M. D. Looper

NEW HIGH TEMPORAL AND SPATIAL RESOLUTION MEASUREMENTS BY SAMPEX OF THE PRECIPITATION OF RELATIVISTIC ELECTRONS

Abstract The precipitation of electrons of 150 keV and 1 MeV in the outer zone have been measured by an instrument aboard the low-altitude, polar-orbiting SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer)satellite. This instrument has an extremely large geometric factor (100 cm2 sr at 1 MeV) and is sampled ten times per second. Broad areas of strong precipitation, extending ∼ 2–3° in latitude, frequently are observed near the high-latitude boundary of the outer zone. These features can persist for hours and are seen in conjugate locations. A transient form of strong precipitation, a microburst, is also seen regularly. Microbursts often are seen lasting for less than a second, indicating that microbursts sometimes occur in a very localized region; the narrow temporal structure is a consequence of the orbital velocity of SAMPEX. In other cases, where the spatial size is greater, the temporal evolution of the microburst can be followed. These sum of these observations clearly indicates that outer-zone electron precipitation frequently results from a strong scattering process, and not by weak diffusion of stably trapped electrons into the drift loss cone.

Response of the Inner Radiation Belt to the Violent Sun-Earth Connection Events of October-November 2003

We report observations of the response of the low-altitude radiation population below L = 3 during and after the strong solar energetic particle events and geomagnetic disturbances of late October and early November 2003, and we place this response in the context of observations throughout the 12-year SAMPEX mission. We find that on 29 October 2003, at the approximately 600 km altitude of SAMPEX, the usual belt of energetic protons (above 19 MeV) around L = 2 almost completely disappeared, recovering only after several months. We also observed the appearance of a new belt of ultrarelativistic (above 10 MeV) electrons centered around L = 2. In the previous twelve years, we have never observed such a large decrease of the energetic protons at low altitude. An injection of very high energy electrons like this has not been seen since February 1994.

Charge State Measurements of Solar Energetic Particles Observed with SAMPEX

We have measured the ionization states of major elements accelerated in two large solar energetic particle events by use of the geomagnetic cutoff technique. The observations were made in 1992 October/November using instrumentation on the SAMPEX satellite, which is in an 82° inclination low Earth orbit. We calibrate the geomagnetic cutoff rigidity by assuming that protons and He observed on SAMPEX are fully stripped. The mean charge states of the heavier elements are then determined from their measured cutoffs, along with this calibrated scale. Our results cover the energy range ~0.5-5 MeV nucleon^(-1) and are in good agreement with earlier studies by Luhn et al. (1985) near -1 MeV nucleon^(-1) in all cases except for Fe. We find a mean charge state of 11.04±0.22 for Fe, while the average from Luhn is 14.09±0.09. The lower ionization state of Fe reported here appears to be consistent with the general range of values recently measured in solar wind Fe on the Ulysses deep space probe. Although the observational picture is still incomplete, overall the present results confirm and extend previous evidence that the solar energetic particles in large events are accelerated from the coronal or solar wind material.

Charge state of anomalous cosmic-ray nitrogen, oxygen, and neon: SAMPEX observations

We report observations of the ionization state of anomalous cosmic-ray (ACR) nitrogen, oxygen, and neon during the period 1992 October to 1993 May, carried out with instrumentation on the Solar, Anomalous & Magnetospheric Particle Explorer (SAMPEX) spacecraft. The low-altitude (510 x 675 km) and high-inclination (82 deg) orbit enables SAMPEX to sample the interplanetary ACR fluxes on each polar pass and then to observe the cutoff of these fluxes by the geomagnetic field at lower latitudes. The arrival time and direction of each ion is recorded by the instruments, allowing detailed calculations of the particles trajectory through the Earths magnetic field and thereby placing upper limits on the ionization state of the particles. We find (a) that ACR nitrogen, oxygen, and neon each contain singly ionized particles and (b) that ACR oxygen is predominantly singly ionized with an upper limit of 10% for higher ionization states. These ionization states confirm theories of ACR origin as neutral interstellar material that is singly ionized near the Sun by UV or charge exchange with the solar wind, and is subsequently accelerated in the outer heliosphere.

Understanding large SEP events with the PATH code: Modeling of the 13 December 2006 SEP event

The Particle Acceleration and Transport in the Heliosphere (PATH) numerical code was developed to understand solar energetic particle (SEP) events in the near-Earth environment. We discuss simulation results for the 13 December 2006 SEP event. The PATH code includes modeling a background solar wind through which a CME-driven oblique shock propagates. The code incorporates a mixed population of both flare and shock-accelerated solar wind suprathermal particles. The shock parameters derived from ACE measurements at 1 AU and observational flare characteristics are used as input into the numerical model. We assume that the diffusive shock acceleration mechanism is responsible for particle energization. We model the subsequent transport of particles originated at the flare site and particles escaping from the shock and propagating in the equatorial plane through the interplanetary medium. We derive spectra for protons, oxygen, and iron ions, together with their time-intensity profiles at 1 AU. Our modeling results show reasonable agreement with in situ measurements by ACE, STEREO, GOES, and SAMPEX for this event. We numerically estimate the Fe/O abundance ratio and discuss the physics underlying a mixed SEP event. We point out that the flare population is as important as shock geometry changes during shock propagation for modeling time-intensity profiles and spectra at 1 AU. The combined effects of seed population and shock geometry will be examined in the framework of an extended PATH code in future modeling efforts.

