EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary

Abstract. The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6° cgmLat) and Longyearbyen (75.2° cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5° cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. During the substorm recovery phase, the footpoints of the Cluster satellites at a geocentric distance of 4.4 RE mapped in the vicinity of EISCAT measurements. Cluster data indicate that outflow of H+ and O+ ions took place within the plasma sheet boundary layer (PSBL) as noted in some earlier studies as well. We show that in this case the PSBL corresponded to a region of enhanced electron temperature in the ionospheric F region. It is suggested that the ion outflow originates from the F region as a result of increased ambipolar diffusion. At higher altitudes, the ions could be further energized by waves, which at Cluster altitudes were observed as BBELF (broad band extra low frequency) fluctuations. The four-satellite configuration of Cluster revealed a sudden poleward expansion of the PSBL by 2° during ~5 min. The beginning of the poleward motion of the PCB was associated with an intensification of the downward FAC at the boundary. We suggest that the downward FAC sheet at the PCB is the high-altitude counterpart of the Earthward flowing FAC produced in the vicinity of the magnetotail neutral line by the Hall effect (Sonnerup, 1979) during a short-lived reconnection pulse.

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IMF effect on the polar cap contraction and expansion during a period of substorms

Annales Geophysicae ◽

10.5194/angeo-31-1021-2013 ◽

2013 ◽

Vol 31 (6) ◽

pp. 1021-1034 ◽

Cited By ~ 4

Author(s):

A. T. Aikio ◽

T. Pitkänen ◽

I. Honkonen ◽

M. Palmroth ◽

O. Amm

Keyword(s):

Recovery Phase ◽

Bow Shock ◽

Polar Cap ◽

Reconnection Rate ◽

Early Recovery ◽

Mhd Simulation ◽

Expansion Phase ◽

Contraction And Expansion ◽

Good Agreement ◽

The Imf

Abstract. The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

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Combined ESR and EISCAT observations of the dayside polar cap and auroral oval during the May 15, 1997 storm

Annales Geophysicae ◽

10.1007/s00585-000-1067-x ◽

2000 ◽

Vol 18 (9) ◽

pp. 1067-1072 ◽

Cited By ~ 8

Author(s):

H. Liu ◽

K. Schlegel ◽

S.-Y. Ma

Keyword(s):

Strong Electric Field ◽

Dynamic Pressure ◽

Time Lag ◽

Recovery Phase ◽

Auroral Oval ◽

Auroral Zone ◽

Soft Particle ◽

Polar Cap ◽

Ion Flow ◽

Storms And Substorms

Abstract. The high-latitude ionospheric response to a major magnetic storm on May 15, 1997 is studied and different responses in the polar cap and the auroral oval are highlighted. Depletion of the F2 region electron density occurred in both the polar cap and the auroral zone, but due to different physical processes. The increased recombination rate of O+ ions caused by a strong electric field played a crucial role in the auroral zone. The transport effect, however, especially the strong upward ion flow was also of great importance in the dayside polar cap. During the main phase and the beginning of the recovery phase soft particle precipitation in the polar cap showed a clear relation to the dynamic pressure of the solar wind, with a maximum cross-correlation coefficient of 0.63 at a time lag of 5 min.Key words: Ionosphere (auroral ionosphere; polar ionosphere) - Magnetospheric physics (storms and substorms)

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Signatures of moving polar cap arcs in the F-region PolarDARN echoes

Annales Geophysicae ◽

10.5194/angeo-30-441-2012 ◽

2012 ◽

Vol 30 (3) ◽

pp. 441-455 ◽

Cited By ~ 9

Author(s):

A. V. Koustov ◽

K. Hosokawa ◽

N. Nishitani ◽

K. Shiokawa ◽

H. Liu

Keyword(s):

