M.-B. Kallenrode

Radial and Longitudinal Dependence of Solar 4-13 MeV and 27-37 MeV Proton Peak Intensities and Fluences: Helios and IMP 8 Observations

We study the radial and longitudinal dependence of 4-13 and 27-37 MeV proton peak intensities and fluences measured within 1 AU of the Sun during intense solar energetic particle events. Data are from the IMP 8 and the two Helios spacecraft. We analyze 72 events and compute the total event fluence (F) and the peak intensity (J), distinguishing between the events absolute maximum intensity and that neglecting local increases associated with the passage of shocks or plasma structures. Simultaneous measurements of individual events by at least two spacecraft show that the dominant parameter determining J and F is the longitudinal separation () between the parent active region and the footpoint of the field line connecting each spacecraft with the Sun, rather than the spacecraft radial distance (R). We perform a multiparameter fit to the radial and longitudinal distributions of J and F for events with identified solar origin and that produce intensity enhancements in at least two spacecraft. This fit determines simultaneously the radial and longitudinal dependences of J and F. Radial distributions of events observed by at least two spacecraft show ensemble-averaged variations ranging from R-2.7 to R-1.9 for 4-13 and 27-37 MeV proton peak intensities, and R-2.1 to R-1.0 for 4-13 and 27-37 MeV proton event fluences, respectively. Longitudinal distributions of J and F are approximated by the form e, where 0 is the distribution centroid and k is found to vary between ~1.3 and ~1.0 rad-2. Radial dependences are less steep than both those deduced from diffusion transport models by Hamilton et al. in 1990 and those recommended by Shea et al. in 1988 to extrapolate J and F from R = 1 to R < 1 AU.

Current views on impulsive and gradual solar energetic particle events

Solar energetic particles (SEPs) are one manifestation of violent energy releases on the Sun. The study of their acceleration and propagation reveals information about basic plasma-physical processes, such as reconnection, shock acceleration and wave–particle interaction, in astrophysical objects, such as stars, magnetospheres or the diluted plasma of the interstellar or interplanetary medium. This paper introduces the current classification scheme for solar energetic particle events, its relation to the underlying acceleration processes, and addresses open questions regarding the better understanding of SEPs as well as the underlying physical processes. Modifications to the current paradigm considering more recent observations will be suggested.

Composition and azimuthal spread of solar energetic particles from impulsive and gradual flares

We began with a list of 77 flare-associated solar energetic particle (SEP) events observed from 1974 to 1985 by at least one of the two Helios space probes. We classified the SEP parent flares as impulsive (25 cases) or gradual (52 cases) on the basis of their soft X-ray durations. We then compared the intensities of the prompt component of ∼0.5 MeV electrons, ∼10 MeV protons, and ∼10 MeV per nucleon helium for the two classes of SEP flares. We find that SEPs from gradual flares have higher intensities than SEPs from impulsive flares

A model study of the impact of magnetic field structure on atmospheric composition during solar proton events

[1] During a polarity transition of the Earth’s magnetic field, the structure and strength of the field change significantly from their present values. This will alter the global pattern of charged particle precipitation into the atmosphere. Thus, particle precipitation is possible into regions that are at the moment effectively shielded by the Earth’s magnetic field. A two-dimensional global chemistry, photolysis and transport model of the atmosphere has been used to investigate how the increased particle precipitation affects the chemical composition of the middle and lower atmosphere. Ozone losses resulting from large energetic particle events are found to increase significantly, with resultant losses similar to those observed in the Antarctic ozone hole of the 1990s. This results in significant increases in surface UV-B radiation as well as changes in stratospheric temperature and circulation over a period of several months after large particle events. INDEXTERMS:0340Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 1535 Geomagnetism and Paleomagnetism: Reversals (process, timescale, magnetostratigraphy); 1650 Global Change: Solar variability; 2716 Magnetospheric Physics: Energetic particles, precipitating. Citation: Sinnhuber, M., J. P. Burrows, M. P. Chipperfield,

Propagation of particles injected from interplanetary shocks: A black box model and its consequences for acceleration theory and data interpretation

