T. T. von Rosenvinge

Two classes of solar energetic particle events associated with impulsive and long-duration soft X-ray flares

For the period 1978 September to 1983 December, 67 solar particle events have been identified for which the instruments detected electrons above 3 MeV and for which there are soft X-ray observations. The events are divided into two classes impulsive and long-duration - based on their signature in soft X-rays, and it is found that they have different properties. The events originating with impulsive flares are associated with intensed meter-wavelength type III bursts with associated type V continuum. The events associated with long-duration flares can originate anywhere on the solar disk, extend to much higher proton energies, and are well associated with coronal and interplanetary shocks; for about half of the long-duration events, the associated meter-wavelength events do not include type III bursts. The results discovered by Evenson et al. (1984) and by Kahler et al. (1984) are extended.

INTERACTING CORONAL MASS EJECTIONS AND SOLAR ENERGETIC PARTICLES

We studied the association between solar energetic particle (SEP) events and coronal mass ejections (CMEs) and found that CME interaction is an important aspect of SEP production. Each SEP event was associated with a primary CME that is faster and wider than average CMEs and originated from west of E45°. For most of the SEP events, the primary CME overtakes one or more slower CMEs within a heliocentric distance of ∼20 R⊙. In an inverse study, we found that for all the fast (speed greater than 900 km s^(-1)) and wide (width greater than 60°) western hemispheric frontside CMEs during the study period, the SEP-associated CMEs were ∼4 times more likely to be preceded by CME interaction than the SEP-poor CMEs; i.e., CME interaction is a good discriminator between SEP-poor and SEP-associated CMEs. We infer that the efficiency of the CME-driven shocks is enhanced as they propagate through the preceding CMEs and that they accelerate SEPs from the material of the preceding CMEs rather than from the quiet solar wind. We also found a high degree of association between major SEP events and interplanetary type II radio bursts, suggesting that proton accelerators are also good electron accelerators.

The interplanetary acceleration of energetic nucleons

Corotating proton and electron streams are the dominant type of low-energy (i.e., 0.1--10 MeV per nucleon) particle event observed at 1 AU. The radial dependence of these events has been studied between 1 and 4 AU using essentially identical low-energy detector systems on IMP-7, Pionner-10, and Pioneer-11. It had been expected that at a given energy the intensity of these streams would decrease rapidly with heliocentric distance due to the effects of interplanetary adiabatic deceleration. Instead it is observed that from event to event the intensity either remains roughly constant or increases significantly (more than an order of magnitude) between 1 and 4 AU. It appears that interplanetary acceleration processes are the most plausible explanation. Several possible acceleration models are discussed. (AIP)

Cosmic ray modulation and the solar magnetic field

We show that the variations of the interplanetary magnetic field strength (B) over a 22-year period are tracked by the inverted profile of the cosmic ray density measured by neutron monitors. We suggest that global changes in the Suns magnetic field are more important for long-term modulation than magnetic field enhancements resulting from the merging of high-speed flows and coronal mass ejections in the outer heliosphere. The unexpectedly close relationship that we find between the “tilt angle” of the heliospheric current sheet and the cosmic ray density away from solar minimum for both polarity states of the solar magnetic field may be accounted for by the anticorrelation between the cosmic ray density and field strength variations.

PET: a proton/electron telescope for studies of magnetospheric, solar, and galactic particles

The proton/electron telescope (PET) on SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) is designed to provide measurements of energetic electrons and light nuclei from solar, Galactic, and magnetospheric sources. PET is an all solid-state system that will measure the differential energy spectra of electrons from approximately 1 to approximately 30 MeV and H and He nuclei from approximately 20 to approximately 300 MeV/nucleon, with isotope resolution of H and He extending from approximately 20 to approximately 80 MeV/nucleon. As SAMPEX scans all local times and geomagnetic cutoffs over the course of its near-polar orbit, PET will characterize precipitating relativistic electron events during periods of declining solar activity, and it will examine whether the production rate of odd nitrogen and hydrogen molecules in the middle atmosphere by precipitating electrons is sufficient to affect O/sub 3/ depletion. In addition, PET will complement studies of the elemental and isotopic composition of energetic heavy (Z>2) nuclei on SAMPEX by providing measurements of H, He, and electrons. Finally, PET has limited capability to identify energetic positrons from potential natural and man-made sources. >

RECORD-SETTING COSMIC-RAY INTENSITIES IN 2009 AND 2010

We report measurements of record-setting intensities of cosmic-ray nuclei from C to Fe, made with the Cosmic Ray Isotope Spectrometer carried on the Advanced Composition Explorer in orbit about the inner Sun-Earth Lagrangian point. In the energy interval from ~70 to ~450 MeV nucleon^(–1), near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997). The elevated intensities reported here and also at neutron monitor energies were undoubtedly due to several unusual aspects of the solar cycle 23/24 minimum, including record-low interplanetary magnetic field (IMF) intensities, an extended period of reduced IMF turbulence, reduced solar-wind dynamic pressure, and extremely low solar activity during an extended solar minimum. The estimated parallel diffusion coefficient for cosmic-ray transport based on measured solar-wind properties was 44% greater in 2009 than in the 1997-1998 solar-minimum period. In addition, the weaker IMF should result in higher cosmic-ray drift velocities. Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations, indicating that significant solar modulation occurs inside ~20 AU, consistent with earlier galactic cosmic-ray radial-gradient measurements. In 2010, the intensities suddenly decreased to 1997 levels following increases in solar activity and in the inclination of the heliospheric current sheet. We describe the conditions that gave cosmic rays greater access to the inner solar system and discuss some of their implications.

