Longitude

About: Longitude is a research topic. Over the lifetime, 2260 publications have been published within this topic receiving 54988 citations. The topic is also known as: angle of longitude.

Time variations in elemental abundances in solar energetic particle events

Abstract: The Solar Isotope Spectrometer (SIS) on-board the Advanced Composition Explorer has a large collection power and high telemetry rate, making it possible to study elemental abundances in large solar energetic particle (SEP) events as a function of time. Results have now been obtained for more than 25 such events. Understanding the causes of these variations is key to obtaining reliable solar elemental abundances and to understanding solar acceleration processes. Such variations have been previously attributed to two models: (1) a mixture of an initial impulsive phase having enhanced heavy element abundances with a longer gradual phase with coronal abundances and (2) rigidity dependent escape from CME-driven shocks through plasma waves generated by wave-particle interactions. In this second model the injected abundances are assumed to be coronal. Both these models can be expected to depend upon solar longitude since impulsive events are associated with flares at longitudes well-connected magnetically to the observer, and shock properties and connection of the observer to the shock are also longitude dependent. We present results on temporal variations from event to event and within events and show that they appear to have a longitude dependence. We show that the events which have been well-explained by model (2) tend to be near central meridian or the west limb. In addition, we show that there are events with little time variation and heavy element enhancements similar to those of impulsive events. These events seem to be better explained by model (1) with only an impulsive phase.

First direct measurement of the beaming of Jupiter's decametric radiation

Abstract: ONE of the most important features of the jovian decametric radioemission comes from the geometry of the observed radiation. The emission occurrence probability appears as a well defined function of the respective positions of Jupiter, the observer and the innermost galilean satellite Io, exhibiting several major sources which correspond to narrow ranges in zenocentric observer's longitude L (the so-called central meridian longitude) and in the synodic orbital phase of Io, Φ. These statistically established directivity properties were defined by Warwick1 and Alexander2, who showed that the repetition of the distinguishable dynamic spectra associated with each source occurring in a longitude range of 20°, is attributable to a ±10° beaming cone into which the radiation escapes in the ecliptic plane. Dulk3 stated more precisely that the emission is received on the Earth when Io makes certain angles with the line of sight, and that the dynamic spectrum varies in shape and frequency range with the magnetic longitude of Io. The beaming in latitude is also important4: the ±3.3° variation of the zenocentric declination of the Earth is sufficient to lead to a 15° variation of the statistical apparent sub-Io longitude of the emission. But these conclusions, based on statistics and on the repeatability of similar dynamic spectra in equivalent geometrical configurations of Jupiter, Io and the observer, are not decisive, as they were obtained from a single direction at a given time. We present here the first direct experimental proof of a narrow beaming of Jupiter radiation, using simultaneous observations in two different directions, during the French–Soviet space radio astronomy experiment Stereo-5.

Spatial variation of aerosol spectral optical depth and columnar water vapour content over the Arabian Sea and Indian Ocean during the IFP of INDOEX