Energetic protons in the hundreds of keV to the tens of MeV range frequently are observed in connection with traveling interplanetary shocks. Occasionally, the particle energies can extend up to about 100 MeV. The intensity time profiles at the observers site are a superposition of the continuous, spatially and temporally variable acceleration at the shock and the subsequent interplanetary propagation. To gain a better understanding of both processes and to derive their relevant parameters, we extend a numerical solution of the model of focused transport to accommodate the shock as a moving source. No assumptions about the acceleration mechanism are made; the shock is treated as a black box. In this paper we introduce the model, discuss its validity, and present model results which have implications for acceleration theory and data interpretation. The main results concerning acceleration and propagation are as follows: (1) In the limit of strong scattering and low particle speeds our model converges toward diffusive shock acceleration. (2) For weak scattering or fast particles, spatial diffusion is an insufficient approximation for particle transport; in this case, the physical consequence is a fast escape from the shock, and the formal consequence is that the standard description of diffusive shock acceleration is insufficient. (3) Because of this fast escape, even a turbulent foreshock region, while it is perfectly capable of keeping 100 keV protons confined to the shock, would allow 10 MeV protons to stream away easily. Important results for data interpretation are as follows: (1) A quasi-exponential intensity increase upstream of the shock is not necessarily indicative of diffusive shock acceleration. (2) The intensity at the time of shock passage is a crude measure for the local acceleration efficiency as long as it stays constant or continues to rise.

Neutral lines and azimuthal “transport” of solar energetic particles

We discuss properties of solar energetic particle (SEP) events observed by both Helios spacecraft in the time period March 1976 to March 1980, in particular variations of the intensity time profiles with angular distance between the flare and the observers magnetic footpoint. Special attention is paid to the neutral lines of the large-scale coronal magnetic field. For individual events we will show that sector boundaries can have a strong influence on intensities and time-scales of SEP events. This can be seen best in events in which both spacecraft are connected more or less symmetrically around the flare site with a sector boundary between them. In a statistical approach we discuss variations of onset and maximum times as well as maximum intensities with angular distance between the flare and the observers magnetic footpoint for 39 SEP events. The use of two spaceprobes has the advantage that unknown flare parameters, such as the number of injected particles and the time of acceleration, can be eliminated by studying only relative variations of time and intensity with angular distance. These variations were analyzed under consideration of the sector boundaries of the large-scale coronal magnetic field. We find (1) particles can be observed on both sides of sector boundaries during both, impulsive and gradual, events, (2) the onset times of ∼0.5-MeV electrons can be ordered by the occurrence of sector boundaries, inside the flare sector we can define some kind of “azimuthal propagation velocity” Δϕ/Δtons with ≥4°/min, outside the flare sector this velocity is ≤1°/min, (3) the azimuthal propagation of ∼20-MeV protons seems to be systematically slower, (4) there seems to be no difference in the variations of onset times or maximum intensities with angular distance between impulsive and gradual events, and (5) within the broad scatter the variation of the maximum intensity with angular distance is not systematically influenced by the crossing of sector boundaries, despite the obvious influence of sector boundary crossings in individual events. These observations support the association of the fast propagation region (FPR) with the large-scale polarity cells of the coronal magnetic field. The “transport” inside the FPR can be understood in terms of an open cone of propagation. The “transport” outside the FPR can be interpreted in terms of a propagation mechanism or acceleration at a shock.

Particle propagation in the inner heliosphere

For 27 energetic particle events observed on one of the Helios spacecraft we present fits on the intensity and anisotropy time profiles performed with the model of focused transport. The events have local mean free paths λ∥ between 0.02 and ≥1 AU, for most events λ∥ lies within or above the consensus range between 0.08 and 0.3 AU. The propagation in general can be described as focused or diffusive transport, therefore in the majority of events interplanetary scattering plays an important role. Comparing with results from earlier studies we find that scatter-free events can be observed inside 0.5 AU as well as close to 1 AU, therefore extreme scattering properties seem not to be related to radial changes in scattering conditions. In addition, any radial variation in the scattering mean free path seems to be small compared to the event-to-event variations. A comparison of fits on ∼18-MeV protons and ∼1-MeV electrons shows that the scattering mean free path λp of the protons is related to the electron mean free path λe, whereby the average ratio λp/λe is 1.6±0.9. This latter observation has important consequences for our understanding of the underlying scattering mechanism, in particular as it is in contradiction with conventional results from quasi-linear theory.