A background correction algorithm for Van Allen Probes MagEIS electron flux measurements

We describe an automated computer algorithm designed to remove background contamination from the Van Allen Probes Magnetic Electron Ion Spectrometer (MagEIS) electron flux measurements. We provide a detailed description of the algorithm with illustrative examples from on-orbit data. We find two primary sources of background contamination in the MagEIS electron data: inner zone protons and bremsstrahlung X-rays generated by energetic electrons interacting with the spacecraft material. Bremsstrahlung X-rays primarily produce contamination in the lower energy MagEIS electron channels (∼30–500 keV) and in regions of geospace where multi-MeV electrons are present. Inner zone protons produce contamination in all MagEIS energy channels at roughly L < 2.5. The background-corrected MagEIS electron data produce a more accurate measurement of the electron radiation belts, as most earlier measurements suffer from unquantifiable and uncorrectable contamination in this harsh region of the near-Earth space environment. These background-corrected data will also be useful for spacecraft engineering purposes, providing ground truth for the near-Earth electron environment and informing the next generation of spacecraft design models (e.g., AE9).

SAMPEX observations of storm‐associated electron flux variations in the outer radiation belt

Flux variations of the outer radiation belt electrons (> 1-MeV) during the main phase and early recovery phase of 25 geomagnetic storms are studied using data obtained by the Heavy Ion Large Telescope (HILT) experiment onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite. Employing a simple model for the ring current field, we examine the degree to which the decrease of electron flux during the early main phase is attributable to adiabatic deceleration processes in response to changes in the magnetic field. Such an adiabatic response is shown to be detected most clearly for 4 < L < 5. In the lower L region (2 < L < 4) the electron flux decrease is less prominent and at times increases during the main phase of intense storms. On the other hand, in the region 5 < L < 7 the level of the electron decrease is larger than that expected from the adiabatic response alone. These observations suggest that the energetic electrons are trapped effectively near the inner edge of the outer radiation belt probably because of sudden inward transport and acceleration of the electrons during the main phase. The reduced flux of electrons returns to the normal level during the early recovery phase, even exceeding the prestorm level after about 1–2 days for intense storms. An outward diffusion process of the electrons at the inner edge, which are trapped during the main phase, could at least account partly for this observation. The low-altitude observation of precipitating electrons supports the recirculation model for radiation belt electron dynamics during magnetic storms.

How efficient are coronal mass ejections at accelerating solar energetic particles

The largest solar energetic particle (SEP) events are thought to be due to particle acceleration at a shock driven by a fast coronal mass ejection (CME). We investigate the efficiency of this process by comparing the total energy content of energetic particles with the kinetic energy of the associated CMEs. The energy content of 23 large SEP events from 1998 through 2003 is estimated based on data from ACE, GOES, and SAMPEX, and interpreted using the results of particle transport simulations and inferred longitude distributions. CME data for these events are obtained from SOHO. When compared to the estimated kinetic energy of the associated coronal mass ejections (CMEs), it is found that large SEP events can extract ~10% or more of the CME kinetic energy. The largest SEP events appear to require massive, very energetic CMEs.

Maps of hydrogen isotopes at low altitudes in the inner zone from SAMPEX observations.

The PET instrument aboard the SAMPEX satellite has provided us with long-term intra-calibrated observations of geomagnetically trapped protons and deuterons in the inner zone, suitable for use in constraining the low-altitude portions of radiation belt models being developed as successors to AP-8. These observations have been summarized elsewhere (Looper et al., 1996). Here we report a detection of geomagnetically-trapped tritum at energies from 14 to 35 MeV/nuc below L = 1.2, at about 1/8 the flux of deuterium previously reported at that location and at similar energy per nucleon. We also demonstrate the utility of the SAMPEX/PET observations for measuring the east-west anisotropy in the trapped particle flux at low altitudes, which is due to displacement of particle gyrocenters from the position of observation in a region of strong flux gradients. This anisotropy is implicitly ignored in omnidirectional radiation-flux models, but it can be important to mission planners considering how to distribute shielding over the surface of oriented spacecraft in low Earth orbit.

Anomalous cosmic ray argon and other rare elements at 1-4 MeV/nucleon trapped within the Earth's magnetosphere

We summarize over 6 years of observations of ∼1–4 MeV/nucleon heavy ions trapped in the Earths magnetosphere on L shells of 1.7–3. We obtained these new results in low Earth orbit with the SAMPEX spacecraft; they extend the observations of trapped heavy ions in this L range to much lower energies than had previously been examined in detail. At 1–4 MeV/nucleon we observed a trapped population with a peak intensity near L∼2.3 that includes the anomalous cosmic ray species O, Ne, and Ar also observed in interplanetary space at 1 AU. We also found elements with low first ionization potential (C, Mg-S, and Fe) trapped with the same spatial distribution. The low-energy trapped population increased in intensity between 1996 and 1997, roughly during solar minimum and minimum geomagnetic activity. It is possible that the 1–4 MeV/nucleon trapped population originates from a number of sources, including high-energy trapped anomalous cosmic rays that have lost energy in the residual atmosphere in the case of O and Ne, and directly incident, singly charged anomalous cosmic rays that have become stripped and subsequently trapped in the case of Ar. The group of trapped elements with low first ionization potential (C, Mg-S, and Fe) have roughly solar wind abundances relative to one another, suggesting a possible link between this trapped component and recently discovered solar wind pickup ions released from dust grains within the inner heliosphere.