scholarly journals Nightside studies of coherent HF Radar spectral width behaviour

2002 ◽  
Vol 20 (9) ◽  
pp. 1399-1413 ◽  
Author(s):  
E. E. Woodfield ◽  
J. A. Davies ◽  
M. Lester ◽  
T. K. Yeoman ◽  
P. Eglitis ◽  
...  
Keyword(s):  
Case Studies ◽  
Open Field ◽  
Local Time ◽  
Frictional Heating ◽  
Spectral Width ◽  
Hf Radar ◽  
Closed Field ◽  
Multiple Peak ◽  
Field Lines

Abstract. A previous case study found a relationship between high spectral width measured by the CUTLASS Finland HF radar and elevated electron temperatures observed by the EISCAT and ESR incoherent scatter radars in the post-midnight sector of magnetic local time. This paper expands that work by briefly re-examining that interval and looking in depth at two further case studies. In all three cases a region of high HF spectral width (>200 ms-1) exists poleward of a region of low HF spectral width (<200 ms-1). Each case, however, occurs under quite different geomagnetic conditions. The original case study occurred during an interval with no observed electrojet activity, the second study during a transition from quiet to active conditions with a clear band of ion frictional heating indicating the location of the flow reversal boundary, and the third during an isolated sub-storm. These case studies indicate that the relationship between elevated electron temperature and high HF radar spectral width appears on closed field lines after 03:00 magnetic local time (MLT) on the nightside. It is not clear whether the same relationship would hold on open field lines, since our analysis of this relationship is restricted in latitude. We find two important properties of high spectral width data on the nightside. Firstly the high spectral width values occur on both open and closed field lines, and secondly that the power spectra which exhibit high widths are both single-peak and multiple-peak. In general the regions of high spectral width (>200 ms-1) have more multiple-peak spectra than the regions of low spectral widths whilst still maintaining a majority of single-peak spectra. We also find that the region of ion frictional heating is collocated with many multiple-peak HF spectra. Several mechanisms for the generation of high spectral width have been proposed which would produce multiple-peak spectra, these are discussed in relation to the data presented here. Since the regions of high spectral width are observed both on closed and open field lines the use of the boundary between low and high spectral width as an ionospheric proxy for the open/closed field line boundary is not a simple matter, if indeed it is possible at all.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities)

scholarly journals Dynamical critical scaling of electric field fluctuations in the greater cusp and magnetotail implied by HF radar observations of F-region Doppler velocity

2006 ◽  
Vol 24 (2) ◽  
pp. 689-705 ◽  
Author(s):  
M. L. Parkinson
Keyword(s):  
Solar Wind ◽  
Open Field ◽  
Electric Fields ◽  
Spectral Width ◽  
Hf Radar ◽  
Doppler Velocity ◽  
Closed Field ◽  
Scaling Exponent ◽  
Central Plasma ◽  
Field Lines

Abstract. Akasofu's solar wind ε parameter describes the coupling of solar wind energy to the magnetosphere and ionosphere. Analysis of fluctuations in ε using model independent scaling techniques including the peaks of probability density functions (PDFs) and generalised structure function (GSF) analysis show the fluctuations were self-affine (mono-fractal, single exponent scaling) over 9 octaves of time scale from ~46 s to ~9.1 h. However, the peak scaling exponent α0 was a function of the fluctuation bin size, so caution is required when comparing the exponents for different data sets sampled in different ways. The same generic scaling techniques revealed the organisation and functional form of concurrent fluctuations in azimuthal magnetospheric electric fields implied by SuperDARN HF radar measurements of line-of-sight Doppler velocity, vLOS, made in the high-latitude austral ionosphere. The PDFs of vLOS fluctuation were calculated for time scales between 1 min and 256 min, and were sorted into noon sector results obtained with the Halley radar, and midnight sector results obtained with the TIGER radar. The PDFs were further sorted according to the orientation of the interplanetary magnetic field, as well as ionospheric regions of high and low Doppler spectral width. High spectral widths tend to occur at higher latitude, mostly on open field lines but also on closed field lines just equatorward of the open-closed boundary, whereas low spectral widths are concentrated on closed field lines deeper inside the magnetosphere. The vLOS fluctuations were most self-affine (i.e. like the solar wind ε parameter) on the high spectral width field lines in the noon sector ionosphere (i.e. the greater cusp), but suggested multi-fractal behaviour on closed field lines in the midnight sector (i.e. the central plasma sheet). Long tails in the PDFs imply that "microbursts" in ionospheric convection occur far more frequently, especially on open field lines, than can be captured using the effective Nyquist frequency and volume resolution of SuperDARN radars.

