Connecting remote and in situ observations of shock-accelerated electrons associated with a coronal mass ejection

Context. One of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun, known as coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism, for example, type II and accompanying herringbone bursts. Aims. In this work, we investigate the acceleration location, escape, and propagation directions of various electron beams in the solar corona and compare them to the arrival of electrons at spacecraft. Methods. To track energetic electron beams, we used a synthesis of remote and direct observations combined with coronal modeling. Remote observations include ground-based radio observations from the Nançay Radioheliograph (NRH) combined with space-based extreme-ultraviolet and white-light observations from Solar Dynamics Observatory (SDO), Solar Terrestrial Relations Observatory (STEREO), and Solar Orbiter (SolO). We also used direct observations of energetic electrons from the STEREO and Wind spacecraft. These observations were then combined with a three-dimensional (3D) representation of the electron acceleration locations, including the results of magneto-hydrodynamic models of the solar corona. This representation was subsequently used to investigate the origin of electrons observed remotely at the Sun and their link to in situ electrons. Results. We observed a type II radio burst followed by herringbone bursts that show single-frequency movement through time in NRH images. The movement of the type II burst and herringbone radio sources seems to be influenced by regions in the corona where the CME is more capable of driving a shock. We found two clear distinct regions where electrons are accelerated in the low corona and we found spectral differences between the radio emission generated in these regions. We also found similar inferred injection times of near-relativistic electrons at spacecraft to the emission time of the type II and herringbone bursts. However, only the herringbone bursts propagate in a direction where the shock encounters open magnetic field lines that are likely to be magnetically connected to the same spacecraft. Conclusions. Our results indicate that if the in situ electrons are indeed shock-accelerated, the most likely origin of the in situ electrons arriving first is located near the acceleration site of herringbone electrons. This is the only region during the early evolution of the shock where there is clear evidence of electron acceleration and an intersection of the shock with open field lines, which can be directly connected to the observing spacecraft.

Relativistic electron beams accelerated by an interplanetary shock

Context. Collisionless shock waves have long been considered to be among the most prolific particle accelerators in the universe. Shocks alter the plasma they propagate through, and often exhibit complex evolution across multiple scales. Interplanetary (IP) traveling shocks have been recorded in situ for over half a century and act as a natural laboratory for experimentally verifying various aspects of large-scale collisionless shocks. A fundamentally interesting problem in both heliophysics and astrophysics is the acceleration of electrons to relativistic energies (> 300 keV) by traveling shocks. Aims. The reason for an incomplete understanding of electron acceleration at IP shocks is due to scale-related challenges and a lack of instrumental capabilities. This Letter presents the first observations of field-aligned beams of relativistic electrons upstream of an IP shock, observed thanks to the instrumental capabilities of Solar Orbiter. This study presents the characteristics of the electron beams close to the source and contributes to the understanding of their acceleration mechanism. Methods. On 25 July 2022, Solar Orbiter encountered an IP shock at 0.98 AU. The shock was associated with an energetic storm particle event, which also featured upstream field-aligned relativistic electron beams observed 14 min prior to the actual shock crossing. The distance of the beam’s origin was investigated using a velocity dispersion analysis (VDA). Peak-intensity energy spectra were anaylzed and compared with those obtained from a semi-analytical fast-Fermi acceleration model. Results. By leveraging Solar Orbiter’s high temporal resolution Energetic Particle Detector (EPD), we successfully showcase an IP shock’s ability to accelerate relativistic electron beams. Our proposed acceleration mechanism offers an explanation for the observed electron beam and its characteristics, while we also explore the potential contributions of more complex mechanisms.