Spectral Width ◽

Global Scale ◽

Auroral Oval ◽

Polar Cap ◽

Common Features ◽

F Region ◽

Radar Network ◽

Radar Measurements ◽

Resolute Bay ◽

Region Plasma

Abstract. Joint observations of the all-sky camera at Resolute Bay (Nunavut, Canada) and the Polar Dual Auroral Radar Network (PolarDARN) HF radars at Rankin Inlet and Inuvik (Canada) are considered to establish radar signatures of poleward moving polar cap arcs "detaching" from the auroral oval. Common features of the events considered are enhanced power or echo occurrence in the wake of the arcs and enhanced spectral width of these echoes. When the arcs were oriented along some of the radar beams, velocity reversals at the arc location were observed with the directions of the arc-associated flows corresponding to a converging electric field. For the event of 9 December 2007, two arcs were poleward progressing almost along the central beams of the Inuvik radar at the speed close to the E × B drift of the bulk of the F-region plasma as inferred from HF Doppler velocities and from independent measurements by the Resolute Bay ionosonde. In global-scale convection maps inferred from all Super Dual Auroral Radar Network (SuperDARN) radar measurements, the polar cap arcs were often seen close to the reversal line of additional mesoscale convection cells located poleward of the normal cells related to the auroral oval.

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Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm

Annales Geophysicae ◽

10.5194/angeo-27-2157-2009 ◽

2009 ◽

Vol 27 (5) ◽

pp. 2157-2171 ◽

Cited By ~ 4

Author(s):

T. Pitkänen ◽

A. T. Aikio ◽

A. Kozlovsky ◽

O. Amm

Keyword(s):

Electric Field ◽

Growth Phase ◽

Continuation Method ◽

Recovery Phase ◽

Temporal Variations ◽

Auroral Oval ◽

Closed Field ◽

Polar Cap ◽

Vhf Radar ◽

The North

Abstract. The dynamics of the polar cap and the auroral oval are examined in the evening sector during a substorm period on 25 November 2000 by using measurements of the EISCAT incoherent scatter radars, the north-south chain of the MIRACLE magnetometer network, and the Polar UV Imager. The location of the polar cap boundary (PCB) is estimated from electron temperature measurements by the mainland low-elevation EISCAT VHF radar and the 42 m antenna of the EISCAT Svalbard radar. A comparison to the poleward auroral emission (PAE) boundary by the Polar UV Imager shows that in this event the PAE boundary is typically located 0.7° of magnetic latitude poleward of the PCB by EISCAT. The convection reversal boundary (CRB) is determined from the 2-D plasma drift velocity extracted from the dual-beam VHF data. The CRB is located 0.5–1° equatorward of the PCB indicating the existence of viscous-driven antisunward convection on closed field lines. East-west equivalent electrojets are calculated from the MIRACLE magnetometer data by the 1-D upward continuation method. In the substorm growth phase, electrojets together with the polar cap boundary move gradually equatorwards. During the substorm expansion phase, the Harang discontinuity (HD) region expands to the MLT sector of EISCAT. In the recovery phase the PCB follows the poleward edge of the westward electrojet. The local ionospheric reconnection electric field is calculated by using the measured plasma velocities in the vicinity of the polar cap boundary. During the substorm growth phase, values between 0 and 10 mV/m are found. During the late expansion and recovery phase, the reconnection electric field has temporal variations with periods of 7–27 min and values from 0 to 40 mV/m. It is shown quantitatively, for the first time to our knowledge, that intensifications in the local reconnection electric field correlate with appearance of auroral poleward boundary intensifications (PBIs) in the same MLT sector. The results suggest that PBIs (typically 1.5 h MLT wide) are a consequence of temporarily enhanced longitudinally localized magnetic flux closure in the magnetotail.

