56 research outputs found
Comparisons of electron energy deposition derived from observations of lower thermospheric nitric oxide and from X-ray bremsstrahlung measurements
No full textThermospheric nitric oxide at higher latitudes: Model calculations with auroral energy input
No full textMesoscale ionospheric electrodynamics of omega bands determined from ground-based electromagnetic and satellite optical observations
No full textInternational audienceWe present ground-based electromagnetic data from the MIRACLE and BEAR networks and satellite optical observations from the UVI and PIXIE instruments on the Polar satellite of an omega band event over Northern Scandinavia on 26 June 1998, which occured close to the morning side edge of a substorm auroral bulge. Our analysis of the data concentrates on one omega band period from 03:18-03:27 UT, for which we use the method of characteristics combined with an analysis of the UVI and PIXIE data to derive a time series of instantaneous, solely data-based distributions of the mesoscale ionospheric electrodynamic parameters with a 1-min time resolution. In addition, the AMIE method is used to derive global Hall conductance patterns. Our results show that zonally alternating regions of enhanced ionospheric conductances ("tongues") up to ~60S and low conductance regions are associated with the omega bands. The tongues have a poleward extension of ~400km from their base and a zonal extension of ~380km. While they are moving coherently eastward with a velocity of ~770ms-1, the structures are not strictly stationary. The current system of the omega band can be described as a superposition of two parts: one consists of anticlockwise rotating Hall currents around the tongues, along with Pedersen currents, with a negative divergence in their centers. The sign of this system is reversing in the low conductance areas. It causes the characteristic ground magnetic signature. The second part consists of zonally aligned current wedges of westward flowing Hall currents and is mostly magnetically invisible below the ionosphere. This system dominates the field-aligned current (FAC) pattern and causes alternating upward and downward FAC at the flanks of the tongues with maximum upward FAC of ~25µA m-2. The total FAC of ~2MA are comparable to the ones diverted inside a westward traveling surge. Throughout the event, the overwhelming part of the FAC are associated with gradients of the ionospheric conductances, and 66-84% of the FAC are connected with ionospheric Hall currents
Effects of energetic electrons on the electrodynamics in the ionosphere
No full textInternational audienceFrom the observations by the PIXIE and UVI cameras on board the Polar satellite, we derive global maps of the precipitating electron energy spectra from less than 1keV to 100keV. Based on the electron spectra, we generate instantaneous global maps of Hall and Pedersen conductances. The UVI camera provides good coverage of the lower electron energies contributing most to the Pedersen conductance, while PIXIE captures the high energy component of the precipitating electrons affecting the Hall conductance. By characterizing the energetic electrons from some tens of keV and up to about 100keV using PIXIE X-ray measurements, we will, in most cases, calculate a larger electron flux at higher energies than estimated from a simple extrapolation of derived electron spectra from UVI alone. Instantaneous global conductance maps derived with and without inclusion of PIXIE data have been implemented in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure, to study the effects of energetic electrons on electrodynamical parameters in the ionosphere. We find that the improved electron spectral characterization using PIXIE data most often results in a larger Hall conductance and a smaller inferred electric field. In some localized regions the increase in the Hall conductance can exceed 100%. On the contrary, the Pedersen conductance remains more or less unaffected by the inclusion of the PIXIE data. The calculated polar cap potential drop may decrease more than 10%, resulting in a reduction of the estimated Joule heating integrated over the Northern Hemisphere by up to 20%. Locally, Joule heating may decrease more than 50% in some regions. We also find that the calculated energy flux by precipitating electrons increases around 5% when including the PIXIE data. Combined with the reduction of Joule heating, this results in a decrease in the ratio between Joule heating and energy flux, sometimes exceeding 25%. An investigation of the relationship between Joule heating and the AE index shows a nearly linear correspondence between the two quantities, in accordance with previous studies. However, we find lower proportionality factors than reported by others when taking geomagnetic conditions into account, ranging between 0.13 and 0.23GW/nT. We also find that the contribution from auroral particles to the energy budget is more important than most previous studies have reported. Key words. Ionosphere (auroral ionosphere; particle precipitation) ? Magnetospheric physics (storms and substorms
Cutoff latitude variation during solar proton events: Causes and consequences
No full textTo accurately quantify the effect of solar proton events (SPEs) on the atmosphere requires a good estimate of the particle energy deposition in the middle atmosphere (60–100 km) and how the energy is distributed globally. Protons in the energy range 1–20 MeV, depositing their energy in the middle atmosphere, are subject to more complex dynamics with strong day-night asymmetries compared to higher-energy particles. Our study targets six SPEs from 2003 to 2012. By using measurements from the Medium Energy Proton and Electron Detector on all available Polar Orbit Environment Satellites (POES), we show that in the main phase of geomagnetic storms the dayside cutoff latitudes are pushed poleward, while the nightside cutoff latitudes have the opposite response, resulting in strong day-night asymmetries in the energy deposition. These features cannot be measured by the frequently used Geostationary Operational Environmental Satellites (GOES). Assuming that the protons impact the polar atmosphere homogeneously above a fixed nominal latitude boundary will therefore give a significant overestimate of the energy deposited in the middle atmosphere during SPEs. We discuss the magnetospheric mechanisms responsible for the local time response in the cutoff latitudes and provide a simple applicable parameterization which includes both dayside and nightside cutoff latitude variability using only the Dst, the northward component of the interplanetary magnetic field, and solar wind pressure. The parameterization is utilized on the GOES particle fluxes, and the resulting energy deposition successfully captures the day-night asymmetry in good agreement with the energy deposition predicted from the POES measurement
Upper-mesospheric temperatures measured during intense substorms in the declining phase of the January 2005 solar proton events
No full textTemperature measurements from the ALOMAR Weber Na lidar together with cosmic
radio noise absorption measurements from IRIS and particle measurements from
NOAA 15, 16 and 17 are used to study effects of geomagnetic activity on the
polar winter upper-mesospheric temperature. On 21–22 January 2005 we have 14 h
of continuous temperature measurement with the Na lidar coinciding
with strong geomagnetic activity in the declining phase of one of the
hardest and most energetic Solar Proton Event (SPE) of solar cycle 23.
According to measurements by the imaging riometer IRIS in northern Finland,
the temperature measurements coincide with two periods of increased cosmic
radio noise absorption. Particle measurements from the three satellites,
NOAA 15, 16 and 17 that pass through and near our region of interest confirm
that the absorption events are probably due to particle precipitation and
not due to changes in e.g. the electron recombination coefficient.
The measured temperature variation at 85 and 90 km is dominated by a 7.6-h
wave with downward phase propagation and a vertical wavelength of
approximately 10 km. Assuming that the wave is due to a lower altitude
source independent of the particle precipitation, we do not find any
temperature modification that seems to be related to the absorption events.
The average temperature is larger than expected above 90 km based on MSIS
and the monthly mean from falling spheres, which could be due to particle
precipitation and Joule heating prior to our measurement period. There is
also a possibility that the identified wave phenomenon is an effect of the
geomagnetic activity itself. Earlier studies have reported of similar
wavelike structures in wind observations made by the EISCAT VHF radar during
SPEs, and found it conceivable that the wave could be excited by the effect
of energetic particles precipitating into the mesosphere
Statistical relationships between cosmic radio noise absorption and ionospheric electrical conductances in the auroral zone
No full textStatistical models expressing the Hall and Pedersen conductances and their ratio as functions of cosmic noise absorption (CNA) are derived for five intervals of magnetic local time. The models are based on simultaneous measurements of electron densities from the EISCAT UHF radar at Tromso (69.6 N, 19.2 E) and absorption from the imaging riometer at Kilpisjarvi (69.1 N, 20.8 E). The Hall conductance and the conductance ratio are found to be rather strongly related to CNA, whereas the Pedersen conductance is less so. The Hall conductance-CNA relationship is strongly dependent on magnetic local time. These results are interpreted as being the consequence of the particular sensitivity of CNA to the typical energy of electron precipitation, the latter changing as a function of local time as the electrons drift around the Earth. The models are compared to a previous study which did not use simultaneous measurements or take into account the local time dependence. There is a significant difference between that study and the results presented here
Seasonal variations in the incidence of auroral radio absorption events at very high latitude, and the influence of the magnetotail
No full textInternational audienceA statistical analysis has been made of the incidence of auroral radio absorption events at South Pole, and of its dependence on basic geophysical parameters such as season, time of day, and magnetic activity level. It is found that at low and moderate levels of activity the incidence of events in the winter season is at least twice that in the summer. However, at high activity no events at all occurred during the local summer night, which appears to be explicable as the effect of the magnetotail and the consequent distortion of the magnetosphere when the southern polar region is tilted strongly towards the Sun. Previous results from even higher latitudes show the effect in an even more exaggerated form, in that both the day and night periods of absorption activity exhibit strong seasonal variations
Variations in cutoff latitude during the January 2012 solar proton event and implication for the distribution of particle energy deposition
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