scholarly journals A case study of HF radar spectral width in the post midnight magnetic local time sector and its relationship to the polar cap boundary

2002 ◽  
Vol 20 (4) ◽  
pp. 501-509 ◽  
Author(s):  
E. E. Woodfield ◽  
J. A. Davies ◽  
P. Eglitis ◽  
M. Lester
Keyword(s):  
Local Time ◽  
Plasma Sheet ◽  
Optical Emission ◽  
Spectral Width ◽  
Radar Data ◽  
Magnetic Local Time ◽  
Hf Radar ◽  
Polar Cap ◽  
Central Plasma

Abstract. The aim of this paper is to advance the current understanding of the spectral width parameter observed by coherent high frequency (HF) radars. In particular, we address the relationship of a frequently observed gradient, between low ( < 200 m/s) and high ( > 200 m/s) spectral width, to magnetospheric boundaries. Previous work has linked this gradient in the spectral width, in the nightside sector of magnetic local time, to the Polar Cap Boundary (PCB), and also to the boundary between the Central Plasma Sheet (CPS) and the Plasma Sheet Boundary Layer (PSBL). The present case study investigates the former by comparison with the 630.0 nm optical emission. No suitable data were available to test the second of the two hypotheses. It is found that during the interval in question the spectral width gradient is within the region of the 630.0 nm optical emission. A comparison of coherent and incoherent scatter radar data is also conducted, which indicates that values of high spectral width are typically collocated with elevated F-region electron temperatures. We conclude that the high spectral width region in the interval under study is associated with particle precipitation and also that the spectral width gradient is not a reliable method for locating the PCB.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities)

scholarly journals Meridian-scanning photometer, coherent HF radar, and magnetometer observations of the cusp: a case study

1999 ◽  
Vol 17 (2) ◽  
pp. 159-172 ◽  
Author(s):  
S. E. Milan ◽  
M. Lester ◽  
S. W. H. Cowley ◽  
J. Moen ◽  
P. E. Sandholt ◽  
...  
Keyword(s):  
Open Field ◽  
Current System ◽  
Hf Radar ◽  
Optical Observations ◽  
Closed Field ◽  
Plasma Convection ◽  
Radar Backscatter ◽  
Range Resolution ◽  
Equatorward Boundary ◽  
Field Lines

Abstract. The dynamics of the cusp region and post-noon sector for an interval of predominantly IMF By, Bz < 0 nT are studied with the CUTLASS Finland coherent HF radar, a meridian-scanning photometer located at Ny Ålesund, Svalbard, and a meridional network of magnetometers. The scanning mode of the radar is such that one beam is sampled every 14 s, and a 30° azimuthal sweep is completed every 2 minutes, all at 15 km range resolution. Both the radar backscatter and red line (630 nm) optical observations are closely co-located, especially at their equatorward boundary. The optical and radar aurora reveal three different behaviours which can interchange on the scale of minutes, and which are believed to be related to the dynamic nature of energy and momentum transfer from the solar wind to the magnetosphere through transient dayside reconnection. Two interpretations of the observations are presented, based upon the assumed location of the open/closed field line boundary (OCFLB). In the first, the OCFLB is co-located with equatorward boundary of the optical and radar aurora, placing most of the observations on open field lines. In the second, the observed aurora are interpreted as the ionospheric footprint of the region 1 current system, and the OCFLB is placed near the poleward edge of the radar backscatter and visible aurora; in this interpretation, most of the observations are placed on closed field lines, though transient brightenings of the optical aurora occur on open field lines. The observations reveal several transient features, including poleward and equatorward steps in the observed boundaries, "braiding" of the backscatter power, and 2 minute quasi-periodic enhancements of the plasma drift and optical intensity, predominantly on closed field lines.Key words. Ionosphere (auroral ionosphere; plasma convection) · Magnetospheric physics (magnetopause · cusp · and boundary layers)

scholarly journals High resolution observations of spectral width features associatedwith ULF wave signatures in artificial HF radar backscatter