Multiple injections of energetic electrons associated with the flare and CME event on 9 October 2021

Context. We study the solar energetic particle (SEP) event observed on 9 October 2021 by multiple spacecraft, including Solar Orbiter. The event was associated with an M1.6 flare, a coronal mass ejection, and a shock wave. During the event, high-energy protons and electrons were recorded by multiple instruments located within a narrow longitudinal cone. Aims. An interesting aspect of the event was the multi-stage particle energisation during the flare impulsive phase and also what appears to be a separate phase of electron acceleration detected at Solar Orbiter after the flare maximum. We aim to investigate and identify the multiple sources of energetic electron acceleration. Methods. We utilised SEP electron observations from the Energetic Particle Detector (EPD) and hard X-ray (HXR) observations from the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter, in combination with radio observations at a broad frequency range. We focused on establishing an association between the energetic electrons and the different HXR and radio emissions associated with the multiple acceleration episodes. Results. We find that the flare was able to accelerate electrons for at least 20 min during the non-thermal phase, observed in the form of five discrete HXR pulses. We also show evidence that the shock wave contributed to the electron acceleration during and after the impulsive flare phase. The detailed analysis of EPD electron data shows that there was a time difference in the release of low- and high-energy electrons, with the high-energy release delayed. Also, the observed electron anisotropy characteristics suggest a different connectivity during the two phases of acceleration.

Solar-MACH: An open-source tool to analyze solar magnetic connection configurations

The Solar MAgnetic Connection HAUS1 tool (Solar-MACH) is an open-source tool completely written in Python that derives and visualizes the spatial configuration and solar magnetic connection of different observers (i.e., spacecraft or planets) in the heliosphere at different times. For doing this, the magnetic connection in the interplanetary space is obtained by the classic Parker Heliospheric Magnetic Field (HMF). In close vicinity of the Sun, a Potential Field Source Surface (PFSS) model can be applied to connect the HMF to the solar photosphere. Solar-MACH is especially aimed at providing publication-ready figures for the analyses of Solar Energetic Particle events (SEPs) or solar transients such as Coronal Mass Ejections (CMEs). It is provided as an installable Python package (listed on PyPI and conda-forge), but also as a web tool at that completely runs in any web browser and requires neither Python knowledge nor installation. The development of Solar-MACH is open to everyone and takes place on GitHub, where the source code is publicly available under the BSD 3-Clause License. Established Python libraries like sunpy and pfsspy are utilized to obtain functionalities when possible. In this article, the Python code of Solar-MACH is explained, and its functionality is demonstrated using real science examples. In addition, we introduce the overarching SERPENTINE project, the umbrella under which the recent development took place.

The 17 April 2021 widespread solar energetic particle event

Context. A complex and long-lasting solar eruption on 17 April 2021 produced a widespread Solar Energetic Particle event (SEP) that was observed by five longitudinally well-separated observers in the inner heliosphere covering distances to the Sun from 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and close-to-Earth spacecraft. The event was the second widespread SEP event of solar cycle 25 and produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare showing multiple hard X-ray peaks over a duration of one hour. The event was further accompanied by a medium fast Coronal Mass Ejection (CME) with a speed of 880 km s −1 driving a shock, an EUV wave as well as long-lasting and complex radio burst activity showing four distinct type III burst groups over a period of 40 minutes. Aims. We aim at understanding the reason for the wide SEP spread as well as identifying the underlying source regions of the electron and proton event. Methods. A comprehensive multi-spacecraft analysis of remote-sensing observations and in-situ measurements of the energetic particles and interplanetary context is applied to attribute the SEP observations at the different locations to the various potential source regions at the Sun. An ENLIL simulation is used to characterize the complex interplanetary state and its role for the energetic particle transport. The magnetic connection between the spacecraft and the Sun is determined using ballistic backmapping in combination with potential field source surface extrapolations in the lower corona. In combination with a reconstruction of the coronal shock front we then determine the times when the shock establishes magnetic connections with the different observers. Radio observations are used to characterize the directivity of the four main injection episodes, which are then employed in a 2D SEP transport simulation to test the importance of these different injection episodes. Results. A comprehensive timing analysis of the inferred solar injection times of the SEPs observed at the different spacecraft suggests different source processes being important for the electron and the proton event. Comparison with characteristics and timing of the potential sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source of the electron event. Conclusions. Different to earlier studies on widespread SEP events, we find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110◦ covered by distinct SEP injections, which is also supported by our SEP transport modeling.

