High-latitude propagation studies using a meridional chain of LF/MF/HF receivers

Abstract. For over a decade, Dartmouth College has operated programmable radio receivers at multiple high-latitude sites covering the frequency range 100-5000kHz with about a 1-s resolution. Besides detecting radio emissions of auroral origin, these receivers record characteristics of the ionospheric propagation of natural and man-made signals, documenting well-known effects, such as the diurnal variation in the propagation characteristics of short and long waves, and also revealing more subtle effects. For example, at auroral zone sites in equinoctial conditions, the amplitudes of distant transmissions on MF/HF frequencies are often enhanced by a few dB just before they fade away at dawn. The polarization and/or direction of the arrival of ionospherically propagating signals in the lower HF range (3-5MHz) show a consistent variation between pre-midnight, post-midnight, and pre-dawn conditions. As is well known, magnetic storms and substorms dramatically affect ionospheric propagation; data from multiple stations spanning the invariant latitude range 67-79° reveal spatial patterns of propagation characteristics associated with magnetic storms and substorms. For example, in the hours preceding many isolated substorms, favorable propagation conditions occur at progressively lower latitudes as a function of time preceding the substorm onset. For some of these effects, explanations follow readily from elementary ionospheric physics, but understanding others requires further investigation.Key words. Magnetospheric physics (annual phenomena) – Radio science (ionosphere propagation; radio-wave propagation)6

Download Full-text

<i>Letter to the Editor:</i> First complementary observations by ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar

Annales Geophysicae ◽

10.1007/s00585-998-1519-2 ◽

1998 ◽

Vol 16 (11) ◽

pp. 1519-1522 ◽

Cited By ~ 1

Author(s):

C. N. Mitchell ◽

I. K. Walker ◽

S. E. Pryse ◽

I. Kersley ◽

I. W. McCrea ◽

...

Keyword(s):

Auroral Zone ◽

Hf Radar ◽

Auroral Ionosphere ◽

Polar Cap ◽

Low Velocity ◽

Radio Science ◽

Ionospheric Tomography ◽

Ionospheric Physics ◽

Incoherent Scatter ◽

Complementary Nature

Abstract. Experimental results are presented from ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar. Tomographic measurements on 10 October 1996, showing a narrow, field-aligned enhancement in electron density in the post-noon sector of the dayside auroral zone, are related to a temporal increase in the plasma concentration observed by the incoherent scatter radar in the region where the HF radar indicated a low velocity sunwards convection. The results demonstrate the complementary nature of these three instruments for polar-cap ionospheric studies.Key words. Ionosphere · Auroral ionosphere · Polar ionosphere · Radio science (ionospheric physics)

Download Full-text

Global morphology of night-time <i>Nm</i>F2 enhancements

Annales Geophysicae ◽

10.5194/angeo-20-1795-2002 ◽

2002 ◽

Vol 20 (11) ◽

pp. 1795-1806 ◽

Cited By ~ 51

Author(s):

A. F. Farelo ◽

M. Herraiz ◽

A. V. Mikhailov

Keyword(s):

Solar Cycle ◽

Data Base ◽

Statistical Study ◽

Large Data ◽

Night Time ◽

Large Data Base ◽

Radio Science ◽

Ionospheric Physics ◽

Latitude Range ◽

Physical Mechanisms

Abstract. An overall statistical study of night-time enhancements of NmF2 has been carried out. All available foF2 observations since 1955 at 53 ionosonde stations distributed worldwide in the latitude range fgeom = 15° - 60° were used in the analysis. More than 200 000 station-nights of data were analysed. This large data base allowed us to study seasonal, solar cycle and spatial variations of the NmF2 nighttime enhancements. Both pre-midnight and post-midnight NmF2 peaks demonstrate distinct variations with geophysical conditions, indicating different physical mechanisms responsible for their formation.Key words. Ionosphere (mid-latitude ionosphere, ionosphere-magnetosphere interactions) Radio science (ionospheric physics)

Download Full-text

Ionosphere dynamics over Europe and western Asia during magnetospheric substorms 1998–99

