This paper presents a new transport theory suitable for the study of energetic particles in the magnetospheric environment. A Fokker‐Planck diffusion equation governing the variation of the particle distribution function, along with its isotropic and bounce average approximations, is established in the phase space of location and momentum, which is similar to how cosmic rays are treated by the heliospheric community. The equation includes essentially all the particle transport mechanisms: streaming, convection, drift, adiabatic energy change, acceleration by parallel electric field, focusing, and diffusions in location, momentum, and pitch angle. All the coefficients of the equation are directly linked to plasma and magnetic configurations including electromagnetic fluctuations. The theory can form a base for developing full‐scale numerical models of energetic particle transport including acceleration in the magnetosphere. Unlike the conventional theory using canonical variables which is only applicable to the radiation belt of the inner magnetosphere, this theory includes the effects of nonuniform magnetospheric convection and thus is valid for the entire magnetosphere. The derivation of the transport equation has identified some important particle transport mechanisms that have not received careful study by the magnetospheric community. It is found that the compression of magnetospheric plasma can play a significant role in particle acceleration and trapping. The compressional acceleration is the first‐order particle acceleration mechanism, very similar to particle acceleration at shock waves in other space plasma environments. This acceleration mechanism is operable to both electrons and ions. A model map of magnetospheric convection flow shows that the compressional particle acceleration is a large‐scale phenomenon and it is the strongest in the near‐Earth nightside plasma sheet. Magnetospheric compression can also drive particles toward the 90° equatorial pitch angle, which is ideal for trapping them in the geomagnetic field.
Abstract Solar energetic particles (SEPs) can cause severe damage to astronauts and their equipment, and can disrupt communications on Earth. A lack of thorough understanding the eruption processes of solar activities and the subsequent acceleration and transport processes of energetic particles makes it difficult to forecast the occurrence of an SEP event and its intensity using conventional modeling with physics‐based parameters. Therefore, in order to provide an advance warning for astronauts to seek shelter in a timely manner, we apply neural networks to forecast the occurrence of SEP events. We use the properties of coronal mass ejections (CMEs) archived in the Coordinated Data Analysis Workshops catalog based on SOHO Large Angle and Spectrometric Coronagraph Experiment observations. We also derive some features based on these properties associated with the CME, and analyze the contribution of each feature to the overall prediction. Our algorithm achieves an average True Skill Statistic of 0.906, an average F1 score of 0.246, an average probability of detection of 0.920, and an average false alarm rate of 0.882. An analysis of the features shows that sunspot number and a feature based on Type II radio bursts contribute the most, but when grouped together, CME speed‐related features are the most important features.
Abstract Interstellar pickup ions are an ubiquitous and thermodynamically important component of the solar wind plasma in the heliosphere. These PUIs are born from the ionization of the interstellar neutral gas, consisting of hydrogen, helium, and trace amounts of heavier elements, in the solar wind as the heliosphere moves through the local interstellar medium. As cold interstellar neutral atoms become ionized, they form an energetic ring beam distribution comoving with the solar wind. Subsequent scattering in pitch angle by intrinsic and self-generated turbulence and their advection with the radially expanding solar wind leads to the formation of a filled-shell PUI distribution, whose density and pressure relative to the thermal solar wind ions grows with distance from the Sun. This paper reviews the history of in situ measurements of interstellar PUIs in the heliosphere. Starting with the first detection in the 1980s, interstellar PUIs were identified by their highly nonthermal distribution with a cutoff at twice the solar wind speed. Measurements of the PUI distribution shell cutoff and the He focusing cone, a downwind region of increased density formed by the solar gravity, have helped characterize the properties of the interstellar gas from near-Earth vantage points. The preferential heating of interstellar PUIs compared to the core solar wind has become evident in the existence of suprathermal PUI tails, the nonadiabatic cooling index of the PUI distribution, and PUIs’ mediation of interplanetary shocks. Unlike the Voyager and Pioneer spacecraft, New Horizon’s Solar Wind Around Pluto (SWAP) instrument is taking the only direct measurements of interstellar PUIs in the outer heliosphere, currently out to $\sim47~\text{au}$ ∼47au from the Sun or halfway to the heliospheric termination shock.