Solar energetic particle time series analysis with Python

Solar Energetic Particles (SEPs) are charged particles accelerated within the solar atmosphere or the interplanetary space by explosive phenomena such as solar flares or Coronal Mass Ejections (CMEs). Once injected into the interplanetary space, they can propagate towards Earth, causing space weather related phenomena. For their analysis, interplanetary in situ measurements of charged particles are key. The recently expanded spacecraft fleet in the heliosphere not only provides much-needed additional vantage points, but also increases the variety of missions and instruments for which data loading and processing tools are needed. This manuscript introduces a series of Python functions that will enable the scientific community to download, load, and visualize charged particle measurements of the current space missions that are especially relevant to particle research as time series or dynamic spectra. In addition, further analytical functionality is provided that allows the determination of SEP onset times as well as their inferred injection times. The full workflow, which is intended to be run within Jupyter Notebooks and can also be approachable for Python laymen, will be presented with scientific examples. All functions are written in Python, with the source code publicly available at GitHub under a permissive license. Where appropriate, available Python libraries are used, and their application is described.

Single-spacecraft techniques for shock parameters estimation: A systematic approach

Spacecraft missions provide the unique opportunity to study the properties of collisionless shocks utilising in situ measurements. In the past years, several diagnostics have been developed to address key shock parameters using time series of magnetic field (and plasma) data collected by a single spacecraft crossing a shock front. A critical aspect of such diagnostics is the averaging process involved in the evaluation of upstream/downstream quantities. In this work, we discuss several of these techniques, with a particular focus on the shock obliquity (defined as the angle between the upstream magnetic field and the shock normal vector) estimation. We introduce a systematic variation of the upstream/downstream averaging windows, yielding to an ensemble of shock parameters, which is a useful tool to address the robustness of their estimation. This approach is first tested with a synthetic shock dataset compliant with the Rankine-Hugoniot jump conditions for a shock, including the presence of noise and disturbances. We then employ self-consistent, hybrid kinetic shock simulations to apply the diagnostics to virtual spacecraft crossing the shock front at various stages of its evolution, highlighting the role of shock-induced fluctuations in the parameters’ estimation. This approach has the strong advantage of retaining some important properties of collisionless shock (such as, for example, the shock front microstructure) while being able to set a known, nominal set of shock parameters. Finally, two recent observations of interplanetary shocks from the Solar Orbiter spacecraft are presented, to demonstrate the use of this systematic approach to real events of shock crossings. The approach is also tested on an interplanetary shock measured by the four spacecraft of the Magnetospheric Multiscale (MMS) mission. All the Python software developed and used for the diagnostics (SerPyShock) is made available for the public, including an example of parameter estimation for a shock wave recently observed in-situ by the Solar Orbiter spacecraft.

The effect of shock wave properties on the release timings of solar energetic particles

Context. Fast and wide coronal mass ejections (CMEs) and CME-driven shock waves are capable of accelerating solar energetic particles (SEPs) and releasing them in very distant locations in the solar corona and near-Sun interplanetary space. SEP events have a variety of characteristics in their release times and particle anisotropies. In some events, specifics of the SEP release times are thought to be difficult to reconcile with the scenario that a propagating shock wave is responsible for the SEP release. Aims. Despite the apparent difficulties posed by the shock scenario, many studies have not considered the properties of the propagating shock waves when making a connection with SEP release. This could probably resolve some of the issues and would help us to delve into and understand more important issues such as the effect of the shock acceleration efficiency on the observed characteristics of the SEP timings and the role of particle transport. This study aims to approach these issues from the shock wave perspective and elucidate some of these aspects. Methods. We constructed a simple 2D geometrical model to describe the propagation and longitudinal extension of a disturbance. We used this model to examine the longitudinal extension of the wave front from the eruption site as a function of time, to calculate the connection times as a function of the longitudinal separation angle, and to determine the shock parameters at any connection point. We examined how the kinematic and geometric properties of the disturbance could affect the timings of the SEP releases at different heliolongitudes. Results. We show that the extension of a wave close to the solar surface may not always indicate when a magnetic connection is established for the first time. The first connection times depend on both the kinematics and geometry of the propagating wave. A shock-related SEP release process can produce a large event-to-event variation in the relationship between the connection and release times and the separation angle to the eruption site. The evolution of the shock geometry and shock strength at the field lines connected to an observer are important parameters for the observed characteristic of the release times

