87 research outputs found
Empirically modelled Pc3 activity based on solar wind parameters
It is known that under certain solar wind (SW)/interplanetary magnetic
field (IMF) conditions (e.g. high SW speed, low cone angle) the occurrence of
ground-level Pc3–4 pulsations is more likely. In this paper we demonstrate
that in the event of anomalously low SW particle density, Pc3 activity is
extremely low regardless of otherwise favourable SW speed and cone angle. We
re-investigate the SW control of Pc3 pulsation activity through a statistical
analysis and two empirical models with emphasis on the influence of SW
density on Pc3 activity. We utilise SW and IMF measurements from the OMNI
project and ground-based magnetometer measurements from the MM100 array to
relate SW and IMF measurements to the occurrence of Pc3 activity. Multiple
linear regression and artificial neural network models are used in iterative
processes in order to identify sets of SW-based input parameters, which
optimally reproduce a set of Pc3 activity data. The inclusion of SW density
in the parameter set significantly improves the models. Not only the density
itself, but other density related parameters, such as the dynamic pressure of
the SW, or the standoff distance of the magnetopause work equally well in the
model. The disappearance of Pc3s during low-density events can have at least
four reasons according to the existing upstream wave theory: 1. Pausing the
ion-cyclotron resonance that generates the upstream ultra low frequency waves
in the absence of protons, 2. Weakening of the bow shock that implies less
efficient reflection, 3. The SW becomes sub-Alfvénic and hence it is not
able to sweep back the waves propagating upstream with the Alfvén-speed,
and 4. The increase of the standoff distance of the magnetopause (and of the
bow shock). Although the models cannot account for the lack of Pc3s during
intervals when the SW density is extremely low, the resulting sets of optimal
model inputs support the generation of mid latitude Pc3 activity
predominantly through upstream waves
Broadband Meter-Wavelength Observations of Ionospheric Scintillation
Intensity scintillations of cosmic radio sources are used to study
astrophysical plasmas like the ionosphere, the solar wind, and the interstellar
medium. Normally these observations are relatively narrow band. With Low
Frequency Array (LOFAR) technology at the Kilpisj\"arvi Atmospheric Imaging
Receiver Array (KAIRA) station in northern Finland we have observed
scintillations over a 3 octave bandwidth. ``Parabolic arcs'', which were
discovered in interstellar scintillations of pulsars, can provide precise
estimates of the distance and velocity of the scattering plasma. Here we report
the first observations of such arcs in the ionosphere and the first broad-band
observations of arcs anywhere, raising hopes that study of the phenomenon may
similarly improve the analysis of ionospheric scintillations. These
observations were made of the strong natural radio source Cygnus-A and covered
the entire 30-250\,MHz band of KAIRA. Well-defined parabolic arcs were seen
early in the observations, before transit, and disappeared after transit
although scintillations continued to be obvious during the entire observation.
We show that this can be attributed to the structure of Cygnus-A. Initial
results from modeling these scintillation arcs are consistent with simultaneous
ionospheric soundings taken with other instruments, and indicate that
scattering is most likely to be associated more with the topside ionosphere
than the F-region peak altitude. Further modeling and possible extension to
interferometric observations, using international LOFAR stations, are
discussed.Comment: 11 pages, 17 figure
Electron precipitation from EMIC waves: a case study from 31 May 2013
On 31 May 2013 several rising-tone electromagnetic ion-cyclotron (EMIC) waves with intervals of pulsations of diminishing periods (IPDP) were observed in the magnetic local time afternoon and evening sectors during the onset of a moderate/large geomagnetic storm. The waves were sequentially observed in Finland, Antarctica, and western Canada. Co-incident electron precipitation by a network of ground-based Antarctic Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK) and riometer instruments, as well as the Polar-orbiting Operational Environmental Satellite (POES) electron telescopes, was also observed. At the same time POES detected 30-80 keV proton precipitation drifting westwards at locations that were consistent with the ground-based observations, indicating substorm injection. Through detailed modelling of the combination of ground and satellite observations the characteristics of the EMIC-induced electron precipitation were identified as: latitudinal width of 2-3° or ΔL=1 Re, longitudinal width ~50° or 3 hours MLT, lower cut off energy 280 keV, typical flux 1×104 el. cm-2 sr-1 s-1 >300 keV. The lower cutoff energy of the most clearly defined EMIC rising tone in this study confirms the identification of a class of EMIC-induced precipitation events with unexpectedly low energy cutoffs of <400 keV
