ACE Observations of the Bastille Day 2000 Interplanetary Disturbances

We present ACE observations for the six-day period encompassing the Bastille Day 2000 solar activity. A high level of transient activity at 1 AU, including ICME-driven shocks, magnetic clouds, shock-accelerated energetic particle populations, and solar energetic ions and electrons, are described. We present thermal ion composition signatures for ICMEs and magnetic clouds from which we derive electron temperatures at the source of the disturbances and we describe additional enhancements in some ion species that are clearly related to the transient source. We describe shock acceleration of 0.3–2.0 MeV nucl⁻¹ protons and minor ions and the relative inability of some of the shocks to accelerate significant energetic ion populations near 1 AU. We report the characteristics of < 20 MeV nucl⁻¹ solar energetic ions and < 0.32 MeV electrons and attempt to relate the release of energetic electrons to particular source regions.     

Dynamics of the boundary of the penetration of solar energetic particles to Earth’s magnetosphere according to <Emphasis Type="Italic">CORONAS-F</Emphasis> data

The dynamics of the boundary of the penetration of solar energetic particles (electrons and protons) to Earth’s magnetosphere during solar flares and related geomagnetic disturbances in November 2001 and October–November 2003 is analyzed using CORONAS-F data. The relationship between the penetration boundary, the geomagnetic activity indices, and the local magnetic time is investigated. The correlation coefficient between the invariant latitude of the penetration boundary and the K _p and D _st indices for electrons with energies ranging from 0.3 to 0.6 MeV in the dayside sector is demonstrated to be higher than that in the nightside sector. The correlation coefficient for protons with energies from 1 to 5 MeV is higher in the nightside sector as compared to the dayside sector. For protons with energies from 50 to 90 MeV, the correlation is high at all MLT.     

Galileo plasma wave observations of iogenic hydrogen

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave–particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvénic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program (Rönmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40°W longitude from the sub-Jupiter meridian, this density is found to be ≥0.5% of the local plasma density (∼4000 cm⁻³), namely ≥20 cm⁻³. Assuming a stationary pickup process and a r⁻ⁿ distribution of pickup protons within several Io radii of Io’s wake, this implies that more than 10²⁶ protons/s are created around Io. The ultimate origin of these protons is an open issue.     

Multiple acceleration of protons on the Sun and their free propagation to the Earth on January 20, 2005

We analyze the observations of solar protons with energies >80 MeV near the Earth and the January 20, 2005, solar flare in various ranges of the electromagnetic spectrum. Within approximately the first 30 min after their escape into interplanetary space, the solar protons with energies above 80 MeV propagated without scattering to the Earth and their time profiles were determined only by the time profile of the source on the Sun and its energy spectrum. The 80–165 MeV proton injection function was nonzero beginning at 06:43:80 UT and can be represented as the product of the temporal part, the ACS (Anticoincidence System) SPI (Spectrometer on INTEGRAL) count rate, and the energy part, a power-law proton spectrum ～E ^?4.7±0.1. Protons with energies above 165 MeV and relativistic electrons were injected, respectively, 4 and 9 min later than this time. The close correlation between high-energy solar electromagnetic emission and solar proton fluxes near the Earth is evidence for prolonged and multiple proton acceleration in solar flares. The formation of a posteruptive loop system was most likely accompanied by successive energy releases and acceleration of charged particles with various energies. Our results are in conflict with the ideas of cosmic-ray acceleration in gradual solar particle events at the shock wave driven by a coronal mass ejection.     

Effect of low energy electron injection on storm-time evolution of radiation belt energetic electrons: three-dimensional modeling

Using the STEERB (storm-time evolution of electron radiation belt) code, we simulate the evolution of radiation belt energetic electrons during geomagnetic storms in the case of low energy electron injection. The STEERB code is used to solve the three-dimensional Fokker–Planck diffusion equation which incorporates wave-particle interaction, Coulomb collisions and radial diffusion. Numerical simulations show that under the short time (～1 h) injection of low energy (0.1 MeV≤E _k ≤0.2 MeV) fluxes of radiation belt energetic electrons can increase during the entire storm period. During the main and recovery phases, such injection efficiently enhances chorus-driven acceleration of radiation belt energetic electrons, allowing fluxes of energetic electrons by a factor of 1–2 orders higher than those in the absence of injection. The current results indicate that substorm-induced electron injection must be incorporated to investigate the evolution of radiation belt energetic electrons.     