Multi-spacecraft observations of particle events and interplanetary shocks during November/December 1982

We present a sample of solar energetic particle events observed between November 18 and December 31, 1982 by the HELIOS 1, the VENERA 13, and IMP 8 spacecraft. During the entire time period all three spacecraft were magnetically connected to the western hemisphere of the Sun with varying radial and angular distances from the flares. Eleven proton events, all of them associated with interplanetary shocks, were observed by the three spacecraft. These events are visible in the low-energy (about 4 MeV) as well as the high-energy (30 MeV) protons. In the largest events protons were observed up to energies of about 100 MeV. The shocks were rather fast and in some cases extended to more than 90% east of the flare site. Assuming a symmetrical configuration, this would correspond to a total angular extent of some interplanetary shocks of about 180%. In addition, due to the use of three spacecraft at different locations we find some indication for the shape of the shock front: the shocks are fastest close to the flare normal and are slower at the eastern flank. For particle acceleration we find that close to the flare normal the shock is most effective in accelerating energetic particles. This efficiency decreases for observers connected to the eastern flank of the shock. In this case, the efficiency of shock acceleration for high-energy protons decreases faster than for low-energy protons. Observation of the time-intensity profiles combined with variations of the anisotropy and of the steepness of the proton spectrum allows one in general to define two components of an event which we term ‘solar’ and ‘interplanetary’. We attempt to describe the results in terms of a radially variable efficiency of shock acceleration. Under the assumption that the shock is responsible not only for the interplanetary, but also for the solar component, we find evidence for a very efficient particle acceleration while the shock is still close to the Sun, e.g., in the corona. In addition, we discuss this series of strong flares and interplanetary shocks as a possible source for the formation of a superevent.

A statistical survey of 5-MeV proton events at transient interplanetary shocks

Between 1974 and 1985 the two Helios spacecraft observed 351 transient interplanetary shocks. For 5-MeV protons the particle events associated with these shocks can be divided into three groups : (1) events without intensity increase above quiet time or increased background (47%), (2) solar and interplanetary particle (SIP) events consisting of particles accelerated on or close to the Sun (solar or near-Sun component) as well as at the interplanetary shock (24%), and (3) pure interplanetary particle (PIP) events (29%) which consist of particles accelerated at the shock in interplanetary space but do not show evidence for significant or even excess particle acceleration on the Sun. This classification shows that (1) only about half of the shocks accelerate MeV protons in interplanetary space and (2) MeV protons accelerated on the Sun are neither a necessary nor a sufficient condition for the acceleration of MeV protons in interplanetary space. Shock parameters such as speed or shock strength alone do not give an indication for the class of the associated particle event, because in the parameter range which covers most of the shocks, all three classes are distributed rather evenly. However, the shocks strongest in these parameters tend to accelerate particles. The intensity at the time of shock passage, which can be used as a crude measure for the local acceleration efficiency, is correlated with the local shock speed and the magnetic compression. The correlation coefficients are small but statistically significant, indicating that (1) the correlations are real and (2) the intensity is influenced by additional parameters, which are not necessarily shock inherent. As an example I will show that the local acceleration at the shock decreases roughly symmetrically with increasing distance from the nose of the shock with a median e-folding angle of 10°. Occasionally, larger e-folding angles are observed close to the nose of the shock. The question of how the shock accelerates protons in the MeV range could not be answered here, but I will suggest future studies that could shed a new light on this problem.

Modeling impacts of geomagnetic field variations on middle atmospheric ozone responses to solar proton events on long timescales

[1] Strength and structure of the Earth’s magnetic field control the deflection of energetic charged particles of solar and cosmic origin. Therefore variations of the geomagnetic field occurring on geological timescales affect the penetration of charged particles into the atmosphere. During solar proton events (SPEs) the flux of high-energy protons from the Sun is markedly increased. In order to investigate the impact of SPEs on the middle atmospheric ozone on longer timescales, two-dimensional atmospheric chemistry and transport simulations have been performed using simulated time series of SPEs covering 200 years. Monte Carlo calculations were used to obtain ionization rates, which were then applied to the atmosphere under the consideration of different shielding properties of the geomagnetic field. The present-day magnetic field configuration and four other scenarios were analyzed. For the first time, field configurations representing possible realistic situations during reversals have been investigated with respect to SPE-caused ozone losses. With decreasing magnetic field strength the impacts on the ozone are found to significantly increase especially in the Southern Hemisphere, and subsequently, the flux of harmful ultraviolet radiation increases at the Earth’s surface. The ozone destructions are most pronounced in the polar regions, and for some field configurations they exceed the values of ozone hole situations after large SPEs. In contrast to ozone holes the depletions due to SPEs are not restricted to winter and spring times but persist into polar summer.