2004 ◽  
Vol 22 (1) ◽  
pp. 169-182 ◽  
Author(s):  
D. M. Wright ◽  
T. K. Yeoman ◽  
L. J. Baddeley ◽  
J. A. Davies ◽  
R. S. Dhillon ◽  
...  
Keyword(s):  
High Resolution ◽  
Plasma Source ◽  
Spectral Width ◽  
Hf Radar ◽  
Mhd Waves ◽  
Small Scale ◽  
Ulf Waves ◽  
Source Population ◽  
Radar Backscatter ◽  
Field Lines

Abstract. The EISCAT high power heating facility at Tromsø, northern Norway, has been utilised to generate artificial radar backscatter in the fields of view of the CUTLASS HF radars. It has been demonstrated that this technique offers a means of making very accurate and high resolution observations of naturally occurring ULF waves. During such experiments, the usually narrow radar spectral widths associated with artificial irregularities increase at times when small scale-sized (high m-number) ULF waves are observed. Possible mechanisms by which these particle-driven high-m waves may modify the observed spectral widths have been investigated. The results are found to be consistent with Pc1 (ion-cyclotron) wave activity, causing aliasing of the radar spectra, in agreement with previous modelling work. The observations also support recent suggestions that Pc1 waves may be modulated by the action of longer period ULF standing waves, which are simultaneously detected on the magnetospheric field lines. Drifting ring current protons with energies of ∼ 10keV are indicated as a common plasma source population for both wave types. Key words. Magnetospheric physics (MHD waves and instabilities) – Space plasma physics (wave-particle interactions) – Ionosphere (active experiments)

scholarly journals Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations: signatures of boundary layer processes in the cusp

2004 ◽  
Vol 22 (2) ◽  
pp. 511-525 ◽  
Author(s):  
K. Oksavik ◽  
F. Søraas ◽  
J. Moen ◽  
R. Pfaff ◽  
J. A. Davies ◽  
...  
Keyword(s):  
Electric Field ◽  
Electric Fields ◽  
Broad Band ◽  
Spectral Width ◽  
Hf Radar ◽  
Optical Data ◽  
Low Energy Electrons ◽  
Field Lines ◽  
Auroral Phenomena ◽  
Electric Fields And Currents

Abstract. In this paper we discuss counterstreaming electrons, electric field turbulence, HF radar spectral width enhancements, and field-aligned currents in the southward IMF cusp region. Electric field and particle observations from the FAST spacecraft are compared with CUTLASS Finland spectral width enhancements and ground-based optical data from Svalbard during a meridional crossing of the cusp. The observed 630nm rayed arc (Type-1 cusp aurora) is associated with stepped cusp ion signatures. Simultaneous counterstreaming low-energy electrons on open magnetic field lines lead us to propose that such electrons may be an important source for rayed red arcs through pitch angle scattering in collisions with the upper atmosphere. The observed particle precipitation and electric field turbulence are found to be nearly collocated with the equatorward edge of the optical cusp, in a region where CUTLASS Finland also observed enhanced spectral width. The electric field turbulence is observed to extend far poleward of the optical cusp. The broad-band electric field turbulence corresponds to spatial scale lengths down to 5m. Therefore, we suggest that electric field irregularities are directly responsible for the formation of HF radar backscatter targets and may also explain the observed wide spectra. FAST also encountered two narrow highly structured field-aligned current pairs flowing near the edges of cusp ion steps. Key words. Ionosphere (electric fields and currents). Magnetosphere physics (magnetopause, cusp, and boundary layers; auroral phenomena)

scholarly journals Differential rotation in neutron stars with open and closed magnetic topologies

2020 ◽  
Vol 494 (3) ◽  
pp. 3095-3109
Author(s):  
F Anzuini ◽  
A Melatos
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Open Field ◽  
Flux Tube ◽  
Differential Rotation ◽  
Time Scale ◽  
Outer Sphere ◽  
Closed Field ◽  
Magnetic Tension ◽  
Field Lines

ABSTRACT Analytic arguments have been advanced that the degree of differential rotation in a neutron star depends on whether the topology of the internal magnetic field is open or closed. To test this assertion, the ideal-magnetohydrodynamics solver pluto is employed to investigate numerically the flow of an incompressible, viscous fluid threaded by a magnetic field with open and closed topologies in a conducting, differentially rotating, spherical shell. Rigid body corotation with the outer sphere is enforced on the Alfvén time-scale, along magnetic field lines that connect the northern and southern hemispheres of the outer sphere. Along other field lines, however, the behaviour is more complicated. For example, an initial point dipole field evolves to produce an approximately closed equatorial flux tube containing at least one predominantly toroidal and approximately closed field line surrounded by a bundle of predominantly toroidal but open field lines. Inside the equatorial flux tube, the field-line-averaged magnetic tension approaches zero, and the fluid rotates differentially, adjusting its angular velocity on the viscous time-scale to match the boundary conditions on the flux tube’s toroidal surface. Outside the equatorial flux tube, the differential rotation increases, as the magnetic tension averaged along open field lines decreases.