PyThea: An open-source software package to perform 3D reconstruction of coronal mass ejections and shock waves

PyThea is a newly developed open-source Python software package that provides tools to reconstruct coronal mass ejections (CMEs) and shocks waves in three dimensions, using multi-spacecraft remote-sensing observations. In this article, we introduce PyThea to the scientific community and provide an overview of the main functionality of the core software package and the web application. This package has been fully built in Python, with extensive use of libraries available within this language ecosystem. PyThea package provides a web application that can be used to reconstruct CMEs and shock waves. The application automatically retrieves and processes remote-sensing observations, and visualizes the imaging data that can be used for the analysis. Thanks to PyThea, the three-dimensional reconstruction of CMEs and shock waves is an easy task, with final products ready for publication. The package provides three widely used geometrical models for the reconstruction of CMEs and shocks, namely, the graduated cylindrical shell (GCS) and an ellipsoid/spheroid model. It also provides tools to process the final fittings and calculate the kinematics. The final fitting products can also be exported and reused at any time. The source code of PyThea package can be found in GitHub and Zenodo under the GNU General Public License v3.0. In this article, we present details for PyThea‘s python package structure and its core functionality, and we show how this can be used to perform three-dimensional reconstruction of coronal mass ejections and shock waves.

The first ground-level enhancement of solar cycle 25 on 28 October 2021

Aims. The first relativistic solar proton event of solar cycle 25 was detected on 28 October 2021 by neutron monitors (NMs) on the ground and particle detectors on board spacecraft in near-Earth space. This is the first ground-level enhancement (GLE) of the current cycle. A detailed reconstruction of the NM response together with the identification of the solar eruption that generated these particles is investigated based on in situ and remote-sensing measurements. Methods. In situ proton observations from a few MeV to ∼500 MeV were combined with the detection of a solar flare in soft X-rays, a coronal mass ejection, radio bursts, and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Timing analysis was performed, and a relation to the solar sources was outlined. Results. GLE73 reached a maximum particle rigidity of ∼2.4 GV and is associated with type III, type II, and type IV radio bursts and an EUV wave. A diversity of time profiles recorded by NMs was observed. This points to the event having an anisotropic nature. The peak flux at E > 10 MeV was only ∼30 pfu and remained at this level for several days. The release time of ≥1 GV particles was found to be ∼15:40 UT. GLE73 had a moderately hard rigidity spectrum at very high energies (γ ∼ 5.5). Comparison of GLE73 to previous GLEs with similar solar drivers is performed.

Multi-spacecraft observations of the structure of the sheath of an interplanetary coronal mass ejection and related energetic ion enhancement

Context. Sheath regions ahead of coronal mass ejections (CMEs) are large-scale heliospheric structures that form gradually with CME expansion and propagation from the Sun. Turbulent and compressed sheaths could contribute to the acceleration of charged particles in the corona and in interplanetary space, but the relation of their internal structure to the particle energization process is still a relatively little studied subject. In particular, the role of sheaths in accelerating particles when the shock Mach number is low is a significant open research problem. Aims. This work seeks to provide new insights on the internal structure of CME-driven sheaths with regard to energetic particle enhancements. A good opportunity to achieve this aim was provided by multi-point, in-situ observations of a sheath region made by radially aligned spacecraft at 0.8 and ∼1 AU (Solar Orbiter, the L1 spacecraft Wind and ACE, and BepiColombo) on April 19−21, 2020. The sheath was preceded by a weak and slowly propagating fast-mode shock. Methods. We apply a range of analysis techniques to in situ magnetic field, plasma and particle observations. The study focuses on smaller scale sheath structures and magnetic field fluctuations that coincide with energetic ion enhancements. Results. Energetic ion enhancements were identified in the sheath, but at different locations within the sheath structure at Solar Orbiter and L1. Magnetic fluctuation amplitudes at inertial-range scales increased in the sheath relative to the solar wind upstream of the shock, as is typically observed. However, when normalised to the local mean field, fluctuation amplitudes did not increase significantly; magnetic compressibility of fluctuation also did not increase within the sheath. Various substructures were found to be embedded within the sheath at the different spacecraft, including multiple heliospheric current sheet (HCS) crossings and a small-scale flux rope. At L1, the ion flux enhancement was associated with the HCS crossings, while at Solar Orbiter, the ion enhancement occurred within a compressed, small-scale flux rope. Conclusions. Several internal smaller-scale substructures and clear difference in their occurrence and properties between the used spacecraft was identified within the analyzed CME-driven sheath. These substructures are favourable locations for the energization of charged particles in interplanetary space. In particular, substructures that are swept from the upstream solar wind and compressed into the sheath can act as effective acceleration sites. A possible acceleration mechanism is betatron acceleration associated with a small-scale flux rope and warped HCS compressed in the sheath, while the contribution of shock acceleration to the latter cannot be excluded.