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Influence of high-latitude geomagnetic pulsations on recordings of broad-band force-balanced seismic sensors

Geoscientific Instrumentation Methods and Data Systems Discussions ◽

10.5194/gid-2-107-2012 ◽

2012 ◽

Vol 2 (1) ◽

pp. 107-148 ◽

Cited By ~ 2

Author(s):

E. Kozlovskaya ◽

A. Kozlovsky

Keyword(s):

Magnetic Field ◽

Geomagnetic Field ◽

Broad Band ◽

Geomagnetic Latitude ◽

Low Frequency ◽

Auroral Oval ◽

Polar Regions ◽

Geomagnetic Pulsations ◽

Seismic Sensors ◽

Magnetic Disturbances

Abstract. Seismic broad-band sensors with electromagnetic feedback are sensitive to variations of surrounding magnetic field, including variations of geomagnetic field. Usually, the influence of the geomagnetic field on recordings of such seismometers is ignored. It might be justified for seismic observations at middle and low latitudes. The problem is of high importance, however, for observations in Polar Regions (above 60° geomagnetic latitude), where magnitudes of natural magnetic disturbances may be two or even three orders larger. In our study we investigate the effect of ultra-low frequency (ULF) magnetic disturbances, known as geomagnetic pulsations, on the STS-2 seismic broadband sensors. The pulsations have their sources and, respectively, maximal amplitudes in the region of the auroral ovals, which surround the magnetic poles in both hemispheres at geomagnetic latitude (MLAT) between 60° and 80°. To investigate sensitivity of the STS-2 seismometer to geomagnetic pulsations, we compared the recordings of permanent seismic stations in northern Finland to the data of the magnetometers of the IMAGE network located in the same area. Our results show that temporary variations of magnetic field with periods of 40–150 s corresponding to regular Pc4 and irregular Pi2 pulsations are seen very well in recordings of the STS-2 seismometers. Therefore, these pulsations may create a serious problem for interpretation of seismic observations in the vicinity of the auroral oval. Moreover, the shape of Pi2 magnetic disturbances and their periods resemble the waveforms of glacial seismic events reported originally by Ekström (2003). The problem may be treated, however, if combined analysis of recordings of collocated seismic and magnetic instruments is used.

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Polar cap patches observed during the magnetic storm of November 2003: observations and modeling

Annales Geophysicae ◽

10.5194/angeo-33-1117-2015 ◽

2015 ◽

Vol 33 (9) ◽

pp. 1117-1133 ◽

Cited By ~ 3

Author(s):

C. E. Valladares ◽

T. Pedersen ◽

R. Sheehan

Keyword(s):

Magnetic Storm ◽

Recovery Phase ◽

Auroral Oval ◽

Polar Cap ◽

Throat Region ◽

Incoherent Scatter ◽

Cross Correlation Analysis ◽

Convection Patterns ◽

Airglow Intensity ◽

Good Agreement

Abstract. We present multi-instrumented measurements and multi-technique analysis of polar cap patches observed early during the recovery phase of the major magnetic storm of 20 November 2003 to investigate the origin of the polar cap patches. During this event, the Qaanaaq imager observed elongated polar cap patches, some of which containing variable brightness; the Qaanaaq digisonde detected abrupt NmF2 fluctuations; the Sondrestrom incoherent scatter radar (ISR) measured patches placed close to but poleward of the auroral oval–polar cap boundary; and the DMSP-F13 satellite intersected topside density enhancements, corroborating the presence of the patches seen by the imager, the digisonde, and the Sondrestrom ISR. A 2-D cross-correlation analysis was applied to series of two consecutive red-line images, indicating that the magnitude and direction of the patch velocities were in good agreement with the SuperDARN convection patterns. We applied a back-tracing analysis to the patch locations and found that most of the patches seen between 20:41 and 21:29 UT were likely transiting the throat region near 19:41 UT. Inspection of the SuperDARN velocities at this time indicates spatial and temporal collocation of a gap region between patches and large (1.7 km s−1) line-of-sight velocities. The variable airglow brightness of the patches observed between 20:33 and 20:43 UT was investigated using the numerical Global Theoretical Ionospheric Model (GTIM) driven by the SuperDARN convection patterns and a variable upward/downward neutral wind. Our numerical results indicate that variations in the airglow intensity up to 265 R can be produced by a constant 70 m s−1 downward vertical wind.

