R. P. Lin

The solar wind interaction with Mars: Locations and shapes of the bow shock and the magnetic pile-up boundary from the observations of the MAG/ER Experiment onboard Mars Global Surveyor

The Mars Global Surveyor spacecraft was inserted into an elliptical orbit around Mars on September 12, 1997. It includes the MAG/ER instrument with two magnetometers providing in-situ sensing of the ambient magnetic field and an electron reflectometer measuring the local distribution function of the electrons in the energy range of 10 eV to 20 keV. This statistical study deals with the identification and the position of the Bow Shock (BS) and of another plasma boundary, the Magnetic Pile-up Boundary (MPB), proved as permanent by MAG/ER. During the first year of the MGS mission, a total of 290 orbits have been considered to fit the geometric characteristics of these boundaries. The position and shape of these boundaries are compared with previous studies. Good agreement is found with the Phobos 2 observations, suggesting than the mean bow shock and MPB locations are independent of solar cycle phase. The great number of crossings shows that the Bow Shock position and nightside MPB position are highly variable.

Solar hard x-ray microflares

Using balloon-borne instrumentation of very high sensitivity, we have detected approx.25 hard x-ray bursts with peak fluxes of above 7 x 10/sup -3/ (cm/sup 2/ s keV)/sup -1/ at 20 keV, in 141 minutes of observation of the Sun on 1980 June 27. These hard x-ray microflares last from a few seconds to several tens of seconds and have power-law energy spectra. They are generally accompanied by small soft x-ray bursts, but H..cap alpha.

Solar He-3-rich events and non-relativistic electron events: A new association

In 15 months of observation by the ISEE-e spacecraft, it was found that virtually all solar greater than or approximately equal to 1.3 MeV/nucleon He-3-rich events are associated with impulsive 2 to approximately 100 keV electron events, although many electron events were not accompanied by detectable He-3 increases. Both the He-3 and the electrons exhibit nearly scatter-free propagation in the interplanetary medium, and the times of onset and maximum for the He-3 and electron increases are closely related by velocity dispertion. The electron events and their related type III solar radio bursts provide, for the first time, identification of the flares which produce He-3-rich events. He-3 appears to be accelerated at the flash phase of solar flares along with nonrelativistic electrons.

First-order fermi acceleration in solar flares as a mechanism for the second-step acceleration of prompt protons and relativistic electrons

We find that for two of the hard X-ray bursts of an energetic flare on 1980 June 27, the time profile of the hard X-rays above 235 keV is delayed by 3 s with respect to the time profiles of the lower energy X-rays and that the high energy spectrum becomes flatter with time during each of these bursts. From these findings we argue that during this flare a second-step mechanism accelerated further some of the high-energy tail population of the first-step electrons. By noticing that all of the flares with second-step delays produced interplanetary energetic protons, and that gamma-ray lines were detected from all of these flares except one that was not observed by any gamma-ray detector, we conclude that the second-step mechanism accelerates not only (mildly) relativistic electrons but also protons and heavy nuclei. Small delays of the nuclear gamma-ray time profiles with respect to the hard X-ray time profiles observed by SMM from the 1980 June 7 and 21 flares are consistent with this conclusion. After estimating the acceleration rate, we conclude that first-order Fermi acceleration operating in a closed flare loop is a very likely mechanism for the second-step acceleration.

Particle acceleration in cosmic plasmas

Introduction.- Ion Acceleration at the Earths Bow Shock.- Particle Acceleration in the Magnetotail andAurora.- Acceleration of Particles to High Energies in Earths Radiation Belts.- High-energy Atmospheric Physics: Terrestrial Gamma-ray Flashes and Related Phenomena.- Particle Acceleration at the Sun, Current Fragmentation and Particle Acceleration in Solar Flares.- Heliospheric Particle Acceleration.- The Acceleration Mechanism of Anomalous Cosmic Rays.- Particle Acceleration in Relativistic Outflows.- Pulsar Wind Nebulae as Cosmic Pevatrons: A Current Sheets Tale.- Observational Signatures of Particle Acceleration in Supernova Remnants.- Particle Acceleration Processes in the Heliosphere: Implications for Astrophysics.- Langmuir Turbulence and Suprathermal Electron Acceleration.- Diffusive Shock Acceleration and Magnetic Field Amplification.- Kinetic Simulations of Collisionless Shocks.- Relativistic Reconnection and Particle Acceleration.- Stochastic Acceleration by Turbulence.- Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration.- Cyclotron Resonant Interactions in Cosmic Particle Accelerators.