Aalto-1, multi-payload CubeSat: Design, integration and launch

The design, integration, testing and launch of the first Finnish satellite Aalto-1 is briefly presented in this paper. Aalto-1, a three-unit CubeSat, launched into Sun-synchronous polar orbit at an altitude of approximately 500 km, is operational since June 2017. It carries three experimental payloads: Aalto Spectral Imager(AaSI), Radiation Monitor (RADMON) and Electrostatic Plasma Brake (EPB). AaSI is a hyperspectral imager in visible and near-infrared (NIR) wavelength bands, RADMON is an energetic particle detector and EPB is a de-orbiting technology demonstration payload. The platform was designed to accommodate multiple payloads while ensuring sufficient data, power, radio, mechanical and electrical interfaces. The design strategy of platform and payload subsystems consists of in-house development and commercial subsystems. The CubeSat Assembly, Integration & Test (AIT) followed Flatsat-Engineering-Qualication Model (EQM)-Flight Model (FM) model philosophy for qualification and acceptance. The paper briefly describes the design approach of platform and payload subsystems, their integration and test campaigns and spacecraft launch. The paper also describes the ground segment & services that were developed by Aalto-1 team.

FORESAIL-1 CubeSat Mission to Measure Radiation Belt Losses and Demonstrate Deorbiting

Today, the near‐Earth space is facing a paradigm change as the number of new spacecraft is literally skyrocketing. Increasing numbers of small satellites threaten the sustainable use of space, as without removal, space debris will eventually make certain critical orbits unusable. A central factor affecting small spacecraft health and leading to debris is the radiation environment, which is unpredictable due to an incomplete understanding of the near‐Earth radiation environment itself and its variability driven by the solar wind and outer magnetosphere. This paper presents the FORESAIL‐1 nanosatellite mission, having two scientific and one technological objectives. The first scientific objective is to measure the energy and flux of energetic particle loss to the atmosphere with a representative energy and pitch angle resolution over a wide range of magnetic local times. To pave the way to novel model‐in situ data comparisons, we also show preliminary results on precipitating electron fluxes obtained with the new global hybrid‐Vlasov simulation Vlasiator. The second scientific objective of the FORESAIL‐1 mission is to measure energetic neutral atoms of solar origin. The solar energetic neutral atom flux has the potential to contribute importantly to the knowledge of solar eruption energy budget estimations. The technological objective is to demonstrate a satellite deorbiting technology, and for the first time, make an orbit maneuver with a propellantless nanosatellite. FORESAIL‐1 will demonstrate the potential for nanosatellites to make important scientific contributions as well as promote the sustainable utilization of space by using a cost‐efficient deorbiting technology.