Abstract: °E and 78°E longitude and 15°N and 20°S latitude) using a ten -channel multi wavelength solar radiometer (MWR) and a 4 -channel EKO Sun photometer on-board the cruise #141 of ORV Sagar Kanya during the Intense Field Phase (IFP) of the Indian Ocean Experiment (INDOEX) from 20 January to 12 March 1999. From these measurements, the columnar aerosol optical depths and columnar content of water vapour are estimated; and their spatial variations examined. The results show occurrence of fairly high values of aerosol optical depths along the West Coast of India. The optical depths decrease gradually as one moves down south, with an e –1 scaling distance of ~ 1200 to 1300 km. Extremely low (near zero) values are e ncountered in mid Indian Ocean due south of 10 ° latitude (i.e. on the south of the ITCZ) followed by a weak increase close to Mauritius. Presence of an exte nsive region of enhanced aerosol optical depths (which we call the West Asian High) is seen in t he mid -Arabian Sea between 5° and 10°N westward of 65 °E longitude which is stronger and wider than those seen along the coastal regions of India. This effect is seen at the lon gest wavelength (1025 nm) also. Variation of columnar water vapour content (W g cm –2 ) shows highest values (> 3 g cm –2 ) around the equator (~ 3°N to 5°S exten ding over the entire longitude region) possibly assoc iated with the ITCZ. Moderately high values are encountered along the coastal regions of India and also close to Mauritius. Compared to these, the northwest Arabian Sea (between 5 to 15°N and 60° to 70°E) shows rather dry atmosphere with W < 2 g cm –2 , ind icating prevalence of drier continental conditions. The implications of the findings are discussed. DURING the period January to March of 1999 extensive and co-ordinated ground based, and ship-borne experiments were conducted to estimate aerosol spectral optical depth over coastal India and the oceanic environments over the Arabian Sea and Indian Ocean between 15°N and 20°S lat itude and 60°E and 78°E longitude. These were carried out as part of the Intense Field Phase (IFP -99) of international programme INDOEX (Indian Ocean Experiment). The chief objective was to study the spatial distr ibution of aerosols along the I ndian Coast and over remote area of the Arabian Sea and Indian Ocean, and the role of the Inter Tropical Convergence Zone (ITCZ) in the spatial distribution of these at locations far away from potential source regions 1

Unseasonal development of post-sunset F-region irregularities over Southeast Asia on 28 July 2014: 2. Forcing from below?

Abstract: This contribution is the first of a two-part investigation into an unseasonal post-sunset equatorial F-region irregularity (EFI) event over the Southeast Asian region on the evening of 28 July 2014. Ground-based GPS scintillation data, space-based GPS radio occultation (RO) data, and ionosonde data show the existence of EFIs shortly after sunset over a region spanning 30° in longitude and 40° in latitude, centered on the geomagnetic equator. This post-sunset EFI event was observed during a time of the year when post-sunset equatorial plasma bubbles (EPBs) are very infrequent in the Southeast Asian longitude sector. GPS RO data shows that the EFI event over Southeast Asia coincided with the suppression of peak-season EPBs in the African and Pacific longitude sectors. Ionosonde data shows the presence of a strong pre-reversal enhancement (PRE) in the upward plasma drift over Southeast Asia prior to the detection of EFIs. Further, it is reported that this PRE was significantly stronger than on any other day of July 2014. An analysis of the geophysical conditions during this event reveals that this enhanced PRE was not caused by disturbed geomagnetic activity. Therefore, it is hypothesized that forcing from lower altitudes, perhaps tidal/planetary waves, was the potential cause of this strong PRE, and the subsequent EPB/EFI activity, on this day over the Southeast Asian sector.

Moist shallow-water response to tropical forcing: Initial-value problems

Abstract: The response of a spherical moist shallow water system to tropical imbalances in the presence of inhomogeneous saturation fields is examined. While the initial moist response is similar to the dry reference run, albeit with a reduced equivalent depth, the long time solution depends quite strikingly on the nature of the saturation field. For a saturation field that only depends on latitude, specifically, one with a peak at the equator and falls off meridionally in both hemispheres, height imbalances adjust to large-scale, low-frequency westward propagating modes. When the background saturation environment is also allowed to vary with longitude, in addition to a westward quadrupole, there is a distinct eastward propagating response at long times. The nature of this eastward propagating mode is well described by moist potential vorticity conservation and it consists of wave packets that arc out to midlatitudes and return to the tropics and are predominantly rotational in character. In all moist cases, initially formed Kelvin waves decay, and this appears to be tied to the off-equatorial organization of moisture anomalies by rotational modes. Many of these basic features carry over to the response in the presence of realistic saturation fields derived from reanalysis based precipitable water. In boreal summer, long time eastward response is restricted to northern hemisphere and takes the form of a wavetrain that passes over Indian landmass into the subtropics, reaching across the Pacific to North America. In boreal winter, the eastward mode consists of a subtropically confined rotational quadrupole along with midlatitudinal disturbances. Thus, in addition to circumnavigating westward Rossby waves, slow eastward propagating modes appear to be a robust feature of the shallow water system with interactive moisture in the presence of saturation fields that vary with latitude and longitude.