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The Low-Frequency Array (LOFAR): Opening a New Window on the Universe

Symposium - International Astronomical Union ◽

10.1017/s0074180900169591 ◽

2002 ◽

Vol 199 ◽

pp. 474-483

Author(s):

Namir E. Kassim ◽

T. Joseph W. Lazio ◽

William C. Erickson ◽

Patrick C. Crane ◽

R. A. Perley ◽

...

Keyword(s):

Angular Resolution ◽

Broad Band ◽

Low Frequency ◽

Electromagnetic Spectrum ◽

Interferometric Imaging ◽

Very Large Array ◽

Long Wavelength ◽

Calibration Techniques ◽

Long Baselines ◽

The Universe

Decametric wavelength imaging has been largely neglected in the quest for higher angular resolution because ionospheric structure limited interferometric imaging to short (< 5 km) baselines. The long wavelength (LW, 2—20 m or 15—150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. This has inspired the Low Frequency Array (LOFAR)—-a fully electronic, broad-band (15—150 MHz)antenna array which will provide an improvement of 2—3 orders of magnitude in resolution and sensitivity over the state of the art.

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The challenging SED of AP Librae

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312009532 ◽

2011 ◽

Vol 7 (S284) ◽

pp. 411-413 ◽

Cited By ~ 3

Author(s):

David Sanchez ◽

Berrie Giebels ◽

Pascal Fortin ◽

Keyword(s):

Spectral Energy Distribution ◽

Broad Band ◽

Low Frequency ◽

Theoretical Models ◽

Intermediate Frequency ◽

High Energy ◽

Spectral Energy ◽

Broad Band Emission ◽

Band Emission ◽

Bl Lac

AbstractMatching the broad-band emission of active galaxies with the predictions of theoretical models can be used to derive constraints on the properties of the emitting region and to probe the physical processes involved. AP Librae is the third low frequency peaked BL Lac (LBL) detected at very high energy (VHE, E>100GeV) by an Atmospheric Cherenkov Telescope; most VHE BL Lacs (34 out of 39) belong to the high-frequency and intermediate-frequency BL Lac classes (HBL and IBL). LBL objects tend to have a higher luminosity with lower peak frequencies than HBLs or IBLs. The characterization of their time-averaged spectral energy distribution is challenging for emission models such as synchrotron self-Compton (SSC) models.

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Observations of AGW/TID propagation across the polar cap: a case study

Annales Geophysicae ◽

10.5194/angeo-29-1355-2011 ◽

2011 ◽

Vol 29 (8) ◽

pp. 1355-1363 ◽

Cited By ~ 11

Author(s):

H. T. Cai ◽

F. Yin ◽

S. Y. Ma ◽

I. W. McCrea

Keyword(s):

Large Scale ◽

Source Region ◽

Polar Cap ◽

Night Time ◽

F Region ◽

Atmospheric Disturbances ◽

Geomagnetic Pole ◽

Ionization Waves ◽

North Polar ◽

Propagation Properties

Abstract. In this paper, we present observational evidence for the trans-polar propagation of large-scale Traveling Ionospheric Disturbances (TIDs) from their nightside source region to the dayside. On 13 February 2001, the 32 m dish of EISCAT Svalbard Radar (ESR) was directing toward the geomagnetic pole at low elevation (30°) during the interval 06:00–12:00 UT (MLT ≈ UT + 3 h), providing an excellent opportunity to monitor the ionosphere F-region over the polar cap. The TIDs were first detected by the ESR over the dayside north polar cap, propagating equatorward, and were subsequently seen by the mainland UHF radar at auroral latitudes around geomagnetic local noon. The propagation properties of the observed ionization waves suggest the presence of a moderately large-scale TIDs, propagating across the northern polar cap from the night-time auroral source during substorm conditions. Our results agree with the theoretical simulations by Balthazor and Moffett (1999) in which poleward-propagating large-scale traveling atmospheric disturbances were found to be self-consistently driven by enhancements in auroral heating.

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