Asymmetries in Mars' Exosphere: Implications for X-ray and ENA Imaging

Observations and simulations show that Mars' atmosphere has large seasonal variations. Total atmospheric density can have an order of magnitude latitudinal variation at exobase heights. By numerical simulations we show that these latitude variations in exobase parameters induce asymmetries in the hydrogen exosphere that propagate to large distances from the planet. We show that these asymmetries in the exosphere produce asymmetries in the fluxes of energetic neutral atoms (ENAs) and soft X-rays produced by charge exchange between the solar wind and exospheric hydrogen. This could be an explanation for asymmetries that have been observed in ENA and X-ray fluxes at Mars.Comment: Submitted to Space Science Review. v2: Minor changes in text and figure

Kinetic-scale magnetic turbulence and finite Larmor radius effects at Mercury

We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near Mercury's space environment, with the emphasis on key boundary regions participating in the solar wind -- magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable time scale ~20 s imposed by the signal nonstationarity, suggesting that turbulence at this planet is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.Comment: 46 pages, 5 figures, 2 table

A Comparison Of Five Mechanical Work Algorithms For Different Footstrike Patterns And Speeds During Distance Running

The mechanical work done by a runner during an average stride cyde has been calculated with a variety of algorithms that generate values that may vary by an order of magnitude. The application of different algorithms to the same data set is uncommon, and does not seem to have been used at all to compare different foot strike patterns (FSP) during distance running. Average stride cycle values from five work algorithms for forefoot strike (ffs) and heel strike (hs) running at three different running speeds are presented. In general order from most to least restrictive: Wn allows no transfer between segments; Ww, within-segment transfer only; WwbAS, transfer within and between adjacent segments only; WwbLT, within and between segments of the same limb and the trunk; and, Wwb, within- and between-segment transfer with no restrictions. The primary difference in these algorithms is the amount of energy transfer they permit between and among body segments. Twelve highly skilled, male distance runners each ran with both FSP at three speeds ranging from 3.58 to 4.58 m-s-l. High-speed video (200 Hz) was used to track eight segment endpoint markers in the left sagittal plane. An ll-segment model was used with symmetry assumed to generate right side values. Among the algorithms, the no-transfer method (Wn) produced the highest work estimates. An absolute difference of -300 joules-stride-1 (-15-20%) existed across speeds between the no-transfer and within-transfer algorithms. There was then a relatively large decrease to the span of values generated from the other three algorithms. WwbAS was slightly higher than the remaining two algorithms, moreso in relative terms as speed increased. WwbLT increased slightly over speed (-40% slow->fast), while Wwb, the least restrictive, demonstrated almost no change across speeds (-1 % slow->fast). On average, these differences converged absolutely (75->20 joules-stride-1) and relatively (9.8%->2.5%) with increased speed; i.e., differences between the two .FSP decreased as speed increased. At all speeds for each algorithm, hs was lower than ffs. Collapsed across speeds, hs as percentage of ffs was 96.7 (Wn), 96.5 (ww)- 96.7 (WwbAS), 95.8 (WwbLT) and 89.4% (Wwb). Wwb across speeds consistently showed the largest relative differences between FSP, due perhaps in part to low absolute values. However, FSP differences still decreased with increased speed. This algorithm, therefore, appears to preserve the ordinal relationship and the trend in relative change between FSP across speeds reflected in the other four algorithms. Overall, the consistency across all algorithms of absolute and relative decrease between FSP with increased speed suggests variations in actual kinematics, not algorithms, are responsible for observed differences

High-power AlGaAs channeled substrate planar diode lasers for spaceborne communications

A high power channeled substrate planar AlGaAs diode laser with an emission wavelength of 8600 to 8800 A was developed. The optoelectronic behavior (power current, single spatial and spectral behavior, far field characteristics, modulation, and astigmatism properties) and results of computer modeling studies on the performance of the laser are discussed. Lifetest data on these devices at high output power levels is also included. In addition, a new type of channeled substrate planar laser utilizing a Bragg grating to stabilize the longitudinal mode was demonstrated. The fabrication procedures and optoelectronic properties of this new diode laser are described

Photoionization of Galactic Halo Gas by Old Supernova Remnants

We present new calculations on the contribution from cooling hot gas to the photoionization of warm ionized gas in the Galaxy. We show that hot gas in cooling supernova remnants (SNRs) is an important source of photoionization, particularly for gas in the halo. We find that in many regions at high latitude this source is adequate to account for the observed ionization so there is no need to find ways to transport stellar photons from the disk. The flux from cooling SNRs sets a floor on the ionization along any line of sight. Our model flux is also shown to be consistent with the diffuse soft X-ray background and with soft X-ray observations of external galaxies. We consider the ionization of the clouds observed towards the halo star HD 93521, for which there are no O stars close to the line of sight. We show that the observed ionization can be explained successfully by our model EUV/soft X-ray flux from cooling hot gas. In particular, we can match the H alpha intensity, the S++/S+ ratio, and the C+* column. From observations of the ratios of columns of C+* and either S+ or H0, we are able to estimate the thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K) thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources are required for the slow clouds to maintain their ~7000 K temperatures at these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa