SM31D-01
Wavelet analysis of low frequency magnetic field fluctuations at the magnetosheath and foreshock regions of the outer planets
Low frequency magnetic field waves in the magnetosheath and foreshock regions of the outer planet magnetospheres are studied in this work with wavelet analysis. Magnetic field data from Voyager-1 and 2 magnetometers are studied during the flybys of Saturn, Uranus and Neptune. The magnetic field data are 3-sec averaged, and are analysed with the wavelet Morlet transform. This wavelet analysis enables the identification of the main frequencies in the magnetosheath and foreshock regions of the outer planets. Further,the non- stationarity of these waves is investigated, as well as the space dependence in relation to bow shock or magnetopause crossings. Finally, the wave frequencies and properties are compared for the different regions and different planets.
SM31D-02
Temporal and Spatial Characteristics of Pc 1-2 Waves Observed by ST5
We present the results of a study of ULF waves in the Pc 1-2 frequency range (0.2-5 Hz) recorded by the three spacecraft of NASA's ST 5 mission, which operated in a dawn-dusk, 300 x 4500 km sun-synchronous orbit in a "pearls-on-a-string" configuration, with spacing ranging from >5000 km down to under 50 km, from March 26 through June 23, 2006. We find that regions with Pc 1-2 wave activity are not only highly localized to rather narrow L shells, as has been known for some time, but they also can appear and disappear on the time scales of ~10 s to 10 min examined by ST5. Only half of the 48 identified events were observed by all three spacecraft as they passed over similar L shells, and five events were observed by only one spacecraft. We interpret the lack of more multi-spacecraft observations as indicating that the regions of the magnetosphere that become unstable to electromagnetic ion cyclotron instabilities are often short-lived during moderate to quiet geomagnetic conditions. Wave occurrence was maximum in the daytime sector, consistent with stimulation by magnetospheric compressions. Only seven events were observed below L = 4, and only one below L = 3.6, consistent with the relatively quiet geomagnetic conditions during this interval. The temporal occurrence distribution of Pc 1-2 events was similar to that recorded at Halley, Antarctica (L = 4.56) during this same interval, in that the number and intensity of events increased during magnetospheric compressions and during the recovery phase of magnetic storms, but was reduced or absent during the main phase and early recovery phase of magnetic storms. This agreement suggests that if Pc 1-2 events occur during main phase, their nearly universal absence in ground records cannot be ascribed to ionospheric screening effects or obscuration by irregular ULF noise generated in the ionosphere. These findings add support to recent suggestions that although EMIC waves might theoretically cause rapid depletion of radiation belt electrons during the main phase of storms, such waves cannot be assumed to occur during the main phase of all storms.
SM31D-03
Structure of ULF Pc3 waves in the Upper Ionosphere: Observations and a Model
Recent low-orbiting observations by satellites with highly sensitive magnetic measurements (Orsted, CHAMP, ST-5) have provided a detailed picture of the Pc3 wave structure in the topside ionosphere. Pc3 waves in space were detected very clearly in the compressional component of the satellite magnetic field data, whereas in ground magnetometer data, their signature was found in the H component. The occurrence of a significant compressional component in Pc3 pulsations in the top-side ionosphere was also evidenced by radio-sounding measurements of ionospheric plasma oscillations. We consider the following two possibilities for ULF compressional disturbance excitation: (a) The incident Alfven wave upon interaction with the anisotropically conducting ionosphere generates an evanescent fast compressional mode; and (b) The transport of ULF wave energy from a distant source towards the ionosphere predominantly occurs by a fast compressional mode. We have developed an analytical-numerical model of the magnetosphere-ionosphere-atmosphere-ground system which enables one to estimate quantitatively the expected relationship between the Pc3 wave magnetic components above the ionosphere and on the ground produced by these two different mechanisms. The results of the model are applied to the interpretation of observations of Pc3 waves by satellites in the upper ionosphere and by mid-latitude ground stations.
SM31D-04
Comparison of techniques to determine intermittency of riometer auroral absorption at South Pole
It is well known that auroral radio wave absorption, as measured by riometers, consists of periods of relative quiescence which are interrupted by short bursts of activity. Such patterns in activity are observed in systems ranging from the stock market to turbulence, i.e. they exhibit intermittency. In the case of the auroral absorption it has also been found that intermittency strongly depends on the magnetic local time, being largest in the nighttime sector. This can be interpreted as indicating that the precipitating particles responsible of the absorption exhibit intermittency, especially near the substorm eye, where the level of turbulence increases. Here, different techniques to determine intermittency of auroral absorption measured by a riometer at South Pole (-90°S) are compared. The techniques are (i) the standard Castings formulation, (ii) the Local Intermittency Measure, (iii) the superstatistic, and (iv) the non-extensive statistics. It is shown that results obtained using the superstatistic and non-extensive statistic techniques confirm previously published results for auroral absorption at South Pole using the Castings and the Local Intermittency Measure techniques. Furthermore, a preliminary comparison between techniques indicates technique differences seem to be more related to the conceptual approach of each one rather than to the numerical results given by them.
SM31D-05
Response of the Magnetosphere-Ionosphere system to a sudden southward turning of IMF
A sudden southward turning of IMF is simulated by the University of Michigan BATSRUS model. After the northward to southward transition of IMF at a solar wind speed of 400km/s encounters the bow shock, it takes about 6 minutes to propagate to the magnetopause, eating up the northward IMF inside the magnetosheath and excites the dayside reconnection. The ionospheric response to this sudden southward IMF transition is delayed by another ~4 minutes, during which, conversions from the cusp reconnection to dayside reconnection and Alf'ven wave propagation take place, carrying dayside plasma down to the Earth; thereafter changes in the ionosphere and ground magnetic perturbations associated with the southward IMF are observed. These responses appear to be globally onset. The time it takes from the bow shock hitness to the ionospheric reaction varies with the solar wind speed.
SM31D-06
Substorm plasma sheet ion pressure profiles
The plasma sheet pressure, temperature, and density profiles inferred from DMSP observations are used to investigate substorm growth, expansion, early-recovery, and late-recovery phases. During the growth phase, the pressure peaks at the inner edge of the plasma sheet. The premidnight pressure peak is associated with the temperature peak, while the postmidnight peak is associated with the density enhancement. After the substorm onset, the pressure at the inner edge diminishes. Instead, the pressure peaks at premidnight from X = –10 to –40 RE, which can be associated with temperature enhancement. During the early and late recovery phases, the pressure peaks at postmidnight, which is associated with a cold, dense ion population, possibly resulting from ion outflow and the substorm current systems. In the near-Earth region, the entropy decreases after substorm onset, but the specific entropy appears to be roughly conserved.