scholarly journals An unusual geometry of the ionospheric signature of the cusp: implications for magnetopause merging sites

2002 ◽  
Vol 20 (1) ◽  
pp. 29-40 ◽  
Author(s):  
G. Chisham ◽  
M. Pinnock ◽  
I. J. Coleman ◽  
M. R. Hairston ◽  
A. D. M. Walker
Keyword(s):  
Magnetic Field ◽  
Travel Time ◽  
Spectral Width ◽  
Hf Radar ◽  
Small Scale ◽  
Convection Flow ◽  
Closed Field ◽  
Plasma Convection ◽  
Ionosphere Plasma ◽  
Local Noon

Abstract. The HF radar Doppler spectral width boundary (SWB) in the cusp represents a very good proxy for the equatorward edge of cusp ion precipitation in the dayside ionosphere. For intervals where the Interplanetary Magnetic Field (IMF) has a southward component (Bz < 0), the SWB is typically displaced poleward of the actual location of the open-closed field line boundary (or polar cap boundary, PCB). This is due to the poleward motion of newly-reconnected magnetic field lines during the cusp ion travel time from the reconnection X-line to the ionosphere. This paper presents observations of the dayside ionosphere from SuperDARN HF radars in Antarctica during an extended interval ( ~ 12 h) of quasi-steady IMF conditions (By ~ Bz < 0). The observations show a quasi-stationary feature in the SWB in the morning sector close to magnetic local noon which takes the form of a 2° poleward distortion of the boundary. We suggest that two separate reconnection sites exist on the magnetopause at this time, as predicted by the anti-parallel merging hypothesis for these IMF conditions. The observed cusp geometry is a consequence of different ion travel times from the reconnection X-lines to the southern ionosphere on either side of magnetic local noon. These observations provide strong evidence to support the anti-parallel merging hypothesis. This work also shows that mesoscale and small-scale structure in the SWB cannot always be interpreted as reflecting structure in the dayside PCB. Localised variations in the convection flow across the merging gap, or in the ion travel time from the reconnection X-line to the ionosphere, can lead to localised variations in the offset of the SWB from the PCB. These caveats should also be considered when working with other proxies for the dayside PCB which are associated with cusp particle precipitation, such as the 630 nm cusp auroral emission.Key words. Ionosphere (plasma convection) – Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (magnetic reconnection)

scholarly journals The accuracy of using the spectral width boundary measured in off-meridional SuperDARN HF radar beams as a proxy for the open-closed field line boundary

2005 ◽  
Vol 23 (7) ◽  
pp. 2599-2604 ◽  
Author(s):  
G. Chisham ◽  
M. P. Freeman ◽  
T. Sotirelis ◽  
R. A. Greenwald
Keyword(s):  
Field Line ◽  
Large Scale ◽  
Spectral Width ◽  
Hf Radar ◽  
Closed Field ◽  
Plasma Convection ◽  
Meridional Direction ◽  
Closed Field Line ◽  
Good Proxy ◽  
Automated Algorithms

Abstract. Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate measurements of magnetic reconnection. This study compares the latitudes of spectral width boundaries (SWBs) measured by different beams of the Goose Bay radar of the Super Dual Auroral Radar Network (SuperDARN), with the latitudes of OCBs determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the accuracy of the SWB as a proxy for the ionospheric projection of the OCB depends on the line-of-sight direction of the radar beam. The latitudes of SWBs and OCBs were identified using automated algorithms applied to 5 years (1997–2001) of data measured in the 1000–1400 magnetic local time (MLT) range. Six different Goose Bay radar beams were used, ranging from those aligned in the geomagnetic meridional direction to those aligned in an almost zonal direction. The results show that the SWB is a good proxy for the OCB in near-meridionally-aligned beams but becomes progressively more unreliable for beams greater than 4 beams away from the meridional direction. We propose that SWBs are identified at latitudes lower than the OCB in the off-meridional beams due to the presence of high spectral width values that result from changes in the orientation of the beams with respect to the gradient in the large-scale ionospheric convection pattern. Keywords. Ionosphere (Instruments and techniques; Plasma convection) – Magnetospheric physics (Magnetopause, cusp and boundary layers)

