion across the average magnetic field, which includes stochastic field line weaving. A constraint on connection to a distant portion of the CIR is energy loss in the solar wind, which is substantial for the relatively slow-moving accelerated ions. Finally, the weaker northern CIR is compared with the southern CIR. It is weak because the inclination of the streamer belt and HCS decreased as Ulysses traveled to lower latitudes so that the spacecraft remained at about the maximum latitudinal extent of the HCS.