The composition of solar energetic particles

The present measurements of elemental abundances in 15 large solar energetic particle events confirm the existence of two major effects: systematic differences between solar energetic particle abundances and photospheric abundances that are approximately correlated with first ionization potential (implying that the solar corona is the likely source population for these particles), and an enhancement of heavy ion abundances relative to a baseline of solar energetic particle abundances whose magnitude increases with rising atomic number. These data suggest the possibility that the degree of heavy ion enhancement has some correlation with spectral slope, and confirm that the aforementioned effects are not due to time or energy variations within individual events. The maximum column density above the solar acceleration region is less than 0.1 g/sq cm, on the basis of these data.

Corotating MeV/amu ion enhancements at ≤1 AU from 1978 to 1986

MeV/amu ion enhancements associated with corotating high-speed solar wind streams in 1978–1986 during pre-solar maximum to near solar minimum conditions are studied using ISEE 3/ICE, IMP 8, and Helios 1 data. Around 50% of corotating streams contain energetic ion increases. These increases extend to ∼25 MeV/amu, where they merge into the galactic cosmic ray background, and are most evident approaching solar minimum. Sunward ion streaming in the solar wind frame (first-order anisotropy ∼20%) and positive radial intensity gradients (∼400%/AU) are consistent with acceleration in the outer heliosphere at corotating shocks followed by streaming into the inner heliosphere. The spectra and intensities show little solar cycle variation. The spectra of ions from protons to Fe at ∼2–20 MeV/amu are approximated equally well by exponentials in momentum dJ/dP ≈ exp (−P/P0), P0 = 11–16 MeV c−1 amu−1, or by distribution functions ƒ ≈ exp (−υ/υ0), υ0 = 0.18–0.25 (MeV/amu)1/2, with equivalent power law in energy slopes in the range ∼ −3 to −4. Ion abundances are correlated with the stream peak solar wind speed. In slower corotating streams (maximum solar wind speed <600 km/s), mean abundance ratios are protons/4He = 43 ± 18; 4He/O = 54 ± 23; C/O = 0.62 ± 0.06; Mg/O = 0.19 ± 0.03, and Fe/O = 0.14 ± 0.02. These show some similarity to the corresponding ratios for “solar energetic particles” (SEP) (protons/4He = 70 ± 10; 4He/O = 55 ± 3; C/O = 0.48 ± 0.02; Mg/O = 0.21 ± 0.01 and Fe/O = 0.16 ± 0.02) which are typically accelerated by shocks passing through slow solar wind. In corotating events in higher-speed streams, these ratios become protons/4He = 19 ± 5; 4He/O = 130 ± 35; C/O = 0.89 ± 0.05; Mg/O = 0.14 ± 0.01, and Fe/O = 0.10 ± 0.01 and more closely resemble the corotating event abundance ratios measured in high-speed streams during the mid-1970s solar minimum (protons/4He = 17 ± 7; 4He/O ∼ 160 ± 50; C/O = 0.89 ± 0.1; Mg/O = 0.13 ± 0.03, and Fe/O = 0.096 ± 0.05). Solar wind plasma may also show similar variations in composition with solar wind speed (based on the limited solar wind composition measurements available) so that the energetic ion compositions are consistent with the acceleration of corotating event ions and SEPs from the solar wind. The ordering of corotating event and solar wind abundances by first ionization potential and their variation with solar wind speed suggest that conditions in the ion-neutral fractionation region in the upper chromosphere determine the abundances and are associated in some way with regulation of the solar wind speed.

Cosmic ray decreases: 1964–1994

We have studied 30 years (1964–1994) of neutron monitor data in order to understand the principle mechanisms causing short-term (< 20-day duration) cosmic ray decreases seen at Earth. By examining the characteristics of associated low energy (<200 MeV) particle enhancements in combination with the neutron monitor data, we have determined the responsible solar wind disturbances for 153 of the 180 ≥ 4% decreases. The vast majority (86% of the 153 events) are caused by coronal mass ejections and the shocks that they generate. The ejecta is intercepted only when the solar event originates within 50° of the Suns central meridian. For more distant events, only the shock is intercepted at Earth. We present a list of all 180 events seen in the years 1964–1994 together with the associated solar event, when this can be determined, and some details about the solar wind structures based on in situ solar wind data, if available. This list represents a compendium of major solar wind disturbances affecting a large section of the inner heliosphere over this time period. We also discuss enhanced daily variations in some events which are related to radial gradients caused by strong disturbances inside the Earths orbit.

Two components in major solar particle events

A study has been made of 29 intense, solar particle events observed in the energy range 25–80 MeV/nuc near Earth in the years 1997 through 2001. It is found that the majority of the events (19/29) had Fe/O ratios that were reasonably constant with time and energy, and with values above coronal. These all originated on the Suns western hemisphere and most had intensities that rose rapidly at the time of an associated flare (and coronal mass ejection). Interplanetary shocks observed near Earth had little effect on particle intensities during these events. The remaining 10 events had different intensity-time profiles and Fe/O ratios that varied with time and energy with event-averaged values at or below coronal. Most of these originated near central meridian and 6 had strong interplanetary shocks that were observed near Earth. There were four events with two peaks in the intensity-time profiles, the first near the time of the associated flare (with high Fe/O) and the other at shock passage (with a lower Fe/O) suggesting that solar particle events have two components. At high rigidities the first component (probably flare generated) usually dominates and interplanetary shock-accelerated particles (forming the second component) make a minor contribution except in the case of unusually fast shocks.