Van Allen probes, NOAA, GOES, and ground observations of an intense EMIC wave event extending over 12 h in magnetic local time
Although most studies of the effects of EMIC waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on February 23, 2014 that extended over 8 hours in UT and over 12 hours in local time, stimulated by a gradual 4-hour rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) appeared for over 4 hours at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5-20 cm-3. Waves were also observed by ground-based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA-POES and METOP satellites near the northern footpoint of the Van Allen Probes observed 30-80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L-shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field-aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC-induced loss processes for this event
Minimum wages in 2020 : Annual review
Aquesta publicació s'elabora a partir de les contribucions de cadascú dels membres nacionals que integren la Network of Eufound Correspondent. Pel cas d'Espanya la contribució ha estat realitzada per l'Oscar MolinaThis report, as part of an annual series on minimum wages, summarises the key developments during 2019 and early 2020 around the EU initiative on fair wages and puts the national debates on setting the rates for 2020 and beyond in this context. The report features how minimum wages were set and the role of social partners. It discusses developments in statutory minimum wages and presents data on minimum wage rates in collective agreements related to 10 low-paid jobs for countries without statutory minimum wages
Energetic electron precipitation driven by electromagnetic ion cyclotron waves from ELFIN's low altitude perspective
We review comprehensive observations of electromagnetic ion cyclotron (EMIC)
wave-driven energetic electron precipitation using data from the energetic
electron detector on the Electron Losses and Fields InvestigatioN (ELFIN)
mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000
keV electrons with good pitch-angle and energy resolution. EMIC wave-driven
precipitation exhibits a distinct signature in energy-spectrograms of the
precipitating-to-trapped flux ratio: peaks at 0.5 MeV which are abrupt (bursty)
with significant substructure (occasionally down to sub-second timescale).
Multiple ELFIN passes over the same MLT sector allow us to study the spatial
and temporal evolution of the EMIC wave - electron interaction region. Using
two years of ELFIN data, we assemble a statistical database of 50 events of
strong EMIC wave-driven precipitation. Most reside at L=5-7 at dusk, while a
smaller subset exists at L=8-12 at post-midnight. The energies of the
peak-precipitation ratio and of the half-peak precipitation ratio (our proxy
for the minimum resonance energy) exhibit an L-shell dependence in good
agreement with theoretical estimates based on prior statistical observations of
EMIC wave power spectra. The precipitation ratio's spectral shape for the most
intense events has an exponential falloff away from the peak (i.e., on either
side of 1.45 MeV). It too agrees well with quasi-linear diffusion theory based
on prior statistics of wave spectra. Sub-MeV electron precipitation observed
concurrently with strong EMIC wave-driven 1MeV precipitation has a spectral
shape that is consistent with efficient pitch-angle scattering down to 200-300
keV by much less intense higher frequency EMIC waves. These results confirm the
critical role of EMIC waves in driving relativistic electron losses. Nonlinear
effects may abound and require further investigation
Experimental evidence of the simultaneous occurrence of VLF chorus on the ground in the global azimuthal scale – from pre-midnight to the late morning
International audienceNight-time VLF (very low frequency) chorus bursts lasting about one hour have been recorded at Finnish temporal station Kannuslehto (CGM: 64.2 • ; 107.9 • , L = 5.3) during two VLF campaigns (on 25 February–4 March 2008 and 27 March–17 April 2011). The chorus bursts were associated with substorm development. They were accompanied by riometer absorption enhancements, which occurred simultaneously within as large longitude areas as from pre-midnight (Sodankylä, ∼22:00 MLT) to the late morning (Tixie, ∼03:00 MLT and Gakona, ∼08:00 MLT) longitudes. It was found that the pre-midnight chorus observed on the ground occurred simultaneously with VLF chorus emissions recorded in the late morning on the low-altitude DEMETER satellite crossing the similar geomagnetic latitudes on the opposite local time sector. For the first time some evidence of simultaneous chorus burst generation in the global longitudinal scale was found (from pre-midnight to the late morning) by using direct comparison with satellite data as well as using non-direct indicator–azimuthally extended riometer absorption enhancements
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