Annales Geophysicae ◽

10.5194/angeo-21-1141-2003 ◽

2003 ◽

Vol 21 (5) ◽

pp. 1141-1151 ◽

Cited By ~ 16

Author(s):

D. V. Blagoveshchensky ◽

O. A. Maltseva ◽

A. S. Rodger

Keyword(s):

Short Term ◽

Magnetospheric Substorms ◽

Expansion Phase ◽

Radio Science ◽

Ionospheric Physics ◽

Western Asia ◽

Storms And Substorms ◽

Temporal And Spatial ◽

Using Data ◽

Short Term Forecasting

Abstract. The temporal and spatial behaviour of the ionospheric parameters foF2 and h'F during isolated substorms are examined using data from ionospheric stations distributed across Europe and western Asia. The main purpose is finding the forerunners of the substorm disturbances and a possible prediction of these disturbances. During the period from March 1998 to March 1999, 41 isolated substorms with intensities I = 60 - 400 nT were identified and studied. The study separated occasions when the local magnetometers were affected by the eastward electrojet (positive substorms) from those influenced by the westward electrojet (negative substorms). The deviations of the ionospheric parameters from their monthly medians (DfoF2 and Dh'F) have been used to determine the variations through the substorm. Substorm effects occurred simultaneously (< 1 h) across the entire observatory network. For negative substorms, DfoF2-values increase > 6 h before substorm onset, To, reaching a maximum 2–3 h before To. A second maximum occurs 1–2 h after the end of the substorm. The Dh'F values 3–4 h before To have a small minimum but then increase to a maximum at To. There is a second maximum at the end of the expansion phase before dh'F drops to a minimum 2–3 h after ending the expansion phase. For positive substorms, the timing of the first maximum of the dfoF2 and dh'F values depends on the substorm length – if it is longer, the position is closer to To. The effects on the ionosphere are significant: DfoF2 and Dh'F reach 2–3 MHz (dfoF2 = 50–70% from median value) and 50–70 km (D h'F = 20–30% from median value), respectively. Regular patterns of occurrence ahead of the first substorm signature on the magnetometer offer an excellent possibility to improve short-term forecasting of radio wave propagation conditions.Key words. Ionosphere (ionospheric disturbances) – Magnetospheric physics (storms and substorms) Radio science (ionospheric physics)

Download Full-text

Local and regional controls of phylogenetic structure at the high-latitude range limits of corals

Proceedings of The Royal Society B Biological Sciences ◽

10.1098/rspb.2017.0915 ◽

2017 ◽

Vol 284 (1861) ◽

pp. 20170915 ◽

Cited By ~ 5

Author(s):

Brigitte Sommer ◽

Eugenia M. Sampayo ◽

Maria Beger ◽

Peter L. Harrison ◽

Russ C. Babcock ◽

...

Keyword(s):

Climate Change ◽

High Latitude ◽

Species Interactions ◽

Phylogenetic Signal ◽

Range Limits ◽

High Turnover ◽

Phylogenetic Structure ◽

Phylogenetic Clustering ◽

Latitude Range ◽

Eastern Australia

Understanding how range-edge populations will respond to climate change is an urgent research priority. Here, we used a phylogenetic community ecology approach to examine how ecological and evolutionary processes shape biodiversity patterns of scleractinian corals at their high-latitude range limits in eastern Australia. We estimated phylogenetic signal in seven ecologically important functional traits and conducted tests of phylogenetic structure at local and regional scales using the net relatedness (NRI) and nearest taxon indices (NTI) for the presence/absence and abundance data. Regional tests showed light phylogenetic clustering, indicating that coral species found in this subtropical-to-temperate transition zone are more closely related to each other than are species on the nearby, more northerly Great Barrier Reef. Local tests revealed variable patterns of phylogenetic clustering and overdispersion and higher than expected phylogenetic turnover among sites. In combination, these results are broadly consistent with the hierarchical filtering model, whereby species pass through a regional climatic filter based on their tolerances for marginal conditions and subsequently segregate into local assemblages according to the relative strength of habitat filtering and species interactions. Conservatism of tested traits suggests that corals will likely track their niches with climate change. Nevertheless, high turnover of lineages among sites indicates that range shifts will probably vary among species and highlights the vulnerability and conservation significance of high-latitude reefs.