Ion beams observed in the near Earth plasma sheet region on May 10, 1996

This Letter reports observations of ion distributions made by the Wind 3D plasma experiment on May 10, 1996 during the 0400 UT substorm. The observations come from the 3D ion analyzer with a geometrical factor two orders of magnitude larger than most of the similar instruments flown through the near-earth geomagnetic tail. This has permitted observations of detailed ion beam characteristics during the passage of the spacecraft across the plasma sheet boundary into the lobe where the density is very low. The plasma initially consists of cold and warm components, with an additional hot component observed as the spacecraft approaches the plasma sheet-lobe interface. The beams originate from the warm component, which is the most dynamic. The Wind detector was even able to detect a weak beam inside the lobe where the density is ≲0.01 cm−3. Some of these observations are new and are not completely explained by current theories.

IMF induced changes to the nightside magnetotail: A comparison between WIND/Geotail/IMP 8 observations and modeling

A 3-D global fluid simulation is used to investigate the changing magnetic field topology of the magnetotail as observed by Geotail and IMP 8. The event studied is of particular interest as the solar wind density and speed as observed by WIND were approximately constant so that the influence of the interplanetary magnetic field (IMF) can be isolated. Loading of the tail fields during southward IMF is seen at high latitudes with IMP 8 moving from the sheath into the magnetosphere while at low latitudes Geotail moves from the plasma sheet into the lobes. The reverse is true for northward turnings. The tail cross-section is shown to be elliptical during southward IMF with an eccentricity of about 0.2 and that this eccentricity is slowly eroded over a period of about an hour during northward IMF.

Quiet-time solar wind superhalo electrons at solar minimum

We survey the statistical properties of ∼2-20 keV superhalo electrons in the solar wind measured by the STEREO/STE instrument during quiet-time periods from 2007 March through 2009 March at solar minimum. The observed quiet-time superhalo electrons have a nearly isotropic angular distribution and a power-law spectrum, f αv−γ, with γ ranging from 5 to 8.7, with nearly half between 6.5 and 7.5, and an average index of 6.69±0.90. The integrated density of quiet-time superhalo electrons at 2-20 keV ranges from 10−8 cm−3 to 10−6 cm−3, about 10−9-10−6 of the solar wind density, and it, as well as the power-law spectrum, shows no correlation with solar wind protons. The density of superhalo electrons decreases by approximately one order of magnitude between early 2007 and early 2009, probably reflecting the decay of solar cycle 23 and the approach to its unusually deep activity minimum, while the power-law spectral index γ has no solar-cycle variation. These quiet-time superhalo electrons are present even in the...

An Observation of SN1987A with a New High Resolution Gamma-Ray Spectrometer

The discovery (Matz et al. [1]) of gamma-ray line emission at 847 and 1238 keV from radioactive 56Co in the recent supernova SN1987A proved that explosive nucleosynthesis occurred in this supernova. Gamma-ray light curves derived from these and subsequent observations (Cook et al. [2], Mahoney et al. [3], Sandie et al. [4], Rester et al. [5], Teegarden et al. [6]) have a broad plateau from August 1987 to October 1988, with an 847 keV flux of ~ 7×l0-4 ph/cm2-sec (Tueller et al. [7]). The early detection of gamma rays required the inclusion of mixing or clumping in the models, (e.g. Pinto and Woosley [8] and Chan and Lingenfelter [9]). The gamma-ray fluxes are predicted, e.g. Bussard et al. [10], to peak at about day 400 and then decrease, by a factor of ~ 6 at day 800, as the effect of increasing transparency becomes dominated by radioactive decay. Then they should depend primarily on the amount of 56Co produced and little on the degree on mixing since most of the 56Co should be exposed. Thus measurements of the 56Co gamma-ray line fluxes and profiles will continue to be important during the decline of SN1987A.