An Empirical Modification of the Force Field Approach to Describe the Modulation of Galactic Cosmic Rays Close to Earth in a Broad Range of Rigidities

On their way through the heliosphere, galactic cosmic rays (GCRs) are modulated by various effects before they can be detected at Earth. This process can be described by the Parker equation, which calculates the phase space distribution of GCRs depending on the main modulation processes: convection, drifts, diffusion, and adiabatic energy changes. A first-order approximation of this equation is the force field approach, reducing it to a one-parameter dependency, the solar modulation potential ϕ. Utilizing this approach, it is possible to reconstruct ϕ from ground-based and spacecraft measurements. However, it has been shown previously that ϕ depends not only on the local interstellar spectrum (LIS) but also on the energy range of interest. We have investigated this energy dependence further, using published proton intensity spectra obtained by PAMELA and heavier nuclei measurements from IMP-8 and ACE/CRIS. Our results show severe limitations at lower energies including a strong dependence on the solar magnetic epoch. Based on these findings, we will outline a new tool to describe GCR proton spectra in the energy range from a few hundred MeV to tens of GeV over the last solar cycles. In order to show the importance of our modification, we calculate the global production rates of the cosmogenic radionuclide 10 Be which is a proxy for the solar activity ranging back thousands of years.

Mini neutron monitor measurements at the Neumayer III station and on the German research vessel Polarstern

Neutron monitors (NMs) are ground-based devices to measure the variation of cosmic ray intensities, and although being reliable they have two disadvantages: their size as well as their weight. As consequence, [1] suggested the development of a portable, and thus much smaller and lighter, calibration neutron monitor that can be carried to any existing station around the world [see 2; 3]. But this mini neutron monitor, moreover, can also be installed as an autonomous station at any location that provides ‘‘office" conditions such as a) temperatures within the range of around 0 to less than 40 degree C as well as b) internet and c) power supply. However, the best location is when the material above the NM is minimized. In 2011 a mini Neutron Monitor was installed at the Neumayer III station in Antarctica as well as the German research vessel Polarstern, providing scientific data since January 2014 and October 2012, respectively. The Polarstern, which is in the possession of the Federal Republic of Germany represented by the Ministry of Education and Research and operated by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research and managed by the shipping company Laeisz, was specially designed for working in the polar seas and is currently one of the most sophisticated polar research vessels worldwide. It spends almost 310 days a year at sea usually being located in the waters of Antarctica between November and March while spending the northern summer months in Arctic waters. Therefore, the vessel scans the rigidity range below the atmospheric threshold and above 10 GV twice a year. In contrast to spacecraft measurements NM data are influenced by variations of the geomagnetic field as well as the atmospheric conditions. Thus, in order to interpret the data a detailed knowledge of the instrument sensitivity with geomagnetic latitude (rigidity) and atmospheric pressure is essential. In order to determine the atmospheric response data from the Neumayer III station here we will perform comparisons to other polar stations, resulting in an atmospheric pressure coefficient of 7.5‰/hPa, while the rigidity dependence will be determined experimentally by utilizing several latitude scans. Thereby, the atmospheric pressure and temperature correction will be discussed in more detail and the results of the latitude scan performed between October 2012 and March 2013 will be presented. Moreover, we will show that this latitude scan can also be described by using the yield function of [4].

Inner heliosphere spatial gradients of GCR protons in the low GeV range

The spacecraft Ulysses was launched in October 1990 in the maximum phase of solar cycle 22, reached its final, highly inclined (80.2°) Keplerian orbit around the Sun in February 1992, and was finally switched off in June 2009. The Kiel Electron Telescope (KET) aboard Ulysses measures electrons from 3 MeV to a few GeV and protons and helium in the energy range from 6 MeV/nucleon to above 2 GeV/nucleon. Because the Ulysses measurements reflect not only the spatial but also the temporal variation of the energetic particle intensities, it is essential to know the intensity variations for a stationary observer in the heliosphere. This was accomplished in the past with the Interplanetary Monitoring Platform-J (IMP 8) until it was lost in 2006. Fortunately, the satellite-borne experiment PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) was launched in June 2006 and can be used as a reliable 1 AU baseline for measurements of the KET aboard Ulysses. Furthermore, we show that measurements of higher nuclei by the Advanced Composition Explorer (ACE), launched 1997 and still operating, can also be used as an extended baseline and to improve the analysis. With these tools at hand, we have the opportunity to determine the spatial gradients of galactic cosmic ray (GCR) protons between several tenth MeV to a few GeV in the inner heliosphere during the extended minimum of solar cycle 23.