Energetic solar-proton precipitation in the auroral zone associated with storm sudden commencements

Measurements by balloon-borne instruments, data from the satellites Explorer 41 and 43 and riometer recordings were used to investigate the influence of magnetospheric processes on the precipitation of energetic solar protons related to the occurrence of two ssc's on 8–9 August 1972. The high-energy protons (E_p ? 30 MeV) had direct access to auroral-zone latitudes. The flux variations of low-energy (some MeV) protons in interplanetary space and the magnetosphere were different from those of the protons precipitated in the auroral zone. These low-energy protons were precipitated mainly during and after the ssc's. The importance of direct proton access, radial diffusion, pitch angle scattering and proton acceleration for the explanation of the low-energy proton behaviour is discussed.     

Long term storage of relativistic particles in the solar corona

A model is presented which shows that large numbers of energetic electrons (0.3-> 10 MeV) and protons (1–30 MeV) can be stored in the solar corona at altitudes around 3 × 10⁵ km for periods in excess of 5 days. Specific reference is made to the time period July 6–16 1968 as an excellent example of energetic solar particle storage. Time histories of interplanetary charged particle intensities observed by the IMP-4 and Pioneer 8 satellites are used to substantiate this contention. Detailed reference is also made to solar X-ray, optical and radio data obtained during the period in question, in addition to interplanetary magnetometer data. This model provides a unique solution to many hitherto unexplained solar particle events, and can also account for the lack of prompt particle emission from certain large solar flares recorded in the past.     

IBEX-Lo observations of energetic neutral hydrogen atoms originating from the lunar surface

In this paper we present quantitative results of observations of energetic neutral atoms (ENAs) originating from the lunar surface. These ENAs, which are hydrogen atoms, are the result of the solar wind protons being reflected from and neutralised at the surface of the Moon. These measurements were made with IBEX-Lo on NASA's IBEX satellite. From these measurements we derive the energy spectrum of the ENAs, their flux, and the lunar albedo for ENAs (i.e., the ratio of ENAs to the incoming solar wind protons). The energy spectra of the ENAs clearly show that their origin is directly from the solar wind via backscattering, and that they are not sputtered atoms. From several observation periods we derived an average global albedo of A_H=0.09±0.05. From the observed energy spectra we derive a generic spectrum for unshielded bodies in the solar wind.     

Particle acceleration during the explosive phase of a solar flare

Starting with the quasi-linear equation of the distribution function of particles in a regular electric field, a combined diffusion coefficient in the momentum space conbining the effects of the regular field and a turbulent field is obtained and a combined mechanism of acceleration by the regular and turbulent fields in the neutral sheet of solar proton flares is proposed. It is shown by calculation that conditions in solar proton flares are such that the charged particles can be effectively accelerated to tens of MeV, even ~1 GeV. It is shown that the combined acceleration by a regular electric field and ion-acoustic turbulence pumps the protons and other heavy ions into ranges of energy where they can be accelerated by Langmuir turbulence. By considering the combined acceleration by Langmuir turbulence and the regular electric field, the observed spectrum of energetic protons and the power-law spectrum of energetic electrons can be reproduced.     

SOLAR ENERGETIC ION AND ELECTRON LIMITS FROM HIREGS OBSERVATIONS

The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is 0.8 ph cm^-2 in the flares, and the upper limit flux is 1.8 × 10^-4 ph s^-1 cm^-2 in the absence of flares. These limits imply that 6 × 10³⁰ (2) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons >30 MeV is 4 × 10²⁶ ergs, but this limit can be more than 10³⁰ ergs if the spectrum extends down to 1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 10³⁰ ergs if the spectrum goes down to 7 keV. Quiet-Sun observations indicate that 10²⁶erg s^-1 are deposited by energetic protons >1 MeV, well below the10²⁷ –10²⁸ erg s^-1 required for coronal heating, while <3 × 10²⁷ erg s^-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least 2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for 10⁴ s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

The energy coupling function and the power generated by the solar wind-magnetosphere dynamo

A solar wind parameter ε, known as the energy coupling function, has been shown to correlate with the power consumption in the magnetosphere. It is shown in the present paper that the parameter ε can be identified semi-quantitatively as the dynamo power delivered from the solar wind to an open magnetosphere. This identification not only provides a theoretical basis for the energy coupling function, but also constitutes an observational verification of the solar wind-magnetosphere dynamo along the magnetotail. Moreover, one can now conclude that a substorm results when the dynamo power exceeds 10₁₈ ergs ^?1.     