At the time of the solar flare on the Bastille Day of 2000, the Ulysses spacecraft was at 3.17 AU from the Sun, 62° south in heliographic latitude, and 116° in longitude east of the Earth. Solar wind and magnetic field measurements by Ulysses indicate that the coronal mass ejection (CME) of this event had a limited size in both latitude and longitude, although it was a halo CME as seen in the Solar and Heliospheric Observatory coronagraph images. The event produced large fluxes of energetic particles up to energies >100 MeV at both Ulysses and the Earth. Enhancements of energetic particles were immediately observed at the Earth, with their onset times consistent with the velocity dispersion due to the streaming of particles along magnetic field lines from the location of particle acceleration in the corona to the Earth. To the contrary, at Ulysses, the energetic particles from the solar event were not detected until 4–11 hours later, and the increases of particle intensity were much more gradual. The onset times of particles in different energy channels were not organized by particle speed; rather they depended on both particle rigidity and speed, indicating that the transport of particles to Ulysses at high latitudes had a diffusive nature. The first‐order anisotropy in the 40–90 MeV proton flux was significantly larger than what is expected from the Compton‐Getting effect for many hours after the onset. The direction of the first‐order anisotropy was not along the projection of local magnetic fields onto the scan plane of the detector and it was not affected by the polarity of the field either, indicating that the particles did not arrive at Ulysses through propagation along magnetic field lines and rather much of the anisotropy was produced by cross‐field diffusion in the presence of a cross‐field density gradient pointing toward the low latitude direction. All these observations are consistent with easy particles transport across the mean heliospheric magnetic fields. The apparent difficulty for the theory is that the observations require a cross‐field diffusion that is too fast to be explained by random walk of field lines due only to supergranulation.
We analyse 9 large solar energetic particle (SEP) events detected by the Ulysses spacecraft at high heliolatitudes during the recent solar maximum polar passes. Properties of time intensity profiles from the Ulysses/COSPIN instrument are compared with those measured by SOHO/COSTEP and Wind/3DP near Earth. We find that onset times and times to maximum at high latitude are delayed compared to in‐ecliptic values. We show that the parameter which best orders these characteristics of time profiles is the difference in latitude between the associated flare and the spacecraft. We find that the presence of a shock is not necessary for the establishing of near equal intensities at Ulysses and in the ecliptic during the decay phase. The model of SEP acceleration by coronal mass ejection driven shocks does not appear to account for our observations, which would more easily be explained by particle diffusion across the interplanetary magnetic field.
Solar Energetic Particle (SEP) events are interesting from a scientific perspective as they are the product of a broad set of physical processes from the corona out through the extent of the heliosphere, and provide insight into processes of particle acceleration and transport that are widely applicable in astrophysics. From the operations perspective, SEP events pose a radiation hazard for aviation, electronics in space, and human space exploration, in particular for missions outside of the Earth's protective magnetosphere including to the Moon and Mars. Thus, it is critical to improve the scientific understanding of SEP events and use this understanding to develop and improve SEP forecasting capabilities to support operations. Many SEP models exist or are in development using a wide variety of approaches and with differing goals. These include computationally intensive physics-based models, fast and light empirical models, machine learning-based models, and mixed-model approaches. The aim of this paper is to summarize all of the SEP models currently developed in the scientific community, including a description of model approach, inputs and outputs, free parameters, and any published validations or comparisons with data.
The focused transport equation without adiabatic energy loss is widely used to model solar energetic particles' (SEP) interplanetary propagation by fitting spacecraft data. We incorporate the adiabatic energy loss effect, provided by the divergence of the solar wind flows, into the focused transport equation. The equation is then solved numerically using a time‐backward stochastic integration method. We show the comparison between solutions of focused transport equations with and without energy loss. We found the effect of adiabatic cooling is significant on the time profile of the intensity of SEPs. It is also shown that without energy loss, for gradual events, we can only fit the initial phase of SEP events. However, with energy loss, we can fit the entire (initial and decaying) phases. In addition, the values of the mean free path obtained by fitting the SEP events with energy loss is always smaller than that without. The results suggest that including adiabatic cooling effect is another way to partially fix the solar energetic particle mean free paths' “too small” problem discussed by Bieber et al. (1994), i.e., the mean free paths obtained by fitting transport equation to observation data are much larger than the quasi‐linear theory results.
We review recent observations and modeling developments on the subject of galactic cosmic rays through the heliosphere and in the Very Local Interstellar Medium, emphasizing knowledge that has accumulated over the past decade. We begin by highlighting key measurements of cosmic-ray spectra by Voyager, PAMELA, and AMS and discuss advances in global models of solar modulation. Next, we survey recent works related to large-scale, long-term spatial and temporal variations of cosmic rays in different regimes of the solar wind. Then we highlight new discoveries from beyond the heliopause and link these to the short-term evolution of transients caused by solar activity. Lastly, we visit new results that yield interesting insights from a broader astrophysical perspective.
The Voyager plasma experiment observed large‐amplitude plasma fluctuations in the Uranian magnetosheath downstream from the planet. This is a region that has not been well sampled in the Earth's magnetosphere. These waves have periods of tens of minutes, are characterized by an anticorrelation between the plasma density and temperature, and are associated with deflections in the flow angle of the plasma. These fluctuations are observed only in regions where the magnetic field is rapidly varying. These waves have time and distance scales placing them in the MHD regime, but their characteristics are not compatible with any known solution of the MHD equations. It is suggested that these fluctuations are produced by the solar wind interaction with the magnetosphere at the bow shock, but the physics governing the production and propagation of these fluctuations is not understood.