Latitudinal and radial gradients of galactic cosmic ray protons in the inner heliosphere - PAMELA and Ulysses observations

Ulysses, launched on 6 October 1990, was placed in an elliptical, high inclined (80.2) orbit around the Sun, and was switched off in June 2009. It has been the only spacecraft exploring high-latitude regions of the inner heliosphere. The Kiel Electron Telescope (KET) aboard Ulysses measures electrons from 3 MeV to a few GeV and protons and helium in the energy range from 6 MeV/nucleon to above 2 GeV/nucleon. The PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) space borne experiment was launched on 15 June 2006 and is continuously collecting data since then. The apparatus measures electrons, positrons, protons, anti-protons and heavier nuclei from about 100 MeV to several hundreds of GeV. Thus the combination of Ulysses and PAMELA measurements is ideally suited to determine the spatial gradients during the extended minimum of solar cycle 23. For protons in the rigidity interval 1.6–1.8 GV we find a radial gradient of 2.7%/AU and a latitudinal gradient of− 0.024%/degree. Although the latitudinal gradient is as expected negative, its value is much smaller than predicted by current particle propagation models. This result is of relevance for the study of propagation parameters in the inner heliosphere.

Modulation of galactic cosmic ray protons and electrons during an unusual solar minimum

During the latest Ulysses out-of-ecliptic orbit the solar wind density, pressure, and magnetic field strength have been the lowest ever observed in the history of space exploration. Since cosmic ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence, the weak heliospheric magnetic field as well as the low plasma density and pressure are expected to cause the smallest modulation since the 1970s. In contrast to this expectation, the galactic cosmic ray (GCR) proton flux at 2.5 GV measured by Ulysses in 2008 does not exceed the one observed in the 1990s significantly, while the 2.5 GV GCR electron intensity exceeds the one measured during the 1990s by 30%–40%. At true solar minimum conditions, however, the intensities of both electrons and protons are expected to be the same. In contrast to the 1987 solar minimum, the tilt angle of the solar magnetic field has remained at about 30° in 2008. In order to compare the Ulysses measurements during the 2000 solar magnetic epoch with those obtained 20 years ago, the former have been corrected for the spacecraft trajectory using latitudinal gradients of 0.25% deg−1 and 0.19% deg−1 for protons and electrons, respectively, and a radial gradient of 3% AU−1. In 2008 and 1987, solar activity, as indicated by the sunspot number, was low. Thus, our observations confirm the prediction of modulation models that current sheet and gradient drifts prevent the GCR flux to rise to typical solar minimum values. In addition, measurements of electrons and protons allow us to predict that the 2.5 GV GCR proton intensity will increase by a factor of 1.3 if the tilt angle reaches values below 10°.

Latitudinal gradients of galactic cosmic rays during the 2007 solar minimum

Ulysses, launched in 1990 October in the maximum phase of solar cycle 22, completed its third out-of-ecliptic orbit in 2008 February. This provides a unique opportunity to study the propagation of cosmic rays over a wide range of heliographic latitudes during different levels of solar activity and different polarities in the inner heliosphere. Comparison of the first and second fast latitude scans from 1994 to 1995 and from 2000 to 2001 confirmed the expectation of positive latitudinal gradients at solar minimum versus an isotropic Galactic cosmic ray distribution at solar maximum. During the second scan in mid-2000, the solar magnetic field reversed its global polarity. From 2007 to 2008, Ulysses made its third fast latitude scan during the declining phase of solar cycle 23. Therefore, the solar activity is comparable in 2007-2008 to that from 1994 to 1995, but the magnetic polarity is opposite. Thus, one would expect to compare positive with negative latitudinal gradients during these two periods for protons and electrons, respectively. In contrast, our analysis of data from the Kiel Electron Telescope aboard Ulysses results in no significant latitudinal gradients for protons. However, the electrons show, as expected, a positive latitudinal gradient of ~0.2% per degree. Although our result is surprising, the nearly isotropic distribution of protons in 2007-2008 is consistent with an isotropic distribution of electrons from 1994 to 1995.