Properties of energetic electrons of magnetospheric origin in the magnetosheath and in the solar wind—correlation with geomagnetic activity

Bursts of energetic electrons (from >40keV up to 2MeV) as distinct from the magnetopause electron layer observed by Domingo et al. (1977) have been observed in the magnetosheath and in the solar wind by HEOS-2 at high-latitudes. Although these electrons are occasionally found close to the bow shock and simultaneously with low frequency (magnetosonic) upstream waves our observations strongly indicate that these electrons are of exterior cusp origin. Indeed, the flux intensity is highest in the exterior cusp region and decreases as the spacecraft moves away from it both tailward or upward. The energy spectrum becomes harder with increasing radial distance from the exterior cusp. The measured anisotropy indicates that the particles are propagating away from the exterior cusp. The magnetic field points to the exterior cusp region when these electrons are observed, being, for solar wind observations, centred at longitude 0° or 180° rather than along the spiral and in the magnetosheath, being usually different from the 90° or 270° orientation typical of that region. We exclude, therefore, that acceleration in the bow shock is the source of these particles because $\text{B}$ is not tangent to the shock when bursts are observed. We have also found a one to one correlation between geomagnetic storms' recovery phases and intense, continuous observations of >40 keV electrons in the magnetosheath, while, on the other hand, during geomagnetically quiet (Dst) periods bursts are observed only if AE is much larger than average.     

Galactic cosmic ray modulation from 1965–1970

Numerical solutions of the cosmic-ray equation of transport within the solar cavity and including the effects of diffusion, convection, and energy losses due to adiabatic deceleration, have been used to reproduce the modulation of galactic electrons, protons and helium nuclei observed during the period 1965–1970. Kinetic energies between 10 and 10⁴ MeV/nucleon are considered. Computed and observed spectra (where data is available) are given for the years 1965, 1968, 1969 and 1970 together with the diffusion coefficients. These diffusion coefficients are assumed to be of separable form in rigidity and radial dependence, and are consistent with the available magneticfield power spectra. The force-field solutions are given for these diffusion coefficients and galactic spectra and compared with the numerical solutions. For each of the above years we have (i) determined the radial density gradients near Earth; (ii) found the mean energy losses suffered by galactic particles as they diffuse to the vicinity of the Earth's orbit; (iii) shown quantitatively the exclusion of low-energy galactic protons and helium nuclei from near Earth by convective effects; and (iv), for nuclei of a given energy near Earth, obtained their distribution in energy before entering the solar cavity. It is shown that the energy losses and convection lead to near-Earth nuclei spectra at kinetic energies ≤100 MeV/nucleon in which the differential intensity is proportional to the kinetic energy with little dependence on the form of the galactic spectrum. This dependence is in agreement with the observed spectra of all species of atomic nuclei and we argue that this provides strong observational evidence for the presence of energy losses in the propagation process; and for the exclusion of low energy galactic nuclei from near Earth.     

Radio,Hard X-Ray,and Gamma-Ray Emissions Associated with a Far-Side Solar Event

The far-side solar eruptive event SOL2014-09-01 produced hard electromagnetic and radio emissions that were observed with detectors at near-Earth vantage points. Especially challenging was a long-duration >?100 MeV \(\gamma\)-ray burst that was probably produced by accelerated protons exceeding 300 MeV. This observation raised the question how high-energy protons could reach the Earth-facing solar surface. Some preceding studies discussed a scenario in which protons accelerated by a shock driven by a coronal mass ejection high in the corona return to the solar surface. We continue with the analysis of this challenging event, involving radio images from the Nançay Radioheliograph and hard X-ray data from the High Energy Neutron Detector (HEND) of the Gamma-Ray Spectrometer onboard the Mars Odyssey space observatory located near Mars. HEND recorded unocculted flare emission. The results indicate that the emissions observed from the Earth’s direction were generated by flare-accelerated electrons and protons trapped in static long coronal loops. They can be reaccelerated in these loops by a shock wave that was excited by the eruption, being initially not driven by a coronal mass ejection. The results highlight ways to address the remaining questions.     

