207 research outputs found
Ionospheric storms at geophysically-equivalent sites – Part 2: Local time storm patterns for sub-auroral ionospheres
The response of the mid-latitude ionosphere to geomagnetic storms depends
upon several pre-storm conditions, the dominant ones being season and local
time of the storm commencement (SC). The difference between a site's
geographic and geomagnetic latitudes is also of major importance since it
governs the blend of processes linked to solar production and magnetospheric
input, respectively. Case studies of specific storms using ionospheric data
from both hemispheres are inherently dominated by seasonal effects and the
various local times versus longitude of the SCs. To explore
inter-hemispheric consistency of ionospheric storms, we identify
"geophysically-equivalent-sites" as locations where the geographic and
geomagnetic latitudes have the same relationship to each other in both
hemispheres. At the longitudes of the dipole tilt, the differences between
geographic and geomagnetic latitudes are at their extremes, and thus these
are optimal locations to see if pre-conditioning and/or storm-time input are
the same or differ between the hemispheres.
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In this study, we use ionosonde values of the F2-layer maximum electron
density (<I>Nm</I>F2) to study geophysical equivalency at Wallops Island (VA) and
Hobart (Tasmania), using statistical summaries of 206 events during solar
cycle #20. We form average patterns of Δ<I>Nm</I>F2 (%) versus local
time over 7-day storm periods that are constructed in ways that enhance the
portrayal of the average characteristic features of the positive and
negative phases of ionospheric storms. The results show a consistency
between four local time characteristic patterns of storm-induced
perturbations, and thus for the average magnitudes and time scales of the
processes that cause them in each hemisphere. Subtle differences linked to
small departures from pure geophysical equivalency point to a possible
presence of hemispheric asymmetries governed by the non-mirror-image of
geomagnetic morphology in each hemisphere
Periodicities in the occurrence of aurora as indicators of solar variability
A compilation of records of the aurora observed in China from the Time of the Legends (2000 - 3000 B.C.) to the mid-18th century has been used to infer the frequencies and strengths of solar activity prior to modern times. A merging of this analysis with auroral and solar activity patterns during the last 200 years provides basically continuous information about solar activity during the last 2000 years. The results show periodicities in solar activity that contain average components with a long period (approx. 412 years), three middle periods (approx. 38 years, approx. 77 years, and approx. 130 years), and the well known short period (approx. 11 years)
The 1999 Quadrantids and the lunar Na atmosphere
Enhancements of the Na emission and temperature from the lunar atmosphere were reported during the Leonid meteor showers of 1995, 1997 and 1998. Here we report a search for similar enhancement during the 1999 Quadrantids, which have the highest mass flux of any of the major streams. No enhancements were detected. We suggest that different chemical-physical properties of the Leonid and Quadrantid streams may be responsible for the differenc
1999 Quadrantids and the lunar Na atmosphere
Enhancements of the Na emission and temperature from the lunar atmosphere
were reported during the Leonids meteor showers of 1995, 1997 and 1998. Here we
report a search for similar enhancement during the 1999 Quadrantids, which have
the highest mass flux of any of the major streams. No enhancements were
detected. We suggest that different chemical-physical properties of the Leonid
and Quadrantid streams may be responsible for the difference.Comment: 5 pages, 1 figure, accepted for publication in MNRA
Brightening of 630.0 nm equatorial spread-F airglow depletions
[1] Observations from the Boston University all-sky imaging system at Arecibo, Puerto Rico (18.3°N, 66.7°W, 28°N mag), show an unusual behavior of nighttime 630.0-nm airglow depletions. Associated with equatorial spread-F (ESF), these structures move eastward before reversing their motion and become airglow enhancements. Few other cases have been found, all during December solstices. For the case study presented here, data from the Arecibo incoherent scatter radar and the Republic of China Scientific Satellite (ROCSAT-1) provide supporting information. The radar shows that around local midnight the background zonal and meridional plasma motions reverse to westward and southward, respectively. ROCSAT-1 shows enhanced ion density, i.e., a low-latitude plasma blob, above the bright feature recorded by the all-sky imager, indicating a possible connection between both phenomena. Drifts parallel to the magnetic field are observed only in the region where the enhancement occurs. One possible interpretation of this change in the brightness of the depleted structure involves the influence of northward meridional winds and a reversal in the zonal drift motion, most likely caused by a zonal wind reversal
Planetary Imaging Concept Testbed Using a Recoverable Experiment-Coronagraph (PICTURE C)
An exoplanet mission based on a high-altitude balloon is a next logical step in humanity's quest to explore Earthlike planets in Earthlike orbits orbiting Sunlike stars. The mission described here is capable of spectrally imaging debris disks and exozodiacal light around a number of stars spanning a range of infrared excesses, stellar types, and ages. The mission is designed to characterize the background near those stars, to study the disks themselves, and to look for planets in those systems. The background light scattered and emitted from the disk is a key uncertainty in the mission design of any exoplanet direct imaging mission, thus, its characterization is critically important for future imaging of exoplanets
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