Download Full-text

Evaluation of the geometry of ionospheric current systems related to rapid geomagnetic variations

Annales Geophysicae ◽

10.5194/angeo-22-63-2004 ◽

2004 ◽

Vol 22 (1) ◽

pp. 63-72 ◽

Cited By ~ 8

Author(s):

S. V. Apatenkov ◽

V. A. Sergeev ◽

R. Pirjola ◽

A. Viljanen

Keyword(s):

Dynamic Pressure ◽

Auroral Oval ◽

Auroral Zone ◽

Middle Part ◽

Magnetic Storms ◽

Geomagnetically Induced Currents ◽

Geomagnetic Variations ◽

Ionospheric Current ◽

Solar Wind Parameters ◽

Current Systems

Abstract. To learn about the geometry and sources of the ionospheric current systems which generate strong geomagnetically induced currents, we categorize differential equivalent current systems (DEC) for events with strong dB/dt by decomposing them into the contributions of electrojet-type and vortex-type elementary systems. By solving the inverse problem we obtain amplitudes and locations of these elementary current systems. One-minute differences of the geomagnetic field values at the IMAGE magnetometer network in 1996–2000 are analysed to study the spatial distributions of large dB/dt events. The relative contributions of the two components are evaluated. In particular, we found that the majority of the strongest dB/dt events (100–1000nT/min) appear to be produced by the vortex-type current structures and most of them occur in the morning LT hours, probably caused by the Ps6 pulsation events associated with auroral omega structures. For strong dB/dt events the solar wind parameters are shifted toward strong (tens nT) southward IMF, enhanced velocity and dynamic pressure, in order for the main phase of the magnetic storms to occur. Although these events appear mostly during magnetic storms when the auroral oval greatly expands, the area of large dB/dt stays in the middle part of the auroral zone; therefore, it is connected to the processes taking part in the middle of the magnetosphere rather than in its innermost region populated by the ring current. Key words. Geomagnetism and paleomagnetism (rapid time variations) – Ionosphere (auroral ionosphere; ionospheric disturbances)

Download Full-text

Role of plasma instabilities driven by oxygen ions during magnetic storms and substorms

Disturbances in Geospace: The Storm-Substorm Relationship - Geophysical Monograph Series ◽

10.1029/142gm12 ◽

2003 ◽

pp. 131-141 ◽

Cited By ~ 2

Author(s):

G.S. Lakhina ◽

S.V. Singh

Keyword(s):

Magnetic Storms ◽

Plasma Instabilities ◽

Oxygen Ions ◽

Storms And Substorms

Download Full-text

A comparison of satellite scintillation measurements with HF radar backscatter characteristics

Annales Geophysicae ◽

10.5194/angeo-23-3451-2005 ◽

2005 ◽

Vol 23 (11) ◽

pp. 3451-3455 ◽

Cited By ~ 8

Author(s):

S. E. Milan ◽

S. Basu ◽

T. K. Yeoman ◽

R. E. Sheehan

Keyword(s):

Broad Spectrum ◽

High Latitude ◽

Auroral Zone ◽

Intersection Point ◽

Density Fluctuations ◽

Hf Radar ◽

Ionospheric Scintillation ◽

Radar Backscatter ◽

Density Perturbations ◽

Coherent Radar

Abstract. We examine the correspondence between high latitude ionospheric scintillation measurements made at 250MHz with the occurrence of 10MHz HF coherent radar backscatter, on 13 and 14 December 2002. We demonstrate that when the ionospheric intersection point of the scintillation measurements is co-located with significant HF radar backscatter, the observed scintillation, quantified by the S4 index, is elevated. Conversely, when the radar indicates that backscatter is observed away from the intersection point due to movements of the auroral zone, the observed scintillation is low. This suggests that scintillation is highly location-dependent, being enhanced in the auroral zone and being lower at sub-auroral latitudes. The coexistence of scintillation and HF radar backscatter, produced by ionospheric density perturbations with scale sizes of 100s of metres and ~15 m, respectively, suggests that a broad spectrum of density fluctuations is found in the auroral zone.

Download Full-text