Electromagnetic and corpuscular emission from the solar flare of 1991 June 15: Continuous acceleraton of relativistic particles

Data on X-,γ-ray, optical and radio emission from the 1991 June 15 solar flare are considered. We have calculated the spectrum of protons that producesγ-rays during the gradual phase of the flare. The primary proton spectrum can be described as a Bessel-function-type up to 0.8 GeV and a power law with the spectral index ≈3 from 0.8 up to 10 GeV or above. We have also analyzed data on energetic particles near the Earth. Their spectrum differed from that of primary protons producingγ-ray line emission. In the gradual phase of the flare additional pulses of energy release occurred and the time profiles of cm-radio emission andγ-rays in the 0.8–10 MeV energy band and above 50 MeV coincided. A continuous and simultaneous stochastic acceleration of the protons and relativistic electrons at the gradual phase of the flare is considered as a natural explanation of the data.     

Excitation of Langmuir waves by the lower energy cutoff behavior of power-law electrons

Langmuir waves (LWs), which are believed to play a crucial role in the plasma emission of solar radio bursts, can be excited by streaming instability of energetic electron beams. However, solar hard X-ray observations imply that the energetic flare electrons usually have a power-law energy distribution with a lower energy cutoff. In this paper, we investigate LWs driven by the power-law electrons. The results show that power-law electrons with the steepness cutoff behavior can excite LWs effectively because of the population inversion distribution below the cutoff energy (E _c ). The growth rate of LWs increases with the steepness index (δ) and decreases with the power-law index (α). The wave number of the fastest growing LWs (kλ _D ), decreases with the characteristic velocity of the power-law electrons ( \(v_{c}=\sqrt{2E_{c}/m_{e}}\) ) and increases with the thermal velocity of ambient electrons (v _T ). This can be helpful for us to understand better the physics of LWs and the dynamics of energetic electron beams in space and astrophysical plasmas.     

Production of organic molecules in the outer solar system by proton irradiation: Laboratory simulations

In the past few years considerable attention has been given to the determination of likely compounds that could account for the various colors observed in the outer solar system: and to possible formation mechanisms for these compounds. Many experiments have been done using electrical discharges (Chadha, M. S., et al., 1971, Icarus15, 39) and ultraviolet light (Khare, B. N., and Sagan, C., 1973, Icarus20, 311) on mixtures of CH₄, NH₃, and H₂S, which are most likely the dominant minor constituents of the atmospheres of Jupiter, Saturn, Titan, and possibly the other satellites early in their histories. Colored polymers, usually brownish-red, have been produced in these experiments. With the passage of Pioneer 10 around Jupiter, there is another source of energy worthy of consideration, energetic protons (and electrons). Preliminary experiments to investigate the formation of colored polymers and other interesting molecules by the irradiation of gas mixtures by protons are discussed. Two to four Mev protons were used, with corresponding beam fluxes (as measured at 6R_J from the planet) equivalent to approximately 80 Earth years at Jupiter per hour of exposure. As in the other types of experiments, colored polymers have been produced. An important feature of this work is the presence or absence of absorption at 5 μm in the different materials produced; Titan is quite dark at this wavelength and Io is fairly bright. Such features may provide criteria for accepting or rejecting various materials produced in these experiments as reasonable coloring agents for the outer solar system.     

Magnetically trapped particles in the lower solar atmosphere

The trapping of energetic electrons and protons in a simple, arched magnetic field imbedded in the lower solar atmosphere was considered. The lifetime of electrons with kinetic energies up to about 1.5 MeV was found to be completely determined by the motion of the mirror points, provided the gyro-synchrotron loss can be neglected. The same motion also influences the lifetimes of more energetic electrons, up to 10 MeV. This was not found to be the case for protons in the range from 1 MeV to 100 GeV. Some fluid and streaming instabilities were also considered; they pull the particles upward, raise their mirror points, and increase their lifetime. The emission of gyro-synchrotron radiation and bremsstrahlung in this model has been related to observations. Using the duration of non-thermal X-ray peaks given by Kane (1969), the altitude of injection of energetic particles was estimated.     

On proton and electron acceleration by shock waves during large solar flares

The data on optical, X-ray and gamma emission from proton flares, as well as direct observations of flare-associated phenomena, show energetic proton acceleration in the corona rather than in the flare region. In the present paper, the acceleration of protons and accompanying relativistic electrons is accounted for by a shock wave arising during the development of a large flare. We deal with a regular acceleration mechanism due to multiple reflection of resonance protons and fast electrons from a collisionless shock wave front which serves as a moving mirror. The height of the most effective acceleration in the solar corona is determined. The accelerated particle energy and density are estimated. It is shown in particular that a transverse collisionless shock wave may produce the required flux of protons with energy of 10 MeV and of relativistic electrons of 1–10 MeV.The proposed scheme may also serve as an injection mechanism when the protons are accelerated up to relativistic energies by other methods.