scholarly journals On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

2000 ◽  
Vol 18 (12) ◽  
pp. 1523-1530 ◽  
Author(s):  
M. Pinnock ◽  
A. S. Rodger
Keyword(s):  
Local Time ◽  
Time Of Flight ◽  
Radar Data ◽  
Accurate Estimate ◽  
Hf Radar ◽  
Closed Field ◽  
Plasma Convection ◽  
Polar Cap ◽  
Radar Backscatter ◽  
Magnetospheric Cusp

Abstract. Previous work has shown that ionospheric HF radar backscatter in the noon sector can be used to locate the footprint of the magnetospheric cusp particle precipitation. This has enabled the radar data to be used as a proxy for the location of the polar cap boundary, and hence measure the flow of plasma across it to derive the reconnection electric field in the ionosphere. This work used only single radar data sets with a field of view limited to ~2 h of local time. In this case study using four of the SuperDARN radars, we examine the boundary determined over 6 h of magnetic local time around the noon sector and its relationship to the convection pattern. The variation with longitude of the latitude of the radar scatter with cusp characteristics shows a bay-like feature. It is shown that this feature is shaped by the variation with longitude of the poleward flow component of the ionospheric plasma and may be understood in terms of cusp ion time-of-flight effects. Using this interpretation, we derive the time-of-flight of the cusp ions and find that it is consistent with approximately 1 keV ions injected from a subsolar reconnection site. A method for deriving a more accurate estimate of the location of the open-closed field line boundary from HF radar data is described.Key words: Ionosphere (ionosphere–magnetosphere interactions; plasma convection) · Magnetospheric physics (magnetopause · cusp · and boundary layers)

scholarly journals Complexity in the scaling of velocity fluctuations in the high-latitude F-region ionosphere

2008 ◽  
Vol 26 (9) ◽  
pp. 2657-2672 ◽  
Author(s):  
M. L. Parkinson
Keyword(s):  
Probability Density ◽  
Spectral Width ◽  
Probability Density Functions ◽  
Closed Field ◽  
Density Functions ◽  
Scaling Exponent ◽  
Scaling Exponents ◽  
Velocity Fluctuations ◽  
F Region ◽  
Field Lines

Abstract. The temporal scaling properties of F-region velocity fluctuations, δvlos, were characterised over 17 octaves of temporal scale from τ=1 s to <1 day using a new data base of 1-s time resolution SuperDARN radar measurements. After quality control, 2.9 (1.9) million fluctuations were recorded during 31.5 (40.4) days of discretionary mode soundings using the Tasmanian (New Zealand) radars. If the fluctuations were statistically self-similar, the probability density functions (PDFs) of δvlos would collapse onto the same PDF using the scaling Ps (δvs, τ)=ταP (δvlos, τ) and δvs=δvlosτ−α where α is the scaling exponent. The variations in scaling exponents α and multi-fractal behaviour were estimated using peak scaling and generalised structure function (GSF) analyses, and a new method based upon minimising the differences between re-scaled probability density functions (PDFs). The efficiency of this method enabled calculation of "α spectra", the temporal spectra of scaling exponents from τ=1 s to ~2048 s. The large number of samples enabled calculation of α spectra for data separated into 2-h bins of MLT as well as two main physical regimes: Population A echoes with Doppler spectral width <75 m s−1 concentrated on closed field lines, and Population B echoes with spectral width >150 m s−1 concentrated on open field lines. For all data there was a scaling break at τ~10 s and the similarity of the fluctuations beneath this scale may be related to the large spatial averaging (~100 km×45 km) employed by SuperDARN radars. For Tasmania Population B, the velocity fluctuations exhibited approximately mono fractal power law scaling between τ~8 s and 2048 s (34 min), and probably up to several hours. The scaling exponents were generally less than that expected for basic MHD turbulence (α=0.25), except close to magnetic dusk where they peaked towards the basic MHD value. For Population A, the scaling exponents were larger than for Population B, having values generally in the range expected for basic MHD and Kolmogorov turbulence (α=0.25–0.33). The α spectra exhibited complicated variations with MLT and τ which must be related to different physical processes exerting more or less influence.

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